C4-107 VOLTAGE DIPS AND SHORT INTERRUPTIONS DIFFERENT STRATEGIES IN CONTRACT FOR THE ELECTRIC POWER SUPPLY

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1 2, rue d'artois, F-758 Paris C4-7 Session 24 CIGRÉ VOLTAGE DIPS AND SHORT INTERRUPTIONS DIFFERENT STRATEGIES IN CONTRACT FOR THE ELECTRIC POWER SUPPLY Grzegorz MATUSZ, Tomasz MAJ, Zbigniew HANZELKA *, Wladyslaw LOZIAK AGH University of Science and Technology (Poland). INTRODUCTION The provisions related to the quality of supply are presently being incorporated into contracts for the electric power supply in Poland. It is important that their formulation will not create a privileged position for any party of the electric power sales-buying transaction - the supplier or the consumer. For the purpose of this task, during seven months have been carried out measurements in one of distribution companies in the south of Poland - the PDC Cracow. The measurements were carried out in three measuring points at the coupling of the transmission (22 kv) and distribution system ( kv). In effect of these measurements has been created the database which contains recorded events in voltage: dips, swells and interruptions. These data were used for the analysis of supply conditions in the considered points of power delivery. The following effects have been investigated: selection of various factors which describe the voltage dip, selection of the dip threshold voltage to which the measuring thresholds are referred, selection of threshold values which define the disturbance and methods for aggregation of the measurement results. The considerations are restricted to single-event and single-site indices only. 2. THE MEASURING SYSTEM Fig.. Block diagram of the PDC Cracow distribution system connections to the transmission system The PDC operated kv distribution system is connected with neighbouring operators' distribution systems and with the 22 kv transmission system by means of three autotransformers, 6 MVA each, installed at three substations (a) LUBOCZA industrial and household customers, (b) WANDA - predominantly industrial customers, (c) SKAWINA near to heat and power generating plant - Fig.. At these points have been installed instruments for the power quality factors measurement. The measuring instruments were provided with modems, which enabled transmission of recorded data via public telecommunications network and their acquisition in the main computer, located at the AGH - UST. * hanzel@uci.agh.edu.pl

2 3. RECORDED VOLTAGE DIPS AND SHORT SUPPLY INTERRUPTIONS 3.. Aggregation methods Tables -3 give the summary of recorded voltage dips and short interruptions. Most of them occurred within a short time interval. Each disturbance is treated separately. Three-phase or two-phase events are counted as three or two independent disturbances, respectively. Table. Voltage dips without aggregation (WANDA) - L/L2/L3 Dips [%] - ms - 5ms5 m s- s - 3s3-2s2 - min - 5 /3/ // // // // // 5-3 //4 2/3/3 // // // // 3-6 //3 // // // // // 6-9 // // // // // // 9 - /2/3 // // //2 2//3 // Number of recorded voltage dips: 44 -minute aggregation the number of recorded events: 25 3-minute aggregation - the number of recorded events: 23 Phase aggregation - the number of recorded events: 38 Table 2. Voltage dips without aggregation (SKAWINA) - L/L2/L3 Dips [%] - ms - 5ms 5ms - s - 3s 3-2s - 2s min - 5 2/3/2 // // // // // 5-3 // 2/4/2 // // // // 3-6 // // // // // // 6-9 // // // // // // 9 - // // // // // // Number of recorded events: 9 - and 3 minute aggregation - the number of recorded events: 5 Phase aggregation - the number of recorded events: 3 Table 3. Voltage dips without aggregation (LUBOCZA) - L/L2/L3 Dips [%] - ms - 5ms5ms - s - 3s3-2s2s - min - 5 //3 2/2/4 // // // // 5-3 // 3/2/ // // // // 3-6 //2 //2 // // // // 6-9 //4 //7 // //7 // //5 9 - // // // // // // Number of recorded events: 73 -minute aggregation - the number of recorded events: 46 3-minute aggregation - the number of recorded events: 44 Phase aggregation - the number of recorded events: 7 Figure 2 summarizes the results of the influence of different types of aggregation on the number of voltage dips in given measuring points. It can be seen that the use of phase and time (- and 3-minute) aggregation significantly reduces the number of disturbances. This does not concern the LUBOCZA substation, where a particularly large number of disturbances were recorded in a single phase (L3). Table 4. Number of dips depending on the selected threshold value (% kv) Threshold SKAWINA WANDA LUBOCZA Dependence of the number of dips on the selected threshold value, which defines the disturbance is presented in table 4 and in Figure 3. For each location can be seen the reduction of the number of disturbances with an increase in the threshold voltage. Figure 4 shows the diagram of the network supplying WANDA and LUBOCZA substations. If a disturbance occurs in the point, it will be also transferred to points 2 and 3. In this case the use of place aggregation can be taken into consideration, that is, voltage dips occurring simultaneously on different lines supplying the same user can be aggregated. 2

