CHAPTER ONE overview of stations. The main objective of Power Systems Protection and control is to isolate a faulty section of the electrical power system from the rest of the live system so that the rest of the system can function satisfactorily without any severe damage due to the fault current flowing. Actually, a circuit breaker is the item that isolates the faulty system from the rest of the healthy system. And this circuit breaker automatically opens during fault conditions due to its trip signal coming from protective relays. The main philosophy about protection is that no protection of a power system can prevent the flow of fault current through a system it can only prevent a continuation of flowing fault current by quickly disconnecting the short circuit path from the system. What this means is that in order to detect a fault, a fault has to happen first, so there's certain amount of current has to flow.

In order to identify that there is a fault in the system. What they need to do or what we need to do is detect that fault condition as fast as possible, and then isolate the system as fast as possible. In this chapter, the overview of stations we'll look at the general makeup and configuration of some of the power system stations out there. And although there are numerous variation of the types of stations and the configurations that are out there, what follows is generically a we'll cover most of the common elements of them as I said before substations or stations Power Station high voltage stations can be made up of various elements and they can take on various configurations. What I'm got in front of you here in the way of a slide is what I would call a generic setup of a station. And we'll go through the various elements here just to become familiarized with how generally a power station is put together.

A station can usually be divided into sections in this case, we've got a primary side, we call it in the diagram a side, but that's essentially where the power is coming into the station. It could be coming in at a high voltage, anywhere from 735 kV down to 66 kV or even lower than that. However, it's usually the elements associated with power flow into the state. The secondary power lines are the beside the station is usually associated with power flow out of the station. And in the case of a distribution station within a municipal utility say the power would be coming in from the wholesale companies such as hydro one might be a stage a power company that's selling power wholesale to the municipal utility and the power flow out would be to generally where they're distributing the power throughout the utility or it could be a power flow anywhere else, but we'll call out the secondary power line side of the station.

The primary power lines just as you can see here, just above number one, as I said they could be any anything from extremely high voltage or very high voltage systems such as 735,000 volts down to 115 kV and the power lines could be on metal structures or on pole structures, or they could even be underground services coming into the station. However, we're representing them here by a metal tower with a double light circuit coming in. And that is what we'd call the power lines into the station. Now most of the power lines as well as across the top of the station, you'll have a ground wire, which is usually earth or it's not usually it is earth through the metal structures where it's attached and you can see that it's attached directly to the top of the power line coming in as well as the tops of the various structures in the station.

The ground wire has a two fold function one it tries to maintain the ground current flowing in a predictable area in order to have a safe environment for people to work in. And it also is a deflector not a deflector, but it's kind of where lightning might hit First, if it's going to be hitting in during an electrical storm, it doesn't always hit there and lightning is terribly unpredictable, but for the most part, they will be the first place of lightning they'll hit and then we can control its flow directly to the ground without damaging the circuits beneath it. The overhead lines in the station are called that because that's usually the highest level of the station and the power lines would flow along the top of the station and continue to flow outwards from there. we designate those as overhead lines. Coming into the station, you will see several points where we want to monitor the voltage and because the voltage is usually at very high levels.

We have to monitor that voltage through what they call a voltage transformer. And a voltage transformers function is mainly just to bring the line voltage or the high voltage down to say 115 kV, which is measurable and will destroy the instruments that are that are trying to measure it. And throughout the station, just above five, you can see that there are things called disconnect switches and there's several disconnect switches throughout a station. The function of a disconnect switch is is exactly in what its name says it disconnects one part of the station from the other for various reasons one for protection, it'll automatically open during a protective relaying scheme in some cases. In other cases, they're manually open through a wheel or a lever at the base of it and that will isolate a section of the station from for maintenance purposes or other reasons, but basically they're just a switch.

Just above the number six he'll see a an element of the substation, which is one of the main working elements of the station. A circuit breaker and a circuit breaker is essentially a device a switch for interrupting power flow under high current flow conditions such as might happen under high load conditions or under fault conditions. breaker ratings are usually designated or calculated to be able to interrupt a largest amount of current flowing in the power station. So when stations are designed In power flow power flow is calculated even under what might be considered the most faulty condition of the system. The circuit breaker has to be able to open that and interrupt that current no matter how high it is. If that is not the case, then the circuit breaker would probably destroy itself by trying to interrupt a high current flow.

