CHAPTER 2 SUBSTATION EQUIPMENTS Major Components Power Transformer

CHAPTER 2 SUBSTATION EQUIPMENTS

Major Components Power Transformer Earthing Switch Current limiting reactor Circuit Breaker Instrument transformer Bus-bar Load Breaking Switch Disconnecting Switch Protective Relay Lightning Arrester Controls Panel Gas Insulations S/s (GIS

Substation Layout

Power Transformer Selection of Transformers: • Transformer size/s must be selected according to the maximum expected load and possibility of future expansions. • The size of transformer may be selected from power ratings to supply present and future loads.

Power Transformer Relative size of HV power transformer

Power Transformer

Power Transformer The three segments based on size ranges are: • Small power transformer: 500 to 7500 k. VA • Medium power transformer: 7500 to 100 MVA • Large power transformers: 100 MVA and above

Power Transformer Dry type transformer is one in which the insulating medium surrounding the winding assembly is a gas or dry compound. Two types of designs of dry type transformers: • Resin impregnated (Vacuum pressure impregnated, VPI) • Cast resin Dry type transformers compared with oil-immersed are lighter, negligible maintenance and nonflammable. • Resin impregnated, permissible winding temperature rise up to 140°C. • The initial cost of dry type transformer is higher compared to similar capacity oil filled transformer.

Power Transformer Table. Dry type Cooling class of transformer • • • Ventilated – Ambient air may circulate, cooling the transformer core and windings Non-ventilated – No intentional circulation of external air through the transformer Sealed – Self cooled transformer with hermetically sealed tank Self-cooled – cooled by natural circulation of air Force-air cooled – cooled by forced circulation of air Self cooled/force-air cooled – a rating with cooling by natural circulation of air and rating with cooling by forced circulation of air.

Power Transformer Oil immersed type transformer Power transformers have cooling class, depend on k. VA rating. Station transformers are commonly supplied with OA/FA/FOA. • OA is open air, • FA is forced air cooling, • FOA is forced air cooling plus oil circulating pumps. ANSI in IEEE C 57. 12. 00 -2000, indicate that First letter – Internal cooling medium in contact with the windings: • O mineral oil or synthetic insulating liquid with fire point = 3000 C • K insulating liquid with fire point > 3000 C • L insulating liquid with no measurable fire point.

Power Transformer Second letter– circulation mechanism for internal cooling medium: • N natural convection flow through cooling equipment and in windings. • F forced circulation through cooling equipment (i. e. , coolant pumps); natural convection flow in windings. • D forced circulations through cooling equipment, directed from the cooling equipment into at least the main windings Third letter – external cooling medium: • A air • W water Fourth letter – circulation mechanism for external cooling medium: • N natural convection • F forced circulation: fans (air cooling), pumps (water cooling).

Power Transformer Table Equivalent Cooling Class ---------------------------------Year 2000 Designations prior Designations to year 2000 ---------------------------------ONAN OA ONAF FA ONAN/ONAF OA/FA/FA ONAN/ONAF/OFAF OA/FA/FOA OFAF FOA OFWF FOW -------------------------------

Power Transformer Maintenance of Transformers require some routine maintenance to reduce accidental outages. Periodic inspections should be made for checking: • connection that are rusty or discolored (indicating excessive heat), • accumulation of dirt on high voltage bushings, rust, • accumulations of refuse from birds, squirrels, etc. , on transformer lids. These inspections should include checks of temperature, liquid level, and leaks in liquid filled transformers

Power Transformer Example: An engineer has to selects a transformer to supply electrical power to a factory which requires maximum of 270 k. VA of power at 400 V on the LV side and no expansion is considered for near future. The power will be provided by connecting the factory to 33 k. V voltage level. Choose the suitable size of transformer.

Power Transformer Solution Powers: 25 k. VA, 50 k. VA, 100 k. VA, 250 k. VA, 400 k. VA, 630 k. VA, 800 k. VA, 1000 k. VA, 1250 k. VA, 1600 k. VA, 2000 k. VA Primary Voltages (line-to-line): 6 k. V, 7. 2 k. V, 10 k. V, 12 k. V, 24 k. V, 31. 5 k. V, 33 k. V, 34. 5 k. V, 35 k. V, 36 k. V Secondary Voltages (line-to-line): 380 V, 400 V The suitable transformer size for the Factory: 400 k. VA, 33 k. V/400 V

Circuit Breaker (CB) is defined as mechanical switching device capable of making, carrying and breaking current under normal circuit conditions and also making, carrying for specified time and breaking currents under specified abnormal circuit conditions such as a short circuit.

