Arresterfacts 037 Insulation Coordination Fundamentals

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ArresterWorks  Insulation Coordination Fundamentals Where Arrester and Insulator Characteristics Characteristics Meet 6/23/2012 Jonathan Woodworth ArresterFacts 037 Insulation Coordination Fundamentals Insulation Coordination Fundamentals Transient overvoltages are a fact of life on power the most complex forms, computer simulations systems. Arresters can be used to effectively are highly recommended; however, for a good control the most frequent type, which are caused first approximation, system performance can be by switching operations. modeled using simple formula presented in IEC lightning somewhat are Insulation co-ordination more difficult to mitigate, but those too can be successfully Transients caused by handled with a judicious effort. How one protects the Selection of the dielectric strength of equipment in relation to the operating voltages and overvoltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of t he available preventing and protective devices. 60071-1, 60071-2 60071-4. These and three standards are very well written, very understandable, and easy to use. They cover 99% of  what a person needs to insulation on a power system is very often an know to perform a lightning or switching surge economic decision. insulation coordination study. It would certainly not be IEEE 1313.1 and reasonable to insulate only for the operating 1313.2 are another excellent source for better voltage and allow all transients to cause insulation understanding this engineering practice. A third failure. It is equally unreasonable to insulate for and highly acclaimed reference is “Insulation all transient events, if it were even possible. Coordination of Power Systems” by Andrew Therefore, a solution that makes a reasonable Hileman. This 1999 publication is invaluable for investment in insulation and protective equipment the student that would like to understand some of  is the compromise most often taken. the This carefully designed combination of insulators and most complex concepts in insulation coordination. arresters is called insulation coordination. This Insulation coordination has become a ArresterFacts is not meant to be a well comprehensive treatise on the subject, but developed engineering practice. This practice is instead a basic coverage of the fundamentals of  where the characteristics of the system, insulation insulation of all forms, and arresters of all forms cross paths. engineer understand when a study needs to be The task of coordinating the insulation withstand done and what might be the benefits of such a coordination levels with the desired Self-restoring Insulation the system can be Insulation which, after a short time, completely recovers its insulating properties after a disruptive discharge during test different if arresters are applied verses if they are not applied. This help the power study. performance levels of  significantly to Non self-restoring insulation (60071-1) Insulation Characteristics All insulation has its limits of withstand capability. Insulation which loses its insulating properties, or does not recover them completely, after a coordinating task and arrester selection/locating task can be quite simple at times, and at others very complex. In Copyright ArresterWorks 2012 2 ArresterFacts 037 Insulation Coordination Fundamentals Figure 1  – Insulation Withstand Figure 2  – Types of Insulation Because it is impossible to insulate high enough to withstand lightning surges, all insulators are indirectly. All stations need connection to the rest designed and tested to determine the level to of the system via incoming and outgoing overhead which it flashes over. Insulation has two conductors. The only exception to this would be fundamental withstand characteristics: lightning GIS or underground station, therefore all the impulse following pertains to overhead, air insulated withstand withstand. These and switching impulse two characteristics are substations only. If lighting strikes either incoming graphically shown in Figure 1 lightning impulse or outgoing lines within a span or two of the withstand voltage (LIWV) and switching impulse station, a surge is likely to enter the stations on withstand voltage (SIWV). The LIWV characteristic the conductors. of external self-restoring insulation is universally tested and verified under dry conditions. The physical straight line length between the insulator terminals is the most significant factor in determining these fast impulse characteristics. The SIWV of external self-restoring insulation is universally tested under wet conditions because this withstand characteristic is a function of the insulation’s creepage or leakage distance when wet. The creepage distance is the distance between the two terminals along the surface of  the sheds. Figure 3 – Backflash into Protected Substation Substations Insulation Coordination for Lightning Surges Substations are subjected to two types of  Even well shielded transmission lines can allow a very fast rising surge to enter a nearby