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Autonomous active power management in isolated microgrid based on proportional and droop control
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Energy Procedia Procedia 00 153(2017) (2018)000–000 48–55 Energy www.elsevier.com/locate/procedia
5th International Conference on Energy and Environment Research, ICEER 2018 5th International Conference on Energy and Environment Research, ICEER 2018
Autonomous active power management in isolated microgrid based Autonomous active power management in isolated microgrid based proportional andondroop The 15thon International Symposium Districtcontrol Heating and Cooling on proportional and droop control Hyeon-Jin Moon, Jae Won Chang, Seok-Young Lee and Seung-Il Moon* Assessing the feasibility of using the heat demand-outdoor Hyeon-Jin Moon, Jae Won Chang, Seok-Young Lee and Seung-Il Moon* School of Electrical and Computer Engineering, Seoul National Univ., 1, Gwanak-ro, Gwanak-gu, Seoul, 08826, South Korea temperature function for a long-term district heat demand forecast School of Electrical and Computer Engineering, Seoul National Univ., 1, Gwanak-ro, Gwanak-gu, Seoul, 08826, South Korea I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, Avenue Dreyfous Daniel, 78520 Limay, France In an isolated microgrid, coordinated active power control291 is required for power balance under any circumstances. We propose a c Département Systèmes Énergétiques et Environnement IMT Atlantique, 4balance rue Alfred Kastler, 44300 Nantes, France In an isolatedcontrol microgrid, coordinated active power control is required forpower powerbalance under anyinsufficient circumstances. We propose proportional method based on the master-slave concept to- achieve even from power supply. Thisa proportional control method on the master-slave concept to achieve powerbattery balanceenergy even from insufficient power supply. This autonomous method based uses the frequency generated by the grid-forming storage system (BESS) as a global autonomous control method uses the frequency by theis grid-forming energy storage system (BESS) as a global data signal. Other units are controlled indirectly. generated The frequency controlled to battery be proportional to the AC voltage deviation of the data signal. Other are controlled indirectly. The frequency controlled to be proportional to the AC of voltage deviationmethod of the grid-forming BESSunits for detecting sudden power shortages and forissharing active power. The effectiveness the proposed Abstract BESS for detecting sudden power shortages and for sharing active power. The effectiveness of the proposed method grid-forming is verified by simulations in MATLAB/Simulink. is verified by simulations in MATLAB/Simulink. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the © 2018 The Authors. Published by Elsevier Ltd. greenhouse gas emissions fromby theElsevier building sector. These systems require high investments which are returned through the heat © 2018 The Authors. Ltd. © 2018 The Authors. Published by Elsevier Ltd. This is an open accessPublished article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article climate under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) sales. Due to the changed conditions and building policies, heat demand in the future could decrease, This is an and openpeer-review access article under the CC BY-NC-ND license renovation (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific committee of the 5th International Conference on Energy and Selection andthepeer-review under responsibility of the scientific committee of the 5th International Conference on Energy and prolonging investment return period. Selection andResearch, peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment ICEER 2018. Environment Research, ICEER 2018. The main scope of thisICEER paper is2018. to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Environment Research, forecast.microgrid; The district of stroage Alvalade, located in Lisbon used asautonomous a case study. Thedroop district is consisted Keywords: energy system; frequency control; (Portugal), active power was management; control; control; coordinatedof 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district control; master-slave Keywords: microgrid;control; energy stroage system; frequency control; active power management; autonomous control; droop control; coordinated renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were control; master-slave control; compared with results from a dynamic heat demand model, previously developed and validated by the authors. results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.The Introduction error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.(the Introduction scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). With enhanced price competitiveness and sustainability, the penetration of renewable energy sources (RES) into The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the With enhanced price competitiveness and the ispenetration of renewable energy sources (RES) the electricity is steadily A sustainability, microgrid, anseason integrated platform storage units into and decrease in thesupply number of heatingincreasing. hours of 22-139h during which the heating (depending on for the supply, combination of weather and the electricity supply is steadily increasing. A microgrid, which is an integrated platform for supply, storage units and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. * Corresponding author. Tel.: +82-10-4626-7527; fax: +82-2-878-1452. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and address:author. [email protected] * E-mail Corresponding Tel.: +82-10-4626-7527; fax: +82-2-878-1452. Cooling.
