1. Personnel are not familiar with arrester ratings and applications Newer engineers replacing older experienced engineers (aging and retiring workforce problem) Personnel not familiar with / aware of / do not have access to IEEE Standards and Guides. Budget limitations do not allow for much training Surge protection not perceived as important as major equipment.
2. Standards for other equipment is more straight forward Breakers and switches – Continuous current ratings and voltage ratings are straight forward – Interrupting ratings have reasoning that with some explanation, can be understood by a novice Transformers – Voltage ratings are straight forward – Power ratings only require a short explanation
3. Arrester Ratings are not as Straight Forward Best illustrated by a question from a project manager: “We are building a 230 kV substation. Why would you install a 192 kV rated arrester? Is this a mistake?”
It’s not obvious to a novice what arrester ratings mean Two voltage ratings: – MCOV – Duty Cycle Concept of Insulation coordination Energy Ratings: Distribution / Intermediate / Station Class Fault duty rating Temporary Overvoltage (TOV) Physical characteristics Other attributes
Confusion over what an arrester does Common Questions are: When does it work? What does it do when it does work? What happens when it really does work? How do I know it worked? What does it do when it does work? Does it have to “fail” to work? Do we even need arresters?
Other Sources of Confusion Why so many building blocks and what are their effects? What is meant by failure mode? What is collateral damage?
What happens if I don’t get it right? Misapplication and it’s effects Arrester failure Equipment failure
Who applies arresters? Utilities: Standards Engineers Design Engineers Maintenance technicians Linemen / mechanics Factories & Other Industry: Corporate Engineers Local Engineers Maintenance workers In each case: Does surge protection get enough attention?
How can these Challenges be Met? One Approach: Produce an Internal Application Guide
Two Objectives Today Explain the rationale of this approach Provide some examples of what an internal guide includes to illustrate the need.
Publish an internal application guide with these objectives: Identify audience and keep them in mind Base the guide on IEEE standards, but only address the parts we need => keep it simple Address problems experienced in the past Answer common questions and clear up confusion Maintain the fewer building blocks approach Describe special applications to guide’s reader Publish and teach the guide to all personnel involved Publish an internal shortcut key to address 90% of applications on our system Review the guide periodically and update as necessary
Consider our arrester applications Where should we apply arresters? – Substations What are they designed to protect? – Major Equipment What good can they do us => How can they help us? – Prevent equipment outages due to failures – Increase reliability Where should we not apply arresters? – Requires development of internal philosophy Economical and practical approach – Reduce building blocks
Consider Application Problems of the Past Internal (arrester) Failure due to – Temporary overvoltage – Being applied at wrong system voltage External (arrester) failure due to – Solid pollution residue on housing – Animals – Improper mechanical forces Collateral damage – Failing arrester caused damage nearby equipment
Minimize the building blocks Fewer options => less chance of misapplications by those unfamiliar with standards Concentrate on the arrester models that solve the most problems Allow for Special applications Eliminate models that had a narrow application. Results in a savings of inventory costs
Results of the New Guide Explained using specific examples to introduce arrester concepts to a novice Resulted in fewer misapplications Better equipment protection
Results of the New Guide (Continued) Only allow Station Class arresters inside the substation to eliminates confusion. => Zero arrester failures applied ≤ 230 kV since 2004. This rule was amended to require the use of energy class H arresters at system voltages up through 230 kV and energy class J arresters at EHV system voltages. This will harmonize with the new C62.11-2012 Test Standard. Require one level of MCOV higher than the minimum recommended by IEEE C62.22 for most applications. =>Eliminated TOV damages Require only polymer-housed arresters to be used. => Drastic reduction in collateral damage risk.
Results of New Guide (Continued) Required a minimum housing height for medium class voltage applications to eliminate some animal outages Required a minimum creep distance for medium class voltages to reduce risk of flashover due to environmental contamination. Both of these are helping reduce external flashovers in high pollution environments
Examples of material to include in the new Guide
Definitions Terminology must be understood to apply concepts. Not always straight forward – Surge Arrester – Line Terminal / Ground Terminal – MCOV, Duty Cycle, and the relationship between the two – System Voltage – Temporary Overvoltage (TOV) Use and explain the IEEE definitions
Proper spelling ARRESTOR Somebody who puts you in jail. Be carful with spell check. ARRESTER This is the version we’re here to learn about today.
Specific Example How to determine proper arrester rating for a given system voltage? Arresters are installed phase-to-ground on the system System voltages all have maximum limits where they can deviate from nominal Must allow for TOV’s specific to our system.
Calculation of Minimum MCOV Where V MCOV = Minimum MCOV rating of applied arrester, kV V SYS = Nominal system operating voltage, kV Utility’s specific application guide requires the use of the next higher entry from Table 1 of ANSI/IEEE C62.11. This is to allow for a longer unanticipated TOV on the system.
Explain the Concept of Insulation Coordination (ABB T&D Book) Crest Voltage (kV) Time ( s) A B Curve A is the demonstrated strength of insulation on a piece of equipment. Curve B is the protective level provided by an arrester. Curve A is protected by the protective device of Curve B which limits the voltage to a value level less than in insulation strength of Curve A.
