Application of Intelligent Voltage Control System to Korean Power Systems

Transcription

1 Applcaton of Intellgent Voltage Control System to Korean Power Systems WonKun Yu a,1 and HeungJae Lee b, *,2 a Department of Power System, Seol Unversty, South Korea. b Department of Power System, Kwangwoon Unversty, South Korea. *Correspondng Author 1,2 ORCID: , Abstract The Korea power system has been operated more closely to stablty lmts n state heavy-load because of growth n load-demand and the reactve power loss have been ncreased due to the nstallaton of long-dstance transmsson lnes, snce the power plants are located far from load-demand regons. The voltage collapse can be caused by reactve power unbalance. Some advanced countres experenced massve blackout due to voltage collapse. In order to solve voltage problems, several advanced countres are operatng voltage and reactve power control system by takng nto account the nherent characterstc of ther ndvdual power system. Recently, secondary control system and ntellgent control system for Jeju power system have been reported n Korea. But voltage control system about manland power system has not been reported. Ths paper presents the applcablty of ntellgent voltage control system for Korean power systems. Keywords: Intellgent voltage control system, Senstve matrx, Expert system INTRODUCTION Recently, massve power outages n Europe and North Amerca were caused by unbalances n reactve power, resultng n voltage collapse. In order to solve reactve power unbalance problem several advanced countres are operatng the voltage and reactve power control system by takng nto account nherent characterstc of ther own power system[1,2]. The Korea power system has been operated more closely to stablty lmts because of rapd growth n load-demand. And reactve power losses are ncreased due to the nstallaton of long-dstance transmsson lnes snce the generaton power plant are located far from the load-demand regons. For ths reason, there s reactve power demand contnues to ncrease. However, the power system voltage s mantaned only by engneerng knowledge and judgment of the operators snce t s not actually easy to secure the ste for voltage compensaton equpment and the regonal systematc voltage control system s not really prepared. Newly, the KEPCO recognzed the need for automaton of the voltage control and developed ntellgent voltage control system and secondary voltage control system[3,4]. The methodology of the voltage control may be classfed nto a numercal optmzaton method and ntellgent control. In case of ntellgent voltage control system t has been developed real-tme voltage and reactve control expert system usng senstvty tree n Canada n the 1980s[5]. Span developed SEGRE[6] and SETRE[7,8] n the 1990s and they has been successfully operatng n domestc power system. They are bascally hybrd system usng ntellgent method and senstvty tree-based numercal computaton. But they are not easy to develop snce numercal module and search method are not reported as well as operaton method of system and structure of power system s varous. The ntellgent voltage control system performs voltage control usng senstvty matrx of generator termnal voltage, parallel capactor/reactor, and transformer Tap. It can adjust abnormal voltage range and target voltage range and can use basc power system data such as PSS/E data. Generally, effectve searchng method s the most mportant factor to mprove performance of expert system snce the process of obtanng a soluton n ntellgent system has to search the state space wth nference of target. Ths paper uses exstng weghted evaluaton functon and least-cost search method. And a contnuous quantty such as generator termnal voltage converted to dscrete quantty through the quantzaton process n order to accurate voltage control. The error range n the senstvty matrx must be precse because the senstvty matrx uses lnearzaton model for nonlnear system. The error of ntellgent voltage control system n a performance test n Jeju power system was precse, up to a maxmum of 1%. But the error range n the senstvty matrx has to be verfed n order to test applcablty of the ntellgent voltage control system for a manland power system, relatvely large power system than Jeju power system. Ths paper presents an artfcal ntellgence based voltage control system to adjust the bus voltage of the power system to wthn the specfed range. STRUCTURE OF INTELLIGENT VOLTAGE CONTROL SYSTEM The structure of ntellgent voltage control system to be explored n ths paper s descrbed n Fg. 1, where the ntellgent controller s made up of the senstvty matrx based numercal module and the knowledge base ncludng a wde 8529