E-mail address: [email protected] 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change This is an open access article underPublished the CC BY-NC-ND 1876-6102 © 2018 The Authors. by Elsevier license Ltd. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific of the 5th International Conference on Energy and Environment This is an and openpeer-review access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Research, and ICEER 2018. under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Selection peer-review Research, ICEER 2018.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 10.1016/j.egypro.2018.10.055
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demand resources located in a local grid, enables local energy management [1]. Meanwhile, most remote areas such as small oceanic islands have no connection with the main power grid due to cost and/or technical problems. The electric power supply for these areas in several countries depends primarily on a diesel generator that can be adjusted to meet the power balance of the grid. However, RES such as photovoltaics (PV), wind turbines (WTs) are more difficult to apply as power systems than diesel generators due to their intermittency, which causes even more stability problems in remote power systems. However, the development of microgrid technology and energy storage system (ESS) has rendered RES an attractive option for supplying electricity in remote areas. In an isolated microgrid without any electrical connection with other systems, the battery energy storage system (BESS) unit is typically operated as a grid-forming unit to control the alternating current (AC) bus voltage and the system frequency. This is because the ESS can be controlled immediately and charged and discharged by the smart inverter technology [2]. When the BESS is used for the grid-forming unit, its state of charge (SOC) has to be maintained within the acceptable range for device protection and stable operation [3], [4]. Several control strategies based on the constant-voltage constant-frequency (CVCF) method for the grid-forming ESS unit have been proposed and used in real microgrids across South Korea [5]. This method controls the frequency and voltage to be a constant and primary active power control is met by the BESS which operates as ideal voltage source. However, the SOC level must be considered to protect the battery and guarantee sufficient spinning reserve. In addition, the active power balance depends primarily on the BESS. To share the burden of the grid-forming BESS, frequency-based droop control is proposed [6]-[9]. In [6], active power and frequency droop control is applied to coordinate the distributed energy sources (DER) and ESSs in inverter-based islanded microgrids. To maintain the SOC value, SOC-frequency droop control is proposed in [7]. However, problems arise from the fluctuations in the SOC or from active power in the droop control region. Further, conventional generators such as diesel generators are not considered. Moreover, master-slave-based SOC control method with droop control in diesel generators is presented in [8]. In [9], the active power control algorithm of diesel generators with the SOC of a BESS using low-bandwidth communication is proposed. This paper proposes an autonomous active power management based on the proportional control of the girdforming BESS and droop control of diesel generators. With CVCF operation, the BESS controls the frequency when an AC voltage is changed which implies active power imbalance. This method minimizes the low voltage problem and power insufficiency without any communication. A simulation in MATLAB/Simulink is performed to show the effect of the proposed control method. 2. System description ACSR/AW-OC 58mm2 : 0.497+j0.438 [Ω/km] ACSR/AW-OC 95mm2 : 0.304+j0.420 [Ω/km]
Diesel Gen.1
40 kW ACSR 58mm2 1.8 km
40 kW
70 kW AC
150kW
ACSR 58mm2 0.2 km
380V/6,900V, Y-Δ
ACSR 58mm2 0.2 km
ACSR 58mm2 0.5 km
ACSR 58mm2 0.4 km
DC 380V/6,900V, Y-Δ
Diesel Gen.2 17 kW
Diesel Gen.3
ACSR 58mm2x1.0km ACSR 95mm2x0.6km
380V/6,900V, Y-Δ
BESS 250kW/500kWh
ACSR 58mm2x0.9km ACSR 95mm2x1.0km
81 kW
PV 100kW AC DC
380V/6,900V, Y-Δ
Fig. 1. Geocha Island microgrid system configuration.
WT 100kW
ACSR 58mm2 0.6 km
ACSR 58mm2 0.3 km
320V/380V, Δ-Y
AC
180 kW
ACSR 58mm2 0.3 km
AC DC
DC
Hyeon-Jin Moon et al. / Energy Procedia 153 (2018) 48–55
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Geocha Island, one of the areas of the remote microgrid project driven by Korea Power Electric Corporation (KEPCO), has an isolated microgrid system. Fig. 1 illustrates the configuration of the microgrid system in Geocha Island. The system has no electrical connection with the main grid and includes a 250 kW/500 kWh of BESS, 100 kW of PV, WT, and three 150 kW diesel generators, respectively. The other data are replicated from the real system of Geocha Island. The nominal system voltage and frequency are 6.9 kV and 60 Hz, respectively. The converters in the system were based on the two-level averaged model. The WT, PV, BESS and diesel generator were modeled based on the MATLAB/Simulink library models. The WT and PV are assumed to be operated by the maximum-power point tracking (MPPT) algorithm to maximize their cost efficiency [10], [11]. The loads were implemented with a three-phase series RLC load model with floating Y and constant Z. 3. Proposed control method 3.1. Grid-forming BESS unit The BESS consists of a battery, LC filter, inverter, and transformer. Fig. 2 shows the configuration of the gridforming BESS unit, which works as an AC voltage source with a given voltage amplitude and frequency that sets the system frequency and AC voltage of the BESS. The BESS is suitable for grid forming because it can adjust the output quickly and accurately and has bidirectional characteristics. However, if the active power in the microgrid changes suddenly beyond the operating range of the BESS, other resources such as DER and controllable loads must be simultaneously controlled for stable system operation. The reference signal of the frequency is given by a voltage-frequency proportional controller (VFPC) based on the AC voltage difference of the BESS, as illustrated in Fig. 3. The BESS forms the system frequency with the VFPC considering the AC voltage from the local measurement. f nom is the nominal frequency of the system and f ref is the reference value of the grid forming BESS. Vc is the value obtained by subtracting the AC voltage from the voltage reference value of the BESS . The coefficient K v denotes the proportional gain of the VFPC. The limiter plays suppresses the sudden changes in the frequency. The frequency reference signal of the BESS is given by
f ref = f nom − K v Vc (1) BATTERY
LC Filter
AC DC
GRID Transformer
VC
abc/ dq
V-F Proportional Controller
PWM Modulation Signal
abc/ dq
IL Voltage Controller
Vref
m
Idq*
Theta
Current Controller
Vdq*
Vabc* abc/ dq
Theta
Fig. 2. Grid-forming BESS configuration with control algorithm.
