International Conference on Electrical, Computer and Communication Engineering (ECCE), February 16-18, 2017, Cox’s Bazar, Bangladesh
Weakest Location Exploration in IEEE-14 Bus System for Voltage Stability Improvement Using STATCOM, Synchronous Condenser and Static Capacitor F.B.K. Mahmood, S.Ahmad, G.Mukit,
Fadi M. Albatsh
M.T.I. Shuvo, S.Razwan and M.N.I. Maruf Department of EEE, Faculty of Engineering American International University-Bangladesh Email:
[email protected] Abstract— In modern power systems, long transmission lines coupled with fast-growing load demand factor have resulted in a more stressed system than ever before. This paper is aimed at static voltage stability analysis(VSA) of IEEE-14 bus standard test case for weakest region exploration using conventional P-V and Q-V curve methods for investigating voltage unstable regions, predicting and preventing system blackouts. Furthermore, the static capacitor, synchronous condenser and STATCOM have been used for effective voltage restoration and regulation purposes. Hence the power grid can effectively manage and provide electricity reliably, safely and economically to consumers. Power World Simulator environment has been used for performance analysis. Keywords—Voltage Stability, P-V Curve, Q-V Curve, STATCOM, Power Factor Improvement
I.
INTRODUCTION
Now-a-days, impending voltage instability has posed a serious threat to the security and reliability of modern power systems. Voltage stability is defined as the ability of the power system to maintain equilibrium operating condition after being subjected to severe contingencies from an initial operating condition [1]. There are mainly two types of approaches for voltage stability analysis. The static approach is based on conventional power flow solutions which are appropriate for studies where precontingencies and post-contingencies cases are identified for voltage stable limits. While the dynamic approach is set to solve highly non-linear differential equations for generator dynamics, on load tap changing transformers, variation of load properties, etc. based on real –time simulations [8],[9]. Since voltage stability is a dynamic phenomenon, the later approach is more preferable in terms of accuracy but the computations are highly complex and laborious for large meshed power systems [9,11,12]. Besides, static voltage stability analysis is more suitable for bulk studies, provided that stability margin for contingency cases is resolved [2]. STATCOM (Static Synchronous Compensator), a FACTS device, mainly used for reactive power compensation. A STATCOM is nothing but a voltage source converter, which takes dc input voltage and splits into 3 phase AC voltages at
Senior Lecturer Electrical Engineering Section International College Universiti Kuala Lumpur
fixed frequency and phase angle [4]. It consist of only one capacitor and reactor, the former being normally used as input and later being used as a filter for an inverter [5]. It is observed that with the increase of reactive power supply from various reactive power compensating devices, pre- contingency Another important and widely used reactive power supplying device is Synchronous Condenser, which simply at overexcited state, acts like a motor spinning at no load condition. Its field is attached to a voltage regulator where it can provide to or absorb reactive power from the system. It’s very suitable for varying load fluctuations, even in the severe cases of short circuit and electric arcing [6]. Shunt Capacitor is perhaps the most common and inexpensive device for reactive power source which are usually connected in load areas. However, owing to poor voltage regulation and limited reactive power support, the reactive power drops during low voltage conditions [5], [6], [7]. In this paper, static voltage stability analysis has been used and the test case under investigation is IEEE-14 bus system. Two conventional methods P-V and Q-V curves are used to identify weak buses and strong buses resp. Later, reactive power compensating devices such as STATCOM, synchronous condenser and shunt capacitor are used at weak buses and the results have been compared in terms of voltage restoration and power factor improvement. II.
VOLTAGE STABILITY ANALYSIS
Voltage stability can be broadly analyzed into two ways; static and dynamic. The static analysis involves only the solution of algebraic equations which is given load level available from power flow analysis and therefore is computationally efficient than the dynamic analysis. Several analysis techniques of static voltage stability are used such as direct method, modal analysis, continuation method, optimization method etc[13-14]. Two most conventional methods are discussed below: A. P-V Curve The P-V curves are the most elementary method for predicting
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voltage instability. They are used to identify maximum allowable safe loading margin for a particular load bus in a power system. For this analysis, the active power, P is systematically increased in steps for a particular load bus and the corresponding load bus voltage magnitude is observed. Then, the curves of those buses under investigation are plotted along X-axis while the corresponding bus voltage is plotted along Y-axis, a simple two bus system is considered as shown
curve. The points where dQ/dV is not equal to zero is the voltage unstable criterion.
