Protection Measurement and Instrument Transformers

Introduction and context

Power-system protection depends on measurements that are sufficiently accurate, fast, insulated from high voltage, and dependable during abnormal conditions. Relays and digital protection devices normally cannot be connected directly to primary conductors. Instrument transformers and modern sensors therefore form the measurement interface between the power system and the protection, control, metering, and monitoring equipment.

The two principal conventional instrument transformers are the current transformer (CT) and the voltage transformer (VT). A CT reproduces primary current at a standardized, lower secondary current, commonly 1 A or 5 A. A VT reproduces primary voltage at a standardized secondary voltage, commonly 100, 110, or 120 V line-to-line, depending on the applicable standard and system arrangement. Capacitive voltage transformers (CVTs) use a capacitive divider and electromagnetic or electronic transformation to measure high voltage economically, particularly at transmission voltage levels.

The measurement interface must be designed as part of the protection system. A CT that is satisfactory for steady-state metering may saturate during a fault and cause a differential relay to under-reach or fail to operate. A VT with excessive burden or an inappropriate grounding arrangement may produce a distorted voltage signal and cause distance or directional protection to malfunction. Correct polarity, burden, insulation, grounding, and secondary-circuit practice are therefore safety and protection requirements, not merely installation details.

Learning objectives

By the end of this chapter, the reader should be able to:

1. Explain the construction and operating principles of CTs, VTs, and CVTs.
2. Distinguish protection, metering, and electronic sensor requirements.
3. Calculate burden, ratio error, phase-angle error, and approximate CT excitation requirements.
4. Explain CT saturation, remanence, knee-point voltage, and transient performance.
5. Interpret accuracy classes and select instrument transformers for common protection functions.
6. Apply polarity, grounding, fusing, shorting, and isolation practices safely.
7. Evaluate conventional transformers against low-power sensors and merging-unit architectures.

1. Measurement functions in protection

A protection measurement chain consists of the primary electrical quantity, a sensing element, an insulation and scaling interface, secondary wiring, an input channel, signal processing, and a protection algorithm. Errors may arise at every stage. The protection engineer must distinguish three questions:

• Magnitude: Is the measured current or voltage sufficiently close to the primary value?
• Phase: Is the phase displacement small enough for directional, differential, and distance elements?
• Dynamic behavior: Does the device reproduce rapidly changing and fault quantities without unacceptable distortion?

Protection measurements differ from ordinary instrumentation. A metering CT is optimized for accuracy around normal load current and may intentionally saturate at high current to protect meters. A protection CT is intended to reproduce large fault currents over a specified range and should delay saturation. The two applications can require different cores, ratios, classes, and burdens even when installed on the same conductor.

2. Current transformers

2.1 Construction and principle

A CT has a primary winding, magnetic core, and secondary winding. In a bar-type CT, the primary may be the conductor passing through the core; in a wound CT, it contains one or more turns. The secondary is connected to a relay, meter, transducer, or test circuit.

In the ideal approximation, ampere-turn balance gives:

where N_p and N_s are primary and secondary turns, and I_p and I_s are rms primary and secondary currents in amperes. Thus, the nominal current ratio is:

where I_pn and I_sn are rated primary and secondary currents. The ideal secondary current referred to the primary is approximately I_s = I_p/K_n. This relation assumes sinusoidal operation, negligible excitation current, and operation within the CT’s accuracy range.

A practical CT requires excitation current to establish core flux. The secondary current is therefore not exactly the ideal transformed current. A useful phasor relation, with currents referred consistently to the secondary, is:

Here I_p' is primary current referred to the secondary, I_s is secondary current, and I_e is excitation current. The excitation current contains magnetizing and core-loss components. The relation is phasorial, not an ordinary scalar subtraction.

2.2 CT polarity and terminals

CT polarity is identified by marked terminals such as P1–P2 on the primary and S1–S2 on the secondary. When current enters P1, the corresponding secondary current leaves S1 under the usual polarity convention. Correct polarity is essential for differential and directional protection. Reversing one CT in a differential scheme can convert restraint current into operating current or cancel a genuine operating quantity.

