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Voltage Stability Control Based on VIPI Including a NAS Battery System
Copyright © IFAC Power Plants and Power Systems Control, Seoul, Korea, 2003
ELSEVIER
IFAC PUBLICATIONS www.elsevier.comllocate/ifac
VOLTAGE STABILITY CONTROL BASED ON VIPI INCLUDING A. NAS BATIERY SYSTEM
Toshinori Esaka
Arisa Takehara
Toshiya Ohtaka
Shinichi Iwamoto
Dept. ofElectrical Engineering and Bioscience, Waseda University, Tokyo, Japan
Abstract: In recent years, the majority of voltage control schemes are local controls on each bus. However there are a few on-line voltage stability control schemes that consider the whole system. Hence, we propose on-line voltage stability control schemes in addition to conventional local controls (Tanaka, et al., 1998; Lobato, et al., 2000; Feng, et al., 1999; Wang, et al., 1998; Tran-Quoc, et aI., 2000). In this paper, we regard an NAS battery system as control equipment and we propose a voltage stability preventive control method based on the VIPI using the NAS battery system. Simulations have been run to verify the effectiveness of the proposed method. The results showed that the proposed methods coped well with severe faults. Copyright © 2003 IFAC Keywords; NAS battery system, Voltage stability control, VIPI, Preventive control,
I. INTRODUCTION Recently, as power system loads are located further away from power plants and are more unevenly distributed, reactive power losses tend to increase due to the heavier power flow with high load growth and long distance transmission. The power demand differences, between the daytime and the night-time or between summer and winter, worsen the load factor. Furthermore, the Power Products and Suppliers (PPS) have been participating in the power generation market since the deregulation of the electric power industry in Japan. There is a possibility that their generators might affect voltage stability. Therefore, concerns about voltage instability phenomena such as a great voltage drop and a voltage collapse are being raised. Thus, an effective voltage control scheme is becoming more indispensable (Ilic, et aI., 1995). A control index is needed when the preventive control and the emergency preventive control are implemented. Most of the proposed indices for the evaluation of voltage stability vary peculiarly near the critical point or scarcely vary for heavy loaded conditions. Compared with them, the voltage stability index VIPI has some considerable characteristics. The value decreases linearly with an increase of loads and becomes zero near
*Arisa Takehara is presently with CRIEPE.Japan
the critical point. It is not a local bus index but an entire system index. The VIPI makes it easier to grasp a margin to the critical point than other indices (Hashimoto, et al., 1988). Therefore, we have adapted VIPI as a voltage stability index. Some energy storage systems have been proposed, such as energy storage batteries, Superconducting Magnetic Energy Storage devices (SMES devices), flywheel devices and compressed air storage devices. Above all, from a technological point of view, the NAS battery system is superior to the others and most practical at present. They can control active and reactive power outputs promptly and individually. In this point, the NAS battery system is more flexible and practical than the other devices. It has been proposed that the NAS battery system should be applied to voltage reactive power control in power systems ( Ohtaka and Iwamoto, 2000). It is adequate to convert the MW margin standard in the whole system to set the reference value of VIPI. If we [md the fault for which the value of VIPI becomes below the reference while on-line contingency analysis is being carried out, we initiate the preventive control against the severest case. We also carry out Optimal Power Flow (OPF) calculations. However, if the entire equipment is already used or the operation amount reaches its limits or the OPF has no solution, we
1109
consider load curtailment as an emergency preventive control in a conventional method. It has become more indispensable to investigate load shedding schemes as corrective control (Begonic, et aI., 1995; Moors, et al., 2(00). In this paper, we can take the NAS battery system and carry out the preventive control again before we consider load curtailment. When we perform load curtailment, we can scarcely adjust the ratio of active power output to reactive power output. As a result, we can consider more effective voltage stability control.
Voltage space Specified value space
Simulations have been run using the 1I bus system to verifY the effectiveness of the proposed methods. The results showed that the proposed methods when compared with the conventional methods, coped with more severe faults owing to the NAS battery system.
Fig.1 Concept of VIPI 45
r-----------------,
40
I--------o••-••- - - - - - - - - - - - - - j
35
I-----'~
1 30
~: 1-------.~ .......- - - - - - _ 1
2. VOLTAGE STABILITY INDEX VIPI
~
Power flow equation has several solutions. The number of these solutions decreases as a power system becomes heavy loaded It is known that two closely located solutions exist near the critical point. As they approach the critical point, these two solutions become closer, and at the critical point they merge. Focusing on this point, VIPI has been proposed (Tamura et al., 1988). VIPI is a scalar index for evaluating the voltage stability margin by using the angle between the critical vector and the specified value vector. In rectangular coordinates, the power flow equation is represented as follows:
Ys = Y(x)
••-•.~. ~_._--_.--------
I---~~...- - - - - - - - - - - -
(l)
where
Ys : specified value
x = (el ,/1 ,e2,h ,···,en,ln f
15 I - - - - - - - .•~ •
...----------1
10
1 - - - - - - - - - -..:.,.~---___l
5
1 - - - - - - - - - - - . -..:..........,..---1
o L-_~_~_~_~_~_.-"••'--__.J 1
12
1.4
1.8
1.8
2.2
2.4
ToUlload [P.uJ
Fig.2 Relationship between VIPI and total load VIPI has the following characteristics. Fig.2 shows that the value decreases linearly with an increase ofloads and becomes zero near the critical point. It is not a local bus index but an entire system index. Que to these characteristics, the VIPI makes it easy to grasp the margin to the critical point and the VIPI is suited for on-line system monitoring and control near the critical point. Therefore, we have adapted VIPI as a voltage stability index.
