IEC 61869-1 sets three electromagnetic compatibility (EMC) requirements for an instrument transformer: the radio interference voltage of the high-voltage side (maximum 2,500 µV at 1.1 times Um/√3, from Um of 123 kV in air-insulated substations), the immunity of parts with active electronics, and the overvoltage transferred from the primary to the secondary, which must not exceed 1.6 kV peak from Um of 72.5 kV.
Short answer
IEC 61869-1, 2007 edition, groups the electromagnetic compatibility (EMC) of an instrument transformer into three requirements. The first limits the radio noise that high-voltage equipment produces. The second requires that active electronics, if present, withstand disturbances. The third limits the overvoltage that passes from the primary to the secondary during a switching operation. Each requirement applies to a different voltage range and is verified with its own test. For the electromagnetic compatibility tests for switchgear, see electromagnetic compatibility of switchgear.
What the standard means by electromagnetic compatibility
The standard defines EMC as the ability of equipment or a system to function satisfactorily in its electromagnetic environment without introducing intolerable disturbances into that environment (clause 6.11.1). For instrument transformers, that definition becomes three specific requirements.
| Requirement | Part or equipment it applies to | Limit value |
|---|---|---|
| Emission: radio interference voltage (RIV) | High-voltage parts; transformers with Um of 123 kV or more in air-insulated substations | 2,500 µV at 1.1 × Um/√3 |
| Immunity | Only parts with active electronic components | Defined in part 9 of the series |
| Transferred overvoltage | Transformers with Um of 72.5 kV or more | 1.6 kV peak on the secondary |
Um is the highest voltage for equipment. It is the value on the rating plate and it defines the insulation level. A conventional instrument transformer, with windings and a core, has no active electronics. For that reason, the immunity requirement does not apply to it.
Radio interference voltage (RIV)
The high-voltage parts of the transformer can emit radio noise. The standard includes the requirement so that installations can comply with electromagnetic compatibility regulations (note to clause 6.11.2).
The limit is 2,500 µV measured at a voltage of 1.1 times Um divided by √3. It applies to transformers with Um of 123 kV or more installed in air-insulated substations. This covers high-voltage and extra-high-voltage equipment in a conventional substation.
This is a type test, according to the list of tests in clause 7.1.2. The manufacturer performs it in the laboratory on a representative unit. Before measuring, the unit receives a pre-stress voltage of 1.5 × Um/√3 for 30 s. The voltage is then lowered to 1.1 × Um/√3 and the measurement is taken. The transformer passes if the radio interference level at that voltage meets the limit (clause 7.2.5.1).
Two clarifications on scope:
- The result is expressed in microvolts and, according to the standard, no atmospheric correction factors are applied. The test is performed with air within a defined range of temperature, pressure and humidity.
- If the transformer is gas-insulated and installed in a GIS, the RIV test and the wet test do not apply (note a to Table 11).
Immunity
The 2007 edition is brief on this point. It states that immunity requirements and tests apply only to parts containing active electronic components, and refers to part 9, which at that time was under study (clauses 6.11.3 and 7.2.5.2).
For an inductive voltage transformer, IEC 61869-3 declares the immunity test not applicable (clause 7.2.5.2 of that part). The same criterion holds for a current transformer with conventional windings. Electronic parts are a matter for low-power instrument transformers and the digital interface, which are covered in other parts of the series.
Transferred overvoltage to the secondary
Switching operations in a substation produce fast voltage oscillations. Part of them passes from the primary to the secondary terminals, where electronic relays and meters are connected. The standard sets a limit to protect that electronics: the transferred overvoltage must not exceed 1.6 kV peak (clause 6.11.4 and Table 9).
