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Anonymous Poster #1

Neutral Isolating Switch

01/18/2022 8:20 AM

of the 14 MW hydrogenerator, 11 kV voltage generation, for neutral grounding switch, 400A switch is chosen. how this value is obtained?

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#1

Re: Neutral Isolating Switch

01/18/2022 8:45 AM

It would be best to ask the Designer, whose details are on the drawing and other documents, or the Commissioning Engineer, whose name is on the commissioning records, rather than a bunch of anonymous contributors to a global internet engineering forum. After all, the latter doesn't even know which country this installation is in, for example, and which national standard is applicable; not a clue. Why one would withhold this information is a mystery too.

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#2

Re: Neutral Isolating Switch

01/18/2022 9:08 AM
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#3

Re: Neutral Isolating Switch

01/18/2022 9:29 AM

Does the grounding equipment have an impedance exceeding 27.5Ω?

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#4

Re: Neutral Isolating Switch

01/18/2022 12:05 PM

It is likely that the 14MW generator is resistance grounded.

Check the scheme and see the Neutral Grounding Resistor nameplate.

The ground fault current is probably limited to a max of 400A!

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Anonymous Poster #1
#5

Re: Neutral Isolating Switch

01/18/2022 8:38 PM

but the grounding resistance value is not shown. only 400 amps nis is shown. i understand that 14 MW, 11 kV means a rated current of 735 amps, which should come out of generator neutral terminals as well. but how 400 amps nis switch chosen?

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#6
In reply to #5

Re: Neutral Isolating Switch

01/18/2022 11:20 PM

After neutral is formed, what is done -

+ isolted from ground

+ Solidly grounded

+ Grounded through a resistor

I am afraid you are missing important information there.

The rated current will be there up to neutral formation. In a balanced three phase supply, vectorial addition of three currents yields zero. So, ideally there will be no current in the neutral under healthy conditions.

Assuming that the NIS is in the neutral circuit, it only sees current when there is a single phase to ground fault.

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#7
In reply to #5

Re: Neutral Isolating Switch

01/19/2022 10:22 AM

Answer: because of design conditions that the forum cannot access.

Ask the question again, in the right place next time; see #1⇑.

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#8

Re: Neutral Isolating Switch

01/19/2022 11:02 AM

Let's say it is about 14 MVA salient poles generator. The rated neutral switch current is calculated for steady state of short-circuit and verified for initial symmetrical short-circuit [ and asymmetrical RMS].

As I found for less than 16 salient poles generator [Siemens Formel und Tabellenbuch for Starkstrom-Ingenieure Tab.7.2]:

xd=1.4;xo=0.2;x2=0.25 then total reactance in p.u. =1.4+0.2+0.25=1.85

Neglecting resistences the total phase-to-neutral impedance it will be 11^2/14*1.85=16 ohm. Then Ik=11/sqrt(3)/16=0.397 kA

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#9

Re: Neutral Isolating Switch

01/19/2022 10:12 PM

The low resistance ground resistor is selected to allow a 400 ampere current. This current is the minimum current that can be reliably detected by zero sequence current transformers (CTs that go around all 3 phase conductors), and gives you protection down to the bottom 10% of the windings. A carefully engineered system can use as low as a 200A ground resistor. 400A is typical for systems engineered in the late 1960s, early ‘70s.

The ground fault sectional protection coordination starts with motor starter ground fault relays, feeder breaker relays, main breaker relays, and ends with the ground fault relay on the resistor. The resistor is sized to allow the 400 ampere current for about 10 seconds before it self destructs, so the relay on the ground resistor is set to a shorter time to protect the resistor, clearing the fault often by interrupting the supply, drive turbine or transformer primary.

The reason to have a switch is to only have one ground point on the system at one time. This reduces the heating in the resistors caused by 3rd harmonic circulating currents, and makes it easier to ensure correct sectionalizing during a system ground fault.

The reason for resistance grounding is to limit stator and iron core lamination damage during a fault, significantly reducing the probable repair cost.

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