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Before
the
MAHARASHTRA ELECTRICITY REGULATORY COMMISSION 13th Floor, Centre No.1, World Trade Centre, Cuffe Parade, Mumbai 400 005. Tel. 22163964 / 22163965, Fax No. 22163976 E-mail mercindia@mercindia.com Website: www.mercindia.com |
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CASE No. 21 of 2005
In the matter of Dr
Pramod Deo, Chairman, |
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ORDER
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Dated - 8th September, 2005
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| Section 86(1)(h) of the Electricity Act, 2003 provides that "the State Commission shall specify State Grid Code consistent with the Grid Code specified under clause (h) of sub-section (1) of Section 79." The work on formulating such Grid Code is currently under progress. Earlier, the Commission under its letter No. 1586 dated 22nd December, 2003 had directed the licensees, namely erstwhile MSEB, BSES Ltd., (now Reliance Energy Ltd. (REL) and Tata Power Company Ltd. (TPC), that all instances of grid failures causing disruptions of power supply in any part of the State should be reported to the Commission by fax or e-mail immediately. While doing so, brief details, extent and probable causes are to be furnished and be put on the Internet within 24 hours of such occurrence. A copy of the letter dated 22nd December 2003 was also endorsed to BEST Undertaking and The Mula Pravara Electric Co-operative Society Ltd for information and appropriate action. | ||||||||||
| 2. | Subsequent to the disruption of power supply that occurred in Mumbai on 16th June, 2005 (morning hours), in compliance of above, preliminary reports dated 16.6.2005 on the partial occurrences of system disruptions in Mumbai area were submitted both by TPC and Maharashtra State Transmission Co. Ltd. [State Load Despatch Centre, Kalwa] (MSTCL). | |||||||||
| 3. | The utilities were further directed under notice dated 17th June, 2005 to submit by 24th June, 2005 the following, and also to attend a hearing on 28th June, 2005: | |||||||||
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(a)
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Further details of the event listing out all affected feeders and generating units, | |||||||||
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(b)
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Detailed causes, | |||||||||
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(c)
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Details of steps and time taken to restore power, including with regard to essential services, particularly Railway traction, | |||||||||
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(d)
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Step envisaged/ taken to prevent such incidents in future. | |||||||||
| 4. | The detailed report on disruption of power supply were submitted by TPC vide their letter 24h June, 2005 [enclosed as Appendix - I]. TPC submitted that at 06:11 hours on 16th June, 2005, 220 kV Tata Trombay (TPC) - Trombay (MSTCL) tie lines 1 & 2 tripped with heavy voltage fluctuation at Tata end. 1330 MW generation on Unit No.4, 5, 6, 7-A and 7-B at Trombay was lost. Being an off-peak period, there was no generation at Hydro stations at the time of disturbance. Power supply to Dharavi, Parel, Mahalaxmi, Carnac, Backbay, Vikhroli, Chembur and Mankhurd areas got affected due to above tripping. | |||||||||
| 5. | MSTCL vide their submission dated 23rd June, 2005 [enclosed as Appendix - II], with copies endorsed to the authorised consumer representatives, stated that a disturbance took place in TPC-MSTCL network on 16.6.2005 at 06.11 hours leading to disruption of power supply in parts of Mumbai. The 100 kV interconnection at Borivali (MSTCL)-Borivli (TPC) and 220 kV Kalwa-Salsette Double circuit lines remained in service. Tripping of the TPC generating units caused shortage of about 1200 MW generation affecting supply to southern parts of Mumbai. The loss of generation in TPC system caused heavy overdrawal of about 500 MW from MSTCL system over Borivali and Kalwa lines. | |||||||||
| 6. | Meanwhile, REL vide their letter dated 21st June, 2005 have alleged partial restoration of power supply to the their area of supply even after the grid stabilization, and being compelled to restrict the load on the TIE line causing load shedding to their consumers till 16:01 hours on 16th June, 2005. REL have also sought explanation from TPC of the procedures being observed by TPC for load shedding in such cases of emergencies and mechanism for timely restoration. | |||||||||
