Insulation Coordination Fundamentals

Insulation Coordination Fundamentals
Rev 0 – Jan 16, 2019 This presentation is brought to you by NEMA Section 8LA. The 8LA section is made up of high voltage surge arrester manufactures. They are interested in helping their customers and interested parties achieve a better understanding of their products and the application thereof. This presentation will give arrester users a better understanding of what is meant when a person refers to an Insulation Coordination Study. We will take it from a vague concept to a clear practical reality. Nema 8LA Rev

Webinar Outline Initial basics of Insulation Coordination Studies
Definitions , Types, Parameters, Purposes Examples of an Insulation Coordination Study Basic Substation , Complex Substation, Transmission Line BIL,BSL The Backflash Traveling Wave Phenomena Arrester Fundamentals Margin of Protection Ground Flash Density The Report Before we start, let’s review what we are going to do. This list covers all the fundamental concepts needed to understand what is being said when an insulation coordination study is recommended. As always basic definitions are needed to begin the understanding, as well as short examples of different studies BIL-BSL etc., are important concepts in the understanding of insulation characteristics The backflash, traveling wave and separation distance are three phenomena that are specific to power systems and must be discussed in the context of Insulation Coordination. Since insulation coordination almost always involves surge protection, a short section on arresters in included. The margin of protection calculation is simple, and essential to all forms of insulation coordination studies. We pick up some loose ends with ground flash density and altitude. And wrap it up with an overview of what should be in a report. When complete you should be prepared to participate in any discussion that is considering the initiation or review of an insulation coordination study.

Resources for this Webinar
Book: “Insulation Coordination of Power Systems” by Andrew (Bob) Hileman, 1999. AR Hileman Software ATP and ATP Draw, XY Plot IEC ,2,3,4 IEEE C and .2 Formerly and .2 (Insulation Coordination Standards) IEEE C62.11 Arrester Test Standards IEEE C62.22 Arrester Application Guide IEEE 1410 and 1243 Improving Lightning Performance of lines With regard to references used in insulation coordination work this is a list that will be helpful to anyone that wants to study the topic in more detail. Insulation Coordination of Power Systems by Bob Hileman is the bible of insulation coordination. His book is a text book on the subject and though it is nearly 20 years old, the principles hold today as much as they did in 1999. From the pages and his works in general, you can see his significate influence in both the IEC and IEEE standards which he was heavily involved. If you can run a DOS program, the AR Hileman software becomes an excellent companion to the book. If you would like to run a complex study, ATP, EMTP, PS CAD, and numerous other software programs are available to use. But as with most software, it is a means, and quite a bit of work using the other references is necessary to make any sense of it. There are some simple rule of thumb formulae that can be used for simple insulation deterministic coordination studies, and they will be covered in this webinar also. They are found in C62.22, IEEE 1410 and IEEE 1243.

Definition of Insulation Coordination
Simple Definition Insulation coordination is the selection of the insulation strength of a system. (Hileman) Better One Insulation coordination is the process where the insulation characteristics of all components of the power system are determined, specified and coordinated to avoid failure due to expected internal and externally occurring surges. (Hileman) Insulator There are two key words to focus on in this definition, process and coordination. Insulation coordination is usually an iterative process where optimization is necessary. Where compromise is the norm, where perfection is the goal but not usually the final result. For example, in a substation, it would be ideal to never experience a flashover and it can be designed to do that, but in some cases, the cost is prohibitive, so it is designed for a potential flashover of an insulator perhaps only once every 500 years. This level of reliability albeit not perfect, is quite good. The word coordination here refers to coordinating clamping voltage of the arresters with the minimum flashover withstand voltage of the insulation in the area of concern. It is important that the arrester clamping voltages are lower (better) than the voltage necessary to damage the equipment. If the arrester limits the surge amplitude at 1000kV peak, and the damage level (BIL) of the equipment is 1500kV peak then we have coordinated properly and have achieved our goal. On the pole to the right, the arrester is sized (coordinated) to limit the voltage across the insulator to less than the voltage that would cause a flashover. This is the simplest form of Insulation Coordination. Arrester

