ArresterFacts 016Selecting Arrester MCOV and Uc Part 1 of Arrester Selection Guide Prepared by Jonathan Woodworth Consulting Engineer ArresterWorks Feb 12, 2009 IEC) Ground Fault –An event There are several necessary where current steps that need to be taken when flows from the selecting an arrester for an power system application. IEC) understanding of the system to which it is to be applied. value of powerfrequency voltage between its terminals at which it is designed to operate correctly under temporary overvoltage conditions as established in the operating duty tests. Tests used to establish the voltage rating in IEC 60099-1.The designated maximum permissible voltage between its terminals at which an arrester is designed to perform its duty cycle test. (IEEE.Rated voltage of an arrester (IEC) Maximum permissible r. Attention is drawn to the fact .000 sec (IEEE. Introduction Definitions MCOV rating (IEEE) .The maximum designated root-mean square (rms) value of power frequency voltage that may be Uc Continuous operating voltage (IEC) The designated permissible r. value of powerfrequency voltage that may be applied continuously between the arrester terminals indefinitely. TOV Curve –A graph that shows the power frequency withstand voltage vs. when a system phase The only voltage rating of an arrester that is important is the conductor is MCOV (Maximum Continuous connected to Operating Voltage IEEE ) and Uc earth either (Continuous Operating Voltage through a IEC). Duty-cycle voltage rating (IEEE) . involve the application of repetitive impulses at nominal current with power frequency voltage applied.m. An early step in this selection is the determination of to ground the voltage rating of the arrester. Ur .s.m. NOTE 2 The rated voltage as defined in this standard is the 10 s power-frequency voltage used in the operating duty test after high-current or long-duration impulses. time for arrester from .Uc rating direct contact however is not always obvious or through an without a fairly good arc. The objective of this ArresterFacts is to make this decision clearer and understandable.01 sec to 100.s. This MCOV . as well as some national standards. NOTE 1 The rated voltage is used as a reference parameter for the specification of operating characteristics.ArresterFacts 016 Selecting Arrester MCOV and Uc Selecting Arrester MCOV and Uc Contents Introducti on Relevant Definition s System Configura tions Determining LineGnd Voltage Determining Voltage Rise due to Fault Using the TOV Curve Transmission Line Arresters Summary Recommended Ratings Tables applied continuously between the terminals of an arrester. When arresters are applied to nominal system the line to protect systems from lightning or phase to phase ground voltage switching surges. For consideration. the above system.73. on the MCOV or Uc system. this too 485kV. on a the regulation is Determining Line-Gnd 760kV 10%. earth. ground voltage is equal to the Arrester 2008. the to earth voltage The MCOV or MCOV of the installed arrester would be 440kV.that these two methods used to established rating do not necessarily produce equivalent values. If example. On three some regulation should be phase systems. they are voltage is 760kV could be 440x installed between the phase and therefore the line 1. Woodworth Copyright Works 2009 Page2 . the line to error. Uc or an arrester must be equal or higher to the Since all for this system at continuous voltage between the systems have a minimum phase and earth.Jonathan J. For this application. phase to phase must be taken voltage divided into by 1. then for Voltage and Minimum transmission example.10 = 485kV. 9 275.0 121 69.3 98.5 13.9 66.2 8.8 34.0 24.0 345. This is also known as an .0 245 142 142 69.6 6.04 2.01 22.0 72 41. the next step in the selection process is to determine the system configuration to which the arrester will be applied.0 100 57.0 72. one must determine if it is a wye or delta system (star or delta in the IEC world).40 