3. NAS BATTERY SYSTEM
ei : real part ofbus voltage
f;: imaginary part ofbus voltage Here, we assume that two kinds of voltage vectors x (operational solution) andx* (ficticious solution) satisfY the same specified value vector. Using critical vector a and deflection vector b, x and x* can be expressed as follows: x = a + b , x* = a - b (2) From these, a and b are expressed as follows:
x+x*
An NAS battery is made of fused sodium as a cathode-active material, fused sulfur and sodium polysulfide as an anode-active material, and utilizes beta alumina, which conducts sodium ions selectively as an electrolyte (Okuno,1996; Kodama and Tanaka, I999; Standing Special Committee of Chemical and Electrical Energy Transformation, 1980; The Institute of Applied Energy, 1992).
x-x*
a=-2-' b=-2-···············(3)
NAS battery systems have the following features. • They can control active and reactive power outputs promptly and individually. • They are capable of discharging the power approximately from two to five times as much as the rated capacity if the capacity of an AC-DC converter device is sufficient to do so. • They can be placed compactly in such a small space as distribution systems and power demand areas. The size of the energy density is approximately three times that of a lead battery. • They have high efficiency for both charging and discharging but do not have self-discharging
Because VIPI is the angle between the specified value vector Y. and the critical vector Yea), VIPI is defmed as follows (see Fig. 1). T
VIPI
-I
Y. yea)
= = cos Ily.ll-lly(a )11 f)
( ) 4
1110
characteristics, i.e. Therefore, they can store electricity effectively. • They can withstand more than 2,250 times of charging or discharging. They have a notable lifetime with IS-year durability. • A clean battery having a completely closed structure ensures safety and no maintenance for operation.
For example, if one finds that the operating point (al) has moved to (b I) after a fault has occurred, we carry out preventive control to move the post fault operating point to a safe state (cl). When a post fault operating point (b2) can be moved only to (c2) by preventive control, such state is defmed as a warning state. Lastly in case that the operating point after preventive control (c3) is still in the emergency state, we carry out emergency preventive control to move the operating point to the warning state (d3).
Load
( Discharging)
e----+
,\. 00 -e 0
,'i 0
0 0
5. PROPOSED METHODS
I I
I Sodium
Beta alumina
I
Power Source
( Charging)
Next, we will look at the conventional method. For further details of preventive control I, see (Takehara, et aI., 2002; Takehara ,et al., 2000; Esaka, et al., 200 I). Formulation of Optimal Power Flow (OPF) is as follows. If the value of VIPI is not above threshold I after contingency analysis is implemented, we implement preventive control I. Cl =assurned fault condition C2=operating condition
e_
e-
I Sodium
e
5.1 PREVENTIVE CONTROL 1
+
Sutfur
Beta alumina
0
0
I Sutfur
c:.
•
Electron Sodium Sodium ion
+
Sulfur Sodium polysufide
Fig.3 Mechanism Chart of Charging and Discharging
• Objective function
Minimize f 4. PREVENTIVE CONTROL AND EMERGENCY PREVENTIVE CONTROL
= (VIPI-thresholdl)2
(5)
• Equality constraints Power flow equation with operational solution at each bus (Cl and C2) and with fictitious solution at each bus (Cl) • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), SC and ratios of a transformer.
The concept of preventive control is proposed to avoid voltage instability phenomena beforehand by improving the voltage stability. However in case the preventive control cannot cope with severe faults, emergency preventive control is carried out to curtail loads.
5.2 PREVENTIVE CONTROL 2
VIPI safety state threshold I r----/f--(>-c--:I-------/f
o
bl
o al
0
/' c2 o b2
d3
Here, we talk about the proposed method. The improvement is that we use the NAS battery system as control equipment. If the value of VIPI is not above threshold I after preventive control I is implemented or the solution is not found, we implement preventive control 2. In this case, all SCs are used. Formulation of Optimal Power Flow (OPF) is as follows.
warning state
threshold 2 r------::7"'--=-~::r""-----
o
a2
o/'
/' c3 o~q;3 a3
emergency state
• Objective function
Fig.4 Concept of preventive control and emergency preventive control
Minimize f = (VIPl- thresholdl)2
(6)
• Equality constraints Power flow equation with operational solution at each bus (Cl and C2) and a fictitious solution at each bus (Cl). • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), ratios of a transformer and the NAS battery's output.
One needs a reference value in order to determine when one should perform and fmish the preventive control. There is considerable validity in converting the MW margin standard in a whole system into the value of VIPI. Thus, we the authors determine "threshold I" and "threshold 2" and divide the power system state into three states; safety, warning and emergency states.
I II I
load bus having the next biggest value of VIPI sensitivity and go back to Step7.
5.3 EMERGENCY PREVENTIVE CONTROL Here is another proposed method. The improvement is that we use the NAS battery system as control equipment (Concerning the conventional method, see [19]-[21]). If the value of VIPI is not above threshold 2 after preventive control 2 is implemented, we implement emergency preventive control. VIPI sensitivity for a change of real power and reactive power at each load bus is calculated in order to select effective loads to curtail (Nanba, et aI., 1998). VIPI sensitivity[22]: dVIPI + dVIPI tan 4> d~
Yes
(7)
dQL Yes
where tan et> is calculated by power factor cos et> . In this case, all SCs are used. Formulation of Optimal Power Flow (OPF) is as follows. • Objective function Minimize f = Total amount ofload curtailment. .. (8) • Equality constraints Power flow equation with an operational solution at each bus (Cl and C2) and with a fictitious solution at each bus (Cl). • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), ratios of a transformer, the NAS battery's output and the arnount of load curtailment at a selected bus.
