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Research on dynamic characteristics of Unified

Power Flow Controller (UPFC)

Yao Shu-jun,Song Xiao-yan, Wang Yan, Yan Yu-xin, Yan Zhi

School of Electrical and Electronic Engineering North China Electric Power University

Beijing, China

 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. Using the UPFC simulation model established in

SIMULINK, a dynamic simulation tool in MATLAB, take a

simple power system with UPFC as an example. the simulation

test has been conducted on a simple system composed of 

synchronous generator and infinite capacity bus, the steady state

and transient characteristics of UPFC in this system areresearched. In the process of simulation, the control strategy of 

UPFC system is also 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 toconsider the dynamic situation of the DC capacitor when

establishment the mathematical of UPFC. The results from

simulation and experiments show that by means of UPFC the

power flow distribution among transmission lines can be give

back rapidly and reposefully. The transient experiments proved

that UPFC can improve the stability of power grid. Simulation

results also confirm that UPFC can restrain the oscillation of 

power angle and power flow.

 Keywords-Unified Power Flow Controller (UPFC); dynamic

characteristics; MATLAB simulation;

I.  I NTRODUCTION

With the rapid development of power system, how toimprove its operational flexibility, controllability and stabilityis becoming an urgent problem in today's society, theemergence of Flexible AC Transmission System (FACTS)

 provides a new way to this, many of which devices have been put into used. They all play an important role in the power 

system, for example, TCSC，SVC，STATCOM. As the mostrepresentative member of the FACTS family, The UnifiedPower Flow Controller (UPFC) has more control variables,Compared with the other FACTS devices, it can change avariety of system parameters during operation, make the

system running more flexible, Therefore, it becoming more andmore valued.

Simulation of UPFC at this stage is essentially based on theideal model of dual-controlled power, but this ideal modelneglected the dynamic process caused by switching converters,so it cannot fully reflect the operating characteristics of UPFC

[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 remainsconstant, ignoring the dynamic process of the device itself,which does not match the actual operation of the system.

In order to make the research more objectively andcomprehensively, this paper provides a detail mode of UPFCconsidering the charging dynamics of its DC link capacitor for simulation, then simulated on a simple power system withUPFC basing on this detailed dynamic model, and thesimulation results verified that the UPFC device can improve

 power system transient stability properly and effectively.

II.  THE DYNAMIC MODEL AND CONTROL MODEL OF UPFC

 A.  Structure of UPFC 

UPFC is one of the typical FACTS devices which can provide simultaneous control of all basic parameters of power system (transmission voltage, line impedance and phase angle)and proceed dynamic compensation to the power system. TheUPFC can fulfill the functions of STATCOM, SSSC and phaseshifter, and content multiple control objectives.

UDC

 

Fig.2.1. General configuration of UPFC

General structure of UPFC like this, it contains two “back to back” voltage source converters which is connected by acommon DC link (Fig.2.1). Converter 1 is connected parallelwith the transmission line and converter 2 as series with theline. The main role of shunt converter is providing active

 power to the series converter through the DC link capacitor.The main function of the UPFC is achieved through the seriesconverter. The series part acted as a voltage source, realize the

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voltage phase shift regulation and series compensation byinjecting an voltage with controllable magnitude and phaseangle, then control the active and reactive power of the line.

 B.  The dynamic model of UPFC 

The dynamic model of UPFC is shown in Fig.2.2. WhereLSH, R SH and LSE, R SE 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 arethe ratio of transformers of parallel side and serial side,separately

[4]. 

Fig.2.2 the dynamic model of UPFC

In general, the system include UPFC can be equivalent asfollows:

θ s∠θ SE∠

θ SH∠

 

Fig.2.3 Equivalent circuit of UPFC connected between node 1and node 2

For simplicity, we often ignore the losses of the deviceitself in the following discussion, only taking the equivalentreactance of the shunt part X and the series part X intoaccount, 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 invalue, that satisfied the expression PSH-PSE= 0. However, in adynamic 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 notnegligible, so the dynamic equation of the DC link can beexpressed as

[5]:

VI CV V PSH PSE (2-1) PSH ReVSH L

PSH R e (VSE) L (2-2) 

Form the equation (2-3), we can see that: during thedynamic adjustment process of the converter, with the input

active power of AC side changing on both sides of theconverter, the power imbalance on both sides appeared, makingthe DC capacitor charging and discharging, then the DCvoltage will change. That is to say, both converters areassociated with the part of DC link. Therefore, it is verynecessary to consider the dynamic situation of the DCcapacitor when establishment the mathematical of UPFC

[6].