So, most designs in a station will take these into consideration far in advance of building the station and make sure that any circuit breakers that are placed in that area will be able to handle the highest current flowing in the in that particular circuit as well as they have to be rated for the voltage level that they are connected app if they're connected at say 735,000 volts so you certainly have to be insulated to handle the voltage that that they are connected to, as well as being able to interrupt and map the mass amount of current that could be flowing through a circuit breaker during a fault condition. Just as voltage transformers were in place so that we could measure safely, the voltage levels proportionally that were the are attached to the system, current transformers do the same thing. However, it's the current that we're measuring in this case, and we could have upwards around thousands of amps flowing in a system.

We want to be able to measure that safely with small instruments or even a metering them. So we want to proportionately bring those currents down to a safe level that our devices can handle it. And we do that using current transformers. current transformers can be of various types or talk more about them later. What you're looking At there is a free standing outside current transformer. There are ones that are inside and some are built right into the bushings of breakers and transformers.

And as I said, we'll talk more about that later. Most stations have lightning arresters. These are devices that are placed on the system so that when lightning does hit and the voltage starts to rise in that particular element of the system, there will be a flashover point where the lightning will actually flash to ground momentarily while it's there, directing the dangerous current and the dangerous voltages to ground at a controlled area and control level. Once a lightning disappears, the lightning arresters usually are self healing and they go back to just monitoring the system and and being in place for late to arrest lightning in a future mode. We'll talk more about that later. Most stations have power transformers in them and power transformers are made up of either three single phase Transformers connected in some configuration or they could be a three phase transformer bank, they will handle three phase power.

The purpose of the transformer of course is either to step the voltage up or step the voltage down depending on the functionality of the station and the Transformers can be various configurations. Again, we will be talking more about that later. Most stations have a control building because we have to house outside outside the weather and under lock conditions. We want to have a place where we can put the monitoring relays that control circuits, the metering circuits and the communication equipment outside the weather and usually under lock can So nobody can interfere with the operation of them. And surrounding all of this is a metal security fence metal, I say because they're usually not usually they are grounded to the, to the earth connections and the earth grid of the station. So that if there is a grounding of some of the power lines under unusual conditions, there won't be a rise in potential in the metal fencing itself.

It'll be safe for an operator to say unlock the gates and be able to go in without having to be worried about the fact that the security fence would be at a higher potential than what they are standing. And lastly, you can see the secondary power lines going out of the station. We call them secondary power lines only by the virtue of the fact that they're usually at a lower voltage. They're usually taking the power out of the station and They're designated as secondary power lines. Early electrical substations required manual switching or adjustment equipment and manual collection of data for low energy consumption and abnormal events. As the complexity of distribution networks grew, it became economically necessary to automate supervision and control of substations from centrally attended points to allow for overall coordination in the case of emergencies and to reduce operating costs.

Early efforts to remote control stations use dedicated communication wires often run alongside the power surface themselves. Today development has seen powerline carrier, microwave radio fiber optic cables as well as dedicated wired remote control circuits applied to what we call supervisory control. And data acquisition for substations or SCADA for short. The development of microprocessors made for an exponential increase in the number of points that could be economically controlled and monitored. Today, standardized communication protocols such as the mp3, IEC 61 850, and mod bus. Two lists just a few are used to allow multiple intelligent electrical devices to communicate with each other and for supervisory control.

Distributed automatic control at substations is one of the elements of the so called Smart Grid. switchyard configurations are classified under two main designs the open profile or air insulated switchgear is and the enclosed or gas insulated switchgear g is the open profile structure is further delineated into two common types. The standard high profile switch yards that can be seen for many kilometers and have been around since the turn of the century and low profile or reduce visual impact RVI switch yards, which may be passed relatively unnoticed. The type of design that is utilized for high voltage stations will be the result of a main factor. These main factors which is cost, land availability, and environmental impact, individual components of GIS switchgear such as SS six breakers and Fs six disconnects, which is maybe installed in open profiles. stations or as complete GIS high voltage stations housed in very large buildings.

However, the open profile still station still remains as the most predominant type of structure in today's system. In the open profile configuration the physical equipment has to be operated and maintained is located at ground level on the lowest tier and the output is on the highest or third tier of the station. This output is connected to the output equipment such as line terminations, switches, transformer isolation switches, or a switchyard bus tie switch. Bus drops are used to connect the various tiers of the station. The layout provides a low cost switchyard that can be easily reconfigured for any equipment change. changes or additions or modifications that could occur in the future.

