66 - 33/11 kV substation Equipment
Equipment in a substation can broadly be categorized as follows:
• power transformers;
• circuit breakers (33 kV and 11 kV);
• isolators or isolating switches (33 kV and 11 kV);
• earthing switches;
• power and control cables;
• control panel;
• lightning protection − surge arrestors;
• instrument transformers (current and power transformers, i.e., CTs and PTs);
• earthing arrangements;
• reactive compensation;
• DC supply arrangement;
• auxiliary supply transformer; and
• fire-fighting system.
The design of the substation equipment must comply with the requirement of relevant Indian Standards.We now briefly describe each one of these.
Structures are required to provide entry from the overhead line to the substation and to extend out required number of feeders. The numbers of structures should be kept to a minimum as large number of structures would not only be uneconomical but give an ugly look to the substation and may prove to be obstructions in extending bus-bar, lines, etc. The main structures required for 33/11 kV substations are:
− incoming and outgoing gantries;
− support structures for breaker, isolators, fuses, insulators, CTs and PTs; and
Switchyard structures can be made of fabricated steel, RCC or PSCC,Rail or RS Joist.
You have learnt about the underlying principle and design of a power transformer in previous unit. The general operation and maintenance practices of power transformers are similar to those of distribution transformers, which are discussed in detail in next Unit.
A bus-bar in electrical power distribution refers to thick strips of copper or aluminum that conduct electricity within the substation (Fig. ). The size of the bus-bar is important in determining the maximum amount of current that can be safely carried. The bus-bar should be able to carry the expected maximum load current without exceeding the temperature limit.The capacity of bus should also be checked for maximum temperature under short circuit conditions.Different types of bus-bars, namely, single bus-bar, single bus-bar with bus sectionalizer, main and transfer bus, double bus-bar, double bus-bar with double breaker scheme and mesh scheme are used in a substation in accordance with the safety and reliability considerations.
A circuit breaker is a switching device built ruggedly to enable it to interrupt/ make not only the load current but also the much larger fault current, which may occur on a circuit.A circuit breaker contains both fixed contacts and moving contacts. The purpose of circuit breakers is to eliminate a short-circuit that occurs on a line. Circuit breakers are found at the arrivals and departures of all lines incident on a substation. When the circuit breaker is closed these contacts are held together. The mode of action of all circuit breakers consists in the breaking of the fault current by the separation of the moving contacts away from the fixed ones. An arc is immediately established on separation of the contacts. Interruption of the current occurs after the arc at these contacts is extinguished and current becomes zero.
Elements of a Circuit Breaker
Circuit breakers contain the following elements, irrespective of the medium for arc quenching and insulation:
− main contact at system voltage;
− insulation, such as porcelain, oil or gas, between the main contacts and ground potential;
− operating and supervisory accessories, of which tripping facilities are most important.
A wide variety of closing and tripping arrangements (using relays with variable time delay) and a number of operating mechanisms (based on solenoids, charged springs or pneumatic arrangements) are available now-a-days.
The types of breakers used in a distribution system are:
− air break type;
− oil break type;
− vacuum type; and
− SF 6 gas breaker.
Circuit Breakers: a) Oil Break Type Breaker; and b) SF 6 Gas Breaker
Circuit Breakers: a) Oil Break Type Breaker; and b) SF 6 Gas Breaker
The rated voltage of circuit breakers for 33 kV level is 36 kV, and for 11 kV,it is 12 kV. The short circuit current rating is 25 kA. The 11 kV switchgear is generally metal enclosed indoor type.
Isolators are mechanical switching devices capable of opening or closing a circuit
− when a negligible current is broken or made, or
− only a small charging current is to be interrupted, or
− when no significant voltage difference exists across the terminals of each pole.
Isolators are capable of carrying current under normal conditions and short circuit currents for a specified time. In open position, the isolator should provide an isolating distance between the terminals. The standard value of rated duration of short time current capacity withstand for isolator and earthing switch is normally 1 second. A value of 3 seconds is also sometimes specified. For 33 kV, horizontal type isolating switches are used. The rated normal current is 630 A at 36 kV. For 11 kV, both horizontal and vertical mounting isolating switches of 400 Amps at 12 kV are used.
Earthing switches are provided at various locations to facilitate maintenance. Main blades and earth blades are interlocked with both electrical and mechanical means. The earthing switch has to be capable of withstanding short circuit current for short duration as applicable to the isolator.
An electrical insulator resists the flow of electricity. Application of voltage difference across a good insulator results in negligible electrical current. Adequate insulation is of prime importance for obvious reasons of reliability of supply, safety of personnel and equipment, etc.The insulators in use at substations are post insulators of pedestal type. The station design should be such that the number of insulators is kept at a minimum at the same time ensuring security of supply. In the areas where the problem of insulator pollution is expected (such as near the sea or thermal station, railway station, industrial area, etc.) special insulators with higher leakage resistance should be used.
•Power and Control Cables
Power and control cables of adequate current carrying capacity and voltage rating are provided at the substation. Power cables are used for 33kV,11 kV or LT system to carry load current. The control cables are required for operating and protection system connections. The cables are segregated by running in separate trenches or on separate racks.
Control panels installed within the control building of a switchyard provide mounting for mimic bus, relays, meters, indicating instruments, indicating lights, control switches, test switches and other control devices. The panel contains compartments for incoming lines, outgoing lines, bus-bars with provision for sectionalizing, relays, measuring instruments, etc.
