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Paralleling Operation

Paralleling Operation and the Advantages:

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We got your project cover! Our industrial models have the capacity of paralleling operation and can deliver years of reliability in this configuration.


Paralleling is the synchronous operation of two or more generator sets connected on a universal bus to provide power to standard loads.

Not all generators can operate in parallel together – the ruler is all the sets need to be the same size and of the same manufacturer.


In deciding whether to install in any project one single large unit or multiple generators, one should consider various factors, such as:


  • Reliability

  • Performance

  • Cost

  • Generator and Room size

  • Efficiency

  • Load variation

  • Flexibility



Reliability is the primary factor in the decision to use for paralleling in prime power or continual applications, such as hospitals, computer centers, or to provide electricity to a town or a city; where the reliability of Power Supply is critical since the loads connected are vital.


In these cases, the use of multiple generator sets and prioritized loading of the system allows critical loads first at the expense of none essential loads.


In systems where all the loads require proper operation – Redundant generator sets are recommended, so that failure of a generator set will not disable the facility.


Paralleling requires the ability to sequence loads in steps and the ability to shed loads to allow the generator sets to operate within their load ratings in the event of generator failure.


A multiple set installation, the unit sizing needs to be 70% of the rated capacity to allow a generator set to be taken out of the system for routine maintenance or repair without jeopardizing the supply to the load.



Performance of the on-site power system can be comparable to the utility service when generators operate in parallel, because the capacity of the aggregated generators relative to individual loads is higher than it would be with single generator sets serving separate loads.


Because the bus capacity is higher, the impact of the transient loads applied to the generator sets by individual loads is minimum.



In general, multiple paralleled generator sets will cost more than a single large generator of the same capacity, unless the capacity requirement forces the design to machines operating at less than 1500 rpm.


The cost of a system should be evaluated as the total cost of ownership and must take into account factors such as:


  • The available building space,

  • A remotely located location area 

  • Layout of cables, substations and interconnections 

  • Switchgear requirements and

  • A system control for multiple installations


The required reliability and the benefit outweighs the increased cost. 


The cost of maintenance is a significant factor with generator sets that run for prime power or cogeneration schemes. Although a single large unit may have a different capital cost, others may mitigate these factors associated with the installation costs of a multiple generator system.


When evaluating the total cost of ownership, the installation will impact on the decision on the degree of redundancy that is associated with the system. Some local codes and standards require continuous service to legally required loads and the critical nature of some facilities may require similar service provisions.


If generators are operating paralleled, the maintenance cost and temporary downtime associated with brief generator sets are avoidable. These considerations may also impact on the number of sets required for the installation.


Generator and Room Size:

Generator and room size can be critical factors and may force a decision toward single or multiple set installations. A single generator set will typically be considerably heavier than a corresponding machine used in a paralleling situation.


For rooftop installations or where the set has to be maneuvered into a basement or other confined space, this may be prohibitive, leading to a decision toward smaller, lighter generators.


However, an important note to consider; the space for access and maintenance must be allowed between the machines of multiple installations, and these inevitably use more room volume per electrical kilowatt generated.



Efficiency is a vital factor if the power generation scheme is producing baseload power or it is being used for tariff reduction or cogeneration.


The versatility of the paralleling system, enabling generator sets to be run at optimal load and maximum efficiency, will often pay back the initially higher installation costs in a short time in prime power situations.


Load Variation:

Load is a critical factor in deciding on the type of installation is required. A single generator will typically be the most economical choice for loads below approximately 100 kW as the cost of the paralleling control and switching equipment will be significant when compared to the cost of the generator.


For small but essential installations, where the protection of two generators is essential but the cost of the paralleling equipment is prohibitive. A mutual standby installation may be a good alternative, where one generator acts as a standby to the other.


For larger loads, the choice is less straightforward and around 2–3 MW, solutions using multiple generator sets are available. Above 3 MW, the choice is almost always multiple generator installations.


Although at first sight more economical, a single generator solution is also the least versatile and may be less cost-effective, particularly at partial loads and in long operating hour installations. In prime power applications, high-speed 1800 RPM generator sets may provide lower overall life cycle cost, due to higher efficiency and lower maintenance cost than larger lower speed machines.


Load variation should be considered in any generator application decision as many applications exhibit large differences between day and night and between summer and winter load profiles.


A large manufacturing facility may have a daytime load of say 1MWh (equal to 10 units with 100kwh capacity); but at night, unless used for continuous process application, the load may fall to just a few hundred kW or even less. Installing a single large generator into this application could lead to many hours of light load running, which is detrimental to the system.


A typical installation of this type might use 10 units with 100kwh capacity generators in a paralleled scheme, where the daytime the load uses 10 sets and where at night, only 1 to 2 or 3 set, what is required to run the facility. Transient loads have a large effect on the required size of a generator and it is important to take into account all combinations of transient and steady-state loads in any calculation to ensure that the power quality is maintained.


Note that some loads present leading power factor load to generator sets, and this is also required to be considered in the generator set sizing and sequence of operation for the system.



Flexibility may be an important consideration where an installation may change in the future.


A single generator set installation is usually difficult to change, whereas sets can be added to a multiple set installation with relative ease, provided that allowance has been made in the initial design.


There are risks associated with the parallel operation of generator sets; both between sets and with the utility supply and these risks should be balanced with the benefits.


Risks to Avoid:

Where adequate load shedding has not been provided or where the load is maintained at a high level, there is a risk that, if one generator fails, the remaining generators on the system may not be able to support the system load.


Load shedding should always be incorporated into a paralleling generation scheme and the reserve capacity at any time during operation should correspond with the amount of load that can be accepted if a running generator fails.


Not all generators can be paralleled together – it is advised that all the sets are the same size and of the same manufacturer, please contact us or one of the closest distributor before proceeding.


When paralleling with the utility, the generator effectively becomes a part of the utility system. If operation in parallel with the utility supply is specified, additional protection is required for the protection of the generator and utility system interconnection. This protection is commonly specified and approved by the utility service provider.


Always consult local codes and standards when considering utility parallel operation.


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