Consider that the pulse width modulation controltechniques (PWM) is used in the two inverter of UPFC, and theinput amplitude coefficient and firing angel of the shunt andseries inverter were represented by m, φ, m, φ.then we canobtain the fundamental output voltage of the two inverter asfollows:

VSH NVSE N(2-3)

Since the phase angle of  V and V are replacement byθ, θ respectively, and they are determined by the firingangle

φ

, φof the two converters and the head voltage

V and

 phase angle

θof the line installed UPFC. so we can get the

following formula.

θSH θ φθSE θ φ (2-4)According to the equation (2-1) ~ (2-4), the dynamic model

of UPFC can be obtained. Equation (2-1) represents the processes of the capacitor charging and discharging, andequations (2-2) and (2-3) are the effects of the inverter and thetransformer s.

III.  THE EFFECT OF UPFC ON SYSTEM POWER  

UPFC can achieve the target of control the active andreactive 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.

Fig. 3.1 two-machine system with UPFC (1) equivalent circuit (2) vector 

relations

From the Fig.3.1, we can see that: the ending power of theline can be expressed as:

P j Q U        (3-1)

Suppose if the UPFC is installed at the end of transmissionline, according to the vector relations, we can get the active andreactive power equations as follows:

P sin δ sin(δ ρ) (3-2)

Q (cos δ 1) cos(δ ρ)  (3-3)

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It can be seen from the above equation that when ρ = 90°-δ,transmission line which the UPFC is installed can obtain thegreatest power, that is to say, at this point USE has the greatestimpact on power flow of the line. make a appropriatetransformation of equation (3-2) and (3-3),we can obtain thereactive power and active power equation as follows:

P

sinδ

Q

(cosδ

1)

 (3-4)

take different values of  δ, the reactive power and active power curve that on the terminal of the transmission line isshown in Fig.3.2

°=0δ  

°=45δ  

°= 90δ  

°=0δ  

°= 45δ  

°=90δ  

 

Fig.3.2. Reactive power and active power curve when δ is different

From the above we can see that UPFC devices can expandsthe operating range of the transmission system greatly.especially when δ 90°, transmission system has reached thelimit point of stable operation if there is no compensation of UPFC devices, Operation range of the system is far beyond theoriginal range, and the system can still running stability after 

the UPFC device inputted in the system. It is important tooptimal operation for the system, improve the stability limit of the system and improve system stability margin if appropriatenumber of UPFC devices are installed in a system.

IV.  CONTROLSTRATEGY OF UPFC

UPFC’s control section includes shunt part control andseries part control. For each part, of which the parallelconverter controller’s target is to govern the node voltage andDC link voltage, to achieve the node voltage stability controland keep the power balance of UPFC, the series converter controller can implement the trend control/series voltagecompensation and series resistance compensation. we havedifferent control model

[7-9].

 A.  Shunt part 

Double-loop decoupling voltage regulation control mode isused in the shunt side of UPFC. the shunt side of UPFC, notonly offered the active power which is needed by the seriesside, 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 iscorresponding to the changes of the node voltage U . Thecontrol principle of the shunt side is shown in Figure 3.2,

where U is the ration of transformer of the shunt side,U , U , Q  are reference values of the DC link voltage,access point voltage and access point reactive, respectively.

Fig. 4.1 voltage regulation control strategy of the shunt side 

 B.  Series part 

Headings, or heads, are organizational devices that guidethe reader through your paper. There are two types: componentheads and text heads

Cross-coupling control mode is used in the series side of 

UPFC, as a representative of the direct control model of the

UPFC, cross-coupling control model adopted the three-loopcontrol method, that is, the power loop is outer ring ,the voltage

loop and the current loop is inner ring, this three aspect cross

coupled, and using the line power as control object directly,

then obtained the series side offset voltage U by inputting the

error of line power into PI regulator. The control principleshown in Figure 3.3:

Fig. 4.2 cross-coupling control mode of the series side

V.  . SIMULATION

Power flow control and voltage control can set and change

active and reactive power distribution and voltage levels on the

installation point of UPFC by controlling and changing the

reference value of UPFC. Obviously, the access of UPFCcontrol can change the operating conditions of power system;

this will definitely affect the dynamic power system operation

and control features[10]

.