No
Yes
Yes No
caluculation of VlPl sensitivity and determination of the loads for curtailment
6. ALGORISM Fig.5 Flow chart
The algorithm is as follows. Step1: Determine the value ofVIPI thresholds. Step2: Carry out on-line contingency analysis every few minutes or ten minutes. If the value of VIPI becomes below threshold 1, we regard that fault as a target. Step3: Initiate preventive control I. Step4: If the value of VIPI is above threshold 1, the preventive control 1 fmishes successfully. If it is between threshold 1 and threshold 2, the operating point is in a warning state. Keep a constant observation. If it is below threshold 2 or the OPF has no solution, initiate preventive control 2. StepS: In a similar fashion to Step4, the following matter can be explained. If the value of VIPI is above threshold 1, preventive control 2 finishes. If it is between threshold 1 and threshold 2, keep a constant observation. If it is below threshold 2 or the OPF has no solution, proceed to Step6. Step6: Calculate sensitivity of VIPI for each load bus. Take the load bus having the biggest value of VIPI sensitivity. Step7: Initiate emergency preventive control. StepS: If the value of VIPI is above threshold 1, the emergency preventive control has finished successfully. If it is below threshold 1, take the
7. SIMULATIONS
Fig.6 I I-bus system Two simulations were carried out in appropriate system conditions using an II-bus system (Klos and Kemer, 1975) shown in Fig.5. Some voltage controllers were added to this system for the purpose of this paper.
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Asswnptions are as follows: VIPI threshold I is 10 degrees and threshold 2 is 9 degrees. • As a contingency, we asswne one transmission line has opened in a double transmission. In general, there are many contingency accidents. Therefore, in this simulation, we set ten monitored contingencies. SCs are set at bus 2, 3, 7 and 10. As one bank is O.lj, 3 banks are set at each bus. • In the initial condition, the amount of SC is 0 and LTC is set at 1.0 for all devices. The NAS battery is set at bus 2 and the capacity is approximately one-twentieth as much as the total amount of the generator capacity of the bus system.
7.1 SYSTEM CONDITION I A result of a contingency analysis is shown in Table. I and a result of the amount of NAS battery is shown in Table2. For the fault of opening a nwnber 1 transmission line for double circuit transmission lines, the value of VIPI was below threshold 1. Thus, we singled this fault out as the target ofpreventive control.
7.2 SYSTEM CONDmON 2 The result of contingency analysis is shown in Table3; a result of VIPI sensitivity for a load bus is shown in Table4, a result of the arnount ofNAS battery is shown in TableS and a result of the amount of load curtailment is shown in Table6. Owing to the result of contingency analysis, we singled this fault out as the target of preventive control. This system condition is heavier loaded than condition I. Therefore, preventive control I resulted in no solution. Subsequently, preventive control 2 was implemented. As a result, the value of VIPI increased to 7.291. However, it was still in an emergency state below threshold I. This situation leaded us to the consideration of emergency-preventive control.
11ablel contingency analysis (s stem condition I) Transmission line opening operating
Initial condition 18.742
The problem, which we have to consider before emergency-preventive control, is the choice of a load bus to curtail. Therefore, we calculated VIPI sensitivity for the load bus. As the Table! indicates, the most effective load bus to curtail is the No.3 bus. This method minimizes the total amount of curtailment.
Preventive contro12 19.767
1
4.949
9.127
2 3 4 5 6 7 8 9 10 11 12 13 14 15
14.629 16.300 13.078 18.699 18.437 17.221 17.856 16.607 18.520 18.212 17.189 15.700 17.214 18.374
14.648 17.622 13.826 20.064 19.730 18.465 19.264 17.911 19.961 20.306 18.587 16.804 18.966 19.929
Table3 contingency analysis (system condition 2) Preventive Emergency Transmission Initial preventive control line opening condition contro12 operating 16.706 18.172 20.368
Conventional preventive control 1 resulted in no solution because it could not satisfy the inequality constraints of the voltage limits at the bus. Hence, as proposed preventive control 2 was implemented. It satisfied all the inequality constraints and the value of VIPI was beyond threshold 2. The operating point moved from the emergency state to warning state, which indicated avoidance of a severe fault. However, a constant observation is needed.
1
3.368
2 3 4 5 6 7 8 9 10 11 12 13 14 15
13.249 15.029 11.727 17.244 16.969 15.813 16.288 14.910 17.021 16.334 15.828 14.053 15.672 16.901
,,
7.291
9.009
12.995 16.046 12.243 18.183 17.843 16.713 17.332 15.856 18.085 18.420 16.881 14.882 17.094 18.079
15.166 18.058 14.080 20.625 20.257 18.976 19.790 18.289 20.547 20.766 19.075 17.173 19.546 20.469
After emergency preventive control, the value ofVIPI increased to 9.009 and it altered its state from the emergency to the warning state. The result shows that the emergency preventive control with an NAS battery is capable of coping with more severe faults in comparison with conventional emergency preventive control. Furthermore, we can safely say that the proposed method can decrease the total amount of load curtailment because the NAS battery system can control active power outputs. This is the notable difference from
The result clearly shows that preventive control 2 with an NAS battery is capable of coping with more severe faults in comparison with conventional preventive control 1.
1113
the conventional control equipment such as a Static Var Compensator (SVC) and Static Synchronous Compensator (STATCOM). Table4 VIPI sensitivity for each load bus (system condition 2) VIPI VIPI load bus load bus sensitivity sensitivity 2 0.0348 8 0.0143 I 0.0381 3 10 0.0103 4 11 0.0280 0.0288 7 0.0179 I
TableS The amount ofNAS battery (system condition 2) NAS battery's Preventive Emergency output control 2 Preventive control 1.207 1.487 P [p.u.l Q [P.u.] 2.746 2.605 S [p.u.] 3.000 3.000 Table6 The amount ofload curtailment 'system condition 2) bus the amount of curtailment 3
[p.u.]
