How to best use the Power System Study Report

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By Jeff MacKinnon, P.Eng., PE

If you have gone through the process of completing the power system study report, or have had an outside consultant provide you a binder with all the deliverables, now the real work begins. Having a power system study completed is not the end: it is the start. The reasons to have a power system study report completed in the first place can vary from getting incident energy values for PPE selection, to future plant expansions.

In this article we will outline some of the more common uses of a complete power system study report. We'll also include suggestions about how to approach the recommendations that were included.

Short Circuit Study

Let's start at the beginning with the short circuit study. The first thing we will look at is the type of recommendations that you will be expecting to receive in the report. First there is going to be any equipment that is not rated for the existing short circuit current that is available within the facility. There are a couple of reasons for this to happen: the first is that the equipment was purchased undersized, however the more likely scenario is that there have been system changes. These changes could be either at the utility level - adding additional transmission capacity - or within the facility itself.

Typically the best way to fix these issues is to replace the equipment.

Another output of the short circuit study will be areas that are marginal. This could mean that a change in the system could exceed the fault rating on the affected equipment. For these, investigate the switching scenario that was used, and figure out if there are any additions to the plant planned in the future. A decision will then be made to determine if the equipment should be replaced now, or flagged as an area of concern when there are system configuration changes in the future.

Protection Coordination Study

TCC from an ETAP example file

The protection coordination study will likely include recommendations for areas that are mis-coordinated. With the help of your consultant you can go through the areas of concern and develop a list of what needs to be changed. With proper selective coordination, the system will operate in a predictable manner in the event of a fault or overload.

In a lot of cases it will be easy to change the settings on the relay, or LSI(G) breaker, to solve the coordination problem. If the settings can't be changed with the existing equipment, the protection device will need to be replaced. If the mis-coordination is something that will not affect a large portion of the system, or is the same circuit (a primary fuse and secondary breaker on a transformer) the decision may be to leave the mis-coordination as it is.

Another consideration of selective coordination is the incident energy in the case of an arc flash event. Clearing the fault as quickly as possible will limit the heat energy available, and will reduce the arc flash boundary and PPE required for energized work. These decisions will be simplfied when going through the electrical safety program.

Incident Energy Study

The incident energy study is a critical input to the arc flash analysis, which is a critical portion of the electrical safety program. If you haven't developed an electrical safety program or an arc flash risk analysis, a great way to start one is with an incident energy study in hand. With the incident energy known at all the equipment, you will know the arc flash boundary and can start developing PPE levels for energized work.

The Electrical Safety Program is critical workplace safety

Arc Flash Risk Analysis

The first step is to develop a risk assessment as associated with the arc flash hazard. Within the company develop a list of the various tasks that will expose workers to an arc flash hazard. From there you can determine what incident energy is considered too high of a risk for energized work, and cross-reference with the incident energy report. You can implement mitigation techniques for any areas that have a high incident energy.

Incident Energy Mitigation Techniques

There are only two methods to lower the incident energy at a bus: one is to lower the time the arc is present before being cleared, and the other is to lower the magnitude of the current. To shorten the time that the arc is present, modifying the protection settings will work well. Sometimes this may mean sacrificing selective coordination, which is a compromise that can be made on a case by case basis.

It is harder to lower the available current. You can add impedance - add inductance or more cable - which is typically not practical, or break up the large motors onto different buses to lessen the back feed current during a fault. Neither of these are very practical, and are typically ignored.

Load Flow Study

Power system study reports can help you save money by saving energy.

Finally the load flow study is the place that you can really start to save money. Using the results of the load flow study report, you can see where in the system you may have opportunity to increase efficiency. We've already written about who can leverage a power system study :engineers aren't the only group that can use the load flow study in their everyday work. A couple of examples are how capital projects (power factor correction) and operations (load scheduling) can help your facility save money and pay back the cost of having a study completed in the first place.

Power Factor Correction

By ensuring that the power factor is matched as close to 1 as possible, you system will be operating with the least kW, meaning that you will be paying for the least amount of energy from the utility. The load flow study will determine the best place to install power factor correction capacitors, whether they should be staged or always online.

Load Scheduling

If your facility has a large energy draw, and a lot of motor load, then your power bill will likely be in multiple parts. Each part will have different charges. For example:

  • Energy -kWh
  • Max Demand - kW with a rolling average
  • Power Factor - too much kVAR draw

The load flow study allows you to figure out your baseline operations. You can then start making changes to how new loads are added, and determine the most cost effective way to operate the facility without hurting production.

Conclusion

There are a lot of useful projects that will optimize the operation of your facility: we have only scratched the surface with this article. If you liked this article, feel free to share it. If you let us know, we'll add more articles like this one, about utilizing a power system study at your facility to give your business a competitive edge.

This post is part 8 of the "Power System Studies" series:

  1. Power System Studies - The What
  2. Power System Studies - The Critical Four
  3. Power System Studies - The Why and How
  4. Data Collection For Power System Studies
  5. Data Requirements for Power System Studies
  6. Protection Coordination Study
  7. What is a Load Flow Study
  8. How to best use the Power System Study Report
  9. 3 Ways to get a Power System Study Completed