Chapter- 6
REVIEW OF ISLANDING SCHEMES
6.1 To avoid total blackout following a grid disturbance, a number of defense mechanisms and System Protection Schemes mainly comprising of generation backing down, contingency based load shedding, under frequency load shedding, df/dt load shedding etc already exist. The success of these schemes in avoiding grid disturbances to a large extent depends upon the severity, area of disturbance and system conditions prior to the disturbance. Also as a last resort some islanding schemes to save the generating stations are also in existence. During the disturbance which took place on 30th and 31st July 2012 some of the generators which survived in NR due to islanding or on house load were NAPP, BTPS , Dadri Gas, Faridabad Gas. The surviving generating units normally help in meeting essential loads and extending supply to other units within the same generating station and also to the nearby generators thereby helping in restoring the grid in reduced timeframe. The Committee reviewed the existing schemes and explored possibility of formulation of more islanding schemes in the NR.
6.2 A meeting in this regard was held on 7th August 2012 wherein members from various state utilities participated. After deliberations it was agreed that criteria for formation of islands should not be the geographical or electrical size but reliability of load-generation balance in the islands. There was agreement on the general philosophy on formation of islands, salient features of which are given below:
6.3 Guidelines for formation of islands
a) For the success of the islanding scheme, the load and generation of these islands should match and also it is necessary that generators within the island are operated with Governor action.
b) All control areas should endeavor to operationalize under frequency based load shedding scheme as first defense. Only if this defense mechanism fails and frequency continues its fall to dangerously low levels, formation of islands should be initiated as a last resort.
c) The probability of survival of islands will be realistic only when all the generating units are on free governor or on restricted governor mode in accordance with provisions of Indian Electricity Grid Code.
d) Islanding scheme could be a two-tier scheme. At frequency level of say 47.9 Hz, signal for formation of islands comprising of more than one generating stations along with pre-identified load could be initiated. However, if after the formation of island, frequency continues to fall further to say 47.7 Hz, these islands could be further broken into smaller islands comprising of single generating station with pre-identified loads.
e) For survival of the Islands, they should be created in such a manner that the possibility of generation exceeding load is more.
f) In case of hydro generators with limited pondage, islands should be created keeping peak generation in mind. This is because, in low hydro season, generation will practically be negligible during off-peak hours and hence creation of island may not serve any purpose.
g) Load-generation balance in pre-identified islands may change based on season, there would be need to review the scheme on seasonal basis. Such review should also capture network changes taking place in the interim period.
h) As far as possible, major essential loads such as hospitals etc should be incorporated in the islands. However, if this was not possible due to some reasons, efforts would be made to extend supply from these islands to essential loads on priority basis.
i) State load Dispatch Centers/ State Transmission Utilities along with the generating stations in their area should explore the possibility of formation of various islands.
6.4 Possibility of islanding of Delhi metro and Indian Railways
6.4.1 During the grid disturbances which occurred on 30th & 31st July 2012, Railways and Delhi Metro services were also affected. During the disturbance on 30th July 2012, Delhi Metro services were affected in the morning to the extent that services were delayed as the disturbance had occurred at 2:35 hours when metro services were off. This did not trouble the passengers. However, during second disturbance at 13:00 hours, the trains were in operation, and the passengers faced difficulties because of sudden stoppage of services. This problem could have been avoided if the metro network would have islanded with some generating station(s).
6.4.2 In view of the importance of Metro and other Rail network, the Committee held discussions with DMRC and Indian Railways on how islanding schemes could be developed for them.
6.5.3 Delhi Metro Rail Corporation (DMRC) have 200 trains running on 185 Kms metro rail network in Delhi fed from 13 nos. 220 kV substations, out of which one each was fed from UP and Haryana side and rest from DTL’s 220 kV network in Delhi. DMRC was using its own 33 kV network for feeding stations and 25 kV network for meeting traction load. The distance between two metro power stations was in the range of 15 to 17 Kms as higher distance resulted in voltage drop and poor traction. The peak load of Delhi Metro was 120 MW with 50 MW station load and 70 MW traction load. Load per train was about 2 MW. DMRC had installed a DG set at each metro station to meet the load of lighting, ventilation and fire-fighting during main supply failures. They needed minimum 50 MW from at least 7 infeeds for traction purpose to keep their skeletal services running only for half an hour during contingencies like islanding of Delhi system from rest of the power grid for pulling the trains to the nearest station. Though they could feed their entire network from a single point, this would result into low voltage at distant locations. It was also noted that at any point of time, 10 trains are running inside the tunnels. If the power supply fails and the train stops inside the tunnel, then battery-backup is used to keep lights & fans running inside the compartments. In the event of power failure, it is not possible to open the doors of the compartments too. Batteries could provide backup supply for about half an hour only. Thereafter, fumes from the batteries start making the environment inside the tunnel suffocating. In view of this, it is essential to move the trains out of the tunnel and bring them to the nearest station within 15 minutes of supply-failure. In case, it is not possible, then passengers needed to be evacuated from the train under the guidance of trained metro staff.
