eee 465 ppt 1

EEE 465: High Voltage EngineeringInstructor: Nabil Shovon Ashraf, Ph.D.

Office Room to Contact: SAC 1047

Lecture 1 In this introductory lecture, you will learn about all the integral elements of a typical high voltage power system transmission and distribution network For high voltage transmission and distribution power network, proper insulation from breakdown of gaseous, liquid and solid materials is unavoidable. Hence the lecture will initiate the comprehensive study of breakdown mechanism of first and foremost gaseous dielectric and continue in lecture 2.

EEE 465: High Voltage Engineering

Pay particular attention to the above power system complete network that you would like to be knowledgeable of. Substation and distribution stages are arranged top-down and their respective step-up and step-down voltage rating are also shown

Figure 1: Schematic representation of a power network


To prevent current leakage or flashover, selection of proper insulating material must be on the power system designer’s forte Air at atmospheric pressure has been an obvious choice as an insulating material. Air can easily withstand 20 kV/cm. The tripping point for flashover initiation is 30 kV/cm for air Sulphur hexaflouride gas (SF6) is used in metal clad gas insulated system (GIS)


Figure: Principle of the generation of electricity, using Faraday’s Law

Generation of Electricity employed by major energy sources and energy conversion obeys Faraday’s Law. In the above figure, water, steam or wind energy causes rotation of the turbine and generator rotor. When the magnetic poles on the rotor move past the stator, electricity is induced in the stator windings, producing three phase power


Viewgraphs of (1) hydro and fossil-fuel power stations & (2) conventional and pebble bed modular reactors (PBMR)

Top figure: hydro-electric power station(left) and coal-fired power station (right)Bottom figure: conventional pressurized water reactor (PWR)(left) and PBMR(right)


Generators: The rating of a typical large generator is between 500 and 900 MVA. The generator voltage is typically 24 kV with an estimate of full load current 10-20 kA. The losses associated with these high currents necessitate water-cooling of the stator winding and the use of air-cooled bus ducts for the connections between the generator and the generator transformer

(a) (b)

Figure: (a) Generator, stator windings and (b) detail of the winding insulation in the stator slot


Substations: The substations are the nodes in the power system where several lines and transformers are connected together. Transmission substations serve as the interconnection nodes on the main power transmission system

Figure: Single Line diagram of typical 132 kV substation


Substations: Conventional outdoor and indoor gas-insulated substation (GIS) impart an alternative to efficient power transmission and distribution. In GIS, compressed SF6 gas with exceptional insulating properties facilitates the design of very compact substations

Figure: Typical 132 KV substation. (a) outdoor and (b) indoor GIS


Power Lines and Cables: High voltage feeders in the form of overhead power lines or underground cables interconnect high voltage substations

Figure: Typical overhead high voltage lines: (a) 400 kV suspension tower and (b) 400 kV double circuit strain tower (c) 22 kV woodpole distributionline with fused cable connection

(a) (b) (c)


Power Lines: towers, conductors, metal ware and insulators. Towers consist of steel lattice structures or wood or steel poles. Conductors are aluminium core steel reinforced (ACSR). Where the conductors are supported at the towers, insulators are used.

Figure: Examples of conductor power line components: (a) Detail of ACSR conductor, (b) Pistol grip connection of a conductor to a strain insulator &(c) Wind vibration dampers

(a) (b) (c)


Power Line Insulators: Traditionally the ceramic materials glass and porcelain were the main insulator materials but now-a-days various non ceramic insulator materials are available.

Figure: Insulator types


Underground power cables: Underground cables are buried in trenches and correct installation is important from a safety point of view. Heat dissipation is also important factor.

Figure: Various types of cable constructions and terminations


Bushings: It is sometimes required to take a high voltage conductor through a wall or the tank of a transformer. In such cases a bushing is required to support the high voltage conductor and to provide the necessary insulation in the axial and radial directions

Figure: (a) Normally straightthrough epoxy line bushing and(b) Capacitively graded paper and oil bushing

Figure: Example of typical graded bushing


Power transformers: The power transformer transforms voltage from one level to another and must be able to handle the full power to be transformed, i.e., the copper windings must be able to withstand the full load current and short term overcurrents and the magnetic circuit and insulation must be able to cope with the rated system voltage, allowing for overvoltages.

Figure: Power transformers


Instrument transformers: Current transformers (CT) and voltage transformers (VT): In an operating power system, it is necessaryto know the system voltages and currents as accurately as possible. Current transformers (CT’s), voltage transformers (VT’s) and capacitive voltage transfomers (CVT’s) are used for this purpose. A VT is a high impedance shunt device, similar to normal power transformer whereas a CT is a low impedance device in series with the power line circuit current


Voltage transformer: Voltage transformers are typically power transformers with a rating of 110 kV: 110 V, a ratio of 1000:1 for measuring, metering and protection purposesCapacitive voltage transformers: These dividers divide the actual high voltage level to a lower level before using a conventional voltage transformer

Figure: (Left) a capacitive voltage transformer


Line Traps: Power lines are provisioned to carry high frequency signal 300 kHz in power line carrier applications. The high voltage capacitors of the CVTs are used as coupling capacitors and air cored inductors (line traps in this illustration) as part of the filter circuits


Circuit breakers and fuses: The duty of circuit breakers and fuses is to rapidly interrupt fault current.

Figure: Schematic representation of the interruption of the fault current by a circuit breaker and associated protection relays


Air blast circuit breakers: In air blast circuit breakers, compressed air at pressures as high as 1 Mpa is used to blow out the arc as the contacts are separated. While the contacts are open, full system voltage appears across the contact and the required insulation is provided by the pressurized gas.


SF6 circuit breakers: In SF6 circuit breakers, the insulation and arc quenching tasks are both performed by SF6 gas. SF6 is an electronegative gas with superior insulation characteristics. The gas also has the ability to assist arc quenching, due to its thermal and electronegative properties. The circuit breakers are usually spring operated.


Fuses are mainly used up to voltage 22 kV. High rupturing capacity fuse elements are used. The fuses are often pole-mounted as drop-out fuse link assemblies.


Isolators: When working on apparatus, such as circuit breakers, it is necessary to disconnect the apparatus from the live system and to apply visual earths. For this purpose isolators and earthing switches are provided. Isolators are different from circuit breakers in that they should be operated under no load current and they have no arc quenching capacity.


Surge arresters and lightning arresters: The power system is subject to transient overvoltages due to lightning and switching. Lightning arresters, also called surge diverters, are applied to limit the peak voltages to values that cannot damage the equipment to limit overvoltages. Lightning arresters are usually fitted with grading rings to ensure a more uniform voltage distribution over the height of the arrester.

eee 465 ppt 1

Environment

high voltage transmission

generator voltage

high voltage engineeringto

high voltage engineeringfigure

power networkeee

distribution power network

high currents

efficient power transmission