Multiple Resistor Grounding 1

[joan271273]

When you have several generators in paralell that are grounded via resistors the "brief in nature" information I have indicate that sizing shall account for circulating currents.This is also stated when discussing having all generators grounded through a single resistor.

Can someone provide input to the following:

1.- Circulating currents in the situations described above.

2.- Single or Multiple resistor grounding. Pro's and Con's

3.- Reference Material. [sig][/sig]

 

[jbartos]

1. All parallel generators are grounded via circuit breakers, one for each (but two only are required and practical to have), to the neutral ground bus which leads to the resistance grounded neutral system grounding arrangement.
2. Single resistor is preferred since the ground fault current is smallest via a single resistor resistance grounded neutral system grounding arrangement.
3. Beeman D. "Industrial Power System Handbook," First Edition, McGraw-Hill Book Co., 1955, page 373 [sig][/sig]

 

[joan271273]

Thanks for the reply.

1.- Do you have a method to estimate the current values.

2.- The breaker system is used in system with units exceeding the 15 MW, in some countries. Gas Units such as the LM 2500 ( 22MW ISO ratings ) use just grounding via distribution transformers.

3.- The use of breakers in systems with gen sets in the range of 2.5MW to 10MW is rarely applied due to cost. [sig][/sig]

 

[jbartos]

1. Reference:
Baker D. S. "Charging Current Data For Guesswork-Free Design of High-Resistence Grounded Systems," IEEE Transactions on Industry Applications, Vol. IA-15, No. 2, pp 136-140, March/April 1979
contains the charging current information to aid the design engineer in determining the magnitude of charging current. There are also listed useful references, e.g. IEEE 142
2. If there are generators connected in parallel then the circuit breakers are preferred or mandatory. If generators are not connected in parallel then the circuit breaker is not needed since there is not any system neutral ground bus and one capacitive charging current level (See D. Beeman Handbook posted above. It explains the system grounding in Chapter 6.)
3. It appears that the cost is not a factor; however, the charging current is and parallel operation of generators is (See D. Beeman Handbook posted above.) [sig][/sig]

 

[CP]

i'm sorry jbartos, i am a little bit nervous with having the circuit breaker for the ground. is it safe to this? if your primary concern is charging current, then an isolator switch will do. breaking up the continuity of ground is not really a good idea. the ground should be the last to go.
shouldn't we have the grounded neutral from the gen sets installed with respective isolator switch so that at only one time only one neutral is closed to the main switch board (to your neutral bus).

 

[jbartos]

That is all right, Madam/Sir. Please, consult the reference, mentioned above, theory, engineering and design before you are sorry for anyone. That is a standard professional engineer practice. The circuit breaker function has very little to do with the ground circuit protection. It is there for maintenance purposes and for very rare short circuits, only. Supposing that the generator has to be serviced, it must be isolated from the grounded neutral system bus since there may be dangerously high voltage. This means that another parallel generator has its neutral breaker turned on before the serviced generator neutral breaker is turned off. However, this is just an excerpt from the reference that I did not post to safe the posting space.

 

[Marcus]

hey, no one puts a CB in the neutral grounding circuit of any sized generator. If you did, the generator could become energized to high voltages in the event that a fault occured in the armature winding.

resistors and the neutral grounding transformer are sized to create a reactance which will limit the amount of ground current in the event of a fault in the winding.

 

[jbartos]

Suggestion to Marcus: Please, become more familiar with medium voltage power distribution system grounding. Many good references are available including standards, handbooks, and papers. An excellent and fairly detailed reference is posted above (D. Beeman).

 

[CP]

u r right about isolation of common neutral from multiple gensets configuration. maybe the defination of cb isn't clear enough. when u say cb, it should mean current limiting device - breaks off the currents under abonormal condition for certain specific time. if we hv ground fault and the cb breaks off the continuity to earth, then we are going to have problem. the isolation service for lock out (for your gensets) can be served by installing isolation switches not circuit breaker.

 

[jbartos]

Please, are you familiar with medium voltage switchear practices, standards, products, etc. as far as medium voltage switchgear, circuit breakers and isolation switches are concerned? If yes, name some.

 

[CP]

no tell me more, please.
thank you.

 

[joan271273]

Please let's not assume that someone is wrong completely about something.

1.- Circuit breakers are used by certain companies in large systems were the generators are in paralell.

2.- Some companies do not use breakers but disconnect switches type arrangments.

3.- Other schemes involved the use of one breaker and several disconnect switches.

Circuit breakers are not the only items used in power systems. References such as Beeman and the GE Industrial Power System Books are a important source of information but be careful in the application of specific schemes because of their age. Certain practices have received modifications due to companies now in a "budget operating mode".

 

[jbartos]

Suggestion: There appears to be some need for distinctions between low voltage and medium voltage grounding practices since the medium voltage disconnects are essentially the same interrupting devices/hardware as medium voltage circuit breakers. When it comes to low voltage, the disconnect devices are different from circuit breakers, e.g. molded case circuit breaker and molded case switch (does not have any protection devices included).

 

[joan271273]

I am not sure on the last posting, this what I am understanding currently:

1.- Disconnect Switches are different in LV and MV applications. I am assuming you are referring to the NO-Load Break and Load Break characteristics of MV disconnect switches when compared to LV switches.

2.- If item #1 is correct breakers are different from disconnects in regards to their ability to interrup abnormal current values:

a) Within specific times/values in LV systems.
b) Within specific time/values via protective relaying schemes in MV systems.

3.- MV CB differ from disconnects in the interrupting medium ( Vacuum, Oil,SF6,Air ) used.

