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Generator Protection: Grounding and Ground Fault Protection

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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 Protect Against Ground Faults

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I’ve already talked a bit about what grounding is, and what types of grounding are most common. What I haven’t really talked about yet are ground faults and ground fault protection. While specific details will depend on the type of grounding that your system uses, some general principles can be applied to most systems.

This certainly caused a ground fault!

What is a Fault?

A fault can be defined as any time electric current in a circuit is going somewhere that it shouldn’t be. You can have faults anywhere in your system. For example, if electricity is arcing from one power line to another, that’s a line-to-line fault.
The most common type of fault that occurs in electrical systems is a ground fault, where the electric current is running from somewhere in your circuit to the ground. Causes of ground faults could include, but are not limited to:

  •  Faults caused by inclement weather. If a tree falls over in a storm and rests on power lines, you’ve now got a path from the lines through the tree to the ground.
  • Reduced insulation from age. If a cable is old and has been in use for a while, the heat from current passing through it could break down the insulation.
  • Reduced insulation from other sources. This could include moisture from high humidity or physical stress placed on power lines when they collect ice in the winter.
  • Excessive overvoltages can also cause faults. You’ll remember in my article about different types of grounding systems, ungrounded systems had unnecessarily high overvoltages. This means that if you don’t immediately clear the first fault, more faults could quickly develop!
Stress from all this ice could definitely cause a fault.

How can I protect against faults?

The goal whenever you have a fault is to clear it (isolate the part of the circuit with the fault so that there is no power going through that particular line). In order to clear a fault, you first have to determine whether or not you have a fault, and then you have to determine where the fault came from.
To determine whether or not you have a fault, you can use a current transformer (CT).

A very basic CT.

A CT is hooked up to a meter that gives a reading based on how much current is flowing through the primary side of the transformer: 5A of current in your CT might mean that you have 500A of current flowing through your ground line. So as soon as you have a fault, the meters hooked up to your CT will let you know that your ground line is getting current when it really shouldn’t be.
You want to have multiple CTs attached to different points in your circuit. For example, most if not all substations in your electrical power grid will have CTs and PTs (potential transformers that measure voltage differentials) so that if anything goes wrong on the grid, the power company can isolate the problem right away.
Once you know that you have a ground fault, you’ve got to clear it. You can do this by using a selection of relays, circuit breakers, and fuses.
A simple relay

A relay is a device with adjustable settings that is used to determine whether or not to trip (open) a circuit breaker. Because the relay is the device that makes the decision of whether or not to cut the power to a portion of thecircuit, it must be able to activate blindingly fast, on the order of milliseconds. This is to make sure that when a fault occurs, the section of the circuit with the fault is isolated as soon as possible to prevent harm to people and damage to equipment.
A circuit breaker is a pathway that is mechanically forced open when too much current passes through it.
A fuse is essentially a wire that is designed to be destroyed when a certain amount of current passes through it. The difference between a fuse and a circuit breaker is that when a fuse is activated, it is destroyed and has to be replaced. When a circuit breaker is activated, it can be re-set by physically closing it again.
 
Circuit breakers. Unlike fuses, you can see that these can be switched open or closed.

 
A fuse. Although this one is small, the operating principle is the same for larger fuses.

 

Protection in Action

In a typical scenario, a CT or PT senses current or voltage along one of your power lines. If there is a fault and the current or voltage level increases to an unacceptable amount, the relay opens the circuit breaker and isolates the area of the circuit. There may also be fuses somewhere in the system that will burn out and instantly open the circuit if too much current passes through them.

A simple protection circuit.

One important thing to be sure of is that your protection devices are all properly coordinated with each other. You want to be sure that a fault at one end of your system does not completely take down your whole system. Your
A GFCI. You should be able to find these just about anywhere, from homes to businesses to schools.

relays should be connected in a network. If one relay trips a breaker, it should be sending a signal to all relays closer to the source, telling them not to trip because the fault has been taken care of. This is another reason that relays need to be fast: if your devices don’t work fast enough, you might end up with multiple relays unnecessarily tripping circuit breakers. Unwanted circuit shutdowns cost time and money to fix.
You can even observe ground fault protection in your home: if you live in a newer home, you should have some ground fault circuit interrupters (GFCIs) installed at your power outlets. These also monitor for current imbalances and will shut off the power to that outlet if it detects a fault.
 
