Intention islanding of distributed generation for reliability ... - IEEE Xplore

INTENTION ISLANDING OF DISTRIBUTED GENERATION FOR RELIABILITY ENHANCEMENT K. A. Nigim, Senior Member, IEEE, and Y.G.Hegazy, Member, IEEE

Abstract _. The paper investigates the coordination requirements for intention islanding to increase DGNtility reliability in providing uninterruptihle power to the connected loads. The

most principal interconnection switchgear components are identified for lhe islanding. The argument of the intention islanding case is carried out using a hypothetical DGNtility model. By identifying the principal switchgear, proper interfacing using programmable logic controllers are recommended to bridge the communication gap between the DGRitility and the protection switchgear. Through the identified interface requirements and permitting DG islanding in the case of main feeder faults, greater reliability and load power continuity was achieved. Index Terms

-- Distributed

Generation, reliability, islanding,

and switchgear coordination

I. INTRODUCTION ISTRIBLITED Generation (DG) or independent power producer (IPP) generating facilities are interconnected at the substation, distribution feeder or at the customer load levels. The premise of DG is to provide electricity to a customer at a reduced cost, higher reliability and efficiency while reducing transmission and distribution losses. With the current worldwide renewable energy and green power initiatives combined with the lack of investment in construction of ncw power generating plants or even the expansion of existing one’s, DG technologies are becoming an attractive niche supplementary energy option with reasonable profitable investment.

D

Recent studies have predicted that by the year 2010, distributed generation will account for up to 25% of all new generation [I]. The keys to the widespread of DGs are the ability to safely, reliably and economically interconnect with the existing utility grid. DGs can be classified into two categories depending on the dispatch-ability nature of the produced energy:

K. A. Nigim and Y . G. H e g u y are with the Drpanment of Electrical and Computer Engineering, University of waterloo. waterloo, Ontario NZL 3G1. Canada. (Email: Lni~meece.uwaterloo.ca)

0-7X01-7YXY-6/03/$l7.00 0 2 0 0 3 IEEE

1.

Dispatchable DGs in which the produced power can be precisely controlled and dispatched in pre programmed purchase agreement. Such DGs are fuel cells, sterlingengine, and internal combustion engine-generators and many co-generation schemes.

2.

Non-dispatchable DGs as the prime source is site specific and the energy produced cannot precisely he controlled. Such DGs schemes are photovoltaic, micro sized water turbine and wind turbine grid connected power generating schemes.

For successful integration of DG with Utilities, clear interconnection requirements must be formulated. A broad range of industry representatives have been participating in the development of a new standard for DG/Utility interconnection. Examples of these activities are the undergoing IEEE standard P1547, which is meant to provide a uniform standard for interconnection of distributed resources with electric power systems. Since the introduction of DG provides an unwanted source for re-distribution of both load and fault current, as well as possible source of over voltage and islanded operation, the investigated requirements are looking at issues such a s performance, operation, testing, safety and maintenance of the interconnection. The current engineering practice for DG/Utility interconnected systems is t o revert the utility systems to its original configuration (radial or meshed distribution system) with all interconnected DG units de-energized whenever an unexpected disturbance occurs in the system. Since most distribution systems comprise radial feeders, this practice leads to the discontinuation of the supply for all the downstream customers. Thus, the system reliability stays at the same level as it was before integrating the DG with the system 12-41, At the same time, there exist in the system some unsupplied loads and unutilized DG capacity. If interconnected DGs are permitted to supply loads during utility outages, the system reliability will be much better and the customers will not experience any discontinuity of their supply. This goal can be achieved by simply coordinating intention Of DG units

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Islanding occurs when a portion of the distribution system becomes electrically isolated from the remainder of the power system, yet continues to he energized by the DG system. Islanding is so far avoided to guarantee maintenance operators safety, customer load damage protection liability and distribution protection complexity [6-8]. However, islanding could he of high potential to sustain power ilow to selective sensitivc loads connected to the network through DGs integration. The islanding option can he coordinated through the identification of the principle protection switchgear elements that need to he interfaced for proper routing of power supply to sensitive loads within the distribution network.

