Review of Proposed New IEC 60099-4 Energy Handling Tests
ARRESTERS
Review of Proposed New IEC 60099-4 Energy Handling Tests
Perhaps the most significant change yet in IEC 60099-4 promises to be good news for engineers involved in the specification and application of surge arresters. Not only is there a change in the way an arrester’s energy handling capabilities are tested, but a better way of classifying arresters is now also emerging. INMR columnist and arrester specialist, Jonathan Woodworth, a member of IEC Maintenance Team (TC 37 MT4) that proposed the new tests, has prepared an article that outlines his perspective on the changes as well as how these tests should be interpreted and applied for maximum benefit. Photo: INMR ©
102 INMR® Q2 2012
INMR Issue 96.indd 102
rEviEw
oF
proposEd iEc 60099-4 nEw EnErgY handling tEsts
16/05/12 2:44 PM
Introduction It has long been apparent to both suppliers and users of arresters that the line discharge classification system used to quantify an arrester’s energy handling capability has not been ideal. After review by members of IEC TC 37 MT4, the following major issues were identified: 1. Line discharge classification could not be fixed using the present standard and therefore a better system to classify arresters was needed; 2. Existing tests do not provide for a standardized way to establish and verify energy handling capability. As such, arrester manufacturers have had to develop their own methodologies to calculate product data, typically resulting in energy ratings that can vary from one supplier to the next; 3. To perform transient studies of their systems to determine protective needs, arrester users require data that is consistent and realistic; 4. Impulse and thermal withstand characteristics are still tested (indirectly) in the present standard using the same procedures and are not discernible from one another. It has become clear that it would be desirable to modify these tests to provide a means to independently verify an arrester’s thermal withstand and its impulse withstand. Previously, the two were intermingled in the operating duty cycle tests and in the transmission line discharge (TLD) tests. The proposed new document to improve these perceived deficiencies is currently at the CD (Committee Draft) stage, which in turn will be followed by a CDV (CD for voting) and an FDIS (Final Draft International Standard) – whereby experts from across the world can provide their input and comments via their respective IEC National Committees. The 3rd Edition of IEC 60099-4 (expected to be published no sooner that mid 2013) will then contain substantive changes in the area of energy handling capabilities as well as in the means of classifying arresters. For example, to address the desired changes in energy testing, two tests have been developed that separately quantify impulse type surge durability and thermal withstand capability.
rEviEw
INMR Issue 96.indd 103
oF
All Photos: INMR ©
proposEd iEc 60099-4 nEw EnErgY handling tEsts
Q2 2012 INMR® 103
16/05/12 2:44 PM
The issues surrounding improved arrester classification have been resolved by adopting an IEEEstyle system whereby arresters are classified as either station class or distribution class. Each has a specific set of tests to pass, which then define arrester class and replace the former line discharge classification.
Important New Definitions
Table 1: Arrester Classification Arrester class
Station
Nominal discharge current
20 kA
10 kA
Switching impulse discharge current
2 kA
1 kA
Qrs (C)
≥ 2.4
≥ 1.2
Wth (kJ/kV)
≥ 10
≥4
Qth (C)
—
—
Distribution Class Arresters Arrester class These include arresters intended Nominal discharge current for use on distribution systems, Switching impulse discharge current typically Us ≤ 52 kV, that are meant to protect components primarily Qrs (C) from the effects of lightning. Arrester Wth (kJ/kV) classification is assigned based Qth (C) on the test series applied during type tests. Note: Distribution class arresters can have nominal discharge Thermal Energy Rating (Wth) currents (In) of 2.5 kA; 5 kA or 10 kA. The energy, given in kJ/kV of Ur, which can be dissipated by an Station Class Arresters arrester or arrester section during a These are arresters used at substations thermal recovery test without leading to protect equipment against to thermal runaway. transient overvoltages and typically (but not necessarily) intended for Repetitive Charge Transfer Rating (Qrs) use on systems of Us ≥ 72.5 kV. The The charge, in coulombs (C), in the classification is assigned based on form of a single event that can be the test series applied during the type transferred through an arrester at test. Note: Station class arresters will least 20 times (at intervals that allow have nominal discharge currents of cooling to ambient temperature) 10 kA or 20 kA. without causing mechanical failure or unacceptable degradation of the Thermal Charge Transfer Rating (Qth) metal oxide varistor blocks. This is the charge, in coulombs (C), that can be transferred through an Charge Transfer arrester or section of arrester during a A unit of measure that quantifies the thermal recovery test without causing current flow through an arrester and thermal runaway. is calculated as the integral of the
Distribution 10 kA
5 kA
2.5 kA
—
—
—
≥ 0.4
≥ 0.2
≥ 0.1
—
—
—
1.1
0.7
0.45
current over the time of the surge event (in coulombs).
