Generator Set parallel-connection

From Electrical Installation Guide

Parallel-connection of the generator set irrespective of the application type - Safety source, Replacement source or Production source - requires finer management of connection, i.e. additional monitoring functions.

Parallel operation

As generator sets generate energy in parallel on the same load, they must be synchronised properly (voltage, frequency) and load distribution must be balanced properly. This function is performed by the regulator of each Generator Set (thermal and excitation regulation). The parameters (frequency, voltage) are monitored before connection: if the values of these parameters are correct, connection can take place.

Insulation faults

(see Fig. N12)

Fig. N12 – Insulation fault inside a generator

An insulation fault inside the metal casing of a generator set may seriously damage the generator of this set if the latter resembles a phase-to-neutral short-circuit. The fault must be detected and eliminated quickly, else the other generators will generate energy in the fault and trip on overload: installation continuity of supply will no longer be guaranteed. Ground Fault Protection (GFP) built into the generator circuit is used to:

  • Quickly disconnect the faulty generator and preserve continuity of supply
  • Act at the faulty generator control circuits to stop it and reduce the risk of damage

This GFP is of the “Residual Sensing” type and must be installed as close as possible to the protection device as per a TN-C/TN-S[1] system at each generator set with grounding of frames by a separate PE. This kind of protection is usually called “Restricted Earth Fault”.

Generator Set operating as a load

(see Fig. N13 and Fig. N14 )

One of the parallel-connected generator sets may no longer operate as a generator but as a motor (by loss of its excitation for example). This may generate overloading of the other generator set(s) and thus place the electrical installation out of operation.

To check that the generator set really is supplying the installation with power (operation as a generator), the proper flow direction of energy on the coupling busbar must be checked using a specific “reverse power” check. Should a fault occur, i.e. the set operates as a motor, this function will eliminate the faulty set.

Fig. N13 – Energy transfer direction – Generator Set as a generator
Fig. N14 – Energy transfer direction – Generator Set as a load

Grounding parallel-connected Generator Sets

Grounding of connected generator sets may lead to circulation of earth fault currents (triplen harmonics) by connection of neutrals for common grounding (grounding system of the TN or TT type). Consequently, to prevent these currents from flowing between the generator sets, we recommend the installation of a decoupling resistance in the grounding circuit.


  1. ^ The system is in TN-C for sets seen as the “generator” and in TN-S for sets seen as “loads”