| 7. | The hearing was held on 28th June, 2005. During the hearing, TPC made a presentation (copies of which were circulated amongst the participants) giving the details of network and interconnections, system condition prior to occurrences, system disturbances, Transmission line tripping, relay operation / protection, zone affected, receiving station-wise status, supply restoration, analysis of faults, and suggestions for interim remedial measures. | |||||||||
| 8. | Shri Prabhakar Kukde, Executive Director, TPC submitted that 220 kV Trombay (TPC)-Trombay (MSTCL) lines 1 & 2 tripped at TPC end on Zone-2 of distance protection. The relays are set at 400 msecs at TPC end. By TPC's analysis, the fault was in MSEB network, either on 220 kV Trombay bus or on any of the outgoing 220 kV lines at Trombay (MSTCL). None of the line breakers had tripped at Trombay (MSTCL) and the fault was cleared by tripping of all the lines at remote end either in zone 2/ 3/ or back up protection. The operation of protection in zone 2 / 3 indicates that the fault was in the section between 220 kV Trombay (MSTCL) bus and 20% of one of the connected lines to Kalwa, Nerul, Mulund or TFPIL. Prima facie the fault is of a transient nature and the exact location of fault has not been established. All the breakers had tripped between 0.4 to 0.45 seconds. It is observed that the fault was cleared at Trombay (TPC) in 485 msecs. On account of the above fault being fed for a long duration by the generating units at Trombay, units tripped one after another on different protection schemes that resulted in subsequent collapse in TPC system. He suggested that Trombay (MSTCL) sub-station is to be probed thoroughly to identify the nature of fault. | |||||||||
| 9. | According to TPC, as an interim measure, zone-2 relay operation timing on TPC-MSTCL tie lines has to be reduced to 0.25 seconds as compared to present setting of 0.4 seconds till such time MSTCL improves the reliability of protection and associated switchgear. With these settings, the tie lines are likely to trip at Tata end for any fault in the MSEB system. Though not desirable, the same has to be done as an immediate measure. | |||||||||
| 10. | Shri Kukde informed about another fault on 220 kV Trombay (MSTCL) - Mulund line on 19th June 2005 due to parting of the jumper on a tower close to Trombay (MSTCL). However the breaker of the above line at Trombay (MSTCL) did not trip and the fault was finally cleared by LBB protection (stuck breaker protection). Interim remedial measure of reducing Zone-2 timing at Trombay (TPC) cited above helped in separating TATA power generation from the fault, thus avoiding the recurrence on 19th June. Shri Kukde pointed out that Trombay locality suffers from very heavy chemical pollution added with saline atmosphere. He further submitted that MSTCL systems do not have carrier aided schemes to clear the faults in the last 20% of the line in Zone-1 timing. If such carrier protection is available in addition to distance protection faster isolation of faults can take place making the system less vulnerable. | |||||||||
| 11. | MSTCL in its report submitted that the fault had been of transient nature. However, no trace of fault could be conclusively arrived at. While analyzing the occurrence of fault MSTCL submitted that the window and relay indications at different stations as well as event log generated at SLDC Kalwa points out that all the 220 kV lines connected at 220 kV Trombay (MSTCL) substation tripped at remote end in Zone-2 distance protection. There was neither any tripping nor any indications at Trombay (MSTCL) substation. The switchyard on inspection however has not revealed anything abnormal. As all the 220 kV lines tripped at remote end in Zone-2, it appears that the fault may be beyond Trombay (MSTCL) bus i.e towards TPC system. MSTCL took steps to check all the relays and physically inspected all the lines connected to Trombay during 20th and 22nd June 2005. Shri Satarke, EE, MSTCL queried about the rationale of tripping of 220 kV Trombay (TPC) - Salsette Double Circuit (D/C) lines in Zone-2 when the TPC - MSTCL tie lines have already tripped in 436 milliseconds. | |||||||||
| 12. | To a query on earlier occurrence of similar nature, TPC submitted that on 13th June, 2005 there had been number of faults but all of them have been correctly isolated by protection equipments. Shri Kukde emphatically observed that the critical analysis of the FDR is necessary to identify the nature of faults and take corrective action in future for better relay co-ordination. The Commission also drew attention of TPC about 18th May, 2004 disturbance wherein a single bus-fault had resulted in tripping of all the units which ideally should not have been the case. | |||||||||