New Arrester Classifications Classification of an IEC rated arrester will be based on the data provided in Table 1. If an arrester is tested according to the tests in the selected column and passes all levels, it can then be rated at that level. This will replace the previous line discharge (LD) class scheme. With the proposed new system, there is no possibility that e.g. a 10 kA station arrester could be classified as 20 kA, as might have been the case in the previous LD system by increasing arrester discharge voltage. Not only must energy dissipation (kJ/kV) now be at an acceptable level, but the charge transfer (in coulombs) must also be acceptable in order to classify an arrester at the next higher level.
Repetitive Charge Transfer Rating (Qrs)
Example of arrester with energy withstand exceeded.
104 INMR® Q2 2012
INMR Issue 96.indd 104
rEviEw
oF
Test Rationale This test is applied to all non-gapped MOV type arresters with the only difference between station and distribution class arresters being in the wave shape of the 20 impulses in the rating test. The Qrs test has been designed to test the capability of an arrester to withstand discharges such as lightning or switching surges and, being performed only on disks, does not need a thermal equivalent section.
proposEd iEc 60099-4 nEw EnErgY handling tEsts
16/05/12 2:44 PM
Examples of arresters with energy rating exceeded. for the repeated impulse, the residual 3. One 8/20 current surge of voltage at In and Vref are measured. 0.5 kA/cm2 peak current Both are measured again after the test as part of the pass/fail evaluation. The last impulse is intended to stress the disk one more time to make sure It is worth noting that for the that the final Vref or residual voltage first time in arrester testing, the impulse did not lead to any internal arrester’s reference voltage is used in cracks in the material that might determining pass/fail criteria. During otherwise have gone undetected. research and development of this test, it was discovered that small The disk is considered passed if it changes in Vref are an indication of has not exceeded the 5% change limit of Vref and residual voltage at degradation in the varistor material. Therefore a change in Vref of greater In and is not physically damaged. If than 5% is a test sample failure. one sample fails, it is permitted that Similarly, residual voltage is sensitive 10 more samples be tested, but this Test Procedure to degradation and a change in time no failures are allowed. The repetitive charge transfer test residual voltage of greater than 5% is replaces the long-duration current also considered failure. Rating Considerations impulse withstand test whereby The Qrs characteristic will be 3 samples are tested with 18 The repeated impulse is a switching quantified in terms of charge impulses each with no failure surge for station class arresters (coulombs) and not energy permitted – a methodology that with the amplitude set by the dissipation (joules). Charge has yielded little in the way of statistical manufacturer at 110% of the unit’s been chosen as a test basis for information about failure probability. desired charge transfer rating. For the purpose of better comparing The new test, by contrast, will distribution class arresters, an 8/20 different models of MOV arresters. offer higher assurance of arrester impulse is utilized while transmission Energy values can be calculated performance by being applied only line arresters use 200 µs to peak from this information by multiplying to the individual disks that make surge. Each sample is impulsed 20 the charge and the related switching up the full arrester. In the case of times in 10 sets of two impulses impulse protection level. station class arresters, a switching about 1 minute apart. Ample time surge or half sine surge is applied. for cooling is allowed between each For the Qrs test and final rating, it is For arresters used on systems of impulse set to allow the disk to expected that the values should fall 52 kV or less (i.e. distribution class return to ambient temperature. between 0.5 and 25 coulombs. The arresters), a half sine wave of 200 µs rating will be expressed in coulombs is used for the test. At the end of this test sequence and not as a class or level. However and once the disks have returned each class of arrester does have Test Procedure Details to ambient temperature, three final a minimum requirement to meet, The test procedure is written for both tests are applied: as stated in Table 1. The rating is station and distribution class arresters determined as 90% of the repeated with the only difference being in wave 1. Vref, impulse level during the 20 shot shapes and amplitudes. In preparation 2. Residual voltage at In series. Since it is the express goal the of the IEC maintenance team to separate thermal energy and impulse energy handling capabilities, 10 sets of two impulses each are to be applied in succession. This is believed to be an acceptable number that will not drive the disk to a temperature that could damage its materials. Moreover, cooling is allowed between sets to ensure that the result is not a one or two shot rating but rather a rating that can be sustained over many surges during the arrester’s service life.