and DG may result in damage to the Utility protection gear andlor to the customer units. The DG operator (IPP) must obey the Utility inter-tie by-laws that could lead to complete re-design of the DG protection algorithm. Thereforc, the DG/IPP identity has to take the appropriate interconnection licensing from the Utility and carry out technical and feasibility studics on the cost and benefits of integration. Moreover, a communication link between the DG and the Utility point of common coupling (PCC) must he established to allow hi-directional power flow and the coordination of the reclosing relays. Such requirements necessitate the application of microprocessor based intelligent agents to interface the DG and the Utility protection devices. In some cases, this could lead to costly DGIUtility venture.

This paper addresses the main issues in the coordination between the protective devices of the Utility and distributed generators for deliberate islanding. DGKJtility islanding Interface requirements arc suggested to he adapted in order to enhance the overall system reliability through islanding. Among the examined requirements is the permitting of distributed generation islanding in case of main feeder faults. The structure of the paper is as follows: Section I1 discusses the interconncction components required between the DG and the Utility. Section I11 presents the existing interconnection and islanding by-laws based on California utility inter-tic regulation requirements [9]. Section IV outlines the intention islanding requirements in which islanding is assumed between DGs with sufficient reserve capacity and the Utility aiming at providing power supply continuity to the connected loads. Finally, Section V concludes the merits of the proposed intention islanding.

IPPowned lraniformer

11. DGNtility INTERCONNECTION COMPONENTS There are many technical constraints on the placing and operation of DG in parallel with the utility due to the protection and power flow coordination complexities [ IO-121. The monitoring of the DG, the communication between the utility system and the DG, the metering, the protection protocols and the control of power flow are among the challenges which face the process of interconnection. In addition, the interconnection has to bc non-disruptive and economical, particularly when applied to smaller units. An example of un-coordinated DGNtility interconnection is shown in Fig. I in which the IPP owns the connecting transformer. In such configuration, the DG protection switchgear is designed to trip the DG upon internal fault. Meanwhile, the utility side switchgear is designed to trip based on its distribution faults and independent to the DG. Such schemes are not allowed to operate in parallel with the utility hut are used as stand by schemes. For proper interconnection with the utility, DG needs to he protected not only from its internal faults, but also from abnormal operating conditions that could he propagated from the Utility side. It is the responsibility of the DG operator to provide the appropriate protection gear design to safcguard the DG. The Utilities are concerned that the installation of

lndpendent Power Producer

(IPP)

Fig. I Typical un-coordinated DG stand-by interconnection Key elements of the reliability of distributed generation power systems are the performance of the electrical switchgear, interconnection, controls, and communication feature. DG growth is hampered, in p"t, by the lack of uniform requirements for interconnecting a paralleled power generation source with electric utility grids. Many electric utilities have guidelines for interconnection, hut these can vary significantly among different utilities. In the USA, with support from the Department of Energy and many other stakeholders, the Institute of Electrical and Electronics Engineers (IEEE) is developing uniform national interconnection standards for DG sources. The PI547 Distributed Resources Working Group is comprised of a cross-section or stakeholders including Utilities and manufacturers as well as Electric Powcr Research Institute (EPRI). In a typical DG/Utility interconnection the main

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protection devices concerns that have control on the system operation are prcsented in Tablc I. In the table, the ANSI protection function number is also indicated. Generally, interconnection and protection requirements are established by each individual utility. The components used in interconnection are engineered and assembled, based on DG technology, s i x and ratings. Table I Interconnection components anticipated protection function _

_

_

_

_

~

IEEE - ANSI (Device Number) vo1rage regulolioa (27 & 27G)