Hyeon-Jin Moon et al. / Energy Procedia 153 (2018) 48–55
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fnom fref
ΔVc
Kv
Limiter
2π
ωref
θref
Fig. 3. V-F Proportional controller block diagram.
The frequency formed by the BESS functions as a global data signal that allows the BESS to command other DESs to adjust their output without any communication. If the AC voltage of the grid-forming BESS is changed, it implies that the power balance of the system is broken. This situation means that the active power reserve of the BESS has reached its limit. At this time, the load is reduced due to insufficient power supply, leading to a voltage drop [12]. When the BESS has sufficient power reserve to meet the power balance, they can maintain the AC voltage immediately because the inverter can control the voltage accurately and quickly. Therefore, when voltage fluctuates in the BESS bus, the grid-forming BESS detects it and changes its frequency reference for sharing active power reserve, as shown in (1)). Subsequently, other DESs adjust their output by droop control in response to the frequency that is a global data signal. In this paper, we focus on the coordinated autonomous active power control between the grid forming BESS and the diesel generator. This is because the PV and WT have limits in their output and they are operated with MPPT to maximize economic efficiency. 3.2. Diesel generator unit Diesel generators are assumed to be operated by the active power droop control method, which mimics the operation of synchronous generators for sharing active power and maintaining power balance in large interconnected power systems [9]. The system frequency is changed when the rotor speed is changed by the effective torque, that is, the difference between the electrical and mechanical torques. This means that power balance is broken and the energy stored in the synchronous generator is used to maintain the power balance of the system. Using this characteristic, active power droop control was proposed such that the generators share the active power for power balance, which can be expressed as
ref = set − K P ( Pm − Pset )
(2)
where ωref denotes the reference angular frequency of a generator, and the values of ωset and Pset correspond to the nominal value of angular frequency and active power, respectively. Kp represents the droop coefficient. Droop control is widely applied for the reliability and improvement of the system performance and stability. This control is based only on local measurements and allows other generators to operate autonomously. Fig. 4 shows the control structure of a diesel generator for active power. It includes an active power droop control, PI controller, time delay model of a valve actuator, diesel engine, and a synchronous machine dynamic model. The synchronous machine was modeled by the synchronous machine SI fundamental block in the MATLAB/Simpowersystems. The parameters related to the control block are referred from [8]. The droop controller generates ωref_di and the PI controller is used to track the reference signal by comparing ωref_di and ωm_di, the latter of which is the measured angular frequency of a diesel generator.
Hyeon-Jin Moon et al. / Energy Procedia 153 (2018) 48–55
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Pm_di
Droop Gain
Pref_di
Pdi
Synchronous Machine
ωm_di
ωref_di fref_di
Kp
Pset_di
fn
1/60
1 1 + sTd
1 1 + sTv
Diesel Engine
Valve Actuator
(pu)
(pu)
PI
Fig. 4. Control structure for active power of a diesel generator.
Fig. 5. (a) VFPC scheme in the BESS; (b) conventional droop characteristic.
3.3. Proposed autonomous active power management The conventional primary control method by grid-forming converters based on CVCF and SOC does not respond well with the large change in active power beyond the active power reserve of the BESS because it only depends on the BESS to maintain the power balance of the system. By using the VFPC of the BESS and the active power droop control of the diesel generator as shown in Fig. 5, the autonomous active power management can be achieved for solving the active power shortage due to the active power limit of the BESS, without any communication equipment. This is because the frequency generated by the grid forming BESS plays the role of a global data signal based on master-slave control. The BESS notices insufficient active power condition in the system by measuring the AC bus voltage of the BESS. Subsequently, they controls frequency proportional to the voltage deviation from the nominal voltage to control the diesel generator with active power droop control. This method enables the utilization of other resources in the system when the active power is rapidly changed. Further, the advantage of the proposed control method includes compatibility with the conventional control structures. 4. Simulation results and discussion To verify the effect of the proposed control method, a simulation is carried out based on the MATLAB/Simulink model of Geocha Island. In this paper, we focus on the low voltage condition, which is more important and more common than the high voltage condition due to the limited active power reserve in the system. The simulation was performed for three successive events in low voltage condition: (i) the load (Dongyuk, 180 kW) is connected at t = 20 s; (ii) WT is tripped at t = 30 s; and (iii) PV is tripped at t = 40 s. We assumed that the wind speed changes from 11 m/s to 14 m/s. At t = 0 s, the total load demand, PV, and diesel generators are set to 248 kW, 90 kW, and 100 kW,
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respectively. The simulation results are shown in Fig. 6 to Fig. 8. Other parameters related to the simulation are shown in Table 1. Table 1. System parameters Parameter name
Symbol
Value
Units
Time constant of diesel engine
Td
0.5
s
Time constant of valve actuator
Tv
0.05
s
Droop coefficients of the diesel generator
Kp
0.6/150e3
-
System nominal frequency
fnom
60
Hz
Low frequency of the BESS
-
59.4
Hz
Rate limit of the BESS frequency
-
0.3
Per sec
Rated voltage
Vref
1
pu
V/f proportional gain
Kv
6
-
(a)
(b)
Fig. 6. Active power profile in Geocha microgrid: (a) CVCF only; (b) Proposed.
