DRPT2008 6-9 April 2008 Nanjing China
A New-style Dynamic Var Compensation Control Strategy Jinxia Gong, Jingjing Lu, Da Xie, IEEE Member, and Yanchi Zhang
Abstract--A new dynamic reactive power compensation technique for dynamic var compensator is presented. This paper presents the DC capacitor voltage control strategy, the loss current variation control algorithm and a new reactive current calculating method. And the desired compensating current is compared with the measurement of output current in the hysteretic current feedback-loop. Based on the analysis, a newstyle dynamic var compensation control strategy is realized. It also analyzes the relationship between DC capacitor voltage variation characteristic and the active and reactive power flow between AC and DC sides. A novel dynamic var compensation prototype is developed. The experimental waveforms of the prototype show that this control algorithm has the expected compensating performance, demonstrating the validity of the design scheme proposed in this article. Index Terms--control algorithm; DC voltage control; Dynamic var compensation; loss current; reactive current; simulation
II. INTRODUCTION
I. NOMENCLATURE
T
List of symbols: us(t) = mains voltage UsM = peak value of mains voltage u ' (t ) = unity voltage in a 90 phase delay with the mains voltage ∆U dc = difference of Udc and Uref Uref = setting voltage Udc = the DC capacitor voltage is, is(t) = mains current iL, iL(t) = load current ic, ic(t) = the output current of the compensator ic* = desired current ipA(t) = active current needed to be compensated iq(t) = reactive current needed to be compensated IpA = amplitude of ipA(t) IL = peak value of load current Ip = peak value of reactive component of load current Jinxia Gong is with Department of Electrical Engineering, Shanghai Jiaotong University, No.800, Dongchuan Road, District Minhang, Shanghai, 200240, China. (e-mail:
[email protected]) Jingjing Lu is with Department of Electrical Engineering, Shanghai Jiaotong University, No.800, Dongchuan Road, District Minhang, Shanghai, 200240, China. (e-mail:
[email protected]) Da Xie is with Department of Electrical Engineering, Shanghai Jiaotong University, No.800, Dongchuan Road, District Minhang, Shanghai, 200240, China. (e-mail:
[email protected]) Yanchi Zhang is with Department of Electrical Engineering, Shanghai DianJi University, District Minhang, Shanghai, 200240, China. (e-mail:
[email protected])
978-7-900714-13-8/08/ ©2008DRPT
Iq = peak value of active component of load current wL(t) = instantaneous power demanded by load wS(t) = instantaneous output power supplied from the mains wA(t) = instantaneous output power supplied from the compensator pA(t) = loss power of the converter pL(t) = real power demanded by load qL(t) = var power demanded by load q = a symbol of reactive power ω = frequency of mains voltage θ = phase of load fundamental current ϕ = practical electric degree C = capacity of DC capacitor L = filter inductor T = 20ms A = fluctuation part of DC capacitor voltage
HE wide applications of electronic appliance generate harmonic and reactive current in the utility system, which cause low power factor and great system loss. The effective harness is paid more and more attention to by scholars both at home and abroad [1-4]. For a compensator, its performance depends on the detection and control algorithm of compensating current [5]. Recently, extraction of reactive current and control algorithm have developed rapidly, most of which are completed with the instantaneous reactive power theory [6, 7]. These methods need lots of mathematics calculation [8]. In paper [9], a simplified control method for the single-phase active power filter is proposed. The purpose of this method is that the mains current is in phase with the mains voltage and the load current is not used. Paper [10] proposes that it is probable to add positive feedback to the control system when the load current has a content of capacitive reactive current in a practical system. And in this situation, the system may cause current oscillation and has poor compensating performance. This paper presents a new dynamic reactive power compensation technique to detect and compensate reactive current for dynamic var compensator, based on DC voltage control and load reactive current feedback control method. Through measurement of the load reactive current, this new proposed control algorithm can control the output current directly, and it avoids the extra procedure to detect active and reactive power. So the control strategy realizes the var compensation simply without complicate calculation of reactive and active current.
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This control algorithm compensates the fundamental frequency reactive current, and the dynamic continuously compensating effect is satisfied in both the inductive and capacitive condition. The compensating system is more stable in theory. The proposed method has simple structure, quickly dynamic character and good compensation performance. Based on this new proposed control method, a dynamic var compensating equipment is realized. Simulation and experimental results prove the validity of the control algorithm.
