Enhancement of Available Transfer Capability by the use of UPFC in Open Power Market

Transcription

1 INDIAN INSTITUTE OF TECHNOLOGY KHARAGUR 7 DECEMBER Enhancement of Available Transfer Capability by the use of UFC in Open ower Maret K.S.Verma Abstract- In deregulated power systems Available transfer capability (ATC) analysis is presently a critical issue either in the operating or planning because of increased area interchanges among utilities. FACTS devices can be an alternative to reduce the flows in heavily loaded lines resulting in an increased transfer capability low system loss improved stability of the networ reduced cost of production and fulfilled contractual reuirement by controlling the power flows in the networ. It is important to ascertain the location for placement of these devices because of their considerable costs. A method to de termine the suitable locations of Unified ower Flow Controller based on the real power flow performance index sensitivity has been suggested in the paper for enhancing the available transfer capability of the interconnected power system. A conceptually reasonable and computationally feasible approach has been developed and is illustrated by an example. Index Terms- Available transfer capability FACTS devices open access total transfer capability UFC I. INTRODUCTION: Transmission open access has been an important issue in the ongoing deregulation and restructuring of power sector in many countries. Transmission open access is a vehicle for promoting competition in generation. Open access to the transmission systems places a new emphasis on the more intensive shared use of the interconnected networs reliably by utilities and third party generators. As system becomes deregulated loop networs introduced technical issues with the definition and calculation of the ATC. In addition the differences between contract path and actual power flow path introduced additional complexity to the uantification of ATC. When systems were isolated and largely radial these capabilities were fairly easy to determine and consisted mainly of a combination of thermal ratings and voltage drop limitations. In most cases these two limitations were easily combined into a single power limitation (either MW or MVA or surge impedance loading). As such ATC for a given transmission line at a given time could be interpreted as the difference between the power limit and the power flow at that time. K. S. Verma is research scholar in the Department of Electrical Engineering IIT Rooree India (telephone: ( svevdee@iitr.ernet.in) H. O. Gupta is rofessor in Deptt. of Electrical Engg. IIT Rooree ( harifee@iitr.ernet.in) K. S. Verma thans the Director KNIT Sultanpur (India) for sponsoring him for doctoral wor under QI Govt. of India in the Department of Electrical Engineering IIT Rooree India. Authors also thans Dr S N Singh for his fruitful discussion on this issue. H.O.Gupta Available transmission capacity as described in the U.S. Federal Regulatory Commission s (FERC) March Notice of roposed Rule maing (NOR) Docet RM95-8- section III-E4f is a new term that has not been universally defined or used by the electric industry. FERC used the term available transmission capacity in its NOR to label the information that is to be made accessible to made all transmission user as an indication of the available capability of the interconnected transmission networs to support additional transmission service []-[]. Owing to the commercial and technological significance of ATC in the power industry deregulated environment more and more institutes and utilities have shown increased interest and are undertaing studies of evaluation and enhancement of ATC. In recent years various approaches have been proposed to modal and calculate ATC. Under open access power system complexity has grown and system stabilities became an important constraint for some areas of the interconnected networ and this reuired the consideration of the third limiting phenomena. The introduction of St. Clair curves were one of the first attempts to include thermal voltage and stability into a single transmission line loading. These results were later verified and extended from a more theoretical basis. Linear load flow and linear programming solutions made transmission transfer capability determination relatively fast and easy. It is highly recognized that flexible AC transmission systems (FACTS) devices specially the series devices such as thyristor controlled series capacitor (TCSC) thyristor controlled phase angle regulator (TCAR) unified power flow controller (UFC) etc. can be applied to increase the ATC of power networ. In [5] a comparative study to improve ATC has been done and it is shown that FACT technology can redistribute load flow and regulate bus voltage so a promising method to improve TTC. In [9] location of FACTS devices has been suggested with increase in total transfer capability. II. ATC DEFINITIONS AND RELATED TERMS: There has been interest in uantifying the transmission capabilities of power system for many years. When systems were isolated and radial these capabilities were fairly easy to determine and consisted mainly of a combination of thermal ratings and voltage drop limitations. 996 document introduces several new terms which refine the concepts of the 995 documents and specifically identify uantities associated with the uncertainty in modeling and system conditions.