3 comparison of aggregation methods number of dips no aggregation phase aggregation -minute timeaggregation aggregation method used Skawina Lubocza Wanda 3-minute timeaggregation 3-minute timeaggregation and phase aggregation number of dips Skawina Wanda Lubocza Substations 9% 8% 7% 6% Fig. 2. Dependence of the number of dips on the selected method of aggregation GPZ Wanda 22kV kv kv 2 3 GPZ Lubocza Fig. 4. Diagram of the network supplying the measuring points WANDA and LUBOCZA Fig. 3. Dependence of the number of dips on the selected threshold value, which defines the disturbance The sum of aggregated dips for the substations WANDA and LUBOCZA after phase and time (3-minute) aggregation is: = 57. Assuming the place aggregation for these two substations, the total number of contractual disturbances will be reduced to 44, for the 3 simultaneous dips have been recorded on these substations. Number of dips recorded in phase-to-phase voltages: WANDA 32; SKAWINA - 4; LUBOCZA Reference voltages for voltage dips recording The residual voltage is normally expressed in percent or per unit values, with respect to the nominal or declared voltage at the considered point of a system. On HV systems, it is more advantageous to measure voltage dips with respect to the voltage U X, preceding the disturbance, determined in a continuous manner in the assumed measuring window. The measurements were performed at the assumed, constant reference voltage kv. Only those disturbances where the voltage decreased below 9% of kv were recorded. It was also checked whether these disturbances might have been recorded if the declared voltage (2 kv), or the voltage determined in the last measuring window, would have been used for the reference voltage. That s why the number of dips for the reference voltage 2 kv and that for the voltage preceding the disturbance, are different; they are denoted with asterisk (*). Table 5. The influence of various reference voltages on the number of dips (WANDA/LUBOCZA/SKAWINA) Reference % of the nominal voltage kv voltage [kv] k 44/74/9 35/59/6 24/45/ 2/42/ 2 44*/74*/9* 4/67/7 3/54/4 2/42/ U X 44*/74*/9* 4/64/2 29/5/4 2/42/ With the lower thresholds defining a dip (8 and 7 % U N ), the difference in numbers of recorded dips can be noticed. No reference voltage less than kv, measured immediately before a dip, has been recorded over the entire period of measurements 4. VOLTAGE DIPS INDICES (SELECTED) 4.. Voltage-sag energy [2, 3] In the event the residual voltage (U) and duration (T) of a dip are known, and assuming that the r.m.s. voltage value during the dip is constant, the disturbance can be described using the relation (), where 3

4 U ref is the reference voltage. For multi-channel events the voltage-sag energy is defined as the sum of the voltage-sag energy in the individual channels: EUS = ( EUS L + EUS L2 + EUS L3 ). Eexample values of indices for the substation WANDA are shown in Figure wartość wskaźnika Evs 3 faz [s numer zaburzenia Fig. 5. Example sag energies for the three-phase system WANDA E US 2 U = T () U ref The values of energy indices are varying within a broad range: from several thousandth to over thirty. This kind of index may be dominated by long-duration dips: a single long disturbance can be equivalent to many short-duration dips. For description of the measuring point in the assumed time-interval have been proposed the so-called Sag Energy Index (SEI), that is the sum voltage-sag energies of all qualified disturbances at the given measuring point and the given time-interval n: determined over a one-month or one-year interval. ASEI = n Table 5. Summary of sag energy indices Indices WANDA LUBOCZA SKAWINA SEI n i= E US SEI = n i= E US i. I These indices are usually The Average Sag Energy Index (ASEI) is the average of voltage sag energies for all qualified events measured at a given site during a given period - Table 5. The value of the average sag energy index indicates how deep and long voltage dips most frequently occur at a given point of the network. The ASEI is dependent on the triggering of the monitor. A sensitive setting will result in a large number of shallow events (with a low sag energy) and this in a lower value for ASEI. The SEI on the other hand will increase for sensitive setting of the monitor. To compare results from site to site and from one period to another, a standardized trigger setting needs to be defined. Most of all long-duration and deep voltage dips recorded at the substation LUBOCZA, hence the ASEI value is so high. The least number of relatively shallow dips have been at the substation SKAWINA, this fact manifests itself in the value of the index The EPRI indices The EPRI indices are summarized in Table 6; their values for the considered substations and for the entire period if measurement are given in Table 7. The value of voltage dips weighting coefficient has been assumed unity for all locations. Table 6. The comparison of the indices EPRI.5 cycle.5 cycle.5 s 3 s 3 s 6 s 6 s.5 s < 9 % SARFI 9 SIARFI 9 SMARFI 9 STARFI 9 < 8 % SARFI 8 SIARFI 8 SMARFI 8 STARFI 8 < 7 % SARFI 7 SIARFI 7 SMARFI 7 STARFI 7 < 5 % SARFI 5 SIARFI 5 SMARFI 5 STARFI 5 < % SARFI SMARFI STARFI Table 7. Aggregated indices of voltage dips acc. to Table 6 (LUBOCZA/ SKAWINA/ WANDA) Dips.5 cycle.5 cycle.5 s 3 s 6 6 s.5 s 3 s s < 9 % 74/9/44 5/7/33 7/2/4 7//7 < 8 % 59/6/35 35/4/24 7/2/4 7//7 < 7 % 45/6/24 2/4/4 7/2/3 7//7 < 5 % 4/6/8 7/4/9 7/2/2 7//7 < % 3//4 2//7 //7 4

