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Exploring frequency control capability of a PV system in a hybrid PV-rotating machine-without storage system
Electrical Power and Energy Systems 60 (2014) 258–267
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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Exploring frequency control capability of a PV system in a hybrid PV-rotating machine-without storage system P.P. Zarina, S. Mishra ⇑, P.C. Sekhar Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi, India
a r t i c l e
i n f o
Article history: Received 16 May 2013 Received in revised form 5 January 2014 Accepted 25 February 2014 Available online 1 April 2014 Keywords: Deloading Distributed sources Frequency control Penetration Photovoltaic systems Renewable sources
a b s t r a c t When the penetration of inertia-free photovoltaic (PV) system is rapidly increasing, it is of timely importance to make the system capable of handling the disturbances safely so that the system stability can be assured by maintaining the system frequency. In this paper a novel control strategy has been formulated for frequency regulation utilizing the output from the PV generators itself without going for any kind of storage technologies. A multi bus system with two conventional generators and twelve PV systems which are working away from the maximum power point of operation is the system used for the study. Frequency control of the deloaded PVs with frequency controller is simulated using Power Factory Software. An improved controller which considers not only the frequency deviation of the system but also the available reserve in the photovoltaic is proposed. This ensures that the PV with more reserve will participate more in the frequency control compared to those with less reserve so that the frequency control capability is evenly distributed throughout the system. Two modes of operation for deloaded PV is suggested to derive maximum benefit when it is used for frequency regulation. A cost analysis is carried out to show that the concept of deloaded PV is economical when compared to battery usage for frequency control. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In conventional power system which is frequently subjected to disturbances, due to the machine inertia, energy exchanges takes place between the rotating machines and the other part of the system. The primary/secondary frequency control is provided by the automatic generation control (AGC) to maintain the required balance between load and generation. In future, the renewable energy sources especially in the form of small capacity generations are expected to outnumber the conventional sources. They are usually integrated with the grid at low voltage as distributed generators (DGs). Since most of these DGs, like the photovoltaic (PV) generators are interfaced with grid by means of power electronic converters, stability will be a big concern for the system operators due to the absence of inertia which makes them incapable of providing inertial/primary frequency response, if not regulated properly. Any generator is said to participate in frequency regulation when its output varies depending on load conditions and hence balances out the generation and demand. Due to expensive nature of solar panel, efforts are made to extract maximum power from the panels by using maximum power point tracking (MPPT) methods as could ⇑ Corresponding author. E-mail addresses: [email protected] (P.P. Zarina), [email protected] (S. Mishra). http://dx.doi.org/10.1016/j.ijepes.2014.02.033 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.
be seen in literature whose comparison study is done in [1]. Authors in [1] have reviewed and compared 19 distinct MPPT methods from 90 papers for the photovoltaic systems. This shows the thrust given for maximum power extraction from a PV panel. Using such MPPT algorithms, under varying conditions of temperature and irradiation, the power output from the solar panel is maintained at their maximum value irrespective of the load condition, implying that no frequency regulation contribution from the PV. With the help of proper control philosophies implemented in a battery storage system the frequency disturbances can be handled effectively. However the expenditure on the battery storages which includes their maintenance and replacement costs is an extra burden. Authors in [2] had done a performance analysis on storage devices and concluded that the battery storage is not economical. When the ancillary services like frequency control are gaining importance in the changing scenario with high renewable penetration, it is important to incorporate these features into photovoltaics itself, which is the main objective of this paper. In [3–9], usage of battery or other storage devices along with renewable energy systems is considered to mitigate the various issues like output leveling, voltage unbalance, voltage rise, frequency regulation etc. However, photovoltaics participating in frequency control without using any external resources like battery, supercapacitors etc. is hardly seen. For wind turbines the frequency
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regulation capability is discussed in many literatures [10–13]. The inertial response of DFIG is analyzed in [10,11]. The dependence of frequency response of wind turbine generator on the initial available active power output of the generator is brought out in [11]. Comparison of performance of variable speed wind turbine generator with a conventional generator done in [11] showed that even though the proposal is useful for initial frequency support, frequency recovery is inferior. In [12], droop of the WTG is adjusted continuously in response to wind velocity, thereby, its primary frequency response is improved in terms of reduced stresses on WTGs during low wind speeds. The frequency fluctuations in a grid connected PV system are reduced using an electric double layer capacitor in [14]. The idea proposed is to control the ramping up rate of the PV output. The electric double-layer capacitor absorbs the fluctuations of PV output and controls the ramp rate of its output based on the moving average of the PV output. In [15] a PV-diesel hybrid system is operated without any storage device. However, it requires the irradiation data and complex control for realization. This paper proposes a novel control philosophy in which the output power of the PV generator itself is controlled to maintain the system frequency by keeping some portion of the PV output as reserve. The highlight of this paper is to make the PV array operate away from its MPPT point and thus make a reserve power available with the PV itself. This reserve power is used whenever there is a demand for increased generation, there by, avoiding the frequency disturbances. So it is assumed that, in situations where frequency control is to be derived from PV, the panel is deloaded by a fixed maximum power percentage depending on the reserve to be made available. A control philosophy is formulated such that, during a disturbance, the power released is proportional to deviation in frequency (Df). During the transients, there is a fast response from the PV and it adjusts its output power depending on the demand of power. This eliminates the usage of costly storage units like battery. The day time scenario of a microgrid with industrial loads in which most of them are not operational at night is discussed. In such a case, the conventional generators are sufficient to meet the partial load at night. In [16], the overloading capacity of battery is utilized for primary frequency regulation. There it was shown that by the time the diesel generator ramps up to meet the increased power demand, the battery is used to take care of the deficient power. But in this paper, apart from using the PV for primary frequency regulation, it is used for secondary frequency regulation also, up to the maximum extent possible (until all the PVs reach the MPP). The idea behind the usage of PV power for secondary control is to minimize the load cycling of conventional generators. The conventional generator is not expected to contribute to frequency regulation until reserve is available with PV. But once the PV reserve gets exhausted due to the PVs reaching the maximum power point, the conventional generators will be taking care of the frequency regulation. Emission is an issue which is much talked about. In [17], one of the objectives of economic analysis was to reduce the emission by reducing the ramping of generators. If PV participates in secondary frequency control, the ramping up and down of conventional generators can be very much reduced. Advantages of this type of control is that operation and maintenance cost due to frequent ramping can be reduced and the emission will also be comparatively less. As compared to usage of battery for secondary frequency regulation, usage of deloaded PV has the advantage that PV being an active source, no charging is required unlike battery. Another advantage is the much lesser disposal issue of PV as compared to battery. This is because, PV has a life expectancy of nearly 25 years which is high compared to battery (life expectancy of battery is different for different types but much less compared to PV). Yet
259
another advantage is that PV is a clean energy. Only disadvantage in terms of secondary frequency regulation application as compared to battery is the unavailability during night. So it is recommended only in systems as considered for this study where the night loads can be met by conventional generators and higher power demand is during day time. Section 2 explains the PV model and its characteristics and also the deloading concept of photovoltaic system which provides reserve for frequency control. Section 3 explains the controllers used for frequency control and simulation results to show the performance of the controllers. As the system proposed is for frequency regulation which can also be achieved using a PV at MPP plus a battery storage unit, it is important to compare the cost of the two options. Hence, a cost analysis is carried out in Section 4, which brings out the economical aspect of using deloaded PV as against the battery unit installation for frequency regulation. In Section 5, modes of operation of deloaded PV is discussed and simulation results for the various modes of operation of PV is shown. The system used for study is modelled using Power Factory Software. 2. PV generator modeling and operation with reserve 2.1. PV modeling The mathematical model of photovoltaic array is represented by its characteristic function as in [18]. The basic theory of a photovoltaic cell is that when light falls on the p–n junction, charge carriers are generated which leads to an electric current. To increase the current rating, a PV panel will have many cells in parallel, and for a higher voltage, they have many cells in series. The output current Ipv is given by Eq. (1),
q V dc 1 Ipv ¼ Np Iph Np Irs exp kTA Ns
ð1Þ
where Np is number of cells in parallel, Ns is number of cells in series. Irs is reverse saturation current, q is the electron charge with a value of 1.602 1019 C, k is the Boltzmann’s constant (1.3806503 1023 J/K), A is the ideality factor, T is the temperature in Kelvin and Vdc is output dc voltage. Iph the short-circuit current for one string of the PV panel is a function of temperature T and irradiation S (W/m2). It is given by Eq. (2).