2 464 NATIONAL OWER SYSTEMS CONFERENCE NSC Available Transfer Capability (ATC) is measure of the transfer capability remaining in the physical transmission networ for further commercial activity over and above already committed uses. ATC can be expressed as: ATCTTC-TRM-ETC-CBM where the ETC is the sum of the existing transmission commitment between those areas. Total Transfer Capability (TTC) is defined as the amount of electric power that can be transferred over the interconnected transmission networ in a reliable manner while meeting all of a specific set of defined pre- and post- contingency system conditions. Transmission Reliability Margin (TRM) is defined as that amount of transmission transfer capability necessary to ensure that the interconnected transmission networ is Recallability is defined as the right of the transmission provider to interrupt all or part of a transmission service for Total Transfer TRM Capability Transmission Reliability ower TRM Flow (MW) Non-recallable Non-recallable ATC ATC ATC Non- reserved Scheduled Fig : ATC and related terms III. Modeling Of UFC The UFC which was first proposed by Gyugi whose euivalent circuit placed in line- connected between bus-i and bus-j is shown in Fig.. UFC has three controllable parameters namely the magnitude and the angle of inserted voltage (V T φ T ) and the magnitude of the current (I ).. UFC Bus-i r ij +j x ij Bus-j V V i I T i V T I i V j jb/ I I T Reserved Reserved Operating Horizon lanning Horizon Time V i jb/ any reason including economic that is consistent with FERC policy and the transmission provider s transmission service tariffs or contract provisions. ATC (RATC) is defined as TTC less TRM less recallable transmission service less non-recallable transmission service (including CBM). Capacity Benefit Margin (CBM) is defined as that amount of transmission transfer capability reserved by load serving entities to ensure access to generation from interconnected systems to meet generation reliability reuirements. Non-recallable ATC (NATC) is defined as TTC less TRM less non-recallable reserved transmission service (including CBM) Curtailability is defined as the right of a transmission provider to interrupt all or part of a transmission service due to constraints that reduce the capability of the transmission networ to provide that transmission service. Transmission service is to be curtailed only in cases where system reliability is threatened or emergency conditions exist. Available transfer capability is the measure of the ability of interconnected electric systems to reliably transfer the power from one area to another area over all transmission lines or paths between those areas under the specified system conditions. It is seen as very important issue in present day power system deregulation. There are several papers those deal with ATC calculations and related issues Fig shows ATC and related terms depicted graphically. They form the basis of transmission service reservation system that will be used to reserve and schedule transmission services in the new open power maret. is gij Vi gijcos( + V j [gijcos( js i ) j ) + bijsin( δ j )] () V j [gijcos( j ) bijsin( j)] () Qij V ii -Vi ( bij + B / ) Vi [ gijsin( i ) + ( bij + B/)cos( i)] Vi Vj (gijsinδ ij bijcosδ ij) Qji -V j ( bij + B / ) + V j ( gijsin( j ) + bijcos( j )) + Vi Vj (gijsinδ ij + bijcosδ ij ) Bus-i S is r ij +j x ij S js Bus-j Fig. : Injection Model of UFC () (4) IV REAL OWER FLOW I SENSITIVITY INDICES FOR LOCATING UFC Fig: Euivalent circuit of UFC The injected active power at bus-i ( is ) and bus-j ( js ) and reactive powers (Q is and Q js ) of a line having a UFC are given below. Its injection model is given in Fig. The severity of the system loading under normal and contingency cases can be described by a real power line flow performance index [] as given below.

