Advanced Power System Automation and Protection APAP 2007, Jeju (Korea), 24.-27. April 2007, Bericht P488
Protecti rotection on Co-ordi o-ordina nati tion on Method thods s un under Changing Network Conditions caused by large I PP Units Jo J ohann J äger
system with a high load transfer on long lines and furtheron a ration capacity capacity growth. growth. A high high num number of T his paper presents protecti protection on co-or co-ordination dination rapid generation methods to be appli applie ed for the integration integration of I PP units to the challenges for the protection systems and its co-ordination trans tr ansmiss mission grid. gri d. A typical system system confi configu gurration of an IP IPP unit targets results results from from this this change changes. s. installati nstall ation on is consi consider dered. The The protection system is also assumed This This paper sh shows advanced protection ion methods and concepts to be commonly which is based on the distance protection consideri consi dering ng these the se cha l l eng es m en tioned tioned above. Advance A dvanced d principl principle e. T he chan changing ging network conditi conditions ons and and their their impacts impacts on the protection protection syste system m cause caused by the IP I PP are described. cribed. Thus T hus the relay features will be not only used, but also co-ordinated to topics of co-ordination of different characteristics, teleprotection fulfil the conditions of the operation of large IPP units in a schemes, distance grading and relay relay loadabili loadabil ity are are investiga ti gated ted proper manner avoiding supply interruptions and influences to and methods methods for covering their adverse impacts mpacts are given. given. I t can foreign neighbouring networks as far as possible. This paper be see seen that only moder moder n relay r elay technol technology ogy is is not le leading to a wil will illustra ustrate te particularly rticularly that in case of an I PP unit unit onl only a proper protection system. Only well co-ordinated and adapted protection relays will result in a system of high reliability of the well co-ordinated protection system ensures a high reliability n-feed and and leads leads to an an econom economic bene benefit for for the I PP in-f i n-fe eed and and in an econom conomic ic bene benefi fitt for the I PP inve i nvestor stor s of the I PP in-fee investors finally [1,3]. finally. Abstract—
I ndex Te Terms—Protective
relaying, transmission network, indepe ndepende ndent nt power power produce producers, distance protec protecti tion, on, protec pr otecti tion on coordination
I. NOMENCLATURE I PP EHV EHV PUT PU TT POT POTT NERC NE RC
I ndependent power producer Extra high voltage voltage Permiss ssiive unde underreach transfer transfer trip trip Permi Permissve overreach transf transfer trip trip North A merica rican n Ele El ectric Relia Reliability bility Council Council II. I NTRODUCTION
T
HE deregulation and privatisation in the power market are significantly changing relations among power generation, transmission and distribution systems. Due to liberalisation programs the utilities will be divided into different independent entities like a transmission company, energy trading trading company company and power generation ration company. company. On the other other hand international consortiums are financing more and more large power plant units as independent power producers (I (I PP) PP) fired by different kind of primary energy depending on the best economic and local availability. Normally the installed output power is rated regardless to the local or regional power demand but only according to financing and amortization aspe aspects. This T his leads to a mul multi-ow ti-owne ner interconnected rconnected power
J . J äger is with ith the Departm rtment of Elec Electrica rical Pow Power Sys Systems, Frie Fried drich richA lexander-U r-University niversity of Erl E rlang angen-Nuremberg en-Nuremberg, Caue Cauerstr. 4, 91058 Erlang Erlangen, Germany. e-mail: jae jaeger@e
[email protected] i.uni-erla i-erlangen.de
STEM CONFIGURATION III. S Y STEM
A. Transm Transmission Network ork The The system configu figuration ion which ich is conside idered in the followi following ngs s is shown shown in Fig. Fi g. 1. It I t is base based on a typical typical EHV E HV-transmission system consisting of parallel lines between substation tion A and B amongst other devices devices of the whole whole transm transmission network. It is is assu assum med that that the IPP unit is is interlinked to one existing parallel line between the substa substations tions A and and B of the transm transmission network network.. The T he existing existing line ends are numbered as 1, 1, 2, 3 and 4. There T hereby by only only one li line is cut and and the new two li line ends 5 and and 6 and the I PP unit unit are connected by the use of the new substation C. This is one of the most common ways of an IPP I PP integration into i nto an existing xisting network. This This configu figuration ion should only serve rve us as an example of a typical IPP I PP unit connection to the EHV EHV -grid. -gri d. But it should should not exclude other cases for which the following considerations can be applied accordingly.
