A REVIEW OF THE NEW AUSTRALIAN HARMONICS STANDARD AS/NZS

Transcription

1 A REVIEW OF THE NEW AUSTRALIAN HARMONICS STANDARD AS/NZS Abstract V. J. Gosbell 1, P. Muttik 2 and D.K. Geddey 3 1 University of Wollongong, 2 Alstom, 3 Transgrid v.gosbell@uow.edu.au Harmonics due to large installations in Australia will be covered at te end of 1999 by a new standard AS/NZS "Limits - Assessment of emission limits for distorting loads in MV and HV power systems" replacing AS wic as been in force for twenty years. Tis new standard is largely based on an IEC standard and proposes a quite different approac to te connection of armonic loads, but tis requires a greater attention to detail wit a consequent increase in complexity. Te paper will discuss limitations of te old standard and ow tese ave been overcome. Te key steps in te new standard will be outlined and some example applications given in bot MV and HV power systems. 1. INTRODUCTION Te main power quality standard in force in Australia for te past twenty years as been AS2279 "Disturbances in mains supply networks" [1]. It was last revised in 1991 and appears in four parts, 1 and 2 concerned wit armonics and 3 and 4 concerned wit fluctuations and flicker. Part 1 is in effect a product standard for ouseold appliances rated at not more tan 4.8 kva wile part 2 concerns large industrial installations, were te point of common coupling migt appear in a MV or HV power systems. Part 2 "Limitations of armonics caused by industrial equipment" is due for replacement at te end of 1999 by AS/NZS , largely based on an IEC standard wit an identical number. Te scope of bot tese standards is to give guidance in determining te maximum size of distorting electrical equipment or converter tat may be supplied from ig or low voltage, especially wen te rating is more tan 4.8 kva or te voltage at te point of common coupling is greater tan 240/415 V. Te point of common coupling or pcc is defined as te nearest point in power system to wic anoter consumer migt be connected. It sould be noted tat te use of te terms "ig voltage" and "low voltage" in tis standard is to be replaced by te IEC terms wic ave te following meanings for line-to-line values V ll : Low voltage: V ll <= 1 kv Medium voltage: 1 kv < V ll <= 35 kv Hig voltage: 35 kv < V l l <= 230 kv Extra ig voltage 230 kv < V ll AS gives armonic voltage limits wic are smaller at iger voltage levels. Tis is meant to assure a reasonable clean system at te next lower voltage level to take up te distortion due to loads connected tere. Te limits are given for odd and even voltages over te range 2nd-40t armonic and for te total armonic distortion. Lower limits are given for te even armonics to minimise te cance of maloperation of rectifying equipment as tis may lead to dc being drawn from te power system. Standards need to be a compromise between a full armonic study giving accurate evaluations at te cost of muc time and simplified approaces wic give quick answers wit poor accuracy. AS allows most cases to be quickly evaluated by aving assessment in tree stages. Stage 1 is a conservative but quick assessment based on te ratio of converter rating to te fault level at te pcc. Stage 2 allows iger ratings if te existing or background armonics ave been measured. Tree possible conditions arise (i) If te background is less tan 25% of te standard levels, Stage 2 allows converter size depending on fault level and converter type. (ii) If te background is between 25% and 75% of te standard levels, converter ratings of alf tat given for (i) are allowed. (iii) For iger background levels, connection must be assessed under Stage 3, tat is a full armonic investigation. Tis standard was one of te first national armonic standards to be developed. Altoug adequate wen first developed, a number of deficiencies ave become apparent in recent times. (i) It gives no account of time variation. Normal background variation is as muc as 3:1 over a day in a distribution system. Converter armonics can vary widely wen used as part of a cyclic load suc as a minewinder. Tis was not important initially because most converters assessed were major cause of armonics in te neigbourood and it could be argued tat te maximum armonics produced ougt to meet te

