Distributed generation control using protection principles

Transcription

1 University of Wollongong Research Online Faculty of Engineering an Information Sciences - Papers: Part A Faculty of Engineering an Information Sciences 2006 Distribute generation control using protection principles An D. T Le University of Tasmania, tale@utas.eu.au Kashem M. Muttaqi University of Tasmania, kashem@uow.eu.au Michael Negnevitsky University of Tasmania, michael.negnevitsky@utas.eu.au Gerar Lewich Queenslan University of Technology, g.lewich@qut.eu.au Publication Details A. D. T. Le, K. A. Kashem, M. Negnevitsky & G. Lewich, "Distribute generation control using protection principles," in Australian Universities Power Engineering Conference (AUPEC 2006), 2006, pp Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.eu.au

2 Distribute generation control using protection principles Keywors principles, protection, istribute, generation, control Disciplines Engineering Science an Technology Stuies Publication Details A. D. T. Le, K. A. Kashem, M. Negnevitsky & G. Lewich, "Distribute generation control using protection principles," in Australian Universities Power Engineering Conference (AUPEC 2006), 2006, pp This conference paper is available at Research Online:

3 Distribute Generation Control using Protection Principles An D.T. Le, M.A. Kashem, M. Negnevitsky School of Engineering University of Tasmania Sany Bay, Tasmania, Australia ABSTRACT In a istribution system, it is essential to maintain the voltage variation within a specifie limit for satisfactory operation of connecte customers equipment. Normally, this goal is achieve by controlling the operation of compensating evices, such as loa tap changing transformers, shunt capacitors, series capacitors, shunt reactors, an static VAr compensators. However, technical an regulatory evelopments are encouraging a greater number of small generator units, known as Distribute Generation (DG), an this has the potential to significantly affect voltage control systems. This paper presents an aaptive voltage control technique which incorporates DG systems into the voltage control system. The control scheme uses On-loa Tap Changing Transformer (OLTC) an DG for voltage corrections, both are riven by avance Line Drop Compensators (LDC). At the substation, the LDC is employe to control step up or step own ecisions of the OLTC, while another LDC will be use at DG connection point to set DG parameters. Also, for a more cost-effective system, voltage control action coorination is propose using magnitue graing an time graing. The control approach is teste on a moifie istribution system with loa variations that are stochastic in time an location. The results show that the integration of these magnitue graing an time graing, protection principles have consierably reuce the DG energy require to achieve the esire control. Inex Terms -- Distribute Generation, Power Distribution System, Voltage Control, On-loa Tap Changing Transformer, Line Drop Compensator. 1. INTRODUCTION In the last few ecaes, customers have become increasingly more sensitive to the voltage violations outsie the preefine limits. The National Electricity Market stanars inicate that the range of voltages in istribution system shoul not excee ± 5%. This is to ensure proper functions of connecte electrical appliances, which are highly require by certain types of customers. This in return leas to an increase necessity for voltage management services. There are four methos which have been use wiely to regulate istribution system voltage, incluing [1]: a) On-loa tap changing transformer (OLTC) b) Capacitor bank (an/or reactor) switching G. Lewich Electrical an Electronic Systems Engineering Queenslan University of Technology Brisbane, Australia g.lewich@qut.eu.au c) Synchronous an static compensators ) Generating unit excitation systems In most applications, the first two methos respon frequently after every short perio to the voltage errors. The later ones, on the other han, usually act continuously an rapily to correct the system voltage within their capacity range. Given that the istribute generation (DG) brings a consierable number of benefits with a more compact configuration an more competitive price, the wish to connect them into low voltage networks by istribution companies is increasing [2]. However, this tenency plus the growth of loa eman an the uncertainties of loa connection/isconnection have been contributing to the complexity of voltage regulation [3]. Traitional voltage control actions, in the absence of DG, epen much on the fact that the voltage profile ecreases along the feeer from the substation to the remote en. In contrast, the integration of DG systems makes this characteristic no longer vali. Other possible ifficulties involve the chance of interaction between ifferent control evices an a DG or among several DGs. As a result, voltage control strategies nee reconsieration [4]. Solutions for voltage control problem in the presence of DG have been reporte in the literature recently. Authors in [5] have propose an algorithm to control voltage with inverter-base DG for a uniformly istribute feeer moel. The control metho in [6] has been establishe by altering the automatic voltage control (AVC) relay target voltage base on the estimate maximum an minimum voltage noes. In [7], an Artificial Neural Network has been applie to esign the settings of AVC relay for OLTC control purposes. Mogos et al. have presente a voltage regulation system for electronic interface gri-connecte DG base on active an reactive power control [8]. Authors in [9] have presente a neste evolutionary programming approach for optimising the voltage control variables, such as voltage reference, tap position, etc. This paper introuces a new voltage control scheme for the presence of OLTC an DG as primary system voltage regulators. Both OLTC an DG are riven by an avance line rop compensator (LDC), which is expecte to provie goo overall performance an viable running cost. Moreover, the magnitue an time graing principles in protection system have been aapte to avoi the risk of interaction between OLTC an DG, as

4 well as to utilise the capacity of taps. Simulations are conucte over a small perio of time with consieration of loa ynamics to show the effectiveness of the metho. 2. OPERATING PRINCIPLES OF LDC The operation of OLTC an DG usually can be obtaine in a simple way by controlling the local voltage at their common coupling points. Due to the high iversity in location an eman of customers with respect to time, the voltage references of these regulators nee to be set relatively high to guarantee no uner-voltage problems in the system. This is sometimes very costly since DG may overrun at some stages. Moreover, unnecessary taps an DG switching are more likely to happen. To overcome these challenges, the LDC has been prove to be very promising. As LDC is more sensitive to the changes of loa an system voltage, it is able to preict voltage rop more effectively, an thus, reuce DG running time if possible. In aition, LDC allows a simpler an more accurate tuning process for voltage control CONVENTIONAL LDC It is quite common in istribution system that the aime point for voltage regulation is neither at the seconary sie of substation transformer nor DG s location, but at some remote loa centre. Ieally, the best way for voltage problem solving in this case woul be using the actual voltage at that point as the feeback to the controller [10]. However, this is not a preferre solution to the istribution companies as it requires extra measurement an communication systems. LDC, on the contrary, uses the local measurements of voltage an current to preict voltage at remote loa with acceptable iscrepancy. Besies the local voltage an current, voltage preiction of conventional LDC also epens on the internal coefficient settings of LDC, R an X. The R an X are usually ajuste to reflect the line resistance an reactance, thus make it possible for LDC to give an inication of the remote voltage. The esign of R an X has been iscusse in [10-12] ADVANCED LDC In practice, it is sometimes very challenging to select an effective R an X as the loa change is unpreictable. Also, the tap changing operation an the inclusion of DG systems have mae this process even more complicate [13]. In this section, an avance LDC which preicts remote en voltage only by using the local voltage an current measurements is propose. The LDC works base on the assumption that the line current rops linearly from measurement point to the en of the feeer. Thus, the estimate current x kilometres from the substation can be written as: I I ( x) = x + I (1) ( l ) where l an are the istances in km from the remote en an regulation point to the substation, I is the measure current at. Voltage preiction at the remote en is etermine by subtracting the estimate voltage rop from the measure voltage at regulation point : l x= ( x) V = V zi (2) pr Eq.