Editor’s Note: The following is Tata Power Company’s description of the islanding system it employs to avoid catastrophic blackouts in Mumbai when the regional grid the utility is connected to goes down. The text was sent to Senior Editor Harry Goldstein as part of the research for his article, ”How to Blackout-Proof a City.” It has not been altered from the original.
ISLANDING OF POWER SYSTEMS
The electrical load on a system at any given point of time in conjunction with the on line generating capacity determines the system frequency. If the load is well within the on line generating capacity of the system, then the frequency can be maintained at 50 Hz. Similarly if the actual generation is less than the load, then the frequency drops and settles below 50 Hz. The type of loads and their distribution in the power system also determine the operational behaviour of the power system.
In any power system, the transmission network is designed to be adequate for normal load flows and also for certain reasonable contingencies. However, when there is a major disturbance with considerable loss of network/generating capacity, the remaining network may not be in a position to cater to the load and generation. This may result in overloading and cascade tripping of the remaining network following the disturbance.
2.0 SYSTEM OPERATION UNDER SUDDEN LOSS OF SUBSTANTIAL GENERATING CAPACITY
In any power system under normal circumstances a load generation balance is achieved which in turn governs the system frequency at any instant of time. Any small difference between generation and load is adjusted by change in generation by the operators or by the frequency settling to a new value. However, when there is a sudden loss of large percentage of generation of the order of 20% percent and above, if no immediate corrective action is taken. It is likely that system might become unstable and in the process cause damage to plant and equipment apart from loss of power supply to consumers. Also, when there is a total system collapse, system restoration takes considerable time, due to the complexity involved in bringing back a major power system, back into service. However, even if a part of the total system remains in service, supplying a portion of the total system, it is relatively quicker to restore the remaining system back into service. In view of the above the corrective action under such situation has to be instantaneous and drastic to arrest the system from collapsing.
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In order to protect the power system integrity during system disturbances certain system protection measures have to be adopted.
Some of the important system protection measures that are adopted are:
a) Automatic under frequency load shedding scheme/line overload trimming scheme/under voltage load shedding scheme
b) System islanding scheme
2.1 Load Shedding Scheme
The principle of automatic under frequency load shedding scheme is that by relieving definite quantum of load at different frequencies attempt is made to arrest the downward drift of the system frequency from reaching the point where the thermal machines are set to trip by their protection. However, when the system is subjected to a disturbance of severity beyond the protection capacity of the scheme, the system frequency may reach the thermal unit trip setting. This would result in the tripping of all the thermal units in the system where such trip protection is provided. In most of the Indian power systems, which are predominantly thermal, such a situation would result in total system collapse.
2.2 System Islanding
In order to provide a further layer of system protection, following major system disturbance a scheme called islanding scheme has been developed. This protection is really a system protection of last resort. This scheme pre supposes that the integrity of the system cannot be maintained in spite of the automatic load shedding, for every possible emergency. Instead of allowing the system to disintegrate by the tripping of generators and transmission lines as the disturbance develops, the islanding scheme itself sectionalises the whole system into sustainable small systems each consisting of a group of generating stations and a group of load that can be supplied by these generating stations. In effect each group becomes a sustainable island and hence the name islanding scheme.
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3.0 TPC SYSTEM ISLANDING SCHEME
3.1 Tata Power had developed and commissioned a system islanding scheme co-ordinated with an automatic under frequency load shedding scheme a number of years back. The scheme had saved the Tata Power system from collapsing on a number of occasions.
3.2 The Tata Power Islanding scheme isolates Tata Power system from MSEB and BSES at all the interconnecting points when the system frequency reaches set . and there is a net reversal of power from Tata Power to MSEB and BSES. Before the islanding scheme is initiated, load shedding by under frequency and rate of change of frequency relays are carried out at higher frequencies to stabilise the system frequency from dropping. Only if frequency continues to drop further, the islanding scheme is initiated. At the end of successful islanding, Tata Power system operates as an islanded system with reduced load but with its own generation with system stabilised.
3.3 System Restoration
Normal thermal units take about 6 to 8 hours to be brought back on line, once these have tripped due to any reason.
Tata Power has 1 x 180 MW combined cycle power plant (which uses gas as fuel) and which can supply 120 MW in 30 minutes and can supply full load within three 3 hours.
In addition, hydro station of Tata Power at Khopoli, Bhivpuri and Bhira can be brought to full load within a short span of 30 minutes and power supply to essential services viz. Railways, essential consumers of BEST like Hospitals, etc. can be resumed at the earliest. The above system provides power supply of the highest reliability.
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