Iph ¼ ½ISC þ K I ðT T ref ÞS
ð2Þ
where ISC is short circuit current of the cell, Tref is the reference temperature, KI is temperature coefficient. Datasheet from manufacturer tells about the expected performance of PV arrays for standard test conditions (STC), which means at a temperature of 25 °C and irradiation of 1000 W/m2. The modeling of PV was done using Power Factory Software [19]. VMPP, the voltage at maximum power point for a given irradiation E is calculated based on VMPP0, voltage at maximum power point at standard test conditions (STC). This value of VMPP0 for a given module characteristics will be specified by the module manufacturer. From these values, the VMPP can be calculated for any irradiation levels as by Eq. (3).
V MPP ¼ V MPP0
lnðEÞ lnð1000Þ
ð3Þ
To incorporate the change in the PV characteristics with variation in irradiation and temperature, correction factor is included in the model depending on the ambient conditions at which the PV is working. The calculation using temperature correction factor av for voltage is given by (4).
V MPP ¼ V MPP0
lnðEÞ ð1 þ av ðT T STC ÞÞ lnð1000Þ
ð4Þ
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The power output of a single PV array is as shown in Fig. 1, whose specifications are given in Appendix A. The variation in the power output with different irradiation is evident from the figure. The MPP values for irradiation levels of 1000 W/m2, 900 W/m2 and 800 W/m2 are marked along with the corresponding Vdc values.
PMPP Preserve
P1
2.2. PV operation with reserve power If the photovoltaic systems are made to work at maximum power point, say for a PV with characteristics as in Fig. 1, under an irradiation of 1000 W/m2, the power output will be 448.5 kW. Irrespective of the load condition, the PV will always track the maximum power point and will give a fixed output if the ambient conditions remain the same. In such a situation, there is no frequency control contribution from the PV. This is because there would not be any reserve power available with them to participate in frequency control. Hence, it is proposed to provide a reserve power in the PV systems which is achieved by making the PV work away from the MPP. When the PV is operating at MPP, dc bus voltage will be VMPP and the power output will be maximum, marked as PMPP, which is the maximum value of power that can be extracted from the PV panel. As seen in Fig. 2, if the operation is at a higher voltage (DV) than VMPP, the power output will be P1. Under this condition the PV is said to be deloaded, as the full available power is not extracted. Here, the difference in P1 and PMPP is the reserve power Preserve which can be utilized for the frequency control. Depending on the percentage of deloading, the available reserve power varies. So if a company wants to install a PV system of capacity P1 and make it participate in frequency control, they have to invest money for an installation which is higher than P1 by the same proportion of deloading. To elaborate more, looking at Fig. 2, if P1 is the power to be produced in steady state after deloading, then the peak capacity has to be PMPP and hence the company has to invest according to PMPP. In this scenario the reserve power (Preserve) available to participate in frequency regulation is given by (5).
Preserv e ¼ PMPP P 1
ð5Þ
3. Controller for frequency control contribution If the PV system is not deloaded, and is working at its maximum power point (MPP) of operation, then there would not be any reserve for frequency regulation contribution. The active power is given by, P = vd * id + vq * iq (where d and q are direct axis and
5.0E+5 V_MPP=694V P_MPP=398kW Insolation 900W/m2
V_MPP=710V P_MPP=448.5kW Insolation 1000W/m2
4.0E+5
VMPP
VMPP+
Fig. 2. Concept of reserve power in the PV array.
quadrature axis components). At point of common coupling, vd = 1 and vq = 0, hence P = vd * id only. Hence active power can be varied by varying the id. For a PV which is not deloaded, the idref will change depending on the ambient conditions (because VMPP will change). In the deloaded PV, where we are finding a provision for frequency regulation in the controller of the PV itself, deloading is done by increasing the dc voltage beyond MPP. This is achieved by increasing the value from VMPP by a voltage Vdeload corresponding to the power reserve to be provided as depicted in Fig. 3. Though with this simple addition, the PV array keeps some reserve power, unless otherwise instructed, the reserve is not released. Since, the PV power has to change as per deviation in system frequency, a signal, proportional to frequency deviation Df, is added to the dc reference voltage as presented in the figure. Looking carefully at Fig. 3 it can be concluded that the variation of power from the PV will not only depend on VMPP value but also on frequency deviation as given by Eq. (6).
V dcref ¼ ðV MPP þ V deload V dc proportional to Df Þ
ð6Þ
As this controller does not consider reserve in the PV, it may lead to a situation that the PV with less reserve also equally contribute to frequency control as compared to PV with more reserve and may thus reach their MPP faster. When MPP is reached, they would not be able to contribute further to any new extra power requirement. This will lead to an uneven distribution of frequency regulation capability. Hence a modified controller is suggested in Section 3.1 which considers a component DVreserve. 3.1. Modified controller for distributed frequency control according to the available reserve Using the controller in Fig. 3, the power output of PV will change based on the change in frequency and change in the VMPP value (which varies depending on the ambient conditions). As shown in Figs. 4 and 5, the point 1 corresponds to the MPP power and point 2 corresponds to power at deloaded condition of PV.
Power
3.0E+5 V_MPP=681V P_MPP=347.5W Insolation 800W/m2
Kp1+Ki1/s
2.0E+5
V proportional to change in freq _ dc VMPP +
1.0E+5
Voc VMPP +
0.0E+0 0.00
200.00
400.00
600.00
800.00
1000.00
Limit
Kp+Ki/s Vdcref
_
idref
Vdc
Vdeload
Vdc Fig. 1. Power vs. voltage curve of PV array at different irradiation.
Fig. 3. Controller for deloaded PV without considering the amount of available reserve.
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When there is a load increase, the controller will decrease Vdc and make the PV work at point 3, with an increase in power output. As the controller in Fig. 3 does not consider how much reserve is still available (DVreserve) with the PV, the PV with more reserve and the one with less reserve will contribute equally to the frequency control. The modified controller in Fig. 6 accommodates the DVreserve component also, which corresponds to the reserve still available with the PV. The Vdcref in this case is given by Eq. 7. The multiplier block, which accounts for the 2nd term of equation will affect the output only when the DVreserve is non-zero. For higher values of DVreserve, the second term of the equation will be higher and Vdcref value will move more closer to VMPP, resulting in more power. Thus, the decrease of Vdc from its deloaded position to the new position will depend not only on Df but on DVreserve also. With this modified controller, as more power is contributed by PVs with more reserve, burden on PVs with less margin will be less and hence the frequency control capability is evenly distributed among the different PVs.
261
Fig. 5. PV with lesser deloading giving less reserve.
V dcref ¼ ðV MPP þ V deload V dc proportional to Df Þ ðDf DV reserv e K p2 Þ
ð7Þ
3.2. Simulation results to show controller performance A multi bus system given in IEEE Standard 399-1997 is taken for the simulation studies. There are two conventional synchronous generators connected at bus 4 and bus 50, and many rotating machines as loads at different buses. The system is modified with the addition of twelve PV generators connected at 18, 35, 37 and 49 as shown in Fig. 7. The PVs are of 448.5 kW each and are connected to 0.48 kV buses. The conventional generators are connected to 13.8 kV level at buses 4 and 50. The effect of controller parameters on the PV output is shown in Section 3.2.1. In Section 3.2.2, the simulation results of two types of controllers shown by Figs. 3 and 6 are compared. 3.2.1. Effect of controller parameters on the PV power The aim of this section is to see the effect of controller parameters on the PV outputs. All the PVs are deloaded with equal percentages of 18% with respect to their MPP. As all PVs are of same rating, equal deloading will result in equal reserve of 81 kW. The total reserve comes to 0.972 MW (81 kW * 12). A load increase is considered at 100 s to check the effect of controller parameters on the performance of the PV system. The effect of proportional gain and integral gain (Kp and Ki) of the PI controller of Fig. 3 is examined on the output of PV. The variation of output power of
Reserve available due to deloading
1 3 2
P
reserve
VMPP
Vdc proportional to change in frequency
VMPP + Vdeload
Fig. 4. PV with higher deloading giving more reserve.
Fig. 6. Improved controller for deloaded PV which considers the amount of available reserve.
PV and output of conventional generator connected at bus 50 with variation in control parameters is shown in Figs. 8 and 9. When Ki is changing from 1000 to 8000, the output power contribution from PV is increasing and correspondingly output power from conventional generator is decreasing. So with a value of 8000, PV is solely contributing for frequency regulation. A comparison of frequency response is shown in Fig. 10. From Fig. 10 it is seen that the area of the curve corresponding to the Ki = 8000 is small as compared to Ki = 2000 or 1000. This is justified by the fact that product of Ki and area of curve will be the power. So for higher values of Ki, the area of curve will be smaller. So in further simulation cases also the value of Ki is kept the same. The variation in Kp is not causing the steady state PV power values to change.
