The arc suppression coil is a single-phase reactor between the network neutral and earth that compensates the capacitive current of a single-phase-to-earth fault. Clause 11 of IEC 60076-6 defines it as a linear reactor up to 1.1 times rated voltage, with adjustable inductance, and sets its rated quantities, temperature-rise limits, tests and tolerances.
What it is and what it does
An arc suppression coil is a single-phase reactor connected between the neutral of the power transformer, or of an earthing transformer, and earth. It compensates the capacitive current that appears in a single-phase-to-earth fault in a network with resonant earthing (IEC 60076-6, clauses 3.1.11 and 11.1). It is also known as a Petersen coil or ground fault neutralizer. On the reactor types and what each one is for: reactor types per IEC 60076-6.
In a single-phase-to-earth fault a capacitive current flows. The coil supplies an inductive current that compensates it. That is why its inductance is tuned to the capacitance of the network (clause 11.2). An overview of neutral earthing methods is in isolated, resonant and solidly earthed neutral. For more on the reactors that limit fault current or connect the neutral to earth: current-limiting and neutral earthing reactors.
How it is built
Clause 11.2 describes the usual design.
- It is a liquid-immersed, naturally cooled reactor, for indoor or outdoor use.
- It has adjustable inductance, in steps or continuously, over a specified range to tune it to the network capacitance.
- It is a linear reactor.
- It can have an auxiliary measuring winding and a secondary winding to connect a loading resistor.
When the earthing transformer and the coil share a tank, that combination is treated in clause 10 of the same standard. See earthing transformer.
Clause 11.4.5 admits these ways of adjusting the current.
- Adding sections of the main winding in steps, with an on-load or off-circuit tap changer. For this case an adjustment range of no more than 2.5 is recommended.
- Reducing the air gap of the magnetic circuit by mechanical means.
- Connecting or disconnecting coils in a set designed to operate in parallel.
Rated quantities
| Quantity | What it is | Clause |
|---|---|---|
| Rated voltage Ur | Voltage across the terminals of the main winding. Set by the purchaser, not lower than the highest neutral-to-earth voltage during an earth fault. It is usually the phase-to-neutral voltage of the network | 11.3.1, 11.4.1 |
| Maximum continuous voltage Umax | Not lower than the voltage that appears at the neutral due to network unbalance in normal operation. If not specified, 10 % of Ur is taken | 11.3.2, 11.4.2 |
| Rated current Ir | Current through the main winding with Ur applied. In an adjustable coil it corresponds to the lowest-inductance setting. Not lower than the highest earth-fault current | 11.3.3, 11.4.3 |
| Duration of rated current Tr | Not shorter than the maximum expected duration of an earth fault, or continuous. With successive faults, the purchaser states the intervals and the number | 11.3.4, 11.4.4 |
| Tuning range | Ratio between the rated current and the current at rated voltage with the highest-inductance setting | 11.3.5 |
| Auxiliary winding | For measurement and control, low voltage and low current. If required, current, voltage and tolerances are specified | 11.3.6, 11.4.6 |
| Secondary winding | To connect a short-time service resistor that increases the resistive component of the fault current | 11.3.7, 11.4.7 |
The standard gives usage references for the windings: a typical auxiliary winding of 100 V and 1 A, and a typical secondary winding of 500 V and 100 A. These are examples, not mandatory values.
Linearity
The coil must be linear, within the tolerance of clause 11.9, up to 1.1 times rated voltage unless otherwise indicated (clause 11.4.8). It is verified with a special test (clause 11.8.10). Voltage is applied in steps of approximately 10 % up to 1.1 Ur. A curve is plotted from the r.m.s. values of voltage and current. The current measured at any point must not deviate more than ±5 % from the straight line from the origin to the rated voltage point. In an adjustable coil, the test is made at the maximum and minimum current settings.
Temperature rise
Temperature rises are calculated starting from the value produced by the maximum continuous voltage. From there the effect of the rated current is added (clause 11.5). The limits for average winding temperature rise and top-oil temperature rise at rated current are:
| Duration of rated current | Windings | Oil |
|---|---|---|
| Continuous | 80 K | 75 K |
| 2 h or less | 100 K | 90 K |
The standard explains that these values take into account that earth faults are infrequent and of limited duration.
