Instrument Transformers - Electrical and Electronic Measurements - Electrical Engineering

Introduction

Instrument transformers are used in AC systems for the measurement of electrical quantities such as voltage, current, power, energy, power factor and frequency. They are also used with protective relays for the protection of power systems. The basic function of an instrument transformer is to step down the high voltage or high current of a power system to a lower, standard level suitable for measuring instruments and relays. Power-system voltages and currents are often very large and it is difficult, unsafe and uneconomic to connect instruments directly to such levels. Typical standard instrument ratings are 5 A for current and 110-120 V for voltage. By using instrument transformers, these standard instruments can be used to measure large system quantities accurately and safely.

Advantages of Instrument Transformers

• Large voltages and currents of the power system can be measured by using small-rated measuring instruments (for example, 5 A ammeter and 110-120 V voltmeter).
• Measuring instruments can be standardised, which reduces manufacturing and replacement cost and simplifies maintenance.
• Instrument transformers provide electrical isolation between the high-voltage power circuit and the measuring/protection circuits, reducing insulation requirements and improving safety for operators and equipment.
• Several measuring devices and relays can be connected to the secondary of a single transformer, allowing multiple circuits to be served from one primary connection.
• Because the measuring and protective circuits operate at low voltages and currents, power consumption in those circuits is small.

Types of Instrument Transformers

• Current Transformer (C.T.)
• Potential Transformer or Voltage Transformer (P.T. or V.T.)

Current Transformer (C.T.)

A current transformer steps down high primary current to a lower, standard secondary current (typically 5 A) so that standard ammeters and protection relays can be used. The primary is connected in series with the power circuit; because of this it is sometimes called a series transformer. A typical connection arrangement is shown below.

Construction and practical features:

• The primary winding has very few turns; sometimes a solid bar (bar primary) is used as the primary conductor.
• The secondary winding has many turns and is connected to the measuring instrument or relay. Because ammeters and relays have low resistance, the CT secondary normally operates close to a short-circuit condition.
• One terminal of the secondary is usually earthed to keep secondary voltages referenced to earth and to improve safety.
• Before opening the secondary circuit, the secondary must be short-circuited through a shorting switch (often labelled S) to prevent dangerously high voltages appearing across the open secondary.
• CTs are designed to maintain accurate current transformation over the specified range and under specified burdens (connected load on the secondary).

Ideal and Practical Behaviour

• For an ideal CT, the ampere-turns of primary and secondary balance exactly and magnetising current is zero, so the secondary current is simply the primary current divided by the turns ratio.
• Practical CTs have a small magnetising current; this causes two main errors: ratio error and phase angle error. These errors depend on the burden, frequency and the flux characteristics of the core.
• Burden is the total impedance (expressed in VA at rated secondary voltage) connected to the CT secondary; CT accuracy is specified up to a rated burden.
• Saturation of the CT core at high flux causes large errors; protection CTs are often designed with a well-defined knee-point to ensure predictable behaviour under fault currents.

Errors and Definitions

• Ratio error (expressed in %) is the relative difference between the ideal secondary current and the actual secondary current at rated conditions. If a is the turns ratio (Np / Ns) and Ip is primary current, the ideal secondary current is Ip / a. Ratio error shows how far Is(actual) deviates from Is(ideal).
• Phase angle error is the angular displacement between the ideal secondary current phasor and the actual secondary current phasor. This affects power and power-factor measurements.
• Accuracy of CTs is expressed by accuracy classes (for example class 0.1, 0.5, 1.0), which indicate allowable ratio and phase errors at specified burdens and conditions.

Types and Uses

• Types: wound-primary CT, bar-type CT, toroidal (ring-core) CT, encapsulated CT, and multi-ratio CTs.
• Uses: measurement (ammeters, energy meters), protection (relays and breakers), and instrumentation (auxiliary circuits).
• Safety rule: never open the secondary of a CT while the primary is carrying current. The secondary must always be shorted or connected to a burden before primary current flows.

Potential Transformer (P.T.)

A potential transformer (also called a voltage transformer, V.T.) steps down a high system voltage to a lower standard voltage (typically 110-120 V) so that standard voltmeters, wattmeters and protective devices can be used. The primary of a PT has many turns and is connected across the line (often line-to-earth); for this reason it is sometimes called a parallel transformer. A typical connection is shown below.