Transmiss ission ion network
3
4
1
2
A
5
6
B
C
Distance protection relays IPP unit
Fig. Fi g. 1. Systemconfiguration confi guration of investigations
Signa Signal transmission issi on channel
the sending and receiving line ends, can reach unexpected high values. Severe line outages lead to high monetary losses B. Protection System immediately and should to be avoided as far as possible. The protection system of EHV-transmission networks is Disturbances of the IPP connecting lines by unnecessary typically based on distance protection relays as already switching should be prevented in general. Otherwise penal illustrated in Fig. 1. Thus distance protection technology will payments can be the consequence. Because of the additional be the basic protection function to be discussed in this paper. rotating masses of the IPP generators connected to the Faults which occur on the protected line e.g. between bus 1 network the transient stability limits are supposed to be and 5, beyond the first distance protection zone of the relay at decreased towards shorter maximum permissible fault clearing bus 5, can only be cleared selectively by this relay after a times. The post fault behavior is most likely characterized by delay time of several hundred milliseconds. With respect of unusual strong power swing phenomena. stability reasons of the surrounding generators, in particular All these mentioned challenges caused by the IPP the IPP, this is usually not acceptable for transmission systems. installation must be covered by the differently composed To achieve a non-delayed and selective tripping on 100% of protection system. For that advanced protection co-ordination the line length, the distance protection relays have to exchange methods are necessary as shown in the following section. information with the opposite line ends and to process teleprotection schemes by means of signal transmission V. ADVANCED CO-ORDINATION METHODS systems as also shown in Fig. 1. There are different schemes to be applied. The most applied scheme is the permissive A. Grading of quadrilateral and circular characteristics underreach transfer trip (PUTT) realized commonly by old The need of the co-ordination of distances relays with fashioned electromechanical or analog electronic relay circular and quadrilateral zone reaches is likely for an IPP technology. This is state of the art in transmission networks. connection as described. The circular relay is graded The new substation C can be assumed to be equipped with according to the apparent line impedance ZL =| RL +jX L | and newest relaying technology as relay 5 and 6. That means the quadrilateral according to the pure line reactance X L . Thus numerical relays which provides quadrilateral zone reaches, the circular relay has a fixed R-reach, whereby the R-reach of time optimized tripping algorithms, advanced methods of a quadrilateral type is freely settable within a wide range. The selective fault clearing and self-monitoring features. problem is to find a common base of grading. The remote ends at substations A and B are equipped with A proper approach is to base the grading on the X-reach of existing relays supposed to be old-fashioned relay technology. the zones at the intersection point of the zone reach An upgrade of this relays is not likely because the investors of characteristic with the line impedance characteristics as shown the IPP unit are not owner of the transmission network in Fig. 2 and 3 [2]. commonly. That means only conventional tripping times, The gray hatched areas illustrate the differences between restricted setting ranges and possibilities e.g. only circular or the different zone characteristics. That means the X-reaches of MHO shaped zone reaches and a limited scope of protection both characteristics can be adjusted approximately equal, at features are available. least for smaller R-reaches. But for higher R-reaches, with In case of the unit protection of substation C, like busbar respect to the arc compensation, larger deviations naturally differential relays, an advancement regarding speed, occur. The greater R-reach of modern relays in any event is a selectivity and dependability can be automatically achieved by positive effect. High fault resistances can already be detected applying the new technology. No any co-ordination with other with short X-reaches for short line lengths, while the circular parties and issues is necessary so far. But with respect to the characteristic may only provide the required R-reach in the line protection the situation is completely different. The new back-up zones. relays installed at substation C will not improve the protection That means if a circular relay is following a quadrilateral behavior only by applying newest technology. To achieve an one, the first zone of the circular relay, e.g. relay 3 of Fig. 1, advancement these have to be co-ordinated with the existing has a considerable shorter R-reach as the back-up zone of the mostly old-fashioned protection systems at the remote ends quadrilateral relay e.g. relay 5. If the relay 3 has not enough and with the changed dynamic networks conditions influenced R-reach to sense a fault within the first zone, relay 5 will trip by the IPP unit. A protection co-ordination study has to be the fault unexpected. An unnecessary interruption of the IPP carried out on this matter. in-feed is the consequence. One remedy would be the change from the existing PUTT IV. S Y STEM CONDITIONS CAUSED B Y A N IPP scheme to a permissive overreach transfer trip (POTT) scheme An IPP installation is aiming at the selling of electrical for the relays 3 and 4. A n extension of the Z-reach as an energy. The return of investment of the IPP unit will be POTT-dependent overreaching zone and of the R-reach of normally the ultimateambition of the IPP investors. Thus high relay 3 consequently can be achieved maintaining selectivity. load flows in the range of nominal currents or above until the Another solution would be the installation of an directional thermal limit can be expected on the connecting lines. The line earth-fault comparison scheme for the relays 3 and 4 to sense angles, as the angle difference between the voltage phasors at high-impedance faults surely and instantaneously.