Polarity should be verified by the manufacturer’s markings and, during commissioning, by a polarity or injection test. A simple battery kick test can indicate polarity for suitable low-energy tests, but it must be performed only with approved procedures and isolated equipment.

2.3 Burden

CT burden is the load connected to the CT secondary, expressed as apparent power at rated secondary current, or equivalently by impedance at the relevant power factor. For a secondary circuit with impedance Z_b and rated secondary current I_sn,

where S_b is the burden in VA and |Z_b| is the magnitude of burden impedance in ohms. This expression represents the apparent power magnitude. If the burden is specified at a particular power factor, the associated resistance and reactance must also be considered because phase angle affects CT performance.

The total burden includes relay input impedance, meters, test switches, terminal blocks, and cable resistance. For a two-wire loop of length l one way, conductor cross-sectional area A, and resistivity ρ, the approximate loop resistance is:

The factor 2 assumes equal outgoing and return conductors of the same length and size. Cable burden is then S_cable ≈ I_sn² R_loop for a predominantly resistive cable. At 5 A secondary current, cable losses are 25 times those at 1 A for the same loop resistance; this is a major reason why 1 A CT secondaries are attractive for long-distance wiring.

Worked example: CT burden. A 5 A CT supplies a relay with 0.20 Ω input impedance and a cable loop resistance of 0.35 Ω. Neglecting terminal resistance, the total burden is Z_b = 0.55 Ω. Therefore,

The CT should have a rated burden and accuracy specification compatible with at least this connected burden, with suitable engineering margin. If the same circuit used a 1 A secondary, its burden would be 0.55 VA, although the relay input and insulation requirements would still need checking.

2.4 Accuracy and error definitions

CT ratio error can be expressed as the difference between the actual ratio and the rated ratio. One common form is:

where K_n is the nominal ratio, I_s is actual secondary current, and I_p is primary current. Sign conventions vary among standards; the applicable standard’s definition must be used when interpreting a certificate.

Phase-angle error is the angular displacement between the primary current and the reversed secondary current, usually expressed in minutes or centiradians. It is especially important for energy metering, directional elements, and differential protection. Ratio error and phase-angle error depend on current level, burden magnitude, burden power factor, frequency, and magnetic condition.

Protection CT classes are defined by standards such as IEC 61869 or corresponding national standards. Common IEC designations include 5P and 10P, where the number indicates a composite error limit under specified conditions and the P indicates protection application. A designation such as 5P20 should not be read as a universal guarantee at every burden and waveform: it indicates a specified accuracy limit up to an accuracy-limit factor of 20 under the standard’s test conditions. Special classes, including PX, are used where knee-point voltage and excitation characteristics are central, particularly in high-impedance differential schemes.

Metering classes, such as 0.2, 0.5, or 1, are intended for specified accuracy over a metering current range. Metering cores often have a saturation characteristic that limits excessive secondary current during faults. Protection cores generally require a higher accuracy-limit factor and a later onset of saturation.

3. CT saturation and transient performance

3.1 Physical mechanism

The CT core flux is related to the integral of applied secondary voltage:

where Φ is core flux in webers, N_s is secondary turns, and v_s is secondary winding voltage. This equation assumes the winding voltage is represented consistently and ignores leakage effects in the simplified model.

During a fault, a large current through the CT burden requires a large secondary voltage. If the voltage-time integral drives the core beyond its approximately linear magnetizing region, a small increase in flux produces a large increase in excitation current. The secondary current then becomes distorted and no longer faithfully represents primary current: the CT is saturated.

The knee-point voltage is commonly defined in a test as the point where a specified increase in applied secondary voltage produces a much larger percentage increase in excitation current. The precise definition depends on the standard. It is not simply the maximum voltage that a CT can ever withstand, nor does it alone determine complete transient performance.

3.2 Causes of saturation

Important causes include:

• high fault current and high burden impedance;
• a large dc component in an asymmetrical fault current;
• remanent flux left in the magnetic core;
• excessive secondary lead length;
• inappropriate CT ratio or insufficient accuracy-limit factor;
• differences between CTs in a differential protection scheme.

The dc component is particularly significant because the integral of a unidirectional component can bias the flux in one direction. Remanence adds an initial flux offset and can make saturation occur earlier on a subsequent fault.