(2) can be simplifie as, V pr = V ( z 2 ) I ( l ) (3) where V is the measure voltage at an z is line impeance per unit length. The avance LDC has eliminate the possibility of inaequate voltage preiction cause by poor esign of LDC internal settings. Therefore, more accurate preiction with higher confience can be obtaine. 3. PROPOSED VOLTAGE CONTROL ALGORITHM In this paper, the mission of maintaining system voltage within the specifie limits is achieve by controlling tap change of OLTC an output current from a single DG. Each voltage regulator is equippe by an avance LDC an they are both responsible for looking after the remote en voltage. Real-time practice of voltage control system also requires taking into account temporary voltage rop circumstances ue to short term loa variations. Such situations usually o not hol for long time an are the system is expecte to automatically recover. Therefore, any tap change or DG operation in response to them is unesire by utilities ue to wear of contacts. This problem, though, can be easily solve by inserting a time elay into the regulators. First tap or DG ajustment takes place after a time elay, then respons instantly to the next. The elay is recommene to be long enough to overcome any unnecessary responses. To improve the performance of the control system, a time elay an a voltage reference setting is integrate for each regulator. This is an imitation of the graing principles in protection system, which are known as time graing an magnitue graing. The graing process will be iscusse in etail later on VOLTAGE CONTROL BY OLTC The status of OLTC can be categorise into three types: o nothing, tap up, an tap own. These statuses are coe as 0, +1, an -1, respectively, an etermine by following rules (with V ref1 is the reference voltage an V pr1 is the estimate remote voltage of OLTC controller): 1) Default status of OLTC is 0 2) If V pr1 < V ref1 ea ban: current status is +1 3) If V pr1 > V ref1 + ea ban: current status is -1 4) Otherwise, current status is 0 A counter is set up in the controller with efault value of zero to make sure tap changes for permanent voltage problems only. The control algorithm of OLTC can be summarise as below (for t > t + 1): Step 1: Determine the current status of OLTC at time t using LDC an local measurements at transformer point. If the status is +1 or -1, go to Step 2. Otherwise, go to Step 6.

5 Step 2: Does the status of OLTC remain the same as that at time t-1? If yes, increase the counter by 1 an go to Step 3. If no, go to Step 5. Step 3: Has the counter equals or greater than the elay time of OLTC? If yes, go to Step 4. If no, go to Step 6. Step 4: Has the OLTC exceee its limit? If yes, go to Step 6. If no, tap up (as status is +1) or tap own (as status is -1) Step 5: Reset counter to zero. Step 6: t = t + 1 an go to Step VOLTAGE CONTROL BY DG The DG control methoology shares some similarities of the OLTC s. Decision making of DG operation is also riven by a variable calle current status an counter is employe to trigger DG action only in actual nee. Default values of both the current status an the counter are zero. Obviously, these variables work inepenently from those of the OLTC. Counter = 0 t = t +1 No Calculate V pr2 using Eq.(3) with local measurements at DG point at time t Determine DG status Is the status = 0? No Is the status the same as t-1? Counter = counter + 1 Is the counter DG elay time? Calculate esire I DG by using Eq.(4) No t = t +1 reference is tolerable while further voltage rop from it is harly accepte. The proportional an integral controller type is use for DG. Voltage error given by the LDC plus some level of tolerance is referre to as feeback signal for the controller. The DG, after receiving feeback signal, will ajust its output current to correct the voltage as, Δ I DG = K P ( Vref 2 V pr 2 + ε ) (4) In this case, DG has been moelle as a constant current source. Its phase angle is etermine such that the DG woul always give maximum voltage change in the feeer [14]. Also for efficient an economic reasons, it is assume that DG works only if its output current is greater than 30% of the DG capacity an DG will be switche of otherwise. The control logic of DG is escribe in the flowchart of Fig.1. Since both OLTC an DG are working towars the same aim of correcting the remote voltage, the two controllers may experience some interactions. These interactions, nevertheless, coul be minimise by setting V ref1 consierably higher than V ref2. By oing this way, it is unlikely that DG has to have substantial run time when the OLTC is not yet saturate, an thus reucing the chance of interaction. The esign of voltage reference level an elay time in the controllers were in fact aapte from the magnitue graing an time graing characteristics, respectively, of the protection system. The employment of these principles is very helpful in improving the control scheme by many ways, such as, - Utilise the capacity of the OLTC, which is consiere as a less expensive voltage regulation metho. Therefore, reucing the running cost of DG. - Control actions of OLTC an DG only take place in case of permanent voltage problems. - Minimise the risk of interactions among controllers. 