3.2.2. Performance comparison of two types of controllers A controller which is regulating the PV output based on the change in frequency alone was shown in Fig. 3. Using such a controller, the behavior of PV under two cases is compared in Fig. 11. In one case PVs are deloaded by 8% and in another case PVs are deloaded by 18%. In the first case the available reserve will be less compared to second case. It is seen that the PV power output increases by the same amount for a load change for both cases when the controller is the one which does not take into account the available reserve. Which means the PV with more reserve and the one with less reserve is contributing equally for a load change. In Fig. 12, the response of PVs which use the modified controller given in Fig. 6, for the same load change in the above case is shown. Here, the increase in power output after the load change is different for both PVs. The PV having more reserve power is contributing more to the frequency control compared to the one with less
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0.46
60.01
0.44
60.00
Frequency in Hz
PV Power in MW
Fig. 7. Multi bus system layout considered for study.
0.42
0.40
0.38
0.36 90.000
59.98
59.97
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: Power output in MW for Ki=8000 PV Generator: Power output in MW for Ki=2000 PV Generator: Power output in MW for Ki=1000
59.96 90.000
96.000
102.00
108.00
114.00
[s]
120.00
49: Bus frequency when the value of Ki is 8000 49: Bus frequency when the value of Ki is 2000 49: Bus frequency when the value of Ki is 1000
Fig. 8. Variation in output power of photovoltaic with different values of Ki.
Fig. 10. Comparison of system frequency for different values of controller parameters of PV.
reserve. This results in an even distribution of frequency regulation capability and hence the reserve.
8.95
Power of Generator in MW
59.99
8.83
3.3. Simulation results showing PV working under different conditions 8.71
8.59
8.47
8.35 90.000
102.00
114.00
126.00
138.00
[s]
150.00
GEN1: Power output in MW for Ki=8000 GEN1: Power output in MW for Ki=2000 GEN1: Power output in MW for Ki=1000
Fig. 9. Power output of conventional generator for different values of controller parameters of PV.
Case 1: PV alone participating in frequency regulation (PV reserve sufficient to meet load change). This type of operation result during high irradiation condition and small load change. As PV reserve will be sufficient to meet frequency regulation, no or little ramping up needed for conventional generators. Responses are given in Figs. 13 and 14. Case 2: PV reserve gets exhausted and conventional generator participates in frequency regulation. This type of operation is needed during high irradiation condition and higher load change. Even though PV irradiation level is same as that in case 1, for higher load change PV will reach MPP as shown in Fig. 15 and conventional generator ramp up to meet the balance load requirement as indicated by Fig. 16.
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0.5000
8.93
Power of Generator in MW
0.5500
0.4500
0.4000
0.3500
0.3000 90.000
102.00
114.00
126.00
138.00
8.49
8.05
7.61
7.17 90.000
[s] 150.00
PV Generator: With more reserve and with controller in Fig.3
102.00
114.00
126.00
138.00
[s] 150.00
GEN1: Active Power in MW
PV Generator: With less reserve and with controller in Fig.3
Fig. 14. Conventional generator power under case 1 operation.
Fig. 11. PV power contribution with unequal reserve using controller considering frequency deviation alone.
frequency regulation is that, in one case, additional PV size is needed and in the other, additional battery is to be used. Hence in this section, the cost of extra panel size needed for reserve is compared with battery unit cost. In Section 4.1, cost of additional PV panel is worked out when we are not using a battery and providing reserve in the PV itself. In Section 4.2, cost of battery units are worked out for different sizes of batteries. Cost analysis is done by assuming a life expectancy of PV panel as approximately 24 years [20]. As the bus voltage where PV is connected is 480 V, and if a battery is used for providing the reserve power, it should also be connected to same bus. In that case the dc voltage Vdc can be arrived at using (8),where Vac is the ac bus voltage. So Vdc should be 800 V.
0.5500
PV Power in MW
0.5000
0.4500
0.4000
0.3500
0.3000 90.000
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: With more reserve and with controller in Fig.4 PV Generator: With less reserve and with controller in Fig.4
Fig. 12. PV power contribution with unequal reserve using controller which considers frequency deviation and reserve.
ð8Þ
4.1. Cost of additional panel providing reserve in case of deloaded PV As explained in Section 2.2, for frequency regulation using deloaded PV, additional PV panels are to be installed as reserve. In the system simulated, the reserve provided was nearly 1 MW. The cost of this extra 1 MW installation, which is to be provided as reserve will be an additional expenditure and is calculated in this section. Table 1 shows the $/W cost of PV generator as given in [21] and Table 2 calculates the extra financial burden with the proposed control strategy by taking the standard cost of the year 2013. So, an additional cost of $2,240,000 is to be expended if a reserve power of 1 MW is to be provided in the photovoltaic system. This is to provide the reserve in the deloaded PV system for frequency regulation in place of a PV at MPP plus battery.
0.51
0.48
PV Power in MW
V dc
rffiffiffi 2 V ac P2 3
0.44
0.41
0.38
4.2. Cost of battery providing reserve in case of PV at MPPT
0.34 90.000
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: Active Power in MW
Fig. 13. PV power under case 1 operation.
4. Cost analysis A cost analysis is important to see if deloaded PV with reserve for frequency regulation (which results in reduced utilization of PV array) is economical as compared to usage of PV at MPP plus battery unit. As seen in Section 2.2, when using a deloaded PV for frequency regulation, even though battery unit is not used, extra panel size is needed. Cost wise difference in the two options of
Instead of deloading the PV system and providing a reserve for frequency regulation, if battery units are used by making the PVs work at MPP itself, the costing of battery unit depends on the size of battery. In [16], the overloading capacity of battery is utilized only for primary frequency regulation and not secondary frequency regulation. In that paper the battery is meeting the load change only for a short duration, that is, till diesel generator ramps up to meet the increased power demand. In such an application, instead of 1000 kW battery, 249 kW was sufficient by considering the overload capacity. But in this paper, apart from using the PV for primary frequency regulation, it is used for secondary frequency regulation also. In such case, the nominal size of battery
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analysis is carried out here with different capacities of battery storage as reserve against the 1 MW extra PV generating capacity. Table 3 calculates the battery cost when replacement of lead acid battery is needed every 2 years and for different battery size options depending on the geographical irradiation level. The calculation is based on Eqs. (9)–(11). The values are based on [22] which shows a value of 300 $/kW, 30 $/kW and 3 $/kW respectively for capital cost, operating cost and salvage value.
0.51
PV Power in MW
0.48
0.44
0.41
Capital cost of battery unit in $ ¼ ðC I NÞ=g 0.38
0.34 90.000
102.00 114.00 126.00 PV Generator: Power output in MW
138.00
[s] 150.00
Fig. 15. PV power under case 2 operation.
Power of Generator in MW
9.37
8.93
8.49
8.05
7.61
7.17 90.000
102.00
114.00
126.00
138.00
[s] 150.00
GEN1: Power output in MW
Fig. 16. Conventional generator power under case 2 operation.
ð9Þ
Operating cost of battery unit in $ ¼ ðO I TÞ=g
ð10Þ
Salvage value in $ ¼ ðS I NÞ=g
ð11Þ
where C is the capital cost of unit in $/kW, I is installed capacity in kW, N is Number of times battery replacement done during life of PV, O is operating cost of unit in $/kW, S is salvage of unit in $/kW, T = time period of study or life of PV, and g is the power efficiency. From the above anaysis, it can be concluded that, providing reserve in the PV itself for frequency regulation is finacially beneficial solutions when compared to battery storages. However, the amount of savings will vary depending on the chosen battery storage capacities. In systems where the reserve provided by extra panel size of 1 MW will deliver 1 MW reserve itself, there is a profit of $2,800,000 because in that system the battery to be used for equivalent frequency regulation will be 1 MW. The benefit decreases with the decrease in size of the battery. If the system is such that with 1 MW additional panel installation, reserve power is equivalent to battery size of 0.444445 MW, (lower due to lower irradiation conditions), then the battery cost is at a break even point when compared to PV with reserve. So usage of deloaded PV for frequency regulation is a recommended option compared to battery usage for systems where the irradiation level is not too low. 5. Modes of operation of deloaded PV
Table 1 Cost of PV installation in $/W [21]. Item
2010
2011
2012
2013
Module Inverter Balance of plant Engineering, procurement and construction Other Total
1.72 0.28 0.47 0.45 0.32 3.24
1.33 0.26 0.45 0.43 0.29 2.77
1.07 0.25 0.43 0.41 0.28 2.44
0.94 0.24 0.41 0.39 0.27 2.24
Bold font indicates the value chosen in calculation.