Types of Insulation Coordination Studies
Transformer Protection Substation Protection Open Air and GIS Line Protection Distribution and Transmission Breaker Protection Generator Protection Determine clearances Determine Separation Distances Determine Arrester Energy and Voltage Ratings. And on and on and on Expand on the following Transformer Protection: This is a very common type of study because the asset is so valuable. Transformer insulation is non-self-restoring meaning it cannot restore its insulating nature if there is a breakdown of the insulation. If the internal dielectric fails, it cannot be re-energized, it must be removed from service. Because of this non-self-restoring nature, this type of study always involves a deterministic approach with no probabilities involved. The reliability is quantified with a Margin-of-Protection ratio and not years between failure. Substation Protection: If this involves an Air Insulated Station (AIS) then the insulation level is chosen that will not allow for a flashover to occur in less than the time period that has been chosen by the designers or owners. In other words, the Mean Time Between Flashover (MTBF) is chosen such as 200 years or 400 years and the study is executed with the maximum surge the station would see in any 200 or 400 year time frame. The insulators as well as the arresters, and both their locations are chosen to ensure no flashover will occur for this maximum surge. Generator Protection: Again, this is a popular study because the assets are so valuable and locating the arresters and surge capacitors can be difficult. In this case the rule of thumb is to locate the proper rated arrester and surge capacitor as close as possible to the terminals of the generator to protect it from breaker generated switching surges. This can be a tenuous task with certain generator configurations.

Deterministic This is the conventional method where the minimum strength of the insulation is equal or greater than the maximum surge stresses. . Transformer insulation is not statistical in nature. It has one lightning withstand value and one switching withstand value. Therefore a deterministic analysis is all that we can do. The two types of analysis used in insulation coordination are Deterministic and Probabilistic (covered on the next slide). A deterministic assessment is used for non-self-recovering insulation such as transformers or cable or any insulation that cannot self-heal after a dielectric breakdown. Equipment of this type is given a very specific withstand voltage known as its BIL (Basic Insulation Withstand Level) and is verified by type tests. The BIL and related BSL and CWW are parameters of this type of insulation and are used in the deterministic study. A margin of protection calculation is a basic deterministic study. (The MOP is covered in more detail near the end of this presentation) If the classification currents are used to determine the arrester clamping levels, then there are absolutely no probabilities involved. If a transient study is carried out to determine the current and voltage at the transformer, then probabilities may be used indirectly. A Margin-of-protection calculation is the simplest form of an insulation coordination study and is very often used when only the protection of a transformer is in question. Example: MP2 = (BIL/LPL)-1 = 95/ = = 171% margin. This is very good, 15-20% is the minimum suggested by IEEE. BIL given by transformer manufacturer. LPL is the 10k discharge voltage of the arrester chosen for the analysis.

Probabilistic This type of analysis consists of selecting the insulation level and clearances based on specific reliability criterion. Since the insulation strength of air is statistical in nature, we can only determine its probability of Flashover for a given surge. Studies of transmission line performance is based on a flashover rate per year per 100km, and because the flashover parameter is statistical, resulting levels are probabilistic. Studies of substation performance is also probabilistic for the same reason. For this type of study we base the performance on MTBF (Mean Time Between Flashover). More later on this. Accurate probabilistic analysis is much more difficult to compete and requires the help of software most of the time. Having said that, IEEE Standards 1410 and 1243 do however offer excellent shortcut formulae to get a first order assessment of probabilistic problems. The flashover characteristic of an insulator is dependent on the rate of rise of the voltage and the CFO of the insulators. Both of these variables have a probability associated with them. As stated above, line performance studies always include this type of assessment. Substations are a combination of probabilistic and deterministic studies.

Lightning Surge Studies This type of study deals strictly with lightning surges and backflash over surges. Is completed for all system voltage levels. Switching Surge Studies This type of study is usually for systems above 240kV since it is this type of system that can produce switching surges of relevance. If a lower voltage system has large cap banks, then a switching study is justified. Insulation Coordination studies can be further divided into two other types of studies. The Lightning Study will be the study that determines the insulator requirements for all systems below 240kV. The Switching Study will determine the minimum insulator strike distance for systems above 240kV. Depending on the goals of an insulation coordination study, one or both studies may need to be carried out. Many times, a lightning or switching goal is established and then the opposite study is carried out to determine if it will also work. Switching studies are much different in that they cover large areas and distances, this is due to the fact that a switching surge can travel great distances over the life of the surge. Lightning studies are a very local event and the effect of lightning is only experienced over a few spans outside the substation.

Parameters of Importance in Studies
Incoming Surge Steepness Backflash Rate (BFR) Calculating BFR Tower Configurations Circuit Physical Dimensions The Transformer Ratings and Capacitance The Arrester VI Curve Selecting the Rating Purpose of Study The Lightning Flash Ground Flash Density Shield Failure rate if known Types of Insulation BIL and CFO MTBS and MTBF Location and Altitude of Study Cable and Isophase specs A very large part of study activities is the collection of all the pertinent data. The data of importance is different for each study type. This means that the purpose of the study needs to be established prior to collecting the data. The next slide covers the different purposes. For Example, if the study is to determine the outage rate of a line due to lightning. Then the following are essential for the study System voltage. Location of the line is needed to know the ground flash density of the area. If the exact location is not known, at least the GFD is needed. Tower Ground Resistance (if not available, then you run the study at low and high levels) Tower configuration (hint; Google Earth and Street View can tell a lot here, if the location is known) Insulator characteristics. (Again, Google comes to the rescue) For shield failure rate and subsequent flashover, the terrain is essential. For lower voltage systems it is necessary to know if trees and buildings are near the lines. More often than not, you will collect too little data and more data collection will be needed as you run a study.

