Generators can be a hotly debated topic throughout the industry. In this article we will discuss the basics of generators, and some of the best practices to use when sizing generators. The NEC does not dictate the sizing of a power generation system for a building but rather leaves it up to the designer and/or manufacturer to make sure it is properly sized. To understand generator sizing we will need to look at some basic electrical theory concepts that come into play.
What type of generator do I need?
Some questions that are important to ask are below:
- What fuel will the generator use? Natural Gas, Propane or Diesel.
- What voltage does the generator need to supply? 120, 240, 208, 480V?
- Is the generator supplying single phase or 3 phase?
- Duty rating: is the generator standby rated or prime rated? Prime means the generator can overload ten percent of its capacity for a certain amount of time.
Answering these questions is the first step in the process of sizing a generator. A diesel generator will function completely differently than a natural gas generator, often requiring some limitations in its capabilities.
Power factor comes into play in most instances when sizing a generator. Power factor is the ratio of real power absorbed by the load (watts) and the apparent power flowing in the circuit (VA). Power factor has no units, and is expressed in a value from 0 to 1 – 1 being a purely resistive circuit that does not draw extra current. The important thing to remember is that the lower a load’s power factor, the more current it will draw. Let’s look at an example.
EXAMPLE: Calculate the current for a 480V 3 phase 45kW load at .8 PF and at .5 PF.
We can observe that the lower power factor requires more current to deliver the same amount of power at a given voltage. This is an important concept to understand when sizing generators. It is also important to understand the difference between apparent power and real power, as we want to make sure we are using the correct units for sizing. For example, all loads should be calculated in apparent power, and converted to real power using the correct power factor. Generators are rated to produce a certain amount of power in watts or kilowatts at a specific power factor. Three-phase generators will have a power factor of .8, while single-phase generators will have a power factor of 1.
What type of loads will the generator serve?
Motor loads are hardest on the generator alternator, due to the large inrush current from the startup. Soft-start motor starters and frequency drives help prevent this to an extent. Across the line motor starters can allow up to six times the motor’s running current to be pulled from the generator itself.
Other loads to consider are non linear loads such as variable frequency drives (VFD), large uninterruptible power supplies (UPS) or even battery charging systems. Non-linear loads can induce unwanted harmonic currents into the system, which can lead to voltage dissertation and unitability.
In situations where a generator is serving large amounts of motor or non-linear loads, the generator should be oversized to prevent voltage and frequency distortion. There are many free generator sizing tools out there that can be used for this, but a good rule of thumb is to provide 50 percent spare capacity.
Generator Fuel type
Generators can be either diesel fuel, or natural gas/propane fired. Diesel generators are much more robust and provide quick start up, providing smaller engine sizes compared to natural gas or propane generator sets. Natural gas and/or propane generators offer an alternative to diesel – onsite fuel storage is not required, they generally run quieter, and burn cleaner than diesel. Oftentimes generators are supplying life safety systems in buildings, which require a minimum of 10 seconds startup. Natural gas generators can have difficulty achieving this in some of the large-size generators. For this reason, diesel generators are generally recommended for sizes 500kW and larger.
Load Shedding/Load Stepping
Similarly, load stepping allows the generator to progressively add loads over a certain amount of time. It’s always best to progressively load the generator up, rather than connect all loads instantly at the same time. The more load is added to it at once, the larger the voltage and frequency dip will be. Remember, the lower the in-rush current, the more stable the system will be. Load stepping is accomplished the same way as load shedding – via transfer switches. The generator controller is programmed to switch over certain transfer switches at given time intervals.
Similarly load stepping allows the generator to progressively add loads over a certain amount of time. Its always best to progressively load the generator up rather than connect all loads instantly at the same time. The more load added it once the larger the voltage and frequency dip will be. Remember the lower the in-rush current, the more stable the system will be. This is also accomplished the same way as load shedding via transfer switches. The generator controller is programmed to switch over certain transfer switches at given time intervals.
As an example, say we have three transfer switches in a building. One transfer switch is for life safety loads (think emergency lighting), one is for optional standby loads (maybe compare systems), and one is for a fire pump. Under normal operation if the power goes out, the generator will energize both the life safety and optional standby loads and not the fire pump, as there is no fire condition. However, during a fire condition, the generator will provide power to the life safety loads and the fire pump, but not the optional standby loads. This allows the sizing of the generator to be based only on the largest two loads, rather than all three.
Let us review and summarize some things we discussed about generator basics. In part two of this article, we will discuss more in-depth sizing techniques.
- Understand all generator parameters required for the project as a first step
- Understand Power Factor and how it plays into generators
- Know the difference between real power (watts) and apparent power (VA)
- Is load shedding right for my application?
- Know the loads being served by the generator and how they affect the system