The integration of the instantaneous var power q L at a
[
t 0 +T
∫q
wS
AC -
wA
(t )dt =
∫ (I U q
s
sin(ωt ) cos(ωt )) dt = 0
wS (t ) = p L (t ) + p A (t ) = ( I pL + I pA ) sin(ωt )u s (t )
(5)
The output power of the compensating equipment is equal to the reactive load power, as is shown in equation (7).
w A (t ) = q L (t ) − p A (t ) = I qU s sin(ωt ) cos(ωt ) − I pAU s sin 2 (ωt ) (6) qL (t ) = I qU s sin(ωt ) cos(ωt )
Load iC
t0 +T
t0 +T
t0
t0
2
∫ p A (t )dt = ∫ (I pAU sM sin(ωt ))dt
AC / DC
Invertor
T I pAU sM 2 2 1 1 2 = C (U dc + ∆U dc ) − C U dc 2 2 1 = CU dc ∆U dc + C（ ∆U dc） 2 2 =
u dc Fig. 1. The single-phase equivalent power circuit.
The controlling purpose is that the mains current iL is in phase with the mains voltage us(t). And the power factor of the load is nearly to one. It compensates reactive power to the shunt load, and the source supply the active power to the load and the compensator. Assuming the mains voltage us (t) is represented as (1) [11]. (1) u s (t ) = U sM sin(ωt ) We choose the phase of the mains voltage as the reference. The system frequency is 50Hz. And the load current can be represented as equation (2). (2)
For the performance of the reactive power compensator, it is expected that iS (t ) = I p sin(ωt ) , which is a sine wave and in
(8)
From equation (6), we can see that ic is divided into two parts: active current and reactive current, just as ic (t ) = i L (t ) − i s (t ) = i pA (t ) + i q (t ) . Where iq (t ) is the reactive current, equal to the load reactive current. It is generated by the compensator and flows to the load. And i pA (t ) is the active current, equal to loss current of the compensator. It flows from the mains to the compensating equipment. According to the principles above, the control scheme can be described simply as figure 2.
phase with the mains voltage. As we know, the real power is generated by the multiple of fundamental active current and fundamental voltage, and other component of the current generates reactive power with fundamental voltage. The instantaneous load power is calculated as following:
wL (t ) = u s (t )iL (t ) = p L (t ) + q L (t ) = I pU s sin 2 (ωt ) + I qU s sin(ωt ) cos(ωt )
(7)
At the same time, the active power injected from the mains also compensates the loss power of the IGBT. The integration of the loss power in a period is as following:
wL
i L (t ) = I L sin(ωt + θ ) = I p sin(ωt ) + I q cos(ωt )
(4)
t0
The output power supplied from the mains is the addition of active load power and loss power of the converter:
iL
+
L
= I pLU s sin 2 (ωt ) + I pAU s sin 2 (ωt )
A. Conventional operation principle The single-phase equivalent compensating power circuit is shown in Fig.1. For a specific load, the other components of the system are regard as a whole. In this system, fundamental reactive current component of load current is very high, and so the power factor is low.
us
t0 +T
t0
III. THE DESIGN OF CONTROL STRATEGY
is
]
random period t 0 , t 0 + T is zero. It is shown as equation(4).
active current
i + pA
+
+
iq
ic i
* c
+
-
+
PWM output
hysteretic control
reactive current Fig. 2. The structure of control strategy.
(3)
The whole control strategy can be divided into three parts: active current calculation, load reactive current calculation and hysteretic current feedback-loop control.
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DRPT2008 6-9 April 2008 Nanjing China *
The crisis is to get the target current ic , which is directly compared with the practical output current of the compensator and detected by the sensor. The compared result is fed to the hysteretic current loop control, to generate the switching signals of the power converter. B. Calculation of loss current The average voltage of the DC capacitor can supply the real power flow information, and the control of DC capacitor voltage is related to the flow of real power. We can get the formula that calculates the equivalent loss current amplitude as equation (9).