Table 2 Additional cost incurred when finding reserve in PV system itself without using battery. Item
Calculation
Cost
Cost for 1 MW additional solar panel installation
2.24 1,000,000
$2,240,000
itself should be taken into account. For example, if the frequency regulation reserve in a deloaded PV is 1 MW, its cost is to be compared with 1 MW size battery only. Since the irradiation is different for each geographical locations, it may happen that 1 MW of additional PV panel installation will give only lower outputs like 0.9 MW or 0.8 MW or even lower. For such systems, comparison of cost of additional PV panels is to be done with battery of smaller size. Say, by installing 1 MW additional PV panel if only 0.8 MW reserve is available for frequency regulation, then its cost need to be compared with a battery of 0.8 MW only. Hence, a cost benefit
As highlighted in Section 1, this paper aims to make use of reserve power of PV both for primary and secondary frequency control. This can be achieved with a battery also with the disadvantages listed in Section 1. The secondary frequency regulation capability in case of deloaded PV varies with irradiation, whereas the regulation capability using a battery is fixed for a given rating of battery. Hence it is important to decide the desirable modes of operation of deloaded PV which make it better in operation compared to battery performance. In Section 5.1, Mode 1 operation in which extra reserve power is available in deloaded PV compared to battery is discussed. In Section 5.2, Mode 2 operation is suggested during the period where PV reserve is lesser when compared to battery reserve. 5.1. Extra reserve power in deloaded PV under higher irradiation (Mode 1) When the irradiation is high, the power output of PV system is high and reserve for frequency regulation is also high. But the reserve level for a battery is fixed irrespective of irradiation. Two systems with same capital cost (one using battery and the other only deloaded PV) are compared to find out the operational benefit in order to decide modes of operation. In Section 4.2, a cost analysis was done to find out the financial benefit of installing deloaded PV for frequency regulation as compared to installation of battery system. It was observed that cost of using a 444.445 kW battery is equivalent to the cost of providing
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P.P. Zarina et al. / Electrical Power and Energy Systems 60 (2014) 258–267 Table 3 Cost for using batteries of 2 years life span, with different sizes. Item
Cost with life span taken as 2 years
Size of battery unit in MW Capital cost (in $) of batteries with efficiency of 0.85 (a) Operating cost (in $) for battery (b) Salvage value (c) Total cost (in $) incurred due to battery usage (d) = (a + b c) Net savings (in $) as compared to deloaded PV usage (d) – cost of additional PV panel
1 4,235,294 847,058 42,352 5,040,000 2,800,000
0.9 3,811,764 762,352 38,118 4,536,000 2,296,000
Table 4 PV of 11 MW deloaded to provide a reserve. Reserve power available for frequency regulation (MW)
11 10 9 8 7 6 5 4 3 2 1
9.999 9.09 8.181 7.272 6.363 5.454 4.545 3.636 2.727 1.818 0.909
1.001 0.91 0.0.819 0.728 0.637 0.546 0.455 0.364 0.273 0.182 0.091
Bold fonts are the values for which reserve of PV is higher when compared to reserve of 0.444445MW battery. a Note: in the case of 9.1% deloading.
Output of PV under MPPT (MW)
Reserve from 0.444445 MW battery (MW)
Total power available in the system (MW)
10 9 8 7 6 5 4 3 2 1 0
0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445
10.444445 9.444445 8.444445 7.444445 6.444445 5.444445 4.444445 3.444445 2.444445 1.444445 0.444445
0.4 1,694,118 338,823 16,941 2,016,000 224,000
Curve 1: maximum power that can be extracted
Curve 2:Power available after deloading
Reserve= Curve 1-Curve 2
1MW
0.444445MW Reserve = 0.444445 MW (= Battery power rating) 1
Table 5 PV system at MPPT which gives 10 MW power at 1000 W/m2 and using a battery of 444.445 kW size.
0.444445 1,882,355 376,471 18,824 2,240,003 3
Power
Power due to 9.1% deloading (MW)
0.5 2,117,646 423,528 21,176 2,519,998 279,998
Extra power available beyond the reserve equivalent to battery of 0.4444445 MW
a
Power if PV at MPPT (maximum power that can be extracted (MW)
0.8 3,388,234 677,646 33,882 4,031,998 1,791,998
5
05.30
9 Time (h)
13
Available reserve higher than battery reserve
21
17
19.30
Fig. 17. Reserve power variation with time.
additional 1 MW PV panel as reserve. Hence, the two systems with equal investment cost considered for comparison are, System 1: 11 MW PV deloaded to give reserve of 1 MW. System 2: 10 MW PV system at MPPT using battery of 444.445 kW. The 9.1% deloading was considered to ensure nearly 1 MW reserve in the system which is having 11 MW power at MPP. The details of power available in both system 1 and system 2 are given in Tables 4 and 5 respectively. Comparing Tables 4 and 5, it can be seen that for higher power output of PV (due to higher irradiation), deloaded PV system has higher reserve power available for frequency regulation as compared to battery system (total power available in the system is also high). The power production curve of the PV system is assumed to have a similar pattern as given in [23]. As per the pattern, with hourly variation, reserve power variation is as shown in Fig. 17. Curve 1 shows the maximum power that can be extracted, curve
2 corresponds to the power under deloaded condition and the net reserve is also shown for system 1. The net reserve of system 1 is varying whereas the reserve of system 2 is constant as shown by the red line. The power above the black dots indicates how much more power the deloaded system has when compared to battery during 05.30–19.30 h. So deloaded PV under this region of operation is said to be in Mode 1. Roughly, the extra energy gain during this period as compared to battery can be calculated as the area of that portion which is 3888.885 kW h.
5.2. Deriving additional reserve by additional deloading (Mode 2) Beyond the duration considered in Mode 1, the system 2 reserve is higher compared to system 1 reserve. Our aim is to have operational reliability in terms of frequency regulation for whole range of reserve levels. So during lower irradiation levels, we will operate the PV in Mode 2 where a deloading percentage is increased to derive additional reserve. This can be achieved by deloading the PV by a higher percentage when the irradiation is less as shown in Fig. 18. It can be seen in Fig. 18 that with the increase in percentage of deloading, we can increase the reserve of the system. As shown in Fig. 18, if a 9.1% deloading at 4 MW MPP power results in 0.364 MW reserve and 3.636 MW deloaded power, an additional deloading will provide 0.44445 MW reserve even if the deloaded power get reduced to 3.5556 MW. Table 6 calculates the deloading percentage for the region of lower power level of deloaded PV. The loss by heavy deloading during the period is less as compared to the benefit achieved in Mode 1, due to the comparatively lesser
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P
1MW reserve available due to 9.1%deloading
Result showed that deloading the PV to provide a reserve is an economical solution when compared to the usage of battery unit. The benefit reduces for batteries of lower sizes than 1 MW and below the 0.444445 MW size of battery, the usage of deloaded PV as reserve is not beneficial. Depending on the irradiation level, two modes of operation is suggested for deloaded PV system, so as to ensure frequency regulation for the maximum extent possible.
11MW
3.636MW
4MW
Appendix A 3.5556MW
Specification of one solar module
V
0.444445MW reserve 0.364MW reserve available due to 9.1% derived by deloading by 11.11% deloading
Number of series modules Number of parallel modules Short circuit current of module MPP voltage and current of module Open circuit voltage of module
20 140 5A 35 V and 4.58 A 43.8 V
Fig. 18. At lower irradiation, deloading more for higher reserve.
References Table 6 During Mode 2, % deloading increased to ensure 0.444445 MW reserve for frequency regulation. Power if PV at MPPT (MW)
Deloaded power which will ensure 0.444445 (MW) reserve
Corresponding % deloading
4 3 2 1
3.5556 2.5556 1.5556 0.5556
11.11 14.813 22.22 44.44
region of operation. So during Mode 2 the deloaded power is represented by line marked by brown colour. Hence we can define two modes of operation for the deloaded PV Mode 1: PV participating in frequency control with fixed deloading percentage. Mode 2: PV participating in frequency control by varying the deloading. So knowing the load pattern and irradiation pattern we can switch the PV modes. For an industrial setup considered in our study, and the irradiation pattern discussed in [23], keeping the deloaded PV system in Mode 1 during 05.30–19.30 h and switching to Mode 2 during 04.00–05.30 h and 19.30–21.00 h is recommended.
6. Conclusion PV penetration in a multi bus system from IEEE Standard 3991997 is studied to address the frequency regulation issues of high renewable energy penetrated hybrid system. Reserve power is made available with PV by deloading them by a certain percentage of their MPP value. A controller is designed to make the PVs contribute to frequency regulation. The proposed controller is modified further, so that the contribution of the different PV are proportional to their available reserve so that PV with more reserve will participate more in frequency control compared to those with less reserve. Cost calculation of the additional requirement of 1 MW PV array which is needed because of deloading of PV array for frequency regulation is carried out. That cost of PV array is compared with the cost of a battery unit of appropriate size which may be required to carry out the same function of frequency regulation.
