In the first part of the Power System Study series I told you what a power system study is, and what power system analysis I consider critical for every facility, plant, building, etc, no matter how large or small.
These are:
Short Circuit
Protection Coordination
Incident Energy
Load Flow
The Fabulous Four
The reason these four studies are so powerful are that they require much of the same data, and when completed together they have the ability to save time and money, versus completing them one at a time. For example, the incident energy at a bus is determined by the amount of current in a fault and the time that current is present. That requires the Short Circuit and Protection Coordination studies to be completed.
Typically, a short circuit study doesn't require impedances other than transformers, however an incident energy study does. With this added information the majority of the data needed for a load flow study is already in the model. To complete the load flow you only need to add some loading scenarios at the various buses on your system.
Short Circuit Study
The purpose of the short circuit study is to determine the fault current available at the various buses on your system. These are worse case single and three phase bolted faults. The data required for this study in particular is the utility impedance, or how much energy can they supply to a fault, and the various transformers size and impedance. It is these impedances that are typically used to determine the magnitude of the fault current at any bus.
With this information you can determine if your equipment is sized appropriately (or if you are designing a new system, you can size the equipment appropriately) and will not fail during a fault event.
Protection Coordination Study
The purpose of the protection coordination study is the verify that the various protection devices in your system, relays, breakers, fuses, etc are coordinated correctly and are sized appropriately for the equipment that they are protecting. For example, this study would verify that a fault on a lighting panel will not activate the main breaker on to the facility, but rather activate the closest breaker to the fault, either the main breaker (or fuse) or the protection on the feed to the lighting panel.
Incident Energy Study
The incident energy study is (IE Study) one part of an Arc Flash Risk Assessment, it determines the thermal energy generated from an arcing fault. From this information the Arc Flash Boundary is determined and proper PPE can be selected for any energized work at that location.
NFPA 70E and CSA Z462 require an Arc Flash Risk Assessment be completed as part of a complete Electrical Safety Program. The incident energy system is one critical component of the Arc Flash Risk Assessment.
The magnitude of incident energy at a bus is proportional to the square of the arcing fault current and the time that it is present. The fault current is based on the currents determined in the short circuit study, typically 80% of the symmetrical short circuit value, and the protection coordination study will determine how long it will take for the arc fault to be cleared by protection.
This is a good place to mention, that all of these studies, by necessity, assume that the equipment has been installed and maintained per the local codes and manufacturers recommendations. If a 4160V breaker hasn't been exercised for the entire time that it has been installed, it may not operate as intended, and the calculations performed by these studies will have no value until the breaker operation has been verified.
Load Flow Study
Now for the hardest of all studies, the load flow study. As we know, a power system is very dynamic, and as such the exact load on a particular circuit is changing all the time. This makes it very difficult to determine the values required to make a perfect model. However that doesn't mean that you can't glean important information from it. With the help of your operators/building maintenance the engineer completing your study will be able to put together a few normal scenarios that can be used to get estimates of the loads and supplies on your system.
The information that is critical from a proper load flow is the voltages and power factor at all your buses, currents or power flow on all your feeders. With this information you will be able to make important decisions on where to add or remove load, and where power factor correction can be added to increase the efficiency of your system.
Getting the word out
This concludes part 2 of our series on Power System Studies, on Friday I will talk about how you can leverage them to make better decisions at your facility. If you missed Part 1 on Monday, it defines what a Power System Study is.
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