Parallel operation of standby and primary generator sets

July 12th, 2017, Published in Articles: Energize


It is becoming common practice to install a number of smaller sized standby or primary power generator sets rather than a single larger machine. This is done to ensure availability, handle variable loads efficiently, and facilitate maintenance. Efficient operation of paralleled sets requires synchronisation and load balancing, as well as operating machines at the optimum load point. Modern control equipment has made it easier to meet these requirements when operating machines in parallel.

Parallel standby power systems have always provided significant advantages over single large generator units. However, implementation of such systems has been limited to large projects or mission critical applications, due to the constraints of higher cost, space, and the high level of complexity involved in setting up and maintaining such plant. With the introduction of advanced digital control technologies, it has now become easier to operate systems in parallel and benefit from the additional advantages these systems can provide.

Fig. 1: Synchronising requires voltage, phase and frequency to be matched [4].

Advantages of parallel operation

The main advantages are:

  • Reliability: The redundancy inherent in parallel operation of multiple generators provides greater reliability than is offered by single generator unit for critical loads. If one unit fails, the critical loads are redistributed among other units in the system on a priority basis.
  • Expandability: When sizing generators to match load requirements, it is often difficult to accurately project increases in load and adequately plan for anticipated additional requirements. By operating systems in parallel, it is easier to allow for variations in load without overrunning budget or piling up expensive units that rarely get used. Redundant generators can be detached from the unit and can be used separately at other sites.
  • Flexibility: Using multiple units in parallel offers greater flexibility than using a single large-sized generator of a high capacity. Multiple smaller generators operating in parallel do not need to be grouped together and can be located in a distributed fashion. Since the units do not require a collective large space that have to be side by side, these can often be installed in small facilities or whenever space is a restricting factor.
  • Ease of maintenance and serviceability: If a generator in the system breaks down or requires maintenance, individual units can be dismantled and serviced without disrupting the functioning of other units. The redundancy inherent in a parallel system provides multiple layers of protection and ensures an uninterrupted supply of power for critical circuits.
  • Cost-effectiveness and quality performance: Individual units operating in parallel are typically of smaller capacities. The engines used in these generators are usually industrial, on-road or high-volume engines produced with advanced manufacturing technology that gives them a high degree of reliability and low cost per unit of power [1].

Fig. 2: Slip frequency synchronising [2].

Most parallel configurations use machines of the same size and same characteristics. Parallel operation of machines of different sizes and/or with different characteristics provides operational challenges, but is not impossible with the advanced control systems available today. There are three critical considerations when operating generators in parallel:

  • Synchronisation: frequency phase and voltage must be the same
  • Load sharing: equal loads must be carried by each machine
  • Circulating currents: these can cause damage and must be kept to a minimum


To connect generators in parallel, they must first be synchronised. Synchronising means that the output voltage waveform of the generator must match the output waveform of another source in terms of voltage, frequency and phase angle. A phase angle difference between the two waveforms creates a difference in potential between the two sources. The potential difference should be as small as possible within practical limits before closing the paralleling circuit breaker (Fig. 1).

Fig. 3: Phase lock synchronising [2].

There are two forms of synchronising in common use: slip frequency synchronising, and phase lock active synchronising.

Slip frequency synchronising

Slip is the difference in frequency (or speed of rotation) of the two sources (measured in Hz). In a slip frequency application there will be alternating moments of being in phase (synchronism) and being out of phase (Fig. 2). A synchronising unit is normally used to match the voltage of the incoming generator set to that of the busbar and the frequency of the incoming generator is set to a fixed difference to that of the busbar. The different frequencies permit an instant of minimal phase angle and therefore potential difference between sources. The paralleling contactor will be closed at the instant of minimum potential difference [3].

Fig. 4: Typical power/frequency droop curve [4].

Phase lock synchronising

Fig. 2 illustrates the phase lock loop (phase match) method of synchronising. There is initially a large phase angle difference between waveforms, which is reduced and maintained. This allows for a sustained period of synchronism. The design of the synchroniser makes this mode of operation possible by controlling voltage, frequency and phase angle. The unit analyses the output voltage and makes corrections to the engine speed (via the governor) and controls the AVR to adjust voltage amplitude and phase angle to achieve sustained synchronism [2].