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Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Exploring frequency control capability of a PV system in a hybrid PV-rotating machine-without storage system P.P. Zarina, S. Mishra ⇑, P.C. Sekhar Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi, India
a r t i c l e
i n f o
Article history: Received 16 May 2013 Received in revised form 5 January 2014 Accepted 25 February 2014 Available online 1 April 2014 Keywords: Deloading Distributed sources Frequency control Penetration Photovoltaic systems Renewable sources
a b s t r a c t When the penetration of inertia-free photovoltaic (PV) system is rapidly increasing, it is of timely importance to make the system capable of handling the disturbances safely so that the system stability can be assured by maintaining the system frequency. In this paper a novel control strategy has been formulated for frequency regulation utilizing the output from the PV generators itself without going for any kind of storage technologies. A multi bus system with two conventional generators and twelve PV systems which are working away from the maximum power point of operation is the system used for the study. Frequency control of the deloaded PVs with frequency controller is simulated using Power Factory Software. An improved controller which considers not only the frequency deviation of the system but also the available reserve in the photovoltaic is proposed. This ensures that the PV with more reserve will participate more in the frequency control compared to those with less reserve so that the frequency control capability is evenly distributed throughout the system. Two modes of operation for deloaded PV is suggested to derive maximum benefit when it is used for frequency regulation. A cost analysis is carried out to show that the concept of deloaded PV is economical when compared to battery usage for frequency control. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In conventional power system which is frequently subjected to disturbances, due to the machine inertia, energy exchanges takes place between the rotating machines and the other part of the system. The primary/secondary frequency control is provided by the automatic generation control (AGC) to maintain the required balance between load and generation. In future, the renewable energy sources especially in the form of small capacity generations are expected to outnumber the conventional sources. They are usually integrated with the grid at low voltage as distributed generators (DGs). Since most of these DGs, like the photovoltaic (PV) generators are interfaced with grid by means of power electronic converters, stability will be a big concern for the system operators due to the absence of inertia which makes them incapable of providing inertial/primary frequency response, if not regulated properly. Any generator is said to participate in frequency regulation when its output varies depending on load conditions and hence balances out the generation and demand. Due to expensive nature of solar panel, efforts are made to extract maximum power from the panels by using maximum power point tracking (MPPT) methods as could ⇑ Corresponding author. E-mail addresses: [email protected] (P.P. Zarina), [email protected] (S. Mishra). http://dx.doi.org/10.1016/j.ijepes.2014.02.033 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.
be seen in literature whose comparison study is done in [1]. Authors in [1] have reviewed and compared 19 distinct MPPT methods from 90 papers for the photovoltaic systems. This shows the thrust given for maximum power extraction from a PV panel. Using such MPPT algorithms, under varying conditions of temperature and irradiation, the power output from the solar panel is maintained at their maximum value irrespective of the load condition, implying that no frequency regulation contribution from the PV. With the help of proper control philosophies implemented in a battery storage system the frequency disturbances can be handled effectively. However the expenditure on the battery storages which includes their maintenance and replacement costs is an extra burden. Authors in [2] had done a performance analysis on storage devices and concluded that the battery storage is not economical. When the ancillary services like frequency control are gaining importance in the changing scenario with high renewable penetration, it is important to incorporate these features into photovoltaics itself, which is the main objective of this paper. In [3–9], usage of battery or other storage devices along with renewable energy systems is considered to mitigate the various issues like output leveling, voltage unbalance, voltage rise, frequency regulation etc. However, photovoltaics participating in frequency control without using any external resources like battery, supercapacitors etc. is hardly seen. For wind turbines the frequency
P.P. Zarina et al. / Electrical Power and Energy Systems 60 (2014) 258–267
regulation capability is discussed in many literatures [10–13]. The inertial response of DFIG is analyzed in [10,11]. The dependence of frequency response of wind turbine generator on the initial available active power output of the generator is brought out in [11]. Comparison of performance of variable speed wind turbine generator with a conventional generator done in [11] showed that even though the proposal is useful for initial frequency support, frequency recovery is inferior. In [12], droop of the WTG is adjusted continuously in response to wind velocity, thereby, its primary frequency response is improved in terms of reduced stresses on WTGs during low wind speeds. The frequency fluctuations in a grid connected PV system are reduced using an electric double layer capacitor in [14]. The idea proposed is to control the ramping up rate of the PV output. The electric double-layer capacitor absorbs the fluctuations of PV output and controls the ramp rate of its output based on the moving average of the PV output. In [15] a PV-diesel hybrid system is operated without any storage device. However, it requires the irradiation data and complex control for realization. This paper proposes a novel control philosophy in which the output power of the PV generator itself is controlled to maintain the system frequency by keeping some portion of the PV output as reserve. The highlight of this paper is to make the PV array operate away from its MPPT point and thus make a reserve power available with the PV itself. This reserve power is used whenever there is a demand for increased generation, there by, avoiding the frequency disturbances. So it is assumed that, in situations where frequency control is to be derived from PV, the panel is deloaded by a fixed maximum power percentage depending on the reserve to be made available. A control philosophy is formulated such that, during a disturbance, the power released is proportional to deviation in frequency (Df). During the transients, there is a fast response from the PV and it adjusts its output power depending on the demand of power. This eliminates the usage of costly storage units like battery. The day time scenario of a microgrid with industrial loads in which most of them are not operational at night is discussed. In such a case, the conventional generators are sufficient to meet the partial load at night. In [16], the overloading capacity of battery is utilized for primary frequency regulation. There it was shown that by the time the diesel generator ramps up to meet the increased power demand, the battery is used to take care of the deficient power. But in this paper, apart from using the PV for primary frequency regulation, it is used for secondary frequency regulation also, up to the maximum extent possible (until all the PVs reach the MPP). The idea behind the usage of PV power for secondary control is to minimize the load cycling of conventional generators. The conventional generator is not expected to contribute to frequency regulation until reserve is available with PV. But once the PV reserve gets exhausted due to the PVs reaching the maximum power point, the conventional generators will be taking care of the frequency regulation. Emission is an issue which is much talked about. In [17], one of the objectives of economic analysis was to reduce the emission by reducing the ramping of generators. If PV participates in secondary frequency control, the ramping up and down of conventional generators can be very much reduced. Advantages of this type of control is that operation and maintenance cost due to frequent ramping can be reduced and the emission will also be comparatively less. As compared to usage of battery for secondary frequency regulation, usage of deloaded PV has the advantage that PV being an active source, no charging is required unlike battery. Another advantage is the much lesser disposal issue of PV as compared to battery. This is because, PV has a life expectancy of nearly 25 years which is high compared to battery (life expectancy of battery is different for different types but much less compared to PV). Yet
259
another advantage is that PV is a clean energy. Only disadvantage in terms of secondary frequency regulation application as compared to battery is the unavailability during night. So it is recommended only in systems as considered for this study where the night loads can be met by conventional generators and higher power demand is during day time. Section 2 explains the PV model and its characteristics and also the deloading concept of photovoltaic system which provides reserve for frequency control. Section 3 explains the controllers used for frequency control and simulation results to show the performance of the controllers. As the system proposed is for frequency regulation which can also be achieved using a PV at MPP plus a battery storage unit, it is important to compare the cost of the two options. Hence, a cost analysis is carried out in Section 4, which brings out the economical aspect of using deloaded PV as against the battery unit installation for frequency regulation. In Section 5, modes of operation of deloaded PV is discussed and simulation results for the various modes of operation of PV is shown. The system used for study is modelled using Power Factory Software. 2. PV generator modeling and operation with reserve 2.1. PV modeling The mathematical model of photovoltaic array is represented by its characteristic function as in [18]. The basic theory of a photovoltaic cell is that when light falls on the p–n junction, charge carriers are generated which leads to an electric current. To increase the current rating, a PV panel will have many cells in parallel, and for a higher voltage, they have many cells in series. The output current Ipv is given by Eq. (1),
q V dc 1 Ipv ¼ Np Iph Np Irs exp kTA Ns
ð1Þ
where Np is number of cells in parallel, Ns is number of cells in series. Irs is reverse saturation current, q is the electron charge with a value of 1.602 1019 C, k is the Boltzmann’s constant (1.3806503 1023 J/K), A is the ideality factor, T is the temperature in Kelvin and Vdc is output dc voltage. Iph the short-circuit current for one string of the PV panel is a function of temperature T and irradiation S (W/m2). It is given by Eq. (2).