ZL X
X' Z'1
R' Z1
Z2
weak in-feed condition, an echo-signal will be generated depending on the signal receive from remote end. This echo signal will acknowledge the hand-shake. An external fault is located outside of the line A-C. It will let the hand-shake failing and block the trip command because one of the relays will sense this fault in reverse direction. If different characteristics are combined to perform a POTT- scheme, as in our case, a special situation arises. This situation is illustrated in Fig. 5 [2]. A MHO-circle relay at bus A and a quadrilateral relay at bus C has to perform a POTTscheme.
R
Fig. 2 Grading of quadrilateral and circular in thefirst zone [2]
ZL X
X' Z'1 Z2
R' Z1
R
Fig. 3. Grading of quadrilateral and circular in the second zone[2]
B. Teleprotection schemes The IPP generator is commonly influencing the network stability towards shorter maximum permissible fault clearing times. That is why, teleprotection schemes are becoming more important ensuring a stable post fault behavior. In that case POTT-schemes are the best adapted schemes regardless of the line lengths. This kind of scheme provides the most flexible zone reaches covering the effects of high impedance faults, mutual coupling etc.. The principle of a POTT-scheme is shown in Fig. 4. Z1(A)
A
Z 1B ( C
C
Z1B A
Z 1( C
Fig. 4. Zone reachesfor POTT- schemes
A fault located on the line A-C will be cleared based on a communication hand-shake between both relays if both relays sense the fault within the dependent zones Z1B. The trip command will be initiated after the hand-shake process was successful, that means the trip command happens nearly instantaneously. If one relay is not picking-up caused by a
Fig. 5. Co-ordination of a MHO-circle with a quadrilateral relay [2]
The reverse reach of the fault detection zone Z(Block) must be greater than the over-reach of the tripping zone Z1B of the relay at remote end. Otherwise an incorrect echo-signal will be produced during external short-circuits and an unnecessary line tripping will be issued. The fault detection characteristic must therefore fully enclose the overreaching zone in the third quadrant, where the impedance of an external fault appears, as the lower diagram of Fig. 5 shows. The same applies for the blocking technique. The reverse transmitting zone instead of the fault detection zone must be analyzed as demonstrated by the upper diagram of Fig. 5. A modern numerical relay provides normally the option of activating different fault characteristics. The change-over of the characteristic will be done by relay settings accordingly. If a communication between such a numerical relay and an oldfashioned circular relay has to be performed, the numerical
relay can be switched over representing a circular relay and the co-ordination task is becoming more appropriate or standard. C. Distancegrading The IPP generator is representing an additional strong infeed into the transmission network. The impedance measurement of distance protection relays is influenced by such an in-feed between the relay and the fault location. Fig. 6 illustrates the influence of an intermediate in-feed in principle [2].
supposed to be highly loaded. If one faulty line will be switched off, e.g. line A-C in Fig. 1, the remaining parallel lines C-B and B-A have to take over the load flow and become particularly high loaded. In this case, the third zones of the distance relays are prone to overfunction and to trip further lines unnecessarily. The load area is encroaching the tripping area of a MHO relay as shown in Fig. 7. A rather spacious blackout could be the consequence. It should be mentioned that this problem is getting more likely with Offset MHO characteristics as illustrated by the lower diagram of Fig. 5.
