Ijcot v5p314

International Journal of Computer & Organization Trends – Volume 5 Number 2 – February 2014

Control Strategy and Dynamic Characteristics of Unified Power Flow Controller (UPFC) Mr.B.Gopinath#1, Ms.M.Anusuya*2, Dr.S.Sureshkumar#3 #1

Assistant Professor, EEE, Vivekanandha Institute of Engineering and Technology for Women,Tiruchengide,India *2 PG Scholar, EEE, Vivekanandha Institute of Engineering and Technology for Women,Tiruchengide,India #3 Principal, IET Chairman, Vivekanandha College of Technology for Women,Tiruchengide,India

Abstract— Based on the basic principle of Unified Power Flow Controller circuit, give a simple analysis about the principle of power flow control of UPFC, and a detailed simulation model of UPFC considering the charging dynamics of its DC link capacitor is provided. In the process of simulation, the control strategy of UPFC system is discussed, its shunt side control the terminal voltage of the system and the firing angle of converter 1 the shunt part of UPFC, in order to keep the terminal bus voltage magnitude of UPFC and the DC capacitor voltage as constant, respectively. its series side control terminal voltage and firing angle of converter 2 the series part of UPFC , so as to keep the real power and reactive power of the line with UPFC device as constant or to act as a series compensator. Further analysis shows that all the active power of the series side is provided or absorbed by the DC capacitor presented among the two converters. The active power is provided by the shunt side convertor of the UPFC. that is to say, both converter is associated with the part of DC link ,therefore, it is very necessary to consider the dynamic situation of the DC capacitor when establishment the mathematical of UPFC. Simulation results also confirm that UPFC can restrain the oscillation of power angle and power flow.

switching converters, so it cannot fully reflect the operating characteristics of Unified Power Flow Controller [1]-[3]. Meanwhile, when doing the transient process simulation study on the system with UPFC, Generally, we supposing that the voltage of DC link capacitors remains constant, ignoring the dynamic process of the device itself, which does not match the actual operation of the system. This paper provides a detail mode of Unified Power Flow Controller considering the charging dynamics of its DC link capacitor for simulation, then simulated on a simple power system with UPFC basing on this detailed dynamic model, and the simulation results verified that the UPFC device can improve power system transient stability properly and effectively.

II. THE DYNAMIC AND CONTROL MODEL OF UPFC

Keywords— UPFC, Control Strategy, dynamic characteristics and DC link capacitor.

A. STRUCTURE OF UPFC

Unified Power Flow Controller is one of the typical Flexible AC Transmission devices which With the rapid development of power system, can provide simultaneous control of all basic how to improve its operational flexibility, parameters of power system (transmission voltage, controllability and stability is becoming an urgent line impedance and phase angle) and proceed problem in today's society, the emergence of dynamic compensation to the power system. The Flexible AC Transmission System provides a new Unified Power Flow Controller can fulfill the way to this, many of which devices have been put functions of STATCOM, SSSC and phase shifter, into used. The Unified Power Flow Controller has and content multiple control objectives. more control variables, Compared with the other FACTS devices, it can change a variety of system parameters during operation, make the system running more flexible, Therefore, it becoming more and more valued.

Simulation of Unified Power Flow Controller (UPFC) at this stage is essentially based on the ideal model of dual-controlled power, but this ideal Fig.1. General configuration of UPFC model neglected the dynamic process caused by I. INTRODUCTION

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International Journal of Computer & Organization Trends – Volume 5 Number 2 – February 2014 B. THE DYNAMIC MODEL OF UPFC

The dynamic model of UPFC is shown in Fig.2.

Where LSH, RSH and LSE, RSE represent the leakage inductances of transformers and losses of inverters transformers in shunt side and series side respectively.NSH1/NSH2=1 and NSE1/NSE2=1 are the ratio of transformers of parallel side and serial side, separately [4].

Consider that the pulse width modulation control techniques (PWM) is used in the two inverter of UPFC, and the input amplitude coefficient and firing angel of the shunt and series inverter were represented by m , φ , m , φ .then we can obtain the fundamental output voltage of the two inverter as follows:

(3)

Fig.2 the dynamic model of UPFC

For simplicity, we often ignore the losses of the device itself, only taking the equivalent reactance of the shunt part X and the series part X into account, in the Steady state, the active power of UPFC shunt part PSH which absorbed from the power system and the real power PSE that the series part send to the system is equal in value, that satisfied the expression PSH-PSE= 0. However, in a dynamic process, the DC storage capacitor of UPFC cannot remain unchanged, so PSH (t)≠PSE (t) . Thus when describing the dynamic characteristics of a power system with UPFC, The dynamic changing of the DC capacitor voltage is not negligible, so the dynamic equation of the DC link can be expressed as[5]: (1)

Since the phase angle of Vsh and Vse are replacement by θsh, θse respectively, and they are determined by the firing angle φ1, φ2 se of the two converters and the head voltage Vs and phase angle θs of the line installed UPFC. so we can get the following formula.

-

(4)

According to the equation (1) - (4), the dynamic model of UPFC can be obtained. Equation (1) represents the processes of the capacitor charging and discharging, and equations (2) and (3) are the effects of the inverter and the transformers.

III. THE EFFECT OF UPFC ON SYSTEM POWER

UPFC can achieve the target of control the active and reactive power on transmission line, and the active power PSE exchanged between the series part of UPFC and system must be provide by the parallel part of UPFC which can absorb power from the transmission line.

