High Current Test Sets

Testing and calibration of solid-state trip devices can be performed by either primary or secondary current injection method.  Current injection testing is generally required and performed in electrical substations and high power distribution systems which have a high working current and voltage. By injecting a known value current into the circuit breaker, it is possible to know if the relay will operate or not at the test current value. It also indicates the time it will take for the trip to occur.  Besides trip tests, you can also check current transformer ratios and polarities, the correct wiring of the protection system, the proper connection of the protective devices,  overcurrent and earth fault relays, protective relays, test busbars, switchgear, and electrical panels, etc.

 

Primary Current Injection

Primary Injection is the test method of applying an AC current through the primary side of a system. This would mean passing a high current through the primary coil of a current transformer, which in turn is passed through the secondary coil lowering the current to a level that can be handled by the protection device. With this method of testing, you can test breaker contacts, current sensors, wiring, and trip unit for a complete test.   Primary current injection testing is very useful for fault-finding in electrical systems.

 

The test set has an integrated high-current transformer that supplies the simulated overload or fault current. Test sets are built with current ratings ranging from 500 to 100,000 amperes.

 

The primary injection test set consists of large transformers that step down line voltage of say 480V to a very low level, such as 2-15V. The large reduction in voltage allows for a large increase in available current output of around 15kA for a short duration.

 

Primary Injection Test

Acceptance and maintenance tests are conducted in the field using a portable primary current injection test set that is specifically designed to be more compact than factory test equipment.

 

Circuit breakers that have electro-mechanical or thermal-magnetic trip technology can be tested by the primary current injection test method. Circuit breakers that have solid-state trip technology can be tested by the secondary current injection method.

 

Common Primary Injection Tests

 

Power Transformers

For these tests a three-phase source is connected to a short-circuited transformer, to supply current through the windings. The current transformer circuits are then checked to confirm that all the test switches and protective relays are receiving the proper current. The test is set up to include both, the high-side and low-side breakers. The test may also simultaneously test the CT circuits from other breakers connected to the same, or adjacent, bus.

 

Relay Testing

In this test primary faults are simulated to see if protective relays work correctly. The trip times are recorded with 1 ms resolution.  For testing the protection relays this is a good tool.

 

Bus-work, Switchgear, and HV Breakers 

Sufficiently high current is applied to the primary conductors. The CT circuits are then checked for magnitude and angle to verify that all test switches and protective relays receive the proper current at the proper angle, as expected.

 

Low Voltage Breakers 

A very high-level current is applied to the breaker for a short duration, to measure how the trip unit reacts.  The trip unit’s time delay is measured at various current magnitudes to check if it meets the manufacturer’s specifications.

 

Switchgear Testing 

Low voltage switchgear and control gear assemblies require high current testing to comply with the relevant product standards, both by manufacturers and users.

 

Stability Test for Current Transformers

It is possible that during installation the CTs may have been connected with the wrong polarities. In this case, there will be a flow of differential current through the relay and if this value exceeds the setting value then it will trip the breaker. This installation error can be discovered during the primary injection test.

 

Loose Connection

A loose connection can be located if sparking is detected at a connection point while the current is flowing during the primary injection test. However, if a low current is flowing during the test then no spark will be detected. This observation ensures that the CT circuits have proper connections with the cores.

 

Secondary Current Injection

Another method is called the Secondary Current Injection test in which AC current is applied on the secondary side of the CT. This tests only the trip unit logic. However, this test can only be done if you can access the secondary side of the CT.

 

Calibration of the High Current Test Sets

Over time the calibration of the High Current Test Set may go out of specification. Since this is equipment is used to test other safety equipment, its accuracy is of utmost importance.

 

Test equipment should be calibrated once a year, without any consideration of how many times it has been used during that period. Calibration can be performed by an accredited calibration laboratory which can give you a calibration certificate.

 

The national laboratories are responsible for maintaining the national reference standards for accuracy and traceability.  A calibration performed at an accredited laboratory fulfills traceability and accreditation requirements as per industry standards.

 

e2b calibration is an ISO/IEC 17025 accredited calibration laboratory and has the necessary standards to test and calibrate various types of High Current Test Sets. Contact e2b calibration for calibration of your High Current Test Sets.

 

IN-HOUSE VS. OUTSOURCED CALIBRATION

Should you be calibrating your instruments in-house or outsourced?