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Oct 22nd , 2012

Tripping of Al Ghail Generators on reverse power or Stator earth fault while charging TRF1 or TRF2 or TR1 at Al Hamra

Oct 22nd , 2012

Energisation of Solkor pilot wire Differetial protection relay in GTG1 at Al Ghail for more than 18 MW load

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Oct 22nd , 2012

There are six number of earthings, two on 33kV and 4 on 11kV, at Al ghail. Whether this is fine or need to rationalized.

Al Hamra, We noticed that there are 2 zig zag transformers and one NER and 2 nos earthing on NER. Whether this is fine or need to rationalized.

Oct 22nd , 2012

We noticed that neutral earthing of 33/11kV transformer is directly connected to 33kV earthing grid. Is this acceptable?

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Oct 22nd , 2012

At Al Ghail, tap at 33kV is not changing in auto despite voltage fall at Al Hamra, therefore to increase voltage at Al Ghail, the tap need to be manually operated. This need to be varied as per the load variation to keep constant voltage at 11kV side of Al Hamra.

Oct 22nd , 2012

Load shedding scheme for following scenarios,1. 4GT running and one trips2. 3Gt running and one trips3. 2 GT running and one trips

Oct 22nd , 2012

Relay coordination as a whole system – Al Ghail Power which includes both power stations and OHL

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21/11/2012

The MVAR of units at Al Hamra while start loading has reached to negative value which can not be raised to positive value by the operators of Al Hamra by giving raise commands

When there was only one source of generation ,why synchroscope was not giving permissive despite voltage being nearly equal on both side of CB7.

When three generators are in parallel,unit A at Al Hamra tripped on reverse powerThe MVAR of units at Al Hamra while start loading has reached to negative value which can not be raised to positive value by the operators of Al Hamra by giving raise commands

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14 21/11/12

When there was only one source of generation ,why synchroscope was not giving permissive despite voltage being nearly equal on both side of CB7

Kindly send the details of calculation of voltage drop on 33kV lines and maximum power which can be transmitted from Al Ghail to Al Hamra with various scenario as mentioned in the operation philosophy

At Al Hamra, the droop settings are at 5% in unit A and 4% in unit B.As the load fluctuation is very fast, please confirm whether these settings are Ok or not while paralleling with Al Ghail units

15 21/11/12 At Al Ghail,when one transformer was loaded 14MW with tap position7,it is observed overcurrent indication and negative active power flow in RTCC2

Reply from ATKINS dtd 24th Oct’12

When generators are working at low power levels the phenomena of power swings are more. i.e they are unstable. In this condition if large inductive load such as transformer or line (with high Mvar) is connected to Generator there may be chance of spurious transient power swings.

To prevent operation of reverse power protection during this time suitable time delay should be given. The present reverse power setting of Al Ghail generators are on the lower side. The time delay of 0.5 sec is too low considering the rating of the generator.

Please ask BHEL o review the settings considering the present configuration of the system (11/33 kV Tx and 33 kV line being connected to generators)

As mentioned in the previous email from Jithendran a current of around 45 A is measured in the cable braid. Is this current is measured after the removal of earthing from switchgear end? Please confirm. Also confirm how the cable is laid? Can you please measure the current in the neutrals of 1. NER of zigzag Tx (station Tx) NGT of Generators which will help us for further investigation

2 earthing of GTG through NGT are necessary for the proper grounding of generator system. Since GTG are high resistance grounded (low earth fault current) it is necessary to have separate grounding - low resistance grounding with earth fault current in the range of 100A to ensure the pick of earth fault relays in the 11 kV system. Hence zigzag Tx earthing with NER is necessary.

However when 2 GTG’s are running in parallel (11kV BC closed) it is enough to have only one zigzag Tx grounded.

At Al Hamra since generators are grounded through NER (earth fault current in the range of 150A) there is no requirement of zigzag Tx.

Connecting transformer earthing directly to the grid is not a good practice. Transformer should be separately grounded to the earth pit and then it should be connected to grid.Please ask the contractor to review the earthing design.

From the discussion we had during with site visit it is clear that for the tap changers the voltage sensing is given from the 11kV side. But at 11kV because of the presence of Generator and AVR voltage will be almost constant. Therefore it will not be able to operate in auto mode for the change in the voltage at 33kV side.

Hence this design is questionable. Please ask designer/contractor to revisit the design.