Circuit Breaker Elementary tripping circuit of a CB

Circuit Breaker classification Classified according to the voltage class i. e • Low voltage CB: used at rated voltage up to 1 k. V • High voltage CB: used at rated voltage 1 k. V above Classified according to installations i. e; • In-door • Out-door

Circuit Breaker Classified according to external design i. e: • the dead tank • live tank Classified according to the interrupting medium: • Air magnetic • Oil • Air blast • Sulphur Hexafluoride ( SF 6 ) • Vacuum

Circuit Breaker •

Circuit Breaker Oil Circuit Breakers (OCBs) • In recently widely used and the are many still service in the utility industry. • Design of OCBs take advantage of excellent dielectricwithstand capabilities of oil and use the oil not only as interrupting medium but also as insulation. • OCBs can be two grouped; 1). Bulk Oil CB and 2). Minimum oil CBs. • The main distinguishing characteristic of bulk OCBs typesis these CBs use oil not only as interrupting medium but also as the primary means to provide electrical insulation.

Circuit Breaker • The advantages of bulk oil CB • Not affective by surround air • Oil is a good insulation • The disadvantages of bulk oil CB • Can explode and inflammable • Periodic maintenance not need • Large relative size

Circuit Breaker The advantages of minimum oil CB • Minimum oil and space require • Possibility of fire is small The disadvantages of minimum oil CB • Frequent maintenance need because • every time operation oil decrease

Circuit Breaker Air Blast. CB (ABCB), • Compressed air at a pressure of 20 -30 kg/cm 2 is employed as an arc quenching medium. • Suitable for operating voltage 132 k. V above and also been used in (11 -33) k. V. • SF 6 CBs are preferred for 132 k. V. The advantages: • Free from fire risk and Light maintenance The disadvantage • Affected by atmosphere • To create loud sound

Circuit Breaker Vacuum Circuit Breakers (VCBs) • The arc product are immediately forced to and deposited on metallic shield surrounding the contacts. • Without anything to sustain the arc, it is quickly extinguished. • The VCB is very simple in construction compared to other types of CB. • VCBs are widely employed for metal-clad switchgear up to 38 k. V class. • When used in outdoor circuit breaker designs, vacuum cylinder is housed in a metal cabinet or oil-filled tank for dead tank construction.

Circuit Breaker Vacuum circuit breaker

Circuit Breaker Gas Circuit Breakers Generally employ SF 6 (Sulfur Hexafluoride) as an interrupting and an insulating medium. SF 6 CBs are available as live tank or dead tank CB live tank CB

Circuit Breaker The advantages of SF 6 CB: • SF 6 is a chemically very stable, non-flammable, non corrosive, nonpoisonous, colorless and odorless gas. • The superiority of dielectric SF 6 was several times better than air. • Need small space. • Heats transfer capabilities is better than air • An excellent arc quenching medium. The disadvantages of SF 6 CB: • Most expensive • Special room need

Circuit Breaker Selections of Circuit Breaker ----------------------------------------------Rated voltage Choice of circuit Breaker Remark ____________________________ Below 1 k. V Air Break CB 3. 3 k. V - 66 k. V Oil CB, Vacuum CB 132 k. V- 220 k. V Air blast CB Min. oil preferred 400 k. V-760 k. V Oil CB, Air blast C. B ABCB Preferred ----------------------------------------------- Operating mechanism To open/close the contacts of a circuit breaker, one of the following mechanisms is employed namely • Spring • Solenoid • Compressed air.

Circuit Breaker Specification of Circuit Breaker 1. The standard ratings according to IEC as follow Rated voltage, rated current, rated frequency 2. Rated breaking capacities 3. Rated making capacities 4. Rated time capacity. 5. Rated operating duty

Circuit Breaker Voltage rated according to IEC for 72. 5 k. V and above are (in k. V): 100, 123, 145, 170, 245, 300, 362, 420, 525, 765 k. V. Rated standard normal current (A): 400, 630, 800, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300 Standard frequency is 50 Hz or 60 Hz • Breaking Capacity. • Symmetrical breaking capacity • Asymmetrical breaking capacity.

