Facts Devices_solution Manual Electrical Engineering BTKIT Uttarakhand TEE 703

8/13/2019 Facts Devices_solution Manual Electrical Engineering BTKIT Uttarakhand TEE 703

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Author: Birendra Kumar Debta , Assistant Professor, Department of Electrical Engineering

SOLUTION MANUAL FACTS DEVICES TEE-703 7TH

Semester 2013-14

DEPARTMENT OF ELECTRICAL ENGINEERING

BIPIN TRIPATHI KUMAON INSTITUTE OF TECHNOLOGY

Dwarahat, Uttarakhand

Q.1.

a) Explain the objectives of FACTs controllers in the power system network ?

Answer:

i) Greater control of power so that it flows on the prescribed transmission

routes.

ii) Secured loading of transmission lines to levels nearer their thermal

limits.

iii) Greater ability to transfer between controlled areas.

iv) Prevention of cascading outages.

v) Damping of power system oscillation.

b) Describe the procedure to locate the FACTs Devices in an electrical network ?

Answer:

Facts devices are basically used for Power control, Voltage control, Stability

control.

i) Devices are modelled.ii) The calculation of available transfer capability involves three majorcomponents: Total Transfer Capability(TTC) & Transmission margin

Transmission Reliability Margin(TRM)

Capacity Benefit Margin(CBM)

TTC minus the base flow and appropriate transmission margin isthe ATC for the selected interfaces

Determination of TTC not only takes normal operation mode butalso takes into account contingency conditions.

iii) The device should be placed in one of the transmission line on whichthe loop flow occurs. The power flow in the lines is being forced to

zero or sent in the opposite direction of the loop flow.

iv) The objective is to maximize the power that can be transferred from aset of generators in a source area to loads in a sink area subject to

voltage limits, line flow limits and FACTs devices operation limits.

v) Load flow is done taking care of line thermal limits & bus voltagelimits using a suitable optimization technique.

vi) Largest flow of power between source and sink is selected as theoptimal solution for a particular FACTs Device.


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c) What are the advantages of slope in the dynamic characteristics of SVC ?

Answer:

The regulation slope is defined bymax

max

max

max

L

L

C

C

IV

IVk

The objective is to limit the steady state reactive power output of the compensator to a

given reference value. As the slope becomes smaller (k0), the terminal voltage

remains constant, independent of the system voltage variation. Similarly, with

increasing slope (k>>X) the terminal voltage becomes unregulated.

d) Describe the V-I characteristics of STATCOM ?

Answer:

The primary objective of applying a static synchronous compensator

(STATCOM) in a power system is to increase the power transmission

capability with a given transmission network, from the generators to the loads.

A typical terminal voltage versus output current characteristic of a static

compensator with a specific slope is shown below together with particular

load lines is shown below.

Load line 1 intersects the compensator V-I characteristic at the nominal(reference voltage), thus the output current of the compensator is zero. Loadline 2 is below load line 1 due to a decrease in the power system voltage (e.g.,generator outage). Its intersection with the compensator V-I characteristic callsfor the capacitive compensating current IC2. Load line 3 is above load line 1due to an increase in the power system voltage (e.g., load rejection). Itsintersection with the compensator V-I characteristic defines the inductivecompensating current IL3. The intersection points of the load lines 2 and 3 withthe voltage axis define the terminal voltage variation without anycompensation. The terminal voltage variation with compensation, in the linearoperating range under steady state conditions and slow system changes isentirely determined by the regulation slope.

0

VT

ILmaxICmax

Load line 1

Load line 3

Load line 2

VT = Vref


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e) Describe in detail the power flow control co-ordination of FACTs controllers?

Answer:

i) The controllable series capacitive impedance provided by the TSSC or GCSCcancels part of the reactive line impedance resulting in a reduced overall

transmission impedance (i.e., in an electrically shorter line) and

correspondingly increased transmittable power. The impedance is controllable

in the range of max00 cXX .

ii) The Static Synchronous Series Compensator (SSSC) injects a continuouslyvariable series compensating voltage in quadrature with the line current. In

contrast to the TSSC & GCSC, the compensating voltage of the SSSC can be

controlled totally independent of the line current (i.e., independent of the

transmission angle ) over the theoretical operating range of zero to rated linecurrent.

iii) A TCSC is capable of providing equal capacitive and inductive compensationand is able to continuously control and maintain the compensating voltage in

the face of decreasing line current.

The reactive power demand at the endpoints of the line is

determined by the transmitted real power in the same way as if the line

was uncompensated but had a lower line impedance

iv) STATCOM operates in two modes, i.e., Reactive Power (VAR) Control Mode(establish the desired current) and Automatic Voltage Control Mode (shunt

converter reactive current is automatically regulated to maintain the

transmission line voltage to a reference value at the point of common

coupling, with a defined droop characteristic).

v) Since the UPFC is a self-sufficient voltage source (whose losses are suppliedby the shunt converter), it can force upto 0.5 p.u. real power in either direction

and also control reactive power exchanges between the sending-end and

receiving-end buses within a graphical circular control region.