2 varety of nformaton related wth power system status and control knowledge. - Reactve power compensaton amount determne Lnear Predcton method Numercal Module Assumng an N bus power system wth M control actons, the relatonshp between the bus voltages and the control actons can be represented as shown n Fg. 2. Fgure 1: Structure of ntellgent voltage control system Knowledge Base The knowledge n a specfc problem doman s classfed by truth and rule and then stored n the database and rule base, respectvely. Database and rule base are n the followng: Database - Upper and lower lmt of each bus voltage - Upper and lower lmt of the voltage regulaton - Upper and lower lmt quantty of compensaton devces - Prorty of compensaton devces - Quantzaton level of generator termnal voltage It s ponted out that the changes n each control acton have sgnfcant mpacts on the voltage n some buses. For a partcular voltage volaton, t s possble to compute the control acton needed to remove ths voltage volaton by the senstvty technque. It s worthy to menton that the control acton should nether exceed the specfed lmts nor ncur new voltage volatons of other buses. The senstvty matrx s a fundamental parameter n the ntellgent voltage control system. By defnng the relatonshp of changes n bus voltages accordng to compensaton changes n the generator termnal voltage, shunt capactor/reactor, and transformer tap, t selects the control actons when the voltage volaton occurs and determnes the quantty of compensaton requrement. Fgure 2: Descrpton of bus voltage and control actons Rule base - If the voltage exceed upper and lower lmt of each bus voltage the system operate the controller - If abnormal voltage occurred, frstly the controller consttute senstvty tree - The controller selects the compensaton devce of largest senstvty - If selected reactve power compensaton devce s capacty s lack, the controller selects the second hghest compensaton devce - The controller operates the specfed prorty of compensaton devces - If abnormal voltage occurred n several bus, the controller operate based on the greatest abnormal bus voltage - If bus voltage don t adjust wthn voltage regulaton by usng compensaton devce of frst rankng, compensaton devce of next rankng s commtted The senstvty matrx s reestablshed by the relatonshp between the voltages and the reactve power n the Jacoban matrx constructed from the load flow equaton. P P Q Q P V Q V V Assumng that the voltage angle s neglgble n the relaton wth the reactve power, the relatonshp between the voltage and the reactve power s encapsulated n (2). [ Q/ V ] Q V (1) Q (2) V Q V 1 V Q s the Jacoban matrx of load flow calculaton n (2). (3) 8530

3 That s, [ 1 Q/ V ] s the nverse matrx of [ Q / V ] and called the senstvty matrx whch estmates the changes of bus voltages aganst the changes of reactve power. The senstvty matrx s gven by the control actons as shown n (4). V S Vg V S sh V S Tap U U Vg sh U STATE-SPACE MODEL OF INTELLIGENT VOLTAGE CONTROL SYSTEM After a gven problem defne the representaton model, to solve the problem we need a several strategy and one of the key strateges s search. The searches wll be defned by the tral process to assess possble soluton paths and reach from ntal state to fnal state. It may be dvded nto two categores: blnd searches and heurstc searches. The blnd searches dvded to breadth-frst search and depth-frst search. But, ths method s not good n case that state-space s large due to t do not contan ntellgent decson. The heurstc searches s a method to contnuously search a soluton after gettng rd of the soluton path that seems napproprate by judgment such as heurstc knowledge or cost functon. Ths method could reduce state-space but mght be occurred that do not solve. Ths paper uses state-space model as shown n Fg. 3. Tap (4) V s newly occurred abnormal bus voltage n normal bus. vo The search process followng: Step 1: About the bus where the abnormal bus voltage, a v1 node that the largest effect quantty (senstvty value control quantty) was selected by the system. And the system expands v1 node as three quantzed effect quantty. Usng three quantzed effect quantty, the system calculated lner predcton and weghted evaluaton functon about all the bus voltage. Evaluaton functon quanttes of expanded node are 11,9,10 as seen fgure 2. The system selected v1_2 node that the smallest of evaluaton functon value and progress a selecton of compensaton devce of step 2. Step 2: The system performs lner predcton by usng effect quantty of v2. As a result, the system selected transformer tap t1 because abnormal bus voltage occurred n normal bus. And through the same process of v1 node, the system selected t1_3 node that the smallest of evaluaton functon quantty. Fnally, f abnormal bus voltage s dssolved, the system selected compensaton devce of step 3. Conversely, f abnormal bus voltage s solved, the system fnshed the search process. Case Study Generally, the voltage n power system fluctuates due to the constantly changng load even f no accdent occurs. Therefore, the power system must be operated wthn the specfed voltage range for voltage stablty and faclty protecton. In the Korean power system, the bus voltage range of the transmsson system s defned as follows < bus Voltage < 1.05 (6) In ths paper, the operatng condtons of the developed voltage control system are set as follows. 1 Upper and lower lmt of abnormal bus voltage : 0.95 < V or V > Upper and lower lmt of the voltage regulaton Fgure 3: State-space model of voltage control : 0.97 < V or V > Upper and lower lmts of generator termnal voltage [p.u.] Least-cost search Ths paper used least-cost search to mnmze weghted evaluaton functon such as expresson (5). mn( V vo Vg Tk Q ) (5) [ ] : 0.95 generator termnal voltage Prorty of compensaton devces : Generator >shunt capactor> transformer tap 5Quantzaton level of generator termnal voltage : 5[step] 8531