4. TEST SYSTEM AND LOAD DATA Is DG current < 30% of its capacity? Desire I DG = 0 Is DG current > 100% of its capacity? Desire I DG = 100% capacity A test system was constructe from real istribution network ata. This is a 69 noe 11 kv feeer moel, with one MV/LV OLTC connecte the feeer to the substation, an one DG at noe 65, as shown in Fig.2. Distance between any two noes is assume to be constant. Ajust I DG as the esire value Figure 1: DG controller s algorithm Current status of DG is efine as follows (with V ref2 is the reference voltage an V pr2 is the estimate remote voltage of DG controller): 1) If V pr2 <V ref2 lower tolerance: current status is +1 2) If V pr2 >V ref2 + upper tolerance: current status is -1 3) Otherwise, current status is 0 Lower tolerance is substantially smaller than the upper tolerance. This is ue to the fact that reference voltage is usually set closer to the lower voltage limit to keep DG from over running. Thus, further voltage rise from the.... DG... OLTC Loa Figure 2: Test system moel The OLTC has the tap ratio of 1:a, where a varies between 0.95 an The tap step is 1.25% an the elay time of first tap is 4 secons. The LDC s ea ban use in the OLTC is 1%. The LDC use for the DG has upper tolerance of 0.5% an lower tolerance of 0.2%.

6 A simulation is carrie out for 100 secons with a time step of 1 secon to emonstrate the effectiveness of the control metho. The LDCs of OLTC an DG receive their local voltage an current measurements an preict the remote en voltage perioically. If the estimate remote voltage is efine to be not safe within the limits, control action will happen. The test network is esire to operate within ±5% from the nominal voltage level. P(kW) an Q(kVars) Loa variation with respect to time Loa real power Loa reactive power the test feeer. Total feeer loa profile with respect to time is given in Fig.4. Total loa in the examine perio is kwh. 5. RESULTS AND DISCUSSIONS Simulations have been conucte in two cases: (1) DG has the elay time of 3 secons for the first ecision an then respons instantly; (2) DG is esigne to respon at every instant to the voltage error signal. The voltage reference of LDC for the OLTC is p.u. an for the DG is p.u. Besies the purpose of maximising tap usage, the reference voltage of LDC at OLTC was set relatively high also because of its less effective voltage preiction. Due to the inclusion of DG as well as the characteristic of the LDC use (base on the linear current rop assumption), the further the LDC from the remote en, the less accurate the voltage preiction Tap position P(MW) an Q(MVars) 0 Time (s) Figure 3: Loa profiles at some selecte customers Total loa with respect to time Loa real power Loa reactive power 0 Time (s) Figure 4: Total feeer active an reactive loa profiles A set of loa ata for 100 secons was prouce for the test, imitating the nature of loa change, which is usually stochastic in time an location. Total feeer loa increases from 2 MVA up to 6 MVA to represent transition from light loae to heavily loae situation. To represent the stochastic nature of loas, 20% of the busses, which were selecte ranomly from the set of 68 busses, to vary at time t from their loa levels at time t-1. Loa variations were calculate by aing a certain amount of variation (ranomly up to 2.5% of the prior loa level) an a correction factor such that the general increasing tren of loa will be followe. Real an reactive power variations were inepenent from each other, thus, customer s power factor is not a constant value. The remaining 80% of customers maintain the same loa as at time t-1. The active an reactive loa profiles of four selecte customers are shown in Fig.3, which illustrate the non-uniformly loa characteristic of Tap ratio P (MW) an Q(MVars) Figure 5: Tap position in case of elaye an nonelaye DG to support feeer loa Real an reactive power of DG DG real power DG reactive power 0 Figure 6: DG injection power with non-elaye DG Fig.5 shows the tap position to control the voltage level, which remains the same for the elaye an instantaneous DG control cases. As the loa increases, the tap ratio also increases until it reaches its saturate state. In Figs.6 an 7, the real an reactive injection power from DG in case 1 an case 2, respectively are shown. The ratio of the DG real an reactive power, as can be observe from these figures, is always kept

