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A write up based on a substation visit Nelamangala,Banglore NEED OF SUBSTATIO SUBSTATION N A substation substation is an essential component of
an electrical generation transmission transmission and distribution
system.They transforms trasnmission trasnmission voltage from highlevel to low level, or the reverse,or perform any of several other important functions.Electric power may flow through several substations at different voltage levels between the generating station and consumer.Generally the operations in the substations is controlled remotely by a control centre. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.
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SUBSTATION EQUIPMENTS
Lightning arresters Capacitive Voltage Transformers Wave Trap Isolators Shunt Reactors Bushings Circuit Breakers Power Transformers Relays Earth Mat
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LIGHNING ARRESTOR
Fig 1.1 A lightning arrester(Fig 1.1) also known as lightning conductor, is a device used on electrical on electrical power systems and telecommunications and telecommunications systems to protect the insulation the insulation and conductors of the system from the damaging effects of lightning. lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.There are different types of arrestors like rod gap arrestor, horn gap arrestor, multigap arrestor, etc. Today the most commonly used arrestor is metal oxide (MOV) arrestor.The metal oxide arresters are without gaps, unlike the silicon carbide (SIC) arrester. This “gap“gap -less” design eliminates the high heat associated with the arcing discharges. The MOV arrester has two-voltage rating: duty cycle and maximum
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continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating. A metal-oxide surge arrester utilizing zinc-oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at operating frequency.When a metal oxide arrester is disconnected from an energized line, a small amount of static charge can be retained by the arrester. As a safety precaution, technicians install a temporary ground to discharge any stored energy.Duty cycle rating: The silicon carbide and MOV arrester have a duty cycle rating in KV, which is determined by duty cycle testing. Duty cycle testing of an arrester is performed by subjecting an arrester to an AC rms voltage equal to its rating for 24 minutes. During which the arrester must be able to withstand lightning surges at 1-minute intervals.Maximum continuous operating voltage rating: The MOV rating is usually 80 to 90% of the duty cycle rating.The metal oxide arrestor used in this substation is zinc oxide arrestor. It is used in 400kV line and has a rating of 390kV.
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CAPACITOR VOLTAGE TRANSFORMER A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a a low voltage signal, for measurement or to operate a protective relay. relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer a transformer to isolate and further step down the voltage for the instrumentation or protective relay. The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the [1] burden of the connected metering or protection devices. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network
Fig 2.1
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WAVE TRAP An electronic filtering device designed to exclude unwanted signals or interference from a receiver is known as wave trap.Line trap also is known as Wave trap. Basically, it is a tuned circuit placed in the antenna path to your rig, which traps and then dissipates the signal, or at least a bunch of it, from a station, allowing you to reduce interference between the stations .What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotectionpanel in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations withoutdependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals. The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow ofthese signals in to the substation busbars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.
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Isolator
Fig 4.1 An isolator is a switch used to isolate a section of a circuit from any energised conductors, by presenting a visible break in the circuit. Isolators are not designed to break load currents (unless fitted with optional arc-breaking feature) or to break fault currents. In high-voltage systems, isolators are used in conjunction with circuit breakers with the isolators opened after a a circuit breaker has opened the circuit, and closed before before the the circuit breaker closes the circuit. For example, before one can perform maintenance work on, say, a high-voltage circuit breaker, the following sequence is followed the circuit breaker is opened, breaking the circuit,isolators located on either side of the circuit breaker are opened, presenting a visible break between the isolator and energised lines. Here we are using the isolator for isolation of apparatus circuit breaker,transformers, and transmission lines, for maintenance
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SHUNT REACTORS
Shunt Reactor and Transformer both appear similar in construction. Reactors are also often equipped with Fans for cooling similar to Power Transformers.However, there are major differences between the two. While a Power Transformer is designed for efficient power transfer from one voltage system to another, a shunt a shunt reactor is intended only to consume reactive Vars. Vars.Thus, there are more than one winding on a Power Transformer with magnetic core which carry the mutual flux between the two. In reactor there is just one winding. The core is not therefore meant only to provide a low reluctance path for flux of that winding to increase the Inductance.In case of a Power Transformer, primary Ampere-Turns (AT) is sum of exciting AT and secondary AT. AT loss (in ( in winding resistance, eddy loss and hysteric loss) loss) is kept to as minimum as possible. Exciting AT is small compared with the secondary AT. Rated current is based on the load transfer requirement.
Fig 5.1
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Bushing An electrical bushing is an insulated device that allows the safe passage of electrical energy through an earth field.When the conductive material is insulated and passes through any earthed material, it is known an electrical bushing. A typical bushing design has a 'conductor', (usually of copper or aluminium, occasionally of other conductive materials), surrounded by insulation, except for the terminal ends. In the case of a bus bar, the conductor terminals will support the bus bar in its location. In the case of a bushing, a fixing device will also be attached to the insulation to hold it in its location. Usually, the fixing point is integral or surrounds the insulation over part of the insulated surface. The insulated material between the fixing point and the conductor is the most highly stressed area. The design of any electrical bushing must ensure that the electrical strength of the insulated material is able to withstand the penetrating 'electrical energy' passing through the conductor, via any highly stressed areas.