Fig.1. Two Bus Representation Model for P-V Curve [9]
Fig.2 P-V Curve and Q-V Curve
It is observed that with the increase of reactive power supply from various reactive power compensating devices; precontingency power system has been established. From [9], ሺȁʹȁʹሻʹሾሺȾሻȀǦȁʹȁʹȀʹሾͳȾʹሿሿൌͲሺͳሻ This is a quadratic equation, eliminating Ʌͳʹ and solving the second order equation, we get, ȁʹȁʹൌሺͳǦȾേξሺሾͳǦሺʹȾሻሿሻሻȀʹሺʹሻ It is seen from the above equation (2) that the load voltage or receiving end voltage is dependent on power delivered, line reactance and load power factor. The solution which has the higher value is the stable solution.
III. REACTIVE POWER COMPENSATION Voltage instability occurs mainly due to shortage of reactive power in the power system [9],[10],[11]. Due to the imbalance of generator and generation, transmission and demand, this problem pertaining to shortage of reactive power supply is more acute in the load buses located far away from the generator side. Though voltage instability is a local phenomenon, it has an adverse effect on the entire power network [9]. So, a localized approach may be adopted to supply reactive power for power factor improvement and voltage restoration purpose [2]. From economical point of view, it is preferable to operate at power factor close to unity. Due to variations of load, that too being mostly inductive, power factor correction methods are applied for achieving higher values of power factor [9]. There are a number of ways for reactive power support: • • • • •
B. Q-V Curve With Q-V curve, it is possible to find out the maximum reactive power that is required by weak regions before minimum voltage limit has been reached [2]. In Q-V curve, the relationship between reactive power (Q) and receiving end voltage (V2) for different load margins(P) is established[6][7]. Let us again cite the example of a simple 2 bus system with equations: ൌȁͳȁȁʹȁɅͳʹ ሺ͵ሻ ൌǦȁʹȁʹȁͳȁȁʹȁ
ɅͳʹሺͶሻ Ʌͳʹ has been derived from the first equation for given values of PD and V2 and taking V1=1.00 arbitrarily. Then from later equation, Q is solved for different values of V2. Hence the Q-V curve is plotted for pre-specified values of PD in the same way as P-V curve [9]. The identical P-V and Q-V curves are shown in Figure 2. For P-V curve analysis, the voltage stability criterion is the distance between current operating point and the nose point of P-V curve. As for Q-V curve, the reactive power margin is the distance between the operating point and the bottom of the Q-V
Shunt Capacitor Static VAR Compensator Synchronous Condenser Phase Advancers FACTS
The reactive power required to inject at a particular load bus is calculated from the following equation: Q = P ( tanɅǦ tanɅǯ)
(5)
Where, P= Active power Q = Reactive power Ʌ= Initial power factor Ʌǯ= Desired power factor A. STATCOM STATCOM is usually connected at the middle of transmission line as the voltage mostly varies most at the midpoint of long distance transmission line. It is a power electronics device which consists of a VSC, a capacitor on the
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electronics device which consists of a VSC, a capacitor on the DC side of converter and a controller circuit to generate pulse for the converter. IGBT based VSC used for obtaining instantaneous output AC voltage. The Voltage magnitude and phase angle plays key role whether a STATCOM absorbs or generates reactive power [3]. If the AC bus voltage is greater than converter output, then reactive power is absorbed. If the angle between current and voltage is 90°, only reactive power is exchanged. Both active and reactive power will be exchanged when the angle exceeds 90°. Figure 3 shows the terminal identical V-I curves for STATCOM [4], [5].
