Substation including generators and parallel operation of transformers

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Only generators connected at MV level are considered in this chapter.

Generators in stand-alone operation, not working in parallel with the supply network

When the installation needs a high level of power availability, one or several MV standby generator set can be used.

In all the stand alone applications the installation includes an automatic changeover able to switch from the utility network supply to the generator(s) in case of failure of the utility supply (see Fig. B45).

Fig. B45 – Automatic change over associated with stand-alone generators

The generators are protected by dedicated protections. For medium size generators the following protections are usually used:

  • Phase to phase and phase to earth over current
  • Percentage biased Differential
  • Negative sequence over current
  • Overload
  • Stator frame fault
  • Rotor frame fault
  • Reverse active power
  • Reverse reactive power or loss of field
  • Loss of synchronization
  • Over and under voltage
  • Over and under frequency
  • Overheating of bearings.

It shall be noted that, due to the very low short-circuit current of the generator(s) compared to the one delivered from the utility supply network, a great attention must be paid to the settings of the protection and the selectivity. It is recommended when ordering a generator(s) to check with the manufacturer its (their) ability in providing a short circuit current ensuring the operation of the phase to phase short circuit protection. In case of difficulties the boosting of the generator’s excitation is required and shall be specified.

Voltage and frequency control

The voltage and the frequency are controlled by the primary regulator(s) of the generator(s). The frequency is controlled by the speed regulator(s), while the voltage is controlled by the excitation regulator(s).

When several generators operate in parallel an additional control loop is required to perform the sharing of the active and reactive power between the generators.

The principle of operation is the following:

  • The active power delivered by a generator increase when the driven machine is accelerated and vice versa
  • The reactive power delivered by a generator increase when its excitation current is increase and vice versa.

Dedicated modules are installed to perform these sharing, generally ensuring other tasks such as the automatic synchronization and coupling of the generators (see Fig. B46).

Generators operating in parallel with the utility supply network

When one or several generators are intended to operate in parallel with the supply network the agreement of the utility is usually required. The utility specifies the conditions of operation of the generators and specific requirements may be asked.

The utility generally requires information concerning the generators, such as:

  • Level of the short circuit current injected by the generators in case of fault on the supply network
  • Maximum active power intended to be injected in the supply network
  • Operation principle of the voltage control
  • Capability of the generators to control the power factor of the installation.

In case of fault on the utility supply network, the instantaneous disconnection of the generators is generally required. It is achieve by means of a dedicated protection specified by the utility. This protection may operate according to one or several of the following criteria:

  • Under-voltage and over-voltage
  • Under-frequency and over-frequency
  • Zero sequence overvoltage

The protection generally orders the tripping of the main circuit breaker ensuring the connection of the installation to the utility while the generators continue to supply the totality of the internal consumers or a part only if they are not sized for the full power required (see Fig. B33). In this case load sheading must be simultaneously executed with the tripping of the main circuit breaker.


When generators at a consumer’s substation operate in island mode (Utility power supply disconnected) the voltage and the frequency at the main substation level are both fixed by the generators and consequently the control system of the generators operate in Voltage/Frequency mode (see Fig. B46).

When the utility power supply is connected the voltage and the frequency are both fixed by the utility and the control system of the generators must be switched from Voltage/Frequency mode (V/F control mode) to Active power/Reactive power mode (P/Q control mode) (see Fig. B46).

The function of the P/Q control mode is to control the exchanges of the active and reactive power with the utility. The typical principle of operation used in most of the applications is the following:

  • The amount of the active and reactive power exchanged with the utility are set by the operator. The settings may be specified by the utility
  • The control system maintains the values of the exchange at the required values by acting on the speed of the generators for the control of active power and on the excitation current for the control of the reactive power
  • The sharing of the active and reactive power between the generators remains in operation.

The P/Q control mode allows:

  • To strictly limit the value of the active power imported from the utility at the amount which can’t be provided by the generators when the demand of the installation exceed their capability.
  • To maintain at zero the imported active power, when the demand of the installation remains below the capability of the generators
  • To maintain the power factor of the installation at the contractual value specified by the utility.

When the capability of the generators in providing reactive power is exceeded, the additional reactive power required to comply with the contractual power factor shall be supplied by a dedicated capacitor bank.

Fig. B46 – Control of generators operating in parallel with the utility supply network

Parallel operation of transformers

The need to operate two or more transformers in parallel may be required when:

  • The level of security of supply to be guarantied requires to duplicate the sources of supply
  • The capacity of an existing transformer is exceeded due to the extension of the installation
  • A single large transformer cannot be installed due to the lack of space
  • The standardisation of the transformers throughout the installation is required.

It is not recommended to connect more than two transformers in parallel because the short circuit current at low voltage level may become too high.

Total power (kVA)

The total power (kVA) available when two or more transformers are connected in parallel, is equal to the sum of the individual transformer’s ratings.

Transformers of equal power rating will each provide a load equal to the total load provided to the installation, divided by the number of transformers working in parallel.

Transformers of unequal power ratings will share the load in proportion to their ratings, providing that their voltage ratios and their short circuit impedances are identical.

Necessary conditions for parallel operation

The following conditions for the connection of power transformers in parallel are required:

It is preferred to connect in parallel transformers having the same characteristics:

  • Same voltage ratio
  • Same rated power
  • Same short circuit impedance.
  • Same coupling symbol of windings as for example D yn 11
  • Same impedances of the LV links between the transformers and the main LV switchboard where the paralleling is realized.

For transformers having unequal rated power their internal impedances are in the ratio of the rated power of the transformers.

Connection in parallel of transformers having a power ratio equal or higher than two is not recommended.

When the transformers do not comply with the above requirements, recommendations for their paralleling shall be asked to the manufacturer.