Purpose of Insulation Coordination Studies
Can be to design proper insulation and arrester location from scratch Can be to validate chosen insulation levels (Very common) Can be to determine where to locate arresters Can be to determine cause of failure of equipment (After an incident) Can be to determine the Width of a ROW (Switching Study) Can be to provide assurance that equipment is protected properly Can be to put in the file for future reference Can be to fulfill a requirement Can be to …………. and more…… This is really just a short list of the many possibilities why one would run an insulation coordination study. Many times, an insulation coordination study is requested and really what is needed is an arrester sizing study, or a shield wire effectiveness study, both of which are parts of a more formal insulation coordination study. It is generally accepted that coordinating insulation levels, arrester ratings and locations and reliability levels are part of every study. Sometimes the purpose of a study is to give the investors’ confidence in the power system design.

Examples of Lightning Studies
Simple Substation from Chapter 12 of “Insulation Coordination of Power Systems”. 500kV Line-Substation-Generator 69kV Line Study These three are selected because they are common types of studies. The substation study chosen is supported by data and explanations in the Hileman text. It is also a clean and simple study that keeps a focus on protection of the transformer. The 500kV study is of a typical gas-powered combined cycle generator. It has portions that deal with the generator, transformer and switchyard. An excellent example of a more complex study. The 69kV line shows how different a line study is to a station study. Elevation and ground resistance become big factors in line studies in the mountains.

ArresterWorks - Surge Protection Seminar
Overhead Shield Wire Simple Substation Station Arresters Disconnect Switch This photo gives you a visual example of a simple substation and its configuration. The only thing missing in the photo is the potential line entrance arrester that would be off the photo to the left. CT or CCVT Breaker Power Transformer arresterworks.com

Basic Substation Lightning Study Incoming Surge Surge at Trans
This is what an ATP model looks like when created and visualized using ATPDraw, a very popular GUI software interface. A few moments should be taken to review and familiarize oneself with the different sections within the model. In a substation study, the purpose is usually to verify that the worst-case incoming surge on one or more incoming phases will not be damage the transformers. Even if the incoming lines are shielded, it is very probable that under certain circumstances a back flash will occur, and the surge initiated and fueled by a lightning flash will find its way to the station. When a surge gets to the station (traveling at nearly the speed of light) it will have an amplitude and steepness that is dependent on the distanced it has traveled from its initiation. It is the task of the person running the study to determine what this amplitude and surge front steepness will be. This is done prior to the study and is based on a number of factors outside our scope but very important for a successful study. When the surge encounters an arrester, its amplitude is clamped, but its surge front steepness is not affected significantly. For transformer protection the surge fronts are not as much of an issue as they are for generators. (next example) Hileman offers simple equations in his text that allows a person to estimate the voltages at different points in the station without the use of transient software.

Complex Study Shown here is a 500kV Station and a 15kV Generator
The purpose of this type of study is typically to check that any forced separation or alternate location of components and reflections, due to traveling wave phenomena, do not cause higher risk of component failure. At 500kV, the yards can be very big and multiple arrester can be necessary to provide the desired protection level throughout the station.

Complex Insulation Coordination Study
Three generators Complex Insulation Coordination Study Cross over line to Generator Station Switchyard with no transformers 3 generator step up Transformers Incoming Line This model shows a three-generator combined cycle facility with a generator substation and a separate switch yard/ substation that is was tied into an existing 500kV line. Studies like this are done to ensure insulator backflash events on the first few spans outside the station do not find their way into the station and cause damage. The blue bubbles show parts of a generator station that are common to almost every generating station. What is not shown in this schematic is the station shield wires and masts. It is assumed that there will never be a direct strike to equipment in the station because 100% of strikes will be intercepted by the shield. IEEE Std 998 provides excellent guidance on station shield wire designs. Although it is common practice to have overhead shield wires, this fact should be verified.

