Ancillary services through Microgrid for Grid Security & Reliability
Microgrids can provide potential Ancillary services to the power system for maintaining its voltage profile and frequency. These services also improve stability, security of system and reduce congestion. These will be source of revenue generation for microgrid owner’s in near future and will be of great importance to the power market players…
- Prof. Mrs. Kalyani M. Kurundkar,
Dr. Mrs. G. A. Vaidya.
India’s sustained growth has led to supply demand imbalance with the peak deficit of 12.7%, which could further increase. Large fraction of population depends on fossil fuels for their energy needs, which increase the green house gas emissions. The government has recognized the importance of renewable energy and it is seen as the environment friendly solution to the problem of increase in energy demand. According to the strategic plan of MNRE for the period of 2011-2017, 20 million solar lights are estimated to avoid the use 1 billion liters of kerosene per annum in 2020 and 5 million sq.m. thermal collectors are expected to save fuel oil of about 350 million liters per annum in 2020. The government is also working towards installation of 10,000 Renewable energy based microgrids in the next five years which will provide access to electricity to 237 million Indians. Government of India has planned for renewable energy (RE) capacity addition of 175 GW by 2022. With technological advances, market shifts, and policy changes that support increasing levels of distributed generation based on renewable energy sources the Indian electricity system is evolving rapidly. Without a proper integration into network operation, Distributed Generation (DGs) based on intermittent energy might jeopardise the stability of the network at higher penetration levels. The possible impacts on power system operation specifically at distribution level due to high penetration of renewable energy sources are discussed below.
Bidirectional Power Flows
While distribution feeders were initially designed for unidirectional power flow, integration of DG units at low voltage levels can cause reverse power flows and lead to complications in protection coordination, undesirable power flow patterns, fault current distribution, and voltage control.
Power Quality Issues
Two main aspects are usually considered under power quality issues: transient voltage variations and voltage harmonic distortion. Depending on several issues such as capacity, type of prime mover, interface, location, etc. the effect of DG on network voltage can be either positive or negative. In practice, meeting the required standards either from the energy converter side or from the grid side causes no problems to grid operation.
The presence of DG may have a considerable impact on the stability of the electric power system since the networks are no longer passive.
In some scenarios, DG may alter branch flows significantly, which may pose additional problems in terms of managing energy flows. This may ultimately cause branch overload, especially, in the case of high levels of renewable-based DG integration, which may inject large amounts of energy into the distribution system.
Protection of the generation equipment from internal faults; protection of the faulted distribution network from fault currents supplied by DG; loss-of-mains protection and impact of DG on existing distribution system protection.
Unlike bulk power systems where high number of synchronous generators ensures a relatively large inertia, DG system might show a low-inertia characteristic, especially, if there is a significant share of power electronic-interfaced DG units.
The economical and reliable operation of DG system requires a certain level of coordination among different sources. This coordination becomes more challenging when taking into account the uncertainty of parameters such as load profile and weather forecast.
This impact of distributed generation penetration in the grid can be reduced without compromising the quality of supply and reliability by ancillary services for grid support. Reliability of supply with best power quality has become the highest priority of consumers. Ancillary Services in power system (or grid) operation are support services necessary to support the power system (or grid) operation for maintaining power quality, reliability and security of the grid, e.g. active power support for load following, reactive power support, black start, etc. For secure and reliable operation of power system in restructured environment, there is a need for ancillary services more at distribution level than at transmission level.
Ancillary Services consist of services required for:
a) Maintaining load – generation balance (frequency control)
b) Maintaining voltage and reactive power support
c) Maintaining generation and transmission reserves
Traditionally, ancillary services were provided by the supply authority itself, in a re-regulated regime, where customers are energy producers too, these may be provided by both supply authorities and microgrid owners. 'Microgrid' fundamentally consist of controlled Distributed Energy Resources (DER’s) which include a master controller with its control system, microsource controller, microsource or distributed generation, storage units and loads. Microgrids can be operated interconnected to the main power grid or in an autonomous mode (standalone or Islanded Mode). The major Ancillary services that are provided by Microgrid to the power system are:
1. Frequency regulation: This service is based on active power control which can be done by controlling the output of DER. Adjusting generation to load minute by minute to maintain specified system frequency within the control area , is known as ‘regulation’. This service can be provided efficiently by the microsources (DG’s) of Microgrid, which are connected to the grid and at the same time located close to the load pockets. Load following takes place over longer periods. Load following is the capability of on-line generation equipment to track customer load variations and control the load as per the requirement.