(a)
(b)
Fig. 7. Local voltage profile in Geocha microgrid: (a) CVCF only; (b) Proposed.
(a)
(b)
Fig. 8. Comparison of system profile in Geocha microgrid: (a) System frequency; (b) Total load.
54
Hyeon-Jin Moon et al. / Energy Procedia 153 (2018) 48–55
4.1. Low voltage condition When the Dongyuk load is connected to the microgrid at t = 20 s, the total load is increased from 248 kW to 428 kW. Consequently, the grid-forming BESS adjust its active power to meet the load demand. The total load profile, which is shown in Fig. 8(b), is the sum of the total load demand and the total loss of the system. The real load demand is smaller than the load rating because the load is modelled by a constant Z model that is proportional to the square of the bus voltage. The voltage drop is greater near the Dongyuk load due to the increase in line flow toward this load. At t = 30 s, when WT is tripped, the active power of the BESS is increased and a further voltage drop in the other generators occurred. The total load demand is decreased due to the additional voltage drop by power shortage. Meanwhile, the other resources retain their active power because the BESS is responsible for maintaining the active power balance. When the PV is tripped at t = 40, the active power reserve of the BESS cannot afford the power balance. An additional voltage drop occurs to reduce the load demand. In the CVCF-only case, the active power of the BESS is lower than its rating power because of the current limit and voltage drop as shown in Fig 6(a). The total load is greatly reduced with the large voltage drop (0.8 pu) as shown in Fig 7(a) and Fig 8(b). However, when the proposed VFPC is applied with CVCF, the frequency is reduced from the voltage dip in the rated voltage, as shown in Fig 8(a). At this time, the diesel generator increases the active power output by the droop control. Therefore, as shown in Fig. 7 and Fig. 8, the voltage drop (0.9 pu) and load reduction (364.5 MW) are mitigated by adding the VFPC. 4.2. Discussion Because the grid-forming BESS is solely responsible for the spinning reserve of the system, a method to supply the reserve using other resources is required when the limit is reached. The proposed VFPC operates when the active power is insufficient, as measured from the AC voltage of the BESS. This control method adjusts the frequency that is proportional to the voltage difference to control the diesel generator with active power droop control. Therefore, this method does not require any communication and can be easily applied with other conventional control methods such as CVCF. 5. Conclusion The VFPC is proposed in this paper for the isolated microgrid to operate as an additional control to the conventional CVCF controllers to solve the low voltage problem due to the limited reserve of the grid-forming BESS. This method uses the system frequency as a global data signal, which is the method to control the other DES indirectly without any communication. The simulation results show that the proposed VFPC method could mitigate active power shortage using the frequency based operation of the DER in the system. Acknowledgements This work was supported by the Human Resources Development program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea government Ministry of Trade, Industry and Energy (No. 20174030201540) and the Korea Electric Power Research Institute (KEPRI) grant funded by the Korea Electric Power Corporation (KEPCO). References [1] Hatziargyriou N. "Microgrids architectures and control." Chichester: Wiley, IEEE-Press,; (2014) XXI, 317 S p. [2] Wang XF, Guerrero JM, Blaabjerg F, Chen Z. "A Review of Power Electronics Based Microgrids." J Power Electron. 12(1) (2012): 181-92. [3] Miao ZX, Xu L, Disfani VR, Fan LL. "An SOC-Based Battery Management System for Microgrids." Ieee T Smart Grid. 5(2) (2014): 966-73. [4] de Matos JG, Silva FSFE, Ribeiro LAD. "Power Control in AC Isolated Microgrids With Renewable Energy Sources and Energy Storage Systems." Ieee T Ind Electron. 62(6) (2015): 3490-8. [5] Hwang WH, Kim SK, Lee JH, Chae WK, Lee JH, Lee HJ, et al. "Autonomous Micro-grid Design for Supplying Electricity in Carbon-Free Island." J Electr Eng Technol. 9(3) (2014): 1112-8.