7 constant at 1.78 for maximum voltage change effectiveness [14]. We can also see that because of the DG immeiate reaction, the generator in case 1 is running more compare to case 2. As the result, a better voltage profile can be expecte in case 1. The remote en voltage profiles without DG, with DG, an voltage preictions at two regulation points, for 2 cases, are illustrate graphically in Figs.8 an 9. The figures obviously inicate that smaller uner-voltage time is achieve with the non-elaye DG. By using the control scheme, the DG is turne on to provie extra support to network voltage only in two scenarios, when the tap has not yet reache its esire level an when the tap is saturate. Otherwise, the voltage is mostly regulate by the OLTC. The remote voltage with an without DG in two cases also reveal that DG has mae a consierable contribution to the control of system voltage Real an reactive power of DG DG real power DG reactive power the total operating cost of the system. As we can obviously be aware, the best control scheme nees to be carefully selecte in trae-off among the priorities. If it is very important to maintain the network voltage within the specification, a non-elaye DG will perform better. Otherwise, a DG with some time elay will be more suitable as an economic choice. Voltage (p.u.) Voltage at the remote en Actual remote en voltage Remote en voltage without DG Remote en voltage preiction at DG point Remote en voltage preiction at LTCT point P (MW) an Q(MVars) Figure 9: Remote en voltage with respect to time for the case of elaye DG Table 1: Comparison of two control systems Case 1 Case Figure 7: DG injection power with elaye DG Voltage at the remote en RMS error in voltage preiction at OLTC point (μp.u. 2 ) RMS error in voltage preiction at DG point (μp.u. 2 ) Total customer minute uner voltage (customer-minute) Voltage (p.u.) Actual remote en voltage Remote en voltage without DG Remote en voltage preiction at DG point Remote en voltage preiction at LTCT point 0.92 Figure 8: Remote en voltage with respect to time for the case of non-elaye DG Table 1 provies the summary of the results in the two cases. In case 1, the non-elaye characteristic of DG makes it working harer, thus provies a better voltage profile with less percentage of customers suffering from uner voltage problem compare to case 2. However, the running cost of this system is also more expensive. Moreover, in several situations, the control scheme in case 1 may cause the DG to turn on more frequently than that of case 2. To certain types of DG, this will also raise Customer minute uner voltage as percent of total time 2.088% 3.809% Total DG working (kwh) DG kwh as percent of total loa 1.569% 1.258% It is seen from Table 1 that the RMS error in case 1 is higher than case 2. This can be explaine by the fact that the reverse current flow from DG has an effect on the accuracy of voltage preiction. As DG is working more in the first case, its errors are also higher. Furthermore, the RMS errors of the LDC at OLTC are consierably larger than that at the DG, which is as expecte. As iscusse earlier, it is actually simpler to control the regulators by using their local voltages. However, this process may result in more expensive operation cost of the system. Another simulation has been carrie out to verify the choice of the LDC. Both OLTC an DG are set to be controlle by their local voltages. The reference voltages of two regulators have been ajuste such that the control scheme provies the same quality level to what we have archive using the LDCs (customer minutes uner-voltage as fraction of total time is