2 standard limits, providing tey lasted for a minimum time of some seconds. (ii) Te Stage 2 approac gives te largest armonic allowance to te first converter to be connected in te neigbourood. Te allowance for subsequent converters of te same rating becomes less as te background level rises. Again tis problem was not too significant at a time wen converters were generally of large rating and connected sparsely in te system. It is inappropriate for te modern situation of many small converters of modest ratings. (iii) Relatively large converters, especially tose connected to te transmission and subtransmission system, are not andled by te standard in Stages 1-2. As a result an industry rule as developed giving an allowance of 1/3 te voltage limit to suc converters. Tis rule as not proved adequate in all cases. (iv) Te crude division of te background into tree classes gave a poor treatment of installations wic were close to te boundary. (v) It was assumed in te Stage 2 formulation tat converters ad a current armonic spectrum wic fell off as 1/n. For example 6-pulse converters ad 20% 5t armonic current, 14% 7t armonic current and so on. Tis applies to inductively filtered rectifiers suc as te dc drives wic were common twenty years ago, but is inaccurate for ac drives wic are now proliferating and wic are capacitively filtered. Studies ave sown tat te 5t armonic current can be as muc as 80% of te rated current [2] so tat assessment under AS can lead to armonic overvoltages four times larger tan expected. Te paper will discuss ow te replacement standard AS/NZS [3] overcomes tese problems. Te standard is more compreensive but at te expense of complexity and te use of new concepts wic will be explained. Some examples will be given to sow ow te standard can be applied to typical problems. Finally some aspects of te standards wic are not clearly described and were tere is scope for uncertainly will be described. 2. THEORETICAL BACKGROUND TO THE NEW STANDARD 2.1 Compatibility levels Te new standard accounts for time variation by limiting a statistic associated wit te time variation of te armonic rater tan just te maximum value. Te measure cosen in general is te 95% probability level, tat is te value wic is exceeded for only 5% of te time. Figure 1 sows and ideal distribution of system disturbances over many points in te power system and taken over many days. In principle, disturbances can occur at very ig levels altoug wit low probability. Equipment immunities ave a similarly saped distribution. Compatibility is acieved if te two distributions are sufficiently separated tat teir overlap occurs at te distribution tails. Te compatibility level is te key limit given in te standard and is defined as tat level wic (i) is rarely encountered in te system, (ii) is lower tan te immunity level of most connected equipment. probability density system disturbance posible Planning levels at different points in te system Compatibility level equipment immunity disturbance level possible equipment Immunity levels at different points in te system Figure 1. Relationsip between compatibility level and te distributions of disturbances and immunities (adapted from IEC Figure 1) 2.2 Planning levels Te utility as to acieve a distribution of disturbances lying below te compatibility level. It does so by coosing a value less tan te compatibility level, called te planning level, limiting all disturbances to tis value. Planning levels may differ in different parts of te power system for example tey may be allowed to be larger in rural areas were tere is less equipment tat may be affected. Te standard suggest typical values of planning levels as summarised in Table I. Compared wit AS , a key feature of te new limits is tat tey are larger for low order armonics and fall off wit increasing frequency to smaller values. Te even armonics are limited to less tan alf te nearby odd armonics. A new class of triplen armonics as been introduced wit limits between te nearby odd and even armonics. Te ig voltage limits are still about alf te medium voltage values.