69kV Sub Transmission Line Study
The purpose of this transmission line study was to determine what it would take to improve this line’s outage rate and if arresters were used, how would/could they be mounted. There was concern also that the arresters were too heavy for the poles. This was shown easily that it was not the case. 69kV Sub Transmission Line Study

The line was already scheduled to be rebuilt due to age. On the left is the existing line (compliments of Google Street View). The image on the right was the proposed build by the utility and arresters added to show their impact and size. Note they add no significant load to the line. The study showed that arresters would need to be added to each pole for its entire length if no shield wire were used. Altitude was a small factor in this study since most of the line was at 2500ft. This reduced the insulator CFO from their design level of 391kV to 357kV. This reduced the minimum stroke current by a few kA that would cause an insulator flashover without arresters. This was a critical step in first calculating the outage rate of the line and then comparing it to the protected configuration.

Insulator that flashes over at a specific voltage 69kV Sub Transmission Line Study This is a simple tower model since it is only one circuit and one single pole. Each insulator is modeled as a voltage-controlled switch. For this type of insulator, the insulator could be modeled as a time/voltage sensitive (or rate of rise sensitive) switch to better represent the upturn in insulator flashover voltages for faster surges. Also note there is inductance in the leads of the arrester. When running a study, many scenarios are ran to check all potential operational situations. Underbuilt Circuit

System Fundamentals Relative to Insulation Coordination
Traveling Waves and Reflections, Backflash, and Separation Distance Tower Grounds and Station Grounds Corona Steepness of Surges Clearances Physical Dimensions Ground Flash Density OHGW Ground Flash Density For the rest of this webinar, we will discuss the many aspect of insulation coordination. This will give you a better idea of the scope of studies. All of these aspects are not found in every study since purposes can be so different.

Types of Insulation Self restoring Insulator Terminator with Self-restoring Insulation on outside and non-self-restoring on inside Underground Cable with Non-Self Restoring Insulation External Insulation The distance in open air or across the surfaces of solid insulation in contact with open air that is subjected to dielectric stress and to the effects of the atmosphere. Examples are porcelain or polymer shell of a bushing, support insulators, and disconnecting switches. Self-restoring Insulation Insulation that completely recovers insulating properties after a disruptive discharge (flashover) caused by the application of a voltage. This is generally external insulation.

Non-Self Restoring Insulation
More On Insulation Internal Insulation The internal solid, liquid, or gaseous parts of the insulation of equipment that are protected by equipment enclosures from the effects of the atmosphere. Examples are transformer insulation, internal insulation of bushings, internal parts of breakers and internal part of any electrical equipment. Non-self-restoring Insulation Insulation that loses insulating properties or does not recover completely after a disruptive discharge caused by the application of voltage. Generally internal insulation. Self Restoring Insulation Non-Self Restoring Insulation

Insulation BIL Basic Lightning Impulse Insulation Level (BIL) The BIL level is the Dry insulation withstand strength of insulation expressed in kV. Is commonly used to describe substations and distribution system voltage withstand characteristics. Statistical BIL is used for insulators means there is a 10% probability of flashover and is used for self- restoring insulation Conventional BIL is used for Transformers and Cable is the voltage level where there is a 0% probability of Flashover and is applied to non selfrestoring insulation Insulator BIL is directly proportional to the strike distance of an insulator BIL ≈ 15kV x S(inches) And is affected by Altitude Note: For self-restoring-insulation, this is a statistical number and represents the voltage that will result in flashover only 1 in 10 impules at this level. For Non-self-restoring insulation, this is the voltage that will never result in a flashover or dielectric failure. The tactical BIL value also an easy value to calculate, it is a linear ratio to the strike distance of the insulator at ~15kV/inch. Note 1: Arresters do not have a BIL rating since their external insulation is self protected by the internal MOV disks. In a sense they have an infinite BIL. Note 2: Arresters close to an insulator give the insulator infinite BIL.

BSL BSL is proportional to the strike distance of an insulator
BSL= 1080e((0.46 x Strike Distance) + 1) And is affected by Altitude Basic Switching Impulse Insulation Level (BSL) The BSL level is the switching surge withstand level of the insulation in terms of kV. BSLs are universally tested under Wet conditions. Statistical BSL of Insulators apply to self restoring insulation and represents a 10% probability of flashover. Conventional BSL of Transformers and solid dielectrics apply to non-self-restoring insulation and represents a 0% probability of flashover The BSL or switching surge withstand voltage is similar to BIL only that it is for a much slower rising and falling surge. Because of its slow nature compared to BIL it is affected by rain, fog and snow. This parameter also has two definitions, one for internal and one for external insulation. Because the withstand voltage is not a linear function of the strike distance, formula for calculating the switching withstand voltage is a bit more complicated. If you are running a switching surge study and do not have the insulator withstand voltages, it can be calculated based on the strike distance for dry withstand BSL= 1080e((0.46 x Strike Distance) + 1) It is also important to note that the minimum low frequency clearance of a power line above 240kV is determined by this value. Below 240kV, the power frequencey withstand is lower and determines the minimum low frequencey clearances. Note 1: Arresters do not have a BSL rating since their external insulation is self protected by the internal MOV disks. In a sense they have an infinite BSL. Note 2: Arresters close to an insulator give the insulator infinite BSL.

