Fault location estimator for series

compensated transmission line

under power oscillation conditions

Abhishek Rai Roll no-31604211

M tech (power system)

Contents• Introduction• Need of compensation• Types of compensation• Objectives • Advantages/ Problems• Principle of the new fault location algorithm• conclusion

Introduction to Compensation

Compensation of transmission lines is meant the use of electrical circuits to modify the electrical characteristics of the lines within the prescribed limit.

Need of CompensationThe line requires the compensation of transmission lines due to:

Reduction of power transfer capability of lines which reduces the margin between the stable and unstable operation of system.

Ferranti effect.Sub-synchronous resonance.

Various Types of Compensation

Various types of compensation techniques are used for compensating the EHV lines:

Series compensationShunt compensation-SVS (Synchronous voltage

Source)Synchronous condenser

Objectives of Series and Shunt Compensation

Main objective of series and shunt compensation of EHV lines for transmitting bulk power over the long distance may be stated as:

To improve transfer capability along with increased steady state and transient stability limits.

To obtain flat voltage profile as possible along the length of the EHV line while maintaining the equal sending end and receiving end voltage.

To avoid degree of compensation leading to sub-synchronous resonance.

SERIES COMPENSATIONThe series capacitor offers an effective and economic

means for improving stability limits of long distance transmission by reducing the net reactance of transmission line.

In the present case of 400kv and 600km transmission system, the maximum value of receiving end power, compensation efficiency, optimum value of series capacitive reactance are obtained using computer program.

Series compensation is used in long lines to increase transmission capacity, improvement of system stability and to obtain correct load division between parallel circuits.

Series capacitors are connected in series with the line to reduce the net impedance of the transmission line. This reduced impedance causes the reduced voltage drop across the line.

Series capacitors are generally installed on special platforms at one or both ends of the lines.

Installation of Series CapacitorSeries capacitors are generally installed on special

platforms at one or both ends of the lines.

Series Capacitor

Current Limiting Reactor

Spark Gap

Bypass Switch

Metal Oxide Resistor

Bypass switchthe bypass switch must be able to carry the rated current and the rated short-circuit current in the closed position, as well as withstand the overvoltages specified across the open gap and phase-to-earth.

METAL OXYDE VARISTORS (MOVs) Metal oxide varistors, which are connected in parallel with the capacitors , provide overvoltage protection of the capacitors during and after power system faults and thus are conducting a large part of the fault current. MOVs are then protected by the spark gap activation against excessive energy absorption

SPARK GAP

• In case of operation of the MOV protection relay, the spark gap is force-triggered by the protection and control system.

Advantage of Series Compensation

There are various advantage of series compensation:

Increase in power transfer capabilityImprovement of system stabilityLoad division among parallel linesControl of voltage

Location of Series Capacitor

Location along the line.

Location at one or both ends of line section on the line sides in the switching stations.

Location between Bus Bars and switching station.

Problems Associated with Series Compensation

Sub – Synchronous Resonance

Sustain oscillation

Unreliable protection of transmission lines

Application of Series Compensation

They have been primarily used to improve system stability and to obtain the desired load division among parallel lines.

High compensation levels also increase the complexity of protective relaying and the probability of sub-synchronous resonance.

A practical upper limit to the degree of series compensation is about 80%.

Following are some of the key considerations in the application of series capacitor banks:

Voltage rise due to reactive current: Voltage rise of on one side of the capacitor may be excessive when the line reactive-current flow is high, as might occur during heavy power transfers.

Bypassing and reinsertion: provision is made for bypassing the capacitor during faults and reinsertion after fault clearing. Speed of reinsertion may be an important factor in maintaining transient stability.

Present trend is to use nonlinear resistors of zinc oxide which have the advantage that reinsertion is essentially instantaneous

SHUNT COMPENSATION- SVS

Shunt compensation may consist of static (using capacitor and reactor) or synchronous compensation to avoid voltage stability.