8 3.809%). The results show that the total DG working in this case is kwh, which is higher than the controller riven by the LDC (1.258 kwh). Even though this oes not seem to be a huge ifference, it is expecte that the LDC woul be much more beneficial if a longer run of the controller is examine. In aition, in reality, the lower uncertainty about the performance of the localvoltage controller usually results in a higher reserve margin i.e. high reference voltage setting. This means that the DG will work more often, as well as having a higher running cost. 6. CONCLUSIONS This paper has introuce a voltage control system for using a OLTC an a single DG. Decisions for these regulators control action have been mae by using a moifie LDC. This LDC manage to preict the voltage at a reasonable accuracy an without taking the risk of ineffective selection for the internal setting as in the case of conventional LDC. The propose LDC is therefore more flexible an precise, especially in the presence of DG. The protection system s principles, which are magnitue graing an time graing, applie in this control scheme have greatly improve its performance by many ways. Not only the capacity of the tap is maximise, but also the interaction level between controllers is minimise. Moreover, the control system has lessene the unnecessary operation of the tap an DG, thus result in a less expensive running cost. Besies, no communication is require to run this voltage controller. The test results reveal that the network voltage has been improve in a by the control system. The analyses have been provie to emonstrate the benefits of LDC rather than the local-voltage control. Also, the comparison of elaye an instant DG is able to help the control engineer in selecting the most suitable control system, to satisfy the utility an the customer s nee. 7. ACKNOWLEDGEMENTS The authors gratefully acknowlege the support an cooperation of Aurora Energy personnel in proviing ata an avice on the operation of istribution systems. REFERENCES [1] Intermittent Generation in the National Electricity Market by National Electricity Market Management Company Limite NEMMCO, Market Development, 18 March [2] A. Bonhomme, D. Cortinas, F. Boulanger, an J.-L. Fraisse, A New Voltage Control System to Facilitate the Connection of Disperse Generation to Distribution Networks, CIRED 16 th International Conference an Exhibition on Electricity Distribution, Part 1: Contributions, June 2001, Amsteram, Netherlans, Vol. 4. [3] R. O'Gorman, an M.A. Refern, Voltage Control Problems on Moern Distribution Systems, 2004 IEEE Power Engineering Society General Meeting, 6-10 June 2004, Vol.1, pp [4] E. Carpaneto, G. Chicco, M.De Donno, an R. Napoli, Voltage Controllability of Distribute Systems with Local Generation Sources, Bulk Power System Dynamics an Control VI, August 2004, Cortina Ampezzo, Italy, pp [5] M.H.J. Bollen, an A. Sannino, Voltage Control with Inverter-Base Distribute Generation, IEEE Transactions on Power Delivery, January 2005, Vol. 20, Issue 1, pp [6] C.M. Hir, H. Leite, N. Jenkins, an H. Li, Network Voltage Controller for Distribute Generation, IEE Proceeings on Generation, Transmission an Distribution, March 2004, Vol. 151, Issue 2, pp [7] S.K. Salman, an I.M. Ria, ANN-base AVC Relay for Voltage Control of Distribution Network with an without Embee Generation, International Conference on Electric Utility Deregulation an Restructuring an Power Technologies, 2000, 4-7 April 2000, pp [8] E.F. Mogos, an X. Guillau, A Voltage Regulation System for Distribute Generation, IEEE PES Power Systems Conference an Exposition, 2004, Oct. 2004, Vol.2, pp [9] F. Batrinu, E. Carpaneto, G. Chicco, M. De Donno, R. Napoli, R. Porumb, P. Postolache, an C. Toaer, New Neste Evolutionary Programming Approach for Voltage Control Optimization with Distribute Generation, Proceeings of the 12th IEEE Meiterranean Electrotechnical Conference, 2004 (MELECON 2004), May 2004, Vol. 3, pp [10] M. Thomson, Automatic-voltage-control Relays an Embee Generation, Part I, Power Engineering Journal, April 2000, Vol. 14, Issue 2, pp [11] T.E. Kim, an J.E. Kim, Voltage Regulation Coorination of Distribute Generation System in Distribution System, IEEE Power Engineering Society Summer Meeting, July 2001, Vol. 1, pp [12] L. Kojovic, Impact of DG on Voltage Regulatio n, IEEE Power Engineering Society Summer Meeting, 2002, Vol. 1, pp [13] Choi Joon-Ho, an Kim Jae-Chul, Avance Voltage Regulation Metho of Power Distribution Systems Interconnecte with Disperse Storage an Generation Systems, IEEE Transactions on Power Delivery, April 2001, Vol. 16, Issue 2, pp [14] An D.T. Le, M.A. Kashem, M. Negnevitsky an G. Lewich, Minimising Voltage Deviation in Distribution Feeers by Optimising Size an Location of Distribute Generation, AUPEC 2005 Conference, Tasmania, Australia, September, 2005, Vol. 2, pp.