GRID FAILURE:A PERSPECTIVE AS A POWER PRODUCERSANTOSH KUMAR V, Dy ManagerRACHANA KAMATH M, Asst ManagerARUN A THOMAS, Asst Manager

Team from Kudgi Super Thermal Power Project

PRESENTATION OUTLINE

Five Regional grids namely

Northern GridWestern GridEastern GridNorth Eastern GridSouthern GridEXISTING POWER NETWORK

NTPC Ltd. Suffered a loss of 38% of its generation capacity.TheJuly 2012 India blackoutwas thelargest power outage in history, occurring as two separate events on 30 and 31 July 2012.GRID LOCKOUT620 MILLION PEOPLE9% OF WORLD POPULATION1/2 OF INDIAS POPULATION32GW OF GENERATION TAKEN OFFLINE

Fig: Affected regions during the grid failure

The NEW grid was operating in an insecure condition due to a large number of line outages particularly near the WR-NR interface.

On this day also, effectively 400 kV Bina-Gwalior-Agra (one circuit) was the only main circuit available between WR-NR. INITIATION OF THE GRID DISTURBANCE

The details of the generation-demand and power export/import scenario in the four regions of the NEW grid on 31.07.2012 are given below. CONDITION BEFORE GRID FAILURE

The over withdrawal by NR utilities, utilizing Unscheduled Interchange (UI), contributed to high loading on this tie line.

Since the interregional interface was very weak, tripping of 400 kV Bina-Gwalior line on protection of distance relay caused the NR system to separate from the WR.

This happened due to load encroachment (high loading of line resulting in high line current and low bus voltage). On 30th , as NR system got separated, it collapsed due to internal power swings. However on 31st , the disturbance affected the eastern and the north-eastern region.

After NR got separated from the WR due to tripping of 400 kV Bina-Gwalior line, the NR loads were met through WR-ER-NR route, which caused power swings in the system.

Subsequently, all the three grids, NR,ER and NER collapsed due to multiple tripping attributed to the internal power swings, under frequency and overvoltage at different places.

The rotors of synchronous machines inter-connected by AC lines tend to run at the same electrical speed in steady state.

Under small disturbances, restorative torques bring back the machines to synchronism .

For large disturbances (e.g faults, loss of critical transmission links), the electrical torques may be unable to bring all the generators to the same electrical speed.

This causes large variations in voltage and power flow in lines.POWER SWINGS

Cascading failure is common inpower gridswhen one of the elements fails and shifts its load to nearby elements in the system.

Those nearby elements are then pushed beyond their capacity so they become overloaded and shift their load onto other elements.It is the power swings that cause these cascading failure.CASCADING FAILURE

DOMINO EFFECT

COULD THE GRID HAVE BEEN SAVED?

Better coordinated planning of outages of state and regional networks, specifically under depleted condition of the inter-regional power transfer corridors.

Under-frequency and df/dt based load shedding relief in the utilities networks.

Better regulation to limit overdrawal/underdrawal under UI mechanism, specifically under insecure operation of the system.

Frequency Control through Generation reserves/Ancillary services.

FACTORS THAT CAN HELP TO IMPROVE GRID STABILITY ( In the perspective as a power producer)Primary control involves the action of turbine speed governors in generating units, which will respond when the speed (frequency) deviates as a result of an imbalance between generation and demand.

Free Governor Mode of Operation, (For short FGMO) is a defense mechanism against grid disturbances and is a standard practice worldwide.

In FGMO, there is no dead band.

Inadequate governor action in Grid Disturbances on 30th and 31st July 2012.

The sudden rise in frequency, close to 50.5 Hz in the WR, indicates inadequate primary response from generating stations.

The provision for putting all generating units on free governor action has been part of Indian Electricity Grid Code (IEGC) for several years.

Many difficulties have also been addressed in the new IEGC issued in April 2010 by providing for restricting the governor action in such zone.

Though impediments in implementation of governor action have been removed, there is still inadequate governor action in Grid Disturbances on 30th and 31st July 2012. This was not getting implemented as generators pointed out few difficulties including wide frequency fluctuations and various commercial issues.

The two system protections include Automatic under frequency load sheddingSystem and islanding scheme.Islanding scheme protection is really a system protection of last resort.

The islanding scheme sectionalizes the whole system into sustainable small systems.

To maintain essential services and quick restoration.The chances of survival of electrical islands would be abysmally low if generating units included in these islands are not on governor action. Some of the generators which survived in NR due to islanding or on house load were NAPP, BTPS , Dadri Gas, Faridabad Gas.

Badarpur TPS survived and operated in Island mode till 06:58 hours on 30.7.2012 when the Island collapsed.

The operation in an island mode could had been continued had all the other generators been operating on free-governor action. Consequently, Faridabad GPS tripped at 0658 hours with the collapse of Badarpur island. A CASE IN POINT :

Generating units are equipped with Power System Stabilizers (PSS), which can save the grid from several potential destabilizing conditions.

However no evidence of stabilizer action was found in Grid Disturbances on 30th and 31st July 2012.

Power grid disturbances cause big oscillations in active and reactive power, low voltage and voltage instability.

Ex. When a generation unit trips, the required electrical power from the remaining generators increases.

The energy furnished by the rotor decreases, which will cause oscillations in rotor angle and can result in severe power flow swings.

Suitably tuned Power System Stabilizers (PSS), will introduce a component of electrical torque in phase with generator rotor speed deviations.

This results in damping of low frequency power oscillations in which the generators are participating.Ensure proper coordination with POSOCO to ensure that exercise of PSS tuning is planned and implemented.

Settings of these dynamic stabilizing devices should be reviewed at appropriate intervals.

The black start procedure has already been prepared by RLDCs and is available with all utilities.

During the recent grid disturbances it has been observed that substantially longer time has been taken by certain generating stations to come on bars.

In case of some units LP diaphragms had burst during the occurrence, for which additional time was taken to rectify/replace the diaphragms.

MOCK EXCERCISESSince the present grid failure occurred after almost a decade, the preparedness and response was perhaps not up to the level expected.

More frequent need of having periodic mock exercises to ensure preparedness of all stakeholders involved as actual grid disturbances needs emphasis.

CONCLUSIONThe grid failure of 2012 has served as a social epiphany about the problems associated with the Indian electricity grid. As pointed out before, We as a power generator can contribute to the strengthening of our grid by

Ensuring IEGC guidelines are followed

Coordinate with agencies for implementation of islanding schemes for all our stationsEnsure the operation of the PSS

Develop procedures by which quick restoration of power plants is possibleBeing a rare occurrence, ensure the level of preparedness and response by mock drills or other ways.

Then, why not think about having a policy to ensure grid discipline? The time is ripe to have a comprehensive policy entailing all the processes and systems that NTPC is conforming to, so as to support the cause of grid discipline.

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