UM FERI laboratorij za energetiko Jože VORŠIČ Kakovost električne energije

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1 Kakovost električne energije Kakovost oskrbe z električno energijo je temelj za gospodarski razvoj predvsem panog z veliko dodano vrednostjo in velikim deležem znanja. Primer za to so visoko avtomatizirani proizvodni industrijski procesi in informacijska podprta tehnologija. V vseh državah EU se zavedamo, da je nujno treba raven kakovosti oskrbe izboljševati oziroma ohranjati na ravni, ki omogoča tak razvoj. Kakovost oskrbe je v splošnem težko meriti, načeloma se meri s treh vidikov, ki so med seboj različni in na katere imajo vpliv različni dejavniki: a. kakovost storitve, ki jo opravlja sistemski operater, in se nanaša čas, ki ga operater potrebuje za nekatera značilna dejanja ali postopke v okviru svoje storitve. Merjena je s splošnimi in posameznimi kazalniki ali parametri in imenovana komercialna kakovost; b. neprekinjenost dobave, merjena s povprečnim trajanjem izpadov in s povprečno pogostostjo izpadov, imenovana zanesljivost napajanja; c. kakovost napetosti, ki se nanaša na odstopanja od idealne napetosti (sinusoide), merjena s pogostostjo in velikostjo različnih motenj (npr. napetostni upadi, flikerji).

2 POWER QUALITY -- THE ADDED VALUE The problem of Power Quality (PQ) cost in industrial customers is particularly important at present in the new scenery of competitive electricity markets. In fact, it represent the added value that the electric suppliers can provide to the users for improving their productivity and the reliability of their industrial process. The industry must compare the cost of the PQ services provided by the utilities with the cost derived from damages to the production output and/or to the plants. While this later is easy to define because is related to the physical replacement of some equipment, the cost of production losses is not simple to monetize. Regina Lamedica

3 Contents and reflections on power quality and energy market Power quality responsibility Economic regulation of network charge Quality parameters Measurements in the past Measurements nowadays Research Model of Slovenian power system

4 Electricity supply has nowadays become a part of every day life and is a service expected by electricity consumers. Their expectations rely on the availability of electricity whenever needed and on a safe and satisfactory operation of all connected electrical devices. In order to carry out economic regulation of network charge, the Energy Agency of the Republic of Slovenia (Slovenian regulator) monitors quality electricity supply, which is divided into: reliability (continuity) of supply, voltage quality and commercial quality. The Agency monitors the parameters of quality electricity supply with the intention of permanently increasing and maintaining its level, respectively.

5 The Council of European Energy Regulators (CEER) has divided quality of electricity supply to: commercial quality, dealing with service between the supplier or system operator and the consumer, continuity (reliability) of supply, dealing with the number and duration of interruptions spotted with consumers, quality of voltage which contains the technical characteristics of voltage measured in the receiving-transmitting point of the consumer.

6 Agency s main task is to prepare and issue the methodology for the calculation of network charge and methodology for setting out the criteria for defining eligible costs while taking into consideration the mechanisms of promotion, manifested in the field of investment, ascertaining technical losses of the system and maintenance. Together with these mechanisms which monitor cost efficiency of system operators, quality of electricity supply in case of consumers also needs to be monitored. A decrease of costs in a company can be most easily reached by means of decreasing costs in the field of maintenance and investment, which can also lead to a lower quality level.

7 Legal bases for quality of electricity supply The Energy Act (Official Gazette of the Republic of Slovenia No.: 26/05; EZ-UPB1) The Decree on the method for implementing public service obligation relating to the activity of distribution system operator The Decree on the method for implementing public service obligation relating to the activity of transmission system operator

8 Quality parameters and methodology of network charge setting Price cap the highest determined price of goods or service which encompasses the general price growth and the anticipated growth in efficiency of the undertaking (1 + CPI X ) (1 + Q ) n p q p q n m t+ 1 N i= 1 j= 1 ij ij m t O i= 1 j= 1 ij ij

9 Commercial quality Agency proposal Legislation CEER General standards Individual standards Time of reestablishing electricity supply in cases of unanticipated interruptions Time of minor work implementation (meter change, manufacture of a new lowvoltage connection) Time needed to connect a consumer to a system Time for replying to consumer s questions (not just a courteous reply) 85% of consumers in 3 hours; 100% in 24 hours In 20 working days 95% of work carried out 90% in 10 working days 16 days (8 + 8) (Article 5 of the Official Gazette 117/2002) 15 days (exceptionally a longer time limit, prescribed in the notification to the consumer, including the reasons) in 24 hours Time for reconnection after debt (Article 78 of the in 1 working day payment Official Gazette in 1 working day 26/2005, EZUPB1) In 24 hours Time for responding to a blown (Article 78 of the 6 hours fuse Official Gazette 3-4 hours 26/2005, EZ-PB1) Time of announced visitation Within 3 hours Within 4 hours Time of drawing up pro forma invoice in 10 working days Time of handling a claim regarding meters in 10 working days In 15 days Time of handling a claim regarding costs or payment in 10 working days In 15 days Time needed to activate connection in 8 working days in 2-5 working days