Protection

Function

To riiainlai,i unmrant vo1tug.e (2 5%) ,fim ull usen derpire the variation in d t q e caused by !he interconnectinn. h i s important rhar rhe DG unit be .spinning at rhr .same elect"r. frequency and deliwring wltagr irirhiri rhe tolerance qf the utiliiy grid's volrage. I n addition,rhe phose sequence and phuse bolonce qf the D G unir wlragr mmt be rhe .some as rhose of the grid at the point of i m e r ~ o n i i e ~ t i ~ n To enure fasi fault clearing under ~~uiious porrible m e r current and rhorr circuit coridiriori,r. To avoid rhr contirruiry of LI ,fault condition arid to rave rhc DG unit from getting damaged, the D C unir should be prevented from re-energizing U de-energized diirriburion .q,v.srem circuit DG i.danding cm1 erpose utili!? workers 10 hoz.ardr by circails rhar othrr-wiTe be deemr2i:ed and may, .P O S ~threat tu the sublic a.5

V"1r"ge Disrurbnricr 159 & 59Gur59N)

protection

,funcrioar

of

Safe and reliable integration requires secure and reliable interconnection between the DG and the Utility as depicted in the simplified schematic shown in Fig. 2. Here, the system integration is far more sophisticated and interrelated with the Utility. For economical operation and reliability, the system control must be coordinated with proper communication protocols using microprocessor based intelligent interfacing switchgear.

the

interconnection ryireni should meamre the qliectiw (mm) or/undo,ner,talfrequenc). value of each phare-to-ncuiral When on? qf the measured volrages is over or under o urility pre-ipecaed range. the DG should ceme IO energize the grid wirhin certain clearing time Under and over.freyuencs proteciivefunctiuns ore among the mmf important memtr qf prevenring rhe emrablishmenr of a DC irlosd. It ir desimblefor there prorectionr IO operare promptly

Power Quolit)

(27TN. 877)

Grounding 164F. 87GD)

The DG unit should not impose m y power qualify problems on the e.xi.rririg grid. Therqbre. the DGfaciIity should be equipped with means q"enmuring the quolity qf their e.rporred power. Eramples of rhese meam are hortnonicJ1rers and isolating tran$omem Addiliond grounding curreni porlrs are forbidden IO avoid malfuricriorir f. the protective relayr. Therefore. rhe D G grounding rhould bP integrated directly with the exirting grid grounding scheme. In addition, 011 DG equipment shall be grounded in accordance with applicable local and norioiiol codes

The main components of interconnection according to the protection functions they perform are categorized as follows: 1.

the connection. Dispatch, communication, and control. These integration and communication componcnts interfacc the DG units with the Utility. Among the functions of these components are: Regional load management, work order management, and billing services Distribution automation Feeder switching Short circuit analysis Voltage profile calculations Trouble calls management 4. DG controls. DG control modules provide the personhachine interface, a communications interface, power management, and monitoring and metering. The DG unit must he controlled for voltage, frequency, and power output. 5. Metering and monitoring. Monitored parameters often include current, voltage, real and reactive power, oil temperature, vibration, and others. Metered parameters also include power output, which may he used for billing that requires utility-grade metering accuracy. 3.

111. Utility I DG INTERCONNECTION GENERAL REQUIREMENTS

hrll. The

2. Interconnection Protection. Relays are normally used to provide protection at both the grid and the DG ends of

Paralleling Transfer switchgear. Paralleling switchgear are switching components used to verify the needs for synchronizing the DG unit to the grid.

T o minimize power flow interruptions, the essential interconnections requirements must he first identified and agreed upon between the utility and the DG operator. For example, the current DGNtilities interconnection requirements stipulated in reference [9] are: The protective switchgear of a DG must include an overhnder voltage trip function (27 & 27G), an ovedunder frequency trip function (81 O N ) , and a means for disconnecting the D G from the utility when a protective function initiates a trip (79). The DG and associated protective switchgear shall not contribute, (unless permitted for higher reliability), to the formation of an unintended island (32). DG switchgear shall be equipped with automatic means to prevent reconnection of the DG with the utility distribution system unless the distribution system service voltage and frequency is of specified settings and is stable for 60 seconds (79). Circuit breakers or other interrupting devices at PCC must be capable of interrupting maximum available fault current (25, 50,51).

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5.

Thc manual disconncct device has a visual break to isolate the DG from the utility distribution system.