station if  overvoltages that can and do stress the insulation there is a backflash to the conductor during a throughout the station. Even if the station is well switching or lightning surge (Figure 3); shielded, lightning surges can enter the station Copyright ArresterWorks 2012 3 ArresterFacts 037 Insulation Coordination Fundamentals however, owing to the high insulation withstand incoming lines to the station. Fortunately, in this on systems above 245kV, back-flashovers are less case, more lines make it harder to flashover probable then on systems below 245kv and are insulators in the substation, but at the same time rare on systems at 500 kV and above. more lines also increase to probability of an Fast rising surges on an incoming conductor have incoming surge. Both these factors are used in the a high probability of flashing over insulation in the formulas used to determine proper coordination. substation if there are no arresters. amplitude of the incoming surge will The be Separation Distance approximately equal to the flashover level of the Separation distance is a very important backflashed insulator. If the only mitigation is an consideration in the protection of substations and arrester at the transformer, it will protect the insulation coordination of substations. Arresters transformer if properly coordinated with the will limit or clamp a fast rising surge according to transformer insulation. The arrester may protect their own clamping characteristics immediately in equipment on the surge side to some extent. In the vicinity of the arrester; however, as the this coordination scenario, the probability of its protected insulation is moved away from the occurrence is quite low over the expected life of  arrester, it is less and less protected from fast the transformer, which is probably 30-40 years; rising surges as described in scenario 1 above however, if the proper arrester is not located near (note there are no separation distance issues for the transformer, it only takes one occurrence to slow rising surges from switching sources). This cause a failure of the expensive asset in the circuit. reduced protection is due again to the effects of  traveling waves and reflections. For this reason, Another important part of this coordination the location and distance scenario is the state of the circuit breaker. If the insulation points in the between critical breaker is in the open position, it will become an endpoint on the circuit. Because endpoints represent a significant change in impedance to the circuit, the voltage will be reflected and cause a doubling effect at the breaker. This voltage doubling effect (see traveling wave theory) will cause the breaker insulator to flashover causing yet another path for power current to flow to ground. This voltage doubling effect can also occur if the breaker is momentarily open and a second lightning surge arrives along the original surge path to find the breaker open. Due to these two potential open breaker scenarios, it is advisable to apply arresters at the line entrance of  the station to eliminate the voltage doubling at the breaker and certain breaker bushing flashover. Yet another variable to consider in substation coordination for lightning is the number of  Figure 4 – Separation Distance substation need to be well known before a proper insulation coordination study can be completed. Of course, the non-selfrestoring insulation of the transformer is generally the insulation of highest Copyright ArresterWorks 2012 4 ArresterFacts 037 Insulation Coordination Fundamentals consideration for separation distance issues. The rejection surges need attention. formula for determining the farthest distance inductive and capacitive currents need particular between an arrester and the transformer is found attention in the stated references above as well as in IEC experience pre-strike or restrike. In this case, the 60099-5. As it turns out, the higher the system range of 2% voltages is 2-2.5pu. when the Switching associated breakers voltage, the shorter the allowable separation distances become because the ratio of the There are two coordination methods used in the transformer practice withstand voltage and system voltages is reduced. of insulation coordination. The deterministic method is used exclusively when applied to non-selfrestoring insulation. When Substation Insulation Coordination for Switching coordinating self-restoring insulation, statistical Surges (also known as probabilistic) methods are almost Switching surges are of concern only on systems universally used. 245kV and above. The magnitudes of switching these two methods is that in the deterministic surges for systems below 245kV generally do not method, absolute maximum and minimum values exceed 1.5pu of the system phase to ground are coordinated. voltage. This is due to the fact that the line residual voltage of an arrester for a slow front capacitance, length, and voltage are not of high surge is coordinated and compared to the enough values to result in challenging surges. minimum withstand level of transformer switching There are numerous sources of