3 2.3 Assessment of disturbance and emission levels Te main steps in assessing te acceptability of a utility's armonic voltage levels are as follows: (i) Measure te armonic voltage continuously over a period of at least a week (te details of te measurement instrument are given in [4]). (ii) Determine te 95% cumulative probability value, tat is te value wic is not exceeded for 95% of te time, and confirm tat it is not greater tan te planning level. Since consumers may ave several converters, te standard limits te total armonic of te consumer installation rater tan te current drawn by a particular converter. Te acceptability of a consumer's installation is assessed by comparing current armonics against an emission standard calculated as will be described later. Time variation is accounted for by a similar approac to tat used in assessing a utility's disturbance levels. 2.4 Notcing Table I Suggested planning levels Odd armonics nontriplen Triplen armonics Even armonics Order Harmonic voltage % Order Harmonic voltage % Order Harmonic voltage % LV/ HV LV/ HV LV/ HV MV MV MV THD MV: 6.5%, HV: 3% Tere is no explicit limit on notcing. It is possible to sow tat te fall of voltage limit wit increasing armonic order provides an indirect limit on noting. Notces more tan 5 wide are limited to a dept of about 20% as in AS For narrow notces, te standard limits te notc area. Tis is a sensible approac as te difficulty in filtering a narrow notc is proportional to te notc area. 2.5 Summation laws for combining armonics Te combined effect of armonic voltages from several different sources is not simply te aritmetical sum of te individual voltages. Te voltages are 95% probability levels of distributions wic are unlikely to be identical and tere will be pase differences between te voltages wic can become significant at iger frequencies. Two summation laws are proposed in te standard (i) First summation law: makes use of diversity factors, similar to AS , but taking more factors into account. (ii) Second summation law: tis is a power law recommended as being more general and will be described ere in more detail. Te net effect U of voltages U 1,... U n is α = α U i i U (1) were α is selected as sown in Table II. Table II Summation exponents Harmonic order α < > Harmonic impedance If tere is no resonance below te 13t armonic, it is suggested tat Z = Z sc (2) Were resonances exist and te network is known in detail, te armonic impedance can be calculated by well-known metods suc as tose given in Appendix B of te standard. Were te network is not known in detail, a worse-case impedance curve approac is recommended suc as given in Appendix I. For te particular case of a typical 11 kv system, tis simplifies to te approac given in Appendix A, ie for 8, use twice te value of eqn(2), oterwise use tis equation uncanged. 2.7 Consumer's sare of emission Central to te implementation of te standard is te concept of agreed power S i, a number wic describes te size of a particular consumer load. Altoug not defined clearly in te standard, it is believed tat it could be measured by te maximum alf our demand. A premise of te new standard is tat every consumer wit te same point of common coupling and aving te same agreed power as te rigt to draw equal distorted current from te supply. Suppose tat after allowing for background armonics, at a particular supply point tere is a voltage G available to be distributed to all connected and potential future consumers. A strategy is needed to allocate a portion of G to eac consumer to satisfy te equal rigts criterion and to take up G fully wen te supply is fully loaded. Define te available power S t as maximum possible sum of te consumer agreed powers. S t = S S n (3)

4 If te second summation law is assumed, a suitable strategy is 1 / α Si G i = G S (4) t Tis can be verified by adding G α i due to every consumer and making use of eqn(3). Eqn(4) gives a larger proportion to eac consumer tan if te allocation was simply proportional to S i /S t. In particular, smaller consumers get a considerably greater sare. For example, a customer taking 1% of te supply capability is allocated 3.7% of te available 5t armonic voltage (α = 1.4) and 10% of te available 11t armonic voltage (α = 2). 3. EMISSION LIMITS IN MV SYSTEMS 3.1 Overview Assessment is made in tree stages, as wit AS Stage 1 as tree tests, any one of wic can give acceptance if passed. Stage 2 combines Stages 2 and 3 of te old standard and contains a number of tests of increasing complexity, te coice of wic depends on ow muc is known about te system. Stage 3 does not correspond to any Stage in AS and is meant to allow connection wit a greater tan normal sare of te emission allocation on a temporary basis wen not all loads are connected or te loads already connected ave not taken up teir full sare. Tis Stage as no tests and will not be considered furter. 