I pA
C（ ∆U dc） 2 T U sM = CU dc ∆U dc + 2 2 2U C C (∆U dc )2 (9) = dc ∆U dc + TU sM TU sM
From equation (9), the amplitude of loss current can be presented by DC capacitor voltage variation. When the system in stable state, the factors, such as C, U sM , T are definite. So the loss current can be computed by equation (9) and the control is related directly to DC voltage stability. The magnitude of loss current is accomplished by PI regulation of the practical DC voltage. And the phase of loss current is the same to that of mains voltage.
and in a 90 phase delay with the mains voltage u s' (t ) . Replaced by u ' (t ) , then simpler result can be presented as I q = 2 q , without calculating the amplitude of the mains voltage. The signal of the practical load current is fed to the controller, to supply the amplitude information of the reactive current. And the phase of reactive current is 90 degree phase delay with mains voltage. The product of Iq and cosθ is the desired reactive current. D. Hysteretic current loop control The current loop control is completed through hysteretic current loop. The input is the difference between the control signal and practical output current of the compensator and the output is the switching signals of IGBT. E. Control block diagram Fig. 3 shows the configuration and control block diagram of the proposed algorithm. is
AC/ DC Invertor
t0 +T
1 T
t 0 +T
=
' s
L
(t )dt
t0
∫U
sM
cos(ωt ){I p sin(ωt ) + I q cos(ωt )}dt
sM
cos(ωt ) I q cos(ωt )dt
t0
1 = U sM I q 2 1 = U sM I L sin θ 2
-
∆Udc
Iq
sampler
iq +
ipA +
trigger
Udc
×2
cos sin
Ur ef
period average
PI controller
i c*
IpA +
driver
PWM output
hysteretic current feedback -loop
perio d average
Fig. 3. Configuration and control block diagram of proposed algorithm.
The detected DC bus voltage is compared with a setting voltage. The compared result is sampled at the crossover point of us(t) and then fed to a PI controller to generate the desired amplitude of the active current. The output of the P-I controller and the reference sine wave are then fed to a multiplier to generate the desired active current i pA (t ) . The load current is multiplied with a voltage that is unity amplitude and in phase with the mains voltage. The result is also sampled at the crossover point of us(t) and then averaged in a period. Two times of the average of the result is the amplitude of the reactive current Iq. Then the desired reactive current iq(t) is the multiplication of the Iq and the cosine wave
t0
∫U
sampler
Udc crossover piont detection
trigger
+
1 = T
∫ u (t )i
θ
+
t 0 +T
iL PLL
+
1 q= T
Load
us
+
5ms, we can get u (t ) = U sM cos(ωt ) , then it multiples with the load current to extract the fundamental reactive current component. Periodic average reactive current is calculated as following. At a period as [t0 , t0+T], we can calculate q as equation (10).
iL
+
iC
C. Calculation of reactive current A new reactive current calculating method is presented. It gets the fundamental reactive load current directly based on the orthogonality of the trigonometric system with high accuracy. Sampling the mains voltage u s (t ) with a delay of ' s
us
po wer system
* u ' (t ) = cos(ωt ) . ic is the addition of i pA (t ) and iq(t). The
(10)
Then the amplitude of load reactive current is calculated as I q = I L sin θ = 2q / U sM . Actually, after PLL and delaying controller, the result is u ' (t ) = cos(ωt ) , with unity amplitude
desired output current and the detected practical output current are fed to the hysteretic current loop to generate the switching signals of the power converter. Through the courses above, the reactive component of load current is compensated by the compensator and the mains current is in phase with the mains voltage. It can be found that four signals, the mains voltage, the DC bus voltage the load current, and the output current of the compensator, are detected in the proposed system. The signal of the mains voltage is used to generate a reference sine wave,
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DRPT2008 6-9 April 2008 Nanjing China
with unity amplitude and in phase with the mains voltage. The DC bus voltage is used to supply the information of the power balance. The load current is used to supply amplitude information of the reactive current. And the signal of the practical output current is fed to as a comparison. F. The control of DC side The variance of DC capacitor voltage can be induced by the flow of active power and reactive power between AC and DC sides of the converter [12]. As the analysis above, the DC capacitor voltage would decrease gradually because of the loss current. The DC bus voltage is used to supply the information of the power balance. If power flow is imbalance in short time, such as the transient course caused by load changing, the DC capacitor can be regarded as another power source and must supply the power difference between the source and the load. However, it will result in the voltage fluctuation of the DC capacitor. Controlled by the PI regulation circuit, the stability of the system can be improved. Fig.3 shows that the regulation of DC voltage formulates a negative feedback loop. When the average capacitor voltage reduces, ipA(t) will be decrease and real power flows from source the DC side. And vice versa. Through this way, the DC voltage can be maintained at an invariant value. In stable state, the fluctuation of DC capacitor is determined by the exchange of reactive power and the capacity of the DC capacitor. The capacity is also critical to the performance and the experiment is performed on this system. When the filter inductor is L=3.75mH and the capacity C changes, the fluctuation of DC capacitor voltage in the same load situation is shown in figure 4.
