How to Test a Shunt Reactor: A Comprehensive Guide

A shunt reactor is defined as a device that absorbs reactive power from a power system to help regulate voltage levels. They are commonly used in high-voltage transmission lines and substations to offset the capacitive effects of long cables and overhead lines. Shunt reactors can be fixed or variable, depending on the needed voltage regulation.

Shunt reactors keep power systems stable and efficient, particularly across long transmission distances and growing renewable generation. Regular testing protects that performance and reliability by measuring parameters such as resistance, reactance, losses, insulation quality, dielectric strength, temperature rise and noise. Testing also uncovers defects before they threaten operation or safety.

Standards and procedures vary with a reactor’s type, rating, application and manufacturer. One widely used reference, particularly in India, is IS 5553, which lays out the test set for extra-high-voltage (EHV) and ultra-high-voltage (UHV) shunt reactors. Under this standard the tests fall into three groups:

• Type tests
• Routine tests
• Special tests

Type tests prove a design, routine tests clear every unit before dispatch, and special tests answer questions the first two never ask.

Type Tests of Shunt Reactor

Type tests verify a shunt reactor’s design and construction features against its specification. They run once per type or model, before that design enters service. The following tests make up the type-test programme:

Measurement of Winding Resistance

This test measures the resistance of each winding in the shunt reactor using a low-voltage direct current (DC) source and an ohmmeter. It is conducted at ambient temperature after disconnecting all external connections. The goal is to check the continuity and integrity of the windings and calculate copper losses.

The measured resistance values should be corrected for temperature using the following formula:

where Rt​ is the resistance at temperature t (°C), R20​ is the resistance at 20°C, and α is the temperature coefficient of resistance (0.004 for copper).

The corrected resistance values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

Measurement of Insulation Resistance

This test measures the resistance of the insulation between the windings and between the windings and the earthed parts of the shunt reactor using a high-voltage DC source (usually 500 V or 1000 V) and a megohmmeter. The test is done at ambient temperature and after disconnecting all external connections. The purpose of this test is to check the quality and condition of the insulation and to detect any moisture, dirt or damage.

The measured insulation resistance values should be corrected for temperature using the following formula:

where Rt​ is the insulation resistance at temperature t (°C), R20​ is the insulation resistance at 20°C, and k is a constant that depends on the type of insulation (usually between 1 and 2).

The corrected insulation resistance values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

Measurement of Reactance

This test measures the reactance of each winding in the shunt reactor using a low-voltage alternating current (AC) source (typically 10% of the rated voltage) and a wattmeter or power analyzer. Conducted at ambient temperature after disconnecting all external connections, this test checks the inductance and impedance of the windings and calculates reactive power consumption.

The measured reactance values should be corrected for voltage using the following formula:

where Xt​ is the reactance at voltage Vt​, and X10​ is the reactance at 10% rated voltage (V10​).

The corrected reactance values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

Measurement of Losses

This test measures the losses of each winding in the shunt reactor using a low-voltage AC source (typically 10% of the rated voltage) and a wattmeter or power analyzer. It is performed at ambient temperature after disconnecting all external connections. The test aims to check the efficiency and power factor of the windings and calculate total losses.

The measured losses consist of two components:

• Copper losses: These are due to the Joule heating effect in the windings and can be calculated by multiplying the measured winding resistance by the square of the rated current.
• Iron losses: These are due to hysteresis and eddy currents in the core and can be calculated by subtracting the copper losses from the total losses.

The measured loss values should be corrected for voltage using the following formula:

where Pt​ is the loss at voltage Vt​, and P10​ is the loss at 10% rated voltage (V10​).

The corrected loss values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

Dielectric Withstand Tests

These tests check that the insulation system survives high-voltage stress without flashover, puncture or lasting damage. Two dielectric withstand tests apply:

• Power frequency withstand test: This test applies a raised power-frequency voltage, commonly a multiple of rated voltage such as 1.5 per unit, between windings and between windings and earthed parts for one minute. It verifies insulation strength at operating frequency.
• Lightning impulse withstand test: This test applies standard 1.2/50 μs lightning-impulse waveshapes from an impulse generator between windings and between windings and earthed parts, three impulses of each polarity. It verifies insulation strength under lightning-surge conditions.

The dielectric withstand tests should follow IS 2071 or IEC 60076-3. Applied voltages must produce no flashover, puncture, sparking or deterioration in the insulation system.

Temperature Rise Test

This test measures the temperature rise in the windings and oil of the shunt reactor under rated load conditions using thermocouples or thermometers. The test is done after running the shunt reactor for several hours until thermal equilibrium is reached. The purpose of this test is to check the cooling system and heat dissipation capacity of the shunt reactor.

Measured temperature-rise values should stay inside the limits set by IS 5553 or IEC 60076-6. The allowable rise depends on insulation class, rating, cooling method and ambient conditions. It generally falls between 40°C and 65°C for oil-immersed shunt reactors.

Routine Tests of Shunt Reactor

Routine tests run on every shunt reactor before dispatch from the factory or installation at site, confirming quality and conformity with the specification. They resemble type tests but are less extensive and less stringent. The routine-test set comprises:

• Measurement of winding resistance.
• Measurement of insulation resistance.
• Measurement of reactance.
• Measurement of losses.
• Dielectric withstand tests.

The procedure, purpose, correction factors, comparison criteria and standards for these tests match those described for type tests.

Special Tests of Shunt Reactor

Special tests happen on request from the customer or manufacturer, gathering information beyond type and routine coverage. They typically serve research, development or design work. The special-test set comprises:

Measurement of Zero Sequence Reactance

This test measures the zero-sequence reactance (or impedance) of each winding. A low-voltage AC source, usually about 10% of rated voltage, feeds the three phase terminals shorted together with the neutral terminal completing the circuit, while an ammeter or a power analyzer records the response. The result shows how much reactive current would flow through the neutral path during a single line-to-ground fault.