Circuit Breaker • Symmetrical breaking capacity is the rms value of the ac component of the fault current that the CB breaker is capable of breaking under specified conditions of recovery voltage. • Asymmetrical breaking capacity is the rms value of the total current comprising of both ac and dc components of the fault current that the circuit breaker can break under specified conditions of recovery voltage.

Circuit Breaker • The making capacity of a circuit breaker is the current that the circuit breaker is capable of making at stated voltage under prescribed conditions of use and behavior. • The rated making current is defined as a peak value of the current (including the dc component) in the first cycle at which a circuit breaker can be closed onto a short circuit.

Circuit Breaker • The sort time current rating is based on thermal and mechanical limitations. The circuit breaker must be capable of carrying short circuit current for a short period • While another circuit breaker is clearing the fault According to BS • 3 seconds : the ratio of Isym to Inorm ≤ 40. • 1 second : the ratio of Isym to Inorm > 40. • Rated Operating Duty. • According to IEC recommendations for rated operating duty for breaker • which are not intended for auto-reclosures, there are two alternatives a) O – t – CO b) O – t”- CO O : opening operation; C : closing operations ; CO : closing followed by opening t, t’ and t” : time interval ; t and t’ is 3 minutes; t” is 15 second.

Air Break Switches • Is a switching device that uses air as the dielectric. Air Break Switches (ABS) are widely installed throughout distribution networks for use as both isolation or switching points. They are usually employed in out door installations. • Can interrupt the exciting currents of transformer, or moderate capacitive currents of unloaded transmission line. It’s cannot interrupt normal load current.

Air Break Switches

Disconnecting Switch • A mechanical device used to change connections within a circuit or isolate a circuit from power source, • Normally used provide isolation of the substation equipment such as circuit breaker, bus-bar for maintenance and also be used for bus sectionalizing. • Its must only be opened and closed when the current is zero, in other word its cannot interrupt in normal load current.

Disconnecting Switch

Disconnecting Switch Location of Disconnecting Switch in Substation

Interlocking • For preventing such incorrect operations interlock are used, i. e. isolating switch is blocked against the respective circuit breaker, earthing switch or other corresponding isolating switches. • There are two ways interlocking namely, mechanically and electrically interlocking.

Interlocking The requirements of interlocking • The disconnecting switch cannot be operated unless the associated breaker is locked in the open position. • The earthing switch shall close only when the line switch is open and locked and is not in its stroke. • The line disconnecting switch shall close only where the corresponding circuit breaker and the earthing switch of the corresponding line are open. • The circuit breaker shall close only after all the disconnecting switch associated with it have been locked either in closed or open position.

Mechanical interlocking single bus arrangement

Load Break Switch • Is a disconnect switch that has been designed to provide making or breaking of specified currents. • This is accomplished by addition of equipment that increasing the operating speed of disconnecting switch-blade and the addition of some type equipment to alter the arcing phenomena and allow the safe interruption of the arc resulting when switching load currents.

Load Break Switch

Earthing Switches Are safety switches that ensure a transmission line is definitely grounded while repairs are being carried out.

Earthing Switches

Lightning Arrester The purposes of these equipment are to limit the over voltage that may occur across transformers and other electrical apparatus due either to lightning or switching surges. Four types of lightning protective device: • Rod gap • Expulsion type lightning arrester • Valve type lightning arrester, • Metal-oxide Surge Arrester.

Lightning Arrester L. A on Bus-bar L. A on transformer

Operation principle of lightning arrester

Rod/spark gap type • Very simple and cheap • It does not cut off follow current • The BD characteristic of a spark gap is also affected by atmosphere conditions • Used at bushing insulator of transformer, circuit breaker, line insulator (arching horn).

Expulsion type surge arrester • A spark gap enclosed in a fiber tube and another external rod spark gap in air, in series. • have better volt-time expulsion type lightning arrester characteristic than rod gaps • Can interrupt the power frequency follow current. • used to prevent flashover of line insulators, isolator and bus insulators.