3.3 Consequences and mitigation

Saturation can cause underestimation of fault current, delayed or incorrect operation, false differential current, directional errors, and waveform distortion that affects numerical algorithms. Mitigation includes reducing burden, using larger conductors, selecting an appropriate 1 A or 5 A secondary, specifying a suitable protection class, selecting a higher knee-point CT where required, using matched CTs for differential protection, and verifying performance by calculation or testing.

For a simple resistive secondary circuit, an approximate maximum secondary voltage is:

where I_s,f is the fault secondary current, R_ct is CT secondary winding resistance, R_lead is loop lead resistance, and R_relay is relay input resistance. This is an approximation: inductive burden, remanence, dc offset, and mutual effects may require a more complete transient model.

4. Voltage transformers and CVTs

4.1 Electromagnetic voltage transformers

A VT is an electromagnetic transformer designed to reproduce voltage at a lower, standardized level. Its ideal ratio is:

where V_pn and V_sn are rated primary and secondary voltages and N_p, N_s are winding turns. A VT secondary must not be short-circuited; its fuses or miniature circuit breakers are therefore selected to provide fault protection without interrupting the circuit unnecessarily during transient conditions.

VT applications include overvoltage and undervoltage protection, distance protection, synchronism checking, directional elements, frequency measurement, revenue metering, and power-quality monitoring. The connection may be phase-to-ground, phase-to-phase, open-delta, or grounded-wye, depending on the system and protection function.

VT burden is the apparent power supplied to connected secondary devices at rated voltage. Unlike CT burden, a VT normally must not be operated above its rated burden because voltage regulation and accuracy deteriorate. Secondary grounding is usually provided at one designated point to establish a reference and prevent hazardous floating voltages; multiple grounds can create circulating currents and measurement errors.

4.2 Capacitive voltage transformers

A CVT uses a capacitive voltage divider followed by an intermediate transformer and associated tuning, damping, and protection components. For two series capacitors C₁ and C₂, the ideal divider voltage across C₂ is:

where V₁ is the applied voltage and the capacitors are assumed ideal and unloaded. In an actual CVT, the electromagnetic unit, burden, stray capacitance, tuning reactor, damping network, frequency, and ferroresonance behavior modify this simple relationship.

CVTs are economical at high voltage and can also provide coupling for power-line carrier communication. Their transient response is not identical to that of an electromagnetic VT. After a fault or switching event, the CVT may produce transient overreach or underreach in distance protection unless the relay and CVT are coordinated. Ferroresonance, caused by nonlinear inductance interacting with capacitance, is mitigated by suitable damping and operating practice.

5. Sensors and transducers

A sensor detects a physical quantity; a transducer converts it into a usable signal. Conventional CTs and VTs are passive electromagnetic transducers. Other technologies include:

• Rogowski coils: air-cored coils producing an output proportional to di/dt; an integrator is required to obtain current. They do not saturate magnetically like iron-core CTs but require accurate integration and auxiliary power.
• Hall-effect sensors: measure magnetic field and can measure dc and ac current, but may have offset, temperature, and bandwidth limitations.
• Optical current and voltage sensors: exploit the Faraday or Pockels effects and provide electrical insulation, but require robust optical and electronic systems.
• Low-power instrument transformers (LPITs): provide standardized low-energy outputs for merging units and digital substations.
• Electronic voltage dividers: use resistive, capacitive, or combined networks, with insulation and frequency-response constraints.

A transducer must be evaluated for range, bandwidth, linearity, noise, offset, temperature dependence, insulation, electromagnetic compatibility, failure mode, calibration, and cybersecurity where it communicates digitally. A sensor with no magnetic saturation may still be unsuitable if its integrator drifts or its output is vulnerable to electromagnetic interference.

6. Safe secondary-circuit practices

6.1 CT circuits

A CT secondary must never be opened while primary current is flowing. With an open secondary, the CT may develop a dangerously high voltage, causing insulation failure, electric shock, equipment damage, and residual magnetization. Before disconnecting a relay or meter, short the CT secondary using an approved shorting terminal block or test switch. The shorting device must be rated for the prospective current and installed so that accidental opening is minimized.