slow front impulse withstand level. switching surges in substations. Circuit breakers statistical method in determining the flashover or switching devices are involved in all forms of  rate of the 25 self-restoring post insulators in the this surge. Fault and fault clearing overvoltages substation the probability of flashover, occurrence are generated in the unfaulted phase when the and magnitude of the surge are used in the fault is first initiated and when the voltage is re- calculation. established. distribution representing the overall switching The basic difference between For example, the maximum The When using the results are a probability surge flashover rate Arrester Characteristics and Substation Insulation Coordination Arresters are a fundamental part of insulation coordination in the substation. universally Figure 5  – 2% Switching Surge Statistical Level Switching surge statistical level is known as the 2% voltage (see figure 5) and range from 1-2 per unit of the crest phase to ground voltage if they are mitigated with pre-insertion resistors or arresters; however, if they are not mitigated, their levels can easily exceed 2.0pu. When energization and re-energization surges are mitigated, load to protect the They are used non-selfrestoring insulation of power transformers. As stated above, the coordination of non-selfrestoring insulation is accomplished using the deterministic method. This is because there are no acceptable test methods that can determine the probability of disruptive discharge in oil/paper insulation systems. Therefore, the only option is to accept the deterministic approach. Copyright ArresterWorks 2012 5 ArresterFacts 037 Insulation Coordination Fundamentals Arresters applied in substations characterized by co-ordination withstand voltage, it should be kept three voltages relative to insulation coordination: in mind that most adverse conditions from a the arrester operating voltage (Uc or MCOV), strength point of view (i.e. low absolute humidity, lightning impulse protective level (LIPL) and low air pressure and high temperature) do not switching impulse protective level (SIPL). They are usually occur simultaneously. In addition, at a shown in Figure 6. given site, the corrections applicable for humidity and ambient temperature variations cancel each other for all intents and purposes. Therefore, t he estimation of the strength can usually be based on the average ambient conditions at the location. When contamination from salt or industrial pollution is present, the response of external insulation to power-frequency voltages becomes Figure 6  – Arrester Protective Characteristics For non-self restoring insulation, a deterministic comparison is completed. After the insulation and arrester characteristics are determined, they are then coordinated to insure that there is ample safety margin between them. The comparative graph is shown in Figure 7 and is referred to as the Margin of Protection. Figure 7 – Non-Self Restoring Margin important and may dictate longer creepage or Environmental Effects on Insulation Coordination leakage distances. This type of contamination Flashover voltages for air gaps depend on the does not adversely affect lightning and fast front moisture content and density of the air. Insulation withstand levels. Flashover of insulation generally strength increases with absolute humidity up to occurs when the surface is contaminated and the point where condensation forms on the becomes wet due to light rain, snow, dew or fog insulator surfaces. Because insulation strength without a significant washing effect. decreases with decreasing air density, longer strike distance is required to attain the same Transmission Line Insulation Coordination flashover voltage at 2000m elevation than at 100 Transmission line insulation coordination is also meters above sea level. A detailed description of  separated into two categories; lightning and the effects of air density and absolute humidity switching. The performance assessment methods are given in IEC 60-1 or IEEE Std 4 for different are based on expected lightning and switching types of voltage stresses. When determining the overvoltages and their corresponding insulation Copyright ArresterWorks 2012 6 ArresterFacts 037 Insulation Coordination Fundamentals levels. Since line insulation is self-recovering, their Switching impulse studies need only be considered performances are usually determined by the for lines exceeding 245kV. For lines below this statistical method. level, the switching surge magnitudes do not overstress normal insulation configurations. For The practices outlined in substation insulation levels above 245kV, the stresses can be significant. coordination also apply to line coordination. The Switching Surge Flashover Rate (SSFOR) is The sum of the back flashover rate (BFR) and determined by numerical integration of the stress- shielding strength relationship. The stress in this case is the failure rate (SFR) determine the flashover rate (FOR) which is expressed in switching flashovers/100km/year. overvoltage (SOV) quantified by a probability The back flashover