Chapter Two reeling in general the purpose of protective relaying is that faulted power equipment must be removed from service quickly to minimize the equipment damage, limit the effect of the disturbance and maintain the stability of the system. protection schemes are required to recognize and isolate all faults rapidly and with a high degree of reliability. protective relays should be dependable that is they should operate when required to and they should provide security that is they should not operate when not required to. They've also often been nicknamed the silent sentinels of the system. In line with these objectives duplicate protections are provided whenever the system stability studies show the requirement for high speed clearance Have faults in addition to duplicate sets of relays. This entails the use of separate CT secondaries and to protections separate v T's and separate CVT secondaries.

Separate batteries separate trip can trip coils. As much as possible control the wiring wiring is also kept separate to ensure that a single contingency will not render both protections on an element inoperative. The following are the protection requirements for a system. selectivity means that the protect protective scheme should accurately identify the problem tripping only the minimum possible number of circuit breakers required to isolate the fault and interrupt the minimum number of customers Ensure that a minimum number of system elements are taken out of service. Unnecessary interruptions, cut into revenue and reduce customer satisfaction. Unnecessary removal of system elements contributes to the overloading of equipment remaining in service and could contribute to the instability conditions.

Speed is required to minimize the damage to the faulted equipment and also minimize the hazard to personnel who may be in the vicinity. Reliable reliability relays remain inoperative for a long time before faults occur. But if a fault does occur, the relays must respond instantaneously and correctly. Traditionally, utilities utility facilities have favored dependability rather than security. That is, they were they prefer For false trips to failure to trip sensitivity of reeling equipment must be sufficiently sensitive so that it can operate reliably when the level of the fault condition just crosses a predefined limit. Really protective schemes are wide and varied depending on their need as well as the age of the system.

Some of the newer IE D relays, which are basically smaller computers in themselves will incorporate many complete schemes in one box and often call it protection management. But a great deal of the old school relays and systems are still in existence today. Regardless if all in one box or installed separately on many panels. These are some of the main categories differential schemes are used as primary protections on buses, transformers, generators and other rotating equipment. Some schemes employ primary and backup relays usually on equipment in smaller and older stations. Newer and larger equipment is protected by duplicate protections.

Transmitted transmission line protection schemes are also many and varied depending on the requirement but usually include both phase and ground fault protection. Some factors considered in the design of line protection, our length of the circuit, importance of the circuit to the bulk electrical system, availability of communication and carrier equipment and if there is any stations tapped off along the line. Bulk electrical system transmission lines if they are short five to three miles long say often employ pilot wire differential schemes. Other high tension lines are protected by distance or impedance relays almost exclusively. These transmission line protection schemes are further divided into two zone distance protections, face comparison protection, directional comparison protection, direct under breaching protection and permissive overreaching protection. And of course, there is the coordinated overcurrent protection used on feeders.

Circuits used for supplying customer loads that are normally operated radially usually employ phase and ground overcurrent schemes like this other protection used on the bulk power system our breaker failure protection. See key and breaker flash protection. Remote tripping remote tripping employs a carrier channel to send remote trip signals to a terminal to operate circuit breakers at another location. The remote trip is initiated by protective relays associated with the faulted equipment and the distance that they're remotely tripping is usually not too far. Transfer tripping over longer distance transfer tripping accomplishes the same end results as remote tripping, but employs a guard signal on a carrier channel which serves as a channel monitoring signal fault detecting relays The transfer trip transmitter and shift the guard signal to trip the frequency which is accepted by the receiver and initiate a tripping at the remote end terminal. More about these transfer, tripping and remote tripping schemes later.

A complete protective reeling scheme consists of the following components or elements, fault sensing or primary relays along with current and voltage transformers. These constitute the means by which the actual system condition conditions are monitored by sampling the system conditions as they occurred during fault conditions. Action selecting or auxiliary relays perform the logic path that the sequence for final tripping takes place. fault isolating or trip output relays and DC control systems starting with the battery banks, which are the source of tripping potential. The station batteries are unaffected by system disturbances and remain a reliable source of energy for operating circuit breakers. Finally, circuit breakers are required to respond to the tripping signal, open their contacts and extinguish the resulting current arc in a very short time, typically two cycles on a 60 hertz system.

