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Parallel Operation of Generator Sets


Utility & Backup Power

Nuclear Power Plant Control Room
As the world becomes more reliant on electric power to function and grow, backup power systems, such as generators, are playing an increasingly significant role in ensuring uninterrupted supply of power. Your choice of a generator depends primarily on the amount of backup power that is required for your specific application.

Oftentimes, you may require just a minimum supply of backup power to ensure uninterrupted functioning of basic appliances or mission critical equipment. Or, you may need to be able to support your full load and then some, which is common in high availability environments.  

In either case, it is not always possible to find a generator that matches your requirements exactly. Sometimes the output power capacity of standard generator units available in the market may greatly exceed your minimum requirement or fall short of your maximum requirement. This is one area where paralleling generators can shine.

Parallel Operation of Generators

The easiest way to setup a parallel system is to use generators that are exactly alike, or at least have the same output rating and alternator pitch.  Another flexible approach to backing up your power requirements is to have two or more generators of variable output. In either scenario, these can be connected in parallel with paralleling switchgear to achieve maximum output during peak requirement or the desired minimal output during other times. Benefits of parallel power generation systems.

Parallel standby power systems have always been significantly advantageous over single large generator units. However, implementation of such systems has historically been limited to large projects or mission critical applications due to the constraints of higher cost, space, and the high level of complexity involved to setup and maintain. Until recently, many businesses both large and small have refrained from parallel operation of generator sets.  With the introduction of sophisticated integrated digital control technologies, it has now become much easier to operate systems in parallel and benefit from the additional advantages these systems can provide. Please read Tips on Buying Used Generators for further information regarding pre-owned gensets that you may be considering to operating parallel:
  1. 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. In many environments, the critical loads that need the highest degree of reliable backup power usually account for only a fraction of the overall power generated by the system. In a parallel system, this means that the most critical elements will have the redundancy necessary to maintain power even if one of the units goes out
  2. Expandability - When sizing generators to match you load requirements, it is often difficult to accurately project increases in load and adequately plan for anticipated additional requirements. If load projections are aggressive, your initial investment in a generator may be higher than necessary. On the other hand, if load projections are inadequate, you may be left without reliable standby power or may have to resort to expensive generator upgrades, or even purchasing another unit altogether
    By operating systems in parallel, it is easier to allow for variations in load without overrunning your budget or piling up expensive units that rarely get used. As long as you have enough physical space, generators can be added for additional power supply when required. Similarly, redundant generators can be detached from the unit and can be used separately at other sites.
  3. 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 lessening the need for one very large footprint for a single, larger generator. Rooftop installations or setting up small-sized generators in limited areas are just a few ways you can creatively find ways to make them fit. 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
  4. 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.
  5. 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 designed with advanced manufacturing technology that gives them a high degree of reliability and low cost of generation per unit of power.

Key Considerations in Setting up Parallel Systems

Kohler Generator with Advanced Controls
In most situations, each individual generator in a parallel system consists of four to six micro-controllers that are hardwired together. The complexity of installation increases if the individual generators have been manufactured by different vendors and the controllers are based on a combination of analog and digital technologies.

Typically, it takes about 3–4 weeks to set up a large capacity generator in parallel and have it ready for supplying backup power. However, in smaller applications like private residences and small businesses, it can take less time. The installation process, in both cases, involves six key features and is a complex process that will require the assistance of an experienced electrical technician. To get an idea of some of the things involved, these key features are discussed below:

Speed Control

Each individual generator operates at its own specified engine speed and frequency. When individual generators are coupled together, their engine speeds are locked into the overall speed of the entire system.
Load Balance: The load shared by each generator determines the speed of its engine. In a parallel system, the entire load is shared by all generators. 


It is of course essential to synchronize the phase of each generator to that of the overall system.  Automatic synchronizing equipment can also be utilized in many situations.
Voltage Regulation: As in the case of engine speeds of individual generators, the voltage of each unit is locked into that of other units in the system. When any unit has a slightly higher voltage set point than other generators in the system, it will end up bearing the entire voltage load of the system. Voltage regulators of individual generators are interconnected in a reactive cross current system, which regulates the voltage set points of individual generators by collecting input from all the transformers controlling individual voltages.

Genset Controller

A controller is installed for checking the parameters of the engine and the alternators of each unit in the system.  Load control and scheduling generators to go on/off can also be done with some of the newer digital controllers available. Proactive Relay: A proactive relay controller checks for proper synchronization, load and voltage balance and reverse power functions. All these features are usually regulated by installing micro controllers into the generators. In traditional parallel operating systems, each of the generators has its own controllers in addition to a master controller that governs the combined system. This is not viable in small set ups, and sometimes even bigger ones, due to the immense complexity and cost of installation.

Each of the controllers have to be installed so they control the working of the individual generator and have to be in sync with the working of the parallel system which is controlled by the master controller. In traditional, older parallel systems, there is also a lot of electric noise due to the simultaneous operation of multiple controllers. This causes a lot of disturbance and these systems are often predisposed to temporary collapse. Therefore, it requires constant supervision for consistent power supply during power failures, which is an added expense and obstacle to overcome.

Some of the issues encountered in traditional parallel systems are overcome by integrating digitally controlled systems. In integrated paralleling, each generator has only one digital controller used to monitor and control all the above key features. This considerably reduces noise and enhances the performance of the parallel power unit. Also, these controllers are plug and play devices. If one fails, it is simply a matter of unplugging it and replacing it with a spare unit. Once everything is setup by trained professionals, the need to reach out for expert technical assistance is also greatly reduced.  Also visit How Generators Work for further details about the function of gensets and how they produce electricity.

Companies opting for integrated generators also do not have to deal with the additional hassle of unending hard wiring, since the number of controllers is reduced to a minimum and all communication is digital. The working of the key features can be monitored on a computer by connecting the master controller to it, making way for uncomplicated troubleshooting. Modern paralleling switchgear can be connected to the computer and the internet for remote monitoring as well.

As you can see, running generators in parallel has some clear advantages and should be a consideration for any company looking to get high level, redundant, backup power.  However, there are cost and other considerations to take into account and because of the complexity involved, technical expertise is an absolute must to design and install a properly configured parallel system.

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