IEEE GUIDE FOR FIELD TESTING OF RELAYING CURRENT TRANSFORMER

IEEE GUIDE FOR FIELD TESTING OF RELAYING CURRENT TRANSFORMER
C57.13.1 A guide for field testing of relaying current transformer

In the application of protective relays, the most widely used input quantity is current. A multiplicity of different

protective relays either utilizes current directly, combines it with other currents as in differential schemes, or combines it with voltage to make impedance or power measurements. The source of relay input current is from current transformers which may be located on the bushings of power circuit breakers and power transformers, on the bus bars of metal clad switchgear, or installed as separate items of equipment located as required.

Relaying accuracy classes have been established in ANSI/IEEE C57.13-1978, Requirements for Instrument

Transformers, to specify the performance of relaying current transformers. During faults on the electric power system, relaying current transformers must operate at high overcurrent levels. ANSI classiÞcations, therefore, deÞne minimum steady-state performance at these levels. Performance is described by using a two-term identiÞcation system consisting of a letter and a number selected from: (C,T) (10, 20, 50, 100, 200, 400, 800), for example, C400.

Insulation Resistance Tests

Insulation resistance between the current transformer secondary and ground is usually checked by the use of

conventional insulation test instruments. The neutral ground must be removed and the current transformer preferably isolated from its burden for this test. Actually, the neutral can be used to test all three phases simultaneously.

If relays are left connected to the current transformers during the test, the relay manufacturer should be consulted before test values above 500 V are used. Many solid-state relay designs have surge-suppression capacitors connected from input terminals to ground which may be damaged by use of a higher voltage.

Ratio Tests

There are two generally accepted methods of checking the tums ratio of all types of current transformers.

• Voltage Method - A suitable voltage, below saturation, is applied to the secondary (full winding), and the primary voltage is read with a high-impedance (20 000ohm//V or greater) low-range voltmeter. The turns ratio is approximately equal to the voltage ratio. Saturation level is usually about 1 V per turn in most low- and medium-ratio bushing current transformers. High-ratio generator current transformers and window-type current transformers used in metal-clad switchgear may have saturation levels lower than 0.5 V per turn. In the case of very high, ratio current transformers, application of a test voltage with an even lower voltage per turn may be required to avoid personnel hazard and possible damage to equipment. The ANSI relay accuracy class voltage rating should not be exceeded during this test.

• Current Method - This method of determining the turns ratio requires a source of high current, an additional current transformer of known ratio with its own ammeter, and a second ammeter for the transformer under test. Any other current transformers that may be in series with the transformer under test should be short circuited and possibly disconnected from their burdens if damage to other meters or relays, or accidental tripping, is likely. This method is not practical for current transformers in an assembled power transformer or generator. See Section 11 for test methods recommended for them.

Polarity Check

There are three generally accepted methods of checking current transformer polarity.

• DC Voltage Test
• AC Voltage Test - Oscilloscope
• Current Method

Winding and Lead Resistance (Internal Resistance)

 In order to calculate ratio correction for a class C current transformer, its internal resistance and the external
impedance (including secondary lead resistance) must be known. The internal winding and lead resistance can be measured with a resistance bridge. Usually, it is suffcient to use the average value of resistance of the current transformers in the three phases for calculations. If it is desired to separate lead resistance and winding resistance to provide data for other calculations, the resistance of the full winding and of a tap should be measured.

Assuming all tums are of equal resistance, the per-turn resistance and lead resistance can be calculated. All measurements should be made at the current-transformer short-circuiting terminal block. Because of possible remanence, the current transformer should be demagnetized after completion of this test as outlined in 4.3. As previously mentioned, proper safety precautions should be taken when connecting and disconnecting the bridge because of potentially dangerous spike voltages.

Excitation Test

Excitation tests can be made on both C and T class current transformers to permit comparison with published or previously measured data to determine if deviations are present.

To perform the test, an ac test voltage is applied to the secondary winding with the primary open circuited. The voltage applied to the secondary of the current transformer is varied, and the current drawn by the winding at each selected value of voltage is recorded. Readings near the knee of the excitation curve are especially important in plotting a comparison curve. For current transformers with taps, the secondary tap should be selected to assure that the current transformer can be saturated with the test equipment available. The highest tap which can accommodate that requirement should be used.

Burden Measurements

Burden measurements and system short-circuit current provide data for calculating ratio-correction factors for class C current transformers. Using these factors it is possible to analyze relay performance. The total burden of the circuit, which is the sum of the internal current transformer burden and the external connected burden, must be determined.

The internal burden is the resistance of the secondary winding plus the lead resistance from the winding to the shortcircuiting terminal block converted to volt-amperes at rated secondary current. The procedure for measuring internal resistance is described in Section "Winding and Lead Resistance (Internal Resistance)." The external connected burden can either be calculated or measured.

To determine the external connected burden in volt-amperes, measure the voltage required to drive rated current through the connected burden. If both resistive and reactive components of the burden are desired, a suitable phase-angle meter can be connected. Burden measurements, when compared with calculated values, help to conÞrm circuit wiring and satisfactory contact
resistances of terminal blocks and test devices.

The following reminders have been found useful in obtaining correct burden data:
1) To represent in-service burden, the relays and other external devices must be on the correct tap.
2) Parallel current transformers should be disconnected.
3) Phase-to-neutral measurements in relay circuits can be high, particularly if ground relays with sensitive
settings are involved.
4) Phase-to-neutral and phase-to-phase measurements of bus differential circuits can be high because of the
impedance of the differential relay operating coil.

source: C57.13-1 : IEEE GUIDE FOR FIELD TESTING OF RELAYING CURRENT TRANSFORMER