In keeping with the concept of duplicate protection schemes, the breaker breakers can carry to trip coils and control wiring is kept separate to minimize the risk of failure to operate. Therefore, there is usually two separate battery Banks used as well. When a system fault occurs, the older schemes or with the older schemes the fault detecting relays in both primary and backup groups of the zone protection involve begin to operate simultaneously to initiate action to clear the fault. Ordinarily, the primary group will complete the operation and remove the faulted element from the service before the backup release can operate and complete their travel. The relays in the backup group will then reset as soon as the faulty condition has been removed. If the primary relays for some reason failed to clear the fault, the backup relays will continue to the travel and the trip and trip the appropriate breakers if the backup relays on the zone concerned also failed to clear the fault the backup release on adjacent zones Are relied on to isolate the defected component to limit the extent of the power system that is disconnected when a fault occurs.

Protection is arranged in zones. Ideally, the zones of protection should overlap so that no part of the power system is left unprotected. A power system is divided into protective zones, which can be conveniently protected by a group of relays and which can also be conveniently disconnected or separated from the rest of the system. The normal protective zones are demonstrated here. For example, the generators have this zone of protection and they will trip the A and a B breakers and isolate any false containers. Within that zone in the dotted lines there, the transformer zones are demonstrated here, you can see that there's a breaker on each side of the transformer and the zone protection is inside the area of the two breakers.

Similarly, buses are protected in the same manner, this time with more than two breakers. So in one case is four and the other is three. But in each of the cases all the breakers would operate to isolate that particular zone. And the transmission line circuits also have their zone of protection and would isolate by tripping the two associated breakers as indicated there. This slide illustrates the principle of zone protection. Note that the zones overlap the associated circuit breakers.

For a fault in a circuit breaker both adjacent zones are required to operate as indicated here. protection schemes older ones generally consist of at least two groups of relays designated primary and backup, the primary group operates instantaneously with no intentional time delay. The backup group operates with an intentional time delay designed to backup the primary relays in the zone in which they themselves are connected, as well as completing the complete relay in the adjacent zones. If for some reason both the primary and the backup relays in those zones fail to function This slide will further demonstrate what is meant by zones of protection and ideally that the fact that the zones of protection have to overlap so that no part of the system is left unprotected. And in fact, it's better to have two protection schemes operating rather than none. protective relays may fail to operate or having operated may fail to clear a fault for one of the following reasons.

Failure of a current or voltage intelligent supply due to false in the CT or VTS or in the secondary surface. Failure of the DC tripping supply, mechanical or electrical failure of the protective relays Failure of the appropriate circuit breakers to trip due to mechanical or electrical troubles in the breaker. And finally, failure of a carrier channel which provided provides intelligence on remote tripping or transfer tripping schemes. For these reason, reasons protections, systems have backup and or redundancy built into them. Many factors may cause protection failure, and there's always some possibility of a circuit breaker failure. For this reason, it is usual to supplement primary protection with other systems to backup the operation of the main system ensure that nothing can prevent the clearance of the fault from the system.

Backup protection may be obtained automatically as an inherent feature of the main protection scheme or separately by means of additional equipment. Time graded schemes such as overcurrent, or distance protection schemes are examples of those providing inherent backup protection. And you'll see this later as we look at those systems more or those protection schemes more closely. The faulty section is normally isolated discriminative Li by the time grading, but if the appropriate relay fails or the circuit breaker fails to trip, the next relay in the grading sequence sequence will complete the operation and trip the associated breaker thereby interrupting the faulty circuit one section further back. In this way, complete backup coverage is obtained. One or more sections may be isolated and is desirable, desirable but this is inevitable in the event of a failure of a circuit breaker.

Duplicate high speed protective schemes or systems may be installed. These provide excellent mutual backup coverage against failures of protection equipment. Ideal backup protection would be completely independent of the main protection, current transformers, voltage transformers, auxiliary tripping, relays, trip coils and DC supplies would be duplicated. This is known as duplicate protection and is becoming the norm in today's relaying systems. The two protections are both first line protections and with both protections in service they will both operate instantaneous instantaneously to clear an end zone fault. breaker failure protection is provided to cover the possibility that even though everything else works properly, a circuit breaker may fail to trip or having trip may fail to interrupt the fault.

In older schemes and older station, the backup scheme operated like this. Sometimes the main protection scheme would have dedicated backup schemes attached to it. this dotted line shows the zone of protection for the main boss protection. As I said the older backups systems are usually were usually timed or are usually timed because some of them are stillness in existence today, in order to give the primary release a chance to clear the problem. And they were usually not not necessarily had to find zones they had to reach into the Next zone quite readily but because of their time definite time characteristic would not operate the instantaneously they would give the primary relays a chance to operate, but they would provide a backup protection even for other zones and other sources of protection. This slide will demonstrate how the older backup reeling systems would operate under certain conditions.