Other essential practices are:

• identify CT cores and polarity clearly;
• ground the CT secondary at one defined point, normally near the relay panel or at the CT location according to the protection design;
• never assume that an unused CT core is harmless—short and ground it as specified;
• use test switches that short CT circuits before isolation;
• verify that terminal blocks cannot interrupt a live CT circuit inadvertently;
• label circuits with ratio, core designation, polarity, and destination;
• perform insulation and secondary-injection tests using approved procedures.

6.2 VT and CVT circuits

VT secondaries must be treated as energized sources. Apply appropriate fusing, isolation, grounding, and phase identification. A VT secondary should not be shorted. Before maintenance, isolate and prove dead according to the switching procedure. For CVTs, allow for stored energy and manufacturer-specific discharge or isolation requirements.

Protection circuits should be segregated from control and communication circuits where practical. Wiring should be mechanically protected, appropriately rated, and documented. Safety depends on design, commissioning, operating discipline, and maintenance; no single label or device is sufficient by itself.

7. Selection and application framework

A practical selection process is:

1. Define the primary voltage, continuous current, fault current, frequency, system grounding, and insulation level.
2. Identify the function: metering, overcurrent, earth fault, differential, distance, directional, synchronism, or monitoring.
3. Select nominal ratio and secondary rating, considering minimum operating current and maximum fault current.
4. Calculate total burden, including the complete cable loop.
5. Check ratio error, phase-angle error, accuracy class, accuracy-limit factor, knee point, and transient requirements.
6. Confirm polarity, core allocation, grounding, insulation, thermal short-time current, and mechanical withstand.
7. Verify the design through manufacturer data, fault studies, excitation curves, secondary injection, and—where necessary—transient simulation.

Protection engineers should avoid selecting a CT solely from the phrase “5P20.” The actual burden, CT winding resistance, fault asymmetry, remanence, and relay algorithm may govern performance. Similarly, a VT or CVT should be selected with attention to both steady-state accuracy and transient behavior.

8. Illustrative applied case: CT selection for transformer differential protection

Background

Consider an illustrative 132/33 kV transformer protected by a percentage-biased differential relay. CTs are installed on both sides. The 132 kV side CT has a 600/1 A ratio, while the 33 kV side CT has a 2400/1 A ratio. The relay compensates for transformer ratio and vector group internally.

Problem

A fault study indicates a maximum through-fault primary current of 18 kA on the 132 kV side. The 132 kV CT secondary winding resistance is 4.0 Ω, the loop lead resistance is 1.2 Ω, and the relay input resistance is 0.10 Ω. Determine the approximate secondary current and voltage demand, then assess why saturation must be considered.

Analysis

The ideal secondary current is:

The approximate resistive voltage demand is:

This is already six times the 1 A rated secondary current and requires approximately 159 V across the secondary circuit under the simplified assumption. A real through-fault may contain a substantial dc component and may begin with unfavorable flux conditions. The CT could therefore saturate even if its steady-state accuracy class appears adequate.

Approach and solution

The engineer should obtain the CT excitation curve, knee-point data, remanence information where available, and the relay manufacturer’s differential stability requirements. The design can then be improved by reducing lead resistance, using suitable CT cores, checking the required knee-point voltage, matching CT characteristics, and validating the scheme with secondary injection or a transient study. The relay’s percentage bias provides restraint against spill current caused by small CT errors, but it cannot compensate indefinitely for severe saturation.

Outcome and learning points

The calculation shows why fault current alone is not enough for CT selection. CT winding resistance and lead burden directly determine voltage demand. Differential schemes are especially sensitive because unequal saturation can create false differential current. A complete design must combine ratio, class, burden, excitation, transient performance, polarity, and commissioning tests.

9. Contemporary digital-substation developments

Modern substations increasingly use merging units to sample CT, VT, or LPIT outputs and publish time-synchronized sampled values over a process bus, commonly using IEC 61850 architectures. This can reduce copper wiring and simplify flexible protection and control. It also introduces requirements for time synchronization, network availability, redundancy, configuration management, cybersecurity, and validation of data quality.