rate is the most significant impulse Shielding Failure Rate (SFR) voltage or distribution. switching Strength is cause of outage on The shielding failure rate is the number of strikes the transmission lines. that terminate on the phase conductors. If the withstand voltage (CFO). voltage produced by a strike to the phase IEC The fast rising surge conductors exceeds the line CFO (critical flashover 1313.2 define this process associated with a back voltage), flashover occurs. in detail. flashover switching 60071-2 impulse and IEEE seldom makes it to the substation due to corona effects; If arresters are used to mitigate the SSFOR, the however, fault current and breaker operation evaluation method is modified to accommodate resulting from the back flashover is felt over the the change in the SOV since it will no longer be a entire length of the system. normal switching surge is Often times, a experienced immediately distribution but instead truncated distribution. following a lightning caused flashover. Distribution Another significant Back Flashover Rate (BFR) Systems Insulation Coordination often The back flashover rate is the number of lightning The practice of distribution involved in lightning strikes that terminate on towers or shield wires and system insulation flashover result in insulator flashover The current impulse coordination is coordination the raises the tower voltage, in turn this generates a limited; however, there are elevations. voltage across the line insulation. If the voltage some The critical flashover across the line insulators exceeds the insulation deterministic practices that voltage strength, a back flashover can be expected from the are quite important. tower onto the phase conductor. margin variable system is (CFO) of a line insulator can be reduced by as much very of calculations very specific The protection for some as 20% at higher elevations. Since transmission system configurations can determine when to and lines often traverse high elevations, this factor when not to use arresters. must be considered. For higher elevations, the underground circuits where voltage doubling is insulators may be lengthened or arrester may be common, a margin of protection calculation can applied. Both are excellent means of mitigation. reveal that applying an arrester only at the riser For instance, on pole for systems above 25kV can be a problem. When this is the case, an open point arrester is Copyright ArresterWorks 2012 7 ArresterFacts 037 Insulation Coordination Fundamentals recommended to provide a lower risk of cable Another factor that can have major impact on failure. insulation coordination on distribution systems is long lead lengths on arresters. Long leads can On ineffectively grounded or delta distribution effectively render an arrester unable to protect systems, a close check of the margin of protection non-self-restoring can often show that there is little margin equipment. insulation of distribution compared to well grounded systems. This is due to the fact that higher rated arresters are applied Conclusion to these circuits to give them ample overvoltage In this brief overview of insulation coordination withstand capability. By raising the operating fundamentals, it can be seen that the many voltage of the arrester, the clamping voltage is variables involved in this engineering exercise can also increased and the margin between the make this task quite complicated. However, using transformer’s this method to optimize the use of arresters can withstand curve and arrester’s clamping curve is decreased. result in significant insulation savings on all systems. ArresterFacts are a compilation of facts about arresters to assist all stakeholders in the application and understanding of arresters. All ArresterFacts assume a base knowledge of surge protection of power systems; however, we always welcome the opportunity to assist a student in obtaining their goal, so please call if you have any questions. Visit our library of ArresterFacts for more reading on topics of interest to those involved in the protection of power system at: About the author: Jonathan started his career after receiving his Bachelor's degree in Electronic Engineering from The Ohio Institute of  Technology, at Fermi National Accelerator Laboratory in Batavia, IL. As an Engineering Physicist at Fermi Lab, he was an integral member of the high energy particle physics team in search of the elusive quark . Wishing to return to his home state, he joined the design engineering team at McG raw Edison (later Cooper Power Systems) in Olean, New York. During his tenure at Cooper, he was involved in the design, development, and manufacturing of arresters. He served as Engineering Manager as well as Arrester Marketing Manager during that time. Jonathan has been active for the last 30 years in the IEEE and IEC standard associations. Jonathan is inventor/co-inventor on five US patents. Jonathan received his MBA from St. Bonaventure University. Jonathan Woodworth ArresterWorks’ Principle Engineer www.arresterworks.com  [email protected] +1.716.307.2431 Copyright ArresterWorks 2012 8