106 INMR® Q2 2012
INMR Issue 96.indd 106
rEviEw
oF
proposEd iEc 60099-4 nEw EnErgY handling tEsts
16/05/12 2:44 PM
New Operating Duty Test For station class arresters (Ur ≥ 54 kV), this will verify the thermal energy rating (Wth) For distribution class arresters (Ur < 54kV), this will verify thermal charge transfer rating (Qth)
be expressed in joules for station class arresters with typical rating of 54 kV or more and coulombs for distribution class arresters rated < 54 kV. Where a station class arrester is rated below 54 kV, it can be tested and rated with a thermal energy rating, i.e. tested in the same manner as those whose ratings are equal to or greater than 54 kV.
The initial Vref and In tests in the sequence are to set the baseline for evaluation after the thermal stresses.
The next two high-current impulses surges are meant to electrically degrade the discs (while a dielectric test has become a separate new test) and the current amplitude of Rationale these impulses is the same as in the This new operating duty test is present operating duty test. This way designed to quantify the energy it will become possible to distinguish The characterization and conditioning dissipation or charge transfer between a thermally prorated section part of the test can be performed on necessary to raise an arrester’s and a dielectrically prorated section the disks in still air. However, they temperature to a level where it is not (which is not the case in the existing can also be tested in a dielectrically stable under operating conditions. arrester standard). pro-rated section to avoid the need For a high voltage arrester, the unit of for subsequent dielectric testing. measure can be joules or coulombs. (The ‘conditioning’ part was removed The fourth and final set of energy For arresters used on distribution inputs to the arrester is the ones because it is really a remnant of old systems, the unit of measure will be used to rate it. Prior to the last set gapped silicon carbide technology coulombs or charge transfer. of impulses, the arrester must be and in any case a CIGRE study has heated to 60°C unless the arrester is shown that 20 impulses have no Again, the use of charge transfer for UHV application (in which case impact on electrical ageing). The eliminates the potential confusion the temperature is determined using thermal recovery part of this test caused by joule ratings with respect another test sequence). As per the must be performed on thermally to residual voltage. As discussed, it is test procedure, energy inputs are as pro-rated sections. A temperature the explicit goal of the developers of follows: sensor must be integrated into the this test to separate the thermal rating sample such that the temperature of an arrester from its impulse rating. Arresters of Ur ≥ 54 kV of the active part can be measured. (for system voltages Us ≥ 72.5 kV) If not dielectrically equivalent, then Test Procedure another test is necessary to qualify This test will become the new Rated energy injection within the dielectrics. (Note: This separate operating duty test. The thermal three minutes by one or more test is not part of this overview) energy/charge transfer rating will long-duration current impulses or by unipolar sine half-wave current impulses or, in case of non-gapped types (NGLAs), by lightning discharge impulses. Arresters of Ur < 54 kV Rated charge transfer within one minute by two lightning current impulses 8/20 μs.
Figure 1: Repetitive charge transfer rating test sequence.