2.52 1.0 12.1 27.32 8.0 170 98.4 10.0 6.8 400.8 57.0 16.6 47.4 18.0 13.57 16.0 10.0 11.5 36.1 7.1 30.3 3.ArresterFacts 016 Selecting an Arresters MCOV or Uc needed for selection is to Typic know how the al IEC Typical IEEE system neutral Syste System m Voltages conductor is Volta Nom Max Max used in the ges Line Line circuit if there Line to Max to Line to Grnd Nominal Typical is one.5 8.9 24.1 110.8 14.0 169 98 230.8 83.0 21.19 scheme 6.5 6. Fortunately the number of system configuration s are limited.9 155.90 7.0 145 83.0 765.3 21.80 5.6 high the line to 12.3 46.8 12.52 neutral kV rms kV rms kV rms kV rms 4.7 2.0 362 209 209 138.5 41. In other words.0 52 30.1 6.8 24.6 7.0 24.9 220.9 13.0 123 71.3 4.8 132.2 15.6 23.74 5.9 ground 12.2 13.0 242 362 525 800 140 209 303 462 System Configurations Once the system voltages are understood.05 determine how 10.0 phases 20.46 and the Voltage Voltage 4.1 22.1 2.2 4.0 300 173 173 115.8 21.28 11.9 26.0 91.0 420 243 243 161.25 4.4 voltage of the 13.91 bonding 3.3 27.9 6.6 29.37 2.0 13.16 4.9 8.2 20.0 500.0 145 83.9 unfaulted 13.0 48.38 33. The most common IEEE configuration is the 4 wire solid multigrounded neutral as shown in figure 2a. The Line Line Voltage Voltage Max Voltage power source Line to Line Minimum to Grnd kV rms kV rms kV transformer rms Line to Line Uc Voltage 2.0 12.6 41.8 330.6 7.1 71.2 14. Also will rise during a ground fault.0 36. Copyright Arrest erWor Figure 2a Solidly Multigrounded 4 wire system ks Jonathan J. 2009 Woodworth Page3 .effectively grounded system. the two unfaulted phases experience an increase in the voltage between the phase and ground. A third common system configuration is an isolated or ungrounded system. Since arresters are most often applied between the phase . When the impedance is high.ArresterFacts 016 Selecting an Arresters MCOV or Uc and 2d show these two A common systems. This is often a cost savings configuration. it can limit the fault current to levels that allow for lower fault current rated equipment to be used on the system. This can be either delta or wye configured. a Petersen coil is used which can offer fault extinguishing capabilities without using breakers to break the fault. Figure 2c Figure 2d Single grounded neutral system (Unigrounded system) Determini ng Phase Voltage Rise due to Earth or Ground Faults When a three phase power system experiences a fault to earth on any one of its phases. This is sometimes referred to as a resonant grounded system. Ungrounded systems The reason for (isolated popularity of this neutral) system is that the fault current to earth is limited by the impedance. Figure 2b Impedance or Resonant Grounded System A common industrial and very common IEC configuration is the 3 wire impedance Figure 2c grounded wye (or star). When low impedance is used. transmission line configuration is the single grounded Wye as seen in Figure 2d . Annex fault. Copyright erWor 2009 Woodworth Page4 . of thumb and This increase in voltage graphs that will remain across the can be used. The voltage rise during a fault in these cases is determined by multiplying the line to ground voltage by The determination of a For voltage rise during a ground fault is not an distribution easy task if a precise systems Arrest ks Jonathan J. subject.conductor and earth. value is then they also see this desired. arrester until a system but these are breaker operates and quit crude and breaks or interrupts the difficult at best to use. This is a very significant event in the C of IEEE life of an arrester and standard must be accounted for C62. There are some rules increase in voltage across their terminals.22 and Annex A of during the voltage rating selection of an IEC 60099-5 cover this arrester. a worst case scenario is used for each type of system. where the system and transformer impedances are relatively unknown. Source Transformer Figure 5 Mixed Configurati on Use the source transforme r grounding scheme to determine the MOV rating Using