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Power Engineering

Generator Protection: Grounding and Ground Fault Protection

Cole Ferguson · Nov 3, 2016 · Leave a Comment

An important part of generator protection is that generators need to be grounded. And because they need to be grounded, generators are susceptible to ground faults. This article is going to talk about general case ground faults on a generator and the appropriate use of generator protection, and then talk about the specific case of a ground fault on the generator field coils.

Your generators need to be grounded for both safety and reliability.
Your generators need to be grounded for both the safety and reliability of your facility.

Ground Fault Generator Protection

One of the most important things to note when protecting against ground faults is that the higher the magnitude of the grounding impedance, the smaller the magnitude of your fault current will be. This makes it difficult to detect weaker faults with high resistance grounding. For example, a differential relay might not pick up a single phase to ground fault because the current magnitude change is not enough to cause the relay to trip.
With high resistance grounding, you need a relay on the grounded neutral to catch ground faults that are single phase to ground, since the relay on the neutral doesn’t care about load current. However, when the magnitude of your ground resistance is high enough, it becomes difficult to differentiate between low magnitude fault currents and harmonics.
An important thing to remember is that all of the relays in your generator protection system need to be coordinated in order to function properly. The last thing you want is for a ground fault detected on a relatively unimportant piece of equipment, like a table saw plugged into the wall, to kill the power to your whole building! You can learn more about coordination here.
The following image is an example of a unit-type grounding system.

A unit type grounding system is a very common form of generator grounding.
A typical unit type grounding system.

This is a fairly common grounding configuration for generators. There is a high resistance ground at the generator, connected to the system through a distribution transformer. The resistor and the relay are grounded in parallel on the grounding transformer.
Now that we’ve talked about generator protection for ground faults in a general case, I’m going to talk specifically about generator protection in the case of field grounds.

Field Ground Generator Protection

A field ground is exactly what it sounds like: a ground on the field of your generator. A single phase to ground fault on the field of a synchronous machine should in most cases produce no immediate damaging effect. Don’t let this fool you though: the field ground fault still needs to be cut off. A single phase to ground fault on a field is dangerous because a second ground fault could short part of the field winding, causing damaging vibrations. A field ground fault must be detected and removed quickly and efficiently.
The following is a circuit that could be used for field ground generator protection.

A field ground protection circuit is important so that you don't fry your field winding.
A possible field ground protection circuit.

In this circuit, the relay uses a voltage divider consisting of two linear resistors and one non-linear resistor. The resistance value of the non-linear resistor varies with applied voltage. When the field becomes grounded, voltage develops at the point between the two resistors and the ground. The magnitude of this voltage depends on the exciter voltage of the generator, and the location of the ground. The maximum voltage value is obtained if the field is grounded at either end of the winding.
There will be a point on the field winding where a ground fault will produce no voltage between the middle point and ground. This “null point” is the point on the field winding where there is an electrical balance between the two field winding resistances and the two relay resistances. The non-linear resistor varies the location of this null point so that a ground can be detected at any point in the field winding.
Finally, you have a pushbutton connected across a portion of the R2 resistor. This permits a manual check for possible ground faults at the center of the winding. This is so that if the generator is base loaded and will not experience periodic excitation variations, you can still check for ground faults.
 

Closing

Hopefully now you have a better understanding of how to use grounding and relays to provide generator protection against ground faults. If you have any more questions, feel free to shoot me an e-mail at cole@jmkengineering.com. You can also sign up for the Sparky Resource newsletter below, which goes out every Sunday. It’s got all kinds of cool stuff in it, possibly including answers to questions you might have after reading this article!
As always, thanks for reading!

How to Detect Generator Faults with Generator Protection

Cole Ferguson · Oct 27, 2016 · Leave a Comment

A fault could interrupt service and must be dealt with immediately.
A fault could interrupt service and must be detected immediately.