Power Frequency Withstand
Power Frequency Withstand Voltage This is the highest power frequency voltage an insulator can withstand under wet conditions (low level of contamination). It is affected by creepage distance and strike distance. Power frequency withstand is a complex function of both the strike distance and the contamination level on the insulator surface. It is not a big factor in insulation coordination studies. Notes 1 – 4 contain essential information and should be reviewed. Note 1: Insulator withstand voltages are often >2-3 times their operating voltage. Note 3: If the housing is highly contaminated, the housing may flashover at levels below the turn-on voltage of the arrester. Note 2: Arresters will go into conduction if the AC voltage across the unit reaches a 1.25 pu MCOV and above. However they cannot sustain this condition for very long or they will over heat and fail. Note 4: In highly contaminated areas, extra creepage distance insulators are used to overcome this potentially low flashover voltage. The same policy should be applied to arresters.

CFO and CWW Critical Flash Over (CFO) Self Restoring insulation only This is the voltage with a 50% probability of flashover of the insulator. It applies to both lightning and switching. It is used to quantify insulation used on transmission and distribution lines. Typically CFO is 4-6% higher than Statistical BIL on an insulator. Chopped Wave Withstand (CWW) This is a withstand level of equipment. A standard lightning impulse is used but the surge is chopped at 3us, which means the stress is applied for a much shorter time than a standard lightning impulse test and must flashover near the crest of the wave instead of on the tail as it can in BIL tests. The value of this characteristic is about 1.10 times BIL for power transformers and 1.15 times BIL for bushings. Caused by insulator flashover just past crest. Can cause winding to winding stress in some transformers

Insulation Withstand Characteristics in Graphic Form
ArresterWorks - Surge Protection Seminar Insulation Withstand Characteristics in Graphic Form Typical Values kVp CWW Chopped Wave Withstand BIL Basic Impulse Withstand Level Another form of Lightning withstand is CFO Critical Flashover Voltage BSL Basic Switching Impulse Withstand Level arresterworks.com

The Backflash When the OHGW on a transmission line is hit by lightning, a rapid series of events takes place. If the system is grounded well than the surge is transferred to earth and there is no effect on the phase conductors. But occasionally a backflash will occur, this series of slides will show you a close up view of the sequence of events. arresterworks.com

The Backflash Time = 0 The first event is the strike. Of course there was already a great deal of activity just to connect this line to the cloud, but that is for another sequence. When the strike pins to the wire, it sets up a voltage surge that travels in both directions down the line. (1-50 million volts) This is all happening at nearly the speed of light and until the surge actually finds ground, there is little current flow. Additional information: The surge amplitude is determined by the conductor impedance and the lightning current amplitude. For example, 200-ohm shield wire impedance x 10,000A strike = 20 million volts. arresterworks.com

The Backflash CFO Time = 1 In a few Nano-seconds, the voltage front meets the down ground and travels toward earth at the tower bottom. While at the same time it is inducing a voltage on to the phase conductors When it reaches earth, the current begins to flow. The voltage along the tower increases rapidly due to ground potential rise. This potential rise is caused by the resistance of the ground rod of the tower. This tower voltage rises as the current begins to flow. Induced Induced arresterworks.com

The Backflash CFO Time = 2 The voltage at the base of the base of the insulators and on the phase conductors increases as the surge increases in amplitude If the voltage at the base of the insulator increases at a faster rate than the induced voltage on phases, it can reach the CFO of the insulator arresterworks.com

The Backflash CFO Time = 3 The voltages continue to increase across all components as the surge crests. arresterworks.com

The Backflash CFO Time = 4 (.5-2 µsec) If the voltage across the insulator exceeds the CFO, it can flashover from the pole down ground to the phase. This is the backflash…… It flashes from the base to the conductor which is intuitively backward since the down ground spends its entire life except for these few microseconds at ground potential. This is the part of the event that we are interested in with insulation coordination studies. What effect this surge will have the substation. But its not over yet….. arresterworks.com

The Backflash Time = 5 (20-50 µsec) The lightning stroke is over and the voltages on the lines revert back to their pre-strike levels. But the air around the insulator is seeping with ions and still highly conductive. When the AC voltage reaches a high enough level, it now flashes forward from the phase conductor to the down ground. Ionized Gas arresterworks.com

