Impacts of Distributed Generation on Restructured Power System

Harsh DhimanFaculty of Technology & Engineering

The Maharaja Sayajirao University of Baroda

Introduction

• Distributed Generation playing an important role in the electricity paradigm of the world. In India also DG is form of renewable energy resources contributing to the Indian power sector.

• As the INDIA addresses the challenges of achieving energy sustainability in the 21st century, the recognition of the need to find alternatives to current practices is surely found in implementing Distributed Generation

Distributed Generation: An Overview

• According to Distributed Power Coalition of America (DPCA) : Distributed power generation is any small-scale power generation technology that provides electric power at a site closer to customers than central station generation.

• A distributed power unit can be connected directly to the consumer or to a utility's transmission or distribution system.

• According to Institute of Electrical and Electronic Engineers (IEEE) : Distributed resources are the sources of electric power that are not directly connected to a bulk power transmission system.

• The Distributed Resources includes generator and energy storage technologies.

• The technologies generally include engines, small (including micro) turbines, fuel cells and photovoltaic.

Distributed Generation Characteristics

• Connected directly to the distribution system or installed at the customer's side of the meter.

• Not dispatched by network operators as in the case of centralized trading.

• Small scale generation <300 MW• The connection of DG to the power system could

improve the voltage profile, power quality and support voltage stability

Types of Distributed Generation

Size of Distributed Generation

Types of DG Capacity

Micro Distributed Generation <5kW

Small Distributed Generation 5kW-5MW

Medium Distributed Generation 5MW-50MW

Large Distributed Generation 50MW-300MW

Single Line Diagram Including DG

Issues related to Distributed Generation

• Power quality and reliability are a bigger issue as the DG’s installed locally tend to produce harmonics and hence hamper the power quality of the grid.

• Bidirectional flow of power along with distorted voltage profile.

• Diminishing stabilizing inertia.• Short circuit levels are changed when a DG is

connected to the network. Therefore, relay settings should be changed

Technical Solutions for DG Issues

• Use of FACTS devices like SSC, D-STATCOM and UPFC and Wide Area Monitoring Systems (WAMS).

• DG can be used as a back up supply, though it is limited in case of Solar powered generation during night time.

• But can be used as an effective tool to store energy in SMES (Superconducting Magnetic Energy Storage).

Distributed Generation : An Upcoming Trend

• Distributed generation has seen a recent spur in its installation in distribution network in European countries like Denmark, Holland, Finland and in India up to an extent as well.

• DG share is usually calculated by an index known as penetration index and is given as :

Penetration Index = (ΣPDG / ΣPL ) x 100%

ΣPDG = Power injected in the network by DG’s ΣPL = Feeder Capacity

• The high costs of wind and solar generation are the most important barriers for their market penetration.

• The capital cost of solar PV is even higher. However, considering only the cost of DG may not give a comprehensive picture of the problem.

• Since a number of other benefits of DG, such as the environmental benefits and reduced transmission losses, may have been neglected

Optimization of Distributed Generation

• Another frequently mentioned benefit of DG is its potential effect on deferring transmission network investments.

• Before the market deregulation, transmission network expansion is conducted sorely by the power utility and is usually modeled as an optimization problem.

• This aims at minimizing expansion investments subject to system reliability and other technical constraints

Market Scenario

• In a market environment, transmission network expansion may also involve other objectives, such as enhancing market competition, minimizing network congestion and facilitating the integration of renewable energy sources.

• A number of technical constraints should be carefully modeled in transmission expansion models. The most fundamental ones are power flow constraints, which represent the physical laws transmission systems must obey.

• To quantitatively measure the impact of DG on transmission network expansion, it is important to determine what portion of the overall transmission expansion investment should be allocated to DG units.

• A number of transmission cost allocation methods have been proposed in the literatures. Two methods, postage-stamp rate method and contract path method have been widely used because of their simplicity.

Closed Loop Pricing

• The price signal introduced in distributed generation is a closed loop signal (i.e. one that incorporates feedback) rather than an open loop signal, as most price signals in the electric supply industry are today.

• One objective in introducing a closed loop price signal to the generation sector is to aid in the creation of the desired competitive market.

• A closed loop price signal will capture the market clearing dynamic of a competitive market in the dynamics of the feedback control, and so incorporate market prices into system control decisions as well as in sitting and investment decisions.

• A second goal of the price signal is to provide a decentralized control mechanism which allows each generator to operate independently while also providing an incentive for the generators in aggregate to produce at the efficient level.

• The price signal facilitates the creation of a decentralized system in which distributed generators are free to act independently, required neither to give control nor any private information to a centralized authority.

• The objective of the price model is to demonstrate that a market-based price signal can be used in conjunction with the existing bulk flow market price to successfully control and coordinate a distribution system.

Closed loop price model

• In the proposed price framework the basic piece of information communicated to the distributed generators from the ISO and the market coordinator (or Power Exchange, PX) is the spot price of energy and/or services.

• This spot price corresponds to the price of the scheduled power flows as determined by the ISO and PX.

Pricing in Distributed Markets

• The full price of energy in the market can thus be expressed as

ρbase ± Δρ• where Δρ is the quantity determined by the price

based control loop and ρbase is the spot price of the scheduled, bulk power flows.

• The price signal can be operated in a flexible time scale. Every k minutes the market or system price, ρbase , is updated to reflect the current price of power delivered to the distribution system.

• The closed loop price signal corresponds to the marginal revenue earned by a participating distributed generator, and as dictated by economic theory the competitive suppliers will produce at the level where their marginal cost equals marginal revenue.

• The market structure envisioned in this model assumes that a competitive market will be developed at the distribution level.

• The distributed generators will be allowed not only to enter into contracts at the wholesale and retail levels, and participate in the Power Exchange, but also provide ancillary services to the ISO and local customers on a competitive basis.

Energy Flow in Distributed System

Ancillary services

• Various ancillary services provided by the DG’s are 1) Voltage support 2) Reactive power compensation by FACTS 3) Black start 4) Spinning reserve.

Relevance of Distributed Generation in India

In India, distributed generation has found three distinct markets.

• Back-up small power generation systems including diesel generators that are being used in the domestic and small-commercial sectors.

• Stand-alone off-grid systems or mini-grids for electrification of rural and remote areas.

• Large-captive power plants such as those installed by power intensive industries.

Conclusion• Thus we can say that flexibility of DG allows the market

participants to respond to changing market conditions, i.e. due to their small sizes and the short construction lead times compared to most types of larger central power plants.

• Also the distributed systems can be coupled with the micro-grid systems which are small scaled and often require lower gestation periods, it enables faster and easy capacity additions when required.

References • Integrating Small Scale Distributed Generation into a

Deregulated Market: Control Strategies and Price Feedback by Judith Cardell and Marija Ili´, Massachusetts Institute of Technology.

• Investigating the Impacts of Distributed Generation on Transmission Expansion Cost, An Australian Case Study, Energy Economics and Management Group School of Economics, University of Queensland

• Distributed Generation : Benefits and issues , KU Leuven, Netherlands

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