4 Fgure 4: Bus voltage profle at steady state Fg 4 shows the power system bus voltage profle at steady state. Lookng at Fg 4, two bus voltages exceeded the voltage range. Fg 5 and 6 show the voltage profles before and after voltage control for voltage volaton bus voltage. After the voltage control operaton, t can be confrmed that all the bus voltage s adjusted wthn the specfed voltage. CONCLUSION Ths paper presents an artfcal ntellgence based voltage control system to adjust the bus voltage of the power system to wthn the specfed range. The developed voltage control system showed satsfactory performance as a result of voltage control. In future, varous cases wll be developed to verfy the performance of the ntellgent voltage control system. Fgure 6: Voltage profles before and after voltage control ACKNOWLEDGMENTS Ths work was supported by Human Resources Program n Energy Technology of the Korea Insttute of Energy Technology Evaluaton and Plannng (KETEP), granted fnancal resource from the Mnstry of Trade, Industry & Energy, Republc of Korea. (No ), and the Research Grant of Kwangwoon Unversty n REFERENCES Fgure 5: Voltage profles before and after voltage control [1] S. J. Cheng et al., An Expert System for Voltage and Reactve Power Control of a Power System, IEEE Trans. on PWRS, Vol. 3. No. 4, Nov. 1988, pp [2] T. L. Le et al., Network Equvalents and Expert System Applcaton for Voltage and VAR Control n Large-Scale Power Systems, IEEE Trans. On PWRS, Vol. 12. No. 4, Nov. 1997, pp

5 [3] H. J. Lee et al., Hybrd Intellgent Voltage and Reactve Power Control System For Jeju Power System n Korea 8th WSEAS Internatonal Conference on POWER SYSTEM(PS 2008), Santander, Cantabra, Span, September 23-25, pp [4] H. J. Lee et al., A study on the Intellgent Voltage Control System, The Transactons of the Korean Insttute of Electrcal Engneers Vol. 61, No. 7, pp. 944~949, [5] S. J. Cheng et al., An Expert System for Voltage and Reactve Power Control of a Power System, IEEE Trans. on PWRS, Vol. 3. No. 4, Nov. 1988, pp [6] J. L. Sancha et al., Spansh Practces n Reactve Power Management and Voltage Control, IEEE Colloquum on Internatonal Practces n Reactve Power Control, Aprl 1993, pp. 3/1-3/4. [7] A. Gomez Exposto et al., Senstvty-Based Reactve Power Control for Voltage Profle Improvement, IEEE Trans. on PWRS, Vol. 8, No. 3, Aug. 1993, pp [8] J. L. Martnez Ramos et al., A Hybrd Tool to Assst the Operator n Reactve Power/Voltage Control and Optmzaton, IEEE Trans. on PWRS, Vol. 10, No.2, May 1995, pp