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[6] Wu D, Tang F, Dragicevic T, Vasquez JC, Guerrero JM. "A Control Architecture to Coordinate Renewable Energy Sources and Energy Storage Systems in Islanded Microgrids." Ieee T Smart Grid. 6(3) (2015): 1156-66. [7] Wu D, Tang F, Dragicevic T, Vasquez JC, Guerrero JM. "Autonomous Active Power Control for Islanded AC Microgrids With Photovoltaic Generation and Energy Storage System." Ieee T Energy Conver. 29(4) (2014): 882-92. [8] Moon H-J, Chang JW, Kim E-S, Moon SI. "Frequency-based wireless control of distributed generators in an isolated microgrid: A case of Geocha Island in South Korea." Universities Power Engineering Conference (UPEC), 2017 52nd International; (2017): IEEE. [9] Kim YS, Kim ES, Moon SI. "Frequency and Voltage Control Strategy of Standalone Microgrids With High Penetration of Intermittent Renewable Generation Systems." Ieee T Power Syst. 31(1) (2016): 718-28. [10] Abdullah MA, Yatim AHM, Tan CWA, Saidur R. "A review of maximum power point tracking algorithms for wind energy systems." Renew Sust Energ Rev. 16(5) (2012): 3220-7. [11] Femia N, Petrone G, Spagnuolo G, Vitelli M. "Optimization of perturb and observe maximum power point tracking method." Ieee T Power Electr. 20(4) (2005): 963-73. [12] Kundur P, Balu NJ, Lauby MG. "Power system stability and control." New York: McGraw-Hill; (1994). xxiii, 1176 p. p.
ScienceDirect ScienceDirect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia Procedia 00 153(2017) (2018)000–000 48–55 Energy www.elsevier.com/locate/procedia
5th International Conference on Energy and Environment Research, ICEER 2018 5th International Conference on Energy and Environment Research, ICEER 2018
Autonomous active power management in isolated microgrid based Autonomous active power management in isolated microgrid based proportional andondroop The 15thon International Symposium Districtcontrol Heating and Cooling on proportional and droop control Hyeon-Jin Moon, Jae Won Chang, Seok-Young Lee and Seung-Il Moon* Assessing the feasibility of using the heat demand-outdoor Hyeon-Jin Moon, Jae Won Chang, Seok-Young Lee and Seung-Il Moon* School of Electrical and Computer Engineering, Seoul National Univ., 1, Gwanak-ro, Gwanak-gu, Seoul, 08826, South Korea temperature function for a long-term district heat demand forecast School of Electrical and Computer Engineering, Seoul National Univ., 1, Gwanak-ro, Gwanak-gu, Seoul, 08826, South Korea I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, Avenue Dreyfous Daniel, 78520 Limay, France In an isolated microgrid, coordinated active power control291 is required for power balance under any circumstances. We propose a c Département Systèmes Énergétiques et Environnement IMT Atlantique, 4balance rue Alfred Kastler, 44300 Nantes, France In an isolatedcontrol microgrid, coordinated active power control is required forpower powerbalance under anyinsufficient circumstances. We propose proportional method based on the master-slave concept to- achieve even from power supply. Thisa proportional control method on the master-slave concept to achieve powerbattery balanceenergy even from insufficient power supply. This autonomous method based uses the frequency generated by the grid-forming storage system (BESS) as a global autonomous control method uses the frequency by theis grid-forming energy storage system (BESS) as a global data signal. Other units are controlled indirectly. generated The frequency controlled to battery be proportional to the AC voltage deviation of the data signal. Other are controlled indirectly. The frequency controlled to be proportional to the AC of voltage deviationmethod of the grid-forming BESSunits for detecting sudden power shortages and forissharing active power. The effectiveness the proposed Abstract BESS for detecting sudden power shortages and for sharing active power. The effectiveness of the proposed method grid-forming is verified by simulations in MATLAB/Simulink. is verified by simulations in MATLAB/Simulink. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the © 2018 The Authors. Published by Elsevier Ltd. greenhouse gas emissions fromby theElsevier building sector. These systems require high investments which are returned through the heat © 2018 The Authors. Ltd. © 2018 The Authors. Published by Elsevier Ltd. This is an open accessPublished article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article climate under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) sales. Due to the changed conditions and building policies, heat demand in the future could decrease, This is an and openpeer-review access article under the CC BY-NC-ND license renovation (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific committee of the 5th International Conference on Energy and Selection andthepeer-review under responsibility of the scientific committee of the 5th International Conference on Energy and prolonging investment return period. Selection andResearch, peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment ICEER 2018. Environment Research, ICEER 2018. The main scope of thisICEER paper is2018. to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Environment Research, forecast.microgrid; The district of stroage Alvalade, located in Lisbon used asautonomous a case study. Thedroop district is consisted Keywords: energy system; frequency control; (Portugal), active power was management; control; control; coordinatedof 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district control; master-slave Keywords: microgrid;control; energy stroage system; frequency control; active power management; autonomous control; droop control; coordinated renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were control; master-slave control; compared with results from a dynamic heat demand model, previously developed and validated by the authors. results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.The Introduction error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.(the Introduction scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). With enhanced price competitiveness and sustainability, the penetration of renewable energy sources (RES) into The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the With enhanced price competitiveness and the ispenetration of renewable energy sources (RES) the electricity is steadily A sustainability, microgrid, anseason integrated platform storage units into and decrease in thesupply number of heatingincreasing. hours of 22-139h during which the heating (depending on for the supply, combination of weather and the electricity supply is steadily increasing. A microgrid, which is an integrated platform for supply, storage units and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. * Corresponding author. Tel.: +82-10-4626-7527; fax: +82-2-878-1452. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and address:author. [email protected] * E-mail Corresponding Tel.: +82-10-4626-7527; fax: +82-2-878-1452. Cooling.
