28 Instrument Transformers - Part-2 - CTs VTs

CTs & VTs – Javed 1

INSTRUMENT TRANSFORMERS – PART-2

CURRENT & VOLTAGE TRANSFORMERS

(FOR ELECTRIC T&D, POWER PLANTS &, INDUSTRIAL APPLICATIONS)

(S. R. JAVED AHMED)


VOLTAGE TRANSFORMERS

Voltage Transformers (VTs) are also classified along with Current Transformers (CTs) as Instrument Transformers. These are

shunt connected devices intended to reproduce Secondary Voltages in fixed proportion to the Primary system Voltage. VTs

contrast with Power, Distribution & Control Transformers (which are also shunt devices) in having very small rated power

compared with Power, Distribution, Grounding & Control Transformers. Main purpose of Power, Distribution & Control

Transformers on one hand is to deliver Power from one system to other system or systems with or without galvanic isolation

between each system (e.g. an Auto-Transformer which may be Power Transformer transforms power from one system to other

without galvanic isolation). Main purpose of a VT is to produce replica of a system voltage which can be used in Metering &

Relaying systems maintain required accuracy over range of Voltage.

Two types of Voltage Transformers are most commonly used in Metering, Protection & control systems are Measuring

(Metering) Voltage Transformers and Protective (Relaying) Voltage Transformers.

IEC 60044-2 defines a VT as:

IEEE C37-13 defines a VT as:

Similar to a CT (refer to INSTRUMENT TRANSFORMERS – PART-2, CURRENT & VOLTAGE TRANSFORMERS), an ideal VT is a VT

which introduces no ratio errors and no phase displacement when it transforms Voltage from Primary system to the secondary

system.

Ratio error or Voltage error is expressed as:


Standard values for VTs as defined in IEC 60044-2 for VTs are:


Standard values for VTs as defined in IEEE C37-13 for VTs are:

Goup-1, Group-2, Group-3, Group-4 & Group-5 are:




ACCURACY CLASSES:

IEC 60044-2 defined accuracy classes for Measuring class VTs are:

IEC 60044-2 defined accuracy classes for Protective class VTs are:


IEC 60044-5 Standard defines accuracy class for Protective class CVTs for their Steady state & Transient performance as:


IEC 60044-5 Standard defines accuracy class for Metering class CVTs as:

IEEE C37-13 defined accuracy classes for Protective class VTs are:


SOME LITERATURES























































































































































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28 Instrument Transformers - Part-2 - CTs VTs