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Protection Coordination Study

Power System Analysis Technical Specification

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Power System Analysis Technical Specification
When starting any project, a power systems analysis technical specification is an integral part of success. A good tech spec eases the difficulty of selecting appropriate analysis software and a high quality service provider. Because a tech spec is a very thorough document, it takes a lot of time to put one together. This is surely time well spent, but why spend that time at all when the solution to your problem is right under your nose?
Our 8 page power system analysis technical specification outlines everything that a full power system analysis should contain. From labels to studies, qualifications to standards, our tech spec tells you everything you need to do and everything you need to have to produce a full, high quality power system analysis that can be used by anyone at your facility to improve decisions related to performance, business, and safety. It outlines what is necessary for the scope, data collection, four different types of power system studies, and the final analysis report, including who is responsible for each.
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Protection Coordination Study

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In power system studies – the critical four I said:

The purpose of the protection coordination study is to 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.

which is very true. To expand on that, a protection coordination study really has two purposes. First it ensures that the electrical equipment is properly protected from over-currents and overloads. Secondly the study determines that the selective coordination is employed such that the system reacts as intended and in a predicable manner. You should (almost) always sacrifice equipment protection for selective coordination.  The cases where the opposite is true is a topic for a future article.

TCCs are the bread and butter for a Protection Coordination Study
TCC from an ETAP example file

A protection coordination study will consist of time-current curves (TCC) of all the electrical equipment, protective devices and large motors. Above is a sample TCC showing a circuit for a 50hp motor, it shows the breaker (CB16) that protects the motor from over-currents, the over-load protection (OL_H1), and the fuse (Fuse3) that feeds the MCC.  The dashed line at the far left shows the motor starting and running curve, the next shaded area is CB16 and OL_H1.  ETAP models them as a system and only shows the parts of the curves that are relevant. At the far right is the fuse.
There is no overlap between the motor curve and the motor protection (CB16 and OL_H1) so therefore the motor will operate as intended, and there is no overlap between the feeder fuse and the motor protection, so if there is a fault on the motor circuit the main fuse won’t activate.  If there was an overlap, then there is a chance of the fuse operating on a motor fault, removing a larger portion of the system from service, and possibly making it difficult to troubleshoot the issue to get the system back into operation.

Why do I need a protection coordination study?

You will need a current protection coordination study at your facility to ensure that your system reacts to a fault in a predictable manner and to know how long a fault will be present before the protection takes action. This time is critical information for an incident energy study, and will greatly affect the severity of an arc fault.

When should a protection coordination study be completed?

A protection coordination study will be completed during the design phase of any facility, however after that it will be reviewed on a periodic basis, but at least every 5 years. As with all power system studies, if there have been changes within the system, you should verify that the protection is still adequate.
Another time to revisit the protection coordination study is when equipment is experiencing nuisance tripping: this can be caused by abnormal currents in the equipment, or a faulty relay.

How do I get one done?

I always recommend that the protection coordination study be completed with software, unlike the short circuit study. It is extremely hard to print curves to scale for overlaying, and to try multiple options when an issue presents itself. However, it is relatively easy to add different devices to the circuit and verify protection and selective coordination when using power system modelling software like ETAP, SKM Powertools or Easypower.
To determine the best way to get a protection coordination study completed check out this post from a couple of weeks ago. In it I explain the various methods that you can use to complete a power system study, which can work for any individual study.
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Power System Study Deliverables

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Now that you have decided on the project framework that you are going to go with and you understand the scope of the power system study, you need to prepare the list of deliverables.

Power System Report drafting
Let’s outline the power system study report

There are 3 items that must be included in every report, and there are some things that should NEVER be included in a power system study report body. There are some sources of information that are important to have as backup to both you as the client to evaluate the consultants expertise in the area, and also to educate the non-electrical workers and managers in your company.

Power System Study Deliverables

The 3 overall deliverables that should be included at the end of the project are:

  • Power System Study Report, complete with recommendations
  • Updated drawings with any new information – onelines, switchgear drawings, etc.
  • Power System model in the software of your choice – complete with any custom libraries.

Depending on the scope of the study, you may want to include the following:

  • Capital Budget estimates to implement selected recommendations
  • Updated Electrical Safety Program Documents
    • Typically labels from the incident energy study
    • Updated information for Energized Work Permits

Now lets describe the critical parts of each of these.

Power System Study Report

We outlined the sections that will be included in the report when we discussed scope, but there we only mentioned the specific study results and a section for recommendations.
The report isn’t the place to educate the reader about what the different studies are, the pro and cons of different methodologies, or discussing why it was a good idea to have the study completed. If the report will be distributed to people that don’t have an electrical background and need to be brought up to speed, here’s the tip I use at JMK Engineering:

Include appendices with articles and short whitepaper reports describing what the studies are and how they can be used. This keeps the report on topic, but expands the audience of the complete report.