E-mail address: [email protected] 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change This is an open access article underPublished the CC BY-NC-ND 1876-6102 © 2018 The Authors. by Elsevier license Ltd. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific of the 5th International Conference on Energy and Environment This is an and openpeer-review access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Research, and ICEER 2018. under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Selection peer-review Research, ICEER 2018.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 10.1016/j.egypro.2018.10.055
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demand resources located in a local grid, enables local energy management [1]. Meanwhile, most remote areas such as small oceanic islands have no connection with the main power grid due to cost and/or technical problems. The electric power supply for these areas in several countries depends primarily on a diesel generator that can be adjusted to meet the power balance of the grid. However, RES such as photovoltaics (PV), wind turbines (WTs) are more difficult to apply as power systems than diesel generators due to their intermittency, which causes even more stability problems in remote power systems. However, the development of microgrid technology and energy storage system (ESS) has rendered RES an attractive option for supplying electricity in remote areas. In an isolated microgrid without any electrical connection with other systems, the battery energy storage system (BESS) unit is typically operated as a grid-forming unit to control the alternating current (AC) bus voltage and the system frequency. This is because the ESS can be controlled immediately and charged and discharged by the smart inverter technology [2]. When the BESS is used for the grid-forming unit, its state of charge (SOC) has to be maintained within the acceptable range for device protection and stable operation [3], [4]. Several control strategies based on the constant-voltage constant-frequency (CVCF) method for the grid-forming ESS unit have been proposed and used in real microgrids across South Korea [5]. This method controls the frequency and voltage to be a constant and primary active power control is met by the BESS which operates as ideal voltage source. However, the SOC level must be considered to protect the battery and guarantee sufficient spinning reserve. In addition, the active power balance depends primarily on the BESS. To share the burden of the grid-forming BESS, frequency-based droop control is proposed [6]-[9]. In [6], active power and frequency droop control is applied to coordinate the distributed energy sources (DER) and ESSs in inverter-based islanded microgrids. To maintain the SOC value, SOC-frequency droop control is proposed in [7]. However, problems arise from the fluctuations in the SOC or from active power in the droop control region. Further, conventional generators such as diesel generators are not considered. Moreover, master-slave-based SOC control method with droop control in diesel generators is presented in [8]. In [9], the active power control algorithm of diesel generators with the SOC of a BESS using low-bandwidth communication is proposed. This paper proposes an autonomous active power management based on the proportional control of the girdforming BESS and droop control of diesel generators. With CVCF operation, the BESS controls the frequency when an AC voltage is changed which implies active power imbalance. This method minimizes the low voltage problem and power insufficiency without any communication. A simulation in MATLAB/Simulink is performed to show the effect of the proposed control method. 2. System description ACSR/AW-OC 58mm2 : 0.497+j0.438 [Ω/km] ACSR/AW-OC 95mm2 : 0.304+j0.420 [Ω/km]
Diesel Gen.1
40 kW ACSR 58mm2 1.8 km
40 kW
70 kW AC
150kW
ACSR 58mm2 0.2 km
380V/6,900V, Y-Δ
ACSR 58mm2 0.2 km
ACSR 58mm2 0.5 km
ACSR 58mm2 0.4 km
DC 380V/6,900V, Y-Δ
Diesel Gen.2 17 kW
Diesel Gen.3
ACSR 58mm2x1.0km ACSR 95mm2x0.6km
380V/6,900V, Y-Δ
BESS 250kW/500kWh
ACSR 58mm2x0.9km ACSR 95mm2x1.0km
81 kW
PV 100kW AC DC
380V/6,900V, Y-Δ
Fig. 1. Geocha Island microgrid system configuration.