0.9990
I I
[%]
8.680
8. CONCLUSIONS In recent years, concerns about voltage instability phenomena, such as a great voltage drop and a voltage collapse, have been raised. In this situation, the importance of voltage stability control schemes cannot be overemphasized. Hence, we proposed an on-line voltage stability control schemes. We focused on the NAS battery system features and regarded the NAS battery system as control equipment. The difference between conventional control equipment and the NAS battery system is that the latter cannot only control reactive power output but also active power output. In this point, the NAS battery system is more flexible and practical than the other devices. Simulations have been run using the 11 bus system to verifY the effectiveness of the proposed method. The results showed that the proposed methods i.e., the preventive control 2 and the emergency preventive control, which compared with the conventional methods, coped with more severe faults owing to the NAS battery system. Curtailment of load bus and output of active power by the NAS battery system have the same effect. We can safely say that proposed method could decrease the total amount of load curtailment due to the active power outputs of the NAS battery system. Thus, the effectiveness of the proposed methods is verified.
REFERENCE Y. Tanaka, S. Iwamoto (1998). A Preventive Voltage Stability Control Using Optimal Power Flow and VIPI, IEEI, PE-98-167 E. Lobato, L. Rouco, M. I. Navarrete, R. Casanova, 1. G Castillejo, G Lopez (2000). An Integrated Too! for Analysis of Power System Constraints in the Spanish Electricity Market, Proceedings ofIEEE PES Summer Meeting, pp.1627-1633. Z. Feng, V. Ajjarapu, D. J. Maratukulam (1999). A Comprehensive Approach for Preventive and Corrective Control to Mitigate Voltage Collapse, PE-290PRS. X. Wang, Gc. Ejebe, J. Tong, J.G Waight (1998). Preventive/Corrective Control for Voltage Stability Using Direct Interior Point Method", IEEE Trans. on Power Systems, Vo1.l3, No.3. T. Tran-Quoc, Ch. Praing, R. Feuillel, 1. C. SabolUladiere, U. La-Van, C. Nguyen-Duc (2000). Improvement of Voltage Stability on The Vietnam Power System, Proceedings ofIEEE PES Winter Meeting M. Ilic, X. Liu, G Leung, M. Athans, C. Vialas, P. Pruvot (1995). Improved Secondary and New Tertialy Voltage Control, IEEE Trans. on Power Systems, Vo. 10, No.4 J. HashimolO, N. Yorino, Y. Tamura (1988). On the Continuous Monitoring of Voltage Stability Margin in Electric Power Systems, The Trans. oflEEl. Vol. 108-B, No. 2 T.Ohtaka, S.IwamolO (2000). Optimal Allocation and PQ Output Ratio of NAS Batteries for Improving Voltage Stability in Distribution Systems, Transaction oflEEl, Vo1.l20-B, No.10, ppI254-1262, M. Begonic, D. FullOn, M. R. Gonzalez, J. Goossens, E. A. Guro, R. W. Haas, C. F. Henville, G Manchur, G L. Michel, R. C. PaslOre, J. Postforoosh, G L. Schrnill, J. B. Williams, K.. Zimmerman (1995). SUMMARY OF SYSTEM PROTECTION AND VOLTAGE STABILITY", IEEE Trans. on Power Delivery, Vol.l 0, No.2 C. Moors, D. Lefebvre, T. Van Cutsern (2000). Design of Load Shedding Schemes against Voltage Instability, Proceedings ofIEEE PES Wmter Meeting Y. Tamura, K.. Sakamoto, Y. Tayarna (1988). Voltage Instability Proximity Index (VIP!) based on multiple load flow solution in Ill-<:onditioned power system, Proceedings of the 27th IEEE Conferr!TK:e on Decision and Control. Texas, USA A.Okuno (1996). Energy Storage NAS Battery - Present Status of Technical Development of NAS Batteries for Load Leve!ing, The New Material ofJapan Industrial Publishing, Vo1.7, No.8 E.Kodama, K.Tanaka (1999). The Newest Battery Technology 5 Energy Storage Battery",JoW71al oflEEl, Vo1.l19, No.7 Standing Special Committee of Chemical and Electrical Energy Transformation (1980). Application of Secondary Batteries for Power Systems, Technical Report ofIEEI, Vol.l 03,No.2, New Energy and Industrial Technology Development Organization, The Institute of Applied Energy (1992). Study on Prevalence of New Battery Power Storage System ( II), NEDO-p9158, A.Takehara, Y. Tanaka, S.Iwamoto (2002). Voltage Stability Preventive and Emergency Preventive Controls Using VIPr Sensitivity, The Trans. ofIEElVol.! Ill-B A.Takehara, T.Ohtaka. S.Iwamoto, (2000). Voltage Stability Control Based on VIPI Sensitivity Considering Contingency Analysis, Power Engineering and Power System Engineering Symposium of ffiEl,PE~I~,~E~I~.
