FACTSFlexible AC Transmission

Systems

ByA.Immanuel

April 19, 2023 1

UNIT-IV

Combined Compensators

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UNIFIED POWER FLOW CONTROLLER (UPFC)

The Unified Power Flow Controller (UPFC) concept was proposed by Gyugyi in 1991.

The UPFC is able to control, simultaneously or selectively, all the parameters affecting power flow in the transmission line (i.e., voltage, impedance, and phase angle), and this unique capability is signified by the adjective "unified” in its name.

It can independently control both the real and .reactive power flow in the line.

The control of real power is associated with similar change in reactive power, i.e., increased real power flow also resulted in increased reactive line power.April 19, 2023 3

UPFC has following features.Instantaneous speed of response

Extended functionality

Capability to control voltage, line impedance and phase angle in the

power system network

Enhanced power transfer capability

Ability to decrease generation cost

Ability to improve security and stability

Applicability for power flow control, loop flow control, load

sharing among parallel corridorsApril 19, 2023 4

UPFC is a generalized synchronous voltage source (SVS), represented at the fundamental (power system) frequency by voltage phasor Vpq with controllable magnitude Vpq(0≤Vpq Vpqmax) and angle ρ (0 ≤ρ ≤2π), in series with the transmission line, as illustrated for the usual elementary two machine system.

SVS generally exchanges both reactive and real power with the transmission system.

SVS is able to generate only the reactive power exchanged, the real power must be supplied to it, or absorbed from it, by a suitable power supply or sink.

In the UPFC arrangement the real power exchanged is provided by one of the end buses (e.g., the sending-end bus), as indicated in Figure 1.April 19, 2023 5

Basic Operating Principles

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Fig 1:Conceptual representation of the UPFC in a two-machine power system.

UPFC STRUCTUREThe general structure of UPFC contains back to back AC to DC

voltage source converters operated from a common DC link capacitor.

The general structure of UPFC is shown in Fig 2.

Fig 2: Circuit Diagram of Unified Power Flow Controller (UPFC)April 19, 2023 7

Fig 3: Implementation of the UPFC by two back-to-back voltage-sourced converters

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In the presently used practical implementation, the UPFC consists of two voltage-sourced converters using gate turn-off (GTO) Thyristor valves.

These converters, labeled “converter1” and “converter2”, are operated from a common dc link provided by a dc storage capacitor.

This arrangement functions as an ideal ac to ac power converter in which the real power can freely flow in either direction between the ac terminals of the two converters, and each converter can independently generate (or absorb) reactive power

at its own ac output terminal

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Converter 2 provides the main function of the UPFC by injecting a voltage Vpq

with controllable magnitude and phase angle ρ in series with the line via an

insertion transformer.

This injected voltage acts essentially as a synchronous ac voltage source. The

transmission line current flows through this voltage source resulting in reactive

and real power exchange between it and the ac system.

The reactive power exchanged at the ac terminal (i.e., at the terminal of the

series insertion transformer) is generated internally by the converter.

The real power exchanged at the terminal is converted into dc power which

appears at the dc link as a positive or negative real power demand.

The basic function of converter 1 is to supply or absorb the real power demanded by converter 2 at the common dc link.

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This dc link power is converted back to ac and coupled to the transmission line via a shunt-connected transformer.

Converter 1 can also generate or absorb controllable reactive power, if it is desired, and thereby provide independent shunt reactive compensation for the line.

It is important to note that, there is a closed path for the real power negotiated by the action of series voltage injection through Converters 1 and 2 back to the line, the corresponding reactive power exchanged is supplied or absorbed locally by Converter 2 and therefore does not have to be transmitted by the line.

Thus, Converter 1 can be operated at a unity power factor or be controlled to have a real power exchange with the line independ- ent of the reactive power exchanged by Converter 2. so, there can be no reactive power flow through the UPFC dc link.April 19, 2023 11

Conventional Transmission Control Capabilities

UPFC can fulfill the all functions of shunt and series compensa -tion to meet multiple control objectives by adding the injected voltage Vpq with appropriate amplitude and phase angle, to the (sending-end) terminal voltage Vs•

Using phasor representation, the basic UPFC power flow control functions are illustrated in Figure 4.

Voltage regulation with continuously variable in-phase/anti-phase voltage injection,is shown in Figure 4(a) for voltage increments Vpq = ±∆V (ρ = 0).

This is functionally similar to that obtainable with a transformer tap changer having infinitely small steps.April 19, 2023 12

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Fig 4: Phasor diagrams illustrating the conventional transmission control capabilities of the UPFC.

