Minutes of the meeting of Expert Group

held on 23.8.2017 to discuss Power Swing disturbance at CGPL

In line with the decision of 34th WRPC meeting held on 28.07.2017 at Mumbai, an Expert Group comprising of representatives from WRPC, WRLDC, CTU/PGCIL, CGPIL, APL and Relay manufacturing Industry was formed to investigate and analyse afresh, the power swing disturbance of 13.07.2016 at CGPL Mundra and recommend measures to avoid such disturbance in future. As decided in the WRPC meeting, after investigation and analysis by the Expert Group, if it is felt necessary, further studies may be carried out by professional bodies such as IIT-B, CPRI, PRDC, etc. through funding from WR

constituents.

The members of the Expert Group formed is enclosed at

Annexure-I and the list of participants is at Annexure-II.

Member Secretary, WRPC welcomed the members of the Expert Group to the meeting. He welcomed the representatives from the Relay Manufacturer Industry, M/s GE T&D, M/s Siemens & M/s ABB and thanked for attending the meeting at a short notice. He said that the Expert Group would discuss the disturbance in detail and come up with recommendations/suggestions. Thereafter, he handed over the proceedings of the meeting to

SE(P) and EE(P).

a) Brief background of the issue

On 13.07.2016 at 20:29 hrs, there was a bus fault (BF) at 400 kV Bus-1 of Versona 400/220 kV s/s and the elements connected to

that bus got disconnected.

Then at 21:50 hrs, there was bus fault (BF) at 400 kV Bus-2 of Versona s/s, and all the elements connected to the bus got

disconnected.

Afterwards, when Bhachau-Versona-1 line was charged from Bhachau end, line fault (LF) in Zone-2 was sensed at Bhachua end of the line and also at Ranchdopura end of Bhachau-Ranchodpura lines and CGPL end of CGPL-Bhachau-1 line and

the lines tripped.

Subsequently, remaining five lines at CGPL end sensed power swing (PS) in Zone-1 and got tripped. At the same time, Generating units (10, 30, 40, & 50) at CGPL got tripped on account of Out of Step (OOS) protection. Loss of generation at CGPL was of the order of 2875 MW.The faults and tripped elements are depicted in the following figure:

EE(P) briefly informed regarding the recommendation of the earlier report dated 13.10.2016 on the power swing phenomenon observed at CGPL during the disturbance of 13.07.2016 and the developments thereafter in various PCM and WRPC forums. He also informed that the recommendation of the WRPC report dated 13.10.2016 was discussed in the 33rd & 34th WRPC meetings. WRLDC have carried out simulation studies on the CGPL disturbance of 13.07.2016. The studies were discussed briefly in 127th PCM. However in light of the decision of the 34th WRPC meeting, a detailed discussion and analysis of the studies carried out by WRLDC and the philosophy adopted by M/s APL at their Tirora plant is required to be done in this meeting, so that this Expert Group could suggest the way forward in this matter to

CGPL

400 kV

Zerda

Varsana

Sami

Hadala

Tappar

Nanikhakhar

Mansar

Jetpur

Choronia

Dehgam

Bhachau

Ranchodpura

APL

220 kV

1

2 Z2

Z2

Morbi

1

2

3

220 kV

Z2 1 PS(Z1)

OOS

BF1, BF2

LF

Fig: Disturbances at CGPL on 13.07.2016

WRPC. WRLDC was requested to explain the studies carried out

by them.

b) WRLDC presentation

WRLDC informed that the transient studies were carried out by them by considering the base case of CGPL disturbance on 13.07.2016, under different simulated fault conditions. The

simulation study results are enclosed at Annexure-III.

WRLDC representative informed that under different scenarios of fault conditions on the evacuating lines from CGPL, the loss of transient stability was accessed and the observations/conclusions of the studies are as follows;

As the Present scenario is the condition where all lines are in service yet under list of contingency without SPS action system is quite vulnerable in terms of Angular stability, Voltage stability, Small signal stability and Relay operation on power swing.