Q.2.

a) What are the various recent advancement in the UPFC design?

The most recent advancement in the UPFC design is an integrated unified

power flow controller with only nine IGBTs to improve grid compliance of renewable

power generation system. The integrated UPFC has two AC outputs, which are served

as an AC injected voltage source (DC-AC inverter) and an AC current source (AC-

DC rectifier). The modulation of the UPFC is developed to provide sufficient voltage

for power flow control as well as minimizing system rating. The schematic is shown

below:


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This nine-switch converter is utilized to replace the inverter 1 & 2 in the

UPFC.

b) Explain the operation of STATCOM with an aid of block diagram?

Fig. STATCOM

If the line voltage V is in phase with the converter output voltage V 0 and has

the same magnitude so that 000 00 VV , there can be no current flowing

into or out of the compensator and no exchange of reactive power with the

line.

If the converter voltage is now increased, the voltage difference between V

and V0appears across the leakage reactance of the step-down transformer. As

a result, a leading current with respect to V is drawn and the compensator

Controller

Voltage

Sourced

Converter

+ -

Vcap

Settings

V

VS VR

P.T.

Step down

transformer

with leakage

reactance XL


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behaves as a capacitor, generating VARs. Conversely, if0VV , then the

compensator draws a lagging current, behaving as an inductor and absorbs

VARs. This operation allows continuous control of reactive power but with

high speed GTOs, MCTs or IGBTs which are forced-commutated.

c) What are the different types of modelling methods of FACTs Devices? What is the

role of DC link in UPFC?

The modelling of FACTs devices is mainly deals with the modelling of

various converter configurations. Mainly for the harmonic analysis a

harmonic modelling is done either in i) Time Domain or in ii) Frequency

Domain

Time domain techniques typically incorporate modular control blocks, capable

of easily and accurately incorporating a variety of control schemes, making

time domain simulation particularly suited to control and transient studies. In

contrast the limitations of time domain simulation become apparent when

considering system-wide power quality issues, which require more than

idealised system equivalents.

The harmonic (or frequency) domain, in contrast to time domain techniques, is

by definition a steady-state form of harmonic analysis based on representing

converters in terms of their harmonic spectra. Converters are therefore viewed

as harmonic modulators, the transfers characteristic of which can be definedusing a range of techniques based on Fourier analysis. The harmonic domain

has the advantage of being capable of easily accommodating frequency

dependent components in the system admittance.

Different types of modelling methods of FACTs Devices:

i) Averaging Methodii) Phasor Methodiii) Small-scale signal modeliv)

Transient model is developed using state space analysis

The Role of DC Link in UPFC: The basic function of converter 1 is to

supply or absorb the real power demanded by converter 2 at the common dc link to

support the real power exchange resulting from the series voltage injection. This dc

link power demand of converter 2 is converted back to ac by converter 1 and coupled

to the transmission line bus via a shunt connected transformer.

In addition to the real power need of converter 2, converter 1 can also generate

or absorb controllable reactive power, if it is desired and thereby providing

independent shunt reactive compensation for the line. There can be no reactive power

flow through the UPFC dc link.


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0

VT

ILmaxICmax

Load line 1

Load line 3

Load line 2

VT = Vref

d) What are the advantages of FACTs controllers? How is co-ordination of FACTs

controllers carried out?

Answer:

Advantages of FACTs controllers:

i) Greater control of power so that it flows on the prescribed transmission

routes.

ii) Secured loading of transmission lines to levels nearer their thermal

limits.

iii) Greater ability to transfer between controlled areas.

iv) Prevention of cascading outages.

v) Damping of power system oscillation.Co-ordination of FACTs controllers is done through various operation modes which

are listed below:

i) Direct voltage injection modeii) Bus voltage regulation and control modeiii) Line impedance compensation modeiv) Phase angle regulation modev) Automatic power flow control modevi) Reactive power control modevii) Automatic voltage control mode

e) Explain about the performances of SVC in controlling voltage in a power system?

Answer: For terminal voltage changes outside of the linear control range, the output

current of the compensator is determined by the basic V-I characteristics of the SVC.

.


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Load line 1 intersects the compensator V-I characteristic at the nominal

(reference voltage), thus the output current of the compensator is zero. Load

line 2 is below load line 1 due to a decrease in the power system voltage (e.g.,

generator outage). Its intersection with the compensator V-I characteristic callsfor the capacitive compensating current IC2. Load line 3 is above load line 1

due to an increase in the power system voltage (e.g., load rejection). Its

intersection with the compensator V-I characteristic defines the inductive

compensating current IL3. The intersection points of the load lines 2 and 3 with

the voltage axis define the terminal voltage variation without any

compensation.

The regulation of the voltage at particular intermediate points and

selected load terminals of the transmission system, limits voltage

variation , prevents voltage instability (voltage collapse) and increasestransient stability limits, whereas appropriate variation of the terminal

voltage can further enhance transient stability and provide effective

power oscillation damping (dynamic stability).