WT 100kW
ACSR 58mm2 0.6 km
ACSR 58mm2 0.3 km
320V/380V, Δ-Y
AC
180 kW
ACSR 58mm2 0.3 km
AC DC
DC
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Geocha Island, one of the areas of the remote microgrid project driven by Korea Power Electric Corporation (KEPCO), has an isolated microgrid system. Fig. 1 illustrates the configuration of the microgrid system in Geocha Island. The system has no electrical connection with the main grid and includes a 250 kW/500 kWh of BESS, 100 kW of PV, WT, and three 150 kW diesel generators, respectively. The other data are replicated from the real system of Geocha Island. The nominal system voltage and frequency are 6.9 kV and 60 Hz, respectively. The converters in the system were based on the two-level averaged model. The WT, PV, BESS and diesel generator were modeled based on the MATLAB/Simulink library models. The WT and PV are assumed to be operated by the maximum-power point tracking (MPPT) algorithm to maximize their cost efficiency [10], [11]. The loads were implemented with a three-phase series RLC load model with floating Y and constant Z. 3. Proposed control method 3.1. Grid-forming BESS unit The BESS consists of a battery, LC filter, inverter, and transformer. Fig. 2 shows the configuration of the gridforming BESS unit, which works as an AC voltage source with a given voltage amplitude and frequency that sets the system frequency and AC voltage of the BESS. The BESS is suitable for grid forming because it can adjust the output quickly and accurately and has bidirectional characteristics. However, if the active power in the microgrid changes suddenly beyond the operating range of the BESS, other resources such as DER and controllable loads must be simultaneously controlled for stable system operation. The reference signal of the frequency is given by a voltage-frequency proportional controller (VFPC) based on the AC voltage difference of the BESS, as illustrated in Fig. 3. The BESS forms the system frequency with the VFPC considering the AC voltage from the local measurement. f nom is the nominal frequency of the system and f ref is the reference value of the grid forming BESS. Vc is the value obtained by subtracting the AC voltage from the voltage reference value of the BESS . The coefficient K v denotes the proportional gain of the VFPC. The limiter plays suppresses the sudden changes in the frequency. The frequency reference signal of the BESS is given by
f ref = f nom − K v Vc (1) BATTERY
LC Filter
AC DC
GRID Transformer
VC
abc/ dq
V-F Proportional Controller
PWM Modulation Signal
abc/ dq
IL Voltage Controller
Vref
m
Idq*
Theta
Current Controller
Vdq*
Vabc* abc/ dq
Theta
Fig. 2. Grid-forming BESS configuration with control algorithm.
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fnom fref
ΔVc
Kv
Limiter
2π
ωref
θref
Fig. 3. V-F Proportional controller block diagram.
The frequency formed by the BESS functions as a global data signal that allows the BESS to command other DESs to adjust their output without any communication. If the AC voltage of the grid-forming BESS is changed, it implies that the power balance of the system is broken. This situation means that the active power reserve of the BESS has reached its limit. At this time, the load is reduced due to insufficient power supply, leading to a voltage drop [12]. When the BESS has sufficient power reserve to meet the power balance, they can maintain the AC voltage immediately because the inverter can control the voltage accurately and quickly. Therefore, when voltage fluctuates in the BESS bus, the grid-forming BESS detects it and changes its frequency reference for sharing active power reserve, as shown in (1)). Subsequently, other DESs adjust their output by droop control in response to the frequency that is a global data signal. In this paper, we focus on the coordinated autonomous active power control between the grid forming BESS and the diesel generator. This is because the PV and WT have limits in their output and they are operated with MPPT to maximize economic efficiency. 3.2. Diesel generator unit Diesel generators are assumed to be operated by the active power droop control method, which mimics the operation of synchronous generators for sharing active power and maintaining power balance in large interconnected power systems [9]. The system frequency is changed when the rotor speed is changed by the effective torque, that is, the difference between the electrical and mechanical torques. This means that power balance is broken and the energy stored in the synchronous generator is used to maintain the power balance of the system. Using this characteristic, active power droop control was proposed such that the generators share the active power for power balance, which can be expressed as
ref = set − K P ( Pm − Pset )
(2)
where ωref denotes the reference angular frequency of a generator, and the values of ωset and Pset correspond to the nominal value of angular frequency and active power, respectively. Kp represents the droop coefficient. Droop control is widely applied for the reliability and improvement of the system performance and stability. This control is based only on local measurements and allows other generators to operate autonomously. Fig. 4 shows the control structure of a diesel generator for active power. It includes an active power droop control, PI controller, time delay model of a valve actuator, diesel engine, and a synchronous machine dynamic model. The synchronous machine was modeled by the synchronous machine SI fundamental block in the MATLAB/Simpowersystems. The parameters related to the control block are referred from [8]. The droop controller generates ωref_di and the PI controller is used to track the reference signal by comparing ωref_di and ωm_di, the latter of which is the measured angular frequency of a diesel generator.
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Pm_di
Droop Gain
Pref_di
Pdi
Synchronous Machine
ωm_di
ωref_di fref_di
Kp
Pset_di
fn
1/60
1 1 + sTd
1 1 + sTv
Diesel Engine
Valve Actuator
(pu)
(pu)
PI
Fig. 4. Control structure for active power of a diesel generator.
Fig. 5. (a) VFPC scheme in the BESS; (b) conventional droop characteristic.
