UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND ALTITUDE CORRECTIONS AS RECOMMENDED IN IEC STANDARDS

Transcription

1 Paper Published on the16th International Syposiu on High Voltage Engineering, Cape Town, South Africa, 2009 UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND ALTITUDE CORRECTIONS AS RECOMMENDED IN IEC STANDARDS Dong Wu 1*, Ming Li 2 and Mats Kvarngren 3 1 ABB HVDC, SE Ludvika, Sweden, 2 ABB Corporate Research, SE Västerås, Sweden, 3 STRI, SE Ludvika, Sweden *Eail: dong.wu@se.abb.co Abstract: The dielectric strength of air is influenced by air density (teperature and pressure) and huidity. Such effects need to be taken into account when external insulation is designed and tested. Since the conditions of application and the conditions of the laboratory tests ay be different, it is often necessary to ake corrections between different atospheric conditions. For engineers at anufacturers, utilities or high-voltage laboratories, they follow the relevant IEC standards. However, atospheric conditions influence the dielectric strength of air in a coplicated way. Siplified and generalized solutions ay cause vacillations especially when different recoendations are given in different standards without sufficient clarifications. It is the intension of this paper to give an outline of such issues that ay lead to uncertainties in the application of various IEC standards regarding the atospheric correction. Soe proposals are also given for discussion. 1. INTRODUCTION The dielectric strength of air is influenced by air density (teperature and pressure) and huidity. Such effects need to be taken into account when external insulation is designed and tested. Since the conditions at the application and the conditions at the laboratory tests ay be different, it is often necessary to ake corrections between different atospheric conditions. Creditable studies and reviews have been published on the atospheric corrections, e.g. [1, 2]. These studies were the base for the recoendations in IEC standards, e.g. [3-10]. For engineers at anufacturers, utilities or high-voltage laboratories, they follow the relevant IEC standards. However, atospheric conditions influence the dielectric strength of air in a coplicated way. Siplified and generalized solutions ay cause vacillations especially when different recoendations are given in different standards without sufficient clarifications. Based on practical engineering experience, it is the intension of this paper to give an outline of such issues that ay lead to uncertainties in the application of various IEC standards regarding the atospheric correction. Soe proposals are also given for discussion. 2. CORRECTION FOR AIR INSULATION 2.1. Related paraeters of air The dielectric strength of air is influenced by the air density (teperature and pressure) and huidity. The influence of teperature and pressure can be taken into account siultaneously, at least as a first approxiation, by the relative air density, δ, [2]: p p 1 0 δ = (1) t 273+ t 1 Where: p 0 and t 0 (in degree C) are the pressure and teperature at the standard reference conditions respectively; p 1 and t 1 are that at other air conditions. Thereafter the atospheric conditions are converted into ainly two paraeters, the relative air density and the absolute huidity. These two paraeters have also certain co-effects. It should be noted that for outdoor conditions, it ight be assued that the effects of abient teperature and huidity tend to cancel each other [4] Influence of air density The breakdown of a non-unifor long air gap takes often the processes as corona inception, streaer propagation, leader foration and propagation, and final jup. The streaer and leader processes are the decisive processes. It has been concluded in literature [1-2] that the influence of air density is ost significant on the streaer foration and propagation. The air density has little influence on the leader process. Therefore, as an approxiation, one ay consider if the streaer doinates the breakdown processes in a gap, the dielectric strength of this air gap is proportional to relative air density. This is in principle the case for shorter gaps, shorter than 2 eters. For longer gaps, the breakdown will be resulted by both the streaer and the leader process. Therefore, the dielectric strength of a longer air gap is, in any cases, less than proportional to air density. According to above approxiations, the change of the dielectric strength of air gaps with air density ay be evaluated by the value of δ. For breakdown caused ainly by streaers, is equal to one (=1). Otherwise, for ost of the non-unifor long gaps, is saller than one (<1). Therefore, the ain task for air density correction is to evaluate. However, the value of is not just a siple function of gap length, it is also the function of gap structure, voltage type, as