Series reactive compensation is shown in Figure 4(b) where

Vpq = Vq is injected in quadrature with the line current I.

Functionally this is similar to series capacitive and inductive line compensation attained by the SSSC.

The injected series compensating voltage can be kept constant, if desired, independent of line current variation, or can be varied in proportion with the line current to imitate the compensation obtained with a series capacitor or reactor.

Phase angle regulation (phase shift) is shown in Figure 4(c) where Vpq=Vσ’ is injected with an angular relationship with respect to Vs, that achieves the desired phase shift (advance or retard) without any change in magnitude.

The UPFC can function as a perfect Phase Angle Regulator which, can also supply the reactive power involved with the transmission angle control by internal var generation.April 19, 2023 14

Multifunction power flow control, executed by simultaneous terminal voltage regulation, series capacitive line compensation, and phase shifting, is shown in Fig4(d) where Vpq =∆V+Vq+Vσ. This functional capability is unique to the UPFC.

The transmitted power P and the reactive power - jQr supplied by the receiving end, can be expressed as follows.

...(1)

where symbol * means the conjugate of a complex number and

j = ejπ/2 = √-1. If Vpq=0, then eq(1) describes the uncompensated system, that is,

…(2)April 19, 2023 15

with Vpq≠ 0, the total real and reactive power can be written in the form

…(3)

substituting

…(4)

and

…(5)

the following expressions are obtained for P and Qr:

…(6)April 19, 2023 16

And

…(7)

where ….(8)

…(9)

are the real and reactive power characterizing the power transmission of the uncompensated system at a given angle δ.

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ρ is freely variable between 0 and 2π at any given transmission angle δ (0≤δ ≤π), it follows that Ppq(ρ) and Qpq(ρ) are controllable between - VVpq/X and + VVpq/X independent of angle δ. Therefore, the transmittable real power ρ is controllable between

…(10)

and the reactive power Qr is controllable between

…(11)

at any transmission angle δ.The wide range of control for the transmitted power that is

independent of the transmission angle δ, observable in the figure, indicates not only superior capability of the UPFC in power flow applications, but it also suggests powerful capacity for transient stability improvement and power oscillation damping.

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Fig 5: Range of transmittable real power P and receiving-end reactive power demand Q vs. transmission angle δ of a UPFC controlled transmission line

A phasor diagram, defining the relationship between V s,Vr,Vx (the voltage phasor across X) and the inserted voltage phasor Vpq, with controllable magnitude (0≤Vpq≤ Vpqmax) and angle (0≤ρpq≤ 360°), is shown in Figure 6(a).

the inserted voltage phasor Vpq is added to the fixed sending-end voltage phasor Vs to produce the effective sending-end voltage Vseff = Vs+ Vpq.

The difference,Vseff -Vr provides the compensated voltage phasor, Vx , across X.

As angle ρpq is varied over its full 3600 range, the end of phasor Vpq moves along a circle with its center located at the end of phasor V s.

The area within this circle, obtained with Vpqmax, defines the operating range of phasor Vpq and thereby the achievable compensation of the line.

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Fig 6: Phasor diagram representation of the UPFC (a) and variation of the receiving-end real and reactive power, and the real and reactive power supplied by the UPFC, with the angular rotation of the injected voltage phasor (b).

The rotation of phasor Vpq with angle ρpq modulates both the magnitude and angle of phasor Vx and, therefore, both the transmitted real power, P, and the reactive power, Qr vary with ρpq in a sinusoidal manner, as illustrated in Fig 6(b).

This process, of course, requires the voltage source (Vpq) to supply and absorb both reactive and real power, Qpq and Ppq , which are also sinusoidal functions of angle ρpq, as shown in the figure.

the UPFC simply controls the magnitude and angular position of the injected voltage in real time so as to maintain or vary the real and reactive power flow in the line to satisfy load demand and system operating conditions.April 19, 2023 22

Independent Real and Reactive Power Flow Control

To investigate the capability of the UPFC to control real and reactive power flow in the transmission line, refer to Figure 6(a).

Let it first be assumed that the injected compensating voltage, Vpq is zero. Then the original elementary two machine (or two-bus ac intertie) system with sending-end voltage Vsreceiving-end voltage Vr transmission angle δ, and line impedance X is restored.