Oncc the DG is connected in parallel with the existing Utility system, its protection switchgear becomes a dependent functional integrity. In the current DGlUtility interconnection rcquiremcnts, islanding is not permitted in order to avoid having customers supplied within the system from sources which are uncontrolled by the Utility. Moreover, the Utility opcrators want to ensure proper transfer of quality power in a timely manner with safety for customer and Utility equipment and to the maintenance personncl. This entails a vigorous regulation for interconnection normally imposed by the Utility on the DGilPP operator and may decrease the economical benefits and any agreed power supply continuity anticipated through the interconnection.

continuity, the DGs operator, the customers and thc Utility will gain [ 5 ] .

JV. INTENTION ISLANDING The proposed intention islanding strategy is cxplained here with thc aid of the schematic of thc assumed simplified distributed generation system depicted in Fig. 3. The system comprises of three distribution transformers (TI, T2 and T3). Three radial feeders (L1 to L3), four laterals (LSI to LS4) and six load points (LA to LF). Four customcr controlled DG units with sufficient reserve capacity are located as shown and assumed successfully intcgrated and running in parallel with the Utility. Each element in the distribution network is protected by its associate protection switchgear (shown as normally opened contacts). The protection element is fully calibrated to fault interruptions as per current DG/Utility regulation. Ulilily Electric

Utility disconnect

1 -

r :

= 5 .I n

Gll!rr*,or PIOlDElion Sullihlral

tndpendenl Power Producer (IPPJ

Fig. 2 DG/Utility interconnection protection requirement Under many Utility connection regulations, once a fault signal is initiated, the DG units are not allowed to reconnect to the utility system unless stable conditions have been reached and lasted for a ccrtain specified period. The current practice of disconnecting the DG once a fault is initiated at coupling, leads to frequent the vicinity of Point of "on power outages and, therefore, defeats the main objectives and benefits o l interConneCtiOn. If DGS are permitted IO Supply vicinity connected Utility loads during main power interruptions, thc reliability of the distribution system will be increased. tinder this operational scenario of power supply

F i g 3 Simplified DGRitility distributed system

L

Based on ~ i g 3. ~ G / U t i l i tdistribution ~ system, once a fault is initiated, say, at the Utility transformer T3, the protection switchgear responds and disconnects the DGs and interrupts power supply to any downstream consumers loads, as shown in Fig. 4. Here the disconnected and isolated pan of the distribution network is shown by dotted lines. It is clear from this operational strategy that the DGs owner, consumer and the utility ace loosing investment due to the power interruptions.

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islanding and continuous supply of power to the assigned sensitive loads. Thc principle protection components need to he integrated can be spotted after the differcnt operational scenarios are tabulated as shown in Table 11.

Power system

discomecl

Tablc II htercOMeCliOn proleclion componenls status during assumed transformers fault with and without DG intentional islanding

Fig. 4 Distribution system under fault initiated at TI without DGs islanding

To sustain power to the sensitive loads, intentional islanding is a practical solution assuming largc reserve capacity in the DG's to supply the nearby connected Utility load. T o uresent the nrotection coordination rcauirements for intentional islanding, Table 2 tabulates three fault scenarios initiated at thc transformer terminals with some of the downstrcam sensitive loads, namely L(A), UB) and L(C) protected by protection devices PA, PB and PC respcctively ( typed in hold fonts) needs to sustain their power flow. The first column in the table lists the protection components that could include several protection functions as listed in Table I. The second column in the table lists the protection components relaying contacts status without the optional islanding. The third and consecutives columns show the protection device dry contact status for the cases of transformer fault under no coordination (NC) and with coordination (WC) for islanding.

