Chapter-4
Analysis of Grid Disturbance on 31stJuly, 2012
4.1 Introduction
While the grid recovered from the black out of 30th July 2011, another major disturbance took place on 31st July 2012 in the NEW grid at 13:00:13 hrs that led to the separation of the NR, NER and ER from the WR and eventually led to the collapse of the NR, ER and NER grids. The pre-disturbance conditions, sequence of events and analysis of the disturbance are described below.
4.2 Pre-Disturbance Conditions on 31st July 2012
The details of the generation-demand as well as import/export of power in each of the four regions in the NEW grid on 31.07.2012 at 12:30 hrs are given below.
A number of EHV lines were out prior to the disturbance and the same are listed in Annexure 4.1. It may be noted that even after grid disturbance on the previous day, similar network operating conditions prevailed on this day as well. The frequency, just prior to the disturbance, was 49.84 Hz.
4.3 Sequence of Events on 31st July, 2012
It may be noted that the NEW grid was operating in an insecure condition even on 31st July 2012 due to a large number of line outages particularly near the WR-NR and ER-WR interfaces. Though an exhaustive list of lines under outage is given at Annexure 4.1, it may be mentioned that the following lines had tripped within an interval of a few hours prior to the grid disturbance.
1. 400 kV Zerda-Bhinmal
2. 400 kV Zerda- Kankroli
3. 220 kV Badod-Modak- tripped a few minutes before the event
4. 220 kV Badod-Kota- tripped a few mintutes before the event
In addition Surat Garh unit-1 also tripped around this time
The following are the sequence of events, which took place on 31st July 2012 leading to the blackouts in the Northern, North-Eastern and Eastern regions.
**After event 64, the NR got practically isolated from the ER+NER and frequency started dropping (observed in the NR system) after a gap of about 1 minutes from the previous major event.
The subsequent events of cascaded tripping led the NR, NER and ER system to practically total blackout.
4.4 ANALYSIS OF GRID DISTURBANCE ON 31st JULY, 2012:
I. It is interesting to note that on 31st July 2012 also, though the frequency of the NEW grid (49.84 Hz) was near to its nominal value (50 Hz), a large number of lines were not available due to either forced outages, planned outages or kept out to control high voltages which, coupled with high demand in the Northern Region, resulted in insecure state of the system operation.
II. NR constituents were instructed by NRLDC to carry out load shedding to reduce the over drawal. Similarly the WR constituents were also instructed by WRLDC to reduce generation to bring down the over injection of power.
However, the quantum of load shedding/generation reduction undertaken by the two constituents seems to be insignificant.
III. Just prior to the initiation of the major disturbance, NR-WR was connected through AC tie links between 400 kV Agra-Gwalior (one circuit), 220 kV Badod-Kota and 220 kV Badod-Modak lines.
IV. Badod-Modak line flow reached 288MW at about 12:58pm on 31st July, 2012 from
V. 103MW and got tripped due to overload. Similarly, 220 kV Badod-Kota line also reached a flow of 298MW from its earlier flow of 113MW and tripped due to overload. The rise in flow of these lines are possibly due to tripping of the Suratgarh generating unit-1 of 250 MW at about 12:50 hours in Rajasthan.
VI. At about 13:00:13 hrs, 400kV Bina-Gwalior-I line tripped on distance relay zone-3 protection, which is also due to load encroachment (as DR records do not show any evidence of fault or swing). As per DR report of PGCIL the loading on this line 1254 MVA and voltage was 362 kV at Bina end (Though the MW loading was less the previous tripping, due to lower voltage the MVAR flow was larger than previous incident).
VII. The load on the 220kV Bina-Gwalior-I&II suddenly increased to 447MW from 330MW and increased further. The power flow on 220kV Gwalior(PG)-Gwalior (MP) line-II was 188MW at 12:58:58pm and got reversed to -180MW. This resulted in the reverse flow of power from Gwalior (MP) to Gwalior (PG) and pumped in to 400kV system.