Your generator protection must have a way to detect generator faults. A fault is any unwanted current flow in an electrical system. Faults can cause all kinds of problems for your generators, including current loss, interruption of power delivery, and damage to the generator due to overheating.
There are three things your generator protection needs to do when your system encounters a fault. First, the protection system has to detect generator faults. Next, the circuit in which the fault occurs has to be tripped. Finally, the specific physical location of the fault has to be found so that the fault can be repaired.

How to Detect Generator Faults: Multiphase Faults and Differential Protection

One of the best ways to pick up multiphase faults on any machine, generators included, is by using differential protection. This is a fairly simple implementation of protective relaying.

Faults can be detected on any kind of protected equipment with a very simple circuit.
A simple circuit that can be used to detect faults on any protected equipment.

The circuit consists of:

  • Two current transformers (CTs), one on either side of the protected equipment. We’ll call these A and B.
  • A relay between the two CTs.

Here’s how this circuit can detect generator faults: the logic is that the current going into the generator should be the same as the current coming from the generator. The relay is tripped if the difference between the current passing through A and B grows too large.
You want to make sure that you use a percentage differential relay here to mitigate error that could be caused by non-identical CT properties. In an ideal world you would have ideal CTs, and they would have the exact same properties. Real world manufacturing processes try to get as close to this as possible, but your CTs will always have some kind of tiny differences. So instead of the relay tripping when the difference is a hard value, it trips when the difference is a percentage away on either side of where the current value should be.
The problem now is that you know the general area of where the fault is, but do not have an exact location.

Locating the Fault

So you know how to detect generator faults, and based on where the relay tripped, you have a rough idea of the location of the fault. What you need to know now is the exact location of the fault so that the fault can be repaired.

Generator faults can interrupt regular service and need to be protected against.
But how do you go about finding a fault?

As usual, before starting work, make sure that the section of the system you are going to be working on is de-energize. It’s time to troubleshoot!
The first thing that you can do is look for any visible signs of damage. Any kind of blackness as though something burned is a good indicator of a fault location (or at the very least something that needs repairs). If the fault occurred very recently you’ll probably be able to smell it too.
The next thing you can do is test the circuit with your trusty multimeter. Start at one end and work your way through the circuit. If you can, get your hands on the circuit diagram and find out what the multimeter is supposed to read if the circuit is operating properly.
If you can’t get at the circuit for one reason or another, you can try a time-domain reflectometer. These send a pulse down the wire and pick up changes in impedance, which reflects the signal back to the device. This will tell you how far down the line your fault is.

Closing

So that wraps it up! Hopefully now you know a little bit more about how to detect generator faults and how to locate a fault once it’s been detected. If you have any more questions, feel free to shoot me an e-mail at cole@jmkengineering.com. You can also sign up for the Sparky Resource newsletter below, which goes out every Sunday. It’s got all kinds of cool stuff in it, possibly including answers to questions you might have after reading this article!
As always, thanks for reading!

Industrial Power System Configuration – Main Tie Main

Cole Ferguson · Aug 1, 2016 · Leave a Comment

Robotworx-arc-welding-robots
Industrial power systems can become very complex, and the related processes need a reliable source of electricity to keep the process running, both for economic and safety reasons. There are many different kinds of distribution systems. This article will only focus on one of them: the Main-Tie-Main system configuration.

Industrial Power System Main-Tie-Main Configuration

Main-Tie-Main, also formally referred to as a “secondary selective system” consists of two independent circuits connected together at the load buses by a tie breaker. See the figure below for details.

A simple main-tie-main system configuration
A simple main-tie-main system configuration

Reliability

The biggest advantage that a for an industrial power system that a main-tie-main configuration has over other system configurations is reliability. Usually, the tie breaker is normally open and the system acts as two independent circuits supplied by two independent sources. For example, we will assume that there is a fault on Source 2. This fault trips CB2, cutting off all power to Load 2. Immediately after power is removed from Load 2, the Tie Breaker closes. Source 1 is now providing power to both Load 1 and Load 2, and the system is able to perform its normal functions until the fault at Source 2 is repaired. When normal power is restored and CB2 is closed, the Tie breaker opens and the system resumes normal operation. Operation is only interrupted for a very brief moment, if it is interrupted at all!
The Main-Tie-Main configuration is also good for maintenance for this very same reason: you can open CB2 and perform repairs upstream from Load 2 de-energized, while still supplying Load 2 with power.