2 well as the relative air density, δ, in a coplicated way. We have =f(u/l, k gap, δ). Here k gap is the gap factor G factor ethod Based on creditable studies and test results obtained at altitude over 3000 eters, the correction ethods using G factor were suarized and presented [1-2]. A factor, G, is introduced to siplify the coplicated relation of =f(u/l, k gap, δ) by a sei-epirical approach =f(g). The value of G is the ratio of the ean electric field, E, at the breakdown voltage of a given gap, and the average electric field of the positive streaer, E s, at the sae atosphere. E G = (2) E s With E=U b /L, E s =E s0 δ k, and E s0 =500 kv/. In the above relations, U b is the breakdown voltage of the air gap and L is length of the discharge. k is the correction for huidity; and E s0 is the average electric field of the positive streaer at standard reference atosphere. It is intended to deterine, by the value of G, how uch a breakdown will be contributed by streaer at the given gap. G value is actually the siplified presentation of the breakdown characterises of the given gap. The functions of =f(g) is given Figure 1, with the value δ k as index. applied with an acceptable accuracy for cases where 0.9 δ k 1.1 [2]. Such liitation ay only be fulfilled when correction is ade between test results obtained near sea level. For high altitude correction other curves give better accuracy. This is an issue being overlooked by this standard. In this standard, it is recoended to use the 50% breakdown voltage and the iniu discharge path to evaluate G. When U 50 is not available, U b can be assued to be 1.1 ties of the voltage level of the withstand test. However, such assuption can only be justified when the voltage level of the withstand test is related to the value of U 50 and the iniu discharge path in such a way that it reflects the discharge characteristics of the test object. Otherwise, errors will be introduced to the value of E=U b /L and thereafter to G and. For the purpose of insulation coordination and the deterination of type test voltage, the value of U 50 and the length of related iniu discharge path are often not available. The test voltage specified has, in any cases, a weak relation with an arcing distance of the equipent. In such a case, E=U b /L can not be correctly estiated as recoended by this standard IEC ( ) In this standard, [4], the altitude correction is given in a forula as: k = e (4) a H 8150 Where: H is the altitude in eters. Actually, we can relate k a in forula (4) with k t in forula (3) by: k a =1/k t =1/δ. Figure 1: The relations of =f(g) given in [2]. 3. RECOMMENDATIONS IN IEC STANDARDS 3.1. IEC ( ) In this standard [3], the correction factor, k t, is defined as: G k t = δ k w = U/U 0 (3) Where U is the breakdown voltage at given site conditions and U 0 is the breakdown voltage at standard reference conditions. G factor ethod is adopted in this standard. Both and w are the function of G. In this standard, the curve with solid line in Figure 1, for δ k=1, is used, for =f(g). This curve ay only be Figure 2: The relations of =f(u cw ) given in [4] Instead of using the relation =f(g), as that in IEC [3], is given here as the function of the coordination withstand voltage, i.e., =f(u cw ). This is a siplified and conservative approach, avoiding the difficulties in obtain the relation of E=U b /L. This is

3 very convenient for the purpose of insulation coordination and the deterination of type test voltage Different values for are recoended for different types of voltage and insulation. For lightning ipulse voltage (LI) and short duration AC voltage, =1 is recoended. For switching ipulse voltage (SI), the value of is to be obtained through a group of curves in Figure IEC ( ) In this standard [5], the noral service conditions including altitudes not exceeding 1000 eters are specified. The altitude correction recoended is said to be applicable to 4000 eters with forula below: H k = e (5) a This is the sae forula as forula (4) but avoided the correction for the altitude not exceeding For SI, further siplifications and approxiations are ade. Instead of the curves in the for of =f(u w ), as given in Figure 2, constant values of for different type of voltages are recoended. The correction is not anyore the function of voltage level: For AC, LI, and phase to phase SI: =1 For Longitudinal SI: =0.9 For phase to earth SI: = IEC ( ) and IEC ( ) In these two standards for power transforers [6-7], the noral service conditions including altitudes not exceeding 1000 eters are specified [6]. The altitude correction is ade directly on the gap distance, i.e. if the transforer is specified for operation at an altitude higher than 1000, the clearance requireents shall be increased by 1% for every 100 by which the altitude exceeds 1000 [7] IEC ( ) In this standard [8], it is said that bushings corresponding to this standard are declared suitable for operation at any altitude not exceeding The altitude correction recoended is applicable to 4000 eters with forula (5). Constant values for are also recoended as: =1 for power frequency and LI; =0.75 for SI IEC ( ) This standard [9] has exactly the sae approach for the altitude correction as that in IEC [5] IEC ( ) In this standard [10], no recoendation is given for altitude correction. If the atospheric conditions at the tie of test differ fro standard reference atosphere, then corrections shall be ade according to IEC [3]. 4. DISCUSSIONS AND PROPOSALS 4.1. Application of IEC The atospheric correction recoended by this standard [3] is ost accurate when breakdown tests have been perfored on air gap or dry insulators. In such cases, the dielectric characteristics of the test objects can be obtained. The value of E=U b /L can be evaluated accurately. This correction becoes difficult to apply when correction is to be ade on withstand test voltage of equipents. This is because in soe cases, the relation between the withstand voltage and U 50 is not 1.1 ties. In soe cases the discharge path ay not be deterined by the voltage level of this test but by other constrains, such as creepage or installation requireent. In soe cases, the shortest discharge path is not the critical insulation under this test voltage. The value of E=U b /L in such cases ay not be correctly evaluated. Even if one ay introduce certain type of iterative calculation procedure between the test voltage and the correction factor, the uncertainty in evaluate E=U b /L can not be solved. However, as long as the corrections are ade for relative sall difference in air density fro standard reference conditions, i.e., 0.9 δ k 1.1, the agnitude of the error introduced ay still be tolerated. The extended application of the relation =f(g) in this standard to high altitude correction will lead to a increased error. The correction as recoended today is not suitable for altitude correction. One solution would be to adopt the coplete relation of =f(g) as that given in Figure 1 into this standard. The other solution is to use the recoendations in IEC for altitude correction, liiting the application of this standard in the range of 0.9 δ k Application of IEC The atospheric correction recoended by this standard is ost suitable for the purpose of insulation co-ordination and the deterination of type test voltage. Since the correction factor is related directly to the co-ordination withstand voltage. One iportance issue in this correction is whether or not to ake correction for the altitude not exceeding 1000 eters. On this point this standards differs fro any other IEC standards [5-9]. In this discussion, there are actually four different atospheric conditions in the context.