(2) Form the equation (3), we can see that: during the dynamic adjustment process of the converter, with the input active power of AC side changing on both sides of the converter, the power imbalance on both sides appeared, making the DC capacitor charging and discharging, then the DC voltage will change. That is to say, both converters are associated with the part of DC link [6].

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Fig. 3. two-machine system with UPFC (1) equivalent circuit (2) vector relations

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International Journal of Computer & Organization Trends – Volume 5 Number 2 – February 2014 From the Fig.4, we can see that: the ending power of the line can be expressed as: (4)

Suppose if the UPFC is installed at the end of transmission line, according to the vector relations, we can get the active and reactive power equations as follows: (6) (7) It can be seen from the above equation that when ρ = 90°-δ, transmission line which the UPFC is installed can obtain the greatest power, that is to say, at this point USE has the greatest impact on power flow of the line. make a appropriate transformation of equation (6) and (7),we can obtain the reactive power and active power equation as follows:

UPFC devices, Operation range of the system is far beyond the original range, and the system can still running stability after the UPFC device inputted in the system. It is important to optimal operation for the system, improve the stability limit of the system and improve system stability margin if appropriate number of UPFC devices are installed in a system.

IV. CONTROL STRATEGY OF UPFC

UPFC’s control section includes shunt part control and series part control. For each part, of which the parallel converter controller’s target is to govern the node voltage and DC link voltage, to achieve the node voltage stability control and keep the power balance of UPFC, the series converter controller can implement the trend control/series voltage compensation and series resistance compensation. we have different control model[7][9].

Fig. 5 voltage regulation control strategy of the shunt side

(8) Take different values of δ, the reactive power and active power curve that on the terminal of the transmission line is shown in Fig.4.

A. SHUNT PART

Double-loop decoupling voltage regulation control mode is used in the shunt side of UPFC. the shunt side of UPFC, not only offered the active power which is needed by the series side, maintained the voltage of the DC link stable, compensated for the power loss of the UPFC system, but also preserved the voltage of the UPFC bus, using the reactive power transferred from the transmission line which is corresponding to the changes of the node voltage U1. The control principle of the shunt side is shown in Figure 5.

B. SERIES PART

There are two types: component heads and text heads Cross-coupling control mode is used in the From the above we can see that UPFC devices series side of UPFC, as a representative of the can expands the operating range of the transmission direct control model of the UPFC, cross-coupling system greatly. especially when δ = 90°, control model adopted the three-loop control transmission system has reached the limit point of method, that is, the power loop is outer ring ,the stable operation if there is no compensation of voltage loop and the current loop is inner ring, this Fig.4. Reactive power and active power curve when δ is different

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International Journal of Computer & Organization Trends – Volume 5 Number 2 – February 2014 three aspect cross coupled, and using the line power as control object directly, then obtained the series side offset voltage U12 by inputting the error of line power into PI regulator. The control principle shown in Figure 6: Fig.8 active power of the transmission line when UPFC out of service and on service

Instantaneous short-circuit fault will cause the system power angle oscillation, then causing oscillation on power flow of the line, considering two cases that the UPFC device not installed in and Fig. 6 cross-coupling control mode of the series side joined in the system, due to the power control of UPFC, system working in single circuit when the V. SIMULATION fault is removed, active power of single-circuit lines Power flow control and voltage control can set can still tracing the reference value, while the active and change active and reactive power distribution power is lower than reference value when UPFC is and voltage levels on the installation point of UPFC out of working, Fig.8 also descript that the dynamic by controlling and changing the reference value of feature of system is better than the status of UPFC UPFC. Obviously, the access of UPFC control can out of service. The system power can restore change the operating conditions of power system; stability after 0.15s running when the fault this will definitely affect the dynamic power system removed, active and reactive power of the system operation and control features [10]. For the purpose still shocking seriously in a long time after the fault of testing UPFC’s regulating efficiency on the removed when UPFC does not work. And at this active and reactive power of line and ability on time the oscillation range of power is greater than keeping voltage stability, now, use the dynamic the case of UPFC working. So that they all directed model and control strategy, taking MATLAB as the that UPFC has a very good improvement on system simulation environment, to implement the dynamic transient stability, it can enhance the power flow of simulation on the system with UPFC. Simulated on transmission line and improve the stability of the following simple network, shown in Fig. 7: system power.

Fig. 7 simple network

The system is composed of a synchronous generator and a infinite system, in which the output of the generator is 1000MW, and the end point of UPFC is connected with the infinite system by the double-circuit transmission line and a transformer, the UPFC system installed between the bus 1 and bus 2.

Fig.9 the phasor angel different between bus 2 and 3 when UPFC out of service and on service

Fig.10 input voltage and reference when UPFC on service and out of service

When the system disturbed, the voltage of access point drops sharply, Fig.10 reveals that, when

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International Journal of Computer & Organization Trends – Volume 5 Number 2 – February 2014 UPFC is on service, the amplitude of the voltage drop smaller than the status of UPFC out of service, and it can also give back to the potential more faster than UPFC out of services. that is to say, at this time the parallel side of UPFC working as a compensator to maintain the system voltage constant and improve system’s stability.

VI. CONCLUSION

All The simulation results and experimental data indicated that: UPFC device in the dynamic simulation system, can adjust the distribution the system power flow among the transmission line quickly and smoothly, and have no significant impact to other operating parameters of the system.

At the same time, the UPFC can improve system’s stability, to keep down the shaking of the line power angle and inhibit the line power flow line oscillation. Dynamic simulation system described in this paper is a simplified power system components, which is composed by generator and infinite system, and the network is not complicated.

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