Chapter-3
Analysis of Grid Disturbance on 30th July, 2012
3.1 Introduction
On 30th July, 2012 there was a grid disturbance in the NEW grid at 02:33:11 hrs that led to the separation of the NR grid from the rest of the NEW grid and eventually NR system collapsed. The pre-disturbance conditions, sequence of events and analysis of the disturbance are described below.
3.2 Pre-Disturbance Conditions
The details of the generation-demand and power export/import scenario in the four regions of the NEW grid on 30.07.2012 at 02:00 hrs are given below.
A number EHV lines were out prior to the disturbance and the same are listed in the enclosed Annexure- 3.1. The grid frequency, just prior to the disturbance, was 49.68 Hz.
3.3 Sequence of Events on 30th July, 2012
The committee studied the data provided by various SLDCs , RLDCs /NLDC , POWERGRID and generation utilities to analyse the sequence of events leading to the blackouts in Northern grid on 30th July, 2012. The committee experienced some difficulty in analysing the available information because of the time synchronisation problems at various stations. The committee, however, established the sequence of events based on correlation of the data from various sources like Disturbance Recorders (DRs), Event Loggers (ELs), few Phasor Measurement Units (PMUs) in the NR and WR at different stations and Wide Area Frequency Monitoring System (WAFMS) of IIT Bombay.
It may be noted that the NEW grid was operating in an insecure condition due to a large number of line outages particularly near the WR-NR interface. Though an exhaustive list of lines under outage is given at Annexure-3.1, it may be noted that the following lines had tripped within an interval of a few hours prior to the grid disturbance.
1. 220 kV Badod(WR)-Modak(NR)
2. 220 kV Badod (WR)-Kota (NR)
3. 220 kV Gwalior-Mahalgaon ckt 2 (in WR but near WR-NR interface)
4. 220 kV Gwalior(PG)-Gwalior(MP)(in WR but near WR-NR interface causing only 220 kV Gwalior-Malanpur as only 220 kV NR-WR interconnection, and 220 kV Bina-Gwalior was no longer in parallel with 400 kV Gwalior-Bina)
Following are the sequence of the events, which took place on 30th July, 2012, leading to the Northern Grid blackout:
Some of the subsequent events of cascaded tripping are listed in Annexure-3.2, which has led the NR system to practically total blackout except a few pockets, such as Badarpur and NAPS (only household loads), which survived in islanded mode.
++ Power Swings: The rotors of synchronous machines inter-connected by AC lines tend to run at the same electrical speed in steady state due to the underlying physics of this system. When this system experiences small disturbances, restorative torques bring back the machines to synchronism (i.e., the same electrical speed). This response is characterized by an oscillatory behaviour since the underlying equations which determine the transient behaviour are like those of a spring-mass system. The oscillations are called “swings” and are seen in practically all parameters including line power flows. The oscillations die down if damping is adequate.
For large disturbances (e.g faults, loss of critical transmission links), the behaviour is non-linear and the electrical torques may be unable to bring all the generators to the same electrical speed. If this happens the angular difference between the generators goes on increasing (Transient Instability or Angular Separation). This causes large variations in voltage and power flow in lines.
Other equivalent terms are “Loss of Synchronism”, “Out of Step”, “Pole slipping”, although the latter two terms are typically used if only one machine loses synchronism. In a multi-machine system groups of machines may separate.
3.4 Analysis of the Disturbance on 30th July 2012
I. It is observed that even though the frequency of the NEW grid (49.68 Hz) was near to its nominal value (50 Hz), a number of lines were not available due to either forced outages, planned outages or kept out to control high voltages. This resulted in a depleted transmission network, which, coupled with high demand in the Northern Region, resulted in an insecure state of the system operation.
II. From WR-NR interface, 400 kV Gwalior-Agra line was carrying about 1055 MW and 400 kV Zerda-Bhinmal was carrying about 369MW, while 400 kV Gwalior-Bina was carrying about 1450 MW. The loading on 400 kV Gwalior-Agra was high. The Surge Impedance Loading (SIL) of the 400 kV Gwalior-Agra and also Gwalior-Bina lines, which are 765 kV lines charged at 400 kV, is about 691 MW (uncompensated), but its thermal loading limit is much higher (for quad Bersimis conductor).
III. NR constituents were instructed by NRLDC to carry out load shedding to relieve the Gwalior-Agra line loading. However, the quantum of load shedding undertaken by the NR constituents seems to be insignificant. WRLDC also issued similar instructions to its constituents for reduction in generation.
IV. The 400 kV Agra-Gwalior line is fed from 400 kV Bina-Gwalior line in the WR.
V. At 02:33:11:907 hrs, the 400 kV Bina-Gwalior line in WR tripped on Zone 3 protection, which is due to load encroachment (DR records do not show any evidence of fault or swing). Prior to tripping the voltage was 374 kV at Bina end and the line was carrying about 1450 MW approximately as per DR report of POWERGRID for this line.