the TOV Curve to Select an Arrester ’s MCOV After the system configuration and potential overvoltage is determined. The voltage MOV1 during a ground fault should be on the unfaulted sized for a phases can reach 8.25 or 10. Figure Potential System 4 Overvoltages . ground voltage is In this case 8.8kV multi-grounded solidly system. it must be compared to the arrester TOV curve. grounded This is the voltage an system. Selecting an Arresters MCOV or Uc Mixed Configuration s It is also important to note that the grounding of the Type of System neutral at the source Solidly Grounded 4 transformer is the wire configuratio systems Uni-grounded 3 wiren referred to in systems determining Impedance grounded the voltage systems rise of the Isolated Ground system. Figure 3 lists the ground fault factors used to determine the unfaulted phase voltage rise during a ground fault. isolated ground system. a Factors delta/delta transformer For example in a is tied to a 13. the maximum grounded continuous line to wye system. Figure 6 shows a comparison of system overvoltage and arrester TOV capability. Systems and Delta Systems For example Figure 3 Ground Fault as seen in Figure 5.38 solidly x 1.47kV rms.38kV. Figure 6 shows TOV curves of several types of arresters.ArresterFacts 016 a ground fault factor or earth fault factor. and arrester will see across MOV 2 its terminals for as long should be sized for an as the fault exists. Woodworth Page5 .Copyright Arr est erW ork s 200 9 Jonathan J. Figure 6 es Example s Arrester TOV of Curve co m pa rin g th e po te nti al sy st e m ov er vol ta Figure 7 ge Comparing TOV an Curve and Potential System d Overvoltage th e In the example in arr Figure 7. the selected es distribution arrester ter would not withstand wit an overvoltage of an hs ungrounded or delta ta system.ArresterFacts 016 Selecting an Arresters MCOV or Uc withstand an overvoltage Fo from a unir grounded di and multistr grounded ib system. it m could s. withstand th even an e ungrounded pr system oc overvoltage. but would nd . uti However if a on gapped sy MOV st arrester was e selected. Se e Fi gu re 8 for thi s re co m m en da tio n. the MCO V or Uc of the arrest er is select ed to equal or excee d the line to line voltag e. F or su bs ta ti o n ap pli ca ti o . for all syste ms other than the multi groun ded syste m. Beca use of this issue. Most manu factur ers also o f f e r a q u i c k l o o k u p t a b l e t o s e l e c t t h e a r r e s t e r r a tin g ba se d on th e sy st e m to wh ich it is att ac he d.capab ility is seldo m compl eted becau se the time of the overv oltage is unkno wn. the comp ariso n of the poten tial syste m overv oltag e and the arrest er overv oltag e withst and capa bility is essen tial in select ing the arrest er MCO V or Uc. Th e be st me an s of obt ain ing the ex pe cte d ov erv olt ag e dur ing a fau lt on a tra ns mi ssi on sys te m is to as . In the case of trans missi on syste ms and subst ations . the expec ted syste m overv oltag e m a g n i t u d e a n d d u r a t i o n a r e k n o w n q u a n t i t i e s s o t h i s c o m pa ris on is qu ite ac cu rat e.ns. They have usuall y model ed the syste m extens ively with proven softwa re.k the person s respon sible for relay setting s. Sin ce ove rhe ad . Use this inform ation to s e l e c t i o n o f t r a n s m i s s i o n l i n e a r r e s t e r s ( T L A ) M C compare against the O target arr curve. In the case of TLA’sonly the obje protect insulators from the undesirable bac kfla sh dur ing a swi tchi ng or ligh tnin g sur ge. V Transmission Line Arresters The r a t i ng or Uc rat in g is dif fer en t th an a dis tri bu tio n or su bs tat io n arr es ter . they can supply both magnit ude and duratio ns of faults at most locatio n on the syste m. insulat ors are genera lly a selfrestori ng type of insulati on it is not impera tive to have the lowest Copyright p o s s i b l e c l a m p i n Arrest erWor g volt ag e for the arr est er to miti gat e ks 2009 