VIII.The power drawl of Auraiya from Mehalgaon resulted in the tripping of 220kV Bina-Gwalior- I&II, 220kV Shivpuri-Sabalgarh-I, 132kV Pichhore-Chanderi and 132kV Pichhore-Shivpuri. On 31.07.12 400kV Bina-Gwalior II and 400kV Gwalior-Agra II lines of POWERGRID were under shut down and 220kV Gwalior (PG)-Mahalgaon (GWL) -I, 220kV Gwalior-(PG) –Malanpur-II of MP were also under shut down since 29.07.2012. This situation led to the isolation of the Gwalior region of MP from WR and formed part of the NR system.
IX. The NR system was isolated from the WR system and the demand, which was earlier fed from the WR got routed through WR-ER-NR systems,
causing increase in the angular separation between the NR and WR systems, similar to the disturbance on 30th July 2012.
X. However, unlike the pattern on 30th July 2012, the electrical center of the angular separation appears to be slightly inside the ER system from the WR-ER interface. This resulted in tripping of lines connecting unlike Ranchi and Rourkela to the rest of the ER. These buses formed part of the WR, which got separated from the rest of ER+NR+NER at about 13:00:20 hrs.
XI. The frequency plots are available from PMUs and the WAFMS from the NR and WR only (see Exhibit 4.1). This shows that the frequency in the WR rose to 51.4 Hz and that in the rest of the NEW grid stabilized close to 48.12 Hz. The sudden rise in frequency, close to 51.4 Hz in the WR, again indicates absence of FGMO controls being activated in several generating stations. In fact, the FGMO operation in the rest of the NEW grid could have possibly recovered the frequency which stayed at 48.12 Hz for about a minute and probably avoided the further catastrophic failure.
XII. The WR system survived with the tripping of Sipat 660MW, DSPM 2*250 MW ESSAR 125 MW and KLTPS 69 MW generating units. APL 660 MW generating unit tripped on Special Protection Scheme, associated with tripping of Adani-Manindragarh HVDC and frequency stabilized at around 51 Hz.
XIII.Further the loss of import from about 3000 MW import from WR resulted in decline of frequency in the rest of the NEW grid, which has Automatic Under Frequency Load Shedding Scheme (AUFLS), that can shed about 5600 MW of loads, and df/dt relays scheme, which can shed about 6020MW of loads, to improve the frequency and save the system under such emergency situations. However, not adequate load relief from the AUFLS and df/dt relays was observed on 31st July 2012 also.
XIV. Subsequently, possibly due to some generator trip in the NR+NER+ER grid led large angular oscillations and drop in system frequency, which resulted in a large number of trippings in the NR, ER and NR-ER links. This cascaded tripping of lines was on overvoltage at few places, power swing or zone-3 protection and tripping of generators on under frequency. This initially separated NR from NER+ER. From PMU records NR systems has collapsed on under frequency. There is no PMU installed so far in ER+NER system. The system is also smaller in size with small Power Number and ER+NER systems collapsed except for few islands, like CESC, NALCO and BSP.
XV. It may be mentioned that with the collapse ER, the Southern Region lost about 2000 MW in feed from Talchar-Kolar HVDC and frequency declined from 50.06 Hz to 48.88 Hz as per SRLDC SCADA. The frequency controller at HVDC Bhadrawati increased the flow of WR to SR from 880 MW to 1100 MW. System Protection scheme at Kolar did not operate. It was informed by the SRPC that there was AUFLS relief of about 984MW in the SR.
XVI. It may be noted that both on 30th and 31st July 2012, lot of tripping of lines were observed due to over voltage and also substantial under voltage at the tail end of the heavily loaded lines were observed, which caused operation of distance protection. These extreme voltage situations could have been avoided with the proper reactive power absorption/support from reactors/capacitors, dynamic compensators as well as synchronous generators.
List of EHV Lines Out on 31.07.2012 Prior to Disturbance
(400 kV and above and Inter-Region 220 kV and above)
(as furnished by NLDC)