Iph ¼ ½ISC þ K I ðT T ref ÞS
ð2Þ
where ISC is short circuit current of the cell, Tref is the reference temperature, KI is temperature coefficient. Datasheet from manufacturer tells about the expected performance of PV arrays for standard test conditions (STC), which means at a temperature of 25 °C and irradiation of 1000 W/m2. The modeling of PV was done using Power Factory Software [19]. VMPP, the voltage at maximum power point for a given irradiation E is calculated based on VMPP0, voltage at maximum power point at standard test conditions (STC). This value of VMPP0 for a given module characteristics will be specified by the module manufacturer. From these values, the VMPP can be calculated for any irradiation levels as by Eq. (3).
V MPP ¼ V MPP0
lnðEÞ lnð1000Þ
ð3Þ
To incorporate the change in the PV characteristics with variation in irradiation and temperature, correction factor is included in the model depending on the ambient conditions at which the PV is working. The calculation using temperature correction factor av for voltage is given by (4).
V MPP ¼ V MPP0
lnðEÞ ð1 þ av ðT T STC ÞÞ lnð1000Þ
ð4Þ
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The power output of a single PV array is as shown in Fig. 1, whose specifications are given in Appendix A. The variation in the power output with different irradiation is evident from the figure. The MPP values for irradiation levels of 1000 W/m2, 900 W/m2 and 800 W/m2 are marked along with the corresponding Vdc values.
PMPP Preserve
P1
2.2. PV operation with reserve power If the photovoltaic systems are made to work at maximum power point, say for a PV with characteristics as in Fig. 1, under an irradiation of 1000 W/m2, the power output will be 448.5 kW. Irrespective of the load condition, the PV will always track the maximum power point and will give a fixed output if the ambient conditions remain the same. In such a situation, there is no frequency control contribution from the PV. This is because there would not be any reserve power available with them to participate in frequency control. Hence, it is proposed to provide a reserve power in the PV systems which is achieved by making the PV work away from the MPP. When the PV is operating at MPP, dc bus voltage will be VMPP and the power output will be maximum, marked as PMPP, which is the maximum value of power that can be extracted from the PV panel. As seen in Fig. 2, if the operation is at a higher voltage (DV) than VMPP, the power output will be P1. Under this condition the PV is said to be deloaded, as the full available power is not extracted. Here, the difference in P1 and PMPP is the reserve power Preserve which can be utilized for the frequency control. Depending on the percentage of deloading, the available reserve power varies. So if a company wants to install a PV system of capacity P1 and make it participate in frequency control, they have to invest money for an installation which is higher than P1 by the same proportion of deloading. To elaborate more, looking at Fig. 2, if P1 is the power to be produced in steady state after deloading, then the peak capacity has to be PMPP and hence the company has to invest according to PMPP. In this scenario the reserve power (Preserve) available to participate in frequency regulation is given by (5).
Preserv e ¼ PMPP P 1
ð5Þ
3. Controller for frequency control contribution If the PV system is not deloaded, and is working at its maximum power point (MPP) of operation, then there would not be any reserve for frequency regulation contribution. The active power is given by, P = vd * id + vq * iq (where d and q are direct axis and
5.0E+5 V_MPP=694V P_MPP=398kW Insolation 900W/m2
V_MPP=710V P_MPP=448.5kW Insolation 1000W/m2
4.0E+5
VMPP
VMPP+
Fig. 2. Concept of reserve power in the PV array.
quadrature axis components). At point of common coupling, vd = 1 and vq = 0, hence P = vd * id only. Hence active power can be varied by varying the id. For a PV which is not deloaded, the idref will change depending on the ambient conditions (because VMPP will change). In the deloaded PV, where we are finding a provision for frequency regulation in the controller of the PV itself, deloading is done by increasing the dc voltage beyond MPP. This is achieved by increasing the value from VMPP by a voltage Vdeload corresponding to the power reserve to be provided as depicted in Fig. 3. Though with this simple addition, the PV array keeps some reserve power, unless otherwise instructed, the reserve is not released. Since, the PV power has to change as per deviation in system frequency, a signal, proportional to frequency deviation Df, is added to the dc reference voltage as presented in the figure. Looking carefully at Fig. 3 it can be concluded that the variation of power from the PV will not only depend on VMPP value but also on frequency deviation as given by Eq. (6).
V dcref ¼ ðV MPP þ V deload V dc proportional to Df Þ
ð6Þ
As this controller does not consider reserve in the PV, it may lead to a situation that the PV with less reserve also equally contribute to frequency control as compared to PV with more reserve and may thus reach their MPP faster. When MPP is reached, they would not be able to contribute further to any new extra power requirement. This will lead to an uneven distribution of frequency regulation capability. Hence a modified controller is suggested in Section 3.1 which considers a component DVreserve. 3.1. Modified controller for distributed frequency control according to the available reserve Using the controller in Fig. 3, the power output of PV will change based on the change in frequency and change in the VMPP value (which varies depending on the ambient conditions). As shown in Figs. 4 and 5, the point 1 corresponds to the MPP power and point 2 corresponds to power at deloaded condition of PV.
Power
3.0E+5 V_MPP=681V P_MPP=347.5W Insolation 800W/m2
Kp1+Ki1/s
2.0E+5
V proportional to change in freq _ dc VMPP +
1.0E+5
Voc VMPP +
0.0E+0 0.00
200.00
400.00
600.00
800.00
1000.00
Limit
Kp+Ki/s Vdcref
_
idref
Vdc
Vdeload
Vdc Fig. 1. Power vs. voltage curve of PV array at different irradiation.
Fig. 3. Controller for deloaded PV without considering the amount of available reserve.
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When there is a load increase, the controller will decrease Vdc and make the PV work at point 3, with an increase in power output. As the controller in Fig. 3 does not consider how much reserve is still available (DVreserve) with the PV, the PV with more reserve and the one with less reserve will contribute equally to the frequency control. The modified controller in Fig. 6 accommodates the DVreserve component also, which corresponds to the reserve still available with the PV. The Vdcref in this case is given by Eq. 7. The multiplier block, which accounts for the 2nd term of equation will affect the output only when the DVreserve is non-zero. For higher values of DVreserve, the second term of the equation will be higher and Vdcref value will move more closer to VMPP, resulting in more power. Thus, the decrease of Vdc from its deloaded position to the new position will depend not only on Df but on DVreserve also. With this modified controller, as more power is contributed by PVs with more reserve, burden on PVs with less margin will be less and hence the frequency control capability is evenly distributed among the different PVs.
261
Fig. 5. PV with lesser deloading giving less reserve.
V dcref ¼ ðV MPP þ V deload V dc proportional to Df Þ ðDf DV reserv e K p2 Þ
ð7Þ
3.2. Simulation results to show controller performance A multi bus system given in IEEE Standard 399-1997 is taken for the simulation studies. There are two conventional synchronous generators connected at bus 4 and bus 50, and many rotating machines as loads at different buses. The system is modified with the addition of twelve PV generators connected at 18, 35, 37 and 49 as shown in Fig. 7. The PVs are of 448.5 kW each and are connected to 0.48 kV buses. The conventional generators are connected to 13.8 kV level at buses 4 and 50. The effect of controller parameters on the PV output is shown in Section 3.2.1. In Section 3.2.2, the simulation results of two types of controllers shown by Figs. 3 and 6 are compared. 3.2.1. Effect of controller parameters on the PV power The aim of this section is to see the effect of controller parameters on the PV outputs. All the PVs are deloaded with equal percentages of 18% with respect to their MPP. As all PVs are of same rating, equal deloading will result in equal reserve of 81 kW. The total reserve comes to 0.972 MW (81 kW * 12). A load increase is considered at 100 s to check the effect of controller parameters on the performance of the PV system. The effect of proportional gain and integral gain (Kp and Ki) of the PI controller of Fig. 3 is examined on the output of PV. The variation of output power of
Reserve available due to deloading
1 3 2
P
reserve
VMPP
Vdc proportional to change in frequency
VMPP + Vdeload
Fig. 4. PV with higher deloading giving more reserve.
Fig. 6. Improved controller for deloaded PV which considers the amount of available reserve.
PV and output of conventional generator connected at bus 50 with variation in control parameters is shown in Figs. 8 and 9. When Ki is changing from 1000 to 8000, the output power contribution from PV is increasing and correspondingly output power from conventional generator is decreasing. So with a value of 8000, PV is solely contributing for frequency regulation. A comparison of frequency response is shown in Fig. 10. From Fig. 10 it is seen that the area of the curve corresponding to the Ki = 8000 is small as compared to Ki = 2000 or 1000. This is justified by the fact that product of Ki and area of curve will be the power. So for higher values of Ki, the area of curve will be smaller. So in further simulation cases also the value of Ki is kept the same. The variation in Kp is not causing the steady state PV power values to change.