3.3. Proposed autonomous active power management The conventional primary control method by grid-forming converters based on CVCF and SOC does not respond well with the large change in active power beyond the active power reserve of the BESS because it only depends on the BESS to maintain the power balance of the system. By using the VFPC of the BESS and the active power droop control of the diesel generator as shown in Fig. 5, the autonomous active power management can be achieved for solving the active power shortage due to the active power limit of the BESS, without any communication equipment. This is because the frequency generated by the grid forming BESS plays the role of a global data signal based on master-slave control. The BESS notices insufficient active power condition in the system by measuring the AC bus voltage of the BESS. Subsequently, they controls frequency proportional to the voltage deviation from the nominal voltage to control the diesel generator with active power droop control. This method enables the utilization of other resources in the system when the active power is rapidly changed. Further, the advantage of the proposed control method includes compatibility with the conventional control structures. 4. Simulation results and discussion To verify the effect of the proposed control method, a simulation is carried out based on the MATLAB/Simulink model of Geocha Island. In this paper, we focus on the low voltage condition, which is more important and more common than the high voltage condition due to the limited active power reserve in the system. The simulation was performed for three successive events in low voltage condition: (i) the load (Dongyuk, 180 kW) is connected at t = 20 s; (ii) WT is tripped at t = 30 s; and (iii) PV is tripped at t = 40 s. We assumed that the wind speed changes from 11 m/s to 14 m/s. At t = 0 s, the total load demand, PV, and diesel generators are set to 248 kW, 90 kW, and 100 kW,
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respectively. The simulation results are shown in Fig. 6 to Fig. 8. Other parameters related to the simulation are shown in Table 1. Table 1. System parameters Parameter name
Symbol
Value
Units
Time constant of diesel engine
Td
0.5
s
Time constant of valve actuator
Tv
0.05
s
Droop coefficients of the diesel generator
Kp
0.6/150e3
-
System nominal frequency
fnom
60
Hz
Low frequency of the BESS
-
59.4
Hz
Rate limit of the BESS frequency
-
0.3
Per sec
Rated voltage
Vref
1
pu
V/f proportional gain
Kv
6
-
(a)
(b)
Fig. 6. Active power profile in Geocha microgrid: (a) CVCF only; (b) Proposed.
(a)
(b)
Fig. 7. Local voltage profile in Geocha microgrid: (a) CVCF only; (b) Proposed.
(a)
(b)
Fig. 8. Comparison of system profile in Geocha microgrid: (a) System frequency; (b) Total load.
54
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4.1. Low voltage condition When the Dongyuk load is connected to the microgrid at t = 20 s, the total load is increased from 248 kW to 428 kW. Consequently, the grid-forming BESS adjust its active power to meet the load demand. The total load profile, which is shown in Fig. 8(b), is the sum of the total load demand and the total loss of the system. The real load demand is smaller than the load rating because the load is modelled by a constant Z model that is proportional to the square of the bus voltage. The voltage drop is greater near the Dongyuk load due to the increase in line flow toward this load. At t = 30 s, when WT is tripped, the active power of the BESS is increased and a further voltage drop in the other generators occurred. The total load demand is decreased due to the additional voltage drop by power shortage. Meanwhile, the other resources retain their active power because the BESS is responsible for maintaining the active power balance. When the PV is tripped at t = 40, the active power reserve of the BESS cannot afford the power balance. An additional voltage drop occurs to reduce the load demand. In the CVCF-only case, the active power of the BESS is lower than its rating power because of the current limit and voltage drop as shown in Fig 6(a). The total load is greatly reduced with the large voltage drop (0.8 pu) as shown in Fig 7(a) and Fig 8(b). However, when the proposed VFPC is applied with CVCF, the frequency is reduced from the voltage dip in the rated voltage, as shown in Fig 8(a). At this time, the diesel generator increases the active power output by the droop control. Therefore, as shown in Fig. 7 and Fig. 8, the voltage drop (0.9 pu) and load reduction (364.5 MW) are mitigated by adding the VFPC. 4.2. Discussion Because the grid-forming BESS is solely responsible for the spinning reserve of the system, a method to supply the reserve using other resources is required when the limit is reached. The proposed VFPC operates when the active power is insufficient, as measured from the AC voltage of the BESS. This control method adjusts the frequency that is proportional to the voltage difference to control the diesel generator with active power droop control. Therefore, this method does not require any communication and can be easily applied with other conventional control methods such as CVCF. 5. Conclusion The VFPC is proposed in this paper for the isolated microgrid to operate as an additional control to the conventional CVCF controllers to solve the low voltage problem due to the limited reserve of the grid-forming BESS. This method uses the system frequency as a global data signal, which is the method to control the other DES indirectly without any communication. The simulation results show that the proposed VFPC method could mitigate active power shortage using the frequency based operation of the DER in the system. Acknowledgements This work was supported by the Human Resources Development program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea government Ministry of Trade, Industry and Energy (No. 20174030201540) and the Korea Electric Power Research Institute (KEPRI) grant funded by the Korea Electric Power Corporation (KEPCO). References [1] Hatziargyriou N. "Microgrids architectures and control." Chichester: Wiley, IEEE-Press,; (2014) XXI, 317 S p. [2] Wang XF, Guerrero JM, Blaabjerg F, Chen Z. "A Review of Power Electronics Based Microgrids." J Power Electron. 12(1) (2012): 181-92. [3] Miao ZX, Xu L, Disfani VR, Fan LL. "An SOC-Based Battery Management System for Microgrids." Ieee T Smart Grid. 5(2) (2014): 966-73. [4] de Matos JG, Silva FSFE, Ribeiro LAD. "Power Control in AC Isolated Microgrids With Renewable Energy Sources and Energy Storage Systems." Ieee T Ind Electron. 62(6) (2015): 3490-8. [5] Hwang WH, Kim SK, Lee JH, Chae WK, Lee JH, Lee HJ, et al. "Autonomous Micro-grid Design for Supplying Electricity in Carbon-Free Island." J Electr Eng Technol. 9(3) (2014): 1112-8.
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