People are often unsure of what parallel generators are or how they work.
Paralleling can be a great solution if you need more power than a single generator can provide. The parallel operation has several advantages over single generators, including increased power output, better fuel economy, and easier portability. But there are also some disadvantages and precautions to be aware of before going for them.
Our guide will help you understand parallel generators’ basics and answer common questions about connecting and using them with our easy-to-follow diagrams.
What are Parallel Generators?
In simple terms, a parallel system of generators involves the operation of two or more generators simultaneously and interconnecting them to a common live system to share the load.
Running generators in parallel is a very common phenomenon for power grids and larger engine-driven generators used at industrial sites and commercial buildings. For these larger generator sets, paralleling may be carried out even with the utility power supply mains. The concept is slowly catching up for small generators as well.
Paralleling generators together requires certain specific requirements to be met, known as the principal conditions of satisfactory parallel operation of synchronous machines. It is not advisable to connect any stationary alternator to the energized busbars. In stationary conditions, the stator-induced EMF is zero, a dead short circuit will occur.
Need for Paralleling of Generators
Before we present you with these specific conditions, let us quickly examine the reasons or advantages associated with running multiple generators in parallel.
This is a case of safety in numbers, where multiple generators paralleled are considered more reliable than one generator of larger capacity. Suppose one takes the fall, and the other sweeps in to make up. The failure of the only available generator in the second case would lead to the entire system shutdown.
Lower Purchase Cost
Aside from dependability, the cost is perhaps another critical advantage.
The bigger the engine, the bigger the price. Therefore, more people tend to go with smaller engines because they are more cost-effective than a stand-alone, giant generator.
Parallels can be a big money-saver due to higher demand for more number of smaller units, prompting the company to lower the price.
Smaller generators loaded to their rating are more efficient. A big generator would run at half the load demand it should optimally, bringing down the efficiency considerably.
When generators run at 75% and over, they begin to use less fuel. The way to manage generating costs is to create parallels.
Cater to Fluctuations in Load Requirements
Load requirements on any generator fluctuate during the day and with seasons. Improved and efficient load management is possible by shutting down some of the generators during low power demand periods and putting them back on when needed.
Continuity of Service during Maintenance
This point is more relevant for commercial applications where generators are the primary power source than a normal portable generator. More generators allow you to plan periodic maintenance or attend to a critical issue on any particular machine without interrupting the electrical power to the load.
The parallel system gives you the flexibility to design and execute various failure modes, wherein the backup power generated is prioritized to feed the critical loads if one or more generators fail.
Increasing the Capacity
If you need an additional power supply due to changes in power demand or additional loads, you can simply add another alternator without going through all the hassles of procuring a bigger generator and disposing of the existing one.
If you frequent RV trips or camping sites, having two smaller units is more convenient due to easy portability than having one unit of larger size. Paralleling generators provide you with enough power to run the 15000 BTU air conditioner, which is difficult without a big generator due to starting watts requirement of the AC unit.
Less Noise & Quiet Operation.
Any smaller generator produces lower noise levels than a bigger generator. Many portable generators offer an Eco mode, resulting in further noise reduction.
What is Synchronisation?
The process of connecting one alternator in parallel with another alternator or a common busbar is known as synchronizing the generators. While synchronizing two generators, the generator first connected to the bus is known as the running generator, and the second generator is known as the incoming generator.
To connect and synchronize two generators in parallel, the following requirements must be met.
- The speed of the incoming generator should adjust such that its alternator should have the same frequency as that of the busbar. You may be aware that the speed of any generator is related to the frequency of the alternator by the relation N = 120 F/P, where N is the speed of the prime mover shaft coupled to the alternator shaft in RPM, F is the frequency in Hz, and P is the number of poles of the alternator.
- The voltage of the incoming machine must match the busbar voltage.
- The phase of the alternator voltage should also be identical to the phase of the busbar voltage. That means the phase angle difference between the voltages must be zero.
- The phase sequence of the three phases of the incoming machine must match that of the busbar. This issue is only encountered during the first installation or after major maintenance is carried out.
The synchronization for the bigger generators is done using lamp methods, a synchroscope, or the synchronization panel.
The above conditions constitute what is also known as the synchronizing window. In this section, we will inform you of more in-depth synchronization considerations.
Frequency: It is recommended to match the frequency within 0.1 Hz. Also, the frequency change rate before operating the circuit breaker of the incoming generator is critical and should be within 0.1 Hz per second. Unstable engine speed can lead to a condition where the engine accelerates and decelerates in fast oscillations, ultimately leading to a very high alignment torque, adequate to damage the mechanical components or drive coupling system.
Voltage & Phase Difference: For paralleling generators, they must have the same voltage within a tolerance of 0.5%, and the circuit breaker closing angle must be less than 5 Deg if the generator is synchronized frequently to have the lowest possible stress. If the generator is put in synchronism and continues to operate for a long time, the angle should be less than 10 deg.
If the incoming alternator voltage is higher than the bus voltage, the reactive power will flow from the generator to the grid and in the reverse direction if the voltage is lower.