3.2.2. Performance comparison of two types of controllers A controller which is regulating the PV output based on the change in frequency alone was shown in Fig. 3. Using such a controller, the behavior of PV under two cases is compared in Fig. 11. In one case PVs are deloaded by 8% and in another case PVs are deloaded by 18%. In the first case the available reserve will be less compared to second case. It is seen that the PV power output increases by the same amount for a load change for both cases when the controller is the one which does not take into account the available reserve. Which means the PV with more reserve and the one with less reserve is contributing equally for a load change. In Fig. 12, the response of PVs which use the modified controller given in Fig. 6, for the same load change in the above case is shown. Here, the increase in power output after the load change is different for both PVs. The PV having more reserve power is contributing more to the frequency control compared to the one with less
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0.46
60.01
0.44
60.00
Frequency in Hz
PV Power in MW
Fig. 7. Multi bus system layout considered for study.
0.42
0.40
0.38
0.36 90.000
59.98
59.97
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: Power output in MW for Ki=8000 PV Generator: Power output in MW for Ki=2000 PV Generator: Power output in MW for Ki=1000
59.96 90.000
96.000
102.00
108.00
114.00
[s]
120.00
49: Bus frequency when the value of Ki is 8000 49: Bus frequency when the value of Ki is 2000 49: Bus frequency when the value of Ki is 1000
Fig. 8. Variation in output power of photovoltaic with different values of Ki.
Fig. 10. Comparison of system frequency for different values of controller parameters of PV.
reserve. This results in an even distribution of frequency regulation capability and hence the reserve.
8.95
Power of Generator in MW
59.99
8.83
3.3. Simulation results showing PV working under different conditions 8.71
8.59
8.47
8.35 90.000
102.00
114.00
126.00
138.00
[s]
150.00
GEN1: Power output in MW for Ki=8000 GEN1: Power output in MW for Ki=2000 GEN1: Power output in MW for Ki=1000
Fig. 9. Power output of conventional generator for different values of controller parameters of PV.
Case 1: PV alone participating in frequency regulation (PV reserve sufficient to meet load change). This type of operation result during high irradiation condition and small load change. As PV reserve will be sufficient to meet frequency regulation, no or little ramping up needed for conventional generators. Responses are given in Figs. 13 and 14. Case 2: PV reserve gets exhausted and conventional generator participates in frequency regulation. This type of operation is needed during high irradiation condition and higher load change. Even though PV irradiation level is same as that in case 1, for higher load change PV will reach MPP as shown in Fig. 15 and conventional generator ramp up to meet the balance load requirement as indicated by Fig. 16.
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0.5000
8.93
Power of Generator in MW
0.5500
0.4500
0.4000
0.3500
0.3000 90.000
102.00
114.00
126.00
138.00
8.49
8.05
7.61
7.17 90.000
[s] 150.00
PV Generator: With more reserve and with controller in Fig.3
102.00
114.00
126.00
138.00
[s] 150.00
GEN1: Active Power in MW
PV Generator: With less reserve and with controller in Fig.3
Fig. 14. Conventional generator power under case 1 operation.
Fig. 11. PV power contribution with unequal reserve using controller considering frequency deviation alone.
frequency regulation is that, in one case, additional PV size is needed and in the other, additional battery is to be used. Hence in this section, the cost of extra panel size needed for reserve is compared with battery unit cost. In Section 4.1, cost of additional PV panel is worked out when we are not using a battery and providing reserve in the PV itself. In Section 4.2, cost of battery units are worked out for different sizes of batteries. Cost analysis is done by assuming a life expectancy of PV panel as approximately 24 years [20]. As the bus voltage where PV is connected is 480 V, and if a battery is used for providing the reserve power, it should also be connected to same bus. In that case the dc voltage Vdc can be arrived at using (8),where Vac is the ac bus voltage. So Vdc should be 800 V.
0.5500
PV Power in MW
0.5000
0.4500
0.4000
0.3500
0.3000 90.000
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: With more reserve and with controller in Fig.4 PV Generator: With less reserve and with controller in Fig.4
Fig. 12. PV power contribution with unequal reserve using controller which considers frequency deviation and reserve.
ð8Þ
4.1. Cost of additional panel providing reserve in case of deloaded PV As explained in Section 2.2, for frequency regulation using deloaded PV, additional PV panels are to be installed as reserve. In the system simulated, the reserve provided was nearly 1 MW. The cost of this extra 1 MW installation, which is to be provided as reserve will be an additional expenditure and is calculated in this section. Table 1 shows the $/W cost of PV generator as given in [21] and Table 2 calculates the extra financial burden with the proposed control strategy by taking the standard cost of the year 2013. So, an additional cost of $2,240,000 is to be expended if a reserve power of 1 MW is to be provided in the photovoltaic system. This is to provide the reserve in the deloaded PV system for frequency regulation in place of a PV at MPP plus battery.
0.51
0.48
PV Power in MW
V dc
rffiffiffi 2 V ac P2 3
0.44
0.41
0.38
4.2. Cost of battery providing reserve in case of PV at MPPT
0.34 90.000
102.00
114.00
126.00
138.00
[s] 150.00
PV Generator: Active Power in MW
Fig. 13. PV power under case 1 operation.
4. Cost analysis A cost analysis is important to see if deloaded PV with reserve for frequency regulation (which results in reduced utilization of PV array) is economical as compared to usage of PV at MPP plus battery unit. As seen in Section 2.2, when using a deloaded PV for frequency regulation, even though battery unit is not used, extra panel size is needed. Cost wise difference in the two options of
Instead of deloading the PV system and providing a reserve for frequency regulation, if battery units are used by making the PVs work at MPP itself, the costing of battery unit depends on the size of battery. In [16], the overloading capacity of battery is utilized only for primary frequency regulation and not secondary frequency regulation. In that paper the battery is meeting the load change only for a short duration, that is, till diesel generator ramps up to meet the increased power demand. In such an application, instead of 1000 kW battery, 249 kW was sufficient by considering the overload capacity. But in this paper, apart from using the PV for primary frequency regulation, it is used for secondary frequency regulation also. In such case, the nominal size of battery
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analysis is carried out here with different capacities of battery storage as reserve against the 1 MW extra PV generating capacity. Table 3 calculates the battery cost when replacement of lead acid battery is needed every 2 years and for different battery size options depending on the geographical irradiation level. The calculation is based on Eqs. (9)–(11). The values are based on [22] which shows a value of 300 $/kW, 30 $/kW and 3 $/kW respectively for capital cost, operating cost and salvage value.
0.51
PV Power in MW
0.48
0.44
0.41
Capital cost of battery unit in $ ¼ ðC I NÞ=g 0.38
0.34 90.000
102.00 114.00 126.00 PV Generator: Power output in MW
138.00
[s] 150.00
Fig. 15. PV power under case 2 operation.
Power of Generator in MW
9.37
8.93
8.49
8.05
7.61
7.17 90.000
102.00
114.00
126.00
138.00
[s] 150.00
GEN1: Power output in MW
Fig. 16. Conventional generator power under case 2 operation.
ð9Þ
Operating cost of battery unit in $ ¼ ðO I TÞ=g
ð10Þ
Salvage value in $ ¼ ðS I NÞ=g
ð11Þ
where C is the capital cost of unit in $/kW, I is installed capacity in kW, N is Number of times battery replacement done during life of PV, O is operating cost of unit in $/kW, S is salvage of unit in $/kW, T = time period of study or life of PV, and g is the power efficiency. From the above anaysis, it can be concluded that, providing reserve in the PV itself for frequency regulation is finacially beneficial solutions when compared to battery storages. However, the amount of savings will vary depending on the chosen battery storage capacities. In systems where the reserve provided by extra panel size of 1 MW will deliver 1 MW reserve itself, there is a profit of $2,800,000 because in that system the battery to be used for equivalent frequency regulation will be 1 MW. The benefit decreases with the decrease in size of the battery. If the system is such that with 1 MW additional panel installation, reserve power is equivalent to battery size of 0.444445 MW, (lower due to lower irradiation conditions), then the battery cost is at a break even point when compared to PV with reserve. So usage of deloaded PV for frequency regulation is a recommended option compared to battery usage for systems where the irradiation level is not too low. 5. Modes of operation of deloaded PV
Table 1 Cost of PV installation in $/W [21]. Item
2010
2011
2012
2013
Module Inverter Balance of plant Engineering, procurement and construction Other Total
1.72 0.28 0.47 0.45 0.32 3.24
1.33 0.26 0.45 0.43 0.29 2.77
1.07 0.25 0.43 0.41 0.28 2.44
0.94 0.24 0.41 0.39 0.27 2.24
Bold font indicates the value chosen in calculation.