TooEsaka, T.Ohtaka, S.Iwamoto (2002). Voltage Stability Emergency preventive Control Considering Priority Order of Curtailment Loads, Proceedings ofthe Twelfth Annual Conference ofPower and Energy Society oflEEl, vol.A.pp.496-497 M. Nanba, Y. Huang, T. Kai, S. Iwamoto (1998). Studies on VIPI based control methods for improving voltage stability, Electrical Power & Energy Systems, Vo120, No.2, pp.141-146 A.Klos and A.Kemer (1975). The Uniqueness of Load-Flow Solution" Proc. PSCC V, 3.1/8 Cambridge
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ELSEVIER
IFAC PUBLICATIONS www.elsevier.comllocate/ifac
VOLTAGE STABILITY CONTROL BASED ON VIPI INCLUDING A. NAS BATIERY SYSTEM
Toshinori Esaka
Arisa Takehara
Toshiya Ohtaka
Shinichi Iwamoto
Dept. ofElectrical Engineering and Bioscience, Waseda University, Tokyo, Japan
Abstract: In recent years, the majority of voltage control schemes are local controls on each bus. However there are a few on-line voltage stability control schemes that consider the whole system. Hence, we propose on-line voltage stability control schemes in addition to conventional local controls (Tanaka, et al., 1998; Lobato, et al., 2000; Feng, et al., 1999; Wang, et al., 1998; Tran-Quoc, et aI., 2000). In this paper, we regard an NAS battery system as control equipment and we propose a voltage stability preventive control method based on the VIPI using the NAS battery system. Simulations have been run to verify the effectiveness of the proposed method. The results showed that the proposed methods coped well with severe faults. Copyright © 2003 IFAC Keywords; NAS battery system, Voltage stability control, VIPI, Preventive control,
I. INTRODUCTION Recently, as power system loads are located further away from power plants and are more unevenly distributed, reactive power losses tend to increase due to the heavier power flow with high load growth and long distance transmission. The power demand differences, between the daytime and the night-time or between summer and winter, worsen the load factor. Furthermore, the Power Products and Suppliers (PPS) have been participating in the power generation market since the deregulation of the electric power industry in Japan. There is a possibility that their generators might affect voltage stability. Therefore, concerns about voltage instability phenomena such as a great voltage drop and a voltage collapse are being raised. Thus, an effective voltage control scheme is becoming more indispensable (Ilic, et aI., 1995). A control index is needed when the preventive control and the emergency preventive control are implemented. Most of the proposed indices for the evaluation of voltage stability vary peculiarly near the critical point or scarcely vary for heavy loaded conditions. Compared with them, the voltage stability index VIPI has some considerable characteristics. The value decreases linearly with an increase of loads and becomes zero near
*Arisa Takehara is presently with CRIEPE.Japan
the critical point. It is not a local bus index but an entire system index. The VIPI makes it easier to grasp a margin to the critical point than other indices (Hashimoto, et al., 1988). Therefore, we have adapted VIPI as a voltage stability index. Some energy storage systems have been proposed, such as energy storage batteries, Superconducting Magnetic Energy Storage devices (SMES devices), flywheel devices and compressed air storage devices. Above all, from a technological point of view, the NAS battery system is superior to the others and most practical at present. They can control active and reactive power outputs promptly and individually. In this point, the NAS battery system is more flexible and practical than the other devices. It has been proposed that the NAS battery system should be applied to voltage reactive power control in power systems ( Ohtaka and Iwamoto, 2000). It is adequate to convert the MW margin standard in the whole system to set the reference value of VIPI. If we [md the fault for which the value of VIPI becomes below the reference while on-line contingency analysis is being carried out, we initiate the preventive control against the severest case. We also carry out Optimal Power Flow (OPF) calculations. However, if the entire equipment is already used or the operation amount reaches its limits or the OPF has no solution, we
1109
consider load curtailment as an emergency preventive control in a conventional method. It has become more indispensable to investigate load shedding schemes as corrective control (Begonic, et aI., 1995; Moors, et al., 2(00). In this paper, we can take the NAS battery system and carry out the preventive control again before we consider load curtailment. When we perform load curtailment, we can scarcely adjust the ratio of active power output to reactive power output. As a result, we can consider more effective voltage stability control.
Voltage space Specified value space
Simulations have been run using the 1I bus system to verifY the effectiveness of the proposed methods. The results showed that the proposed methods when compared with the conventional methods, coped with more severe faults owing to the NAS battery system.
Fig.1 Concept of VIPI 45
r-----------------,
40
I--------o••-••- - - - - - - - - - - - - - j
35
I-----'~
1 30
~: 1-------.~ .......- - - - - - _ 1
2. VOLTAGE STABILITY INDEX VIPI
~
Power flow equation has several solutions. The number of these solutions decreases as a power system becomes heavy loaded It is known that two closely located solutions exist near the critical point. As they approach the critical point, these two solutions become closer, and at the critical point they merge. Focusing on this point, VIPI has been proposed (Tamura et al., 1988). VIPI is a scalar index for evaluating the voltage stability margin by using the angle between the critical vector and the specified value vector. In rectangular coordinates, the power flow equation is represented as follows:
Ys = Y(x)
••-•.~. ~_._--_.--------
I---~~...- - - - - - - - - - - -
(l)
where
Ys : specified value
x = (el ,/1 ,e2,h ,···,en,ln f
15 I - - - - - - - .•~ •
...----------1
10
1 - - - - - - - - - -..:.,.~---___l
5
1 - - - - - - - - - - - . -..:..........,..---1
o L-_~_~_~_~_~_.-"••'--__.J 1
12
1.4
1.8
1.8
2.2
2.4
ToUlload [P.uJ
Fig.2 Relationship between VIPI and total load VIPI has the following characteristics. Fig.2 shows that the value decreases linearly with an increase ofloads and becomes zero near the critical point. It is not a local bus index but an entire system index. Que to these characteristics, the VIPI makes it easy to grasp the margin to the critical point and the VIPI is suited for on-line system monitoring and control near the critical point. Therefore, we have adapted VIPI as a voltage stability index.
