Paralleling of alternators – Methods and Troubleshooting

Paralleling of alternators is required to share total load that exceeds the capacity of the single machine, to reduce operating cost and cost of energy generation, to ensure greater security of supply. Especially during critical operations like cargo operation, tank cleaning, manoeuvering …

  • The changeover of main and standby generator unit require a brief synchronization period to achieve a smooth transition without blackout situation
  • The Alternators already connected to bus-bar is called running machine and generator which has to be brought into service is called incoming machine.

Paralleling of alternators can be done in auto as well as in manual mode with the help of

  1. Synchroscope
  2. Lamp methods
  3. Voltmeter (Analog type)

If we need to operate the alternator in parallel we need to synchronize the alternators

Conditions required for synchronizing

The incoming machine must have equal

  1.  Line voltage

    Synchronizing Panel Arrangements
    Synchronizing Panel Arrangements

  2. Frequency
  3. Phase sequence
  4. Phase angle
  5. Waveform

to that of the bus-bar which it is being synchronized.

The condition of the same waveform (all alternators should have the same sinusoidal waveform) and phase sequence (RYB terminals of an alternator outlet is connected with RYB of the main bus bar) is fixed by the construction of the alternator and can be checked by the phasing out test during the synchronizing of the alternator. So Voltage, Frequency & Phase angle must be controlled each time an alternator is to be connected to the bus-bar.


Syncroscope Method of Paralleling of alternator
Synchroscope Method of Paralleling of the alternator


  • Consists of a small motor with coils on the two poles connected across any two phases of the incoming machine and armature winding supplied from the same two phases of the switchboard bus bar
  • The bus bar circuit consists of an inductance coil and resistance connected in parallel. Inductor circuit delays the current by 90 degrees relative to the current in resistance.
  • These two currents are feed into the synchroscope with the help of slip rings to the armature winding which produces a rotating magnetic field


  • The polarity of the poles will change alternatively in the north-south direction with the change in the phases of the incoming generator.
  • The rotating field will react with the poles by turning the rotor to either in clockwise or anticlockwise direction.
  • If the incoming machine field is strong (means running faster) then the rotor will move in clockwise direction.
  • If the incoming machine field is weaker (means running slower) then the rotor will move in an anticlockwise direction
  • Generally, it is preferred to adjust the incoming machine slightly higher than the bus-bar which will move the pointer of the synchroscope in the clockwise direction. The breaker is closed just before the pointer reaches 12 o’clock position at which the incoming machine is in phase with the bus-bar.

Lamp Method

Lamp Method of Paralleling of alternator
Lamp Method of Paralleling of the alternator

In this method, three lamps are used. but they have deliberately connected asymmetrically (for Bright lamp method), as shown in fig. and symmetrically (for Dark lamp method – they would dark out or glow up simultaneously because the phase rotation is same as that of the bus- bar)

This transposition of two lamps, suggested by Siemens & Halske helps to indicate (apparent direction of rotation of lamp) whether the incoming machine is slow (anticlockwise) or fast (clockwise). The synchronization is done at the moment the uncrossed lamp is dark, other two crossed lamps are dimly but equally bright. Hence, this method of synchronizing is also known as “Two Bright lamps & One Dark” methods.

It should be noted that synchronization by the lamp is not quite accurate because to a large extent, it depends on the sense of the correct judgment of the operator. Hence, to eliminate the element of personal judgment in the routine operation of alternators, the machine is synchronized by a more accurate device by Synchroscope.

Voltmeter method

Voltmeter Method of Paralleling of alternator
Voltmeter Method of Paralleling of the alternator
  • Connect voltmeter accept up to 500 volts on a 440 system across any one of the phase of the incoming generator and the same phase of the running generator
  • Connect voltmeter as shown in fig.
  • This procedure is easily and safely performed at the synchroscope terminal  or key lamp terminals behind the door of the synchronizing panel at the front of the main switchboard
  • Check circuit diagram before such testing
  • Adjust generator speed until voltmeter very slowly move from zero to maximum and close the breakup when the voltmeter indication passes through zero
  • Note:- for this operation and analog type voltmeter is preferred.

Protective Relaying for paralleling generators


  • Volts/Hertz Over Excitation
  • Loss of Excitation Relay
  • Negative Sequence Overcurrent
  • Differential Overcurrent Relay
  • Ground Fault Relay


1). Why do we prefer to close the switch at 11 O’Clock and not at 12 o’clock?

When the synchroscope is approaching 12 o’clock the slip difference of sine waves and the voltage difference between the phases is minimum or nearly zero. We generally close the switch at 11 o’clock because by doing so we are achieving nearly close to 12 o’clock position to compensate time taken for closing synchroscope switch.

2). Why it is necessary that incoming generator frequency more than busbar?

The incoming alternator is always kept it slightly higher frequency than the bus because as soon as we connect the incoming alternator on load its RPM will fall because the sudden demand of load and when result in low frequency. In this case, the generator will behave like motor and start growing current which then is tripped by reverse power relay

  • During load shift, two things are happening demagnetization effect and cross magnetization effect
  • Due to demonetization more current flow through the rotor in case of load increases and result and voltage dip, this is taken care by AVR
  • Due to cross magnetization magnetic lock of excitation flux between rotor and stator will be more this reduces the speed and then frequency deep this is taken care by the Governor
  • Magnetic lock when sudden load is applied then flux is not sufficient to produce enough current and due to this starvation causes magnetic lock

3). Why is it desirable to operate parallel alternator with same power factor?

  • Same power factor is desirable for two or more alternator in parallel because for fixed KVAR  (reactive load)
  • KVAr is the unit to measure the rate of energy that is sloshing back and forth between AC source and load (capacity or inductive). So phase difference between the components of voltage wave and current reviews for this measure COSΦ = KW/KVAR
  • If power factor is not same then the reactive load will also vary which makes alternator load current to vary.
  • In this case of KW load sharing will be equal but KVAR load sharing is not same, the alternator having less power factor have more KVAR load, so it impacts on the machine.


4). What are the likely consequences of attempting to close incomer’s circuit breaker when generator voltage or not in synchronism?

If an attempt to make the breaker of an unsynchronised incoming generator with the running generator then the volt phase difference will cause large circulating current between the Machines.This large circulating current produces large magnetic forces to pull the generator voltage into synchronism. This means a Rapid acceleration of one machine and deacceleration of other machines. These large magnetic force may physically damage the generator and the prime mover. These large circulating current will trip each generator breaker and cause a blackout. To avoid this reverse power relay is there which is set to trip generator breaker at -10% of full load capacity.

Mechanical damage

  • Deformation of the stator winding
  • Movement between stator and frame
  • Failure of rotor diode on Brushless machine
  • Twisted rotor shaft
  • Localised cracking of shaft and keyway and brushless coupling

Method to avoid damages

To achieve proper synchronization reduce damage caused due to improper synchronization the operators to ensure that incoming voltage is within 5% of the bus for voltage and I delete incoming frequency should be within. 2% of the bus the frequency which is indicated by one revolution of synchroscope every 4 second

5). For two alternators operating in parallel the consequences of

  1. Reduced torque from Prime mover of one machine

    • The voltage generated by Prime mover is directly proportional to the torque produced by Prime mover.
    • The magnetic force exerted on the rotor due to KW loading of generator causes the rotor to slow down. This reduction of speed is caused by the governor.
    •  But if torque is reduced, it directly affects the KW loads sharing. which leads to total load shedding of the generator.
    • If other generator is unable to take the load, the preferential trip may occur.
    • Under frequency trip, may also occur if excitation of alternator affected because it is mounted on the same rotor.
  2. Reduce excitation on one machine

  • System voltage my falls slightly because the excitation of the other machine will increase and will affect any tendency for the voltage to fall
  • Reactive current in form of large circulating current flow between the faulty and other machines.
  • Current will cause damage to Falty machine and tripping of other machines.
  • Excitation loss system should be fitted to trip the main breaker.
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He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. He has total sailing experience of 18 months. Presently working with Shipping Corporation and India. He is a member of Nielsons Educational Forum. and pursuing his Master’s from Narottam Morarjee Institute Of Shipping. Specialty : Tanker's Equipment and Electrical

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