Resistive Reach Guidelines for Distance Protection

1

Resistive Reach Guidelines for Impedance Relay Applications R. M. Naidoo and E. Stokes-Waller, Member, SAIEE

Abstract— Impedance relays must accommodate all types of faults. In order to enable this, it is necessary that the correct voltage and current quantities be applied to the measuring elements. The impedance characteristics are obtained from these three-phase voltage and current measurements. The main aim is to detect the faulted phases and react appropriately based on the applied relay settings, scheme design and philosophies. The difficulty arises in the distinction between the faulted and healthy phase impedance measurements. Continuous research and development of impedance relays are in progress to improve its operating performance. Unfortunately, the impact and limitations of such improvements on the power system applications are not studied with the same vigour. As such, it is necessary to investigate the influence of network topology, power system loading, fault resistance etc. on the reach capability of impedance relays when applied to power systems. In this paper, the reach capability of impedance relays is investigated. A set of resistive and reactive reach guidelines for impedance relay applications are presented. The results from the research also show that the network topology and faulted phase selection algorithms play a major role in power system healthy phase impedance measurements. New impedance ratios namely SWIR and SNIR are introduced, and the well-known system impedance ratio (SIR) is enhanced for ease of illustrating network topology and associated impedance measurement.

characteristics may pick up unnecessarily depending on the power system fault resistance, position and impedance relay characteristic reach. Unnecessary pickup may result in incorrect tripping of a healthy transmission thereby reducing the reliability of the total power system [1]. This paper presents a set of guidelines that can be practically applied by a protection engineer to determine the reach capability of an impedance relay. This paper also introduces concepts that may form the basis for further research to follow. Results of this research can be used for a wide variety of impedance relay applications and assist in identifying applications that it may not be suited for. II. DEFINITIONS Before the guidelines can be applied, it is necessary to understand two new definitions that have been created. A. SWIR Consider the following system single line diagram. Zs1

ZA

ZB

Zs2

Index Terms— Impedance Relay, protection, fault.

A

I. INTRODUCTION

Location

B

Vs1

Vs2

I

mpedance relay characteristics and reach greatly influence relay behavior. Relay characteristics can be set to have different reaches in active and reactive impedance limits. The impedance relay measures the system current and voltage, applies basic assumptions and attempts to accurately detect and react on a power system fault condition. The assumptions include: • • • •

zero fault resistance, perfect earth fault compensation perfect replica measurements of voltages and currents perfect applied impedance relay characteristic, all of which do not necessarily hold in a practical power system.

It is necessary that the faulted and healthy characteristics accurate. If incorrect, the healthy phase impedance

Reverse (R)

Forward (F)

Figure 1: Basic network illustrating ratios

At any particular location in a balanced power system network (no fault connected), with the network cut in half at this location, the Thevenin equivalents can be obtained in both directions. This is referred to as forward and reverse Thevenin equivalent directions (thereby also introducing directionality to the SWIR ratio). If we obtain the Thevenin equivalent impedance values in each direction (forward and reverse at the location specified), using symmetrical component impedance values (positive-, negativeand zero-sequence impedance circuits respectively), and dividing the forward over reverse Thevenin equivalent values, we obtain the Impedance Ratio SWIR.

2

This is represented in matrix format as:

 Z F + / Z R +   SWIR+      SWIR = Z F − / Z R − = SWIR−      Z Fo / Z Ro   SWIRo 

…(1)

F = Forward Thevenin equivalent impedance R = Reverse Thevenin equivalent impedance + = Positive sequence impedance circuit - = Negative sequence impedance circuit o = Zero sequence impedance circuit For the circuit in Figure 1, the SWIR Ratio at the fault position is equal to:

D. Healthy phase impedance measurement In the context of this research, healthy phase measurements are considered to be from measuring elements (phase-ground and phase-phase) that are not representative of the fault connection. III. NETWORK SIMULATIONS

…(2)

SWIR comprises of three different ratios namely, positive-, negative- and zero sequence impedance ratios. B. SNIR At any particular location, the following definitions apply:

Z F + / Z Fo if   SNIRF +   Z Fo / Z F + if  SNIR  =  Z / Z if R+     R + Ro  Z Ro / Z R + if

…(4)

As for SWIR, the System Impedance Ratio (SIR) also has directionality associated with it. In this paper, the ratios SWIR, SNIR and SIR are used to summarize network impedance relationships but the same principle could be used to illustrate other kind of relationships as well (e.g. SNIR(I+)).

where

( Z B+ + Z S 2+ ) /( Z A+ + Z S1+ )   SWIR = ( Z B − + Z S 2− ) /( Z A− + Z S 1− )    ( Z Bo + Z S 2o ) /( Z Ao + Z S 1o ) 

 Z SI + / Z A+   Zs    Enhanced SIR =   =  Z SI − / Z A−   Z L  Z + / Z  A0   SI 0

Z F + ≥ Z Fo   Z F + < Z Fo  …(3) Z R + ≥ Z Ro    Z R + < Z Ro  

 SNIRF if SNIRF ≥ SNIRR  SNIR+ =    SNIRR if SNIRF < SNIRR  SNIR- is defined as a similar relationship to SNIR+ but utilises negative- and zero-sequence quantities instead. In the context of this paper SNIR- is chosen to be equal to SNIR+. C. Enhanced System Impedance Ratio (SIR) Using the system representation defined earlier the system impedance ratios is for the relay at A, in the context of this research defined (and enhanced) as:

Two methods commonly used for displaying impedance measurements in protective relaying applications include • •

Positive sequence Loop impedance plane

The loop impedance plane is useful for illustrating protection behaviour of radial networks whereas the positive sequence plane is used for meshed networks as well [2]. All protection relays operate on the fundamental principle of positive sequence characteristics (or some slight variation) and utilise the well-known earth fault compensation factor for ground faults. A. Positive sequence impedance measurements: The measurement quantities indicated below are sometimes referred to as the “generic” formula and are widely used in power system protection. It is also accepted widely as the standard on which protection relays base their behavior. •

Phase-Phase measurement quantities

 V A − VB     Z AB   I A − I B   Z  = VB − VC   BC   I − I   Z CA   B C  VC − V A   I C − I A  •

Phase-ground measurement quantities

…(5)

3

 VA    + I KI N   Z Ag   A    VB   Z Bg  =  I + KI  B N  Z Cg       VC   I C + KI N 

where

…(6)

For a purely reactive network, this gives the following distribution of impedance values measured:

I N = I A + I B + IC

The earth fault compensation factor is: 1 Z − Z1 K= ( o ) 3 Z1

…(7)

In a balanced system, no neutral current will flow and the measured impedance becomes a simple ratio of V/I [3]. Slight variations exist in different relays for the earth fault compensation factor and its implementation. The aim is to improve the positive sequence impedance measurement under different conditions. Typically variations in impedance measurements from the generic formula tend to improve faulted phase measurement but may degrade the healthy phase measurement performance (refer to case study).

Figure2: Impedance values measured for equal symmetrical component currents

Note that the magnitudes of the impedance measurements are comparable and relatively “close” to each other. b) Symmetrical component currents not equal If we make the assumption I1=I2 and vary I0 while measuring the impedance For this condition, we find that IW is equal to IB, which in effect means that the measurement ZWB will always be infinite. I1=I2

B. NETWORK TOPOLOGY Network topology is understood as the complete network in geographical terms (including symmetrical impedance values, phase shift components, generation sources, shunt paths, etc.). To study the effect of network topology in impedance measurements of a protection relay, a reverse approach was followed. First, the effect of unbalanced symmetrical currents or voltages on relay impedance measurement was studied and then networks that may cause this were identified. This approach greatly assisted in identifying limitations and general criteria on which impedance reach limitations was based. Effect Of Symmetrical Component Currents If we assume a balanced source (all three phases voltage equal and shifted 120 degrees) and currents I1, I2 and I0 being supplied by the source, the impedance values measured (phase-ground and phase-phase) are:

Zw (Io< I1)

Zrw

Zbr

Zb (Io< I1)

Z(im )

Zr

Zw (Io>I1)

Zb (Io>I1)

movem ent as Io dec reases

m ovem ent as Io dec reas es

Z(re )

Figure 3: Example of generic impedance measured values

Figure 3 implies that network topology plays a major role in the healthy phase impedance values measured by impedance protection relays. IV. IMPEDANCE REACH GUIDELINES

 Z rw  (Vr − Vw ) /( I r − I w )   Z  = (V − V ) /( I − I ) w b   wb   w b  Z br   (Vb − Vr ) /( I b − I r ) 

…(8)

a) Symmetrical component currents equal

These guidelines were established to assist the Engineer in determining the reach limits of an impedance relay. These guidelines are used in conjunction with the graphs developed in Appendix A. A. Methodology for Application

With the symmetrical component currents equal the impedance values are:

•

 Z rw  (Vr − Vw ) /( I r ) Z  =   ∞  wb     Z br   (Vb − Vr ) /( I r ) 

• …(9) •

Read the y-axis value “Rx” from the guidelines based on a number of other values that needs to be pre-determined from the network and relay settings. Model the network in terms of the +ve, -ve and zero sequence networks. Determine the direction of the impedance relay (normally towards the feeder) and label this as forward.

4

• • • •

•

•

At the relay location, determine the forward Thevenin positive sequence equivalent impedance, Z(F+). Determine SWIR and SIR at the relay location and in the forward direction. Determine SNIR+ at the relay location. From the impedance protection relay settings, determine the value of Kn (earth fault compensation factor, normally in the range between 0.6 and 1.3). From the graph, start on the x-axis utilising the determined SWIR or SIR and read from the appropriate Kn curve the associated y-axis value or “Rx”. Use this y-axis value “Rx” and determine the maximum resistive reach recommended.

Venus-Georgedale line impedance (both the same)

Venus

Georgedale

R-g Fault

Klaarwater

Relay

z+={0.00138,0.01151 } zo={0.01178,0.0408}

A. GeorgedaleKlaarwater line impedance Klaarwater Source impedance

z+={0.00783,0.01634 } zo={0.00299,0.02038}

100 MVA base and 275kV system voltage used as reference (base on per unit values)

• •

V. CASE STUDY For this case study, the developed guidelines are applied. During a single phase to earth reverse fault on the VenusGeorgedale feeder, approximately 10% from Georgedale, the Georgedale, Klaarwater protection relay operated in zone 1 for the reverse fault. The positive sequence impedance plane plot as shown in Figure 4, indicates that the healthy phase measurement Zbg (as seen by the impedance relay) caused the incorrect operation.

z+={0.00933,0.05820 } zo={0.06236,0.20447}

•

• •

For our particular network, SWIR+ is equal to 0.41. Earth fault compensation factor set on the relay was Kn=0.9 The system ratios SNIRF and SNIRR are 2.5 and 3.5 respectively (select the average between SNIRF and SNIRR, thus select SNIR+=3 as it is more appropriate). Comparison with the meshed network guideline (with SNIR+=3) we can see that Rx = 0.55. The recommended resistive reach can be derived as follow: R(reach)< Rx * Z(F+) < 0.55*(0.018+0.011) per unit < 0.55*0.029 per unit < 0.016 per unit