4 1. Standard reference conditions with teperature of 20 o C, air pressure of kpa, and absolute huidity of 11g/ 3 2. Noral service conditions (conditions that specified for various HV equipent in relevant standards) with axiu abient teperature of, e.g., 40 o C, altitude not exceeding 1000 eters, and 3. Specific site conditions (application conditions) with altitude of, e.g., 1600 eters, and 4. Laboratory test conditions (at the day of testing) with abient teperature of, e.g., 25 o C, air pressure of, e.g., kpa and relative huidity of, e.g., 40%. These different conditions represent different severities for the external insulation design and test. In Figure 3, the relations between the different atospheric conditions are sketched out. Severity Theoretically k t Standard reference conditions k t Specific site conditions Noral service conditions k t Lab. test conditions Figure 3: Atospheric correction between different abient conditions Theoretically, for the design and test of external insulation, atospheric corrections should be applied between the specific site conditions, laboratory test conditions and the standard reference conditions. However, for ost equipent, there are noral service conditions specified in the relevant IEC standards. For econoical reasons and industrial practice, equipents have to be designed to withstand the required withstand voltage within the range of noral service conditions. The differences between the standard reference conditions and the noral service conditions have been included in the design. This eans that for equipent used at a location with altitude not exceeding 1000 eters; no altitude correction will be necessary. This industrial practice has been supported by vast operational experience and adopted by any IEC standards. This is especially true for the cases when deterinistic ethod is used for insulation coordination, e.g. for HVDC systes (IEC , under revision). The effect of the air density in this range, where H1000 eters, has already been included in the argins coonly adopted for insulation design. For equipent that will be used at the specific site conditions ore severe than the noral service conditions, e.g., at an altitude higher than 1000 eters, atospheric corrections between these two conditions are necessary for both the design and test of the external insulation. For equipent which will be tested at a laboratory where conditions at the day of testing differ fro standard reference conditions, atospheric correction is necessary for the test voltage between these two conditions. Taking this discussion into account, it is justified to use forula like that in IEC , i.e., (5) instead of forula in IEC , (4) Values of for different types of voltage In several IEC standards, the values of for different voltage types are recoended. The recoendation of =1 for LI is justified. The ean field of a rod-plane gap under positive LI is higher than 500 kv/, i.e., E=U b /L 500 kv/. The recoendation of =1 for AC is a conservative approach. The non-linear characteristic of the AC breakdown voltage against gap length indicate the sae breakdown process as that appears under SI [2]. For long gaps, there will be <1. On the other hand, a rod-plane gap of 2 eters will have a U 50 for short duration AC of 620 kv. This voltage corresponds to a phase to phase voltage of 1074 kv. Therefore, if AC is the diensioning voltage, with the syste voltage available today, relative short gaps will be required. Taking this into consideration, the conservative approach of using =1 ay be justified. Otherwise for longer gaps, the value of should be siilar to what is used for SI. Therefore, the recoendations would be: For gaps of lengths shorter than 2 eters, =1 For gaps of lengths longer than 2 eters, use the sae value as that for SI obtained in Figure 2 Note that peak AC voltage should be used as SI in Figure 2. No correction for DC was included in ost of these standards. In Cigré report a linear relation between the short duration DC breakdown voltage and the length of rod-plane gap has been reported up to 1000 kv with ean breakdown field in the level of 500 kv/ [2]. Since the gap length is short, =1 would be used. However, other literature has reported slightly nonlinearity fro 2 to 4 eters of gap lengths [11]. The ean breakdown field is in the level of 400 kv/. This will result in: G=0.8 and =0.6. More laboratory studies are needed for longer gaps. However, with the liited results today, the recoendations would be: For gaps of lengths shorter 2 eters, =1 For gaps of lengths longer than 2 eters, =0.6.