Fig.3. Terminal V-I characteristics of STATCOM
C. SHUNT CAPACITOR The most feasible and simple reactive power support device in power system is the shunt capacitor. It’s unexacting to install and maintain. Shunt capacitor reduces reactive components from the loads by superimposing the load current with fixed amount of leading current drawn from the system. So, usually the shunt capacitors are installed near the reactive loads [5]. The reactive power supplied by shunt capacitor is square of the terminal voltage, so if line voltage drops at any instant, which is highly likely for modern power systems, reactive power drops as line voltage drops [7]. This is a major disadvantage of shunt capacitors as it has poor voltage regulation. Figure 5 demonstrates the V-I curve for shunt capacitor,
B. SYNCHRONOUS CONDENSER As stated earlier, synchronous condenser acts like a motor, whose shaft remains unconnected and is free to rotate without obstruction. On increasing the field excitation, the corresponding reactive power supplied to system is increased. At overexcited state, synchronous condenser produces leading pf. The major advantage of using a synchronous condenser is its ability to continuously adjust the amount of reactive power in terms of varying loading margins and hence offers great flexibility for ceaseless PFI operation. Figure 4 shows the V-curve for synchronous condenser’s characteristics [6].
Fig.5. V-I characteristics for Shunt Capacitor
IV. SIMULATION AND RESULTS IEEE-14 bus test case consists of 11 load buses, two synchronous generators, three static compensators and a total of 21 transmission lines. The total loads amount to 259 MW and installed capacity of the generators are 272 MW. For P-V curve analysis, keeping constant load current, the active power was systematically subjected to increment of 1MW at each step and corresponding buses which are under investigation, their respective bus voltages were observed. Approximately, 10-12 observations were taken for each bus and even more for stronger load buses until desired curves were obtained The following Figures 6 and 7 show the dynamic modeling of IEEE-14 bus system and P-V curve obtained for IEEE-14 bus test case resp:
Fig.4. V-Curve for Synchronous Condenser
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At Bus 2, when load was increased, the voltage drop was minimal. Initially V2,pu = 1.04 pu and load margin is 21.7 MW. After load increment of 40%, the bus voltage didn’t exhibit vulnerability, so as to conclude this bus is the strongest among all. At Bus 3 and 4, after increasing load, their critical voltages have been identified. This two buses exhibits almost same characteristics and hence can be classified as moderately strong. The rest of the buses starting from 10, 11, 12, 13, 14 are all clustered together to form the weakest/most vulnerable buses of the test case. A slight augmentation of active load in these buses would render the system unstable and worst at most would lead to voltage collapse. The more the distance from generator buses to load buses, the more fragile and weak the load buses are due to redundancy of power in transmission loss. Under no condition these vulnerable load buses are to be subjected to load increment, not even by a small margin particularly at bus 12, being the weakest of all. Fig.6. Dynamic Modelling of IEEE-14 Bus Test Case
A brief summarization has been done about the facts and findings from the P-V curve analysis of IEEE-14 bus test system in the following table 1. Table I. IEEE-14 Bus Stable Operation Characteristics Using P-V Curve:
2
0.76
Stable operating Regions ( pu) 1.04-0.79
3
0.87
0.99-0.90
4
0.87
0.99-0.89
5
0.81
1.05-0.85
6
0.89
0.98-0.93
9
0.87
0.97-0.91
10 11 12 13 14
0.89 0.93 0.91 0.87 0.89
0.96-0.93 0.97 0.96 0.96 0.94
Load Buses
Vcritical ( pu)
Safe Load limits ( MW) 21.730.38 94.2131.8 47.867 7.610.64 11.215.68 29.535.3 09 3.5 6.1 13.5 14.9
Strength
The methodology of generating Q-V curve is almost as same as that of P-V curve except for the fact that the reactive power would be plotted along X-axis instead of active power and bus voltage along Y-axis in the same way. The reactive load margin was increased by 1 MVAR in each step and the tally of observations stood at 5-10 or so on for each load bus, much less for weaker load buses. Table II. IEEE-14 Bus Stable Operation Characteristics Using Q-V Curve
Load Buses
Vcritical ( pu)
Moderate
2
0.7
Stable operating Regions ( pu) 1.04-0.71
Strong
3 4 5 6 9 10 11 12 13 14
0.82 0.83 0.82 0.91 0.90 0.88 0.88 0.86 0.87 0.84