Just as an example. If a fault occurs on G two, and the primary reeling fails to trip breaker B, the G to backup relays will also attempt to trip breaker B. But after a slight time delay, if the breaker B still fails to open for for any reason, the backup reeling associated with a transform zone will attempt to trip breakers B and D even though D is not in G zone. Hence, the backup reeling would act clear the fault on G two. Similarly, if a fault occurred on the Zed bus, the bus zone protection should operate to trip breakers g h and I. However if breakers G and or H did not trip for any reason, then breakers EMF should be tripped from the transmission line backup protection again, isolating the fault again at the risk of repeating ourselves.

Protective relays are of two types fault detecting relays and auxiliary relays. faulty testing relays are those which monitor system conditions and by their design and settings determine whether these conditions are within permissible operating limits or whether they represent a danger or fault condition. Auxiliary relays operate only when initiated from faulty detecting relays. In older schemes. Both the primary and backup relay groups contain fault detecting and auxiliary relays. In some cases, some of the exhilarate relays may be common to both groups.

In the newer standards, protection schemes with duplicate protections both fault detecting and auxiliary relays are found. But the policy is to keep each group independent of the other even to the extent of having separate trip coils in the zone. In his own circuit breakers, there are a number of other factors which are not false as such, but which may be indicative of faults and which are used to initiate automatic removal of equipment from service. Some of these are for example, pressure the buck holds really that is embedded in the tank of a of a power transformer containing insulating oil would react to pressure inside the tank of the transformer and either alarm or trip the breaker to isolate the transformer. There's also temperature sensing devices including the temperature of transformers, which will either give a signal to the operator that an unsafe condition is is happening and he would then or she would then remove that piece of equipment from service Or if the temperature was high enough, it would automatically initiate a trip to isolate that particular piece of equipment.

In the case of rotating equipment, you also have speed protection, overspeed protection. For example, if a if if a generator starts to disconnect from the system and it still has the the water pressure say driving it, it could enter into an overspeed condition which could be damaging to the equipment so you'd want to make sure you isolate that piece of equipment including the prime mover which would be the water that's causing the the the rotor to rotate too fast. You also in the case, so usually diesel type generators or reciprocating type generators, the have they have inherent vibration monitors and if the vibration exceeds a certain preset level Then it would either send an alarm signal to the operator or it would automatically trip the unit itself. These are just a few that would be you might find in a system actual electrical faults are characterized by one or a combination of the following.

Add abnormally high current in one or more of the phases unbalanced currents in what would normally be a balanced system. This would also tend to generate neutral current that normally is zero. And abnormally add normally low voltages in one or more of the phases. These characteristics enable the protective relays to identify the location and locate the fault and initiate action to clear the fault of equipment. The types of electrical faults usually fall into one of these categories, you can have a face to ground short circuit where one of the phases will actually touch the ground or it could touch the steel power which is grounded. It is classified as a face to ground short circuit.

You could also have the situation where two conductors would touch, in this case is called a face to face short circuit. just hypothetically could happen in a long tower span or between two towers or between two pole structures where wind gusts would cause the conductors to swing into each other, where two of the phases would touch and you would have a phase two phase short circuit. You can also have a face to face to ground short circuit And you could have a three phase short circuit. Now three phase short circuits seldom occur naturally because usually a face to face or face ground situation would trip any of the protection ahead of the time of three conductors being simultaneously connected together. However, there is one case where three phase short circuits occur and that is, during maintenance procedures. If ground clamps which are placed in in position during the maintenance are forgotten and left on when the line is energized, then you would have a situation where the three phases would be shorted together, and you'd have a three phase short circuit condition.

These four types of faults are all called short circuits. You can also have a chance where one of the phases would have a hybrid resistance or face to ground connection either by virtue of the fact that the resistance is high or the fact that you've got a return path with high impedance resistance in series with the conductor. Regardless This is caught is classified as an open circuit, very difficult to detect. And lastly, you could have just a straight open phase or open circuit where one of the conductors breaks without touching another conductor or ground. This just contributes to an unbalanced condition but you don't have any large currents flowing that could be detected easily. So these this type of a fault is very difficult to detect as well.