3. NAS BATTERY SYSTEM
ei : real part ofbus voltage
f;: imaginary part ofbus voltage Here, we assume that two kinds of voltage vectors x (operational solution) andx* (ficticious solution) satisfY the same specified value vector. Using critical vector a and deflection vector b, x and x* can be expressed as follows: x = a + b , x* = a - b (2) From these, a and b are expressed as follows:
x+x*
An NAS battery is made of fused sodium as a cathode-active material, fused sulfur and sodium polysulfide as an anode-active material, and utilizes beta alumina, which conducts sodium ions selectively as an electrolyte (Okuno,1996; Kodama and Tanaka, I999; Standing Special Committee of Chemical and Electrical Energy Transformation, 1980; The Institute of Applied Energy, 1992).
x-x*
a=-2-' b=-2-···············(3)
NAS battery systems have the following features. • They can control active and reactive power outputs promptly and individually. • They are capable of discharging the power approximately from two to five times as much as the rated capacity if the capacity of an AC-DC converter device is sufficient to do so. • They can be placed compactly in such a small space as distribution systems and power demand areas. The size of the energy density is approximately three times that of a lead battery. • They have high efficiency for both charging and discharging but do not have self-discharging
Because VIPI is the angle between the specified value vector Y. and the critical vector Yea), VIPI is defmed as follows (see Fig. 1). T
VIPI
-I
Y. yea)
= = cos Ily.ll-lly(a )11 f)
( ) 4
1110
characteristics, i.e. Therefore, they can store electricity effectively. • They can withstand more than 2,250 times of charging or discharging. They have a notable lifetime with IS-year durability. • A clean battery having a completely closed structure ensures safety and no maintenance for operation.
For example, if one finds that the operating point (al) has moved to (b I) after a fault has occurred, we carry out preventive control to move the post fault operating point to a safe state (cl). When a post fault operating point (b2) can be moved only to (c2) by preventive control, such state is defmed as a warning state. Lastly in case that the operating point after preventive control (c3) is still in the emergency state, we carry out emergency preventive control to move the operating point to the warning state (d3).
Load
( Discharging)
e----+
,\. 00 -e 0
,'i 0
0 0
5. PROPOSED METHODS
I I
I Sodium
Beta alumina
I
Power Source
( Charging)
Next, we will look at the conventional method. For further details of preventive control I, see (Takehara, et aI., 2002; Takehara ,et al., 2000; Esaka, et al., 200 I). Formulation of Optimal Power Flow (OPF) is as follows. If the value of VIPI is not above threshold I after contingency analysis is implemented, we implement preventive control I. Cl =assurned fault condition C2=operating condition
e_
e-
I Sodium
e
5.1 PREVENTIVE CONTROL 1
+
Sutfur
Beta alumina
0
0
I Sutfur
c:.
•
Electron Sodium Sodium ion
+
Sulfur Sodium polysufide
Fig.3 Mechanism Chart of Charging and Discharging
• Objective function
Minimize f 4. PREVENTIVE CONTROL AND EMERGENCY PREVENTIVE CONTROL
= (VIPI-thresholdl)2
(5)
• Equality constraints Power flow equation with operational solution at each bus (Cl and C2) and with fictitious solution at each bus (Cl) • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), SC and ratios of a transformer.
The concept of preventive control is proposed to avoid voltage instability phenomena beforehand by improving the voltage stability. However in case the preventive control cannot cope with severe faults, emergency preventive control is carried out to curtail loads.
5.2 PREVENTIVE CONTROL 2
VIPI safety state threshold I r----/f--(>-c--:I-------/f
o
bl
o al
0
/' c2 o b2
d3
Here, we talk about the proposed method. The improvement is that we use the NAS battery system as control equipment. If the value of VIPI is not above threshold I after preventive control I is implemented or the solution is not found, we implement preventive control 2. In this case, all SCs are used. Formulation of Optimal Power Flow (OPF) is as follows.
warning state
threshold 2 r------::7"'--=-~::r""-----
o
a2
o/'
/' c3 o~q;3 a3
emergency state
• Objective function
Fig.4 Concept of preventive control and emergency preventive control
Minimize f = (VIPl- thresholdl)2
(6)
• Equality constraints Power flow equation with operational solution at each bus (Cl and C2) and a fictitious solution at each bus (Cl). • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), ratios of a transformer and the NAS battery's output.
One needs a reference value in order to determine when one should perform and fmish the preventive control. There is considerable validity in converting the MW margin standard in a whole system into the value of VIPI. Thus, we the authors determine "threshold I" and "threshold 2" and divide the power system state into three states; safety, warning and emergency states.
I II I
load bus having the next biggest value of VIPI sensitivity and go back to Step7.
5.3 EMERGENCY PREVENTIVE CONTROL Here is another proposed method. The improvement is that we use the NAS battery system as control equipment (Concerning the conventional method, see [19]-[21]). If the value of VIPI is not above threshold 2 after preventive control 2 is implemented, we implement emergency preventive control. VIPI sensitivity for a change of real power and reactive power at each load bus is calculated in order to select effective loads to curtail (Nanba, et aI., 1998). VIPI sensitivity[22]: dVIPI + dVIPI tan 4> d~
Yes
(7)
dQL Yes
where tan et> is calculated by power factor cos et> . In this case, all SCs are used. Formulation of Optimal Power Flow (OPF) is as follows. • Objective function Minimize f = Total amount ofload curtailment. .. (8) • Equality constraints Power flow equation with an operational solution at each bus (Cl and C2) and with a fictitious solution at each bus (Cl). • Inequality constraints Limits of generator outputs (Cl and C2), voltages at buses (Cl and C2), line flows (Cl and C2), ratios of a transformer, the NAS battery's output and the arnount of load curtailment at a selected bus.