Parallel connection and synchronisation procedures

Two systems of parallel connection and synchronisation are commonly used:

  • Random access paralleling
  • Dead bus paralleling

Fig. 5: Load sharing with similar machines.

Random access paralleling

In a random access paralleling system all generator sets receive a start command at the same time and independently build up their voltage and speed to rated values, at which point they are ready to close to the paralleling bus. The generator sets will not be in synchronism with each other, so the generator set controls have an arbitration scheme which allows only one generator set to close to the dead bus. When one generator set “wins” the arbitration it sends a signal to the other generator sets preventing them from closing their breakers and then closes its own paralleling breaker to the bus. At this point the other generator sets recognise that the bus is now live and they synchronise and close to the bus. In a random access system it is not predetermined which generator set will close to the dead bus. It is a robust paralleling method because if a single generator set fails or is slow to come up to speed the rest of the generator sets are not affected. There is no single point of failure [3].

Dead bus paralleling

In a dead bus paralleling system all generator sets start with their paralleling breakers closed to the bus, and with their excitation circuits disabled. This allows generator sets to be connected in parallel without being in synchronisation because no voltage is generated. As engines reach a preset speed the generator set controls turn on and ramp up excitation levels. This causes the voltage on the bus to build up and forces the generator sets to come into synchronisation with each other. Because there is no need for arbitration or synchronising multiple generator sets, dead bus paralleling can bring a generator to rated speed and voltage quickly. This, however, is a less robust method of paralleling as each generator set represents a single point of failure [3].

Fig. 6: Load sharing on generators with different droop characteristics.

Circulating currents are caused by generators building up internal voltage at different rates as excitation is increased. Although the terminal voltages of the paralleled generator sets will be the same because they will be electrically connected, the internal voltages of the generators may be different due to different characteristics. Current will flow from generators with higher internal voltage to generators with lower internal voltage resulting in some generators being back fed which causes stress to the windings and excitation system.

In a random access paralleling system the closed loop load sharing algorithm will effectively eliminate any circulating current. During the excitation ramp in a dead bus paralleling system the voltage reference of the AVR is increased linearly with no feedback. The control is not correcting for any of these differences so there could be substantial current circulating between the generator sets [3].

Load sharing

There are two main methods of load sharing:

  • Passive load sharing or droop load sharing: this relies upon the settings of the generators.
  • Active load sharing: this uses a load sharing controller which interacts with the voltage and speed controls of the machines to achieve balance.

Passive load sharing

Frequency or voltage droop load sharing

Droop is a characteristic of a generator and describes the variation of either power or voltage with speed of rotation. Speed of rotation determines frequency and droop is often defined a change of output power with frequency.

Fig. 7: The effect of changing the load on dissimilar paralleled generators.

As the load increases, the speed of the generator, and hence the frequency will decrease. In the isochronous case the control system will adjust the operation of the machine to maintain the frequency at a set frequency. In Isochronous operation the no-load frequency is the system frequency. In a droop load sharing system, the proportion of the load carried by each machine is determined by the droop characteristic of the machine. The operation of droop controlled load sharing can be understood by considering the case of two machines connected in parallel. If the two machines have similar ratings and the same droop characteristics, the load sharing is as shown in Fig. 5.

Because the machines have the same droop characteristics power is shared equally between the two generators, i.e. PG1 = PG2. If the power drawn increases, the frequency will change, but the power drawn from each generator remains the same.

Droop load sharing dissimilar machines

When machines with different droop characteristics are operated in parallel, the proportion carried by each machine will depend on the total load and the droop characteristics of each machine. This is illustrated in Fig. 6.

Fig. 8: Changing the sharing ratio between generators.

From Fig. 6 it can be seem that Gen. 1 carries a higher portion of the load than Gen. 2. Changing the load changes the frequency as shown in Fig. 7, and the proportional sharing has also changed because of the different droop rates. The load sharing will depend on the droop characteristics of the two machines. The no load frequency of the machines in this example differs which is unusual, as most machines would be operated or set to have the same no load frequency.

In order to compensate for the variation in frequency, the generator set points can be adjusted to provide constant system frequency. Note that both generator no-load frequencies must be adjusted in order to maintain the balance of power between generators 1 and 2.