C
I C
B
I C ZA-C
IA
⋅
ZC-B
ZA-C
IC
⋅
ZC-F
IA
⋅
ZC-F
Fig. 7. Load encroachment: load area is entering the tripping area
During system disturbances the voltage often drops due to reactive power control problems if the ceiling voltage of the The impedance appears to be greater as the fault is voltage controller of the IPP has been reached [5]. At the same apparently moving away from the relay. The relay may only MVA loading, the measured relay impedance are therefore trip in a higher zone and the back-up zone becomes reduced with the square of the voltage. Therefore this third underreaching. A delayed tripping can be the consequence zone problem, also called loadability problem, has contributed which endangers the post fault stability behavior of the system to blackouts several times in the past. It has caused NERC to issue recommendations to prevent and mitigate the impacts of [5]. One solution may be to extend the second zone reaches of future cascading blackouts. Following recommendations are the relays 1 and 2 of Fig. 1 according to Fig. 6. But also the given inter alia [4]: reverse zone reaches of the relays 5 and 6 are strongly Zone 3 relay should not operate at or below 150% shortened by the intermediate in-feed caused by the IPP. They of the emergency ampere rating of a line must be extended accordingly to ensure a proper POTT(maximum permissible thermal current), assuming scheme operation as described above. 0.85 per unit voltage and a line phase angle of 30 If the IPP is out of operation or is running only with an degrees reduced output power, the zone extension can lead to an Relay should be set to ride through all recoverable severe overreaching and unselective tripping for the relay 1 power swings and 2 in particular. For that an adaptive setting change-over based on the input power of the IPP can cover this problem Fig. 8 shows typical loading values to be considered in case which is representing a relative complex solution. Another idea is to do it without zone extension but with of a MHO characteristic. The data are based on a twin-bundle doubling of the protection relays. That means each line should EHV line. The maximum load angle should be 30 degree. be equipped with to main protection system. Then if one protection fails there is no need of the back-up protection from adjacent lines and the (n-1)-principle is maintained. Fig. 6. Influence of an intermediatein-feed on the distance measuremen t [2]
•
•
D. Relay loadability Lines in the neighborhood of an IPP installation are
VII. REFERENCES [1] J . Jäger, R. Krebs,“Reliability Improvement of MV-Power Systems by Co-ordinated Network Protection”, presented at the 13th IEEE Conference PSP 2002, Bl ed, Slovenia, 2002. [2] G. Ziegler, Numerical Distance Protection, 2nd edition, Erlangen, Publicis Communication Agency GmbH, GWA , 2006. [3] S. H. Horowitz, A. G. Phadke, Power SystemRelaying, 2nd edition, New York, Wiley, 1996. [4] North American Electric Reliabili ty Council: August 14, 203 Blackout: “NERC Actions to Prevent und Mitigate the Impacts of Future Cascading Blackouts”, February 10, 2004 (www.nerc.com). [5] P. M. Anderson, A. A. Fouad, Power SystemControl and Stabili ty, 1st edition The Iowa State University Press, Iowa, USA, 1977.
VIII. BIOGRAPHIES Fig. 8. Loadings of a EHV li ne to beconsidered for zone 3 co-ordination (1) normal load; (2) thermal limit; (3) loadability limit; (4) NERC recommendation [2]
The normal load (1), the thermal limit (2), the loadability of the relay (3) and the loading according to NERC (4) recommendation is marked in Fig. 8. It can be seen that the calculation according to the NERC recommendation with 150% of the maximum thermal current would result in an impedance which appears in the MHO circle. A load blocking cutout as shown in Fig. 9 would then be necessary.
J ohann J äger was born in 1964 in Erlangen, Germany. He received the Dipl.Ing. and Dr.-I ng. degrees in 1990 and 1996 respectively in Electrical Engineering and Power Systems from the University of Erlangen. In 1990 he joined the Institute for Power Systems at the same University working on the analysis and calculation of FACTS-devices. From 1996 he was with the Power Transmission and Distribution Group and the System Planning department at Siemens AG in Erlangen, Germany. He was working on different fields of network planning and protections in worldwide projects. Since 2004 he is in charge of a full professorship for Power Systems at the University of Erlangen. He is member of V DE/ETG, IEEE and CI GRE as well as convenor and member of several national and international working groups.
.
Fig. 9. Load blocking cutout of a MHO relay
Combining the NERC recommendation and modern relay technology providing an flexible loading cutout function, a zone 3 reach setting can be adjusted which is stable for the most critical loading cases and prevents maltripping caused by overload consequently. VI. CONCLUSION The changing network conditions of an IPP installation are comprehensive and their impacts on the protection system multiple. Concerning the topics of co-ordination of different tripping characteristics, teleprotection schemes, distance grading and relay loadability, methods for covering their adverse impacts could be developed. For their implementation the whole systemmust be kept in mind. In this way a well coordinated and adapted protection system can be achieved and high reliability of the IPP in-feed and an economic benefit for the IPP investors can beensured finally.
