Resistance Earthing OR Solid Earthing for 3.3kV Diesel Generators

[SK07]

In our Project we have On-site Power Generation with no "State Grid" Connection.

On-site Power Generation is by 2 nos. of 3.3kV, 2000kVA, 50Hz Diesel Generators operating in parallel and 3rd Diesel Generator of identical rating connected but on standby mode.

Earthing system adopted is Solid earth.

Project is at Construction stage. Now proposal from client is to review / change above solid earthing to low Resistance earthing.

My query is what type of earthing is best suitable for above system and why?
Also is Solid Earthing technically acceptable method for above system.

 

[waross]

Project is at Construction stage. Now proposal from client is to review / change above solid earthing to low Resistance earthing.

I would choose between solid earthing or a high impedance scheme to limt the fault current to about 5 Amps.
When a system with multiple generators is earthed at one point only there may be earthing issues with generators that are running but not yet synchronized. This depends to some extent on the switching arrangement used to connect the generators to the system.
With solid earthing, each generator may be earthed.
With high impedance earthing, the damage due to an earth fault current is limited.
With high impedance earthing the phase to ground voltage on the system will rise on the unfaulted phases to phase to phase voltage.
Eg: A 3.3/1.9kV system will run with the phase conductors at 1.9 kV to ground. In the event of a solid single phase to earth fault on a high impedance earthed system the phase to earth voltage will rise to 3.3 kV
Consider connecting all the generator neutrals to a common bus or connection and then grounding a common point through an impedance that will limit fault to about 5 Amps.
Consider differential protection for the generators.
Low resistance earthing may combine the worst features of both solid earthing and high impedance earthing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eeprom]

I agree with either the low impedance or high impedance grounding, with a ground detection relay. The ground detection relay will force the gen to trip on a ground fault (maybe after some delay time), but there will be damage due to the fault. Post Glover makes a "pulsar" ground detection system that sends a pulsed signal out so that the grounded conductor can be located with an ammeter.

The biggest risk in using impedance grounds is that operators may not feel a pressing need to react to a ground alarm, and will leave it as is. Because the ground fault current is so low, it is easy to ignore. However leaving the ground fault to exist adds the increased hazard of a line to line fault. I've set mine up to trip after 60 minutes if the fault is left unattended. 60 minutes is usually more than enough time to locate the fault.

EE

 

[SK07]

If inductive reactance is used for earthing, does it cause resonant inductive-capacitive over voltages.
Is it because of this reason solid earthing is used upto and including 3.3kV.

 

[waross]

High impedance earthing includes high resistance earthing.
A distribution transformer may be used for earthing. The primary is connected from the wye point to ground but the transformer is rated for phase to phase voltage. The current limiting resistor is connected to the transformer secondary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Unless there are line to neutral (single-phase) loads (this is not common), then never solid grounding unless you have miles of overhead lines. I'd suggest low-resistance grounding to allow selectivity on ground faults.