3 INDIAN INSTITUTE OF TECHNOLOGY KHARAGUR 7 DECEMBER N n l w m lm I n max m lm (5) max lm where lm is the real power flow and is the rated capacity of line-m n is the exponent and w m a real nonnegative weighting coefficient which may be used to reflect the importance of the lines. N l is the total number of lines in the networ. In this study the value of exponent has been taen as and w m.. Real power flow performance index gives good measure of the system congestion during the normal operating condition. The real power flow I sensitivity factors with respect to the control parameters of UFC can be defined as I c I c I c I sensitivity with respect to I I Using euation (5) the sensitivity of I with respect to UFC parameter X (V T φ T and I ) connected between bus-i and bus-j can be written as I N l w m lm max m lm 4 lm The real power flow in a line-m ( lm ) can be represented in terms of real power injections using DC power flow euations [] where s is slac bus as N b Smnn n n s lm N b Smnn + js n n s I sensitivity with respect to V T I sensitivity with respect to φ T for m for m where S mn is the mn th element of matrix [S] which relates line flow with power injections at the buses without UFC and N b is the number of buses in the system. Observe that line- from bus-i to bus-j is the line containing the UFC as illustrated in Fig.. js therefore is the addition flow at bus-j in the line containing the UFC due to the presence of the device. Using () and () the following relationship can be derived S is js mi + Smj for m lm (8) S is js js + S + m mi mj for (7) (6) The terms I I and I can be obtained using euations () and ()respectivelyand are given below. V igijcos( i) + Vj( gijcos( j) + bijsin( j)) (9) V igij sin( δi) φ T () + Vj ( gij sin δ j + bij cosδ j) I Vj ( gij cos δ j + bij sin δ j ) V T V j ( gijsin δ j bij cosδ j ) φ T js I () () () (4) The sensitivity factors c c and c can be obtained using euations (9-4). The derivatives of real and reactive power with respect to phase angle of UFC are considered around zero although the phase angle in UFC can be control from to 6. The angle difference for both ends of line are generally very small and it is limited to due to stability reasons. V- CALCULATION OF ATC WITH FACTS DEVICES The total transfer capability with FACTS devices can be calculated using optimal power flow formulation as mentioned in section-ii which is given below pi min (5) i I G Ci( pi ) subject to L ( Q V θ X ) (6) G ( Q V θ X ) (7) where I G a set of generator buses pi active power of generator-i C i cost of generator-i vector of power injections/extraction Q vector of reactive powers injection/extraction V vector of voltage magnitudes θ vector of voltage angles

4 466 NATIONAL OWER SYSTEMS CONFERENCE NSC X FACTS device control parameter Euality constraint (6) corresponds to power flow euations for both real and reactive power. Ineuality constraint (7) is the limits on the operating constraints such as real and reactive power flow limits of generations transformer taps line flow limits bus voltage limits and the limits on the FACTS device control parameters. VI SIMULATION RESULTS To establish the effectiveness of the proposed methods it has been tested on a 5-area system as shown in Fig.. The two circuits - and -5 are of impedance.58 + j.866 pu each while other four lines have an impedance of.9 + j.48 pu each all to a MVA base. The line flow limit is set to MW. Fig. 4: 5-Area system From base case OF it was found that power injections from the different areas 4 and 5 were and 8. MW respectively. Keeping these injections constant total transfer capability from area- to area-5 (TTC 5 ) was calculated as described as above and found to be 7. MW. With this flow of power in lines the sensitivities were calculated for each UFC placed in every line one at a time for the same operating conditions. The sensitivities of real power performance index with respect to UFC are presented in Table. The highest negative sensitivities in case of voltage control and the highest absolute value of sensitivities in case of phase angle control are presented in bold type. From the load flow it was found that real power flows in line was.5 pu which is its line loading limit. It can be observed from Table that placement of UFC in line-6 is suitable for reducing the I for voltage control as the value of sensitivity is highest negative. lacement of UFC in other line will not reduce the I value as it less effective than placing a UFC in line-6 as can be seen from its sensitivity factors. Table : Sensitivities of 5-area system for TTC 5 Line- No. i - j 4 Line flow (pu) Sensitivities of UFC c c Table shows that placement of UFC in line-6 is more sensitive than the placement in other lines for voltage control. lacing of UFC in line- will reduce the loading of lines and 4 (heavily loaded lines) but it will increase the loading of lines and 6 that are under-loaded (highest negative sensitivity). Table also shows that the placement of UFC in line- in the next choice as the magnitude of sensitivity factors is the second highest. This sensitivity is positive which indicates that phase angle shift of the UFC should be negative. The value of cases is constant due to constant voltage profile. Table II: TTC 5 (in MW) with UFC UFCi ATC Vt φ I (pu) nline (p.u) (pu) c in all the The ATC of each line with UFC placed is shown in the table. The above table shows that the highest ATC is obtained while placing the UFC in the line 5. The optimal parameters of UFC has been also given in the table. Injecting power at one location and extracting it at another location affects system flow patterns. As injected power increases between two locations some lines or interfaces become limiting and therefore no additional power can be transferred between the two locations. If there is one path between two locations the ATC between them will be eual to the ATC of the path. If there is more than one transmission path between the two locations the ATC from location (say ) to location (say ) will be different from location to location. The line flows without UFC is shown in Table. The placement of UFC in different lines one at a time changes the available transfer capability and therefore the line flow is also changed which can be seen from Table. Table 4 shows the line flows after placing the UFC in each line for TTC 5. Line- is the approximate limiting line for all the cases. The parameters of UFC is fixed i.e Vt. (pu) φ.8 (degree) and I. (pu) Table III: Line flows (in pu) with UFC in different lines Line UFC in line nos

5 INDIAN INSTITUTE OF TECHNOLOGY KHARAGUR 7 DECEMBER TTC from area 5 to area (TTC 5 ) also calculated for the same system without FACTS device in lines and operating condition and it was found to be. MW however it was 7. MW for area to area-5.. With this flow of power in lines sensitivities were calculated the direction of power flow in lines has been reversed and therefore the sign of sensitivities. TTC from area-5 to area- was calculated with placement of UFC in lines. TTC 5 with UFC placed in each line one at a time is calculated and it is observed that placement of UFC is suitable in line 4. However it is slightly less than placement in line but the reuire phase is small in the former case Line number Figure 5: Line flow UFC placed in line Line number Figure 6: Line flow UFC placed in line 4 Figure 5 and 6 show the line flows in different cases for TTC 5 calculations. From these figures it is evident that line- is limiting in all the cases. VII CONCLUSIONS American Electric Reliability Council rinceton New Jersey June N.G. Hingorani Flexible AC transmission IEEE Spectrum April 99 pp L. Gyugyi A unified power flow control concept for flexible AC transmission systems IEE roc. art-c Vol.9 No.4 July 99 pp G.D. Galiana Assessment and control of the impact of FACTS devices on power system performance IEEE Trans on ower System Vol. No. 4 Nov. 996 pp I.O. Elgerd Electric Energy System Theory- An Introduction McGraw Hill Inc. New Yor G.C. Ejebe and B.F. Wollenberg Automatic contingency selection IEEE Trans. on ower Apparatus and Systems Vol.98 No. January/February 979 pp A.J. Wood and B.F. Wollenberg ower Generation Operation and Control John Wiley New Yor K.S. Verma S.N. Singh H.O. Gupta FACTS device location for enhancement of total transfer capability IEEE ower Winter meeting Jan8- Ohio USA. S.N. Singh and H.O. Gupta "Location of UFC for enhancing power systems security in deregulated Environment" International Conference on Energy Automation and Information Technology December - IIT Kharagpur India.. K.S. Verma S.N. Singh and H.O. Gupta "Optimal location of UFC for congestion management" Electric ower Systems Research Vol. 58 No. July pp M.D. Illic Y.T. Yoon and A. Zobian Available Transmission Capacity (ATC) and Its Value Under Open Access IEEE S Vol. No. May 997 pp G.C. Ejebe J. Tong J.G. Frame X. Wang and W.F. Tinney Available Transfer Capability Calculations IEEE Trans on ower Systems Vol. No. 4 Nov 998 pp Y. Xiao Y.H. Song Y.Z. Sun Application of unified power flow controller to available transfer capability enhancement IEEE ower Engineering Review April. 5. Yan Ou Chanan Singh Improvement of total transfer capability using TCSC and SVC IEEE Summer ower Meeting July Canada. 6. Ying Xiao Y.H. Song Application of stochastic programming for available transfer capability enhancement using FACTS devices roc. IEEE/ES SM Seattle WA July. A sensitivity-based approach has been developed for finding suitable placement of these devices for enhancement of available transfer capability. Test results obtained on test system show that new sensitivity factors could be effectively used for optimal placement in response to enhance the ATC of the two areas. The placement of a UFC from static point of view can be decided based on the maximum sensitivity for voltage control however the maximum absolute value of sensitivity will be the choice for a phase angle control. The UFC should be placed on the most sensitive lines. With the sensitive indices computed following criterion can be used for their placement. The UFC should be placed in a line having most negative sensitivity factor with respect to change in V T. The UFC should be placed in a line having largest absolute value of the sensitivity factor w.r.t φ T. VIII. REFERENCES. Transmission Transfer Capability Tas Force Transmission Transfer Capability North American Electric Reliability Council rinceton New Jersey May Transmission Transfer Capability Tas Force Available Transfer capability Definition and Determination North