The requirement applies to transformers with Um of 72.5 kV or more. The standard defines two impulse shapes, which represent switching oscillations:
| Type A impulse | Type B impulse | |
|---|---|---|
| Installation | Air-insulated substation | Gas-insulated switchgear (GIS) substation |
| Front time | 0.50 µs ± 20 % | 10 ns ± 20 % |
| Tail | 50 µs or more (time to half value) | More than 100 ns |
| Secondary limit | 1.6 kV peak | 1.6 kV peak |
It is a special test (clause 7.4.4). That means it is neither a type test nor a routine test: it is performed only if manufacturer and purchaser agree. Table 11 indicates that this test is performed at reduced pressure in gas-insulated equipment, not with the service gas.
The test applies a low-voltage impulse between a primary terminal and earth. The voltage that appears on the open secondary is measured with a 50 Ω coaxial cable and an oscilloscope with a bandwidth of 100 MHz or more. The transferred voltage is calculated from the ratio between the measured and applied voltage, scaled to the impulse specified on the primary. If the transformer has several secondaries, each one is measured. If a secondary has intermediate taps, only the full-winding tap is measured.
The transformer passes if the transferred voltage does not exceed the limit in Table 9.
What it means for a project
- Specification. If the substation has electronic relays and meters near the yard, request the transferred overvoltage special test at purchase. If it is not agreed, the manufacturer has no obligation to perform it.
- GIS or air-insulated. The impulse type depends on where the unit is installed. A transformer for GIS is tested with the type B impulse, which has a much shorter front.
- Receiving inspection. The type test report must include the RIV if the unit is 123 kV or more for air-insulated service. The special test report exists only if it was requested.
- Tested unit. Check that the delivered unit is of the same type as the one tested. The standard allows minor design deviations only if manufacturer and purchaser agree.
What not to do
- Do not require RIV for a gas-insulated transformer installed in a GIS: the standard does not apply it.
- Do not assume the transferred overvoltage test was performed. It is a special test and requires agreement.
- Do not confuse the 2,500 µV RIV with the 1.6 kV transferred overvoltage. They are different requirements, with different units and different purposes.
- Do not treat the immunity of a conventional transformer as if it were that of one with electronics: a different part of the standard applies.
Related topics
Electromagnetic compatibility is complemented by partial discharge testing and by impulse tests. For low-power equipment, see the electromagnetic immunity of an LPIT. The general guides on voltage transformers and current transformers summarize the rest of the equipment.
When to contact us
At Tiza Group we verify instrument transformer test reports at receiving and check them in the field at commissioning. If you need to review which special tests are worth agreeing for your substation, see our testing and commissioning service, available in Colombia, or elsewhere on request. Write to us and we will send you a quote.
Related guides
- Partial discharge in instrument transformers
- Impulse tests on instrument transformers
- Electromagnetic immunity of a low-power LPIT
- Inductive and capacitive voltage transformer: tests
- Current transformer (CT): parts, functions and tests
- Electromagnetic Compatibility of Switchgear (IEC 62271-1)
References
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition: clauses 6.11, 7.1.2, 7.2.5, 7.4.4, Table 9 and Table 11.
- IEC 61869-3, Instrument transformers - Part 3: Additional requirements for inductive voltage transformers, 2011 edition: clause 7.2.5.2.
- IEC 61869-9, Instrument transformers - Part 9: Digital interface for instrument transformers, cited by part 1 for immunity.
Frequently asked questions
Must every instrument transformer meet the 2,500 µV radio interference limit?
No. IEC 61869-1 applies the limit to transformers with Um of 123 kV or more installed in air-insulated substations. For a gas-insulated transformer installed in a GIS, the standard states that the RIV test does not apply.
Is the transferred overvoltage test mandatory?
It is a special test. The standard classifies it that way: it is performed only if manufacturer and purchaser agree. The 1.6 kV peak requirement exists for transformers with Um of 72.5 kV or more, but verifying it depends on that agreement.
What is the difference between type A and type B impulse in the transferred overvoltage test?
Type A, with a 0.50 µs front time, represents a transformer in an air-insulated substation. Type B, with a 10 ns front time, represents a transformer installed in a gas-insulated switchgear (GIS) substation. The secondary limit is the same for both: 1.6 kV peak.