5 The recoendation of use =0.75 for all SI level is a conservative approach. This value ay be justified for EHV systes. For UHV syste, relations in Figure 2 should be followed Altitude correction for creepage Along hydrophilic surfaces, when pollution has been wetted, dry-band related discharge activities ay take place. This is a short gap with streaer breakdown. The dry-band can becoe wetted again after the current flow though the arc in air. Such activities can occur several ties and a full flashover of the insulator ay take place. In this process, the change of air density will change the dielectric strength of the air. If it was pure air breakdown in the for of streaer, then the correction would be k t =δ with =1. However, the conditions of both surfaces in series and parallel to the dry-bend activity contribute to the effects. For real insulator, the change of pollution level and the change of creepage distance in parallel with the air gap will change the interaction between the air gap and surface activities. The insulator shed profile play also an iportant role. This is again a coplicated phenoenon [12]. Although wide differences exist in literature, it is recoended that a constant value of should be used with =0.5 for AC and 0.35 for DC [12]. The correction is to be applied for voltage, the sae as in equation (3): k t = U/U 0 =δ It is the sae as for air insulation correction; no correction for the creepage distance is needed for altitude up to 1000 eters. In pollution test, for the insulators of the sae shed profile and at a given pollution level, the relationship between the U 50 and the creepage distance is in ost cases linear [12]. Therefore, one can replace the voltage in equation (3) with creepage distance, L: k c = L/ L 0 =δ a - (6) Here L is the creepage distance for a high altitude and L 0 is the creepage distance for altitudes up to 1000 eters. 5. CONCLUSIONS For the recoendations in IEC standards on the atospheric and altitude correction the following ay be considered. 1. The correction ethod recoendation in IEC is ost accurate for breakdown test perfored at the condition where 0.9 δ k 1.1. It is inaccurate when used for high altitude correction. It is inconvenient and inaccurate when used for insulation coordination and the deterination of withstand test voltages. 2. The correction ethod recoendation in IEC is convenient to use for purpose of insulation co-ordination and the deterination of withstand test voltages. It can be applied for altitude correction for altitudes up to 4000, as being recoended by several other IEC standards, before any research results prove otherwise. It is a conservative approxiation of G factor ethod. 3. For equipent with a specified noral service conditions including altitudes not exceeding 1000, altitude correction should only be applied to correct the altitude exceeding 1000 as that given in forula (5): k a = e H the recoended value for is proposed to be related to gap length as: For LI: =1. For SI: using curves in Figure 2. For AC: when gap length is shorter than 2 eters, =1. When gap length is longer than 2 eters, use the sae value of for SI with the sae voltage level to AC peak voltage. For DC: when gap length is shorter than 2 eters, =1. When gap length is longer than 2 eters, = Altitude correction for creepage should be introduced into IEC standard as that recoended by Cigré Review [12]. 6. REFERENCES [1] K. Feser, A. Pigini, Influence of atospheric conditions on the dielectric strength of external insulation Paper prepared at the request of the Chairan of SC 33, Electra No [2] WG 33-07, Guidelines for the evaluation of the dielectric strength of external insulation Cigré Brochre 72. [3] IEC , High-voltage test techniques Part 1: General definitions and test requireents. Edition 2.0, [4] IEC , Insulation co-ordination Part 2: Application guide. Third edition, [5] IEC , High-voltage switchgear and controlgear Part 1: Coon specifications. Edition 1.0, [6] IEC , Power transforers Part 1: General. Edition 2.1, [7] IEC , Power transforers Part 3: Insulation levels, dielectric tests and external clearances in air. Second edition,

6 [8] IEC 60137, Insulated bushings for alternating voltage above 1000 kv. Edition 6.0, [9] IEC , Instruent transforers Part 8: Electronic current transforers. First edition, [10] IEC 60168, Test on indoor and outdoor post insulators of ceraic aterial or glass for systes with noinal voltage greater than 1000 V. Edition 4.2, [11] S.J. Huang, Z.H. He, Z.B. Wen, M.G. Wang, Flashover tests on large air gaps with DC voltage, ICPST 94 Beijing, China [12] Taskforce : Polluted insulators: a review of current knowledge Cigré Brochre 158