A(%)
In the stable state, the DC capacitor voltage is shown as figure 6. And the setting voltage is 380V.
Fig. 6. The voltage of DC capacitor.
When more reactive load wok at 0.1s after the stable running, the variation of DC capacitor voltage is shown in figure 7. We can see that when the load changes, the output current of the equipment will increase and the loss is bigger than before. So the stable average DC voltage decreases correspondingly and the fluctuation increases.
1.4
Fig. 7. The variation of DC capacitor voltage when the load changes.
1.2
From fig.6 and fig.7, we know that the DC voltage fluctuates around a constant value and the amplitude value of fluctuation is related to reactive power flow.
1 0.8 0.6 0.4 0.2 0 0
5000
10000 15000 20000 25000 30000 35000 C(μF) Fig. 4. The fluctuation of capacitor voltage as the capacity C changes.
From Fig.4, we can get that the fluctuation of capacitor voltage decreases as the capacity is increasing. When C>13000µF, the fluctuation can be within 1% in this proposed system. IV.
Fig. 5. System voltage and current compensated at 0.08s.
NUMERICAL SIMULATION AND EXPERIMENTAL RESULTS
A. Simulation results Simulation results are shown as figure 5 when the compensating equipment works at 0.08s. Relevant parameters are L=3.75mH and C=20000µF. From figure 5, we can see that the mains current is 60 degree in advance of mains voltage before 0.08s. There is a small disturbance when it switched and then the mains current is in phase with main voltage. The dynamic course will be shortened if the compensator is connected at the zero point of system voltage.
B. The experimental waveforms A prototype is also developed to demonstrate the performance of this proposed system. Figure 8 shows the test results of the model. The upper is the mains voltage waveform and the latter is mains current waveform. The horizontal axis unit is second. The vertical axis is the value with different amplification coefficients. In this experiment, the rated mains voltage is 220V and the rated current is 50A. The test results show that the proposed control algorithm has the expected performance. The experiment course includes that charging for the DC capacitor, switching the compensator, working in stable state, and disconnecting the compensator, as shown in figure 8(a) to 8(d). The whole working course can be seen from figure8 (a). It is obvious that the system current when the compensator is working is smaller than that before it is switched.
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DRPT2008 6-9 April 2008 Nanjing China
Fig.(f) Spectrum of mains current in figure 8(b). Fig. 8. Experiment results. Fig.(a) The whole working course.
It can be detected from the real testing waveforms that mains voltage precedes 0.003s than mains current. The electric degree is as following:
ϕ=
Fig.(b) Waveforms of system voltage and current in stable state.
0.003 × 360° = 54° 0.02
cosϕ =0.588 As it is shown in figure 8(b), system current is synchronous to system voltage. Through practical detection, the power factor is cosϕ =0.992, and the compensating results are as expected. From figure8(f), we can see that the spectrum amplitude of mains current at 50Hz is much higher than others. The experiment results prove the validity of the design scheme proposed in this article. V. CONCLUSION
Fig.(c) The output current of the compensator in stable state.
Fig.(d) Waveforms of system voltage and system current when the compensator is switched.
From the design and the simulation and experimental results above, it can be found that the proposed control strategy has several characteristics as following: i. Choose the practical output current of the equipment as the comparing current, which is fed to the current-mode controller to trace the desired output current directly. The mains current is not used. ii. Without the complicated detection of active and reactive current component, the proposed control circuit is simpler, and the implementation cost is cheaper than the conventional method. iii. When the power factor is very low, the compensation performance is fine, no matter what fundamental reactive component is. iv. The dynamic test of the prototype developed with the proposed method in this paper has demonstrated that the control algorithm has the expected performance, proving the validity of the design scheme proposed in this article. Successfully developing dynamic var compensator prototype provides great technical support for the industrialization of dynamic var compensator. VI. ACKNOWLEDGMENT The authors gratefully acknowledge the contributions of Junpeng Zhuang, Wei He, Wei Dai and Guanlan Shu, the engineers in Shanghai Feiping Electronic Technology Co. Ltd., for their work on the accomplishment of this prototype.