2. Voltage Control, congestion management and optimization of grid losses: Ancillary service of voltage control, Congestion Management and Optimisation of Grid Losses can be provided by controlling the reactive power which not only depends on control capability of the grid-coupling technology but also on the active power control capability of the DER unit. Reactive power and voltage control are needed for regulating distribution voltage within specified limits. Microgrid can perform smooth voltage regulation locally in response to controller settings. Moreover, local supply of real and reactive power from microsources of Microgrid significantly reduces feeder losses and is useful for congestion reduction. For power utilities, reactive power and voltage control is generally accomplished at the cost of generating capacity. If Microgrids provide this service, it would help the utility generators to generate at their maximum capacities, thus enhancing overall generation. The Improvement of Voltage Quality depends on the grid-coupling technology.
3. Supply of reserves: Microgrids can sell three ancillary services, viz. (i) frequency responsive spinning reserve, (ii) supplemental reserve and (iii) backup supply in open competitive market. These services are aimed at restoring the real-time energy balance between generators and loads in case of any sudden contingency.
4. Network Restoration / Black Start: Black start is defined as the capability of a power system to restart its generation after a total system collapse, without importing any external power. Microgrids can easily sell power for system black start.
5. The Fault-Ride-Through Capability depends on the grid-coupling technology which has to balance the power flow on the grid-side and the DC-side in case of voltage disturbances.
6. Islanded Operation: Islanded Operation requires the capability to control active power, reactive power, voltage and frequency. Active power control can be done by control of the distributed generation.The reactive power control is possible by proper grid coupling technology.Distributed storage is mostly designed for islanded operation for uninterruptible power supplies.
7. Network Stability: Microgrids are capable of sensing the low-frequency oscillations and providing adequate damping. This may be accomplished by making the microsource supply power at 180 degree out of phase from the oscillation.
Figure 1: Technological Framwork For Ancillary Services through Microgrid
Harmonic compensation and peak shaving are some of the other ancillary services that can be provided by microgrid.
The Table no 1 shows the Microgrid components and the corresponding ancillary services provided by them. The cost reduction in Information and Communication technology has given the boost to increase the capability of the microgrid master controller (MC) to control the output of DER in Standalone and grid-connected mode. The capability of the microgrid as a whole to provide the ancillary service depends on the grid coupling technologies used to couple DER with the main Grid and also the Controlled DER in the Microgrid. There are different types of grid coupling technologies used like Doubly Fed Induction generators(DFIG),directly coupled Synchronous Generators(SG), directly coupled Induction generators(IG), Inverters (Including Inverter coupled IG or SG). The grid coupling technologies transform the available power input into a power output of a different characteristic.
Fig 1. shows the technological framework, for providing ancillary services through microgrid to main grid. In order to analyze the technological capability of a microgrid to provide the ancillary service, firstly the Grid coupling technology is to be analyzed and then the controlled DER as a whole. Economic Framework of Provision of ancillary services through microgrid depends upon decision making capacity of Energy Management System (EMS) that will suggest most promising technologies for the provision of ancillary services and most feasible service which will result in incremental revenue for community microgrid owners, providing security of supply to the customers. Microgrid’s Master controller has Energy Management System (EMS).
Figure 2: Functions of Energy Management System for Microgrid
The energy management system as shown in figure 2 controls the power flows in the microgrid by adjusting the power imported/exported from/to the main grid, controls the dispatchable DERs, the controllable loads based on the present and forecasted information of the market, the generations, and the loads in order to meet certain operational objectives (e.g., minimizing costs) and maximizing the financial gains. For many ancillary services, the EMS would make a decision in a day-ahead market as to whether it would be profitable to supply the service, and at what price. The EMS would then bid into the market and find out if the bid was successful. If successful, the EMS would plan to supply the service the next day. Thus, ancillary service market can be a large source of revenue generation to Microgrid owners. In many parts of the world the microgrid owners are presently making profits from energy markets.
The success of any technology depends upon its commercial acceptance and use. Similar, Microgrid technology will be completely successful only if its participation in the energy and ancillary services market will bring profit to its market players without compromising on its assets. For successful implementation of microgrids, in India, the energy and ancillary services market must come forward with strong financial incentives for both i.e. microgrid owners and power utilities. The independent system operators, aggregators and market players will play a very important role in this context. In this case, it is the Microgrids EMS which responds to the pricing signal and act accordingly. Thus, in an Indian Power system with significantly increased levels of renewable energy sources penetration in distribution networks, the opportunities for microgrids to provide ancillary services at distribution level may increase. Ancillary services provision at distribution level has manifold advantages to utility as well as customers. With ancillary services from microgrids, the utility generators can be used to their full capacity for generating electricity. If utility and microgrid owners are able to participate in the ancillary services market, it will bring a lot of benefit to all the stakeholders, improving resource utilization, security, power quality and increasing overall system reliability.
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