LPIT Electromagnetic Immunity Tests (IEC 61869-6)

Short answer

A low-power instrument transformer (LPIT) with active electronics lives with the disturbances of a digital substation: disconnector switching, lightning, portable radios and magnetic fields from conductors. IEC 61869-6, 2016 edition, lists the immunity tests it must pass and defines how much the equipment may degrade while it withstands them.

The standard separates two types of LPIT. The electronic one, with active components, is tested. The passive one, without them, is not.

Which tests it asks for

Clause 6.11.3.601 summarizes the type tests in Table 602, with the reference standard of each, the test level and the acceptance criterion. Clauses 6.11.3.602 to 6.11.3.614 explain the purpose of each test. These are the thirteen:

Three tests depend on the type of supply. The harmonics test applies only to an LPIT with an AC supply port. The slow variation and dips tests have one version for AC and another for DC.

Table 602 also gives the level of each test. Those levels refer to the IEC 61000-4 series, and for their exact value you have to read the standard’s table. This guide does not repeat them, because what decides the purchase is the acceptance criterion and the port the test applies to.

What the standard says about each risk

The standard explains what each test is for in terms of the substation. For communication and protection equipment, the equivalent is IEC 61850-3 and IEEE 1613. Some examples:

• The surge test is very important at medium and high voltage, because of the likelihood of lightning exposure.
• Fast bursts come from switching of small inductive loads, relay contact bounce and the switching of high-voltage switchgear, particularly SF6 or vacuum.
• Oscillatory waves appear in the low-voltage circuits of medium- and high-voltage stations because of switching, above all of disconnectors on high-voltage busbars.
• Electrostatic discharge is of little concern, because the electronics usually sit on a concrete floor and in well-earthed metal cabinets. It is still tested.
• The power-frequency magnetic field is important because of the expected closeness of the electronics to the main circuits.
• The radiated field matters because of the use of portable radios and mobile phones at the installation.

Acceptance criteria A and B

Table 602 assigns each test one of two criteria.

• Criterion A. Normal operation within accuracy limits, in steady state at rated primary current or voltage or lower.
• Criterion B. A temporary degradation of measurements that are not relevant to protection or self-diagnosis is allowed, as long as the equipment recovers by itself. A reset is not allowed. An output overvoltage above 500 V is not allowed. And in an electronic protection transformer, a degradation that causes false trips is not allowed.

The criteria answer a practical question: what does the relay see while the disturbance occurs. With A it sees nothing different. With B it may see an instant of bad measurement, but never a false trip command.

Ports and subassemblies

Clause 7.2.5.2.601 says the tests are done port by port. The ports the standard identifies are:

• the high-voltage line;
• the enclosure;
• the signal earth;
• the signal;
• the control;
• the communication;
• the AC supply;
• the DC supply.

An LPIT usually has several main subassemblies: circuits in the control cabinet and circuits in the yard or in the switchgear. The tests are done on each, including the merging unit when the LPIT has it built in, with the complete LPIT running or with the missing parts simulated. For more on the electromagnetic compatibility tests for switchgear: electromagnetic compatibility of switchgear.

Figures 605, 606 and 607 show three common structures: an air-insulated high-voltage substation, a medium-voltage switchgear cell and a gas-insulated high-voltage substation. In the air-insulated one there is a subassembly in the yard and another in the control cabinet, joined by a link. In the medium-voltage one there is no galvanic isolation between the switchgear and the cabinet, because everything is in an earthed metal enclosure. In the gas-insulated one, there is also no galvanic isolation between the switchgear and the local control panel.

Transferred overvoltages and emission

Clause 6.11.4 covers transferred overvoltages. The main cause is the switching of high-voltage equipment, and the requirement does not apply if the transmission system is non-conductive, such as an optical fiber.

Clause 6.11.601 adds emission requirements. In addition to the radio interference voltage test and the transferred overvoltage test, the LPIT must meet the limits of CISPR 11:2015 for group 1, class A equipment. Passive LPITs are not subject to this requirement.

What not to do

• Do not ask for the immunity test on a passive LPIT as if it were mandatory.
• Do not accept a report made with the whole device without stating ports and subassemblies.
• Do not confuse criterion B with permission to lose the protection measurement. The standard does not allow it.
• Do not assume the magnetic field near a high-current conductor is covered without checking the power-frequency field test.

Related topics

Electromagnetic immunity relates to insulation and auxiliary power, to faults and reliability and to accuracy with harmonics. Primary equipment is covered in electromagnetic compatibility. The overview is in merging unit and non-conventional instrument transformers.

When to call us

At Tiza Group we support the specification of the digital substation, from the choice of LPIT to how protection behaves under disturbances. We offer IEC 61850 engineering and current and voltage transformer testing, in Colombia or elsewhere on request. If a digital device trips for no apparent reason at your installation, we quote the relay trip analysis, which we can do remotely, worldwide.

Related guides

• Insulation and auxiliary power of a low-power LPIT
• LPIT faults, reliability and temperature
• Accuracy with harmonics and noise in an LPIT
• Merging unit: IEC 61869-9, 9-2LE and testing
• Non-conventional instrument transformers (LPIT)
• Electromagnetic Compatibility of Switchgear (IEC 62271-1)

References

• IEC 61869-6, Instrument transformers - Part 6: Additional general requirements for low-power instrument transformers, edition 1.0 of 2016, clauses 6.11.3.601 to 6.11.3.614 (Table 602), 6.11.4, 6.11.601 and 7.2.5.2.601 (figures 605 to 607).
• IEC 61000-4 series and CISPR 11:2015, to which Table 602 and clause 6.11.601 refer.

Frequently asked questions