Valve type surge arrester Classification of the valve type L. A based on application: • Station type to protect main equipment in substation > 70 k. V • Line type to protect equipment in substation < 70 k. V • Station type to protect equi. like motor/generator (2. 2– 5 k. V) • Secondary Arrester to protect low voltage system 120 V-750 V

Metal-oxide surge arrester • Zinc oxide (Zn. O) connected in series and elements • Having a highly non-linear resistance. • The non-linear characteristic of zinc oxide element has enabled to make surge arresters without series connected spark gaps. • Suitable for system protection up to the highest voltages.

Instruments Transformer Current Transformer (C. T. ) and Potential Transformer (P. T. ) • The both of transformer have low VA rating Current transformers • To reduce the heavy current • To isolate the relay circuit from the primary circuit

Current transformer C. T. ’s Specifications 1. Rated primary and secondary current 2. Rated short time current 3. Rated burden 4. Accuracy class 5. Ratio error 6. Insulation level (for only primary)

Current transformer • The secondary current rating is usually 5 or 1 ampere. • C. T used for instrumentations is required to be accurate over the normal working range of currents. • C. T. used for protection is required to give a correct ratio up to several times the rated primary current. The rated primer current • 2. 5 A; 5; 7. 5; 10; 15; 20; 25 30 up to 10, 000 Ampere. Rated short time currents and duration for example • 750 A for 0. 5 sec; 525 A for 1 sec; 300 A for 3 sec. The accuracy of a current transformer is expressed in terms of the departure of its ratio from and is called the ratio error.

Current transformer Per cent error = x 100 % N : Nominal ratio rated IP to rated IS. IS : Secondary current; IP: Primary current. • Accuracy class of current transformers are quoted 5 P and 10 P which gives a composite error at rated accuracy limit of 5% and 10%. • A current transformer information: 15 VA, Class 5 P 20 which means that it is rated for a burden 15 VA and will not have more than 5% error at 20 times I (rated). • C. T. burden (VA) is defined as the loads connected across its secondary

Current transformer Saturation with Asymmetrical Fault Current (with DC Component)

Typical exciter curve of a Class C or Class K multi-ratio C. T.

C. T. excitation curve

C. T. excitation curve

C. T. excitation curve Example A current transformer is designated as 600/5 A, 25 VA, 5 P 20, RCT = 0. 2 ohm. Determine the knee point voltage.

C. T. s construction • Current transformers can be classified into • Window type C. T. which includes bushing type ; bar type C. T split-core type CT. The window type CT is the simplest form of instruments transformer • Wound type C. T. the latter having both a primary and secondary winding. The most common type is that wound on a torroidal core

Current transformer

Polarity of C. T. : Polarity marking

Figure Polarity convection for current transformer Figure Wye connection of CT

Burden on C. T. s for various types of C. T. s connections and faults

Burden on C. T. s for various types of C. T. s connections and faults

Current transformer

Potential transformer • To reduce the system voltage to level lower (suitable) • The secondary voltage rating is usually 110 V, 220 V, 240 Volt. Potential transformer specification 1. Rated primary and secondary voltage 2. Rated burden 3. Accuracy class 4. Ratio error 5. Number of phases

Potential transformer Percentage ratio error = x 100 K: Nominal voltage ratio, VS : Secondary voltage, VP : Primary voltage. Potential Transformer Construction The following two types are used for P. T. s namely i). Electromagnetic wound type and ii). Capacitive coupled PT

Potential transformer The standard accuracy classes at the rated burden are 0. 3, 0. 6, and 1. 2. For medium and high-voltage systems, VT is selected with an accuracy class of 0. 3 at the rated burden. The standard burden designations are W, X, M, Y, Z, and ZZ.

Potential transformer

P. T. construction 69 k. V Single bushing Dual bushing out-door PT 242 k. V single bushing

Current Limiting Reactor • Connected in series with each phase of the feeder • To prevent short-circuit should occur on one of the feeders while impedance of the MV bus is usually very low.

Current Limiting Reactor

Control Panel • The control panels may be of the cubicle type or corridor (duplex) type. • In the cubicle type each panel is fabricated as an independent box closed from sides but with a door at the back.

Control Panel