The measured zero sequence reactance values should be corrected for voltage using the same formula as that used for the measurement of reactance in type tests.

The measured zero sequence reactance values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

The zero sequence reactance of a shunt reactor is important for predicting the current flow during single-line-to-ground faults and for designing the neutral grounding system¹.

Measurement of Mutual Reactance

This test measures the mutual reactance (or impedance) between two windings of the shunt reactor using a low-voltage AC source (usually 10% rated voltage) applied between one phase terminal of one winding and one phase terminal of another winding while shorting the other terminals of both windings and an ammeter or a power analyzer. The purpose of this test is to check the coupling and leakage flux between the windings and to calculate the mutual inductance.

The measured mutual reactance values should be corrected for voltage using the same formula as that used for the measurement of reactance in type tests.

The measured mutual reactance values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

The mutual reactance of a shunt reactor is important for calculating the equivalent circuit parameters and for analyzing the transient behavior of the device.

Measurement of Acoustic Sound Level

This test measures the acoustic sound level emitted by the shunt reactor under rated load conditions using a sound level meter and a microphone. The test is done after running the shunt reactor for several hours until thermal equilibrium is reached. The purpose of this test is to check the noise level and vibration of the shunt reactor and to ensure its compliance with the environmental standards.

The measured acoustic sound level values should be corrected for background noise and frequency range using the methods described in IEC 60076-10 or IEC 60076-6² standards. The sound pressure level and sound power level should be calculated and reported.

The measured acoustic sound level values should not exceed the limits specified by IS 5553³ or IEC 60076-6 standards. The maximum allowable sound level depends on the type, rating, cooling method, location, etc., but generally ranges from 40 dB(A) to 65 dB(A) for oil-immersed shunt reactors.

Measurement of Vibration and Stress on Tank

This test measures the vibration and stress on the tank of the shunt reactor under rated load conditions using accelerometers, strain gauges, or other sensors. The test is done after running the shunt reactor for several hours until thermal equilibrium is reached. The purpose of this test is to check the mechanical strength and integrity of the tank and to detect any cracks, leaks or deformations.

The measured vibration and stress values need correction for temperature, frequency and loading using appropriate formulas or methods. Vibration amplitude, frequency spectrum, stress distribution and fatigue life should then be calculated and reported.

The measured vibration and stress values should not exceed the limits specified by IS 5553³ or IEC 60076-6 standards. The maximum allowable vibration and stress depend on the type, rating, cooling method, material, etc., but generally follow the guidelines given in IEC 60068-2 or IEC 60076-6 standards.

Measurement of Magnetizing Characteristics

This test measures the magnetizing characteristics of the core. A low-voltage AC source, usually about 10% of rated voltage, drives the shorted phase terminals against the neutral while a wattmeter or a power analyzer records the input. The readings expose core saturation and hysteresis and allow calculation of magnetizing current and losses.

The measured magnetizing characteristics consist of two components:

• Magnetizing current: This is the current required to establish the magnetic flux in the core and can be calculated by dividing the applied voltage by the zero sequence reactance.
• Magnetizing losses: These are due to hysteresis and eddy currents in the core and can be calculated by multiplying the magnetizing current by the applied voltage.

The measured magnetizing characteristics should be plotted as a curve of magnetizing current versus applied voltage or magnetic flux density versus magnetic field intensity. The curve should show the linear and nonlinear regions of the core operation.

The measured magnetizing characteristics should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

The magnetizing characteristics of a shunt reactor are important for determining the reactive power consumption and losses at different voltage levels and for designing the protection system.

Measurement of the tan delta of winding

This test measures the tan delta (or dissipation factor) of each winding. An AC test source, commonly near 10% of rated voltage, energizes the winding arrangement described above while a bridge circuit or a power analyzer captures the loss angle. Low readings indicate healthy dielectric losses and a strong quality factor; rising values point toward moisture, aging or deterioration.

The measured tan delta values should be corrected for temperature using appropriate formulas or methods. The tan delta values should also be corrected for stray capacitances using appropriate methods.

The measured tan delta values should be compared with the manufacturer’s data or previous test results to detect any abnormality or deviation.

The tan delta of a shunt reactor winding is important for estimating the dielectric losses and heating effect at different voltage levels and frequencies.

Dissolved Gas Analysis of Insulating Oil

This test analyzes the dissolved gases in the insulating oil of an oil-immersed shunt reactor using a gas chromatograph or other instruments. The test is done after sampling the oil from different locations in the tank according to IS 10593 or IEC 60567 standards. The purpose of this test is to check the condition and quality of the oil and to detect any faults or defects in the shunt reactor.

The analyzed dissolved gases consist of various components, such as hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, nitrogen, oxygen, etc. The concentration and ratio of these gases indicate different types of faults or degradation processes in the shunt reactor.

The analyzed dissolved gases should be interpreted using various methods, such as gas ratios, the Duval triangle, the Rogers ratio, etc. The interpretation should indicate the type, location, severity and evolution of the fault or degradation process in the shunt reactor.

The dissolved gas analysis of insulating oil is important for monitoring the condition and health of the shunt reactor and for planning preventive or corrective maintenance actions.

Conclusion

Systematic testing keeps shunt reactors performing reliably inside the grid. Each measurement, from winding resistance through dissolved gas analysis, adds to a picture of the unit’s electrical, thermal, mechanical and dielectric health. Caught early, a drifting reading costs a maintenance visit; missed, it can cost an outage.

For any specific reactor, let the governing standard lead: type results establish what the design can do, routine results certify the individual unit, and special results answer the questions left open. Record every corrected value beside factory data so trends stand out at the next test date.