No
Yes
Yes No
caluculation of VlPl sensitivity and determination of the loads for curtailment
6. ALGORISM Fig.5 Flow chart
The algorithm is as follows. Step1: Determine the value ofVIPI thresholds. Step2: Carry out on-line contingency analysis every few minutes or ten minutes. If the value of VIPI becomes below threshold 1, we regard that fault as a target. Step3: Initiate preventive control I. Step4: If the value of VIPI is above threshold 1, the preventive control 1 fmishes successfully. If it is between threshold 1 and threshold 2, the operating point is in a warning state. Keep a constant observation. If it is below threshold 2 or the OPF has no solution, initiate preventive control 2. StepS: In a similar fashion to Step4, the following matter can be explained. If the value of VIPI is above threshold 1, preventive control 2 finishes. If it is between threshold 1 and threshold 2, keep a constant observation. If it is below threshold 2 or the OPF has no solution, proceed to Step6. Step6: Calculate sensitivity of VIPI for each load bus. Take the load bus having the biggest value of VIPI sensitivity. Step7: Initiate emergency preventive control. StepS: If the value of VIPI is above threshold 1, the emergency preventive control has finished successfully. If it is below threshold 1, take the
7. SIMULATIONS
Fig.6 I I-bus system Two simulations were carried out in appropriate system conditions using an II-bus system (Klos and Kemer, 1975) shown in Fig.5. Some voltage controllers were added to this system for the purpose of this paper.
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Asswnptions are as follows: VIPI threshold I is 10 degrees and threshold 2 is 9 degrees. • As a contingency, we asswne one transmission line has opened in a double transmission. In general, there are many contingency accidents. Therefore, in this simulation, we set ten monitored contingencies. SCs are set at bus 2, 3, 7 and 10. As one bank is O.lj, 3 banks are set at each bus. • In the initial condition, the amount of SC is 0 and LTC is set at 1.0 for all devices. The NAS battery is set at bus 2 and the capacity is approximately one-twentieth as much as the total amount of the generator capacity of the bus system.
7.1 SYSTEM CONDITION I A result of a contingency analysis is shown in Table. I and a result of the amount of NAS battery is shown in Table2. For the fault of opening a nwnber 1 transmission line for double circuit transmission lines, the value of VIPI was below threshold 1. Thus, we singled this fault out as the target ofpreventive control.
7.2 SYSTEM CONDmON 2 The result of contingency analysis is shown in Table3; a result of VIPI sensitivity for a load bus is shown in Table4, a result of the arnount ofNAS battery is shown in TableS and a result of the amount of load curtailment is shown in Table6. Owing to the result of contingency analysis, we singled this fault out as the target of preventive control. This system condition is heavier loaded than condition I. Therefore, preventive control I resulted in no solution. Subsequently, preventive control 2 was implemented. As a result, the value of VIPI increased to 7.291. However, it was still in an emergency state below threshold I. This situation leaded us to the consideration of emergency-preventive control.
11ablel contingency analysis (s stem condition I) Transmission line opening operating
Initial condition 18.742
The problem, which we have to consider before emergency-preventive control, is the choice of a load bus to curtail. Therefore, we calculated VIPI sensitivity for the load bus. As the Table! indicates, the most effective load bus to curtail is the No.3 bus. This method minimizes the total amount of curtailment.
Preventive contro12 19.767
1
4.949
9.127
2 3 4 5 6 7 8 9 10 11 12 13 14 15
14.629 16.300 13.078 18.699 18.437 17.221 17.856 16.607 18.520 18.212 17.189 15.700 17.214 18.374
14.648 17.622 13.826 20.064 19.730 18.465 19.264 17.911 19.961 20.306 18.587 16.804 18.966 19.929
Table3 contingency analysis (system condition 2) Preventive Emergency Transmission Initial preventive control line opening condition contro12 operating 16.706 18.172 20.368
Conventional preventive control 1 resulted in no solution because it could not satisfy the inequality constraints of the voltage limits at the bus. Hence, as proposed preventive control 2 was implemented. It satisfied all the inequality constraints and the value of VIPI was beyond threshold 2. The operating point moved from the emergency state to warning state, which indicated avoidance of a severe fault. However, a constant observation is needed.
1
3.368
2 3 4 5 6 7 8 9 10 11 12 13 14 15
13.249 15.029 11.727 17.244 16.969 15.813 16.288 14.910 17.021 16.334 15.828 14.053 15.672 16.901
,,
7.291
9.009
12.995 16.046 12.243 18.183 17.843 16.713 17.332 15.856 18.085 18.420 16.881 14.882 17.094 18.079
15.166 18.058 14.080 20.625 20.257 18.976 19.790 18.289 20.547 20.766 19.075 17.173 19.546 20.469
After emergency preventive control, the value ofVIPI increased to 9.009 and it altered its state from the emergency to the warning state. The result shows that the emergency preventive control with an NAS battery is capable of coping with more severe faults in comparison with conventional emergency preventive control. Furthermore, we can safely say that the proposed method can decrease the total amount of load curtailment because the NAS battery system can control active power outputs. This is the notable difference from
The result clearly shows that preventive control 2 with an NAS battery is capable of coping with more severe faults in comparison with conventional preventive control 1.
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the conventional control equipment such as a Static Var Compensator (SVC) and Static Synchronous Compensator (STATCOM). Table4 VIPI sensitivity for each load bus (system condition 2) VIPI VIPI load bus load bus sensitivity sensitivity 2 0.0348 8 0.0143 I 0.0381 3 10 0.0103 4 11 0.0280 0.0288 7 0.0179 I
TableS The amount ofNAS battery (system condition 2) NAS battery's Preventive Emergency output control 2 Preventive control 1.207 1.487 P [p.u.l Q [P.u.] 2.746 2.605 S [p.u.] 3.000 3.000 Table6 The amount ofload curtailment 'system condition 2) bus the amount of curtailment 3
[p.u.]