On closing the synchronizing circuit breaker, the transient surge current in the incoming machine should not exceed 50% of its rated current. The synchronizing equipment’s control system should ensure that the breaker closure occurs within the closing angle window and the angle dynamically decrease towards 0 Deg.
How is Load Shared Among Generators?
The different generators, in parallel, get a proportional division of the total kW and kVAR load on the system. This is called load sharing. Load sharing avoids stability and overloading problems on individual generator sets.
Active Power (kW) Sharing
In parallel mode, the speed governor of the individual generator determines its share out of the system’s total active power (kW) requirements. You can increase or decrease the share by changing the fuel supply to the engine.
If the fuel supply to one of the generators in parallel is increased or decreased, there is no change in its frequency, as is expected if you are running the generator alone. But there will be a change in the active power supplied by it.
The control system monitors and controls the active load sharing in proportion to the engine ratings of all the generators through the speed control system of the engines.
Reactive Power (kVAR) Sharing
In the same way, the alternator’s field excitation system of the individual generator determines its share of the total kVAR requirements of the system operating in parallel. The share can be altered by varying the excitation.
If the excitation to any one of the generators in parallel is changed, the voltage of its power output does not change like in the individual operation of the generator. But, the kVAR load output will change accordingly along with the change in power factor.
On the downside, undesirable circulating currents, known as the cross-currents, will be generated in the system if the excitation levels of the alternators are different.
The voltage control system monitors and controls the kVAR load sharing of the paralleled system in proportion to the ratings of the alternators via their excitation systems.
Paralleling Portable Generators
Before you actually decide to run portable generators in parallel, you need to check out the following things for the generators under consideration.
Are generators parallel capable?: While most inverter generators have this capability, many of the traditional generators are not parallel capable, particularly the older models. New machines are always more user-friendly and with more controls. But you need to check about your specific model.
Look out for compatible generators: Two generators are considered compatible if they have compatible alternators with similar alternator pitch, same voltage, frequency, engines, load sharing controls, rpm, etc., with the possibility to interface with the other control and monitoring systems.
Can you parallel two different brand generators?: The answer to this question depends on which brands you are considering. Many manufacturers like Honda only recommend paralleling two or more generators from the same brand and even the same model. Some of their models have designated companion models, which can also be used.
It is always better to follow the manufacturer’s recommendations, while technically, running generators in parallel with other compatible generators may be possible.
How To Parallel Portable Generators
The method to parallel two generators will vary according to the brand and model. The two commonly used methods are using parallel cables or parallel kits. The preliminary activities include
- You first need to find the proper location with a ventilated, level surface to place and secure your generators properly.
- Go through the pre-start checklist in the owner’s manual thoroughly.
- Ensure that the units are properly grounded.
- Use adequately rated cables to the common bus or the automatic transfer switch.
Using A Parallel Cable
You can parallel many inverter generators by using a specially designated parallel cable supplied by the manufacturers. Usually, three cables are connected to each inverter generator, two as connection cables and one for a ground connection. Honda uses only parallel cables to connect and run parallel generators on most of its EU series models.
In this arrangement, the appliances are directly connected to the 30 A outlet on the generator itself.
Using A Parallel Kit
Some manufacturers like Generac and Champion generators provide a separate parallel kit, which is like a central outlet, to connect your RV cords or other appliances. A typical paralleling kit for connecting two generators of about 3000 watts contains the following components on the control panel:
- 120V, 30A, NEMA L5-30R, a twist-lock receptacle to power 120V, 60Hz, single-phase loads.
- 30 A circuit breaker to protect against electrical overloads.
- 50 A circuit breaker to protect against electrical overloads.
- 120V, 50A, NEMA 14-50R receptacle to power 120V, 60Hz, single-phase loads.
While sizing generators to run together with these kits, it is always better to go for equal sizes, though in most cases, it is possible to run generators with different ratings in parallel.
Carry out the following steps for connecting generators in parallel:
- Turn off the generators after disconnecting all loads before installing the kit.
- Place and align the inverter generators in the same direction with a minimum separation of 45 cm. Some manufacturers also recommend aligning the paralleling generators in a “V.”
- Clamp the kit to the handle of one generator.
- Remove the connector end cap on each generator and insert the power leads from the kit – the red lead in the red receptacle and the black lead in the black receptacle.
- Connect the grounding wire from the parallel kit to the grounding terminal of the inverter generator.
- Start both generators one after another. In many generators, a green light will blink until the loads are connected.
- Allow the generators to run for some time and stabilize before connecting the loads.
- Connect the devices to the receptacles, ensuring never to exceed the rated amperage of any receptacle.
Portable generators are a great way to provide backup power for your home or office during an outage, and with the right preparation, they can also be used in parallel to increase the amount of power available. By understanding the benefits of parallel operation and how to parallel portable generators, you can ensure that your family or business stays powered up during an emergency.
Do let us know through the comments section if you would like to add something to the article or clarify any of your concerns.