Generator-Source Fault Current and Delayed Current Zeros

Short answer

In a short circuit fed only by the generator, the AC component decays faster than the DC one. If the degree of asymmetry exceeds 100 %, the current does not cross zero for a time. IEC/IEEE 62271-37-013 requires demonstrating by calculation, with the breaker's arc voltage, that it forces a current zero within its maximum arcing time.

Short answer

A short circuit between the generator and its step-up transformer is fed only by the generator, with no transformation in between. In that case, the AC component of the current decays quickly, while the DC component decays more slowly. If the DC component becomes larger than the peak of the AC component, the current stops crossing zero for a time and a circuit-breaker cannot interrupt it during that interval. IEC/IEEE 62271-37-013, 2015 edition, deals with this phenomenon in clauses 4.101.3 and 8.103.6.3.6.

What the standard defines

Clause 4.101.3.1 calls the generator-source rated breaking current the highest short-circuit current fed by a generator, at the instant of contact separation, that the breaker must be able to interrupt. That current is sought in a circuit with a power-frequency recovery voltage equal to the rated voltage of the breaker and with the transient recovery voltage of clause 4.105. For three-pole breakers, the AC component corresponds to a three-phase-to-ground short circuit.

Two values characterize it:

the rms value of its AC component at contact separation, Iscg;
the degree of asymmetry at contact separation, Asycs.

The standard adds three clarifications:

if the degree of asymmetry exceeds 100 %, there are delayed current zeros;
if it does not exceed 20 %, the current is characterized only by the rms value of its AC component;
at voltages equal to or lower than the rated voltage the breaker must be able to interrupt that current, but above the rated voltage none is guaranteed.

The degree of asymmetry (clause 3.7.144) is the ratio in percent between the DC component and the peak value of the AC component, taken from the envelope of the wave. It equals 100 % when both are equal.

Why the current does not cross zero

Clause 8.103.6.3.6.2 explains the origin. A short-circuit current flows if the breaker closes onto a short circuit, or if there is an arc between at least two phases or between phases and ground. If the fault starts at the voltage zero crossing of one phase, the current in that phase has the maximum degree of asymmetry.

Both components decay exponentially, but with different time constants:

Component	How it decays	Time constants
AC	With the subtransient and transient time constants of the generator (τ”d, τ”q, τ’d, τ’q)	τ”d and τ”q of 25 to 45 ms; τ’d of 0.8 to 1.5 s; τ’q of 250 to 400 ms
DC	With the armature time constant, τa = X”d / (2π f Ra)	τa of 150 to 400 ms

These are typical values given by the standard. They vary over a wide range depending on the size and design of the generator. If the AC component decays faster than the DC one, delayed current zeros appear for some time.

When it is worst: underexcited generator

Clause 8.103.6.3.6.1 states that the highest asymmetry occurs when, before the fault, the generator operates underexcited at a leading power factor. In that condition, the DC component can be larger than the peak of the AC one. The AC component of the fault is then lower than the rated one.

If, on the other hand, the generator delivers power at a lagging power factor, the asymmetry is lower and the AC component is higher.

The analysis of many generators gave a degree of asymmetry of about 130 %, with an AC component of only 74 % of the rated one. This is where the two asymmetry values of class G1 of the generator circuit-breaker come from. The standard recognizes that the asymmetry can exceed 130 %.

How the current zero is forced

Clause 8.103.6.3.6.3 describes the mechanism: an additional resistance in series with the armature resistance makes the DC component decay faster. That additional resistance can be:

the connection of the generator up to the fault point;
the resistance of the arc at the fault location, if there is one;
the resistance of the breaker arc after contact separation.

Normally these values are enough for the DC component to fall quickly and current zeros to appear. The standard gives an example in which, within one cycle after contact separation, there are already current zeros. The breaker’s arc voltage suddenly changes the DC decay at the instant of separation.

How a current zero is reached in a generator-source fault

Generator: Generator-source short circuit. The AC component decays faster than the DC one
Generator: Asymmetry above 100 %. The current does not cross zero
Breaker: Contact separation. The arc voltage appears
Breaker: Series resistance increases. The DC component decays faster
Breaker: Current zero. The breaker extinguishes the arc

How the capability is demonstrated

Testing a current with delayed current zeros under real conditions in a high-power test station can be difficult (clause 8.103.6.3.6.4). That is why the capability to interrupt it is verified by calculation, using results of the generator-source short-circuit tests (clause 6.105.12). If three-phase tests are made in the station, the conditions of clause 6.105 are followed.

The calculation follows these rules:

it starts from the arc voltage versus current characteristic of a breaker interruption, taken from interruption tests;
that characteristic is converted into a non-linear, time-varying arc resistance, which is introduced into the simulation at the instant of contact separation;
the technical data of the actual generator are used;
two three-phase fault cases are studied: start at the voltage zero of one phase, with maximum asymmetry, and start at the voltage maximum, with symmetrical current;
for each one, the generator at no load with the breaker closing onto the fault, and the generator in service at a leading power factor with an arc in at least two phases.

The capability is demonstrated if the breaker forces the current to zero within the maximum permitted arcing time, that is, the interval in which it can interrupt a current.

How it influences the choice of breaker

Clause 8.103.6.3.7 gives a guide for choosing the class. The AC component and the asymmetrical current of a three-phase fault between the breaker and the transformer are calculated, with the generator loaded at lagging and with the generator at no load, and the requirements are met at the same time. The calculated degree of asymmetry, below 110 %, between 110 % and 130 % or above 130 %, defines which class is suitable and which calculation with the arc voltage must be presented.

The generator circuit-breaker classes are explained in generator circuit-breaker: classes G1 and G2. Current asymmetry relates to the rated short-time withstand current and peak withstand current. The analysis of a generator trip is in fault analysis in generators.

The other parts of IEC 62271 are arranged in the standards for primary equipment testing.

What data a study needs

The generator reactances and time constants: subtransient, transient, armature and open-circuit ones.
The armature resistance, which sets the DC time constant.
The prior operating point: load and power factor, and whether the generator can operate underexcited.
The expected instant of contact separation of the breaker.
The arc voltage characteristic of the breaker, supplied by the manufacturer.

With an oscillography record of an actual fault you can review the asymmetry with the COMTRADE viewer and the guide on fault analysis with oscillography records.

When to call us

At Tiza Group we calculate the short-circuit currents of your plant and verify that the generator circuit-breaker and the protection match them. See our engineering studies and primary engineering. Write to us and we will send you a quote.

References

IEC/IEEE 62271-37-013, High-voltage switchgear and controlgear - Part 37-013: Alternating-current generator circuit-breakers, 2015 edition, clauses 3.7.144, 4.101.3 and 8.103.6.3.6 to 8.103.6.3.7.

Frequently asked questions

What are delayed current zeros?

It is the condition in which the short-circuit current does not cross zero for a period, because the DC component exceeds the peak of the AC component. The standard associates it with a degree of asymmetry above 100 %. Without a current zero, a circuit-breaker cannot extinguish the arc until the DC component decays or the arc voltage forces the crossing.

When is the asymmetry of a generator fault highest?

The standard states that the highest value occurs when, before the fault, the generator operates underexcited at a leading power factor. In that case the AC component of the fault is lower than the rated one. With a lagging load, the asymmetry is lower but the AC component is higher.

Is the current with delayed current zeros tested in the laboratory?

Not always. The standard recognizes that it can be difficult to demonstrate in high-power test stations. That is why the capability to interrupt it is verified by calculation, with the breaker's arc voltage measured in short-circuit tests, and it is considered demonstrated if the breaker forces a current zero within its maximum permitted arcing time.