10 Reliability (continuity) SAIFI = N SAIFI (System Average Interruption Frequency Index) This index is designed to give information about the average frequency of sustained interruptions per costumer over a predefined area. Ni NT N i Number of interrupting customers served for each interruption event during reporting, Total number of customers served for the area being indexed. f ( t t, ) CS, = j N N T j

11 SAIDI ri N = N SAIDI (System Average Interruption Duration Index This index is commonly to as customer minutes of interruption or customer hours, and is designed to provide information about the average time the customer are interrupted. ri - Restoration time for each interruption event, Ni - Number of interrupting customers served for each interruption event during reporting, NT - Total number of customers served for the area being indexed. T i U ( t t, ) CS, = i j N T t ij

12 Voltage characteristics of electricity Power frequency Magnitude of the supply voltage Supply voltage variations Rapid voltage changes Supply voltage dips Short interruptions of the supply voltage Long interruptions of the supply voltage Temporary power frequency overvoltage between live conductors and earth Supply voltage unbalance Harmonic voltage Interharmonic voltage Mains signalling voltage on the supply voltage

13 The highest quality demands are set on the voltage level of the consumers. The rule of parallel connection of consumers to the same voltage applies when connecting consumers. Many consumers are very sensitive to oscillation or deviation of voltage, which logically calls for the strictest rules in this area. Therefore, the SIST EN standard has been applied, which determines what characteristics of low supply voltage need to be observed. When electricity supply is reliable enough, we need to ensure to the consumers power quality also in those parameters which are seemingly invisible. By that we mean especially voltage form (voltage dips, rapid changes and harmonics). Such disturbance can appear on the voltage level of consumers or it can be a consequence of lightning or switching surges on higher voltage levels. Disturbance transiting among voltage levels equals disturbance crossing a transformer. With the remuneration of quality supply manager it is essential to pinpoint the place of disturbance.

14 Rules for recording power quality Recording rules establish an obligation for all (or at least all the major) companies to register continuity data. Secondly, they indicate which data are required for a correct identification of an incident. Usually these include the time interval of the supply interruption, its cause, the network device where it originated, the affected installations and the number of consumers involved.

15 In the past we had only one utility and it was responsible to the government for the quality of electricity supply. Frequency at the moment of connection of Yugoslav power system to UCPTE (16. September 1974)

16 Measurements in the past Project quality of electrical energy (June 1972) (Honneywell s 10 channels loop oscilograph mirror galvanometer)

17 Measurements in the past Harmonics and influence of power system growing elektroliza A prof. Pehani meritev z resonančnim V-metrom elektrolizi A in B meritev s HP elektrolizi A in B meritev s HP THD = 3,464 THD = 1,889

18 Measurements in the past Harmonics and influence of power system growing elektrolize A, B in ½ C meritev s Tectro elektrolizi B in ½ C meritev s Tectro THD = 0,842 THD = 0,70

19 Measurements in the past Place Flicker Pst,95 Pst,99 Plt,95 Plt,99 Dravograd 2,95 3,21 2,57 2,74 Kidričevo 0,37 0,44 0,31 0,33 Pekre 0,62 0,80 0,59 0,66 Hudo 0,47 0,58 0,41 0,43 Kleče 0,59 0,71 0,51 0,58 Okroglo 1,22 1,70 1,00 1,04 Selce 0,14 0,19 0,17 0,22 Nova Gorica 0,24 0,29 0,22 0,37 Ravne železarna 3,58 4,82 3,22 3,70

20 Measurements nowadays Monitoring of all voltage characteristics at each nod connecting transmission and distribution network Monitoring recommended (by decree) at each substation and wished at each customer

21 The telegraph equation u u u u u u 2 u i 2 2 = Z = Y Z u = R G + ( R C + G L) p + L C p u = γ u 2 x x 2 i i 2 2 = Y = Z Y i = R G + 2 ( R C + G L) p + L C p i = γ i x x

22 Computer program To calculate transient states in individual systems and in the electricity system as a whole, software such as ATP, EMTP, PSCAD and Matlab with the Powersys module is at disposal. All mentioned software has similar characteristics since it is based on the Dommel approach and it uses similar mathematical data which then causes similar problems. Out of major software from the mentioned field the Siemens Netomac needs to be mentioned but it is not directly related to the above software. Matlab with its Powersys module has been selected as the software for modelling the electricity system of Slovenia due to its user-friendliness and possibility of fixed calculations. Since the dynamic model of the entire electricity system of Slovenia surpasses the capacities of available software and hardware, we were forced to use simplified models of individual elements.