5 Assuming different voltage dips weighting coefficient (e.g. depending on the number of users, their powers, the importance of loads at the given point, etc.) these coefficient can be assigned an additional meaning, which is significant in terms of a contract The index S e referred to the reference characteristic [3] The index (S e ), defined by the relation (2), is determined from the known: the voltage p.u. value and duration of disturbance where: U residual voltage during the voltage dip of duration d; U ref (d) - the voltage dip magnitude on the reference characteristic (e.g. CBMEA, ITIC, SEMI) for the disturbance duration d. For the disturbances on the curve, the index assumes the value Fig. 6. For the disturbances above the characteristic, the index value is less than, below - greater than. For the disturbances of the voltage value (per unit) equal, the index assumes the value zero. The longer is the duration of a disturbance and smaller the residual voltage, the greater is the index value. The values of coefficients in Table 8 represent the numerical description of the situation at given substations. Figure 7 shows example values of the coefficient S e, determined for the substation WANDA. The values of index are represented on vertical axes, on horizontal axes are ordinal number of subsequent disturbances. 2 wartość współczynnika Se numer porządkowy zaburzenia Fig. 6. The index of the voltage dip severity, referred to the reference characteristic (solid line) for disturbances of various duration and residual voltage [2] Table 8. Total value of index S e GPZ LUBOCZA SKAWINA WANDA TOTAL Σ S e S Fig. 7 The values of the coefficient S e at the substation WANDA e U = U (d) ref (2) 4.4. Sag Score [4] The sag score is determined on the basis of so-called qualified dips. The threshold of dip recording is 75% of the nominal voltage and is aggregated over 5-minute period. The sag score calculated from the formula: sag score = (U A + U B + U C )/3 where: U A, U B, U C p.u. value of respective phase voltages during the disturbance. The following values of sag score target (total of the all sag scores for the given measuring point compared with the value set forth in a contract) have been obtained: WANDA - sag score target: (8 qualified dips); LUBOCZA - sag score target: (22 qualified dips); SKAWINA: - sag score target: ( qualified dip). On the basis of the sag score value, which varies over the range.833 to, it could be only possible to estimate very roughly how dangerous a given voltage dip was. The range of aggregation time is too long for actual assessment of a given voltage dip: for instance, the same value can be obtained for a dip with duration of several tens milliseconds and a dip with duration of several seconds, or at least, several dips within a specified interval of time. 5