Now these types of faults that we just mentioned, have been categorized and logged and statistics have been checked out on these type of faults and this is what we found was that single face to ground faults are predominantly The most common type of faults in a power system 70 to 80% of the faults are single face to Ground Type faults. Face to face to ground faults constitute a smaller percentage 10 to 17%. Of these type of faults occur naturally. Face to face fault faults are even lower in ratio there's eight to 10% and three phase faults are very, very low. Again because they don't occur naturally, and hopefully people remember to take the ground clamps off when they're finished doing the maintenance. So three phase faults are usually two to 3% And as we said open circuits are very rare.

I don't have any statistical values on that, but they're rare than all the rest. In a large system with many scattered sources of generation, the transmission grid, which operates at the highest voltage level is the central part of the system. Thus, as the voltage class of equipment increases towards the to 3500, or 735 kV transmission level, the interdependence with the system as a whole becomes greater and outage or malfunction of the equipment has a greater system impact. For this reason, protection schemes tend to be more complex for transmission systems, with more safeguards for system security and a higher And higher voltage levels, the voltage class of equipment has a fairly direct relationship to its cost, other factors being constant, high voltage equipment is more valuable than low voltage counterparts of the same capacity. In addition, in a large system, short circuit values are usually greater at higher voltage levels.

High Voltage lines are longer and generally with greater exposure to the elements and other influences. This has a bearing on the type of relaying protection that is desirable. The various components of an electrical system may be grouped under the following main headings according to the function as shown in this table. However, generally speaking We often classify the system components into either generation, transmission or distribution. Just out of interest, most faults in electric utility system with a network of overhead lines are one face to ground faults resulting primarily from lightning induced transient high voltage and from falling trees or limbs. And in overhead distribution systems, momentary tree contact caused by wind or other other major causes of faults.

Also in Northern Hemisphere's where there is winter, such as in Canada, ice freezing snow And when during severe storms can cause many faults and much damage. We're going to talk a little bit now about electrical devices in a power system and their representation, that you may find them on electrical engineering drawings or designations or identifications in the design of electrical power system. The NC or ANSI American National Standards Institute applies standard device numbers that identify the features of devices in a in an electrical system such as a protection system. A relay or a circuit breaker will have a specific number attached to it and that is a standard number that you will come to recognize as a power system analyst. They are unique numbers and they are used worldwide. So this standard applies to just about everywhere.

One physical device may correspond to one function number for example, 29 is an isolating switch, or a single physical device may be may have several functional numbers associated with it. And I'll show you examples of that in a few minutes. They will add suffixes, suffixes and prefixes to that number to further identify and describe it. And again, you'll see that as we go along. The function descriptions are given by the I triple E Standard C dot three seven dot two dash 1991. You can look that up and you find these numbers as I've indicated them here.

You can also go on the internet To find a lot of reference to these numbers if indeed you want to build them and remember them yourself. And this is a continuation of that listing, I triple E c dot three, seven dot two dash 1991. And comprehensively, every one of these numbers would uniquely describe a function of a particular electrical device on a drawing or on type of any type of a schematic. For instance, this is a small little section of a drawing, where we have numbered some of the functions that are attached to it. We have a boss with a generator attached to it and a breaker and you can see that for instance, the 21 g is on the current and voltage transformers. And it's indicating that there is a distance really attached to that particular part of the drawing.

You can also see that there's over excitation protection on this generator, and it's designated by the number 24. And excitation is associated with the number 40. So loss of excitation is designated as 40. As seen here, you also have over voltage protection on either this boss or this generator, and it's designated by the over voltage number 59. And lastly, there is differential protection and its differential ground protection. In this case, it's 87 G, and it's designated by the number 87 and the letter G As I stated earlier, suffix and prefix letters may be added to further specify the purpose and function of a device.

Relay identification is usually designated in a fractional configuration, such as you see here, the number in the numerator b 94, capital R capital T little our little key. In this case the numbers are significant depending on the utility requirements. This is an old Ontario hydro now hydro one standard where the Big B would indicate that they are using the B battery coil. Number 494, of course is the assay designation for a an auxiliary relay trip device funds The capital R capital T in the case of the the customer or the client that's using it, that is to specify a remote trip and the little r and the little sorry, the little r is a receive function. And the small t means that this relay is a time relay. And you can see that the numbers in the denominator are rk and dash three or seven are the manufacturers relay designation as an was at this time and ASEA or an A C A type now ABB sia Bronco very type relay.