23 Model evaluation The proposed simplified dynamic model of Slovene power system was confirmed in two ways. The steady state accuracy of the dynamic model was confirmed through the comparison of calculated results with those obtained by the professional program for steady state analysis and network optimization NEPLAN. The agreement of steady state voltages during normal operating conditions is very good, while the steady state voltages during the three-phase faults can differ up to 4 % in 0,4 kv network. The dynamic response of the model was partially confirmed by the comparison of calculated results with the results of field testing performed on 20 kv in the substation Rogaška Slatina.

24 Steady state evaluation Comparison of voltage drops calculated by NEPLAN and by dynamic model on 110, 20 and 0,4 kv busbars in substation Cerkno caused by a three-phase short circuit in the middle of 110 kv overhead line Divača Ajdovščina. Cerkno Dynamic model NEPLAN Differences [%] 110 kv 14,6 kv 14,54 kv 0,41 20 kv 2,27 kv 2,27 kv 0,0 0,4 kv 57 kv 55 kv 3,63

25 Field tests at Rogaška Slatina dynamic evaluation Voltages and currents in L3 at 3phase short-circuit on 20 kv level in Rog. Slatina Del EES Slovenije v Matlab/Simulink: Model in meritev napetosti (L3) na 20 kv: 3 % Model in meritev toka (L3) na 20 kv: Model in meritev napetosti (L3) na 110 kv: 4,5 % 1,1 % Model in meritev napetosti (L3) na 0.4 kv: Model in meritev toka (L3) na 110 kv: 4 % 5 %

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28 Model of Slovenian power system

29 Results The proposed simplified dynamic model of Slovene power system was applied to evaluate the impact of different events on power quality in 0.4 kv distribution networks. Simulations were performed for the following list of events: a three-phase short circuit on a 110 kv busbars in the substation Kleče, a switch-off of a 300 MVA power transformer in the substation Okroglo, a three-phase short circuit on a 20 kv busbar in the substation Rogaška Slatina, switch-on in a pumping regime of pump-turbine plant Kozjak and switch-on in a pumping regime of pump-turbine plant Avče.

30 The impact of these events on the power quality in 0.4 kv distribution networks is in this work analyzed in the following points: 0.4 kv busbars in the substation Cerkno, 0.4 kv busbars in the substation Škofja Loka, 0.4 kv busbars in the substation Šentjur and 0.4 kv busbars in the substation Rače.

31 Voltages on 0.4 kv busbar in substations a) Cerkno, b) Škofja Loka, c) Šentjur and d) Rače, during a three-phase short circuit on 110 kv busbar in substation Kleče. u 0.4 kv u 0.4 kv a) b) u 0.4 kv c) u 0.4 kv d) a) b) 250 c) d)

32 Voltages on 0.4 kv busbar in substations a) Cerkno, b) Škofja Loka, c) Šentjur and d) Rače, during switch-off of 300 MVA power transformer (400 kv / 110 kv) in substation Okroglo a) b) c) d)

33 Voltages on 0.4 kv busbar in substations a) Cerkno, b) Škofja Loka, c) Šentjur and d) Rače, for the case of a three-phase short circuit on 20 kv busbar in the substation Rogaška Slatina a) b) c) d)

34 Voltages on 0.4 kv busbar in substations a) Cerkno, b) Škofja Loka, c) Šentjur and d) Rače, calculated for pumping regime switch-on of pump-turbine plant Kozjak a) b) c) d)

35 Voltages on 0.4 kv busbar in substations a) Cerkno, b) Škofja Loka, c) Šentjur and d) Rače, calculated for pumping regime switch-on of pump-turbine plant Avče. 235 a) b) c) d)

36 Voltage dips are normally caused by faults in consumers systems or in the public distribution system. They can also be a result of incidents in highvoltage parts of the system. These are unpredictable, mostly accidental incidents. Annual frequency of voltage drops largely depends on the type of a system and the place of observation. Apart from this, the annual distribution of voltage dips can be fairly irregular. They can be caught by means of constant monitoring of electricity quality or an intentionally provoked incident. A far more elegant approach is the modelling approach. In order to carry out economic regulation of network charge, the Energy Agency of the Republic of Slovenia (Slovenian regulator) monitors quality electricity supply, which is divided into: reliability (continuity) of supply, voltage quality and commercial quality. The Agency monitors the parameters of quality electricity supply with the intention of permanently increasing and maintaining its level, respectively. As a regulator it is also the arbitrator in cases of disputes among the system managers. The verdict is based on a thorough analysis of past events, performed only by means of a (computer) model.