0.99-0.88 0.99-0.86 1.01-0.86 0.98 0.97 0.96 0.97 0.96 0.96 0.94
Strongest Moderate
Weak Weak Weakest Weakest Weakest Weakest Weakest
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Safe Q margin (MVAR)
Strength
12.716.51 19-24.7 -3.9-2 1.6-2.08 7.5 16.6 5.8 1.8 1.6 5.8 5
Strongest Moderate Moderate Moderate Weak Weak Weak Weakest Weakest Weakest Weakest
From the table generated from Q-V curve, bus 2 seems to be the most robust bus which can absorb a high reactive power while maintain stable system voltage. As this bus is close to generator, the voltage drop due to reactive power consumption of load is partially neutralized by the reactive power supply of the generator itself. The curves obtained for buses 3, 4 and 5 can be classified as moderately strong. And the load buses 6 and 9 are classified as weak buses as they can accommodate a certain reactive load margin against redundancy but stable system voltage. But compared to load buses 3, 4 and 5; the buses 6 and 9 hits the nose point earlier indicating the fact that they cannot sustain huge reactive power consumption. Rather a marginal reactive load can be directed towards load buses 6 and 9 for stable functioning. Rest of the buses starting from 10, 11, 12, 13 and 14 are classified as weakest buses. They are the most vulnerable buses and thus, prone to unstable operation. As far as the voltage stability in concerned, the optimal location for reactive power compensation is the weakest bus of the system. Evidently, from PV and QV curves, we can conclude bus 14 to be the weakest of all, that too being far away from the generation side. Owing to poor voltage regulation, shunt capacitor serves the purpose upto a few MVARS only. In normal operating and loading condition, the use of static capacitor may lead to voltage drops or unacceptably over voltages. As reactive power depends upon terminal bus voltages, the use of shunt capacitors might boost power transferring capacity, but won’t be tacit on the voltage stability issues as compared to Synchronous Condenser and STATCOM.
For rough estimation, at first a synchronous condenser without any restrictions to reactive power is connected at bus 14 with the same loading and operating condition as indicated in tables 1 and 2. The reactive power generated at such was found to be 15 MVar. Unlike shunt capacitor the principle advantage of using Synchronous Condenser is variable amount of reactive power can be generated by simply increasing the field excitation irrespective of voltage magnitude and hence ensuring sound voltage regulation. STATCOM is connected exactly at the middle of transmission line 13-14, just like shunt capacitor, as line voltages varies most at the midpoint of transmission line [3]. STATCOM can provide reactive power support as well as limited active power to the system, ensuring additional support for line losses as well. A comparison of improved P-V curve at bus 14 is shown below:
Fig.7 Improved P-V Curve After Introducing Reactive Power Compensation
Table III. Comparative PFI scenario using Shunt, Synchronous Condenser and STATCOM at different transmission lines nearby distributive loads
Improved Vcri
Voltages (pu) at line 13-14
Improved Vcri
%PFI
Vcri
Voltages (pu) at line 12-13
0.95
N/A
0.94
N/A
0.96
N/A
N/A
Shunt
0.96
0.85
0.96
0.9
0.97
0.87
2.13%,1.97%,3.43%
Synchronous Condenser
0.98
0.81
0.98
0.83
0.99
0.81
5.37%,6.02%,6.11%
STATCOM
1.00
0.79
1.00
0.83
0.99
0.8
8.45%,7.7%,9.3%
Voltages (pu) at line 9-13
Improved
Base Case
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V. CONCLUSION
It is observed from Fig.7, the P-V curve declines smoothly after the introduction of compensating devices. The nose point shifts further right to improve the robustness of the system and delaying the incipient voltage collapse point. Though shunt capacitor is the cheapest form to resort to reactive power compensation, it is applicable to serve the purpose of just few MVARs. Though power transfer capacity may be boosted using shunts, yet voltage stability issue might still be unresolved. Synchronous Condenser and in particular STATCOM can deliver much better results as far as both the terminal voltage and stability issue is concerned. Synchronous condenser once switched on, will restore the bus voltage to near unity irrespective of how low critical voltage at particular bus might be. Usually, the range of synchronous condenser extends from 10-200 MVAR. However, synchronous condensers and static capacitors could be used only in distribution substations of power systems whereas STATCOM when connected is integral part of transmission network.