6 4.5. Weighting coefficients [7,, 4] Tables 9 and present the conception of equivalent, weighted voltage dips (calculated on annual basis), which are the basis for determination of mutual financial commitments of the supplier and consumer of electric power. The actual, aggregated voltage dips index, determined on this basis, (total of weighted dips acc. to the tables) can be subtracted from the value set forth in the contract and the difference is multiplied by the agreed compensation rate. Table 9. The concept of the voltage dips weighting coefficients (No ) Dips Weight SKA. LUB. WAN [%] coeff.. / * 4/4 6/6 <5% / 38/38 3/ / 4/2.8 5/ /4 7/6.8 6/ /.9 /. 4/.4 TOTAL * number of dips /weighted dips Table. The concept of weighting coefficients (No 2) Voltage dip Duration magnitude ms- -3s 3-2s 2-6s [%U n ] ms 5ms s Aver Total of weighted dips SKAWINA:.957 LUBOCZA: WANDA: Tabele ESKOM [8, 9, 4] Table. Classification of voltage dips according to the ESKOM table Number of dips per year Voltage Voltage dip window class magnitude Z T S X Y kv 8//8* 5//9 3/2/8 7/4/7 5/3/9 * LUBOCZA/SKAWINA/WANDA The threshold value of a voltage dip is.9 of the declared voltage. The depth of a voltage dip equals the maximum voltage change during the disturbance, and its duration is the maximum time in the most disturbed phase. Voltage dips are graphically represented in the co-ordinate system: the residual voltage value vs. dip duration Table. The limitation of a voltage dip duration to 3 seconds (according to the original ESKOM proposal) has significantly influenced the number of dips. Additional voltage dip windows, which are essential for the parties to the contract, can be included into contractual provisions Characteristics of voltage dip severity The useful tool for comparing measuring points in terms of voltage dips are IEEE severity characteristics. They are created in the co-ordinate system, where the severity indices (SI), determined as the product of the depth and duration of a voltage dip, are indicated on the vertical axis. The horizontal axis gives information on how many of the recorded disturbances has the magnitude, which exceeds the severity indices being assumed as threshold values, given as an example in Table 2. These characteristics allow for fast interpretation of results of voltage dips recording. 5. CONCLUSIONS The paper presents several selected strategies for formulating the issue of voltage dips and short supply interruptions in a contract for the electric power supply. A database, obtained as a result of seven month monitoring of power quality factors at the metering points at the coupling of transmission and distribution system, has been used for illustration of the procedures proposed in bibliography. In the authors opinion, a large part of the proposed procedures of contractual assessment of voltage dips is weakly founded, excessively complicated, and their practical usefulness has not been recognized yet. Sometimes it is proposed to categorize the voltage dips in transmission systems, taking into consideration only their magnitude (residual voltage), assuming that in most cases the disturbance duration is constant, and results from the settings of short-circuit protection times. The obtained results of measurements do not confirm this thesis. 6

7 Table 2. The number of events exceeding the assumed threshold of severity index (SI) SKAWINA LUBOCZA WANDA Severity Index Number of events Severity Index Number of events Severity Index Number of events wartości indeksu uciążliwości (SI),, Liczba zapadów powyżej SI Lubocza Wanda Skawna Fig. 8. Comparison of severity indices (SI) at individual substations 6. ACKNOWLEDGMENTS The authors would like to thank the PDC Cracow for giving their consent to the measurements and for help in carrying out the measurements, the LEM Company and its representative in Poland SEMICON for technical advice and making the measuring instruments available free of charge, and EPRI for encouraging the experiment and making the software available free of charge. 7. BIBLIOGRAPHY. Bollen M.H.J.: Understanding power quality problems voltage sags and interruptions. IEEE Press Series on Power Engineering Bollen M.H.J.: On going standard work on statistical presentation of voltage dips. (material obtained from CIGRE). 3. G Beaulieu, MHJ Bolen,R Koch, S Malgarotti, X Mamao, J Sinclair. Power Quality Indices and objectives. CIGRE WG36.O7/CIRED Progress Paper for the CIRED Conference in May Brooks D.L., Gunther E.W., Sundaram A.: Recommendations for tabulating rms variation disturbances with specific reference to utility power contracts. CIGRE 36.5/CIRED 2 CC Dettlof A., Sabin D.: Power quality performance component of the special manufacturing contracts between power provider and customer /IEEE. 6. Gosbel V.J., Robinson D., Petra S.: The analysis of utility voltage sag data. 7. McGranaghan M., Gunther E.: The economics of custom power. Power Quality and EMC in Power Quality. Colloquium of Czech National Committee and 36 Study Committee of CIGRE, Dep. of Power Eng., CTU Prague, Sep. 28, 2, Prague. 8. NRS 48-2: 996 Electricity supply quality of supply. Part 2: Minimum standards. ISBN Published in Republic of South Africa by the South African Bureau of Standards. 9. NRS 48-4:999 Electricity supply quality of supply. Part 4: Application guidelines for utilities. Published in Republic of South Africa by the South African Bureau of Standards.. Robert A., Hoeffelman J., De Jaeger E.: CIRED 2 Session 2: Power Quality & EMC, Presentations and Contributions for Thursday 2 June 2 (material obtained from the authors).. Transmission Power Quality Benchmarking Methodology. EPRI Final Report, December Voltage sag indices draft 4, working document for IEEE P564, August Voltage sag indices draft 2, working document for IEEE P564, November Understanding premium power grades. EPRI Final Report, Nov. 2. 7