0.9990
I I
[%]
8.680
8. CONCLUSIONS In recent years, concerns about voltage instability phenomena, such as a great voltage drop and a voltage collapse, have been raised. In this situation, the importance of voltage stability control schemes cannot be overemphasized. Hence, we proposed an on-line voltage stability control schemes. We focused on the NAS battery system features and regarded the NAS battery system as control equipment. The difference between conventional control equipment and the NAS battery system is that the latter cannot only control reactive power output but also active power output. In this point, the NAS battery system is more flexible and practical than the other devices. Simulations have been run using the 11 bus system to verifY the effectiveness of the proposed method. The results showed that the proposed methods i.e., the preventive control 2 and the emergency preventive control, which compared with the conventional methods, coped with more severe faults owing to the NAS battery system. Curtailment of load bus and output of active power by the NAS battery system have the same effect. We can safely say that proposed method could decrease the total amount of load curtailment due to the active power outputs of the NAS battery system. Thus, the effectiveness of the proposed methods is verified.
REFERENCE Y. Tanaka, S. Iwamoto (1998). A Preventive Voltage Stability Control Using Optimal Power Flow and VIPI, IEEI, PE-98-167 E. Lobato, L. Rouco, M. I. Navarrete, R. Casanova, 1. G Castillejo, G Lopez (2000). An Integrated Too! for Analysis of Power System Constraints in the Spanish Electricity Market, Proceedings ofIEEE PES Summer Meeting, pp.1627-1633. Z. Feng, V. Ajjarapu, D. J. Maratukulam (1999). A Comprehensive Approach for Preventive and Corrective Control to Mitigate Voltage Collapse, PE-290PRS. X. Wang, Gc. Ejebe, J. Tong, J.G Waight (1998). Preventive/Corrective Control for Voltage Stability Using Direct Interior Point Method", IEEE Trans. on Power Systems, Vo1.l3, No.3. T. Tran-Quoc, Ch. Praing, R. Feuillel, 1. C. SabolUladiere, U. La-Van, C. Nguyen-Duc (2000). Improvement of Voltage Stability on The Vietnam Power System, Proceedings ofIEEE PES Winter Meeting M. Ilic, X. Liu, G Leung, M. Athans, C. Vialas, P. Pruvot (1995). Improved Secondary and New Tertialy Voltage Control, IEEE Trans. on Power Systems, Vo. 10, No.4 J. HashimolO, N. Yorino, Y. Tamura (1988). On the Continuous Monitoring of Voltage Stability Margin in Electric Power Systems, The Trans. oflEEl. Vol. 108-B, No. 2 T.Ohtaka, S.IwamolO (2000). Optimal Allocation and PQ Output Ratio of NAS Batteries for Improving Voltage Stability in Distribution Systems, Transaction oflEEl, Vo1.l20-B, No.10, ppI254-1262, M. Begonic, D. FullOn, M. R. Gonzalez, J. Goossens, E. A. Guro, R. W. Haas, C. F. Henville, G Manchur, G L. Michel, R. C. PaslOre, J. Postforoosh, G L. Schrnill, J. B. Williams, K.. Zimmerman (1995). SUMMARY OF SYSTEM PROTECTION AND VOLTAGE STABILITY", IEEE Trans. on Power Delivery, Vol.l 0, No.2 C. Moors, D. Lefebvre, T. Van Cutsern (2000). Design of Load Shedding Schemes against Voltage Instability, Proceedings ofIEEE PES Wmter Meeting Y. Tamura, K.. Sakamoto, Y. Tayarna (1988). Voltage Instability Proximity Index (VIP!) based on multiple load flow solution in Ill-<:onditioned power system, Proceedings of the 27th IEEE Conferr!TK:e on Decision and Control. Texas, USA A.Okuno (1996). Energy Storage NAS Battery - Present Status of Technical Development of NAS Batteries for Load Leve!ing, The New Material ofJapan Industrial Publishing, Vo1.7, No.8 E.Kodama, K.Tanaka (1999). The Newest Battery Technology 5 Energy Storage Battery",JoW71al oflEEl, Vo1.l19, No.7 Standing Special Committee of Chemical and Electrical Energy Transformation (1980). Application of Secondary Batteries for Power Systems, Technical Report ofIEEI, Vol.l 03,No.2, New Energy and Industrial Technology Development Organization, The Institute of Applied Energy (1992). Study on Prevalence of New Battery Power Storage System ( II), NEDO-p9158, A.Takehara, Y. Tanaka, S.Iwamoto (2002). Voltage Stability Preventive and Emergency Preventive Controls Using VIPr Sensitivity, The Trans. ofIEElVol.! Ill-B A.Takehara, T.Ohtaka. S.Iwamoto, (2000). Voltage Stability Control Based on VIPI Sensitivity Considering Contingency Analysis, Power Engineering and Power System Engineering Symposium of ffiEl,PE~I~,~E~I~.
TooEsaka, T.Ohtaka, S.Iwamoto (2002). Voltage Stability Emergency preventive Control Considering Priority Order of Curtailment Loads, Proceedings ofthe Twelfth Annual Conference ofPower and Energy Society oflEEl, vol.A.pp.496-497 M. Nanba, Y. Huang, T. Kai, S. Iwamoto (1998). Studies on VIPI based control methods for improving voltage stability, Electrical Power & Energy Systems, Vo120, No.2, pp.141-146 A.Klos and A.Kemer (1975). The Uniqueness of Load-Flow Solution" Proc. PSCC V, 3.1/8 Cambridge
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