If a short-time load on the secondary winding is specified, up to 10 s, its temperature must not exceed the values of IEC 60076-5, clause 4.1.4, for transformer windings under short circuit. The oil must not rise more than 90 K.
Insulation
Unless otherwise indicated, the insulation level is that of the neutrals of the network transformers. The terminal that goes to earth admits a lower level, with non-uniform insulation (clause 11.6).
Tests
| Group | Tests |
|---|---|
| Routine | Winding resistance; current; no-load voltage of the auxiliary and secondary windings; dielectric tests of the main winding; applied voltage on the auxiliary winding, the secondary winding and the wiring; operation of the changer, the air-gap mechanism and other switching equipment |
| Type | Current at rated voltage; temperature rise; dielectric tests |
| Special (on request) | Losses; linearity up to 1.1 Ur; sound level; endurance tests of the tuning mechanism; demonstration of the ability to withstand the dynamic stresses of rated current |
Current measurement
As a type test, current is measured at rated voltage and frequency. As a routine test, it can be measured at a lower voltage and corrected to rated. In both cases it is measured over the whole tuning range: at each step if adjustment is in steps, and at no fewer than five evenly spaced settings if continuous (clause 11.8.5).
Dielectric tests
They include separate-source AC withstand voltage, induced voltage and lightning impulse (clause 11.8.8). The impulse is applied at the terminal that goes to the neutral. A front time of up to 13 µs is accepted. If the winding has taps, the impulse is repeated at the maximum and minimum taps. If the induced voltage test is impractical, it can be replaced by a lightning impulse test, by agreement in the order.
Dynamic stresses
They are demonstrated by test or by reference to tests on similar units, per IEC 60076-5, clause 4.2. Unless otherwise indicated, two tests of 0.5 s (± 0.05 s) are made with the coil at the rated-current setting. The first peak is the rated current multiplied by the factor k of IEC 60076-5, with a minimum of 2.55 (clause 11.8.13). For the rated quantities and tests of a shunt reactor, see shunt reactor: rated quantities and tests.
Tuning mechanism
If the coil has a mechanism to adjust inductance, the purchaser can request endurance tests (clause 11.8.12). As an example, the standard mentions 1 000 adjustment operations over the whole range, at an agreed ambient temperature.
Tolerances
| Quantity | Tolerance | Clause |
|---|---|---|
| Main winding current at minimum inductance and rated voltage | ±5 % of the rated value | 11.9 |
| Current at the other settings | ±10 % of the specified values | 11.9 |
| No-load voltage of the auxiliary and secondary windings, over the whole range | ±10 % of the specified values | 11.9 |
| Linearity | ±5 % | 11.9 |
When you need support
The coil is sized with a study of the network: capacitance, fault current and tuning range. At Tiza Group we carry out interconnection and system earthing studies and transformer and reactor testing, the latter in Colombia or elsewhere on request. For the associated protection, see neutral overvoltage protection 59N.
Related guides
- Isolated, resonant and solidly earthed neutral
- Earthing transformer
- Neutral overvoltage protection 59N
- Power reactor
- Reactor Types per IEC 60076-6 and What Each Is For
- Shunt Reactor: Rated Quantities and Tests
- Current-Limiting and Neutral Earthing Reactors
References
- IEC 60076-6: power transformers, reactors (clauses 3.1.11, 10 and 11).
- IEC 60076-5: ability to withstand short circuit (clauses 4.1.4 and 4.2).
- IEC 60076-3: insulation levels and dielectric tests.
Frequently asked questions
What rated current must an arc suppression coil have?
Not lower than the highest line-to-earth fault current expected. The purchaser sets it (clause 11.4.3). If the inductance is adjustable, the rated current corresponds by definition to the lowest-inductance setting, unless the purchaser indicates another setting (clause 11.3.3).
Why must it be linear?
Because its function is to compensate the capacitive current of the network. If the core saturates, the current stops being proportional to voltage and compensation loses accuracy. The standard requires the coil to be linear within ±5 % up to 1.1 times rated voltage (clauses 11.4.8 and 11.8.10).
What durations does the standard use for the rated current?
The purchaser must set a duration not shorter than the maximum expected duration of an earth fault. The note to clause 11.4.4 gives 10 s, 30 min, 2 h and continuous as commonly used durations. For durations over 2 h, continuous is usually specified.