Transformer Sizing

One characteristic of the industrial power system main-tie-main configuration is that both transformers must be sized to appropriately handle the load of both buses. In our example, we will assume that both Load 1 and Load 2 are the same size. Transformer 1 and Transformer 2 must each be sized so that in normal operation they are only loaded to 50%. This way when the tie breaker closes, the transformer that is now supplying both loads doesn’t become overloaded and blow up in your face. The downside to main-tie-main is that the added reliability inherently costs more due to the system requiring larger transformers than a system that would not tie both circuits together.

power transformer
Unfortunately, reliability costs money, and you’ll need to oversize your transformers to add reliability to your system in a main-tie-main configuration.

You can also reduce strain on each individual transformer when it is supplying both loads by adding external cooling to the transformers (like a fan cooling system). You can also simply accept that the transformers will have a reduced life in the event of a fault on a transformer or source.
You will not have just two loads in every case. In systems with multiple loads supplied by a single transformer, transformer size (and therefore cost) can be reduced by designing the system so that only essential operational loads (such as emergency lighting) are supplied power when the tie breaker closes.

Summary

For industrial power systems, a main-tie-main configuration is an extremely reliable power system distribution model, able to maintain power during a fault with little to no interruption. Unfortunately, this added reliability has a cost, whether in the form of larger transformers, extra cooling systems, or shorter transformer lifespans. These factors should all be considered when designing an industrial power system with main-tie-main in mind.
As always, thanks for reading! If you like this post but want some actionable advice, tips and information, check out our newsletter. You can sign up here or below. By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.
 

4 Ways to save money with a power system analysis at your pulp and paper mill

Jeff MacKinnon · May 31, 2016 · Leave a Comment

Paper Mill
Sometimes it seems that the number of requirements for running a plant keeps going up, and regulations keep changing cutting into the bottom line. This is how a lot of clients I have worked with in the past have approached getting a power system analysis completed, they needed labels on their equipment to satisfy the requirements of CSA Z462 and NFPA 70E, so they needed an arc flash study. It was an expense, nothing more.
However, if you define the scope (the first step in performing a power system analysis) smartly, you can duplicate effort without duplicating cost, and use the report to make smart decisions that will either save money immediately or identify capital projects with very fast payback.

1. Reduced number of Electrical Safety Incidents

According to NFPA (source-pdf) the average shock or arc flash injury event can cost $80k in direct costs, if indirect costs are included this can be much higher. However, according to that same paper there are no valid ratios to estimate this.
With a power system analysis in hand, an effective electrical safety program can be developed that will directly affect the number of these incidents. The number of these incidents are typically very low, however the high cost of a single incident will pay for the power system analysis and electrical safety program many times over.

2. Better preventative maintenance program

Motor with pump industry in factory
Competitive pulp and paper mills have at least a preventative maintenance program, and a lot are moving to a predictive maintenance program. The most important part of these programs, like a valid power system analysis, is the quality of input data. When going through the process of validating all the input data for a power system analysis, it would be a simple matter of gathering the information for your maintenance program without duplicating effort. It is likely the same staff that will be doing the work in anycase.
With better information, including short circuit values, expected load flow voltages, etc, you can feed this information into the maintenance program and understand when equipment may fail, allowing you to plan its replacement without affecting the process.

3. Power Quality Improvements

When going through the system and gathering all the necessary input data for the incident energy study, you have all the inputs for proper load flow study, and all you need is some existing load information that can be gleaned from the power meters throughout your mill.
This is where you will see an opportunity to get the most value from the power system analysis. Like most industrial plants you are likely charged:

  • a energy fee (MW-HR),
  • a peak charge (rolling MW) and
  • sometimes a power factor charge.

You can minimize your power factor and energy fees by making sure that you are running your system as close to unity power factor as possible. With a proper power flow study, you will be able to identify areas within you plant that would be best suited for adding power factor correction capacitors and quickly identify the potential payback.