It is better to operate in manual mode till above issue is resolved and the whole system comes to a stable operating condition.

This task is identified under the scope of work for parallel operation of 2 power plants. We are waiting for the techno commercial proposal from the vendors.

Is this regarding to the proposed complete relay co-ordination of Al Ghail and Al Hamra system. Please elaborate.  

ATKINS reply dtd 31st Oct’12Reply from AGP dtd. 29th Oct’12

We are taking up with BHEL and will update you.

BHEL has recommended for reverse power protection Power1,trip delay1-3 seconds.Power2,trip delay2-2 seconds.Please advise whether this can be implimented

BHEL recommendation of 3 sec can be adopted. We feel P1 setting is also on the lower side. Please get the confirmation on this from BHEL.

1. After removal of earthing at one end, no current will flow.2. How to make sure that Cable sheath is damaged and what is remedial action for it3. We will let you current at neutral of NER of Zigzag4. Current in the neutral of NER of zigzag transformers are 0.1Ampere5. Current in the NGT of generator 2 is 0.2AmpereAnd generator 1 is now in OFF condition

Noted.

The only way to check cable sheath is to physically approach the cable and inspect.

Measured current is in order.

1. After removal of earthing at one end, no current will flow.2. How to make sure that Cable sheath is damaged and what is remedial action for it3. We will let you current at neutral of NER of Zigzag4. Current in the neutral of NER of zigzag transformers are 0.1Ampere5. Current in the NGT of generator 2 is 0.2AmpereAnd generator 1 is now in OFF condition Can I take as recommendation for keeping only one Zigzag Transformer . Please confirm.If only one zigzag transformer is to be kept in line when both the generators are running parallel,the EDG needs to be kept in manual mode which we don’t prefer

In Al Hamra,now ,the zigzag transformer of C section is in the ON condition as 11kV bus section between A and B section is OFF.Can we consider it as a recommendation to keep this zigzag transformer in OFF condition when Al Hamra generators are running in parallel without Al Ghail power supply available at C section?

The current through the earthing cables connected to the earth bus of 11kV panels at Al Ghail were observed as 7.4 ampere when the generator was supplying 23MWl oad.

Please elaborate the relation between EDG manual mode and 1 zigzag tx is earthed.

We understand that Bus section between B & C is closed. Then earth path is available through NER of Gen B. Hence zigzag Tx can be kept OFF.At any time if bus coupler is opened and if bus section is not having generator in the circuit (no earth path) then zigzag Tx should be kept ON.

There is no other contractor or consultant with us. Can you please make your recommendation for implementation as a part of 33kV review and commissioning assistance.

You can consider this as our recommendation.However we stress to take the opinion of contractor who designed this

Noted. closed

There is no other contractor or consultant with us. Can you please make your recommendation for implementation as a part of 33kV review and commissioning assistance

You can consider this as our recommendation.However we stress to take the opinion of contractor who designed/commissioned.

This is to avoid any warranty/contractual issues later.

Yes, this consist of relay coordination as one system.

We recommend complete relay co-ordination of the system as suggested in our relay setting audit report.

Reply from AGP dtd 21/11/12Implimented time delay for reverse power as 3seconds for Power1 and 2seconds for Power2 in both the units. Verifivation is yet to be done at next opprotunity

When we isolated both the zigzag transformer from line, after starting EDG ,still it is found that the operating coil of Solkor in unit 1 carries current of 1.5mA when unt 1 was loaded with 7MW.It is also found that the pilot wires of Solkor in unit 2 are transposed which indicates that operation of Solkor is not the issue with unt 1 alone.Please confirm whether the operating current of 12mA (at which Solkor operates)is a permissible value or not.

Noted

closed

Voltage control needs to be made operational in auto mode at both Al Ghail and Al Hamra as taps need to be adjusted manually every time for keeping voltage as per requirement at both 11 and 33kV levels.

we will comment after reviewing report.Anyway please confirm whether the following issues will be resolved with the current settings:At Al Ghail and Al Hamra, keeping the relay settings recommended by Atkins ,it is not possible to energize the transformers from 11kV side without causing tripping of the units whether on stator earth fault or on reverse power.Whenever the line is charged without load, the generator trips on reverse power.Kindly suggest settings of relays with which any type of switching of transformers and lines becomes possible without causing trippings of generators

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