LV Generators are hardly ever grounded via a resistor,therefore hardly any practice of LV neutral CB.Also a large population of LV generators are used for the task of standby systems.

Now the story chages with MV generators, these are used for all type of systems ( Prime power, standby,etc ). A large population of the 4.16 kV generators used in industrial enviroments are grounded via a neutral grounded resistor. NO Neutral CB.

Utilities and large industrial complex ( refineries ) have their generators grounded via neutral distribution resistors

a)With only load break disconnect switches.
b) One CB and the a string of Load Break disconnects.

My original posting was based on how to calculate circulating currents as these will be present in the step up XTR and in the resistor. Beeman does not address this type of calcs.

 

[jbartos]

My posting from Oct 20, 2000 cites a suitable reference leading to ground resistance calculations. Many A/E firms keep these calculations on a proprietary side since their business may depend on them. Please, would you provide samples of medium voltage disconnect switches suitable to regular swichear frames? They appear to be overdue from you and from CP(electrical). ABB, General Electric, Siemens, Square D, etc. will be fine.

 

[peterb]

Back to the original question -
The concern with generators operating in parallel on a common bus is with third harmonic current circulating through the common neutrals. There are several approaches to avoiding this problem, some of which are -
- Provide each generator with a neutral disconnect switch (NOT a circuit breaker). Only one disconnect is closed at any one time, so that there is no posssibility of circulating current.
- Provide each generator with an individual neutral grounding resistor; the resistors have to be sized to carry the expected level of circulating third harmonic current. This approach has the disadvantage that the ground fault level is variable, depending on the number of generators on line.
- Install a separate grounding transformer with a neutral grounding resistor (this can be a wye/delta station service transformer or a zigzag grounding transformer). None of the generators are grounded in this case, all generator neutrals operate isolated.

In all of the above, sensitive and selective ground fault relaying is of paramount importance. There is a need to minimize the ground fault current, so as to minimize stator iron burning for generator ground faults, while still providing a high enough current for selective tripping.
One relaying approach that works well with the separate grounding trasformer approach is to install directional ground relays on each generator circuit, with time delayed overcurrent protection on the grounding transformer itself.

On balance, I believe that the separate grounding transformer is perhaps the best solution for a new installation, where there is a transformer isolating the generation from the distribution system. I have worked recently with a utility 11.5 kV system, connecting 4x10MW generators to a common bus, which uses this method of grounding. No problems to date, and no circulating currents. Am also presently working on an industrial system which has individual NGRs, and am wrestling with setting relays for the range of ground fault current available.
One other point re the above responses - under no circumstances should there be a circuit breaker in the neutral circuit which will trip under fault conditions. The faulted generator itself and its associated main breaker would be tripped for the ground fault condition.

If a disconnect switch is provided in the neutral circuit, it can be a manually operated metal-enclosed switchgear device or an open switch, depending on the type of construction in the station. I have seen older stations with the neutral busbar and individual switches mounted on the wall in a switchgear room, but I wouldn't recommend this for modern practice.
Another useful reference, if you have access to a copy, is the old Westinghouse Electrical T&D Reference Book - see chapter 19. Hope this helps to clarify the issue.

 

[joan271273]

Thanks Peterb for the response.

1.- The only thing to add would be that several operating companies fell comfortable in the application of a common resistor as then the ground fault value will never exceed the sole resistor rating.

2.- If I have several generators in paralell each with a NGR, the maximum ground fault level will be equal to number of NGR's in the system. If I used for the ground relaying a setting value identical to the minimum number of generators (NGR's) allowed in the system will this cause other problems????.

 

[peterb]

1. Common resistor works, but circulating current between the generators connected to the neutral bus still needs consideration.
2. Assuming that your NGRs are the same value, then setting to the minimum number of generators will work. Problem arises when there are different values involved, hence many permutations to cover. Also, note that generator damage becomes more of a concern with increasing ground fault current. While a generator rewind is painful enough, rebuilding burnt stator laminations will spoil your whole day.

 

[jbartos]

Please, notice that D. Beeman reference Figure 6.21 on page 373 shows one neutral circuit breaker "on" at one high resistance grounded paralleled generator only. Therefore, there are no circulating current anywhere among the parallel generators neutrals. The second circuit breaker function is to connect it "on," when the existing paralleled generator with its circuit breaker needs to be serviced or malfunction, and the second paralleled generator must be conected to the ground bus, and from there to the high resistance system grounded neutral scheme, before the first paralleled generator circuit breaker is disconnected ("make before break&quot and used on a stand-by at another paralleled generator neutral. All remaining paralleled generators have their neutral ungrounded. However, their neutrals have the plug in arrangements for the switchgear, if for some reason the existing (second) paralleled generator needs to be serviced or malfunction. Obviously, to keep cost down, the first paralleled generator circuit breaker or switchgear is unplugged from its plug-in arragement and used in one of those paralleled generators that have the plug-in arrangement ready to accept that switchgear from the first paralleled generator. Incidentally, D. Beeman Hadbook is so great and popular that McGraw-Hill decided to reprint it several decades after its first edition in 1955 (in 90s?). This speaks for itself in engineering field and many other engineering handbooks have a long way to go for this kind accomplishment.

 

[joan271273]

I know of the whole statement included in the D.Beeman book, which is an extract of the GE Industrial System Handbook ( 1950 and 1960).

If in any moment you have detected a "negative" statement towards D.Beeman publication, disregard it as the book is a "must read ".

What needs to be understood is that practices have changed or adapted with time due to cost, standard changes and equipment engineering modifications.

At the present time the TOP 3 Oil Companies and Main Western Utilities do not use CB. THis is just a reference point.