To improve economic viability and efficiency of power network, restructuring of power sector has been promoted. Unbundling of generation, transmission and distribution services has provided the option to the customers to choose those services that they need and pay for the services that they use. Unbundling has encouraged the competition among suppliers, and many private players have entered into market thereby lowering the costs of these services. All over the past years, the electric utilities were vertically integrated, owning and operating the whole chain of electricity from generation through transmission to distribution and supply. An ancillary service was therefore an integral part of electric supply, and was not dealt separately. Since the deregulation/restructuring of the electricity supply industry, the resources required for reliable operation have been treated as an ancillary service that the system operator has to obtain from other industry participants.
In a deregulated power system, the system operator often has no direct control over individual power stations – and has to purchase these services from other service providers. Ancillary services are support services in power system or grid operation, which facilitate basic function of generation, transmission and distribution of electricity by ensuring reliable delivery of power at stable frequency and voltage alongwith security of grid. Now, ancillary services are an indispensible part of the electricity industry.
Ancillary services
Ancillary services are all those activities on interconnected grid that are necessary to support transmission of power while maintaining reliable operation and ensuring required degree of quality and safety. As per Indian electricity grid code, ancillary services are services necessary to support power system or grid operation in maintaining power quality, reliability and security of the grid. Ancillary services may include scheduling and dispatch, frequency regulation, voltage control, generation reserves etc. General classification of ancillary services is:
- Frequency Controlled Ancillary Services
- Network Controlled Ancillary Services
Frequency controlled ancillary services:
These are used to maintain frequency within the desired range by balancing load and generation in real time. Three levels of control are generally used to achieve frequency controlled ancillary services i) primary frequency control requires response period of 5 to 10 seconds, ii) secondary response requires response period of 10 seconds to 10 minutes and iii) tertiary frequency control requires response period of 10 to 30 minutes.
Network controlled ancillary services:
These are required to maintain network parameters within permissible range. They are further classified as:
- a) Power flow control ancillary services:- These are used to control the flow on interconnectors within the limits.
- b) Voltage controlled Ancillary services :- These are used to maintain system voltage within desired range, three levels of control are required for voltage controlled ancillary services – they are primary, secondary and tertiary voltage control.
- c) System restart ancillary services :- These help in restarting system post blackout situations. These are required to backup capacity of system and capacity required to return to normal operation after major or partial blackout. So, system restart ancillary services are the services reserved for contingency situations – in which there has been a whole or partial system blackout and the electrical system must be restarted.
Classification of Ancillary services can also be based on common approach of how and when frequently these services are required by the system operator.
- Services required for routine operation
- Services required to prevent an outage from becoming catastrophe
- Services to restore system after blackout.
Needs for ancillary services
Consider Fig. 1 in which all generators, loads and interchanges are connected to the same bus. Frequency remains constant when generation is balanced by load, but in practice there are fluctuations in load, inaccurate control of generation, sudden outages of interconnectors and generators. Imbalance between load and generation cause frequency variation. Generators operate within narrow range of frequencies, protection system comes into action when frequency is too low or high, and the system operator must maintain frequency within limits. Local imbalance in an interconnected system causes change in tie line flows. Inadvertent flow overloads tie lines, protection system acting can cause imbalance. Frequency deviations have to be corrected quickly so that system can withstand further problems and weakening of system. Balancing services are needed to handle imbalances occurring at different time. Regulation service provided by generating units should handle rapid fluctuations in load, unintended variations in generation, should maintain frequency close to normal, interchange at desired values. Thus, ancillary services are either related to generation load balancing issues or network related issues.