Q.3.

a) With a neat schematic diagram, explain the various basic types of FACTs controllers

in detail ? Answer: Refer to Understanding FACTs by Hingorani Page No. 298.

Fig. Group of Conventional thyristor controlled FACTs Controllers

SVC TCSC

TCSC TCPAR


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Q.4.

a) With a neat block diagram, explain the different modes of operation of TCSC ?

Answer :

Refer Understanding Facts by Hingorani Page No. 226.

b) Explain the modelling procedure of UPFC for power flow studies ?

Answer:

A simple transient model of UPFC is shown below.

A basic series branch dq control model of UPFC with respect to the bus voltage is

shown below:


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c) Explain the various control attributes for different FACTs controllers ?

Answer:

The various control attributes for power and current flow is mentioned below:

i) Applying a voltage in the midpoint can also increase or decrease themagnitude of power.

ii) Applying a voltage in series with line and in phase quadrature with the currentflow, can increase or decrease the magnitude of current flow. Because the

current flow lags the voltage by 900, there is injection of reactive power in

series.

iii) If a voltage with a variable magnitude and a phase is applied in series, thenvarying the amplitude and phase angle can control both the active and reactive

current flows. This requires injection of both active power and reactive power

in series.

iv) Increasing and decreasing the value of the reactance X cause a decrease andincrease of the power height of the curves respectively.

v) Power flow can also be controlled by regulating the magnitude of sending andreceiving end voltages Vs and Vr. This type of control has much more

influence over the reactive power flow than the active power flow.

Q.5.

a) Explain the various type DC links ? How are FACTs controllers used to control the

HVDC links ?

Answer:

An HVDC system is basically constant-current controlled. The reasons are the

need to limit over current and minimize damage due to faults and to prevent the

system from closing down in the case of ac system disturbances.

Basic Principles: The single line diagram in figure below shows a monopolar

link or one pole of a bipolar link. An equivalent circuit and a voltage profile are also

shown in figure (b) & (c).


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Fig (a)

Fig. (b)

The direct current Id, flowing from the rectifier to the inverter is :

ciLcr

doidor

d RRR

UU

I

coscos

And the dc voltage at the rectifier dc terminals can be expressed as :

dcrdordr IRUU cos

Udor and Udoiare the no load direct voltage in the rectifier and the inverter

respectively. Rcr and Rciare the equivalent commutating resistances which account

for the voltage drop in the converters due to commutation overlap and RLis the dc

line resistance.

Facts controllers control the internal voltages, the dc voltage at any point on

the line so control the dc current or dc power. This is done either by controlling the

rectifier thyristor firing angle (or extinction angle gamma of an inverter) or by

control of ac voltage through transformer tap changing

DC Line

3 Phase

AC System

3 Phase

AC System

Id

Rcr RL -Rci

Udorcos Udoicos

+

Ud

+

Udi


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b) With basic circuit arrangement explain the working and characteristics of TSSC ?

Answer:

A thyristor switched series capacitor (TSSC) consists of a number ofcapacitors in series, each shunted by a switch composed of two anti-parallel thyristors.

Refer: Understanding Facts by Hingorani Page No. 225

c) Find the range of Active and Reactive power if a voltage Vpqis injected in series for a

unified power flow controllers ?

Answer:

The power flow control capability or the range of active and reactive power is

found as follows:

[Fig. A Two Machine Model Power System using UPFC]

With reference to the shown figure, the transmitted power P and the reactive powerjQr

supplied by the receiving end can be expressed as follows:

*

jX

VVVVjQP

rpqs

rr [ *conjugate of a complex number]

With reference to the shown figure, the transmitted power P and the reactive powerjQr

supplied by the receiving end can be expressed as follows:

*

jX

VVVVjQP

rpqs

rr [ *conjugate of a complex number]

If Vpq=0,then above equation describes the uncompensated system,


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i.e.,

*

jX

VVVjQP rsrr

and with Vpq0,

*

jX

VVVVjQP

rpqs

rrjX

VV pqr

*

As,

2sin

2cos2

jVVeVj

s and

2sin

2cos2

jVVeVj

s

and }2

sin2

{cos)

2(

jVeVV pqj

pqpq

The following expressions are obtained forPand Qr:

pqPPP 0,

2cos.sin

2

X

VV

X

V pq

and

pqrr QQQ 0,

2sin.cos1

2

X

VV

X

V pq

For between (0,2) & between (0,), Active Power ()

& Reactive Power Qpq() are controllable between ,

X

VVto

X

VV pqpq independent of angle .

Where sin2

0X

VP and cos1

2

X

VQor

are the real and reactive power characterizing the power transmission of the

uncompensated system at a given angle .


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Therefore the transmittable real power P is controllable between

X

VVP

pqmax

0 0P X

VVP

pqmax

0

And the reactive power Qris controllable between

X

VVQ

pqmax

0 rQ0 X

VVQ

pq

r

max

0

at any transmission angle .

Facts Devices_solution Manual Electrical Engineering BTKIT Uttarakhand TEE 703

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