So, to quickly recap:

  • A ground fault is an unwanted electrical connection between a line and the ground. It can be caused by insulation breakdown, bad weather, or physical stress.
  • You can protect against faults using protective devices, including CTs, PTs, relays, circuit breakers, and fuses.
  • Your protective devices must be coordinated for the best protection and efficiency in the case of a fault

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Equipotential Zones: Keeping you safe during energized work

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If you are working on high voltage equipment (like power lines), you need to be sure that you are protected from shock hazards that could arise from an unexpected fault. Grounding makes sure that current will be directed along the ground or neutral wire in a system during a fault, but there is still the risk that if you touch the live wires, the current could travel through you into the ground instead of through the neutral line.

How can we stop the current from flowing in to your body at all?

How can we make sure that a lineman is safe even if he accidentally touches the power lines?

We know that electrons need a voltage to flow: without a difference in electric potential energy between two points, there is no voltage, and therefore no current. Grounding can be used to create an “equipotential zone”.

What is an Equipotential Zone?

This matches all the objects in the zone to the same potential, so the voltage between any two objects in the equipotential zone is zero.
Let me take a step back and explain. The reason this works is because in mains electric systems (AC power being delivered from the power plant to your home, or office, or warehouse, etc.), they use the ground in the earth as their “reference voltage”. So all voltages are calculated considering the ground as “zero volts”. What this means is that when you hear people talking about 120 volts in your house, they’re saying that the difference in voltage between the wires in your house and the ground is 120 volts.
Normally, if your system is balanced 3 phase power (like properly operating electricity in your home), there will never be any current flowing into your neutral wire: the potential of the neutral is the same as everywhere else. This changes if there is a fault. A fault is just a low resistance (meaning high current by Ohm’s Law) path for the electricity to travel back to its source (since the circuit has to be complete for current to flow).
If there is a fault somewhere in your system from a main line to some metal surface not designed to carry current, you now have a path through the ground for the electricity to go to. This means that if, for example, you touch a metal door frame, electric current could go through the door frame, to you, to the ground.

We can prevent this by creating an equipotential zone

An equipotential zone is created by grounding all of the metallic surfaces that are not part of your electric circuit. For example, in buildings, in addition to having the ground wire connected to the earth, the grounding wire is also connected to all of the structural metal, all of the metal water pipes, all of the metal everywhere. So, if there’s a fault, it’s not as big of an issue because instead of raising the doorframe voltage relative to the ground, you’ve raised the voltage of the whole building, including the ground, to match the system voltage.
You’ve experienced this type of grounding, called “bonding earthing conductors” if you’ve ever had to jumpstart a car: the negative jumper cable on the side of the dead car is always connected to the metal of the car frame instead of the car battery itself, right?

The black (negative) cable connected to the dead car is always ground somewhere on the engine itself, not the battery terminal.

What if the neutral pathway to ground is lost?

By now you’re probably wondering, what happens if the pathway to ground is lost, or broken? With your neutral wire no longer directing the current to the ground, you would normally now be in danger of the current traveling through you. However, you are still in an equipotential zone: now that everything around you has been brought to the same voltage as the power lines, there will not be a voltage difference between any two points that you can possibly touch. No voltage difference means no current, and not current means no danger!
Of course, this is why bonding earthing conductors are so useful: if all of the metal in a building is connected electrically, including water pipes and structural steel which extend into the ground, then your whole building has essentially become a giant ground rod and you actually never lose your path to ground!
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Three Different Types of Grounding

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Today I’m going to give you a brief overview of three different types of grounding systems that are important.

A basic representation of a grounding system

These three systems are:

  1. Ungrounded Systems
  2. Resistance Grounded Systems
  3. Solidly Grounded Systems

I’ve already talked a little bit about what grounding is, including giving a brief overview of why we do it and what it’s used for. If you haven’t read that article yet, go give it a read before continuing.
Finished reading about what grounding is? Alright then, let’s get to the meat and potatoes of today’s topic.

Ungrounded Systems

“Whoa, hold on”, you might be thinking, “We just finished reading about how important grounding is for safety! Why would we have ungrounded systems?” The answer is that we shouldn’t really have ungrounded systems, but they do exist and they do have their purposes.
You see, an ungrounded system isn’t really ungrounded. Electrically, your system is connected to ground through the capacitance between the lines and the earth, so you can say that it’s a capacitance grounded system. We just call it ungrounded because of convention, and because there isn’t a direct physical connection between any of your power lines and the ground.