3.2 Stage 1 Test 1 is passed it te condition S i /S sc 0.1% is met. Tis test is unfair to very large installations wic migt ave a small fraction of distorting load. Tests 2 and 3 are intended to cope wit tis situation. In test 2, te distorting load MVA is summed wit weigting factors of as given in Table 6 of te standard, depending on converter type, to give te weigted distorting power S Dwi. Wen te converter caracteristics are unknown, a weigt of 2.5 can be used. Te test is passed if te condition S DWi /S sc < 0.1% is met. Test 3 can be used if te current armonic distortion is known for te installation for eac armonic order of interest. 3.3 Stage 2 Tis stage as several tests of increasing complexity and only a broad outline can be given ere. We will concentrate on te simplest case were te installation supply point is close to all loads and te LV load varies in a similar manner to te MV load, ie tere is no diversity. Te next step ten depends on te coice of summation law. We sall adopt te second law as generally recommended. It is assumed tat te armonic voltage at te MV level is te combination of tat allocated to te MV and LV loads G MV+LV and tat propagating down from te HV supply system. Assume tat te HV system is fully distorted to te planning level L HV and tat a fraction T HM appears in te MV system. From te second summation law G MV+MV = α L α ) (T L α MV HM HV (5) T HM is usually taken as 1. Tis as to be allocated to eac customer using eqn(4) giving 1/ α Si E Ui = G MV+ LV S (6) t Te armonic current allocated to consumer "i" is ten found from Z as discussed in Section 2.6 E Ii = E Ui /Z (7) Tis approac can be furter developed to take into account some inconsistencies wic can sometimes arise. Wen te LV load is significant and as its maximum at a different time to te MV load, te above approac is conservative. If te coincidence between te peaks of te MV and LV loads is known, ten an approac described in Appendix J.2.1 can be used to give a more generous allowance. Anoter problem occurs wen te MV loads are spread along a long feeder wit a significant impedance. Te approac of using eqns(6, 7) gives less armonic allowance to loads towards te end of te feeder, sometimes by a factor of four, and it may be argued tat principle of equal rigts is violated. Tere are a couple of modifications to te above approac, given in Appendix D, wic boost te allowance at te end of te feeder and reduce it at te sending end. Tere is a general trend in tese approaces tat increasing te equity for consumers reduces te armonic absorption capability of te power system compared wit te original approac. 3.4 Example - Connection of a large inductively filtered rectifier in a MV network Te following example as some simplifications to make clear te approac of te new armonic standard and compare it wit tat used in AS In particular we sall concentrate only on te 5t armonic. Strictly, similar calculations need to be made on all armonics up to te 40t as well as te THD, altoug wit some experience only a few of tese need be made. A 10 MVA base is used.

5 We sall consider te case of a 10 MVA zone substation supplying a small industrial estate wit no significant LV loads. Te fault level at te 11 kv bus is 150 MVA. A consumer proposes to establis a factory wit a total demand of 3.5 MVA. All of tis load as a low armonic content except for a 1.8 MVA induction furnace. Its front end is a six-pulse tyristor rectifier wit inductive filtering. Te furnace can be operated up to 24 ours non-stop, depending on te production demand. A armonic survey (required for AS Stage 2) sows tat te existing background armonics vary wit time wit a 95% value of 2% AS Assessment Stage 1: Is S converter /S sc < 0.3%? S converter /S sc = 1.8/150 = 1.2% and te installation fails to meet Stage 1 Stage 2: AS is vague regarding time variation. Let us assume te background can be taken as te 95% value of 2%. 2% is between 25% and 75% of te value in Table 1 of te standard. Hence a converter can be connected up to te rating sown in te standard's Figure 1. For a 6-pulse controlled converter tis is 1% of te fault level or 1.5 MVA. As S converter = 1.8 MVA > 1.5 MVA, te installation fails to meet Stage 2 and a full Stage 3 study is required AS/NZS Assessment We sall use te second power law wit a value α = 1.4 for te 5t armonic. Stage 1: Is S i /S t < 0.1%? S i /S t = 3.5/10 = 35% and te installation fails tis part of Stage 1. Stage 2: First calculate te voltage emission available to all MV loads G 5MV+LV = 5 2 = 4% Calculate te portion of te voltage emission available to tis load. 1 /1.4 1/ 1.4 Si 3.5 E U5i = G MV+ LV S = 4 = 1.9% t 10 Calculate te system impedance at te 5t armonic. Z 5 = 5 Z sc = 5 (10/150) = pu Calculate te armonic current available to te installation. E I5i = E U5i /Z 5 = 1.9/0.333 =5.7% or pu. Te actual 5t armonic current can be estimated from I 5 = I 1 /5 = 0.18/5 = pu. Te installation passes Stage 2. Te new standard is more generous tan te old in tis case and does not require a armonic survey. 4. EMISSION LIMITS IN HV SYSTEMS 4.1 Overview 4.2 Stage Stage Example - Similar descriptions as for MV will be presented. 