Shunt capacitors supply reactive power and boost local voltages. They are used throughout the system and are applied in a wide range of sizes.

The principal advantages of shunt capacitors are their low cost and their flexibility of installation and operation.

The principal disadvantage of shunt capacitors is that their reactive power output is proportional to the square of the voltage.

Application of shunt compensation

Shunt capacitors are used to compensate for the XI2 losses in transmission system and to ensure satisfactory voltage levels during heavy loading conditions.

Switching of capacitor banks provides a convenient means of controlling transmission system voltages.

Types of SVS

Self Saturated Reactor (SR)Thyristor Controlled Reactor (TCR)Thyristor Switched Capacitor (TSC) Fixed Capacitor (FC), Thyristor Controlled Reactor

(TCR) scheme Thyristor Switched Capacitor (TSC), Thyristor

Controlled Reactor(TCR) scheme

Advantages of SVS

Increased power transfer capability Enhancement of transient stabilityThe dynamic response of SVS is very fastSteady –State and temporary overvoltage can be

controlled

SYNCHRONOUS CONDENSER A synchronous condenser is a synchronous machine running

without a prime mover or a mechanical load. By controlling the field excitation, it can be made to either generate or absorb reactive power. With a voltage regulator, it can automatically adjust the reactive power output to maintain constant terminal voltage.

Synchronous compensators contribute to system short-circuit capacity. Their reactive power production is not affected by the system voltage.

UB = UE − US

US = IS*1/(jw0CS)

IB = IE

Principle of the new fault location algorithm

Terms can be calculated

(1) Static-fault locator algorithm for transmission lineBased on the distributed parameter model of transmission line, it is easy to obtain the voltage and current at a distance x from the terminal N

Where UBF and UNF are the voltage estimates at point F, respectively.Since the UBF is equal to UNF, the fault location can be expressed as

(2.1) Dynamic fault locator for two-terminal transmission Lines  Dynamic parameters estimator

From here we can calculate characteristics impedance (Zc) and propagation constant (gamma)

Zc = sqrt (Z/Y) gamma = sqrt (Z.Y)

(2.2)Analysis of the compensation capacity’s influence …….(a)

here US(x, t) = us (x, t) * exp(jw0t)

……..(b)This is Equivalent admittance

…….(c)

uS(x, t) = Us (x, t) * exp(-jw0t) ……..(d)

From equation (b), (c), and (d)

Hence, a corrected voltage drop across the SC can be obtained Us = Is * (1/Ys)

Ys

2.3 Dynamic fault locator algorithm:

The fault section needs to be identified since the fault could be occurred randomly in any side of the SC. A method is employed to identify the fault section. The criterion is expressed as follows

fault located at the left-hand side of SC

fault located at the right-hand side of SC

where 1/C is an indicator of the fault location relative to SC, (1/C)set is the setting, it can be set by (1/C)set = 0.5 × (1/Cs).

Fig. flowchart of the iteration

uS(x, t) = Us (x, t) * exp(-jw0t) ………..(21) Ys ……(22)

Us = Is * (1/Ys) ……..(23)

………(22)

………(26)

Us = Is * (1/Ys) ……..(23)

UB = UE – US ……..(27)

UBF = (0.5*(UB–IB .ZBC))*exp(gammaB * ((1-k)*l - x)) + ( 0.5*(UB+ IB .ZBC))*exp(gammaB*((1-k)*l – x)) ……(28)

 

Performance evaluation

Performance evaluation for 300km, 500kV line and

ConclusionThis paper proposes a novel fault location algorithm for SC compensated transmission line under dynamic conditions by using synchronised phasor measurements obtained by PMUs. The influence on the SC caused by power oscillation has been fully considered by DFLSC. The algorithm gives better fault localisation estimate under power oscillation condition with comparison to the fault location algorithms which did not consider the dynamic characteristics of line or the SC.

Thank you