The panel is provided with:
− suitable over-current and earth fault relays to protect the equipment against short circuit and earth faults; and
− measuring instruments such as ammeter, voltmeter and energy meter for 33kV and 11 kV systems.
•Lightning Protection−Surge Arrestors
Large over voltages that develop suddenly on electric transmission and distribution system are referred to as “surges” or “transients”. These are caused by lightning strikes or by circuit switching operations. Surge arrestor is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current.The surge arrestor which responds to over-voltages without any time delay is installed for protection of 33 kV switchgear, transformers,associated equipment and 11 kV and 33 kV lines.The rated voltage of arrestors for 33 kV should be 30 kV for use on 33 kV systems and with nominal discharge current rating of 10 kA. The rated voltage of lightning arrestors should be 9 kV (r.m.s.) for effectively earthed 11 kV system (coefficient of earth not exceeding 80 % as per IS: 4004) with all the transformer neutrals directly earthed. The nominal discharge current rating should be 5 kA.
•Instrument Transformers (Current and Voltage Transformers)
The substations have current and voltage transformers designed to isolate electrically the high voltage primary circuit from the low voltage secondary circuit and, thus, provide a safe means of supply for indicating instruments, meters and relays.
Current Transformer (CT)
Current transformers are used in power installations for supplying the current circuits of indicating instruments(ammeter, wattmeter, etc.), meters (energy meter, etc.) and protective relays. These transformers are designed to provide a standard secondary current output of 1 or 5 A, when rated current flows through the primary. A fundamental characteristic of CT is its transformation ratio, expressed as the ratio of the rated primary to rated secondary current. Current transformers have two inherent errors: the current ratio and phase displacement. These two errors serve as a basis on which current transformers are classified for accuracy.
Voltage Transformer or Potential Transformer (PT)
These instrument transformers are used for supplying the voltage circuit of indicating instruments, integrating meters, other measuring apparatus and protective relays or trip coils. These may be of single phase or three phase design and of the dry or oil immersed types. A voltage transformer or PT is rated in terms of the maximum burden (VA output) it will deliver without exceeding specified limits of error. On the other hand, a power transformer is rated by the secondary output it will deliver without exceeding specified temperature rise. All voltage transformers are designed for a standard secondary voltage of 110 V or 110 / 3 V.
Earthing has to be provided for
− safety of personnel,
− prevention of and minimizing damage to equipment as a result of flow of heavy fault currents, and
− improved reliability of power supply.
The basic grounding system is in the form of an earth mat with risers.
Risers of MS flat are generally provided. Earth mat is provided within the
substation area. The earth rods are connected to the station earth mat.The earthing must be designed so as to keep the earth resistance as low as possible. Earthing practices have been discussed in Unit 6 of the course BEE-002.
Reactive compensation (as indicated by system studies of the network) has to be provided. It is always a good idea to ensure a power factor correction for transformers, since even when they are operating on low load (e.g., during the night) they absorb reactive power, which must be compensated to avoid unnecessary loadings and losses. You can recall this aspect from Appendix 1 to Unit 4. Shunt capacitors (Fig. 5.6) are connected on the secondary side (11 kV side) of the 33/11 kV power transformers. The capacitors are generally of automatic switched type.The automatic system of the capacitor bank has the task of switching in the necessary capacitance according to the load requirements at each given moment.
•Station Battery/DC Supply Arrangement
Station batteries supply energy to operate protection equipment such as breakers and other control, alarm and indicating equipment (Fig.). The station batteries are a source for operating DC control system equipment during system disturbances and outages. During normal conditions the rectifier provides the required DC supply. However, to take care of rectifier failure, a storage battery of adequate capacity is provided to meet the DC requirements.
Normally, in a 33/11 kV substation, the DC system is of 30 cells consisting of 15 lead acid storage batteries or Nickel-Cadmium batteries. The battery is connected in parallel with a constant voltage charger and critical load circuits. The charger maintains the required voltage at battery terminal and supplies the normally connected loads. This sustains the battery in fully charged condition. The correct size battery charger has to be selected for the intended application.
•An Auxiliary Supply Transformer of adequate capacity is required to be provided for internal use for lighting loads, battery charging, oil filtration plant, etc. The supply should be reliable. In a substation it is normally provided from a station transformer connected on 33 or 11 kV bus bar.
Auxiliary Supply Transformer
Fire Fighting System
In view of the presence of oil filled equipment in a substation, it is important that proper attention is given to isolation, limitation and extinguishing of fire so as to avoid damage to costly equipment and reduce chances of serious dislocation of power supply as well as ensure safety of personnel. The layout of the substation itself should be such that the fire should not spread to other equipment as far as possible. Fireextinguishers of the following type must be provided:
− Carbon dioxide extinguisher, and
− Dry chemical powder extinguisher.
Carbon dioxide (CO 2 type) extinguisher and Dry chemical powder type extinguisher should conform to IS: 2878 and IS:2171, respectively. For oil fire, foam type extinguishers are used (see Unit 7, BEE-002 also). The fire fighting equipment should be maintained and kept in top condition for instant use as per IS: 1948-1961 “Fire Fighting Equipment and its Maintenance including Construction and Installation of Fire Proof Doors-Fire Safety of Buildings (General)”.So far we have described the equipment in a 66-33kV/11kV substation