5. DISCUSSION POINTS It is intended tat IEC standards sould be adopted by Australia wit minimum cange. In general some sligt canges are made to IEC standards to clarify expression. In tis case tere as been a more extensive cange is te removal of sections from te main text to new Appendices I-K. Tis arises because te IEC recognise a ierarcy of documents wit international standards as te most important, followed by Tecnical Reports Types I to III. IEC is classified as a Type III report, a collection of data, te lowest IEC classification. Witin Standards Australia, tere is only te two classifications of Standard or Tecnical Report. It was felt by te Standards Australia Committee tat muc of te main body of te IEC document warranted full standards classification. However tere were parts tat were not based on well-known engineering practices and contained several alternative approaces witout a clear recommendation. Tese were removed to te Appendix to empasise tat tey were for information only. Some utilities ave been unappy wit AS for several years and ave adopted all or parts of IEEE 519 [5]. Te standard is called up by te Victorian Office of te Regulator-General in teir Distribution Code [6]. An attractive feature of tis standard is tat tere is a table giving te armonic current to a consumer depending on teir power demand and te sort-circuit rating at te pcc, and little furter calculation is required. Care must be used wit tis standard in Australia since te currents ave been calculated to be compatible wit a different set of voltage limits and wit a distribution system not necessarily aving te same design practices as out own. A particular issue is te level of sort-circuit current for a given supply capacity. Te proposed new Australian standard is better in tat it gives a general calculation tecnique wic can be adapted to a wide variety of practices. Te use of equations and general principles avoids te

6 "quantization" problem tat occurs wen data is presented in a table. It also allows for issues suc as te long feeder mentioned at te end of Section 3.3. Tere as been some discussion in te industry regarding te relative merits of allocating customers voltage limits or current limits. Te advantages of current limits are (i) Te current drawn by an installation can be measured unambiguously irrespectively of background effects (but see (ii) in te next paragrap). (ii) Tey can be estimated at te equipment design stage by calculation or by readings taken in te manufacturer's testing laboratory altoug supply conditions migt differ from tose of te final installation. Te problems are (i) An acceptable installation at one time can create problems later on if tere is a cange in te network impedance, especially if tere is a cange in a resonance condition (ii) Tere can be differences between consumers and utilities as to wic part of te armonic current is caused by teir load and wic part is due to te background. For example an induction motor or capacitor can draw a large armonic current altoug tey are not armonic sources. 6. CONCLUSIONS Te review as given te teoretical background to te proposed armonic standard sowing te key concepts of compatibility levels, te treatment of timevarying disturbances by cumulative probability levels, te summation laws for combining disturbances and te allocation of rigts as a function of agreed power. Te MV and HV approaces ave been summarised sowing te tree Stage approac, somewat modified from tat in te old standard. Witin eac Stage tere are a variety of approaces depending on te need for accuracy and te availability of system data. Two examples sow tat te application of te standard to routine cases is not too difficult. Tere is no need for a preliminary armonic survey and te allocation procedure is more fair and generous tan in AS Various codes are being developed in Australia at State and Commonwealt level to address te issue of power quality including armonics. Tere will need to be close contact between tese bodies and te appropriate Standards Australia Committees as a large number of IEC standards are adopted over te next few years. 7. REFERENCES [1] AS "Disturbances in mains supply networks Part 2: Limitation of armonics caused by industrial equipment" Standards Australia, 1991 [2] Gosbell, V.J. and Mannix, D.J., "Distortion load modelling for distribution system armonic studies", AUPEC98, Hobart, Sep. 1998, pp [3] AS/NZS (Draft) "Electromagnetic Compatibility. Part 3.6: Limits- Assessment of emission limits for distorting loads in MV and HV power systems" [4] AS/NZS :1999 "Electromagnetic compatibility. Part 4.7: Testing and measurement tecniques General guide on armonics and interarmonics measurements and instrumentation, for power supply systems and equipment connected tereto", Standards Australia, 1999 [5] IEEE Std , "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems" [6] Office of te Regulator-General, Victoria, "Electricity Distribution Code", April, LIST OF SYMBOLS Te symbols used are te standard ones used in IEC documents, excluding obvious ones. Main symbols Subscripts E: Emission limit : Harmonic order G: Global allowance HM: From HV to MV L: Planning level I: Current S: Apparent power i: Index of single consumer T: Transfer coefficient MV+LV: Combined MV & LV loads Z: Impedance sc: Sort circuit α: Summation law exponent t: Total available U: Voltage DW: Weigted distorting