VI. With the tripping of the above line, the supply to NR from 400 kV Agra-Gwalior was lost. 400 kV Zerda-Bhinmal-Bhinmal (220 kV)-Sanchore (220 kV) and Dhaurimanna (220 kV) was the only AC tie link left between WR-NR. Subsequently 220 kV Bhinmal–Sanchore line tripped on power swing, and as per SLDC Rajasthan 220 kV Bhinmal-Dhaurimanna tripped on Zone 1distance protection. This resulted in loss of the WR-NR tie links. A small load at Bhinmal remained connected with WR system through the 400 kV Zerda-Bhinmal line.
VII. In some cases the impedance measured by a distance relay at one end of the line may reduce to a point where it is less than the tripping condition for that relay for back-up protection (Zone 3). This may happen even if there is no fault in the nearby transmission system, and may occur when the line carries a very heavy load. This phenomenon of the mal-operation of the distance relays is known as ‘Load Encroachment’. Generally, it is an unintended tripping for distance relays since no fault has actually occurred.
It may be noted that at the time of disturbance, the 400 kV Bina-Gwalior line experienced a lower voltage and higher load current (resulting in less impedance, seen by the relay, which, possibly, was below the zone-3 reach setting of the relay) caused the relay operation under load encroachment. It was informed by POSOCO that this line had not tripped earlier due to zone-3 operation under load encroachment, although few incidences of such operation of distance relays in Western Region are observed in prior disturbances.
VIII. The tripping of the 400 kV Bina-Gwalior line initiated a very large angular deviation between NR system on one side and ER+WR+NER system on the other side. The power from WR to NR was now routed via WR-ER-NR interface, which is a very long path.
IX. An illustrative simulation to understand angular separation of the WR and NR regions was carried out. The simulation confirms that the systems may separate under such conditions. The simulation details are given at Annexure-3.3
X. Due to large power flows in the WR-ER-NR route, 400 kV Jamshedpur-Rourkela double circuit (in ER) tripped on Zone 3 (Exhibit 3.1 shows the angular separation).
XI. Though the NR system, at this stage, was still connected to the ER system (which was connected to the WR), the machines in the NR system had started to slow down as compared to those in rest of the NEW grid. Therefore, angular separation between NR and the rest of the grid continued to increase. This situation would eventually lead to angular instability (loss of synchronism).
XII. It is well established that under such situations, the distance relays near the electrical center of this separation are prone to pick up. Accordingly 400 kV ties between ER and NR (BiharSharif-Balia, Muzzafarpur-Gorakhpur, Patna-Balia, and Sasaram-Balia) tripped.
XIII. Since 220 kV Pasauli-Sahupuri (ER-NR) line was operated in radial mode, Sahupuri loads remained fed from the ER system and survived.
XIV. The NR system was thereby isolated from the rest of the grid. In the NR system, there was loss of about 5800 MW import and resulted in decline of frequency. NR System has Automatic Under Frequency Load Shedding Scheme (AUFLS), which can shed about 4000 MW of loads, and df/dt relays scheme, which can shed about 6000MW 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 and the NR system collapsed except for a few pockets at Badarpur and NAPS.
XV. With the separation of NR from the rest of the grid, the ER+WR+NER grid had a surplus of about 5800 MW power exported to NR prior to the separation. This system had more generation and the frequency rose to 50.92 Hz and stabilized at 50.6 Hz. There was tripping of Korba (E) 2*250 MW, APL Mundra 2*660 MW, Dhuvaran 80 MW, Parli 210 MW and Nasik 210 MW units in WR and Mejia-B 400MW, DSTPS 250 MW and MPL 450MW in ER took place. APL Mundra units tripped on Special Protection Scheme. The reported loss of generation is of the order of 3340 MW.
XVI. The sudden rise in frequency, close to 51Hz in the WR, also indicates inadequate primary response from generating stations. The primary response if enabled in NR could also have helped in curtailing the initial frequency dip in the Northern region.
XVII. During restoration, at 03:39 hours, several units and transmission lines at NTPC Vindhyachal STPS tripped in Western Region which also affected the start-up process.
After the grid was restored on 30.07.2012, another grid disturbance took place on 31.07.2012 , the details of which are given in the next chapter.
List of EHV Lines Out on 30.07.2012 Prior to Disturbance
(400 kV and above and Inter-Region 220 kV and above)
(as furnished by NLDC)
Subsequent tripping of lines in ER and NR systems after separation on 30/07/2012
(only those given in the DRs are listed below)
An Illustrative Example to demonstrate angular separation of NR-WR System
In order to illustrate that the angular separation can occur with the loss of a tie, a simplified two machine system was simulated, approximately representing NR and WR systems. We can look upon this system as a simplified representation of a two area system (NR and ER-WR-NER). We consider two tie lines, one short and one long.
In the simplified system, “NR” part draws 800MW on short tie and 400 MW on the longer tie. With the tripping of the shorter tie, Fig S-1 clearly shows that both systems go out of phase (in about 2.3 sec for this simplified Illustrative example). Fig S-2 shows severe power swings and oscillatory nature of voltage, MW and MVAR flows under this condition.
This simulation illustrates that angular separation between two systems followed by power swings is possible on loss of short tie. However as it is a simplified system, for specific answers to the collapse of the grids on 30th and 31st July 2012, a detailed load flow and transient stability simulation of the NR, ER, NER and WR grids is required.