The selection of the optimum station configuration requires the weighing of primarily these factors, cost, reliability, flexibility and maintainability costs which include equipment procurement, the construction, and the long term maintenance cost must be factored into the design switchyard configurations that increase reliability, flexibility and maintainability will generally add to the cost. The expected equipment life and replacement cost and availability must also be taken into consideration. System related Ability reliability is the measure of resilience to single contingency fault and capability to quickly recover from fault occurrences. Multiple supplies and dedicated switch gear will increase the reliability but it will add to the costs. Aside from the increased amount of equipment required increased fault levels and more expensive equipment are are are also involved. Care must also be observed to ensure a single contingency failures will not jeopardize the reliability of adjacent switchyard elements.

Flexibility is the ability to maintain normal switchyard operation with one or several elements out of service. redundant equipment and multiple supplies do not always equate to increase increased reliability single control tangency faults may place additional stress on individual station elements and decrease the reliability. The bus is a critical element of a power system as it is the point of convergence of many circuits, transmission generation Transformers or loads etc. Due to the concentration of several supply circuits involving high current magnitudes, the fault levels are the most severe on the power system at this point. Besides the basic design requirements, reliability and flexibility, dependability, high speed bus protection must be ensured. This is usually in the form of an instantaneous high impedance differential, designated at seven.

We'll talk about those a little bit later. protections with differentially connected Current circuits and we're going to study this more later on. showing here. This configuration is known as a single bus or a radio feed configuration. This is the least expensive of all switchgear bus configurations. However, if a breaker fails under a single contingency, the entire supply is lost until the breaker is repaired or replaced.

The arrangement consists of a single main bus with radio elements lying are to the lines to the generators to the Transformers etc. The bus protection relay has a current input from each of the connected elements. This basic bus arrangement is a simple and economical design with a minimal amount of equipment. Although it has a minimal cost, it is severely limited. reliability, flexibility and maintainability. Let's look at a double bus double breaker configuration.

The double bus double breaker configuration shown here is highly reliable. There are two buses. And there are two breakers for each circuit radiating from each of those buses. Each bus and its breakers are protected by a separate differential relay system. A fault on either bus clears only that one bus, the other bus and all circuits remain in service. Also a fault on any circuit will not affect any of the other circuits even if one of the associated breakers were to fail as a result of a trip of an adjacent zone, and this would include breaker failure, which we'll talk about later.

Although this bus arrangement is very reliable, flexible and maintainable, it is also very costly. It requires two complete sets of equipment breakers, CP switches, controls, and associated protection for each element. The ring bosses another configuration shown here, the ring bus configuration is connected with a bus in a ring as its name suggests, with the elements, a line generator transformer etc, between each of the four breakers as illustrated. Note that the ring is limited to four circuits in must not be confused with a double bus double breaker arrangement. The main advantage of a ring bus configuration is that the bus does not require four separate bus differential protections. The bus section between each pair of circuit breakers is protected as part of the connected circuit.

This bus arrangement is relatively low cost but it is less reliable than a double bus double breaker and also quite operationally inflexible. For example, if circuit a is out of service, the loss of circuit c would remove circuits B and D. Also, a breaker failure of any of the breakers would virtually disable the switchyard. The breaker and to have bosses The most common bus arrangement used for high voltage 500 to 30 kV stations. It is a very reliable and cost effective bus arrangement that uses two buses tied with three breakers in a string or diameter. The breaker and a half is similar in operating advantages and bus really, to the double bus double breaker arrangement, but has slightly decreased reliability. Its advantage Is economic in that three breakers instead of four are required for each two circuits.

This bus arrangement is ideal for 500 or higher kV switch yards, where the cost of breakers and freestanding CTS are very, very high. This is an example of a combining station configuration. Now, this is probably very unique and it's just an example of how things evolve as they exist in the station starts to grow. However, I thought I would put it here is just an example of what you might run into when you're out there. As things evolve, notice from this slide that the composite station configuration may be a combination of separate separate bus arrangements. In this example, the 500 kV switchyard of the 502 30 kV station has only one set of freestanding 500 kV CTS per breaker.

The lines l one and l two are on one and a half breaker buses along with generators one and generators two. However, a very important timeline is located on a double breaker bus. The relay protection zones are indicated by the dashed lines. tripping zones are of course, dictated by the location of the breakers, auto Transformers T one and T two supply the 230 kV switchyard buses, C and D. The 230 kV yard might also be a combination of double breaker, double bus and breaker and a half buses. As I said These stations are not necessarily designed this complicated, but rather evolve and grow over time. The dual elements spot network or DTS and transformer station is at the heart of the planning philosophy.