6.5.4 Indian Railways were having a supply point every 30 – 50 Km distance to feed a section. In case of requirement of reduced load by the SLDCs / RLDC due to any contingency, they could manage to keep the trains running with availability of supply at each alternate section. They have allocation of 100 MW from NTPC’s Dadri and Auraiya GPPs to meet the load of Delhi – Mughal Sarai section and their own dedicated transmission lines to draw power from the grid for the purpose. They also have supply from 2 locations in Delhi viz. Dhaula Kuan and Narela. Railways felt that this section could be considered for islanding during grid contingencies as this was one of the most important sections of Railways. They do not have allocation of power from any other central sector station to meet the load in any other section of their network in the country. For other sections, they have arrangement of supply from Distribution Companies of the concerned states and had a very good communication system between their control room and concerned SLDCs. During grid disturbances on 30th & 31st July 2012, Railways received full cooperation from SLDCs/RLDCs in restoration of supply to their network on priority, except in the Eastern Region, where supply was restored late reportedly due to non-availability of start-up power to the power stations in that region. Railways requested to get this examined and improve the arrangement of extending start-up supply in that region. Railways would abide by the advice of the power station / SLDC / RLDC in the matter of connecting load on restoration of supply after grid disturbance. They also requested their services to be given priority at the time of restoration of grid.
6.5.5 It was noted that subsequent to grid failure at 1300 hours on 31st July 2012, two gas turbines (30 MW each) were started by Delhi and charged DMRC-I & II feeders after charging other important feeders. However, within a few minutes of charging of DMRC feeders, large fluctuations in the load were observed and the GTs tripped due to fall of frequency to the level of 47.4 Hz. It was felt that this might have been caused due to sudden connection of large quantum of traction load. If the load was connected by DMRC gradually in close coordination with GT control room, the machines could have continued to operate.
6.5.6 The Committee also examined the possibility of islanding of states including Delhi in the Northern Region under a grid contingency and recommends creation of four islands in Delhi. Delhi Metro’s emergency load and a part of Indian Railways load could continue to remain connected with these islands at its minimum four different sub-stations in case of grid contingencies.
6.5.7 In case of failure of formation of islands in Delhi, Delhi Metro while availing supply from any source e.g. IP GTs, Dadri GPS, etc., should connect load in small steps in close coordination with Delhi SLDC and the generating station to avoid the possibility of tripping of the generating station. DMRC should also make necessary changes in the technical and communication arrangements in their system to ensure this. There should be reliable communication arrangement between DMRC and GT station at IP extension in Delhi. DMRC should re-distribute its load so as to make it balanced in all three phases for stable operation of connected power stations. Power could be supplied to Delhi Metro from Rithala GT station of TPDDL (one of the Distribution Companies in Delhi) as well if this station had black-start facility. There being some possibility of malfunctioning of islanding of Delhi in case of grid disturbances and delayed extension of supply thereafter, DMRC might consider installation of DG set(s) of appropriate capacity to move the trains stuck in the tunnels so as to ensure safety of passengers.
6.4.8 As regards Indian Railways, islanding scheme could be prepared for Auraiya GPS along with Railways’ and other loads. This could feed about half of Delhi-Mughal Sarai section. Remaining half could be fed from Dadri GPS, which is envisaged to be islanded with a part of Delhi’s load. In case of failure of formation of Auraiya island, Railways while availing supply from Auraiya GPS after its black-start, should connect load in small steps in close coordination with the power station to avoid the possibility of its collapse again. Keeping in view the fluctuating nature of traction load, no unit should be started with such load. However, the supply should be extended to Railways / DMRC by the power station / SLDC on priority after starting the unit(s) with other types of balanced and more or less constant loads.
6.5 As per the resolution adopted in meeting taken by Hon’ble Minister of Power with Chief Ministers and Power Ministers of Northern State on 6th August 2012 the schemes prepared by States would be deliberated by them with CEA, POWERGRID and NRPC. Indian Railway and DMRC may further firm up the islanding schemes in consultation with CEA, POWERGRID and RPCs. Other islanding schemes should also be prepared on similar lines.
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