In the following example, intentional islanding provides continuous power flow to the assumed sensitive loads as shown in Table 11. For successful operation, the principal protection switchgear must he identified for proper interfacing through communication and command signaling with the Utility and the DG. The identification of the appropriate switchgear component for DG/Utility interface is essential to cnsure safe

NC: No coordination WC: with coordination 1 3 protection contact is closed (power flows) 0 protection contact is opened (power is interrupted) contact don,t caTe(not critical)

+ ~

Under particular assumed DGNtility operational scenario shown as the one presented in Table 11, the switchgear components that requires changing of state upon particular fault is considered principal protection components that must be integrated for safe islanding. For example during a fault on T3, protection PA will close if PT3 and PL3 are opened to sustain power flow to load A from DG-1. Thus, PT3 and PL3 are considered principal protection components for communication with PGI and PA protection assemblage. To sustain power to load B from DG-2, contacts PSI will close if PL3 and PS2 are to open. Again, P S I and PS2 are considered principal for communication with PG2 and P B protection assemblage. Similarly, to sustain continuity to load C from DG-3 and PS3 contacts will close if PS2 and/or PS4 are opened. Therefore, PS3 and PS4 are principal for communication with PG3 and PC protection assemblage. Of course, different logical operational scenario's can exist and ultimately it is left for DG and Utility to decide upon the optimum operational strategy. Once the operational strategy is formulated, a modular design of programmable logic controller (PLC) could be adopted to interface the principle existing protection switchgear components and the DGs. PLCs are considered

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versatile modular microprocessor units with signaling, control and communication capabilities [ 131 and proved their reliability in many industrial and power distribution based applications. By using PLC intcrfacing, minimum protection components are needed or madc redundant. Moreover, the versatility of including industrial PLC is that operational strategy can he altered simply by changing the PLC programming interfacing procedurcs without any rewiring reconfiguration.

V. CONCLUSIONS The paper presents the main integration components and requirements between IPP owned DGs and Utility. The existing current DGlUtility integration does not allow islanding that results in power losses to sensitive loads and reduces system reliability and weak utilization of DG capacity. To enhance the overall system reliability through intention islanding, a proper interfacing operational strategy between the different protection switchgear components must be identified. Power flow to sensitive loads can he sustained once the principal protection elements are identified under different fault scenarios. Through the adaptation of microprocessor PLC modular units interfacing agents, DG operator can maximize their energy delivery without jeopardizing Utility safety requirements. Meanwhile, Utility can increase the distribution reliability with out violating Utilitylconsumer relationship. However, considerable engineering efforts, control functionality and communication infrastructure are still necessary to make intention islanding a practical solution.

"Cnrical Considerations for UtilitylCogenerationInter-Tie Protection Scheme Cnnfiguntion". IEEE Transactions on Industry Applications, Vol. 31. No. 5 . SrpliOCT 1995, pp. 973 977. 1121 S. Brahma and A. Girgijs '' Microprocessor-Based Kcclosing tu Coordinate Fuse and Recluser in A Syntrm with High Penclration of Distributed Generation". IEEE conference publications 2002, pp. 453 [ I11 R. M . RiLat.

~

458. [I31 J . G. Gilbrn and G. R. Diehl " Application of Propmmable Logic Controllers to Substation Contrul and Protection". IEEE Transactions on Power Delivery. Val. 9, No. I , January 1994. pp. 384 393. ~

K. A. N i g h received BSc. Electrical Engineering in 1979. Subsequently. he obtained the Doctorate in Engineering at the University of Leicester, Leicester. United Kingdom, in 1983. Has been at the Electrical Engineering department at Birzeit University, West Hank. Palestine. From 1994 to 2000, he was the director of project management for infrastructure development. In 2000, He was visiting Fulbrighl senior grant Professor Associate at Arizona State University, Engineering Research Center, USA. Since 2001. he is research Professor Associate at the department of Electrical and Computer Engineering. University of Waterloo, Canada. Dr. Nigh's research interests include decision-making and risk analysis, renewable energy resources integration, Self-excited wound rotor induction generators control, distributed generation. industrial power electronics and drives. Y. G . Hegazy received the B.Sc and M.Sc from Ain Shams

University. Cairo, Egypt and his Ph.D. from the university of Waterloo. Waterloo, Ontario, Canada all in electrical engineering in 1986, 1990 and 1996 respectively. At present, he is an assistant professor in the department of electrical power and machines at Ain Shams University, Egypt and a visiting assistant professor to the University of Waterloo. His interests include power distribution systems, distributed generation, power quality and probabilistic analysis of power systems.

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