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Chapter-5

MODELING OF UNIFIED POWER FLOW CONTROLLER

5.1 Introduction

There are a number of FACTS devices that control power system

parameters to utilize the existing power system and also to enhance

the dynamic performance and stability of the power system. Out of

these, UPFC is the most flexible multi functional FACTS device. UPFC

perform the functions of a shunt reactive current injection to control

bus voltage and inject series reactive voltage to control power flow in

transmission line [23][30][33]. If PI Controllers equipped by the UPFC

shunt and series controllers are slow or if PI controllers are not

properly tuned or if the UPFC operates manually, the UPFC is not in a

position to effectively damp the power system oscillations [28]. To

achieve this, power oscillation damping control stability loop or

auxiliary controller is added along with power flow controller [23].

5.2 Unified Power Flow Controller (UPFC)

5.2.1 Configuration

The basic concept diagram of UPFC is shown in Fig.5.1. It

contains two back to back AC to DC synchronous voltage sourced

converters (VSC1 and VSC2) operated with common DC link capacitor

[23] [28] [29]. VSC1 is connected in shunt through shunt-connected

transformer and VSC2 is connected in series through series connected

transformer. The shunt branch of UPFC comprised of a DC Capacitor,

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VSC1 and a shunt-connected transformer corresponds to a

STATCOM. It can absorb or generate only reactive power because the

output current is in quadrature with the terminal voltage. The series

branch of UPFC is comprised of a DC Capacitor, VSC2 and a series

connected transformer corresponds to a SSSC. It can act as a voltage

source injected in series to the transmission line through series

connected transformer; the current flowing through the VSC2 is the

transmission line current (I) and it is function of the transmitted

electric power and the impedance of the line. The injected voltage (Vse)

is in quadrature with the transmission line current (I) with the

magnitude being controlled independently of the line current. Hence,

the two branches of the UPFC can absorb or generate the reactive

power independent of each other.

If the two converters (VSC1 and VSC2) are operating at the

same time, the shunt and series branches of the UPFC can basically

function as an ideal ac to ac converter in which the real power can

flow in either direction through the dc link and between the AC

terminals of the two converters. The real power from VSC1 to VSC2

and vice versa, and hence it is possible to introduce positive or

negative phase shifts between V1 and V2. The series injected voltage

Vse can have any phase shift with respect to the terminal voltage V1.

Therefore, the operating area of the UPFC becomes the circle limited

with a radius defined by the maximum magnitude of Vse, i.e.,

Vse.max.

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The VCS2 is used to generate the voltage Vse 0 ≤ Vse ≤ Vse.max

and phase shift 0 ≤ θ ≤ 2π at the fundamental frequency. This voltage

is added in series to the transmission line and directly to terminal

voltage V1 by the series connected coupling transformer. The

transmission line current passes through the series transformer, and

in the process exchanges real and reactive power with the VSC2. This

implies that the VSC2 has to be able to absorb and deliver both real

and reactive power.

The shunt-connected branch associated with VSC1 is used

primarily to provide the real power demanded by VSC2 through the

common DC link terminal. Also, it can generate or absorb reactive

power independently of the real power, it can be used to regulate the

terminal voltage V1; thus, VSC1 regulates the voltage at the input

terminals of the UPFC.

Another important role of the shunt branch of UPFC is a direct

control of the DC capacitor voltage, and consequently an indirect

regulation of the real power required by the series UPFC branch. The

amount of real power required by the series converter plus the circuit

losses have to be supplied by the shunt converter. Real power flow

from the series converter to shunt converter is possible and in some

cases desired, in this case, the series converter would supply the

required real power plus the losses to the shunt converter.

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Fig.5.1 The basic scheme of UPFC

5.2.2 UPFC transmission control capabilities

The power transmission with UPFC based on the reactive shunt

compensation, the series compensation and the phase angle

regulation. The UPFC can meet multiple control objectives by adding

the series injected voltage with appropriate magnitude and phase

angle to the terminal voltage V1. Using phasor representation, the

basic UPFC power flow control functions are illustrated in Fig.5.2.

Voltage regulation with continuously variable in phase / anti

phase voltage injection is shown in Fig.5.2 (a) for voltage

increments Vse = ±ΔV (σ =0).

Series reactive compensation is shown in Fig.5.2 (b), where Vse

= Vq is in quadrature with line current I. functionally this is

similar to series capacitive and inductive line compensation by

the SSSC.

Phase angle regulation is shown in Fig.5.2(c), where Vse = Vσ is

injected with an angular relationship with respect to Vs that

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archive the function as a perfect phase angle regulator, which

can also supply the reactive power involved with transmission

angle control by internal VAR generation.