4. Update Drawings of the System

Finally, and likely an over-looked part, is that you will have updated drawings as part of your power system analysis that are very accurate, current, as-built conditions. These drawings are now trusted inputs for any capital or maintenance projects that may take place in the future.
IMG_0004One of the biggest risks to integrating into any existing system, is the quality of the existing documents, and the cost of having a consultant on site developing as-built drawings will increase the cost of a small project quickly. Having these drawings on hand, and assuming you have a document control procedure in place, the consultant replacing a motor or adding a new system will have the best information starting, reducing the design time, construction issues, and start-up concerns.

Next Steps

If you like what you read consider joining our newsletter where you will get every post in your inbox at the start of the month. You will never miss a thing.  If you would like to learn more about how a power system analysis can help you manage your system better and safer, contact us here, or visit our services page here.
And if there is someone you think would benefit from this post we appreciate any share on facebook, linkedin or the old fashioned email.

Getting a Cement Plant Power System Report Completed

Jeff MacKinnon, P.Eng · May 24, 2016 · Leave a Comment

Cement SiloOver the last couple of weeks I’ve talked about the steps to get a power system analysis completed, and the different execution strategies.  Today I want to talk specifically about cement plants.
Cement plants have a cyclical profile where they are producing product before the building cycle, and full out throughout the building cycle (spring to winter) to keep with demand, then annual maintenance in the winter until it is time to get started again.

Hybrid Approach

With leaner engineering groups in all of industry, we recommend a hybrid approach with you using internal electrician and engineering resources to gather the data needed for the complete power system analysis.  If there aren’t any power quality concerns at the plant, or your power system study isn’t current we recommend getting started with:

  • Short Circuit,
  • Protection Coordination, and
  • Incident Energy (Arc Flash)

Recommended Scope of the Power System Analysis

The cement plant power system is large and complex, with a range of voltage levels from 100kV to 120/208V. If you are starting from scratch at the plant, or if you (or the person running the project) don’t have a lot of experience with power system studies, it is best to chunk out the system in bit sized chunks. But make sure that how you break out the system is done in such a way that you will have a useable product at each stage.
Business cartoon showing two businessmen looking at complex writing on a whiteboard. One man says, 'when you put it like that, it makes complete sense'.

How much of the Cement Plant Power System

For example, we recommend that you model the entire MV system, from the incomming from the utility down to any 5kV that is on the plant.  This will allow you to address all the large motors, generation and utilities on the site that will account for the majority of the fault availability. Depending on the size of the system and budget, this may be all that you get done in the first round.
The next step is to pick one of the feeders in your system, or a single area of the plant and complete the model (and report) to the 600 V (or 480V in the US) system and include all motors 25hp and over, and to the secondary of any 120/208 distribution panels. Motors less than 25hp will likely not affect the available back feed into a fault, and therefore can be neglected.

120/208V system

Example of a full label
Example of a full label at 208V

The 120/208V distribution panels may be contentious and not needed at your facility. I have typically neglected them in the past, however there are still a lot of electricians that will work on a panel at this voltage energized without a second thought, I have started recommending labeling these panels with full Arc Flash/Shock Warning labels with the intent of raising the awareness that the hazard is still present, and PPE is required.

Electrical Safety Program

In the 2015 revision of NFPA 70E mining was removed from the exceptions, and MSHA has endorsed NFPA 70E as the standard for PPE selection with regards to arc flash. What this means to you is that NFPA 70E is the de-facto workplace electrical safety standard for cement plants.
Having a current (every 5 years) arc flash analysis, which includes a power system study, is a requirement for any successful electrical safety program.

Getting Started today

If you aren’t in a position to get started with a power system analysis at your cement plant today, sign up for our power system newsletter and you will receive a technical spec that you can use to get pricing when you are ready to get started.
If you are ready, to go NOW, give us a call and we can provide a free quote and execution plan to ensure that the report meets all operational needs of your engineering and safety groups.
As always, if you have any comments or questions, don’t hesitate to give me a call.
Regards,
JM

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