These stations take power from a high voltage transmission lines usually at 115 or 230 kV and they step it down using Transformers to supply approximately eight outgoing low voltage feeders emanating from the station. These low voltage feeders are usually 13.8 or 28 or 44 kV pole lines carry this voltage and power along the roadways and supply communities with power the stations can be found in many municipal arrangements or even some rural arrangements. The key factor here is duplicity. And being able to switch circuits back and forth as you'll see in the next slide. Do dual elements spot networks are D ies and is a concept that provides redundancy in the form of duplication. For most station components, there are two Incoming transmission lines.

There are two transformers and there are two voltage buses. Any one of these components can fail without seriously affecting supply reliability as the companion equipment is capable of Carrying the total station load. This is a one line schematic of what used to be Ontario hydro standard is now hydro one standard and quite typical of any type of a similar type station that's out there in the world today, you'll notice the redundancy in the form of duplication for most station components and its capability through switching to maintain supply with a single contingency outage which is the characteristic of this type of a station. as has been said earlier, these systems by virtue of the fact that they are at a very high voltage and usually carrying very high currents and high high energy they need to be protected in various fashions. They're in carrying out this protection and control the system.

Elements may be divided into three categories. One is the breakers and switches themselves. And two is the battery bank, which is the heart of the system. And we'll provide the control circuitry for the breakers and switches and the relays which are regulating devices and sensing devices which will pick up the fact that there's a possibility of of a fault out there on the system, and they will react in order to do it. These three elements are what we're going to look at in the following slides. In an electrical system, a protection device is one whose function it is to detect defective or faulted power equipment or dangerous or undesirable system conditions and to initiate suitable switching or give adequate warning.

The simplest and best known electrical system protective system that is, is found in the ordinary household. The fuse or breaker box is the power source and all various circuits branch out from it. If a fault develops in a branch circuit or in any device connected to it, a fuse or a breaker will trip or blow, which will interrupt the flow of current into the defective equipment. Only the circuit associated with defective equipment is removed from service the remaining circuits and the load which they supply continue in a normal operation. This same basic principle is applied to the protection of a power system. Only the minimum of equipment should be removed from service In order to clear the fault, a power system is considerably more complex than that of the wiring in a house great numbers of lines, interconnected transformers and generating stations and there is usually more than one source of infeed to the fault.

The high fault currents and multiple sources of in feed and large power systems render fuses totally inadequate for this type of protection. circuit breakers are used to interrupt fault currents on a power system. Many circuit breakers presently used on the utility system have interrupting capabilities. In many thousands of MVA circuit breakers in themselves cannot identify fault in equipment, their function and capability is solely the switching apparatus in and out of service protected relays are used to identify faulted apparatus and initiate tripping of circuit breakers. System circuit breakers consist mainly of the following types that are in existence today. The bulk oil circuit breaker which has its contacts in Merced in oil, and will interrupt the circuit and extinguish any arc in that interruption with the oil itself.

Air blast circuit breakers which actually use a puff of compressed air to blast out the arc created by opening the circuit. sf six circuit breakers are used for interrupting currents in a an atmosphere of SS six gas and we'll talk about that later. Also in More lower voltage circumstances you'll find a vacuum circuit breakers, which have been really refined in today's technology, and they basically interrupt the circuit in a vacuum, reducing the amount of parking that takes place when circuit breaker is open. Different operating mechanisms such as solenoid spring, pneumatic, hydraulic, etc are employed in circuit breakers. A circuit breaker is the main part of protection system in a power system. It automatically isolates the follow faulty portion of the system by opening its context.

It is also used to control the switching of system elements for the purpose of load flow control and isolation. However, in fact, it is just a switch station batteries. All circuit breakers of electrical power systems are DC or direct current operated because DC power can be stored in these batteries and be available for use during a time where the station is completely blacked out, you still have energy stored in these batteries and control and operation of breakers and control systems within the station can still be operated regardless of the availability of power to the station. Sometimes these batteries are referred to as the heart of the electrical substation. The electrical substation battery is on charge at all times. So when the station is in operation, in other words when AC power is available to to the station, these batteries are under charging conditions.

They discharge only for a brief moment during the time when these circuits have to be operated either through normal switching operations or through relaying and fault conditions. If the station is completely blacked out, then the station can still be controlled and operated in a normal fashion. protective gear consists mainly of power system protection relays like current relays, voltage, relays, impedance, relays, power relays, frequency relays, etc. Based on their operating parameters, sometimes they are definite time delays or inverse time delays or stepping time delays. Various functionalities of these relays and characteristics are used for the protection of the system. During faults.

The protection relay gives a trip signal to the Associated breaker and operates the opening of the contacts of the breaker. This ends chapter one