Introduction

The introduction is the place in the report where the reader is orientated as to what this report is all about. The introduction should include the purpose and scope of the report, the methodology used to gather data and study the system, and standards that were referenced during the work. The sub-headings include:

  • Purpose
  • Scope
  • References
  • Definitions
  • Assumptions and Limitations
  • Project Methodology

Study Analysis Sections

Each study – short circuit, protection coordination, incident energy and load flow – will have their own chapter in the report.  Each of these will have the following sub-sections:

  • Introduction
    • Study specific scope items
    • Study specific source information – utility supply, motor loading assumptions for load flow, system configurations reviewed, etc
    • Thresholds used for warning and critical for the results.
  • Study Results
    • This is only a summary of the results including separate tables for the warning and the critical items.
  • Study Recommendations
    • These recommendations are for the what needs to be done, not the how.  It could be that all the critical lines need to be addressed, but it could be a procedural change (don’t put the system in that configuration)

Appendices

This is where the report starts to grow. The appendices will include the complete analysis outputs for the various studies, including all TCCs for the protection coordination studies. The appendices should also include any pertinent source information that was used in developing the model and report. Here is an example list of appendices that I have included in past reports:

  • Original Proposal (including scope, etc)
  • Data Collection Sheets
  • Relevant Correspondence
    • With the client, utility, suppliers and anyone else that provided useful information that was used in the analysis.
  • Incident Energy Labels
  • What is Short Circuit, Selective Coordination, Incident Energy and Load Flows.
    • This is 4 different articles in one appendix.

Updated Drawings

Next the drawings that are affected by the study and its recommendations need to be updated. What I have found is the drawings lag behind the changes in the field over time, and during the data collection effort we find a number of conflicts, equipment added or removed and settings changed.  The power system study is the perfect opportunity to as-built all these drawings.
The oneline is always caught, but there are other drawings that should be looked at for inconsistencies.  These can include:

  • Threeline diagrams
  • Motor Schematics
  • Switchgear and MCC Drawings
  • Panel Schedules

Power System Model

If you are having an outside resource develop the model, we still recommend you receive a copy of the model and any custom library items electronically. Ideally this will be included on a thumbdrive and included in the report as an appendix.  The best reason for this is that the power system study report is not a static thing, as the system changes it will be updated, and per CSA Z462 and NFPA70E it should be updated no less than every 5 years. After doing all the work – and spending the money – to get the original completed, it is much easier to update with changes than go through the entire process every 5 years.
If the contractor moves along, or doesn’t have good data retention practices, you will lose this data. Get a copy for your records.

Optional Deliverables

The list of deliverables is not exhaustive, and depending on the scope of the power system study you will include others.  Two that I recommend are capital budgets and electrical safety program updates.

Capital Budget Development

You likely have a capital budget cycle that is used to determine what capital projects will be completed in the coming years.  Based on the recommendations outlined  in the report, there are typically additional engineering and installation that will be needed to fulfill the recommendations and make the system safe. To develop these budgets there is some initial engineering design required, but the entire design does not need to be completed in most cases. To present a complete package to management, and allow business decisions to be made, the capital budget report should include the following for each recommendation:

  • Recommendation and why it is needed (Scope)
  • Options – if there are any
  • Capital cost for each option in today’s dollars
    • This should include engineering, procurement, and installation/construction

If there are similar recommendations, like setting changes to relays, these can be grouped.

Electrical Safety Program Updates

If you included an incident energy study in your report, and you have an existing electrical safety program, you will want to make any necessary updates. This may include adding information to the energized work permits, adding new labels and updating the training information

Conclusion

The deliverables related to a power system study are not just labels and an output for the software. To be useful, informative, and actionable we need to include everything mentioned above.  A good consultant will include all the critical deliverables even if you haven’t included them in the RFP, however the lowest price may cut out most of them and provide the bare minimum.
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Getting a Power System Study Completed – Who does what?

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Now that you have determined that you need a power system study, and you have developed the scope of the project, you need to determine how you are going to get the study completed. To start with there are a lot of questions that need to be answered. Do you buy the software and tackle it yourself? Do you hire an outside company to put it together? How you answer these questions will determine the next steps, but we are 90% sure what the best option will be at the end.

Teamwork is key for a successful power system study
Teamwork is key for a successful power system study – Flickr

 

Getting Started

There a three basic way to approach getting a power system study completed:

  • Completely In-House
  • Completely contracted out
  • Hybrid of both

These all have their pros and cons, and depending on the size and complexity of your company and system the method that is chosen will be self-evident. I will explain the pros and cons of the two extremes, all in-house and all contracted out and then explain why I think a hybrid option is typically the best answer, even with the added administration over completing the project inhouse.

InHouse

The main advantage with completing the power system study inhouse is control. If you, or a co-worker, is completing the study they already have access to the equipment, or know who does.

Outsourcing

The advantage of having an outside resource do the work for you is that you can put together the project scope, pick your favorite consultant and walk away for the most part until they have questions, and finally are ready to present the report.  With this method you have to facilitate the consultant with getting the information and then answering any questions that they may have.

Hybrid Model

The hybrid model is a combination of both, completing the areas that you are competent and have resources available for inhouse, and then outsourcing the areas that you may be weaker – or don’t have the resources available – to a competent consultant.  A common split would be for the owner to gather all the data required and update the drawings, and the consultant build the model with this information and produce the report.
The reason I think that this is the preferred method, and will help keep the project schedule and costs reasonable is because it will allow you to leverage your strengths (knowledge of the system, process, business, etc) and limit your weakness such as knowledge of the modeling software, interpreting the results, working with utilities, etc. These are all tasks and areas that you may not work with on your day-to-day business, but a reputable consultant will.

Our Recommendations

There is no right model for every situation, however we almost always recommend some type of hybrid model. The site should retain control of the Electrical Safety Program, developing the risk matrix, and even gathering the input information. However, outside resources will typically be more efficient for developing the system model, the incident energy report and at times even the data gathering around the site.
Always work with your consultants to divvy up the work in a way that leverages the strengths and limits the weaknesses of each group.
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Power System Studies – The Critical Four

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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

Power SystemsThe 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.

Bloomberg
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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