Under the condition of any additional 400 kV line outages from Choronia/Jetpur/Ranchodpura/Bhachau the CGPL becomes more vulnerable from the aspect of security. So, under full generation line outage from 400 kV remote end substation should not be allowed.

Further, if the tripping is associated with the fault the chances of relay entering swing zone is quite more as observed in the case study.

It can be observed from last case that opening of several line on High voltage and higher generation at CGPL causes relay vulnerability of 400 kV CGPL-Bhachau.

c) Comments from APL Tirora

APL representative informed the special arrangement made at APL Tirora to negate the power swing effect, which is as follows;

The evacuation at APL Tirora consists of 400kV Tirora-Warora-Wardha D/c lines & 765kV Tirora-Koradi-Akola D/c with one 500MVA ICT between 400kV & 765kV Bus. 5x660MW capacity is connected to the above system.

On 400kV Tirora Warora D/c lines all the Zones on power swing is blocked for 2secs from tripping and if the power swing is detected on these lines, a 660MW unit is tripped immediately.

Further if any one ckt trips then a backing down by around 500-600MW is done at APL Tirora.

APL representative informed that earlier during tripping of one of the 400kV lines, the other line on two occasions tripped on instantaneous power swing trip in Z-1. To avoid tripping of other line on power swing and lose both the 400kV lines, study was carried out and based on the studies, 1x660MW unit instantaneous trip was envisaged in the scheme, whenever a power swing is detected on the lines, with DPS (Distance Protection Scheme) power swing blocking on lines for 2 secs.

d) Discussions on the APL Tirora & WRLDC studies:

To various queries of members WRLDC/APL representative gave

following explanation;

(i) WRLDC representative to a query whether simulations of Units tripping along with critical Lines was done & the quantum (Number of Units) required to be tripped to maintain the system stable was checked. WRLDC representative informed that such simulation was not done.

(ii) Simulation for the system on 13.07.2016 was done. On 13.07.2016 many GETCO lines and PGCIL lines were under outage.

(iii) WRLDC representative informed that transient studies on the Siemens PSSE software was carried out by modelling the case similar to that existed during the disturbance. APL representative stated that transient studies on

mypower software were done.

EE(P) WRPC stated that transient stability simulations studies are done over a very small period of time equal to the time required for one swing, which approximates to around 1 sec or even less. If the system is found to be stable during this first swing, it is assumed that the disturbance will reduce in the subsequent swings, and the system will be stable thereafter. The transient stability studies include identifying critical fault clearing time, checking generator rotor angle stability etc. However with transient studies, the response characteristics of the control equipment on the generators, the dynamic characteristics of the loads, the control equipment installed is modelled with assumptions. The machine dynamic response to

any impact in the system is oscillatory. The equipment used for excitation controls are also fast acting. He further informed that Unit tripping would definitely improve the transient stability, but it may not improve the dynamic stability. Since an instantaneous Unit tripping, leads to another disturbance to the system, which

may be trying to recover.

e) Settings around CGPL Complex:

GETCO representative stated that there is lot of inconsistency in settings adopted by PGCIL and CGPL around CGPL complex. He pointed out that the Zone-2 settings adopted at Ranchodpura end on 400kV Ranchodpura-Bacchau Lines is 500msec. The Z-2 setting adopted at Bacchau end on 400kV Bacchau-Vearsana lines is 250 msec. The Z-2 setting adopted at CGPL end on 400kV

CGPL-Bacchau lines is 350 msecs.

MS WRPC stated that with standard guidelines available there

should not be any coordination issue.

EE(P) WRPC & PGCIL representative informed that before the disturbance there were lot of relay coordination issues around CGPL complex. However after the disturbance all these issues were resolved by the PCM forum in consultation with CGPL, GETCO & PGCIL. The above settings were approved in the 129th PCM. The rationale for adopting the above settings was

explained.