| 13. | The Commission wanted to know whether there has been any detailed analysis or simulation done to ascertain generator's capacity to continue its operation beyond 450 msecs. TPC indicated that last year fault was caused by a lightening strike at a particular span of the Bus bar and subsequent re-striking of arc had resulted in a complex multiple fault at a single instance which can give rise to ascending unstable condition leading to a complete shut down. He submitted that this total occurrence has been analyzed and presented in an IEEE Paper that is proposed to be furnished by TPC before the Commission for information. | |||||||||
| 14. | On the question of TPC's observation on MSEB data on Nerul yet to be made available till the date of hearing, and detailed restoration of such service lines, MSTCL submitted that as per their standard practice, being chemical zone, lines were kept charged and put into service only after routine patrolling checks. The TIFIL and Nerul lines were taken on service after 7-11 hours respectively. He further informed that many of the breakers are of vintage technology and efforts are being initiated for appropriate R&M scheme to improve reliability. Similarly, de-capping of insulators is also being looked at. SE - MSTCL further pointed out the possibility of corona creation during the first seasonal rains particularly in the morning period and the unwanted tripping that it may cause. | |||||||||
| 15. | The Commission observed that the objective should be necessarily to take corrective measures to avoid recurrence of similar disturbances in future, particularly when there is a report on total system failure occurrence on 18th May, 2004. It further observed, while expressing regret that the nature of the fault is still to be identified, (a) whether there exists any common time synchronization and (b) whether relay-setting coordination exists between TPC and MSTCL. There may be a case for review of the conventional relay settings. The Commission expressed unhappiness over the statement that the fault nature / location could not be identified on patrolling, which was not done in requisite time, of the suspected zones of fault. | |||||||||
| 16. | The Commission further observed that the area of concern is lack of identification of the exact nature / location of fault that has caused such widespread system disturbances and loss of supply to the consumers. Further, the interim remedial measures already adopted are based on assumption out of relay indications and not supported by any simulation or physical identification of the fault. | |||||||||
| 17. | The Commission enquired why it took such a long period for restoration of Unit 6. TPC replied that since unit had tripped on differential protection, the restoration was carried out only after thorough check up and eliminating all possible causes. Shri Kukde requested the Commission to restrain other Licensees from making adverse publicity keeping the competitive background in mind, while including identifying the real reasons for such occurrence. | |||||||||
| 18. | The Commission notes that during regional grid disturbances (external) in the recent past, TPC system islanded on all occasions ensuring reliability in Mumbai System but same reliable supply is not attained for nature of faults (internal) now and on 18th May 2004 when total generation loss took place. | |||||||||
| 19. | The Commission observes that to improve reliability of power supply in Mumbai System during such faults and to avoid such recurrence of fault in future TPC in association with MSTCL may review existing protection schemes including bus bar protection. If need be, these may also be taken up in appropriate regional forum. The TPC in coordination with MSTCL shall review relays as well as any other protection equipments in switchyard or in transmission lines for its non-performance relating to the above disturbance. | |||||||||
| 20. | The complete report of the findings, as sought in the above para, recommendations and action taken by the TPC and MSTCL should be furnished before the Commission within two months from the date of this Order. | |||||||||
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Encl. Appendix-I
& Appendix-II
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