Digital processing does not eliminate measurement error. An algorithm may detect bad data, saturation, loss of synchronization, or channel failure, but it cannot reconstruct reliable information from an incorrectly installed sensor. Artificial-intelligence methods are being investigated for anomaly detection, waveform classification, and predictive maintenance; however, safety-critical tripping should remain governed by deterministic, validated protection principles, with AI outputs appropriately supervised and fail-safe.

10. Limitations, governance, and professional practice

Instrument-transformer specifications are standard-dependent. Terms such as composite error, knee point, transient performance, and accuracy-limit factor must be interpreted using the referenced edition of the applicable standard, such as IEC 61869 or relevant IEEE standards. Manufacturer test certificates should be checked for ratio, polarity, insulation, burden, excitation, and routine-test results.

Protection measurements also have a safety dimension. Incorrect grounding can expose personnel to dangerous touch voltage; an open CT can generate lethal voltage; an incorrectly fused VT can defeat protection; and undocumented modifications can invalidate a studied protection setting. Engineers should follow approved switching rules, permit-to-work systems, lockout/tagout, arc-flash procedures, calibration controls, and independent verification. The professional obligation is to design for predictable failure, clear isolation, and demonstrable compliance—not merely nominal operation.

Chapter Summary

CTs and VTs provide the scaled, insulated measurements on which power-system protection depends. CT operation follows approximate ampere-turn balance, but excitation current, burden, core nonlinearity, remanence, and transient fault components produce errors. Burden includes relay inputs and the complete secondary wiring loop; at 5 A, cable losses are much greater than at 1 A for the same conductor resistance. Protection and metering cores have different accuracy objectives, and classes must be interpreted under the relevant standard.

CT saturation is driven by the required secondary voltage and voltage-time integral, especially during asymmetrical faults. Knee-point voltage and excitation curves are useful selection information, but transient application must also consider burden, remanence, and the protection scheme. VTs and CVTs provide voltage measurement but differ in construction and transient behavior. Sensors such as Rogowski coils, Hall devices, optical sensors, and LPITs broaden the available technologies while introducing their own bandwidth, calibration, insulation, and signal-integrity issues.

Safe practice requires that CT secondaries never be opened under load, VT circuits be protected against short circuits, secondary circuits be grounded deliberately at designated points, and polarity and wiring be verified. Digital substations reduce copper and enable sampled-value architectures, but they do not remove the need for sound measurement engineering, testing, cybersecurity, and professional governance.

Key Terms

Instrument transformer: A transformer or sensor that scales and isolates a primary electrical quantity for measurement or protection.

Current transformer (CT): An instrument transformer that produces a reduced secondary current proportional to primary current.

Voltage transformer (VT): An electromagnetic transformer that produces a reduced secondary voltage proportional to primary voltage.

Capacitive voltage transformer (CVT): A voltage-measuring device using a capacitive divider and electromagnetic or electronic transformation.

Burden: The load connected to an instrument-transformer secondary, commonly expressed in VA or impedance.

Ratio error: The deviation between actual transformation and the nominal ratio.

Phase-angle error: The angular displacement introduced between primary and secondary quantities.

Excitation current: CT current required to establish core flux and supply core losses.

CT saturation: Nonlinear operation in which the CT no longer reproduces primary current accurately.

Knee-point voltage: A specified point on a CT excitation characteristic where excitation current increases sharply relative to voltage.

Remanence: Residual magnetic flux remaining in a CT core after excitation is removed.

Accuracy class: A standardized designation describing measurement error under specified conditions.

Accuracy-limit factor: The multiple of rated current up to which a protection CT is specified to meet its accuracy requirement.

Polarity: The correspondence between marked primary and secondary terminals and their instantaneous current directions.

Rogowski coil: An air-cored current sensor whose output is proportional to the rate of change of current.

Low-power instrument transformer: A sensor designed to provide a low-energy standardized output for electronic measurement systems.

Merging unit: A device that samples instrument-transformer outputs and communicates digitized measurements to protection and control systems.

Ferroresonance: A potentially sustained nonlinear resonance involving capacitance and magnetic-core inductance.

Secondary injection: A commissioning test in which known current or voltage signals are applied to relay secondary inputs.