The Backflash Time = 6 (50 µsec to 200ms) When the insulator flashes over for a second time, power frequency current flows to ground and a fault is now underway on the circuit and will remain there until a breaker interrupts the event. At that point the event is over assuming no damage occurred on the insulator. AC Follow current causing a Line to Ground Fault Until breaker interrupts arresterworks.com

The Backflash CFO The surge that is transferred onto the phase conductor has entered the station within a few µsec, even before the fault was initiated. This is the impulse that becomes the concern of insulation coordination in substations. arresterworks.com

The Rest of the Story on Lightning Initiated Traveling Waves

Initial Strike to OHGW As stated in the earlier slide sequence, once a strike lands on an OHGW, it generates a surge that heads off in both directions. When it meets a tower with a down ground, it also travels down toward earth. This is the first part of the surge travel on the transmission line. Of course, we also know that the surge is also inducing lower voltage surges on nearby phase conductors due to capacitive coupling.

As shown earlier, every tower has a lightning current level that will cause a backflash across the phase insulation. This backflash now sets up another traveling wave on the phase conductor also heads in both directions. You must remember that this is a simplified overview of the event to keep it comprehensible. Voltages are induced in all directions from not only the surges, but from the initial lightning strike. This is the way a lightning surge challenges the insulation of substation, not directly through the station shield wires. Of course, any nearby lightning strike will also induce surges in a substation, but the amplitude of indirect induced surges is very low compared to that generated by a backflash.

This fast rising surge with a long tail travels on the phase conductor at nearly the speed of light, toward the transformer. The amplitude is approximately equal to the CFO of the line insulator. If this backflash is more than 5-10 spans out from the station, the corona on the line caused by the high voltage surge actually reduces the steepness of the surge. At the point of the backflash, the rate of rise of the surge is nearly infinite. By the time the surge has traveled a few spans, its steepness is reduced significantly. The rate of decrease is very well understood and has a big positive effect on the ability to protect the station. This is why in some areas with a lower GFDs, where lines are not shielded, they have a shield for only a few spans out of the station. This is the only area on a line where a backflash can cause a fast enough rising surge to cause a flashover in the station.

As it reaches the arresters, its tail may still be as far back aa the original tower. Remember the surge has a long tail µs due to the lightning. The front of the surge is perhaps .5 µs. It took less than a few microseconds to travel from the backflash to the transformer. It is a hard to visualize, but the charge transfer from the cloud to the line is not even complete when the surge arrives at the transformer.

The surge is clamped by the arresters at the transformer and a reflection begins to occur as the wave front meets the transformer winding. The winding is like an open circuit to a fast rising surge. Note the voltage at the transformer is clamped by the arresters. This part is too hard to simulate with simple diagrams, so please use your imagination a bit. We know a few things for sure. The voltage at the arrester is the lowest it will get. The voltage at the transformer just a few feet away will be similar, but not quite the same as the voltage at the arrester. This is due to the reflection that occurs when the surge meets the high impedance (sort of like a wall) The surge has both a voltage and current component. The current cannot pass through the high inductance of the winding and is reflected back. CCVTs Arresters

The transformer is protected by the arresters, but a surge is reflected back into the system
Here we see that as the tail of the incoming wave meets the reflected front, we see the voltage increase at that locations. This is known as voltage doubling because it can double at worst case with one reflection. Depending on many factors however the voltage can increase more than 2 times. It is even possible for it to reach levels that can flashover insulators back toward the original strike as you see in the next slide.

In C62.22 there is a separation distance calculator that can be used to determine voltages at other locations in a substation. In this case the bushing of the CCVT has flashed over due to a reflection. Again, keep in mind these are example wave shapes to make a point. The next slide shows an EMTP output that is more realistic.

Also if the arresters are mounted away from the transformer, voltages at the transformer can be higher than at the arrester due to reflections 30 m separation 3 m separation Note the voltage at the transformer is higher than at the arresters. This is due to traveling wave reflection This slide shows the effect if arresters are mounted at a distance from the protected transformer, this is referred to as the “Separation Distance”. Note the transient program output shows how the voltages looks if they were to be measured at the transformer and arrester. The right most graphic has a 30 m separation distance and the left graphic is only 3 m, a much more realistic distance. The difference in the voltage at the transformer is about 150kV raising the voltage at the transformer to 690kV. For a 230kV transformer with a BIL of 650kV, this separation distance would easily result in a damaged transformer winding. Arresters Separation Distance Red = Arrester Green = Transformer

Arresters the other half of Insulation Coordination
Not to be too repetitive, but here is another definition of insulation coordination to consider. Insulation coordination is the selection of insulation characteristics and where appropriate the selection of arrester characteristics to properly protect the insulation when extraordinary surges are present. We have discussed the more important insulation characteristics, so now we will cover the arrester characteristics of importance for insulation coordination.