Advantages

There are a few advantages to having an ungrounded system. The first is that since your system is never physically connected to the ground, you have negligible ground fault current. For example, in a 3-phase system, because all of the ground fault current is capacitive, when you have a single line-to-ground fault in an ungrounded system, the current and voltage you would lose is negligible, and is instead carried by the other two lines. This allows you to continue operation unimpeded during a single line-to-ground fault.
The other big advantage is that because of the negligible ground fault current, special ungrounded systems can be used to minimize shock risk to people. An excellent example would be medical equipment in a hospital: a patient is directly connected to the machine, and if a fault were to occur, electricity might be able to flow through the patient and into the ground. Because the ground fault current is negligible in an ungrounded system, no power current will pass from the machine through the patient into the ground.

Disadvantages

Of course, the disadvantages of an ungrounded system are obvious. If there is a fault, you are now using two wires to carry an amount of current that was allotted for three wires: the increase in current and voltage will jack up the heat, and the extra heat will wear out your insulation much faster. Worn out insulation could lead to unnecessary damage to your electrical system, particularly at motors.
The other big disadvantage of an ungrounded system is that it is incredibly difficult and time consuming to locate any faults. Each line must be tested individually, which is a very slow process that completely interrupts service. The opportunity cost of a fault in an ungrounded system is very high.
Ungrounded systems were the norm back in the 40’s and 50’s, but because their disadvantages outweigh their advantages in most scenarios, you won’t see too many new ungrounded systems today.

Resistance Grounding

Resistance grounding is when you have a connection between your neutral line and the ground through a resistor. This resistor is used to limit the fault current through your neutral line: if your voltage doesn’t change, then your current is dependent on the size of the resistor according to Ohm’s law (V=IR).

Advantages Over Ungrounded Systems

Because the current in the neutral is controlled instead of negligible, system overvoltages are also controlled. This reduced current and reduced overvoltage means reduced heat, which keeps the wear and tear of your electrical system to a minimum. This is especially important for keeping your motors safe, since the reduced current will not damage the magnetic iron of the motor (which is costly to repair). The reduced currents also reduce the risk of shock and arc flash/blast hazards.
There are two types of resistance grounding: high resistance grounding and low resistance grounding.

High Resistance Grounding

High resistance grounding is typically used to limit ground fault current to < 10 amps. The low ground-fault current also means that, just like an ungrounded system, you can continue to operate the system on a single line-to-ground fault. The low current will typically not trip your protective devices during a single line-to-ground fault.
Overall, you want to use high resistance grounding when you need low fault current and still want to operate with a single fault. High resistance grounding is typically seen in retrofits of previously ungrounded systems in addition to new systems.

Low Resistance Grounding

Low resistance grounding typically limits ground fault current to between 100 and 1000 amps. This offers a similar advantage to high resistance grounding in that you can control the ground fault current, which means you can design your system to withstand the currents without damage.
Low resistance grounding systems have the benefit of tripping your protective devices when there is a fault. Their purpose is to immediately cut the power to the circuit, and so unlike the high resistance grounding systems, a low resistance grounding system will not maintain operation during a single line-to-ground fault.
Low resistance grounding also reduces overvoltage, and is used in medium voltage systems of 15kV or less, typically where big generators/motors are used.

Solid Grounding

Solid grounding is what you get when you connect your system directly to the ground, without any sort of resistance in the way. The ground is typically connected to the system at a neutral point, like the neutral terminal of a generator or transformer.

Pros and Cons

Solid grounding, like resistance grounding, can greatly reduce overvoltages in your electrical system. However, solidly grounded systems have the potential to have huge amounts of ground-fault current. As a result, solidly grounded systems cannot operate with a ground fault (since all of the current in the system is going from fault to ground).
Solid grounding has two main uses:

  • In systems with voltages of 600V or less, solid grounding can be used if it is not necessary to maintain operation of a faulted circuit.
  • In systems with voltages of 15kV or greater, solid grounding can be used if high ground fault currents are desirable of any reason, such as quick ground fault detection (since the high current will most definitely trip protective devices).

Recap

  • You can use ungrounded systems when you want to have negligible ground-fault current.
  • Resistance grounding offers the advantages of ungrounded systems without the risk of large overvoltages.
  • Solid grounding reduces overvoltages but has high ground-fault currents.

At the end of the day, the type of grounding you use for your system will depend on which type of grounding best suits your needs and budget.

What is Grounding?

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What is grounding in an electrical sense? Anyone with a little bit of a background in electronics will tell you that grounding is something you need to keep your circuit safe and it keeps the electrical parts of your circuit from blowing up. But what is it, exactly?

