Substation Equipment

 

To do its task in a proper way, substations contain much equipment. The most important and common equipment in the transformers substations are the following:

1)    Bus Bars

When a number of lines operating at the same voltage must be directly connected electrically, busbars are used as the common electrical point. Busbars are rigid aluminum or copper bars (generally of rectangular cross-section) and operate at constant voltage and frequency. The incoming and outgoing lines in a substation are connected to the bus bars. Busbars receives power from incoming circuits and deliver power to outgoing circuits.

There are many arrangements of busbars in substations. Some of them are:

1- Simple single bus bar.

2- Sectionalized single bus bar system.

3- Double bus bar system.

4- Double sectionalized bus bar system.

While the system in (2) is commonly used for medium and low voltages (11KV and less), the system in (4) is commonly used for high and extra high voltages (66 KV and more)

2)     Insulators:

The porcelain insulators employed in the substations are of past and bushing type. They serve as supports and insulations of the busbar. A past insulator consists of porcelain body, an iron cap and a flanged cast iron base.

Bushing insulators are used to pass the conductor through a wall or a tank transformer. A bushing consists of porcelain shell body and upper and lower locating washers used for fixing the position of the busbar or rod in shell. For current rating above 2 KA, the bushings are designed to allow the main busbars to pass directly through them. 

3)    Lightning  Arrestors and Surge Arrestors:

Lightning and surge arrestors are shunt resistors used to divert the lightning and high voltage surges to earth and protect other equipment from H.V surges. They are connected generally between phase conductor and ground. They are located where the first equipment is seen from the incoming overhead line and also near transformer terminals phase to ground. There are two types of surge arrestors; Gapped Arrestors and Gapless Zinc-Oxide Arrestors

4)     Isolators (disconnecting switch):

In substation, it is often desired to disconnect a part of the system for general maintenance and repairs. This is accomplished by an disconnecting switches or isolators. They are located at each side of the circuit breaker. They are disconnected after tripping the C.B and closed before closing the C.B. That's why they don't have any rating for current breaking or current making. From the common types of isolators: center rotating horizontal swing isolators, vertical swing and pantograph type isolator (for 420 KV). Isolators are interlocked with circuit breaker

5)    Earthing switch:

Its function is to discharge the trapped charges on the circuit to earth for safety. They are mounted on the frame of isolators.

6)    Current Transformer (CT):

It is used to step down the current for measurement, protection and control. The need for a CT comes from the fact that the measuring, control and protection instruments are designed for working at low ratings (usually 110V and 5A). The C.T usually has three secondary coils; one for measuring, the 2^nd for protection and the 3^rd for controlling.

7)     Voltage (potential) transformer (PT):

It is used to step down the voltage for measurement, protection and control. Its location is at the feeder side of the circuit breaker. Its secondary voltage is usually 110 V.

8)     Circuit Breaker (C.B):

Circuit breakers are installed to perform the following duties:

§  Switching during normal and abnormal operating conditions

§  Interrupting short circuit currents.

C.Bs are located at both ends of every protective zone. Types of C.B depend on the rated voltage and the medium of arc quenching. Among the types of C.B: SF6, Vacuum, Air blast and minimum oil.

9)     Power transformers:

The power transformer used to step down the voltage from 66 KV to 11 KV. The common connection of the power transformers is delta/star-earthed to trap the zero sequence and third harmonic components and prevent them from reaching the secondary side and thus the distribution networks.            The rating of power transformers depends on the loads of the substation zone. In general the most common used ratings for power transformers used in the distribution substations are 25 and 35 MVA. Power transformers are usually oil filled. They have two or three windings. They are provided with coolers.

Power transformers have tapped windings, which permit adjusting the output voltage to broaden the range of primary voltage inputs. The transformer will have a manual tap changer, which can be operated if the transformer is de-energized. An automatic on load tap changing (OLTC) feature installed on a transformer provides automatic tap changing under load, and normally varies the voltage to 10% of the system’s rated voltage in steps by changing tap connections using a motor-driven, tap-changing switch. Sometimes voltage regulation is needed and the system transformers do not include the OLTC feature. Voltage regulators are used to supply the control for the variations in load.

Industry standards classify transformers as outdoor and indoor transformers. An outdoor transformer is constructed of weather-resistant construction, suitable for service without additional protection from the weather.

10)  Marshalling Kiosk:

They are used in the outdoor substations. They are used to mount both monitoring instruments and control equipment and to provide access to various transducers. Marshalling kiosks are located in the switchyard near every power transformer.

11)   Metering, Control and Relay Panels:

They are used in the indoor substations. They are used to house various measuring instruments, control Instruments and protective relays. They are located in air-conditioned building. Control cables are laid between switchyard equipment and these panels.

12)   Shunt reactors:

They are used with extra high voltage transmission lines to control the voltage during low-load period by compensating the capacitance of the transmission line during these periods.

13)   Series reactors (current limiting reactors):

They are used to limit the short circuit currents and to limit current surges associated with fluctuating loads.

14)  DC Busbars

The trip coils of all circuit breakers operate using a dc supply, thus we need dc busbar in the substation. This is achieved using two auxiliary transformers to step down from 11 KV to 380 V. These transformers feed two rectifying units supplying two chargers. These are charging two battery cells which are kept floating on the supply. If the dc busbars are de‑energized for any reason, the batteries can fill in its place. The 380 voltage supplies necessary lighting, air conditioning, motors for the cooling fans and any other auxiliaries.

15)  Station Earthing System:

It is used to provide a low resistance earthing for doing the following tasks:

§  discharge currents from surge arrestors, overhead shielding and earthing switches

§  for equipment body earthing

§  for safe touch potential and step potential in substation

§  for providing path for the neutral to ground currents for the earth fault protection

Earthing electrodes are placed below the ground level and connected to the equipment structures and neutral points for the purpose of equipment earthing and neutral point earthing. Detailed discussion of earthing schemes is introduced in a coming chapter.

16)   Neutral Grounding Equipment:

They are either resistors or reactors. They are used to limit short circuit current during ground faults. They are short time rated. They are connected between neutral point and ground.