Review Questions

Short Answer Questions

1. Why is a protection CT generally designed differently from a metering CT?
2. Define CT burden and identify three components that may contribute to it.
3. Why must a CT secondary never be opened while primary current is flowing?
4. State two important differences between an electromagnetic VT and a CVT.
5. What is the purpose of establishing a single designated ground in a secondary measurement circuit?

Long Answer Questions

1. Explain the operating principle of a practical CT, including ampere-turn balance, excitation current, burden, ratio error, and phase-angle error.
2. Describe the physical causes and protection consequences of CT saturation during an asymmetrical fault.
3. Compare protection and metering accuracy classes and explain why one CT core may not serve both functions adequately.
4. Explain the construction, operating principle, applications, and limitations of a CVT.
5. Develop a complete engineering procedure for selecting CTs for transformer differential protection.

Analytical / Application Questions

1. A 1 A CT supplies a relay burden of 0.15 Ω through a cable loop of 2.0 Ω. Calculate the total apparent burden, assuming a predominantly resistive circuit, and explain how the result changes if a 5 A CT is used with the same wiring.
2. A CT has a 400/1 A ratio and is subjected to a primary fault current of 8 kA. Calculate the ideal secondary current and explain why the actual waveform may be substantially distorted.
3. A distance-protection CVT shows a transient voltage error after a fault. Analyze two possible effects on relay operation and propose at least three engineering or commissioning measures to reduce the risk.

Multiple Choice Questions with Answers

Question 1

A 600/1 A CT carries a primary current of 3,000 A. Neglecting excitation current, what is the secondary current?

A. 0.2 A

B. 5 A

C. 600 A

D. 1,800 A

Correct Answer: B. 5 A

Question 2

For a 5 A CT secondary and a total burden impedance of 0.40 Ω, the apparent burden is approximately:

A. 0.08 VA

B. 2 VA

C. 10 VA

D. 25 VA

Correct Answer: C. 10 VA

Question 3

The most dangerous immediate consequence of opening a CT secondary while primary current flows is:

A. The primary voltage becomes zero

B. A high secondary voltage may develop

C. The CT ratio becomes exactly unity

D. The VT fuse operates

Correct Answer: B. A high secondary voltage may develop

Question 4

Which condition most directly increases CT saturation risk during a fault?

A. Reduced secondary lead resistance

B. Lower fault current

C. High burden and a large dc component

D. Use of a short-circuited VT secondary

Correct Answer: C. High burden and a large dc component

Question 5

A 5P20 protection CT designation generally indicates that the CT is intended to:

A. Have zero phase-angle error at all currents

B. Meet a specified protection accuracy up to a stated multiple of rated current

C. Operate only at 20 Hz

D. Have a 20 VA burden under every condition

Correct Answer: B. Meet a specified protection accuracy up to a stated multiple of rated current

Question 6

Which device commonly uses a capacitive divider at high voltage?

A. Rogowski coil

B. Capacitive voltage transformer

C. Hall-effect current sensor

D. Bar-type current transformer only

Correct Answer: B. Capacitive voltage transformer

Question 7

For a two-wire secondary circuit, cable loop resistance is approximately doubled when:

A. Only the one-way conductor length is considered instead of outgoing and return paths

B. The conductor area is doubled

C. The resistivity is halved

D. The frequency is reduced to zero

Correct Answer: A. Only the one-way conductor length is considered instead of outgoing and return paths

Question 8

Which sensor normally requires integration to obtain current from its output?

A. Rogowski coil

B. Electromagnetic VT

C. Resistive voltage divider

D. Metering CT

Correct Answer: A. Rogowski coil

Question 9

The primary purpose of correct CT polarity in differential protection is to ensure that:

A. The CT secondary is never grounded

B. Currents combine correctly for restraint and differential quantities

C. The CT burden is independent of cable length

D. The relay input impedance is zero

Correct Answer: B. Currents combine correctly for restraint and differential quantities

Question 10

A recommended practice for an unused CT core is generally to:

A. Leave it permanently open

B. Short and ground it according to the design and manufacturer instructions

C. Connect it directly across a VT secondary

D. Remove its insulation

Correct Answer: B. Short and ground it according to the design and manufacturer instructions