We see the symbol for grounding all the time, but what does it actually mean?

Grounding is a connection between an electrical circuit and an arbitrary reference point.
Grounding is a method of protecting your electrical system and the people using it from harm by setting a known “reference” point for voltage. When you hear someone mention ground, you should ask them to clarify: is it the reference voltage, or is it earth itself? Or is that both? In order to explain grounding, we need to know a little bit about voltage and current. Before you continue, read our article “What is electricity?” It has a bunch of the background knowledge that I’m going to assume you know when I talk about grounding.

Grounding: A History

The earliest instance of grounding that I can think of is when Benjamin Franklin invented the lightning rod.

A lightning rod in action

A lightning rod is a metal rod attached to the top of the house,  connected by a wire to another rod stuck in the ground. When lightning would strike, it would seek out the rod (since the rod has a lower resistance than the rest of the house) and electricity would flow into the ground. This prevented houses from burning down when struck by lightning, which can generally be considered a good thing.
If a lightning rod can be seen as the first real instance of grounding, then we can look at grounding as one part of the overall electrical protection system for your circuit. So in order to keep things safe, you need to figure out a way to create a pathway for the electricity to travel that has less resistance than your body, fragile circuit components, and building materials.
In the event that something goes wrong in your circuit for whatever reason, you need to direct the electric current along a very low resistance pathway. We’ve already talked about how to figure out what resistance is, but what kind of wire would have a low enough resistance to draw all the current?

Why not use the ground itself?

Think about it: use the planet earth as a conductor. Not only does the electricity not have very far to go to get in to the ground, but it also has a HUUUUUUUUUGE cross sectional area, meaning it has very, very little resistance. Electrons always want to go to the place of least resistance, which makes going into the ground their favourite place.
The lightning rod used this concept to great effect, and modern day power companies do the exact same thing. This also doubles as a useful way to complete your circuit: by using the earth itself as your ground wire, you can be sure that every point in your circuit (which in this case is the entire power grid) is connected to the same reference point. This means that when we now talk about voltage (for example, the 120V wires in your house) we know that we’re talking about it with reference to the voltage of the ground itself. There are 120V between the wires in your house and the ground you’re standing on, there are 1000kV between the overhead power lines and the ground, etc. What this also means is that the current in your circuit now has a clear and easy return path to its source: the current can just flow into the ground near, for example, your house, and travel back through the ground to the generators in the power plant.

But isn’t the system still unsafe?

You might see the new problem with that: if electrons want to go in to the ground, how do we get them there safely? If you are standing on the ground and you touch a wire with electric current moving through it, won’t the current just go through you and into the ground, instead of going through the wire?
Current flows through all available pathways at once. This means that even if the ground wire path is the least resistive path for current to flow through, you could still harm yourself by adding yourself to the circuit when you touch it (making a path from the wires through you to the earth). Initially, I said that grounding was to keep your circuit safe, and I wasn’t lying to you. The reason grounding keeps the circuit safe is that grounding makes up a part of your circuit’s protection system.
Most circuits are designed so that when there is a fault to ground in the circuit, the majority of the current will flow through this fault, into the ground wire, and into the ground. Because we know where the current will flow, we can put protective devices along the ground path in such a way that as soon as the fault occurs, the protective devices (such as a fuse or a breaker) are tripped very abruptly. When this happens, the complete pathway for the circuit is broken, and current cannot flow. If current can’t flow, we have no voltage difference, and if we have no voltage difference, we have no real danger. This is the most common way that grounding protects circuits: it forces protective devices to turn the circuit off ASAP.

A fuse is a protective device that gets destroyed when too much current goes through it, cutting off power by breaking the circuit

In order for grounding to work in tandem with your protection system, you need to know how much current can be expected to flow through your ground wires in the event of a fault. You can figure out what amount of current to expect by performing a protection and coordination study. This will let you pick the appropriately sized fuses or breakers.

Wrap up

In summary, we can think of grounding as a circuit protection technique that doubles as an easy way to measure voltages. Electrical grounding can be:

  • A common point of reference in your circuit that is used to measure all voltages
  • The earth itself, when the common point of reference is stuck into the ground that you walk on
  • A technique used to direct fault current along a low resistance path to protective devices that will immediately turn off your circuit
  • All of the above

Hopefully you’ve enjoyed reading this article, and now know a little bit more about grounding.
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