Jonathan J. Woodworth Page6 . ************* Figure 8a IEEE MCOV Suggested Ratings (based on historical preference and TOV analysis) . Sometimes it Selection of an is also desirable to size arrester the arrester so that it MCOV rating does not absorb any or Uc rating significant energy can be during a switching daunting at surge. then the known it is a arrester MCOV should simple be similar to that of the comparison. In this case times. substation arresters. configuration However if the TLA is and being applied to overvoltage mitigate switching potentials are surges.ArresterFacts 016 Selecting an Arresters MCOV or Uc ************ **** Summary flashover. but increasing the MCOV or once the Uc rating is an effective system means to do just this. 8 14.9 26.0 72.0 13.3] 22-24 23.7] 21 22.65 10.2 20.37 2.5 13.0 12.Typical IEEE System Voltage s Nom Line to Line Max Max Line Line to Line to Grnd Voltage Voltage Voltage kV rms kV rms kV rms Suggested IEEE Arrester MCOV Rating Solid Multigrounded Systems ( 4 wire) Uni-grounded Systems (3 wire) Impedance grounded.2 12.8 17 34.4] 20.2 13.5 8.57 7.6 29.0 145 83.28 7.7 15.8 84-98 106-115 98-115 161.5 36.52 4.3 19.4 [15.38 8.7 [8.4 [17] 24-29 29 [22] 36-39 [22] 29-36 15.16 4.01 8. Woodworth Page7 .0 16.7 [7.1 2.8 12.0 121 69.3 22-24 27.3-17 24.3 22 [15.2 14.6 12.4] 13.0 169 98 98-115 115-131 115-131 230.55 5.19 7.90 7.3 [12.9 42-48 53-67 48-67 115.40 2.1 7.05 5.3] 15.4 12. Ungrounded and MCOV MCOV [*] Delta Systems MCOV [*] 2.4 12.74 5.0 242 140 140-152 182-190 152-190 230-289 245-289 345.2 15.65] Transmission Line Arresters for Lightning Protection Only 7.9 15.7 [8.0 800 462 462-490 >490 [*] MCOV rating of a Gapped MOV arrester Copyright ArresterWorks 2009 Jonathan J.65 13.04 2.1 15.1 12.65 8.8 70-76 84-98 76-98 138.65 12.1 6.3] 24-29 24.0 48.9 22 46.0 24.52 1.91 5.6 7.9 24.3 27.8 21.32 8.4] 24.46 4.9 8.55 2.1 5.0 15.5 41.9 29 39 29-39 69.25 4.0 362 209 209-245 500.0 525 303 318-452 >452 765.3 [8.5 [15.80 5. Isolated and Delta Source Transformer Systems Uc 4.3 4.2 4.4 6.4 12 18 18 22.1 77-86 132 145 83.0 10.0 6.5 6.9 9.0 11.3 3.0 72 41.0 36.2 10.8 66-77 102 77-102 125 86-125 110 123 71.1 Uc 2.8 7.8 96-115 145 115-145 155 170 98.0 100 57.8-24 24 24 33.0 24.0 12.ArresterFacts 016 Selecting an Arresters MCOV or Uc Figure 8b Suggested Uc for IEC systems Typical IEC System Voltag es Nominal Typical Max Line to Line Max Line to Grnd Line to Line Voltage Voltage Voltage kV rms kV rms Suggested Uc for IEC Systems Solidly Earthed Neutral at the Impedance Earthed.1 33-43 53 43-53 66.0 24-36 36 36 47.6 43-58 72 58-72 91.6 7.6 12 12 16.7 2.9 16.6 7.3 21.4 18.0 kV rms Transmission Line Arresters for Lightning Protection Only 3.3 110-125 170 125-170 220 245 142 154-188 245 188-245 275 300 173 182-192 300 192-300 330 362 209 221-230 360 230-360 400 420 243 269-288 420 288-420 318 420-440 550 440-550 500 550 Handling Issue Other ArresterFacts Available Arrester Lead Length Field Testing Arresters Infrared Thermometer Guide for Selecting an Arrester Field Test Method VI Characteristics The Externally Gapped Arrester (EGLA) The Disconnector Understanding Mechanical Tests of Arresters What is a Lightning Arrester? The Switching Surge and Arresters The Lightning Surge and Arresters Understanding the Arrester Energy U n d er st a n di n g Di sc h ar g e V ol t a g e W h a t i s a R i s e r P .0 52 30.0 13.2 12 11. erWor Woodworth Page8 .please give ArresterWorks proper credit.com as your source of information on high voltage surge arresters. however Copyrig ht using www. If you choose to copy any part of this document for teaching purposes you have my permission.ArresterWor ks. ole Arrester? Thank you for ArresterFacts Usage ArresterFacts are Copyrighted documents intended for the education of arrester users and stakeholders. Jonathan Woodworth Principal Consultant ArresterWorks Arrest ks 2009 Jonathan J. Woodworth Page9 .ArresterFacts 016 Selecting an Arresters MCOV or Uc Copyright ArresterWorks 2009 Jonathan J.