Multifunction power flow control executed by simultaneous

terminal voltage regulation, series capacitive line compensation

and phase shifting is shown in Fig.5.2 (d) where Vse = ΔV + Vq +

Vσ. This functional capability is unique to the UPFC. No single

conventional equipment has similar multi functional capability.

Fig.5.2 Phasor representation of UPFC power flow control

functions

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5.2.3 UPFC control system

The general control scheme of UPFC [31] [33] is as shown in

Fig.5.3. The UPFC is a multi variable control device with four inputs

(magnitude and phase angle of the shunt and series converter output

voltages) and four outputs (real and reactive output powers of the

shunt and series converters). The series converter controls the active

and reactive powers flow through transmission line by adjusting the

magnitude and phase angle of the series injected voltage. The shunt

converter controls the dc voltage and the bus voltage (V1) at the shunt

converter transformer. In this thesis, the shunt converter is used to

control the sending-end bus voltage magnitude by locally generating

and absorbing reactive power. The series converter directly controls

real line power by the magnitude of the series injected voltage.

Fig.5.3 Basic control structure of UPFC

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5.2.3.1 Shunt converter controls

The shunt converter has two duties, namely, to control the

voltage magnitude at the sending-end bus (Bus V1 in Fig.5.1) by

locally generating or absorbing reactive power, and to supply or

absorb real power at the dc terminals as demanded by the series

converter. It is possible to achieve real power balance between the

series and shunt converter by directly controlling the dc voltage Vdc,

as any excess or deficit of real power will tend to increase or decrease

the dc voltage, respectively.

By varying the magnitude and angle of the shunt converter

output voltage the real and reactive power flow in and out of the shunt

converter is controlled [23][31]. The PI bus voltage regulator as shown

in Fig.5.4 (a) sets the reactive current reference and PI dc voltage

regulator sets real current reference as shown in Fig.5.4 (b) This

control scheme is basically the same as a STATCOM control.

The d–q decoupled current control strategy for shunt converter

[45] is implemented as shown in Fig.5.3.

The control system consists of:

A phase-locked loop (PLL): it is used to synchronize the Shunt

converter current with sending-end bus voltage (V1) at the

point of UPFC connection.

An AC voltage regulator (Bus-voltage regulator): it gives the

reference reactive current Iqref required by the system to

maintain bus voltage at constant value or in specified range.

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A DC voltage regulator: it gives the reference active current

Idref to maintain the capacitor voltage at a constant value or in

specified range.

The inner current regulator: it controls the magnitude and

phase of the voltage generated by the PWM converter of Shunt

converter to deliver or absorb required reactive current by the

Shunt converter as per reference valve given by the AC and Dc

voltage regulators.

Fig.5.4 Shunt converter current controller

The shunt converter controls the bus voltage by injecting

reactive current in quadrature with sending-end voltage V1. The

magnitude of the shunt voltage can be calculated by the following

equation

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Vsh = Vref + XS.I ---------------5.1

Where

Vsh = Positive sequence voltage (pu) of shunt converter

I = Reactive current (pu/Pnom)

XS = Slope (pu/Pnom: usually between 1% and 5%) or the leakage

reactance of shunt connected transformer and series reactance

connected between converter and power system

The voltage Vsh is controlled through the changes in the

amplitude modulation ratio msh, as the output voltage magnitude is

directly proportional to msh according to the following equation

Vsh = (1/2√2)*msh*Vdc -------------5.2

5.2.3.2 Series converter controls

Two different control schemes for the series converter were

implemented. One scheme to control real power flow through

transmission line and voltage magnitude at the receiving-end bus;

another control scheme for controlling the real and the reactive power

flows through the transmission line.

From the basic principle of UPFC, series converter does main

function of UPFC. The series converter active and reactive powers are

controlled by using two separate PI controllers, taking advantage of

the UPFC ability to independently control reactive and real power. The

basic principle of real power flow being directly affected by changes in

phase angles, while reactive power flow is directly associated with

voltage magnitudes, is used here to design the UPFC control.