In this paper, static voltage stability analysis for American Standard IEEE-14 Bus Test System, using two analysis methods, P-V and Q-V curves, followed by comparative study of Shunt Capacitor, Synchronous Condenser and STATCOM has been carried out. The weak regions have been identified and the maximum allowable load increments at all load buses were found out as well. For stability enhancement, shunt capacitor has been ruled out due to poor voltage regulation. In this regard, Synchronous Condenser and STATCOM provide a much better result. However, a comprehensive cost analysis study is required for rating and size selection as these devices are way too expensive than simple shunt capacitors.
STATCOM does an excellent job maintaining acceptable bus voltage and enhancing voltage stability in modern multivariable and multimachine complex power systems. When maximum condition is reached, STATCOM acts like a shunt capacitor, so correct rating should be carefully ensured while selecting the size of STATCOM. It is not economical to use STATCOM for smaller loads. The optimal position for introducing reactive power compensating device is the weakest bus of the system and that would be Bus 14. The following bus graph shows voltage restoration at buses 10-14:
Fig 8. Voltage Restoration at Clusters of Weak Buses in IEEE-14 Bus System
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VI. REFERENCES [1]
S.F.B. Shakil, N. Husain, M.D. Wasim and S.Junaid “Improving the voltage stability and performance of power networks using power electronics based FACTS controllers”, Energy Systems and Policies (ICESP), November 2014. [2] N.A. Yusof, H. Mokhlis, M.Karimi, J.A. Laghari “Under-voltage load shedding scheme based on voltage stability index for distribution network”, Clean Energy and Technology (CEAT), November 2014. [3] M. Gerarld Geafori Titus “Reactive power compensation using STATCOM for single phase distribution system”, Circuit, Power and Computing Technologies (ICCPCT), July 2015. [4] G. Hong, W.Yuhang and LI Yuan, “An improved control strategy of STATCOM for grid voltage unbalance compensation”, Proceedings of TENCON 2015 - 2015 IEEE Region 10 Conference, November 2014. [5] K. Miyazaki and T. Takeshita, “Line loss minimization in radial distribution system using multiple STATCOMs and static capacitors”, Power Electronics Conference (IPEC-Hiroshima 2014 - ECCE-ASIA), May 2014. [6] P. Hsu, E. Muljadi, Z.Wu and W.Gao, “Voltage regulation using a permanent magnet synchronous condenser with a series compensator”, Energy Conversion Congress and Exposition (ECCE), October, 2015. [7] H.E.Z. Farag, E.F.E-Saadany, “Optimum Shunt Capacitor Placement in Multimicrogrid Systems With Consideration of Islanded Mode of Operation”, IEEE Transactions on Sustainable Energy, Volume: 6, Issue: 4, pp- 1435-1436, October. 2015. [8] C.Reis and F.P. Maciel Barbosa, “A Comparison of Voltage Stability Indices”, in proceeding of 13th IEEE Mediterranean Electrochemical Conf. Melecom, Malaga, Espanha, pp 1007-1010, May 2006. [9] S.B. Bhaladhare, A.S. Telang and P.P. Bedeker, “P-V, Q-V Curve- A Novel Approach for Voltage Stability Analysis”, IJCA, NCIPET 2013, pp-346-354, 2013. [10] N.R. Bujal, A.E. Hasan and M.Sulaiman, “Analysis of Voltage Stability Problems in Power System”, ICE2T, 2104 4th Int. Conf.,pp 278-283, Aug 2014. [11] S. Ahmad, F. M. Albatsh, S. Mekhilef, and H. Mokhlis, "A placement method of fuzzy based unified power flow controller to enhance voltage stability margin," Power Electronics and Applications (EPE’14-ECCE Europe), pp. 1–10, Aug. 2014. 12] S. Ahmad, F. M. Albatsh, S. Mekhilef, and H. Mokhlis, "Fuzzy based controller for dynamic unified power flow controller to enhance power transfer capability," Energy Conversion and Management, vol. 79, pp. 652–665, Mar. 2014.
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