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CIRCUIT BREAKER
A circuit Breaker is a piece of equipment whi ch can make or break b reak a circuit either manually or b y remote control under normal condition.It also break a circuit auto matically under fault condition. Accordingly the circuit breakers are classified into
Oil circuit breaker :
This employ some insulating oil for arc extinction
Air Blast circuit breakers
In this high pressure air blast is used for extinguishing the arc
Vaccum circuit breaker
Here vaccum is used for arc extinction
Sulphur hexafluoride SF6 circuit Breaker
Sf6 gas is is used as the arc quenching medium.The contacts of the breaker are opened in a high pressure flow of sf6 gas and arc is struck between them.It consists of fixed and moving contacts enclosed in a chamber containing SF6 gas.In the closed position of the breaker ,the contacts remain surrounded by the SF6 gas .When the breaker operates the moving contact is pulled apart and arc is struck between the contacts.the high h igh pressure flow of SF6 rapidly absorbs the free electrons electrons in the arc path to form immobile negative ions,then they build up high high dielectric strength and causes the extinction of arc.the pressure of sf6 gas is maintained at 14 kg/cm2.SF6 k g/cm2.SF6 breakers have low maintenance cost ,and minimum auxillary requirements.SF6 breakers have been developed for voltages 115kv to 230kv,power ratings 10MVA to 20MVA and interrupting time less than 3 cycles
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The following issues are associated with SF6 circuit breakers:
Toxic lower than order gases When an arc is formed in SF6 gas small quantities of lower order gases are formed. Some o f these byproducts are toxic and can cause irritation to eyes and respiratory systems. Oxygen displacement SF6 is heavier than air, so care must be taken when entering low confined spaces due to the risk of oxygen displacement. A transformer is a static machine used for transforming power from one circuit to another without changing frequency.
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Power Transformer Generation of Electrical Power in low voltage level is very much cost effective. Hence Electrical Power are generated in low voltage level. Theoretically, this low voltage leveled power can be transmitted to the receiving end. But if the voltage level of a power is increased, the electric current of the power is reduced which causes 2 reduction in ohmic or I R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low leveled power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with help of step down transformer. These are the use of electrical power transformer in the Electrical the Electrical Power System.
Fig 8.1 Two winding transformers are generally used where ratio between High Vo ltage and Low Voltage is greater than 2. It is cost effective to use auto use auto transformer where the ratio between High Voltage Vo ltage and Low Voltage is less than 2. Again Three Phase Single Unit Transformer is more cost effective than a bank of three Single S ingle Phase Transformer unit in a three phase system. But still it is preferable to use later where power dealing is very large since such large size of Three Phase Single Unit Power Transformer may not be easily transported from manufacturer's place to work site.
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RELAYS A relay is an electrically an electrically operated switch. operated switch. Many Many relays use an electromagnet an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with c omplete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance tele graph circuits, repeating the signal coming in from one c ircuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations.
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Earthmat An earthing system has two objectives:
1. provide for a uniform potential gradient over o ver a physical area. This is needed need ed so that individuals who are walking in the area are not exposed ex posed to excessive voltages across the length o f a stride ('step potential') or when they touch a metallic structure ('reach potential').
2. provide a connection to earth through which lightning or system fault current can be dissipated. In effect, this objective amounts to managing the potential different relative to a remote location.
Earthing mats normally consist of a rectangular (or square) grid of copp er wires that are buried in underground trenches. This grid must lie underneath the entire area to be protected, and must extend beyond any security fencing that may be present to prevent intrusion by b y civilians. Calculations that take into account the resistivity of the soil and any surface treatment in the protected area (it is customary to put a layer of c rushed stone over the protected area) to achieve maximum step and reach potentials.
Earthing mats also may include driven copper copp er rods at some or all of the grid intersections. The rods are more required to dissipate current than to manage step and reach potentials.
In some instances, it is also necessary to treat the soil surrounding the mesh with chemicals to enhance soil conductivity. And it may be necessary n ecessary to install driven rods in bore holes that a re backfilled with special chemicals.
In especially challenging applications (for example, in a reas where there is a lot of granite underneath the surface soil), it may be necessary to supplement the mesh with either metal plates that are sunk into ne arby bodies of water, or by connecting the mesh to water wells that are bored through the granite into conductive soil.
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Onsite Recordings Recordings
Earth pit For the entire substation we are digging out at one place and there th ere we are putting earth mats and its nodes are connected to earth electrodes.
Neutral earthing The neutral should be earthed at zero potential.
Difference between isolator and circuit breaker Isolator is used to interupt the voltage and circuit breaker is used to interupt the currents.
Floating neutral For auxilarry purpose we are using the floating neu tral.If we dont have a floating neutral it will leads to the destruction of the equipments present in the control room.
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Conclusions This visit to the substation was one of the source t o know about the switch yards and various equipments that is using in the switch yards and their practical demonstrations.This visit was an extremely useful trip for us, as it helped us to gain an idea about the basic operations and the significance of a substation.