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The outputs of the PI controllers are d and q components of the

series injected voltage Vse, i.e., Vsed and Vseq respectively. The

magnitude of the series voltage can be calculated by the following

equation

Vse-q = (Kp + Ki/S)*(Pref - P) ------------ (5.3)

Vse-d = (Kp + Ki/S)*(Qref - Q) ------------ (5.4)

Vse = √ (Vse-d2 + Vse-q

2) ------------ (5.5)

The amplitude modulation ratio

mse = √ (8*Vse/Vdc) ---------------------(5.6)

The phase angle of the series injected voltage with respect to the

reference waveform, i.e., the sending-end voltage V1 is given as

follows

β = -tan-1 (Vse-q/Vse-d)

Fig.5.5 Series converter injected voltage controller

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The series converter controls active power flow in line by

controlling the magnitude of the series injected voltage, injecting in

qudrature with the line current I.

5.3 SIMULINK Modeling of UPFC

The SIMULINK model of UPFC developed as a phasor model, to

perform dynamic and transient stability studies in 3-Ph power

systems. The series converter (VSC2) injected voltage (Vq) is controlled

to meet the power demand in the line set by the reference power set

point (Pref) and shunt converter (VSC1) delivers or absorbs the reactive

current as per the output of ac voltage regulator.

5.3.1 PI Voltage Controller of shunt converter

The SIMULINK model of PI voltage controller block diagram for

UPFC shunt converter is shown in Fig.5.6. This controller gives

appropriate shunt reactive current injected into the power system at

which UPFC located for appropriate change in bus voltage with

respect to the reference voltage.

Fig.5.6 PI Voltage Controller block diagram of UPFC shunt Converter

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5.3.2 FLPOD controller along with PI Voltage Controller of shunt converter

The SIMULINK model of FLPOD controller along with PI Voltage

Controller block diagram for Shunt Current Controller of UPFC is

shown in Fig.5.8. FLPOD shunt controller is fed by one input namely

change in power or difference in power (DP) of a constant resistive

load connected parallel to the shunt converter to the UPFC. This gives

the appropriate shunt current (Iq), which is required by the system

during transient period and it gives zero output for steady state.

The rules for the proposed FLPOD shunt controller are:

i) If „DP‟ is „DPN‟ (DP Negative) Then „Iq‟ is „IqN‟ (Iq Negative)

ii) If „DP‟ is „DPZ‟ (DP Zero) Then „Iq‟ is „IqZ‟ (Iq Zero)

iii) If „DP‟ is „DPP‟ (DP Positive) Then „Iq‟ is „IqP‟ (Iq Positive)

These rules are in matrix form as given below

error (DP)

Out put (IQ)

DPN IQN

DPZ IQZ

DPP IQP

The membership functions for input and output of FLPOD

shunt controller, Change in power or difference in power (DP) and

shunt injected current (Iq) are given in Fig.5.7.

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Fig.5.7 (a) Input membership function (DP) and (b) Output Membership function (Iq) of FLPOD shunt controller

Fig.5.8 FLPOD controller along with PI Voltage Controller block diagram of UPFC shunt converter

5.3.3 PI Power Flow Controller of series converter

The SIMULINK model for PI Power Flow controller of series

converter block diagram is shown in Fig.5.9. This controller gives

appropriate series injected voltage for appropriate change in line

power with respect to the reference power.

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Fig.5.9 PI power flow controller block diagram of UPFC series converter

5.3.4 FLPOD controller along with PI Power flow controller of series converter

The SIMULINK model for Series voltage controller of UPFC with

FLPOD controller along with PI power flow controller is shown in

Fig.5.11. FLPOD controller is fed by one input namely change in

power or difference in power (DP). This gives the appropriate series

injected voltage (Vq), which is required by the system during

transients and it gives zero output under steady state.

The rules for the proposed FLPOD series controller are:

i) If „DP‟ is „DPN‟ (DP Negative) Then „Vq‟ is „VqN‟ (Vq Negative)

ii) If „DP‟ is „DPZ‟ (DP Zero) Then „Vq‟ is „VqZ‟ (Vq Zero)

iii) If „DP‟ is „DPP‟ (DP Positive) Then „Vq‟ is „VqP‟ (Vq Positive)

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These rules are in matrix form as given below

error (DP)

Out put (Vq)

DPN VqN

DPZ VqZ

DPP VqP

The membership functions for input and output of FLPOD

controller, Change in power or difference in power (DP) and series

injected voltage (Vq) are given in Fig.5.10 (a and b)

Fig.5.10 (a) Input membership function (DP) and (b) Output

Membership function (Vq) of FLPOD series controller

Fig.5.11 FLPOD controller along with PI power flow controller block diagram of UPFC series converter

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5.4 Summary

In this chapter details of UPFC have been discussed. SIMULINK

implementation of the UPFC has been discussed. The UPFC with PI

and FLPOD controllers allows the controls of the amplitude of both

shunt reactive current and series injected reactive voltages.