M/s GE T & D representative informed that Zone-5 be introduced to coordinate the Z-2 settings. However PGCIL representative informed that any time setting below 250msec is not possible, as this would mean that this protection will act before the LBB timings. This is not a good practice to trip the line before LBB protection other than Zone-1. EE(P) WRPC informed that most of the lines are D/c lines and it would not be possible to restrict the Z-2 reach of these lines to less than 130%. This would result in under reaching due to mutual coupling effect. EE(P) WRPC suggested that Line differential protection is the only solution for the Distance protection Coordination. The 8.5 km long 400kV Bacchau-Varsana lines should be provided with Line differential

protection.

Conclusion:

After detailed discussion, all the members were of the view that preliminary transient studies be carried out by WRLDC involving GETCO and any other member who likes to get associated with the studies. Based on this studies, further course of action may be suggested by the Expert Group. WRLDC would coordinate for the preliminary transient studies.

Further, a simulation for a complete system without outage of lines in GETCO and PGCIL system shall also be done to ascertain whether with the complete system,

the CGPL is vulnerable to such disturbances.

MS, WRPC informed that as decided in the 34th WRPC meeting held on 28th July, 2017, the Expert Group has to carry out investigation/studies and submit report within a month (i.e. by

31st August, 2017).

The meeting ended with thanks to the Chair.

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Annexure-I

Members of the Expert Group

1. Smt Pushpa Sheshadri, Assit. GM, WRLDC Mumbai.

2. Shri Ankit Vaish PGCIL, Vadodara.

3. Shri Janak Pancholi (officer on special duty), GETCO CO, Vadodara.

4. Peeyush Sharma, Superintending Engineer(Protection), PAC Unit Airoli.

5. Shri Uday Trivedi, AVP, M/s APL.

6. Shri G.T. Jawale, M/s TATA Power.

7. Shri Alok Uppal, M/s CGPL.

8. Shri P. D. Lone EE(P) WRPC

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Annexure-III

Few Case Studies on CGPL showcasing the vulnerability of the system

under N-1-1 Contingency with Full generation.

1. Full generation at CGPL and tripping of 400 kV Bhachau-Ranchorpura D/C without Any SPS

Action

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2. Full generation at CGPL and tripping of 400 kV Bhachau-Ranchorpura D/C and CGPL-

Bhachau 1 without Any SPS Action

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3. Full generation at CGPL and tripping of 400 kV Bhachau-Ranchorpura D/C and CGPL-

Bhachau D/C without Any SPS Action

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4. Full generation at CGPL and tripping of 400 kV Bhachau-Ranchorpura D/C and CGPL-

Bhachau D/C (with 100 ms of three phase fault followed by its tripping) without Any SPS

Action

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5. Full generation at CGPL and tripping of 400 kV CGPL-Choronia D/C followed by 400 kV

Bhachau-Varsana 1, Bhachau-Ranchodpura 1 and Bhachau-Varsana 2 without any SPS

Action

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6. Full generation at CGPL and tripping of 400 kV CGPL-Choronia D/C (10 sec) followed by

opening of

a. 400 kV Jetpur-Amreli one circuit (20)

b. 400 kV Amreli-Choronia (30)

c. 400 kV Choronia-Asoj one circuit (40)

d. 400 kV Choronia-Ranchodrpura one circuit (55)

e. 400 kV Ranchodrpura- Zerda one circuit (70)

f. 400 kV Choronia-Kosamba one circuit (90)

g. 400 kV Rajgarh -Kasor one circuit (110)

without any SPS Action

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Observation

As the Present scenario is the condition where all lines are in service yet under list of

contingency without SPS action system is quite vulnerable in terms of Angular stability,

Voltage stability, Small signal stability and Relay operation on power swing.

Under the condition of any additional 400 kV line outages from

Choronia/Jetpur/Ranchodpura/Bhachau the CGPL becomes more vulnerable from the

aspect of security. So, under full generation no line outage from 400 kV remote end

substation should not be allowed.

Further, if the tripping is associated with the fault the chances of relay entering swing

zone is quite more as observed in the case study.

It can be observed from last case that opening of several line on High voltage and higher

generation at CGPL causes relay vulnerability of 400 kV CGPL-Bhachau.