Now that we have an understanding about the importance of good data collection for power system studies, and how to prepare a plan for data gathering, now we need to understand exactly what is needed for each power system study type and specific pieces of equipment. However, there is some information that is needed no matter what study is being completed.
What do they all need?
- Utility information
- Nominal system voltage levels
- System configuration with the following equipment:
- Transformers (with impednance, voltage and configuration),
- Configuration switches and circuit breakers
- Switchgear, mccs, switchboards, etc down to the voltage level you are interested in
- Any large motors and other generation on the site.
With this minimal information you can prepare an adequate short circuit study that will give you values for the short circuit on any of the buses above. These will be very much worse case as we haven’t gathered any additional impedance information. This study would be useful for design and specification purposes when you are purchasing equipment for a new facility.
What information needs to be added to get a decent short circuit, well it is everything above and the following:
- Distribution equipment short circuit withstand and full load information
- Transformer ratings
- Protection equipment manufacturer information, complete with full load rating and short circuit withstand ratings.
- Cables connecting distribution equipment, complete with size, type, configuration and length
- Lumped motor and non-motor loads at all distribution equipment
With this information you will be able to determine the worse case fault scenarios based on your system configuration and determine if there is any equipment that hasn’t been designed to withstand that level. If there is, this is the first thing that must be addressed in your facility.
The additional information that is required for your protection system includes everything that is needed to determine how your protection is going to activate in the event of a fault as determined by the short circuit study.
For this you will need to determine the following:
- Trip characteristics of all relays
- Tripping times for all breakers
- Complete manufacturer information for the fuses in your system
- Manufacturer information for all moulded case breakers
- Any large motors, and their protection broken out. This is to ensure that the protection is coordinated with the worse case starting current.
- For large motors the interial load, and starting times may be required, along with any design type information.
With this information in the model, time current curves, commonly called TCCs can be developed and reviewed to determine if the system will operate in the event of a fault as intended. If there are any overlapping areas that may be cause for concern, this is where they should be addressed. If there is a fault, predictability in how it will be cleared is very important in the aftermath. If multiple protection devices activate it is much harder to determine the cause if there isn’t much visable damage.
Getting into the meat and potatoes of the studies. As I mentioned in the previous episode, incident energy, and the resultant arc flash risk analysis, is the single most cited reason when I ask why the prospective client is interested in getting a study completed. This is where a lot of time and care needs to go into gathering the data necessary for the study, and where the previously stated numbers related to time come into play.
Along with all the information that was gathered for the barebones short circuit and protection coordination, more detailed informaiotn on the imedances withint the system and loads need to be gathered. All information, for any place that the electrical safety program required a detailed label, based on whether or not there will be energized work, needs to be put together.
What type of loads?
This information amounts to the following, starting at the load. All motors 25hp and above need to be broken out. The actual size whether it is 25hp or 50hp isn’t as important as they are broken out. Induction motors contribution fault current for short periods of time, and the magnitude is proportional to the motor size and intertia. This fault current, is not “seen” by the protection relay upstream and doesn’t account for how quickly the protection will clear the fault, however it does contribute energy into the incident energy value. However, it is not a linear relationship, therefore a large number of small motors will not contribute the same values as a single large motor, therefore it is important to break out the larger motors, I like to use 25hp as the break point, and then lump the remaining motor load.
How are the loads fed, what is the impedance?
The cable information, including the size, configuration, raceway material and length is needed from the load to the MCC, or distribution panel. This all contributes to the overall impedance of the cable, and affects how much energy that motor can contribute. If the length is hard to determine, walk it down and err on the shorter side. Since this fault current doesn’t affect clearing time of the protection, erring on less impedance will give you a worse case incident energy everytime. In a future episode I will discuss why this is important, in short it has to do with the time portion of the incident energy equation.
The motor protection, including overloads and MCP is good to have here, it is not critical, but since you will likely be in the bucket gathering the data, it is better to have it now. It will be important if qualified workers are needed to complete energized work at a local motor disconnect, or within the bucket itself.
If the detailed cable information mentioned for the motor feeds wasn’t gathered during the short circuit it is important to get it here.
When you have all this information, the load flow is mostly complete, we need to get into the operational details. We have already mentioned the motors, and breaking them out, now we need to start thinking about the non-motor loads, and just as importantly, what is a good “demand factor” that can be applied to these.
For example, for all the other studies we assume that that motor or load is running at 100% kW, however this is rarely the case. For an accurate load flow, load factor percentages need to be added to determine the loads and then the resultant report can then be checked against any metering information that you may have at your facility to determine that it is accurate.
If this information is hard to come by, a good starting point may be as follows:
- Motors greater that 250hp – 100%
- Motors 100hp to 250hp – 75%
- Motors less than 100hp – 50%
- Non-motor load – 75%
From this iterations can be made to get closer to actual values that can be used as a basis of the report.
Now that you know why you may need a power system study at your facility, how to get started with getting the studies completed, and with this series how to gather the data necessary for great reports. The next step is to get started.
If you need assistance send us a email with a brief description of your system and how to get back to you. We look forward to hearing from you.
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