MICROGRIDS – Large Scale Integration of Micro-Generation
cnki翻译助手to Low Voltage Grids
Nikos Hatziargyriou
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
School of Electrical and Computer Engineering
Electric Power Division, Electric Energy Systems Laboratory
9, Heroon Polytechniou str., 15773 Zografou, Athens Greece
怎么才能快速美白Tel.: +30210-7723661, Fax: +30210-7723968
e-mail: ua.gr
INTRODUCTION
Key economic potential of the installation of Distributed Generation (DG) at customer premis lies in t
he opportunity to utili locally the waste heat from conversion of primary fuel to electricity. Therefore there has been a significant progress in developing small, kW-scale, CHP applications. The systems are expected to play a significant role in the local generation of Northern EU countries. PV systems are expected to become increasingly popular in Southern EU countries. The application of micro CHP and PVs potentially increas the overall efficiency of utilising primary energy sources and conquently provides substantial environmental gains regarding carbon emissions, which is a critically important benefit in view of meeting the Kyoto objectives. Furthermore, the prence of generation clo to demand could increa rvice quality en by end customers.
From the Utility point of view, DG located clo to loads reduces flows in transmission and distribution circuits with two important effects: loss reduction and ability to potentially substitute for network asts. Thus, the application of distributed energy sources can potentially reduce the demand for distribution and transmission facilities. The inability to control an incread number of distributed sources however creates huge difficulties in operating and controlling the distribution network. Microgrids offer the possibility of coordinating the distributed resources in a more or less decentralized way, so that they behave as a single, controlled entity. In this way, distributed resources can provide their full advantages in a consistent way.
Microgrids compri Low Voltage (LV) distribution systems with distributed energy sources, such as micro-turbines, fuel cells, PVs, etc., together with storage
devices, i.e. flywheels, energy capacitors and batteries, and controllable loads, that behave as a coordinated entity, thus offering considerable control capabilities over the network operation. The systems are interconnected to the Medium Voltage Distribution network, but they can be also operated isolated from the main grid, in ca of faults in the upstream network. Thus, Microgrids can provide network support in times of stress by relieving congestions and aiding restoration after faults. From the customer point of view, Microgrids provide thermal and electricity needs, and in addition enhance reliability, reduce emissions, improve power quality by supporting voltage and reducing voltage dips, and potentially lower costs of energy supply.
This paper outlines the key issues regarding technical and economical operation of Microgrids and provides some findings of the EU funded project “MICROGRIDS – Large Scale Integration of Micro-Generation to Low Voltage Grids”, EU Contract ENK5-CT-2002-00610 [1].
IMPACTS OF MICROGRIDS ON SERVICE QUALITY
Prent distribution networks are designed such, that performance of the MV and LV networks have
a dominant impact on the quality of rvice en by the end customers, while faults in HV distribution and transmission networks do not normally affect the continuity of supply of customers connected to MV and LV networks. In the majority of EU countries, more than 80% of the customer interruptions and the customer minutes lost have their cau at MV and HV levels. One of the key benefits of Microgrids is the potential to increa rvice quality by providing generation redundancy, where most needed. The capability of Microgrids to operate in islanding mode can potentially relieve the effects of faults in the upstream networks. This capability requires sophisticated protection, control and communication infrastructures, in order to be able to isolate external faults and provide stable autonomous operation.
Given that average customer in the EU experiences about 1 outage per year lasting for about 60 minutes, it is important to answer if the introduction of Microgrids, can be justified on this basis. The analysis showed that considering typical ast lifetime and discount rate, islanding operation of small-scale DG, such as 2kW DG supplying one houhold, would in average permit an expenditure of around €45 to
upgrade the system for islanding operation, while a 1MW network area of commercial customers would support an investment of around € 300,000.
Some Microgrids could, in addition to the main activity of generating electricity, market the islanded operation rvice and open up a condary income source. In prent networks, islanding operation is expensive since the costs of the generation plant and additional system automation and control must be borne. However, in future, large numbers of DG could already be connected to actively managed Microgrids.
IMPACTS OF MICROGRIDS ON EXPANSION PLANNING
Microgrids have the ability to postpone the reinforcement of high voltage distribution and transmission circuits as DG is located clo to loads and there is likely considerable correlation between generation and local loads. Furthermore, the number of generators will be very large, so that the status of individual units will not make a particularly strong impact on the overall aggregate output. Clearly, Microgrids will play a critical role in replacement strategies of distribution and transmission networks. The exact evaluation of the effect of Microgrids on the network capital expenditure needs to be carefully studied.
MICROGRIDS OPERATION
Technical challenges associated with the operation and control of Microgrids are immen. Effective
energy management is a key to achieving vital efficiency benefits by optimising production and consumption of heat, gas and electricity. The coordinated control of a large number of distributed sources with probably conflicting requirements and limited communication is a very challenging problem imposing the adoption of distributed intelligence techniques.