Table 2 Additional cost incurred when finding reserve in PV system itself without using battery. Item
Calculation
Cost
Cost for 1 MW additional solar panel installation
2.24 1,000,000
$2,240,000
itself should be taken into account. For example, if the frequency regulation reserve in a deloaded PV is 1 MW, its cost is to be compared with 1 MW size battery only. Since the irradiation is different for each geographical locations, it may happen that 1 MW of additional PV panel installation will give only lower outputs like 0.9 MW or 0.8 MW or even lower. For such systems, comparison of cost of additional PV panels is to be done with battery of smaller size. Say, by installing 1 MW additional PV panel if only 0.8 MW reserve is available for frequency regulation, then its cost need to be compared with a battery of 0.8 MW only. Hence, a cost benefit
As highlighted in Section 1, this paper aims to make use of reserve power of PV both for primary and secondary frequency control. This can be achieved with a battery also with the disadvantages listed in Section 1. The secondary frequency regulation capability in case of deloaded PV varies with irradiation, whereas the regulation capability using a battery is fixed for a given rating of battery. Hence it is important to decide the desirable modes of operation of deloaded PV which make it better in operation compared to battery performance. In Section 5.1, Mode 1 operation in which extra reserve power is available in deloaded PV compared to battery is discussed. In Section 5.2, Mode 2 operation is suggested during the period where PV reserve is lesser when compared to battery reserve. 5.1. Extra reserve power in deloaded PV under higher irradiation (Mode 1) When the irradiation is high, the power output of PV system is high and reserve for frequency regulation is also high. But the reserve level for a battery is fixed irrespective of irradiation. Two systems with same capital cost (one using battery and the other only deloaded PV) are compared to find out the operational benefit in order to decide modes of operation. In Section 4.2, a cost analysis was done to find out the financial benefit of installing deloaded PV for frequency regulation as compared to installation of battery system. It was observed that cost of using a 444.445 kW battery is equivalent to the cost of providing
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P.P. Zarina et al. / Electrical Power and Energy Systems 60 (2014) 258–267 Table 3 Cost for using batteries of 2 years life span, with different sizes. Item
Cost with life span taken as 2 years
Size of battery unit in MW Capital cost (in $) of batteries with efficiency of 0.85 (a) Operating cost (in $) for battery (b) Salvage value (c) Total cost (in $) incurred due to battery usage (d) = (a + b c) Net savings (in $) as compared to deloaded PV usage (d) – cost of additional PV panel
1 4,235,294 847,058 42,352 5,040,000 2,800,000
0.9 3,811,764 762,352 38,118 4,536,000 2,296,000
Table 4 PV of 11 MW deloaded to provide a reserve. Reserve power available for frequency regulation (MW)
11 10 9 8 7 6 5 4 3 2 1
9.999 9.09 8.181 7.272 6.363 5.454 4.545 3.636 2.727 1.818 0.909
1.001 0.91 0.0.819 0.728 0.637 0.546 0.455 0.364 0.273 0.182 0.091
Bold fonts are the values for which reserve of PV is higher when compared to reserve of 0.444445MW battery. a Note: in the case of 9.1% deloading.
Output of PV under MPPT (MW)
Reserve from 0.444445 MW battery (MW)
Total power available in the system (MW)
10 9 8 7 6 5 4 3 2 1 0
0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445 0.444445
10.444445 9.444445 8.444445 7.444445 6.444445 5.444445 4.444445 3.444445 2.444445 1.444445 0.444445
0.4 1,694,118 338,823 16,941 2,016,000 224,000
Curve 1: maximum power that can be extracted
Curve 2:Power available after deloading
Reserve= Curve 1-Curve 2
1MW
0.444445MW Reserve = 0.444445 MW (= Battery power rating) 1
Table 5 PV system at MPPT which gives 10 MW power at 1000 W/m2 and using a battery of 444.445 kW size.
0.444445 1,882,355 376,471 18,824 2,240,003 3
Power
Power due to 9.1% deloading (MW)
0.5 2,117,646 423,528 21,176 2,519,998 279,998
Extra power available beyond the reserve equivalent to battery of 0.4444445 MW
a
Power if PV at MPPT (maximum power that can be extracted (MW)
0.8 3,388,234 677,646 33,882 4,031,998 1,791,998
5
05.30
9 Time (h)
13
Available reserve higher than battery reserve
21
17
19.30
Fig. 17. Reserve power variation with time.
additional 1 MW PV panel as reserve. Hence, the two systems with equal investment cost considered for comparison are, System 1: 11 MW PV deloaded to give reserve of 1 MW. System 2: 10 MW PV system at MPPT using battery of 444.445 kW. The 9.1% deloading was considered to ensure nearly 1 MW reserve in the system which is having 11 MW power at MPP. The details of power available in both system 1 and system 2 are given in Tables 4 and 5 respectively. Comparing Tables 4 and 5, it can be seen that for higher power output of PV (due to higher irradiation), deloaded PV system has higher reserve power available for frequency regulation as compared to battery system (total power available in the system is also high). The power production curve of the PV system is assumed to have a similar pattern as given in [23]. As per the pattern, with hourly variation, reserve power variation is as shown in Fig. 17. Curve 1 shows the maximum power that can be extracted, curve
2 corresponds to the power under deloaded condition and the net reserve is also shown for system 1. The net reserve of system 1 is varying whereas the reserve of system 2 is constant as shown by the red line. The power above the black dots indicates how much more power the deloaded system has when compared to battery during 05.30–19.30 h. So deloaded PV under this region of operation is said to be in Mode 1. Roughly, the extra energy gain during this period as compared to battery can be calculated as the area of that portion which is 3888.885 kW h.
5.2. Deriving additional reserve by additional deloading (Mode 2) Beyond the duration considered in Mode 1, the system 2 reserve is higher compared to system 1 reserve. Our aim is to have operational reliability in terms of frequency regulation for whole range of reserve levels. So during lower irradiation levels, we will operate the PV in Mode 2 where a deloading percentage is increased to derive additional reserve. This can be achieved by deloading the PV by a higher percentage when the irradiation is less as shown in Fig. 18. It can be seen in Fig. 18 that with the increase in percentage of deloading, we can increase the reserve of the system. As shown in Fig. 18, if a 9.1% deloading at 4 MW MPP power results in 0.364 MW reserve and 3.636 MW deloaded power, an additional deloading will provide 0.44445 MW reserve even if the deloaded power get reduced to 3.5556 MW. Table 6 calculates the deloading percentage for the region of lower power level of deloaded PV. The loss by heavy deloading during the period is less as compared to the benefit achieved in Mode 1, due to the comparatively lesser
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P
1MW reserve available due to 9.1%deloading
Result showed that deloading the PV to provide a reserve is an economical solution when compared to the usage of battery unit. The benefit reduces for batteries of lower sizes than 1 MW and below the 0.444445 MW size of battery, the usage of deloaded PV as reserve is not beneficial. Depending on the irradiation level, two modes of operation is suggested for deloaded PV system, so as to ensure frequency regulation for the maximum extent possible.
11MW
3.636MW
4MW
Appendix A 3.5556MW
Specification of one solar module
V
0.444445MW reserve 0.364MW reserve available due to 9.1% derived by deloading by 11.11% deloading
Number of series modules Number of parallel modules Short circuit current of module MPP voltage and current of module Open circuit voltage of module
20 140 5A 35 V and 4.58 A 43.8 V
Fig. 18. At lower irradiation, deloading more for higher reserve.
References Table 6 During Mode 2, % deloading increased to ensure 0.444445 MW reserve for frequency regulation. Power if PV at MPPT (MW)
Deloaded power which will ensure 0.444445 (MW) reserve
Corresponding % deloading
4 3 2 1
3.5556 2.5556 1.5556 0.5556
11.11 14.813 22.22 44.44
region of operation. So during Mode 2 the deloaded power is represented by line marked by brown colour. Hence we can define two modes of operation for the deloaded PV Mode 1: PV participating in frequency control with fixed deloading percentage. Mode 2: PV participating in frequency control by varying the deloading. So knowing the load pattern and irradiation pattern we can switch the PV modes. For an industrial setup considered in our study, and the irradiation pattern discussed in [23], keeping the deloaded PV system in Mode 1 during 05.30–19.30 h and switching to Mode 2 during 04.00–05.30 h and 19.30–21.00 h is recommended.
6. Conclusion PV penetration in a multi bus system from IEEE Standard 3991997 is studied to address the frequency regulation issues of high renewable energy penetrated hybrid system. Reserve power is made available with PV by deloading them by a certain percentage of their MPP value. A controller is designed to make the PVs contribute to frequency regulation. The proposed controller is modified further, so that the contribution of the different PV are proportional to their available reserve so that PV with more reserve will participate more in frequency control compared to those with less reserve. Cost calculation of the additional requirement of 1 MW PV array which is needed because of deloading of PV array for frequency regulation is carried out. That cost of PV array is compared with the cost of a battery unit of appropriate size which may be required to carry out the same function of frequency regulation.
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