Construction and practical behaviour:

• The primary winding has many turns and is connected across the high-voltage line or between two line conductors as required by the application.
• The secondary winding has fewer turns and is connected to the voltmeter, wattmeter or relay. Because voltmeters and relays present a high resistance, the PT secondary normally operates close to open-circuit condition.
• One secondary terminal is usually earthed so that secondary voltages remain at safe values with respect to earth.
• PT accuracy depends on the load on the secondary (burden) and on core magnetising characteristics. PTs are designed to have small excitation current to keep transformation accurate under rated conditions.

Errors and Definitions

• Voltage ratio error (expressed in %) is the difference between the ideal secondary voltage (primary voltage divided by turns ratio) and the actual secondary voltage at rated conditions.
• Phase angle error is the angular displacement between the ideal and actual secondary voltages; this affects power and power-factor measurements.
• Like CTs, PTs have accuracy classes (for example class 0.1, 0.2, 0.5, 1.0) specifying permissible combined ratio and phase errors at defined burdens.
• Types of PTs include electromagnetic PTs and capacitive voltage dividers (capacitive VTs) used for very high voltages.

Difference between C.T. and P.T.

• Function: C.T. steps down current; P.T. steps down voltage.
• Connection: C.T. is connected in series with the circuit (series transformer); P.T. is connected across the circuit (parallel transformer).
• Turns: C.T. primary has few turns (often a bar), secondary has many turns; P.T. primary has many turns, secondary has fewer turns.
• Secondary condition: C.T. secondary operates almost short-circuited (low impedance); P.T. secondary operates almost open-circuited (high impedance).
• Typical instrument rating: C.T. secondary current is normally 5 A; P.T. secondary voltage is normally 110-120 V.
• Earth connection: One terminal of the secondary of both CT and PT is often earthed for safety, but particular earthing arrangements depend on system requirements.
• Hazards: An open-circuited CT secondary can develop dangerously high voltages; incorrectly applied PTs can supply incorrect voltages to meters and relays if burden or saturation occurs.

Accuracy, Burden and Classes

• Burden is the total load (in VA) connected to the secondary winding; CTs and PTs are specified to maintain accuracy up to a rated burden.
• Accuracy classes define permissible ratio and phase errors under specified burden and current/voltage conditions. For example, classes for CTs and PTs used for metering and protection differ: metering classes are tighter (e.g. 0.1, 0.2, 0.5), while protection classes permit larger errors but limit saturation behaviour (class P, 5P, 10P, etc.).
• Knee-point voltage is a parameter for protection CTs that indicates core saturation onset; a well-defined knee point helps predict CT behaviour during faults.

Tests on Instrument Transformers

• Polarity test: Confirms correct polarity marking so that instruments and relays are connected with correct phase relationships.
• Ratio test: Verifies the turns/voltage/current transformation ratio under specified conditions.
• Burden test: Checks accuracy of transformation when the secondary is loaded with the rated burden.
• Excitation or magnetisation curve test: For PTs and CTs, the excitation characteristic (V-I for PT, V-φ or flux characteristics for CT core) is determined to detect saturation and magnetising current behaviour.
• Knee-point test (for protection CTs): Determines the knee-point voltage to ensure the CT will not saturate prematurely under fault currents.

Safety and Installation Precautions

• Never open the secondary circuit of a CT while the primary is energised; always short the secondary before opening.
• Earthing one terminal of CT and PT secondaries is important to avoid floating potentials and ensure operator safety.
• Observe polarity markings when connecting CTs and PTs to instruments and relays to maintain correct phasor relationships.
• Keep secondary wiring as short as practical and use correctly sized conductors to minimise additional burden and avoid measurement errors.
• Select CTs and PTs with appropriate accuracy class and rated burden for the intended metering or protection application.

Applications

• Measurement of line currents and voltages using standard ammeters, voltmeters and wattmeters.
• Energy metering in distribution and transmission systems using metering CTs and PTs.
• Protection circuits: feeding protective relays for overcurrent, distance, differential and other protection schemes.
• Isolation of measuring circuits from high-voltage systems to provide safety and to enable standard instrumentation.

Summary

Instrument transformers - current transformers and potential transformers - are essential devices in power systems to reduce high voltages and currents to standard, measurable levels. They enable safe, accurate measurement and reliable operation of protection systems. Correct selection, installation and testing of CTs and PTs, together with awareness of their limitations (ratio and phase errors, burden, saturation), are necessary to ensure accurate measurement and dependable protection.