Furthermore, the management of instantaneous active and reactive power balances, power flow and network voltage profiles impos unique challenges in the context of Microgrids. Traditionally, power grids are supplied by sources having rotating mass and the are regarded as esntial for the inherent stability of the systems. In contrast, Microgrids are dominated by inverter interfaced distributed
sources that are inertia-less, but do offer the possibility of a more flexible operation. A further particular problem of Microgrids is the high resistance to reactance ratio of the low voltage networks, resulting in strong coupling of real and reactive power. Hence the control of voltage and frequency can no longer be considered parately.
A key challenge of Microgrids is to ensure stable operation during faults and various network disturbances. Transitions from interconnected to islanding mode of operation are likely to cau larg
e mismatches between generation and loads, posing a vere frequency and voltage control problem. Storage technologies, such as batteries, ultra-capacitors and flywheels may become important components of Microgrids, with the duty to provide stable operation of the network during network disturbances. Maintaining stability and power quality in the islanding mode of operation requires the development of sophisticated control strategies and needs to include both generation and demand sides.
Microgrids Control Levels
In order to achieve the full benefits from the operation of Microgrids, as outlined in the Introduction, it is important that the integration of the distributed resources into the LV grids, and their relation with the MV network upstream, will contribute to optimi the general operation of the system. To achieve this goal, a hierarchical system control architecture comprising three critical control levels, as shown in Figure 1, can be envisaged [1]. The different control levels compri:
Figure 1 – Micro-Grid control architecture
•Local Microgenerator Controllers (MC) and Load Controllers (LC)
•MicroGrid System Central Controller (MGCC)
•Distribution Management System (DMS).2013托福考试时间
The Microgenerator Controller (MC) takes advantage of the power electronic interface of the micro source and can be enhanced with various degrees of intelligence. It us local information to control the voltage and the frequency of the Microgrid in transient conditions. MCs have to be adapted to each type of micro source (PV, fuel cell, micro turbine, etc.) Local Load Controllers (LC) installed at the controllable loads provide load control capabilities.
airfieldThe Microgrid Central Controller (MGCC) functions can range from monitoring the actual active and reactive power of the distributed resources to assuming full responsibility of optimizing the Microgrid operation by nding control signal ttings to the distributed resources and controllable loads.
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MicroGrids connected on the feeders of Distribution Management Systems (DMS) should ideally look like concentrated loads. The issues of autonomous-non-autonomous operation of the MicroGrid
s and the related exchange of information are new important issues. Disconnection and re-synchronization of Microgrids during and post-fault periods need to be evaluated.
It is clear that in order to operate a Microgrid in a coordinated manner it is important to provide a more or less decentralized decision making process in order to balance demand and supply coming both from the distributed resources and the MV distribution feeder. There are veral levels of decentralization that can be possibly applied ranging from a fully decentralized approach to a basically centralized control depending on the share of responsibilities assumed by the MGCC and the MCs and LCs. The levels need to be explored and relative benefits identified.英文在线
Islanded vs. Interconnected Mode of Operation
kernel>ytooIn interconnected mode of operation, decisions on local generation are bad on maximization of the Microgrids value, according to the availability of the primary energy sources and the energy prices. Network restrictions, namely capacity of the MV/LV transformer or LV network congestions have to be of cour respected.
When failures occur in the MV or HV system, the Microgrid is automatically transferred to isolated islanding operation, supplied by itlf. Seamless transition between the interconnected to the islandi
ng mode is crucial for uninterrupted
continuity of supply. With an intelligent distributed approach, MCs and LCs act as independent agents and making efficient u of the local resources maintain system operation in islanded conditions.
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If a system disturbance provokes a general blackout at the upstream HV or MV networks, such that the Microgrid is not able to parate and continue in islanding mode, and if the MV system is unable to restore operation in a specified time, the MC s can provide local Black Start capabilities, exploiting autonomous agent concepts. Moreover, the MGCC can support re-connection during Black Start, helping in this way the upstream DMS system that is managing the MV distribution network.
Management of voltage and frequency
In isolated operation mode frequency and voltage control are challenging problems. The conventional power system employs conventional droops, as shown in Figure 2. In principle, this concept can be also adopted by the MCs of the DG, in order to provide load sharing capabilities. However, this is not straightforward to implement in Microgrids due to the clo coupling of P&Q effects. For example, voltage regulation bad on reactive power injection alone is impossible, unles
s excessive amounts are available. It has been shown, that the concept of Figure 2 can be effectively applied, as long as the frequency and voltage droops have the same sign.
The role of IT
IT tools can play an important role in the operation and control of Microgrids. A future decentralized system might require information from any MC or LC attached to