Arrester Definition A device that is connected between phase and earth that will clamp a surge to levels below the damage levels of nearby insulation. All the blue devices in this slide are arresters. You can see they are installed phase to earth. When a surge enters the station, each arrester will clamp the voltage in its immediate area with some effect ft away. In the case of the arresters on the transformer, the arresters are protecting both the bushings and the internal windings.

What’s Inside Station Arrester Distribution Arrester Polymer Housing
Metal Oxide Varistor (MOV) Conductive Spacer Strength Member (Fiberglass) Spring for Compression Rubber Seals End Vents and Diaphragms Let’s take a quick look at what is inside an arrester before we review the electrical characteristics. The parts can be divided into two categories. MOV Disks – The nonlinear semi-conductive element that does all the voltage clamping and surge control. Structure – External housing, end caps, seals, compression seals, strength member etc. … The MOV disks are all that really matter with regard to insulation coordination. They are a stack of smaller disks to make a tall column that is proportional to the operating voltage and desired protective level (LPL). Each disk is in fact a short arrester in itself and stacking them is the universal means of attaining higher voltages. Distribution arresters have similar components plus Distribution Arrester

Voltage Current (V-I) Characteristics
VI Characteristics of an Arrester or Disk is the essence of the MOV. The resistance of the MOV disk is a function of the voltage stress across the terminals. There is much more detail on the next slide, but for now let’s step back and look at this composite curve from a simpler perspective. In any insulation coordination study from the simple margin-of-protection calculation to a complex transient analysis using software, the data from this curve must be used. This data is not readily available for the left end of the curve, but the data on the right side is in every arrester cut sheet and this is the part that is used for IC assessment. Example 50kV MCOV Arrester

Typical Varistor/Arrester V-I Characteristics
| Breakdown Region | Pre-Breakdown Region | | High Current Region | | Leakage Current Region V1ma or Reference Voltage Region TOV Region Switching Surge Region Lightning Impulse Region Normal Operating Region 20C 200C Physicists Terminology Engineering Terminology LPL SPL V10kA or U10kA The VI Curve can offer a great deal of data to an interested party. For Insulation Coordination purposes, the VI characteristics in the switching surge and lightning surge regions are the most important. If energy handling of the arrester is part of the study, then characteristics in the lower current region also become important. Another parameter of importance for an arrester is its MCOV rating. This rating describes the maximum voltage that can be applied across the arrester during steady state operation. This along with the LPL (lightning Protection Level) are the two relevant IC study parameters. Vref or Uref Rated V or Ur peak MCOV or UC (peak)

Arrester Protective Characteristics in Graphic Form
Arrester Discharge Voltage Curve Fast Front Voltage 10kA Lightning Protective Level LPL Switching Surge Protective Level SPL Arresters protective characteristics are also somewhat frequency sensitive. There is an inherent inductance like characteristic in the MOV material. For current surges with fronts that crest in a fraction of a µs, the clamping (discharge voltage) is 5-10% higher than for similar surge current amplitudes that crest in 8µs. Because of this mild frequency dependence, there is another way of looking at arrester characteristics relative to I.C. Instead of plotting the voltage vs the current, the voltage is contrasted to the surge front time. These parameters align with insulator characteristics. With these parameters of arresters, at least the most basic I.C. Study can be completed as shown next. Faster Front Surges Slower Front Surges

Calculating Margin of Protection
Insulation Withstand Curve Arrester Discharge Voltage Curve Chopped Wave Withstand CWW Front of Wave Voltage FOW BSL BIL 10kA Lightning Protective Level LPL Switching Surge Protective Level SPL MP1= (CWW/FOW)-1 MP2= (BIL/LPL)-1 MP3= (BSL/SPL)-1 Now we come back to the Deterministic I.C Study referred to as the Margin-of-Protection. With only the cut sheets of transformer and arresters, one can determine if at a basic level, transformers are protected in a substation or on a distribution pole. This should always be the first pass on a substation study. For each of the surge front times, the ratio of the insulation withstand voltage to the arrester discharge voltage is calculated. If these levels all show difference of at least 15% between the two levels, then all is well. Even if a probabilistic study was done to determine the discharge voltages, in the end, a margin of protection study of the transformers should be part of the final study. The short coming of this type of study is that the surge currents are assumed, and not really known. This is ok for a conservative assessment. If lead lengths and/or separation distances are small, this type of study can be very accurate. This same assessment can also be carried out for self-restoring insulators to get rough estimate of the margin of protection they experience for locations near to the arrester. Once there is ample separation distance, then other methods need to be employed. IEEE recommends > .15 or 15% IEEE recommends >.15 or 15% IEEE recommends >.20 or 20%

Clearances and Altitude
Just a few more concepts and we will be done. Next let’s discuss clearances and the altitude effect on it.