Fig. 1: Load generation interaction in power system network…
Functions of ancillary services
As per Electric Power Research Institute (EPRI) functions of ancillary services can be broadly given as i) Regulation :- Use of generation or load power to maintain minute to minute generation load balance within control area. ii) Load following :- This service refers to load generation balance towards end of scheduling period. iii) Operating reserve :- It’s provision of unloaded generation capacity that is synchronised to the grid and can immediately respond to correct for generation load imbalances, caused by generation and /or transmission outages that is fully available for few minutes. iv) Reactive power and voltage control support:- Injection or absorption of reactive power from generators or capacitors to maintain system voltage within required ranges. v) System black start capability:- Ability of generation unit to proceed from shutdown condition to operating condition without assistance from the grid – and then to energise the grid to help the other unit start after blackout occurs. vi) Network stability services from generation sources:- Use of special equipments example dynamic braking resistances to maintain secure transmission.
Ancillary services for India
The following factors are driving forces for development and need for ancillary services in Indian power sector. i) Multiple buyers and sellers for power in power market, ii) Power market is under restructuring process, iii) Lack of adequate reserves, iv) Loose power pools, v) Absence of tight frequency control, vi) Lack of adequate primary, secondary and tertiary response, vi) Frequent unplanned load shedding and vii) Consistent shortages during peak hours.
Issues with growth of ancillary services
- i) Stranded generation capacity :- The various reasons for stranded generation capacity are inadequate evacuation infrastructure, changing loading methodology from natural to surge impedance, inability of distributed companies to buy expensive power, absence of funding to purchase capacities in need. All possible generation capacities should be mainstreamed in order to reduce power cuts in country. ii) Variation in availability of renewable power into the grid :- The three key impacts of RES on ancillary services are: first variability and uncertainty increases ancillary services requirement, affecting scheduling and pricing, secondly impacts vary depending on system condition, thus it’s more difficult to predict and thirdly allowing renewable to participate in markets can offer more liquidity and challenges. iii) Peaking power requirements:- consistent shortages during peak hours due to supply demand gap. iv) Network issues related with contingency, destabilisation can be caused due to thermal overload, transient instability, and voltage instability. iv) Identification and providing effective solution: – Determining which ancillary services are needed, to what extent ancillary services are required to maintain system reliability, whether suppliers services can be metered and billed cost effectively. Who will pay for ancillary services and to whom? What will be the provisions for payment?
Obtaining ancillary services
Two approaches can be followed to obtain or procure these ancillary services: i) Compulsory provision and ii) Market for ancillary services. The choice is influenced by type of service, nature of power system, history of power system.
Compulsory provision:- This ensures minimum deviation, system security and quality of power supply. Service provider has to meet some obligations.
Market for ancillary services:- They are long-term contracts, spot market and combination of both spot and long-term contracts. Long-term contract: For services where quantity needed does not change and availability depends on equipment characteristics, for example: black start capability, inter trip scheme, frequency regulation. Spot market: Needs change over the course of day, price changes because of interaction with energy market, for example: reserves. Market-based ancillary services are economically more efficient than compulsory provision, so buys only amount of service needed, only participants that find it profitable provide services, opens up opportunities for innovative solutions.
Beneficiaries due to ancillary services
Promotion and growth of ancillary services provide opportunities for generating units towards helping consumers meet higher demand and obtaining monetary incentives, utilisation of fragmented stranded capacity thus harnessing undispatched generation and optimisation, helping hand towards higher renewable power penetration thus complementing intermittency, can meet the peak power requirements leading towards better and stable system.
Steps for overcoming the issues faced in providing ancillary services
Regulation on market design and procurement mechanism, policy framework for provision of ancillary services, regulation on tariffs for power procurement aspects, price pooling and affordability, distribution generation sources are important resources for various kinds of ancillary services, free governor mode operation can be mechanism to provide frequency controlled ancillary service and promoting energy storage solutions.
Conclusion
This article highlighted the issues and importance of provision of ancillary services for power sector. Due to diverse nature and requirements of Indian power sector, ancillary services are required to enhance the reliability and quality of power system network.
- Network controlled ancillary services Generators, Capacitors, Inductors, Synchronous condensers, FACTS controllers, Distributed energy resources
- Frequency controlled ancillary services Governors, Automatic generation and control, Rapid loading unit, Rapid deloading unit, Demand side management
- System restart ancillary services Generators, Distributed energy resources.
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