The normalized transmitted power, Po(δ) = {V 2/X} sin δ = sin δ, and the normalized reactive power, Qo(δ) = Qos(δ) = -Qor(δ) = {V2/X }{1 - cos δ} = 1-cos δ, supplied at the ends of the line, are shown plotted against angle (δ) in Figure 7(a)

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The relationship between real power Po(δ) and reactive power Qo,(δ) can readily be expressed with V2/X = 1 in the following form:

…(12)

or

…(13)Equation (13) describes a circle with a radius of 1.0 around the

center defined by coordinates P = 0 and Qr = -1 in a {Qr, P} plane, as illustrated for positive values of P in Figure 7(b).

Each point of this circle gives the corresponding Po and Qor values of the uncompensated system at a specific transmission angle δ.

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Fig 7:Transmittable real power Poand receiving-end reactive power demand Qorvs. transmission angle δ of a two-machine system (a) and the corresponding Qor vs. Po loci (b).

For example,at δ = 0, Po=0 and QOr=0; at δ=30°, Po=0.5 and QOr = -0.134; at δ=90°,Po=1.0 and QOr=-1.0; etc

Refer again to Figure 6(a) and assume now that Vpq ≠ O. It follows from eq(2),or (6) and (7), and from Figure 6(b), that the real and reactive power change from their uncompensated values, Po(δ) and QOr(δ), as functions of the magnitude Vpq and angle ρ of the injected voltage phasor Vpq.

Since angle ρ is an unrestricted variable (0 ≤ρ ≤ 2π), the boundary of the attainable control region for P(δ, ρ) and Qr(δ, ρ) is obtained from a complete rotation of phasor Vpq with its maximum magnitude Vpqmax •

It follows from the above equations that this control region is a circle with a center defined by coordinates Po(δ) and Qor(δ) and a radius of VrVpq/X. With Vs = Vr = V, the boundary circle can be described by the following equation:

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….(14)

The circular control regions defined by (14) are shown in Figures 8(a) through (d) for V = 1.0, Vpqmax = 0.5, and X = 1.0 (per unit or p.u. values) with their centers on the circular arc characteriz -ing the uncompensated system (13) at transmission angles δ= 0°, 30°, 60°, and 90°.

Consider first Figure 8(a), which illustrates the case when the transmission angle is zero (δ = 0). With Vpq = 0, P, Qr (and Qr) are all zero, i.e., the system is at standstill at the origin of the Qr, P coordinates.

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Fig 8:Control region of the attainable real power P and receiving-end reactive power demand Q, with a UPFC-controlled transmission line at δ = 0° (a), δ = 300 (b), δ = 60° (c), and δ = 90° (d).

The circle around the origin of the{Qr, P} plane is the loci of the corresponding Q, and P values, obtained as the voltage phasor Vpq is rotated a full revolution (0 ≤ ρ ≤ 360°) with its maximum magnitude Vpqmax.

The area within this circle defines all P and Q, values obtainable by controlling the magnitude Vpq and angle ρ of phasor Vpq.

In other words, the circle in the {Qr, P}plane defines all P and Q, values attainable with the UPFC of a given rating.

It can be observed, for example, that the UPFC with the stipulated voltage rating of 0.5 p.u.is able to establish 0.5 p.u. power flow, in either direction, without imposing any reactive power demand on either the sending-end or the receiving-end generator.

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Figures 8(a) through (d) clearly demonstrate that the UPFC, with its unique capability to control independently the real and reactive power flow at any transmission angle, provides a powerful, hitherto unattainable, new tool for transmission system control.

Control StructureThe superior operating characteristics of the UPFC are due to its

unique ability to inject an ac compensating voltage vector with arbitrary magnitude and angle in series with the line upon command, subject only to equipment rating limits.

With suitable electronic controls, the UPFC can cause the series-injected voltage vector to vary rapidly and continuously in magnitude and/or angle as desired.

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It is not only able to establish an operating point within a wide range of possible P, Q conditions on the line, but also has the inherent capability to transition rapidly from one such achievable operating point to any other.

April 19, 2023 31Fig 9: Basic UPFC control scheme

April 19, 2023 32Fig 10: Overall UPFC control structure

Different operating modes of UPFC Active and reactive power flow control Power flow control by voltage shifting General Direct Voltage Injection Direct Voltage Injection with Vse in phase with Vi

Direct Voltage Injection with Vse in Quadrature with Vi

Voltage Regulation with Vse in phase with Vi

Phase Shifting Regulation Line Impedance Compensation

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There fore the series converter has following control modes

Direct Voltage control mode .Line impedance emulation mode.Phase angle Shift emulation mode.Power flow control mode

Shunt converter has following control modesReactive power control mode.Voltage control mode.

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Fig7 : Equivalent circuit of UPFC April 19, 2023 35

Various Applications of UPFC

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