For the purpose of testing UPFC’s regulating efficiency on

the active and reactive power of line and ability on keeping

voltage stability, now, use the dynamic model and control

strategy, taking MATLAB as the simulation environment, to

implement the dynamic simulation on the system with UPFC.

Simulated on the following simple network, shown in Fig. 5.1:

Fig.5.1 simple network 

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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 theinfinite system by the double-circuit transmission line and a

transformer, the UPFC system installed between the bus 1 and

 bus 2. suppose that three-phase short circuit fault occurs on one

of the double-circuit transmission line that no UPFC device

installed in. doing transient simulation on this system, and the

simulation time is 1.5s, during 0.1s to 0.15s, three-phase shortcircuit fault occurs on one of the double lines, at 0.15s the fault

is removed, then maintain the single-loop operation. Pre-fault

system is in normal operation, and the reference as

follows: P =4.95pu，Q =0.54pu，V =1.0pu.

Fig.5.2 active power of the transmission line when UPFC out of service andon service

Fig 5.3 the phasor angel different between bus 2 and 3 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 joined in the system, due to the power control

of UPFC, system working in single circuit when the fault is

removed, active power of single-circuit lines can still tracing

the reference value, while the active power is lower than

reference value when UPFC is out of working, Fig.5.2 also

descript that the dynamic feature of system is better than thestatus of UPFC out of service. the system power can restore

stability after 0.15s running when the fault removed, active and

reactive power of the system still shocking seriously in a long

time after the fault removed when UPFC does not work. And at

this time the oscillation range of power is greater than the case

of UPFC working. So that they all directed that UPFC has avery good improvement on system transient stability, it can

enhance the power flow of transmission line and improve the

stability of system power.

When the system disturbed, the voltage of access pointdrops sharply, Fig.5.4 reveals that, when 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 fast than UPFC out of services. that is to say, at this timethe parallel side of UPFC working as a compensator tomaintain the system voltage constant and improve system’sstability.

Fig.5.4 input voltage and reference when UPFC on service and out of service

VI.  CONCLUSION 

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 flowline oscillation.

Dynamic simulation system described in this article is a

simplified power system components, which is composed by

generator and infinite system, and the network is not

complicated, the flow adjustment is easy to implement, but it isstill worthy of further discussion on how to control the flow on

complex multi-machine systems .

R EFERENCES 

[1]  Zheng Sanbao, Cheng Shijie. Dynamic Simulation of UPFC.Automation of Electric Power Systems.2000.8(10)

[2]  Eskandar Gholipour, Shahrokh Saadate. Improving of Transient Stabilityof Power Systems Using UPFC. IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

[3]  R. Natesan, G. Radman. Effects of STATCOM, SSSC and UPFC onVoltage Stability. 2004 IEEE

[4]  S.H. Hosseini, A. Ajami. Voltage Regulation and Transient StabilityEnhancement of a Radial AC Transmission System Using UPFC. The30th Annual Conference of the IEEE Industrial Electronics Society, November 2 - 6,2004, Busan, Korea

[5]  Zhang Liangdong, CenWenhui, Liu We. Modeland Control of UPFC.Automation of Electric Power Systems.1998,22(1)

[6]  Yan Wei, Zhu Jizhong, Sun Hongbo, Xu Guoyu. Study On Mode lAndControler Of UPFC. Automation of Electric Power Systems. 1999,6(23)

[7]  Cai Song,Duan Shan-xu,Kang Yong. Application of UPFC in DynamicSimulation System. Power System Technology.2007,5,9(31)

[8]  Huang Zhenyu, Ni Yixin, Chen Shousun. Realization of UPFC DynamicModel in Dynamic.Analysis of Power System . Automation of ElectricPower Systems.1999, 6(23)

[9]  Haoming Liu, Haojun Zhu, Yang Li, Yixin Ni. Including UPFCDynamic Phasor Model intoTransient Stability Program. Power Engineering Society General Meeting, 2005. IEEE, 302 - 307 Vol. 1

[10]  Cuong Vu The, Khanh La Minh, Tuan Tran Quoc, Nguyen Boi Khue,Lam Du Son. FACTS Devices Applications on Power System toImprove the Angle Stability. IEEE Asia Pacific Conference on Circuitsand Systems, 2006. APCCAS 2006. 1358 - 1363

[11]  Xiao-Ping Zhang and Keith R Godfrey. Advanced Unified Power FlowController Model for Power System Steady State Control. IEEEInternational Conference on Electric Utility Deregulation andRestructuring and Power Technologies(DRPT 2004),Hong Kong, 2004:(1): 228-233

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