GROUNDING OF 11kV DIESEL GENERATOR 1

[Elecme]

Is it a MUST that all MV (11kV) generators to be grounded through impedance? or can we connect a 11kV generator solidly to ground?...when do we need each type of the following grounding system:

· SOLIDLY GROUNDING
· LOW IMPEDANCE
· MEDIUM IMPEDANCE
· HIGH IMPEDANCE



Elecme

 

[busbar]

A useful reference may be IEEE Std C62.92.2-1989 Guide for the Application of Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generator Systems See: shop.ieee.org

 

[alehman]

You may wish to consult the manufacturer. I'm not aware of any medium voltage generator installations that are solidly grounded.

 

[RAgrawal]

Normally grounding of Generator is suggested by the manufacturer / designer of the machine. Generally, neutral is so grounded, earth fault current should not exeed the full load current of machine.

 

[bigamp]

There are lots of possibilities for earthing the neutral and there are many considerations. You have to select the method of earthing that is most appropriate for your installation.

Things to consider include is the generator going to run parallel with other generators or with the grid connection that is star on the 11kV side? Does the generator need to be able to provide earth fault current so that the earth fault detection on the rest of the 11kV system can function if the generator is the only source of supply?

If the generator is to operate in island mode only then its neutral should be permanently connected to earth via an earthing resistor. The earthing resistor has to be sized to allow sufficient earth fault current so that downstream earth fault protection can operate in the event of an earth fault in downstream equipment. The downstream earth fault protection system should be designed to be able to operate effectively at the level of earth fault current that you are going to limit the generator to. A good typical value is to size the resistor to limit the current to 400A. Some would say to size the resistor to limit the earth fault current to no greater than generator full load current. I guess sizing the resistor for 400A would be medium resistance earthing, and sizing it for full load current (provided full load current was greater than 400A) could be considered low resistance earthing?

If the generator has to operate in parallel with the grid you could operate the generator as a three wire machine whilst in parallel but it would have a neutral earthing resistor connected via switching device which would need to close in if the genrator were to run in island mode. The neutral earthing resistor would be selected as per above. I have seen one installation where the switched connection was directly to earth (i.e. no resistor). It worked fine but obviously would have produced a high level of earth fault current. There is no real need for a high level of earth fault current, so why not limit it.

If the generator has to operate in parallel with others, it is a good idea to tie its neutral earthing system in with that of the other generator(s) so that only one generator neutral is ever earthed. This will still provide earth fault current and will eliminate the possibility of the dreaded circulating third harmonic current problem. A typical setup will involve all generator neutrals connected via a switching device to a neutral bus which is connected to earth via the earthing resistor.

High resistance earthing limits the earth fault current that the generator can produce to the 5A to 10A range, which is insufficient fault current to damage generator core laminations. It is mostly used only for generators that are connected via a delta - star unit transformer (delta facing generator). The generator itself does not need to provide earth fault current to downstream equipment and its earth fault protection does not need to grade with any downstream protection.

 

[Elecme]

bigamp, you got a star.
Can you eleborate more on your paragraph below

"If the generator has to operate in parallel with others, it is a good idea to tie its neutral earthing system in with that of the other generator(s) so that only one generator neutral is ever earthed. This will still provide earth fault current and will eliminate the possibility of the dreaded circulating third harmonic current problem. A typical setup will involve all generator neutrals connected via a switching device to a neutral bus which is connected to earth via the earthing resistor"

Elecme

 

[bigamp]

A generator's output voltage is not 100% pure sinusoidal, there are harmonics present. If there are third and triplin (odd numbered multiples of three) harmonic voltages present, these will produce third and triplin harmonic currents. The third and triplin harmonic currents in each phase are in phase with each other and therefore summate via the starpoint of any load and return to the generator via the connection to the generator starpoint.

If you have two or more generators connected in parallel then there is a path for third and triplin harmonic currents to circulate from one generator to the other even if no load is connected. If the generators are identical and produce the same third and triplin harmonic voltages there will be no net third or triplin hrmonic voltage difference between the generators and hence no circulating third and triplin current between the generators. If the generators are different (as they can easily be because they may be of different capacity, by different manufacturers and of different winding pitch) the chances are that you will get a net third and triplin harmonic voltage and one machine will drive third and triplin harmonic currents through the other generators with the current returning to the starpoint of the driving generator.

The circulating third harmonic current can be unexpectedly high and can upset protective relaying (lots of modern relays have third harmonic filters for this reason). In one extreme case I was told about by a commissioning engineer, there was enough circulating third harmonic current for neutral earthing resistors to be getting decidedly hot.

You can stop circulating currents by breaking the circuit to all generator neutrals except for one. You must have one generator neutral earthed so that you can provide earth fault current for the earth fault protective schemes to work.

Another way to prevent third and triplin harmonic currents is to use generators that are two thirds pitch wound. Such generators produce essentially zero third and triplin harmonic voltages.

I'm sure there are lots of ways to do neutral switching. I will suggest one (expensive) way of doing it but it has the advantage of using standard equipment. It is as follows:

You could use a conventional low capacity 11kV switchboard as a neutral bus, call it the neutral switchboard. On the neutral switchboard, short the three phase busbars together (this becomes the neutral bus) and connect the neutral bus to the neutral earthing resistor which you then connect to earth. At the neutral switchboard, provide a (3-pole) circuit breaker for each generator neutral. At each circuit breaker, short the three "circuit" side connection points together other and connect each generator neutral to its neutral circuit breaker. Install logic to ensure that only one neutral circuit breaker can be closed at any time. The neutral circuit breakers are used as automatic switching devices only, not as protective devices. All would be open when no generators were in service. When the first generator was being run up, its neutral circuit breaker would close in. If that generator should trip, its neutral circuit breaker would also trip. If other generators were in service, another generator's neutral circuit breaker would automatically close in. You would have a permissive in each generator's main circuit breaker that it could not close in unless a neutral circuit breaker was closed.

Also, just a note on my previous posting. If you had a generator whose full load current was less than 400A, you would almost certainly size its neutral earthing resistor to allow generator full load current, not 400A.

Finally, just to re-emphasise that there are many considerations regarding generator neutral earthing. It is well and good to make generalisations (as my postings on the subject have been) but it is important to realise that each installation must be considered as a unique case.