Combine Earthing of 50hz & 60hz Power Systems

Grounding & Bonding Philosophy - 50hz or 60hz Earthing Considerations for 132KV Grid & 60MW Power Generation

Dear My fellow expert.

We are setting up a coal base Power plant & facing an unusual Engineering scenario regarding Power System Grounding of two different power systems operating on different power frequencies that is 60 HZ & 50 HZ.

Although this issue have been highlighted & discussed with our engineering Consultant team but many points are still open and un-addressed. As you are well aware that improperly designed Grounding System can lead to to serious operational and safety issues which can not be resolved just by having good operational and maintenance practices once the system is commissioned. Also grounding problems are very difficult to analyse and point out.

Brief description of our system is below for your advise please.
We have a system of 4 Steam Turbine Generators of following denominations:

1. 2x24 MW, 60 Hz, 13.8KV (for industrial plant supply)

2. 1x10 MW, 60Hz, 13.8KV (for industrial plant supply)

3. 1x60MW, 50 Hz, 11KV (for Power Export to National Grid)

The 60 MW, 50Hz machine shall be used to export power to National Grid at 132 KV through a 11/132 kV , 80 MVA GSU Transformer. The fault level of grid at 132 KV is 8KA from the short circuit report.

Remaining 3 machines of 60 Hz are being used to supply / feed the industrial plant having motors and Transformers of different ratings.

Please note, all Generators (2 x 24MW ,60hz + 1 x 10MW,60hz + 1 x 60MW,50hz) are installed in a same turbine hall and 132 KV Grid station is located around 250 meters away from the Generator- Turbine Hall.

Power Plant Turbine Hall has Separate Earthing Mesh which would be Physically & Electrically isolated from Earthing mesh of 132kV Grid Station.

Concept diagram Link is below:

https://drive.google.com/file/d/0BwhsamhHA_O9T3dRNUZUckVmbWs/view?usp=sharing

Please consider and clarify following points on subject issue :

1- Where should we earth 11kv ,60MW /50hz generator structure : with 60hz industrial Grounding Mesh or Grid station earthing system.

2- Where should we earth 60MW ,50hz Generator Star point : with 60hz industrial Grounding Mesh or Grid station earthing system.

3- Between Generator and Step-up Trafo there is 11kv Bus duct which is connecting Industrial plant and Grid Trafo (132kv side Star point is grounded with 50hz system earthing mesh).

Now ,Where should we ground the Bus duct structure : with 50hz Earthing System or with 60hz plant system ?

OR Ground with, bus duct generator end with generator grounding & Trafo end with Transformer grounding System.

4- Any passible implication in Generator and Transformer protection coordination. (Differential & Block Differential protection . etc.) As Generator CTs will be grounded with generator earthing system and transformer CTs are grounded with Grid Station earthing system.
(Note: No CB in b/W generator and transformer , only CB is at 132kv side only).

5- Initially two system were designed with integrated philosophy , however later on, after reveling initial calculations , it was noted that in case of earth fault from 132KV network, Touch Potentials (TP) are rising enough & are getting out of safe Standard limits . And to meet the standard criteria, very strong and heavy Earthing mesh is required in entire Plant area (Generation Plant + Grid station), which would be difficult to implement due to practical and cost limitations.
Hence it was decided to segregate (physically & electrically) two earthing system for Plant (60hz) and Grid (50hz). Please comment Should the 50 Hz (Grid station) Earthing System be kept separated from 60 Hz plant or can be interconnected to each other , which would be the batter practice ?

6- Grid Station Axillary Transformers (2 x 300 KVA, 480/400V ~3PH & 230V~1PH , 60hz) are to be fed from plant area from 60hz , 480V supply but physically installed at Grid Station . How the grounding should be accomplished ? Is primary side grounded with Plant and Secondary with Grid?

7- Further, I want to know, if there is any recommendations / Standards of having different earth system for 50 Hz and 60 Hz.

8- Please also suggest, if we need to have a separate earthing for DCS /control system and Power system.

9- Earthing and Bonding of 132KV Power Cable & Link boxes.

10- Possibility of having separate Earthing Mesh at Power Transformer Area for 60 MW Generator & GSU Star Points.

please advise.

kind regards,

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