Impulse Wave Example (IEEE Std. 4) T 1 = 1.67 x T T 1 = Virtual Front Time T 2 = Virtual Time to Half Written as T 1 / T 2 wave Typical is 8/20 wave Time ( s) Amplitude (kV) T2T2 T 100% 90% 50% 30% 0 O1O1
Protective Levels Chopped Wave Withstand (CWW) 1.2 s rise time, crest value of 1.10 x BIL BIL = Basic Impulse Insulation Level BSL = Basic Switching Insulation Level BSL = 0.83 x BIL Front of Wave (FOW) 0.5 s rise time and crest is a function of arrester rating Lightning Impulse Protective Level (LPL) 8/20 s impulse w/ 1.5, 3, 5, 10, & 20 kA crest Switching Impulse Protective Level (SPL) classifying current w/ rise time of 45-60 s Transformer Insulation Levels Arrester Protective Levels Sources: IEEE C57.12.90 IEEE C57.12.00 Sources: IEEE C62.22 IEEE C62.11
Transformer Insulation Coordination Per IEEE Guide (C62.22) Values for FOW, LPL, & SPL can be obtained from the arrester manufacturer’s catalogue. PR 1 = CWW FOW 1.1 x BIL FOW > 1.2= PR 2 = BIL LPL > 1.2 PR S = BSL SPL = 0.83 x BIL SPL > 1.15 Voltage at Protected Equipment Insulation Withstand Level of Equip. PR x =
Catalogue Data Example Arrester Rating (kV rms) Arrester MCOV (kV rms) TOV (kV rms) Front-of-wave Protective Level (kV Crest) Maximum Discharge Voltage (kV Crest) 8/20 μs Current Wave 1 sec10 sec 1.5 kA3 kA5 kA10 kA20 kA40 kA 192152.0222.5212.0482386403417444487544 CWW FOW = 1.1 x BIL FOW = 1.1 x 825 482 = 1.88 > 1.2 PR 2 = BIL LPL = 825 487 = 1.69 > 1.2 PR 1 =
Catalogue Data Example (Continued) Switching Surge Protective Level (kV crest) 125 A250 A500 A1000 A 344352362375 PR S = BSL SPL = 0.83 x BIL SPL = 0.83 x 825 375 = 1.82 > 1.15 IEEE recommends using a switching surge coordinating current of 500 A for system voltages up to 150 kV, 1000 A up to 325 kV, and 2000 A up to 900 kV.
Resistively-Grounded Transformer Concern in on the secondary of the transformer Neutral bushing (Xo) voltage shifts during phase-to-ground faults Produces over voltage on un- faulted phases during phase-to- ground faults Requires an arrester with higher MCOV Do NOT depend on TOV curve of arrester for this application.
Why not depend on the TOV curve? Because it’s not necessary. Solidly Grounded Apply a 12 kV, 10.2 kV MCOV arrester PR 1 = 3.36 PR 2 = 2.98 PR S = 3.51 Resistively Grounded Apply an 18 kV, 15.3 kV MCOV arrester PR 1 = 2.25 PR 2 = 1.99 PR S = 2.34 Both cases assume 110 kV BIL on the secondary winding of the transformer. Even by increasing the arrester MCOV rating by two steps, the protective ratios are still well above the IEEE recommended value.
Transformer High Voltage Side Fault When DG is present Scenario: DG is present on low voltage side of transformer Ground Fault on HV bus HV bus cleared either by local breakers or remote end breakers. Backup relaying may take up to 20 cycles to trip the LV BKR. Question: Will our standard arrester withstand TOV caused by the delta winding on the isolated bus?
Analyze the DG Scenario Standard arrester applied at 230 kV is a 192 kV (duty cycle) 152 kV MCOV When the fault occurs and the bus is cleared from the high side, voltage on un-faulted phases rises to 230 kV phase-to-ground. This is 1.5 per unit MCOV. It was determined that that BACK UP relaying would take 20 cycles to clear the LV breaker. This would exceed the TOV curve of the arrester.
What are these Challenges? 1.6 1.5 1.4 1.3 1.2 0.011.00.1 ` Max Time Duration in seconds Voltage per unit MCOV 20 cycles = 1/3 second. Yes the arrester’s TOV curve is exceeded and it will fail. TOV Curve 10.0
So which arrester do we apply? Would an arrester with an MCOV higher than the anticipated TOV work in this case? No, because the discharge voltages would be too high to coordinate with the BIL of the transformer? Therefore, we have to select an arrester with low enough discharge voltages to protect our transformer, but still be able to withstand the TOV.
Determine the proper MCOV From the TOV curve, we know that at 20 cycles, the previous arrester can withstand 1.45 x MCOV for 20 cycles. Use that as a starting point: MCOV ’ > TOV 1.45 = 158.6 kV The next available size arrester is a 198 kV, 160 kV MCOV model.
Will it work? TOV: It will withstand 234.2 kV for 1 sec. Prev. Ratio FOW = 508 kV PR 1 = 1.78(1.88) LPL = 512 kV PR 2 = 1.61(1.69) SPL = 395 kV PR S = 1.73 (1.82)
Do we need arresters at transmission line terminals in substations? SUBSTATION BUS BREAKER ARRESTER Lightning Flash may contain many strokes Arresters must be located on the line side of the breaker Mandatory at EHV, should be considered at lower voltages Should be based on -Economics -Importance of substation -MTBF of breaker’s voltage class -Presence of automatic tripping schemes in areas around EHV switch yards
Conclusion A company-specific guide is necessary to explain and solve arrester standards challenges that are specific to your system. Specific Explanations -> less confusion Fewer building block -> fewer mistakes Specific application examples -> fewer misapplications
Bibliography IEEE Std. 4-2013, IEEE Standard for High Voltage Testing Techniques. IEEE Std. C57.12.00-2010, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std. C57.12.90, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std. C62.11-2012, Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV). IEEE Std. C62.22-2009, IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems (>1 kV). Central Station Engineers of the Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book, re-published by ABB.