Generator protection

From Electrical Installation Guide

Figure N2 below shows the electrical sizing parameters of a Generator Set. Pn, Un and In are, respectively, the power of the thermal motor, the rated voltage and the rated current of the generator.

Fig. N2 – Block diagram of a generator set

Overload protection

The generator protection curve must be analysed (see Fig. N3).

Standards and requirements of applications can also stipulate specific overload conditions. For example:

I/In t
1.1 > 1 h
1.5 30 s

The setting possibilities of the overload protection devices (or Long Time Delay) will closely follow these requirements.

Note on overloads

  • For economic reasons, the thermal motor of a replacement set may be strictly sized for its nominal power. If there is an active power overload, the diesel motor will stall. The active power balance of the priority loads must take this into account
  • A production set must be able to withstand operating overloads:
    • One hour overload
    • One hour 10% overload every 12 hours (Prime Power)
Fig. N3 – Example of an overload curve t = f(I/In)

Short-circuit current protection

Making the short-circuit current

The short-circuit current is the sum:

  • Of an aperiodic current
  • Of a damped sinusoidal current

The short-circuit current equation shows that it is composed of three successive phases (see Fig. N4).

Fig. N4 – Short-circuit current level during the 3 phases

Subtransient phase

When a short-circuit appears at the terminals of a generator, the current is first made at a relatively high value of around 6 to 12 In during the first cycle (0 to 20 ms).

The amplitude of the short-circuit output current is defined by three parameters:

  • The subtransient reactance of the generator
  • The level of excitation prior to the time of the fault and
  • The impedance of the faulty circuit.

The short-circuit impedance of the generator to be considered is the subtransient reactance [math]\displaystyle{ x_d^{''} }[/math] expressed in % by the manufacturer. The typical value is 10 to 15%.

We determine the subtransient short-circuit impedance of the generator:

[math]\displaystyle{ X_d^{''}(ohms) = \frac {{U_n}^2 \cdot x_d^{''}} {100\ S} }[/math]


[math]\displaystyle{ S = \sqrt3\, U_n\ I_n }[/math]

Transient phase

The transient phase is placed 100 to 500 ms after the time of the fault. Starting from the value of the fault current of the subtransient period, the current drops to 1.5 to 2 times the current In.

The short-circuit impedance to be considered for this period is the transient reactance x’d expressed in % by the manufacturer. The typical value is 20 to 30%.

Steady state phase

The steady state occurs after 500 ms.When the fault persists, the output voltage collapses and the exciter regulation seeks to raise this output voltage. The result is a stabilised sustained short-circuit current:

  • If generator excitation does not increase during a short-circuit (no field overexcitation) but is maintained at the level preceding the fault, the current stabilises at a value that is given by the synchronous reactance Xd of the generator. The typical value of xd is greater than 200%. Consequently, the final current will be less than the full-load current of the generator, normally around 0.5 In.
  • If the generator is equipped with maximum field excitation (field overriding) or with compound excitation, the excitation “surge” voltage will cause the fault current to increase for 10 seconds, normally to 2 to 3 times the full-load current of the generator

Calculating the short-circuit current

Manufacturers normally specify the impedance values and time constants required for analysis of operation in transient or steady state conditions (see Fig. N5).

Fig. N5 – Example of impedance table (in %)
(kVA) 75 200 400 800 1,600 2,500
x”d 10.5 10.4 12.9 10.5 18.8 19.1
x’d 21 15.6 19.4 18 33.8 30.2
xd 280 291 358 280 404 292

Resistances are always negligible compared with reactances. The parameters for the short-circuit current study are:

  • Value of the short-circuit current at generator terminals

Short-circuit current amplitude in transient conditions as per CLC/TR 50480[1] is:

[math]\displaystyle{ I_{sc3}=\frac{U_n}{X_d^{'}}\cdot\frac{1}{\sqrt 3} }[/math]   ([math]\displaystyle{ X_d^{'} }[/math] in ohms )


[math]\displaystyle{ I_{sc3}=\frac{I_n}{x_d^{'}}\cdot 100 }[/math]   ([math]\displaystyle{ x_d^{'} }[/math] in % )

Un is the generator phase-to-phase output voltage.

Note: This value can be compared with the short-circuit current at the terminals of a transformer. Thus, for the same power, currents in event of a short-circuit close to a generator will be 5 to 6 times weaker than those that may occur with a transformer (main source).

This difference is accentuated still further by the fact that generator set power is normally less than that of the transformer (see Fig. N6).

Fig. N6 – Example of a priority services switchboard supplied (in an emergency) from a standby generator set

When the LV network is supplied by the Main source 1 of 2,000 kVA, the short-circuit current is 42 kA at the main LV board busbar. When the LV network is supplied by the Replacement Source 2 of 500 kVA with transient reactance of 30%, the short-circuit current is made at approx. 2.5 kA, i.e. at a value 16 times weaker than with the Main source.


  1. ^ European Technical report CLC/TR 50480 "Determination of cross-sectional area of conductors and selection of protective devices" proposes this type of calculation for cable sizing and protection according to IEC 60364-4-43.
    Please note that the right method shall be selected according to the objective of the calculation: first peak estimation for electro-dynamic withstand or making capacity, first period rms value for breaking capacity of overcurrent protective device, steady state current for minimum earth fault calculation …