Clearances Phase to phase and phase to ground clearances are often the purpose of a study. They are easily calculated once the maximum voltage on a line is determined. With arresters, the NEC clearances can be reduced near the arrester and along ROW if studies are completed. Clearances are an issue of importance for both self-restoring and non-self-restoring insulation. In an air insulated substation where air is the insulation medium between phases and ground, the minimum distance is determined by the maximum surge voltage the system will experience based on its location and line configurations. As stated here, at sea level, if a maximum voltage of 1500kV is expected on a system, then the minimum clearance would be 1500/450 = 3.33 meters or 10.8 ft. The same calculations need to be carried out for switching surges and power frequency. For example, Lightning Impulse withstand of Air at STP is a linear function at 450kV/m

Clearance and Altitude/Elevation
All external insulation is affected by altitude. Specifically in this case, the clearance between lines needs to be increased to attain the same withstand voltage at sea level. This is a self-evident slide that shows how the withstand voltage of external insulation is reduced by altitude. It is about 3% per thousand meters. This means if you wish to maintain your system CFO or BIL as the line passes over a 5000-foot mountain, then you would need to increase the length of the insulators by 15%. 'δ=e-A/26710

Physical Dimensions It seems a bit odd that the physical layout of a station can have a significant effect on the outcome of a I.C.S. but it does. This is due to the traveling wave phenomena

Elongated Substation 6000ft ft ft Backflash 6000 ft out on the line At Arrester Surges travel at ~980ft per µs on an overhead line. In this elongated station, It can be seen here that the surge first appears at the metered points at different times based on the distance from the initial surge. What is shown here is both how the surge is modified by the arresters and transmission line, but also how the distance between the points in the station result in the surge effect being seen at different times. This makes reflections a big part of I.C. Studies. At Breaker At Station Entrance

Ground Flash Density This is the last concept we will cover in this webinar. When an ICS is focused on transmission or distribution line lightning outage performance the Ground Flash Density of lightning is important. Also, if the reliability level of a substation is being assessed, the incoming line outage rate is needed. If the backflash rate on the line has not been measured or recorded, then it will need to be calculated. The calculation requires an understand and value for Ground Flash Density.

Lightning Strike Rate Worldwide
Optical Flash Density = Flashes/year/km2 Divide by 3 to get GFD The ground flash density for any point on earth can be estimated by dividing the optical flash density which is online and free from NASA satellites.

Lightning Incidents and Intensity
In the US and most parts of the world there are ground based lightning detection systems that can be accessed to gain accurate ground flash density information on lightning. Vaisala is the leading US supplier of this information. With this information you can calculate the number of challenges a power systems experiences over any given time period. IEEE Standards 1410 and 1243 give very specific formulae on how to translate this data into direct and indirect strikes to power systems.

Ground Flash Density Is used to calculate the Backflash rate on a line
The challenge rate to a line The outage rate of lines Steepness of a surge on a line The MTBF of a substation Studies that involve lines have several variables that do not appear in station studies. Ground resistance, AC voltage effect, probabilities of front times, probability of amplitudes and more. These studies are 100% probabilistic unless you include transformers in the mix.

The Insulation Coordination Study Report
A detailed – comprehensive study report should be completed for every study. At a minimum it should include a summary of results. Most likely the study had some iterations and each iteration should be explained. Output should be tabulated for quickest comprehension.

Webinar Overview Subjects covered Definitions Examples of Studies
Insulation Fundamentals Backflash Concept Traveling Wave Concept Arrester Fundamentals Clearances and Physical Dimensions Lighting Ground Flash Densities The Report We covered numerous definitions from ICS to components. Simple and complex studies were presented We learned that Insulators are not as simple as it may seem, and they have numerous parameters to consider. The backflash along with traveling waves result in surges getting into substations. They are one of the biggest reasons for ICS. There is much more that can be discussed about arresters, but for ICS purposes, only MCOV, LPL and Energy are considered. Clearance and altitude are important. At the end learned a little about lightning and how often it strikes a power system. A Report should be required for every study, even if it is an internal task. We hope that his IC Fundamentals overview has given you a much better idea on what to expect form a study and why one may need to be completed on a project. NEMA Section 8LA thanks you for your attention and wishes you the best in your ICS endeavors.

Insulation Coordination Fundamentals

Similar presentations

Presentation on theme: "Insulation Coordination Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Insulation Coordination Fundamentals

Similar presentations

Presentation on theme: "Insulation Coordination Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback