Current and Voltage Transformers for Protective Relaying, Summaries of Power Distribution and Utilization

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5_CT & VT

A H Chowdhury, PhD

Professor

EEE, BUET

January 2022

Introduction

• Relays require accurate data of normal and fault conditions for correct sensing and operation
• CT, VT, CVT (capacitor voltage transformers)—provide insulation from higher-system voltages and reduction of primary current and voltage quantities
• Problems may be transient or permanent; may involve a change in primary current or voltage over a wide range - Sudden current change due to fault → few amperes to several hundred amperes - Voltage can collapse during faults → from rated value down to zero

Introduction Voltage transformer

Introduction Capacitor voltage transformers CVT is used to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay

Steady-state Performance of CT

• Primary winding of CT connected in series with power network → primary current I´ 1 dictated by network → leakage impedance of primary winding Z´ x1 has no effect on CT performance, and may be omitted
• Referring all quantities to secondary winding, simplified equivalent circuit is obtained CT equivalent circuit and its simplification Turns ratio ( 1 : n)

Zb CT burden → external load applied to CT secondary

• Impedance of relays, meters, and leads connecting secondary winding terminals of CT located in substation yard to protection equipment located in control room
• Lead impedance is a significant part of total load impedance
• Burden specified in volt-amperes ( I 2 Zb VA)
• If 5 amp is rated secondary current at which burden is specified, burden would be 25 Zb Zm magnetizing impedance Steady-state Performance of CT

Steady-state Performance of CT

• small Zb → small Eb, Em → small Im
• Per unit CT error defined by
  • So, CTs work at their best when connected to very low Zb
  • In the limiting case of zero burden impedance (and a small Zx2 ) I 1 = I 2 , and CT error is zero
• CT error more often presented in terms of a ratio correction factor

Steady-state Performance of CT Ratio-correction factor → Factor by which nameplate ratio of a CT must be multiplied to obtain true ratio

• Ratio error of relaying CT are such that, for a given magnitude of primary current, secondary current is less than the marked ratio - hence, ratio-correction factor is greater than 1. Ratio-correction- factor curve

Steady-state Performance of CT

• Magnetizing branch of CT nonlinear → Zm is not constant → actual excitation characteristic of CT must be taken into account in determining factor R for a given situation Steady-state Performance of CT Magnetizing characteristic of a typical CT

Transient performance of CT CT performance on dc component

• CT performance affected significantly by DC component of current
• The DC results from necessity to satisfy two conflicting requirements that may occur: a) in a highly inductive network, current wave near maximum when voltage wave is near zero, and b) actual current at the time of change is determined by prior network conditions
• Example - energizing a circuit with current zero, before closing circuit at the instant when voltage wave is zero presents a problem - By requirement (a) current should be at or near maximum at that moment → a counter current produced to provide zero required by condition (b) - This is the DC component equal and opposite to required AC current by condition (a), with the two adding to zero at the instant of closing the circuit

Transient performance of CT Typical possible distortion in CT secondary current resulting from DC saturation

• DC is no longer required after providing initial requirement
• It decays according to L/R time constant of power system
• Decaying DC acts like a low-frequency AC
• It can saturate iron → secondary current can be severely limited and distorted

Transient performance of CT

• For a later fault, residual flux can either add or reduce CT capabilities
• In figure, residual flux points s and t are closer to saturation level for flux excursions to right, but are far away from saturation for excursions to left
• Consequently, performance depends on which half cycle next fault occurs
• Because this is not predictable in theory, residual flux can cause saturation and problems in protection Air-gap CT minimize residual flux; but performance not nearly as reliable
• < Does not saturate as rapidly as non-gap designs > < exciting current higher > < loss of steady-state accuracy and of transformation of DC transient >

Voltage Transformer

• Two types of VTs are used for protective-relaying purposes 1. Potential transformer - conventional transformer having primary and secondary windings 2. Capacitance potential device - voltage-transforming equipment using a capacitance voltage divider connected
• VT Burden → total external VA load on the secondary at rated

secondary voltage

• If a voltage transformer has acceptable accuracy at its rated voltage,

it is suitable over the range from zero to 110% of rated less voltage

• Operation in excess of 10% overvoltage may cause increased errors

and excessive heating