And sometimes they're really parameters are listed and in this case, the relate is rated for 110 to 120 five volts DC it has a time delay range of point 05 2.5 seconds on pickup. And the coils in this squiggly shape line are the positive and negative signs are indicated as to where they're connected in the system. The normally open and normally closed contacts are indicated as in this slide. There is also relaying terminology out there or jargon that is used and we should we kind of fall into the habit of using that once we get associated with it. But it's worth mentioning here so that the newcomer or it's a good review for us as we look at it in the slides really operation and a electromechanical relay is said to have operated when sufficient current is passed through the operating coil to cause movement of the mechanical components and move the contacts to open or closed depending on the design and the purpose of the relay for solid state relays.

The relay is said to have operated when the quantity to which it responds has reached a value where the logic circuit initiates action to cause a set of contacts to open or close depending on the purpose of the relay. relay resetting most electromechanical relays operate against them restraints spring or gravity and with the result that when the actuating quantity disappears or is reduced below a percent Set pickup value, the relay will reset. These relays are called self resetting. However, some relays once they have operated will not reset themselves. These are known as manually resetting or lockout relays. Solid State relays are similar in that once the actual waiting quantity disappears or drops below a pickup value, the logic circuit allows the resetting of the contacts of that relay, relay pickup and relay drop out if the actuating quantity apply to the relay is gradually increased, a point will be reached at which the relay will operate.

This minimum operating value is called the relay pickup value. If the actual quantity is then gradually decreased, a point will be reached where the relay contacts reopen. This value is called a relay drop out value paleis switches are auxiliary switches provided in a circuit breaker and sometimes a disconnect switch and light to the operating mechanism in such a way that they are open or closed by the operation of the main device. Those switches which open when the device opens are called a pally switches. Those which open when the device closes are called B palace. switches and you can see in the diagram here I've got a 52 a and a 52. b 252 being it's a breaker and the A and B says whether it's an A or a B poly switch.

If we open the breaker, you can see that the a poly is open and they'd be poly as closed. If we close the breaker you can see that the a poly is closed and the B poly is open. normally open and normally closed contacts. A contact which is open if the relay has not operate is called a normally open contact if it is closed when there really has not operate. It's called a normally closed contact. On electrical drawings.

All contacts are showing open or closed as the When the relay is not operated, even though in normal operation, it may be that the relay is picked up symbols for normally open and normally closed contacts are shown here. Under certain circumstances it may be desirable to ensure that once a really has operated it remains in the operated position or picked up for a definite period of time or until certain other events have occurred. In such cases a relay ceiling ceiling is provided. A typical ceiling circuit is shown here. And in the interest of simplicity, only the wiring associated with directly with involved directly involved with the ceiling is showing and in this diagram assume that contact X has closed contact Why is normally closed And therefore, the relay will be fed with positive DC current and it will pick up. In doing so, it will cause contacts one and two to close and contacts three and four to open.

In closing contact one applies DC positive potential to the relay coil. Regardless of whether contact x is now open or closed, the relay will stay picked up until contact y is open. Thus, de energizing the relay coil and permitting the contacts to return to the normal position. In this case, there really is said to have been sealed in during night period of time. Continuing on with relay terminology, we're going to talk about definite time relays and inverse time relays. a definite time relay is one in which the time delay induced remains constant over one operation to the next, regardless of the severity of the fault condition.

So diagrammatically you can see here that the current is feeding the coils of this particular relay, which will pick up a contact at a pre determined level. And the timer is a basically a DC timer here will operate after a period of time regardless of the current that's flowing, there will still be a time induced and that's called a definite time. Really an inverse time really is one In which the rate of travel of the moving contact assembly increases with an increase in magnitude of the actual waiting quantity, the time required to close the contact decreases as the fault current increases. This is used to be a very popular relay before the time of electronic relays. And there's still a lot of them in the system today. And they work and function very well in that the more severe or the higher the fault current, the faster the relay will operate.

And depending on how they're built, there's different characteristics as to you know, how fast that disk will move. The time current curve for both of these type reliefs are demonstrated here. You can see the definite time curve is red, and as the current increase It will trip the relay and it won't trip at any faster or slower once the initial value is met. However, in the case of the inverse time relay, you can see that it's dependent on the curve. And the higher the the amperage the faster the relay contacts will close. This ends chapter two