Fig.(e) Waveforms of system voltage and system current when the compensator is disconnected.
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VII. REFERENCES
VIII. BIOGRAPHIES
Periodicals: [1] [2] [3]
[4] [5]
[6] [7] [8] [9] [10] [11]
[12]
HAO Jiang-tao, LIU Nian, XING Jin-yu. "Study on interhamonic of power system," Electric Power Automation Equipment, 2004, 24(12): 36-39. QIAN Zhao-ming, YE Zhong-ming, DONG Bo-fan. "Harmonics suppression techniques", Automation of Electric Power Systems, 2000, 24(2):66-70. Y. Nishida, O. Miyashita, T. Haneyoshi, H. Tomita, and A. Maeda, “A predictive instantaneous-current PWM controlled rectifier with AC-side harmoinc current reduction,” IEEE Trans. Ind. Electron., vol. 44, pp. 337–343, June 1997. W. L. A. Neves, H.W. Dommel, and W. Xu, “Practical distribution transformer models for harmonic studies,” IEEE Trans. Power Delivery, vol. 10, pp. 906–912, Apr. 1995. E Zhong-ming, DONG Bo-fan, QIAN Zhao-ming. "Comparison of tow approaches for harmonic current extraction", Automation of Electric Power Systems, 1997, 21(12):21-24. LIN Tianshun, HUANG Fumin, ZHANG Yao. "Three phase harmonic and reactive currents detecting based on single-phase adaptive detection", Electric Power Automation Equipment, 2004, 24(3):66-69. C.Y. Hsu, H.Y.Wu. "A new single-phase active power filter with reduced energy-storage capacity", IEE Proc-Electr. Power Appl., 1996, 143(1): 25-30. Qian Ting, Lu Zhengyu, Hu Jin. "Dual-loop scheme for unified constant-frequency integration control for active power filter", Proceedings of the CSEE, 2003, 23(3):34-37. J.-C.Wu, H.-L.Jou, "Simplified control method for the single-phase active power filter", IEE Proc.-Electr. Power Appl., Vol. 143, No. 3, May 1996 219: 219-223. WU Fei, XIE Da, ZHANG Yanchi. "The study of compensative characteristics of a novel active power filter", Electric Power Automation Equipment, 2007, 1, 27(1): 66-69. Xie Da, Zhang Yan-chi, Wu Fei, Shu Xiao-qiong. "A New-style Shunt Active Power Filter Based on DC Capacitor Voltage Control and Compensation Current Feedback Control", Power System Technology, Vol.30 No. 3, Feb.2006:18－21. Li Gengyin, Chen Zhiye, Yang Yihan, Ning Yu. "A Study of DC Voltage Control System for an Active Power Filter", Journal of North China Electric Power University, July 1997, 3th:1-7.
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Jinxia Gong was born in Hubei, China in 1984. She graduated from Yanshan University in 2006. Now she is a graduate student of Department of Electrical Engineering, Shanghai Jiaotong University in Shanghai, China. She mainly focuses her research on FACTS and power system simulation study.
Jingjing Lu was born in Hunan, China in 1985. She graduated from Shanghai Jiaotong University in 2007. Now she is a gradate student of Department of Electrical Engineering, Shanghai Jiaotong University in Shanghai, China. She mainly focuses her research on FACTS and power system simulation study.
Da Xie was born in Ha’erbin, Heilongjiang, China in 1969. He graduated from Shanghai Jiao Tong University in 1991, and in 1996 he received master degree in Ha’erbin Institute of Technology. He received Ph.D. degree in 1999, Shanghai Jiao Tong University. Now he is associate professor in Shanghai Jiao Tong University, EE department. He mainly focuses his research on FACTS and power system simulation.
Yanchi Zhang was born in Beijing, China in 1967. He graduated from Beijing University in 1989, and in 1997 he received master degree in Dalian Institute of Technology. His employment experience included the Xinjiang Institute of technology and Xinjiang University. He studied in Goldwind Science and Technology Company limited for wind power. His special fields of interest include wind power and its control.