Changing load sharing

The load sharing ratio may be altered by changing the settings of the generators. Adjusting one generator changes both the load balance and the frequency of the system. This is illustrated in Fig. 8 where the no-load frequency of
Gen. 1 has been changed, with a resultant change in both load sharing and the system frequency.

Fig. 9: Reactive droop compensation [7].

Active load sharing

Active load sharing is used to ensure a constant voltage and frequency output from the paralleled generator sets. Active load sharing requires the interconnection of control units of each machine. Control units monitor the voltage, current, active and reactive loads on each machine and adjust control parameters to ensure balanced loading, constant voltage and frequency.

Isochronous kW and kvar load sharing

In an isochronous load sharing system, the frequency of the system is maintained constant throughout load variations. Isochronous load sharing control systems are active control systems that calculate the percentage of real and reactive load on a specific generator set, compare those values to the percentage of real and reactive load on the system, and then provide control to the fuel and excitation system of the generator to drive the percentage of load on the generator to the same value as the percentage of load on the system. Load sharing is critical to paralleling compatibility because the load sharing communication is the only point where generator controls interact with each other when operating on an isolated bus [7].

There are several methods of doing this, amongst which are reactive load sharing and cross current compensation.

Fig. 10: Cross current compensation [7].

Reactive load sharing

Otherwise known as reactive droop compensation, this system is shown in Fig. 9.

In this system the voltage regulators operate to obtain equal sharing of the reactive load.

Cross current compensation

Cross current is a flow of electrical current between generator sets that is caused by dissimilar excitation levels in those sets. Cross current compensation is a term describing the operation of paralleled generator sets without intentional voltage droop. This is achieved by the insertion of a current transformer (CT), on one phase of the generator output and interconnecting the CTs together to provide an identical voltage bias to each AVR in the system. Using cross current compensation results in no intentional droop in voltage from no load to full load on the system, so it is considered to be superior to a reactive droop compensation system from a performance perspective. Fig.10 shows a cross current compensation circuit [7].

Fig. 11: Generators with different winding pitch produce different voltage waveforms [8].

Circulating currents

One of the more common concerns with paralleled generator sets is circulating currents, where current flows between the generators. Circulating currents can be due to two things:

  • Difference in voltage settings
  • Difference in generator characteristics

Difference in voltage settings or voltage droop will result in circulating current flowing in the line or phase conductors of the alternators and can be corrected by adjusting the voltage settings, either manually or automatically.

Although the use of generators with dissimilar characteristics is an unusual situation is does occur and has its own set of challenges. The most common problem is the different shape of the voltage waveforms from generators with different winding pitches or patterns. This can also be described as the different harmonic contents of the two waveforms. This is illustrated in Fig. 11. Although the two waveforms have the same peak voltages and more or less sinusoidal waveforms, there are instantaneous differences in voltage. This difference results in circulating neutral currents between the two machines. These circulating currents can cause overheating in the generator windings and false tripping of overcurrent protection equipment, particularly ground fault detection schemes [5].

Fig. 12: Different voltage waveforms result in circulating currents in the neutral [8].

Where the load current contains harmonics, this can also introduce extra harmonics on the generator voltage waveform, as machines with different winding pitch have different reactance to harmonics [4]. Flow of harmonic current in the neutral can be reduced by the introduction of reactance into the neutral conductor.


[1]  Diesel service: “Parrallel operation of generator sets”
[2]  R Patrick: “Considerations when Paralleling Generating Sets”, Cummins.
[3]  R Scroggins: “Random access vs dead bus paralleling”, Cummins.
[4]  A Knight: “Electrical machines: Operating generators in parrallel”,
[5]  A Hoevenaars: “Preventing Neutral Circulating Currents when Paralleling Generators”, Mirus International, 2011.
[6]  MJ Thompson: “Fundamentals and Advancements in Generator Synchronizing Systems”, SEL Journal of Reliable Power, March 2012.
[7]  D Kristensen: “Emergency Generator – Paralleling Switchgear Power Switching ControlMethodologies for LV and MV Applications”,
[8]  G Olsen: “Paralleling Dissimilar Generators: Part 1 – An Overview”, Cummins.

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