Figure 2: Thermal rating test sequence (new operating duty test).
rEviEw
INMR Issue 96.indd 107
oF
Within 100 ms from the application of energy or charge, a voltage equal to the elevated rated voltage (Ur) shall be applied for 10 s. Thereafter, a voltage equal to the elevated continuous operating voltage (Uc) shall be applied for a minimum of 30 minutes to demonstrate thermal stability. The resistive component of current or power dissipation or temperature or any combination of these shall be monitored until the measured value is reduced appreciably (i.e. success), or a thermal runaway condition becomes evident (i.e. failure).
proposEd iEc 60099-4 nEw EnErgY handling tEsts
Q2 2012 INMR® 107
16/05/12 2:44 PM
Table 2: Comparison of the Old and New Energy Rating Levels Old LCD
Required minimum test energy kJ/kV
Corresponding new thermal energy rating Wth kJ/kV
Estimated current at old LD test A
Charge calculated with the same current and duration as old LDC to give the required minimum energy C
Correspond new repetitive charge transfer rating Qrs C
Approximate range of system voltage kV
1
1.0
2
277
0.56
0.4
â&#x20AC;&#x201D;
2
2.1
4
538
1.10
1
up to 300
3
3.3
7
721
1.78
1.6
up to 420
4
5.0
10
962
2.75
2.4
up to 525
5
6.9
14
1118
3.75
3.6
up to 800
The thermal charge transfer or thermal energy rating will be 100% of the sum of the thermal charge transfer or thermal energy rating of these two impulses. Also, the sample must not experience a change in residual voltage at In of more than 5%. Rating System There are minimum rating requirements for station and distribution class arresters, however the actual thermal rating will not be mandated by this test. Station class arrester will have a thermal rating as given by the manufacturer and tested per the above test. Station class arresters will have thermal energy ratings (Wth) from 4 kJ/kV-Ur to 30 kJ/kV-Ur. Distribution class arresters will only have thermal charge ratings and must meet the minimum requirements as per Table 1.
Comparison of Old & New Classification Systems
selecting the arrester with a thermal energy rating (Wth) that is above the system response. The prospective energy that a system will require of an arrester can be determined using transient analysis software, but if that is not available a simplified formula is in IEC 60099-5, as follows:
and is derived based on the assumption that the entire line is charged to a prospective switching surge voltage and is discharged through the arrester at its protective level during twice the travel time of the line (for switching surges). W:
energy in Joules that will be dissipated by the arrester for the given surge level; L: line length; c: speed of light; Z: line surge impedance; Ups: arrester residual voltage at the lower of the two switching impulse currents; Urp: representative maximum switching voltage.
Steep Front Residual Voltage Test This test has new methods required to reduce the potential of misunderstanding the data. Disk Ageing Tests In the past, this test was a procedure that allowed for increasing watts at the end of the test. With the new standard, disk ageing with an upward trending watts loss will no longer be allowed. As a result, there is no required voltage adjustment for ageing in the thermal evaluation tests. 5000 hr Ageing Test of Housings This test is no longer suggested as an alternative to the 1000 hr test. UHV Arresters Requirements and tests for UHV arresters (for highest system voltages Us > 800 kV) are introduced.
Conclusions
With publication of this new standard, industry will have a test Annex K of the new standard shows standard that finally reduces the a detailed comparison of the old and ambiguity of energy rating and energy new classification methods. Table 2 is an example of the potential rating For example, if the calculated energy testing in general. Moreover, arresters having different designs can be and the equivalent rating from the dissipated by an arrester using the properly compared and evaluated. current system and comes directly above formula is 7 kJ/kV, then the from this annex. The old LDC Class 1 desired thermal energy rating (Wth) of is very similar to the new distribution the arrester should be a minimum of Specifiers of arresters can request a specific energy rating and all class arrester and the old LDC 2, 3, 7 kJ/kV. potential suppliers submitting quotes 4 and 5 will become station class will now be offering the same type arresters. Other Tests Changing in the of arrester. Users of arresters can Upcoming Edition of 60099-4 therefore be confident that they are Selecting the Right Station Several important additional changes applying the correct arrester for the Class Arrester Energy Rating application at hand. are coming with Edition 3.0 of With this new system, the required 60099-4 including: energy rating of an arrester can At the same time, arrester be determined by first calculating manufacturers can now run a test Temporary Overvoltage Test the level of energy the system will This is now a mandated test and not that is standardized and meaningful discharge into the arrester and then to both specifiers and users. ď ¸ an option as in the past.
108 INMRÂŽ Q2 2012
INMR Issue 96.indd 108
rEviEw
oF
proposEd iEc 60099-4 nEw EnErgY handling tEsts
16/05/12 2:44 PM