The entire power system from generation to distribution is the largest and most expensive of all man made machines. It is needed to light a lamp, switch on a fan, cool a room with an air conditioner or heat it with a heater, drive motors, sustain life supporting medical equipment – the list is endless.
An electric power system essentially consists of generating stations, transmission and distribution network and the end consumers. Generators are the producers of electric power which is done in different ways using different fuels. Transmission network primarily evacuates the bulk power generated at generating stations, which finally reaches the end consumer through the distribution lines. This meshed network connecting generating stations, substations and the consumer utilities forms the grid. The management of a grid is a complex procedure that not only involves technical but also commercial, legal, social, and political issues.
Indian Power System is demarcated into five regions, Northern, Eastern, Western, North Eastern and Southern. The first four regions have an installed capacity of 1,11,000 MW and are connected through synchronous links, i.e. operating at the same frequency, collectively known as the NEW grid. The power flow in the NEW grid takes place as per the power generation and demand scenario in the respective four constituting regions. The southern region however, with an installed capacity of around 42,000 MW, is connected to the rest of the country through asynchronous links which implies that the power flow is manually controlled and altered.
Frequency is like the pulse of a power system and depicts its healthiness. In India we are following 50Hz (cycles/second) system, whereas some countries follow 60 Hz system. Frequency also signifies the balance between load and generation. If demand is more than generation, the frequency would be less than 50 Hz and vice versa. The load variation follows a diurnal as well as a seasonal pattern. Summer, winter and the monsoon makes separate demands on power usage. Diurnally load picks up in morning with switching on of domestic and office load. A second peak occurs in the evening as light, fan, temperature controlling equipments etc. are turned on. Then there is a dip when domestic load is switched off and finally load again picks up in the morning. A typical load curve is given in the figure below.
After Independence the responsibility of generat-ion, transmission and distribution was handed over to State Electricity Boards (SEBs). Mid seventies onwards central generating utilities like NTPC and NHPC were formed. These utilities came up with large generating stations (Super Thermal Power Stations) and the grid development started on a regional basis. However later in 1989, the transmission wings of these central generating utilities were separated to set up Power Grid Corporation of India (POWERGRID) which is designated as the central transmission utility (CTU) to give thrust to implementation of transmission system associated with central generating stations and intra-regional transmission programme based on the perspective planning. At present there are central generation utilities (CGUs), central transmission utility (CTU), and state transmission utilities (STUs). In some states distribution is privatised but in others it is in state government’s control. For power system operation, a hierarchical structure is followed. Every State has its state load despatch centre (SLDC) for control of generation and load. At regional level, there is a regional load despatch centre (RLDC) for every region and there is national load despatch centre (NLDC) to coordinate among all RLDCs. At inter-state level, Central Electricity Regulatory Commission (CERC) operates to promote competition, efficiency and economy in bulk power markets, improve the quality of supply, promote investments and advise government on the removal of institutional barriers to bridge the demand supply gap and thus foster the interests of consumers. Planning and coordinated development of electricity sector is done by Central Electricity Authority (CEA). Ministry of Power makes policies and guidelines for the sector.
Load Despatch Centres
The Indian power system is one of the largest in the world and requires extensive coordination for the purpose of which many control centres have been setup in each region. The RLDCs are the apex body, as per the Electricity Act 2003 (EA 2003), to ensure integrated operation of the power system in the concerned region. The RLDCs for North, East, West, South and Northeast regions are located at Delhi, Kolkata, Mumbai, Bangalore and Shillong, respectively. The RLDCs coordinate with different SLDCs for safe and reliable operation of the power system. Recently the NLDC has also been set up at Delhi and is designated as the apex body in the country to ensure integrated operation of the national power system. The RLDCs and NLDC are presently owned, managed and operated by the CTU, Powergrid while the SLDCs in the state are owned operated and managed by the respective STU or the SEB as the case may be.
Functions of Load Despatch Centres
The grid management functions can be broadly classified into ex-ante, real time and post facto functions. Ex-ante functions include activities like load forecasting, outage planning etc. The real time functions involve time to time monitoring and balancing of various system parameters like voltage, frequency and power flow. The limits and operating bands for these parameters are provided in the Indian Electricity Grid Code (IEGC) and the planning criteria. Post facto functions consist of reporting, event analysis and settlement processes.
EA 2003 has the provision of non- discriminatory open access in power sector. Power purchasing or buying in India is done either on long term, medium term and short term basis. About 40-45 per cent of generation is contributed by central sector; 45-50 per cent by state sector and nearly 10 per cent by private sector. Most of this power is tied up in long term contracts. Rest of the power is either sold through short term bilateral contracts or traded through trader or power exchange. Long-term users pay higher charges and have a higher priority over short-term users.
In India currently two power exchanges are operating namely Power Exchange of India Ltd and Indian Energy Exchange which provide a platform for buyers and sellers to do their business through a predefined process of price and volume determination. The other way of purchasing/selling power is through trading. It is a transaction which is mediated by a trading company.
Scheduling and Despatch
The grid operator at regional level declares anticipated power transfer capability available in the transmission system during the forthcoming three months of his respective region. The process of preparing interchange schedules of each regional entity (whose metering and accounting is done by RLDC) is known as scheduling. Similarly SLDCs prepare the interchange schedule of utilities which are under the control of states.
Transfer capability is scheduled first for the long term customers. If the margin permits, then the medium and short term transactions are allowed. This preference is important for the transmission system in terms of reliability and certainty of power flows for delivering better quality of power. The utilities also have the option of reviewing and revising the scheduled interchanges to suit the demand/supply position.
The manifold growth in capacity and network augmentation has made power system operation a very complex task. Continuously changing network variables and patterns across the system, continuous public scrutiny and a very small response time requirement makes the function of grid management indeed challenging. Planning in deregulated structure with multiplicity of stake holders, coordination between various utilities involved and conflict of interest pose new problems. A system operator has to quickly switch roles as a planner, a strategist, an administrator, a consultant, an economist and a soldier, which makes his job highly demanding. Thus it is required on part of the grid operator to possess tremendous presence of mind and multidimensional skills.