19-09-2012, 01:08 PM
Electrical Substation components
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A substation is a high-voltage electric facility containing equipment to regulate and distribute
electrical energy. While some substations are small with little more than a transformer and
associated switches, other substations are large and complex.
Functions of a substation include receiving power from a generating facility, regulating
distribution, stepping voltage up and down, limiting power surges, and converting power
from direct current to alternating current or vice versa.
Components in an electric substation
Many electrical components work together in a substation to carry out its functions, these
include
Lightning Arresters
protect a substation from voltage surges and are installed on power
poles, towers, transformers and circuit breakers to protect them from damage during electrical
storms. Lightning Arresters look similar to standoff insulators and bushings, but their unique
characteristics is that they have earthing terminals at the bottom where a large ground cable is
connected and runs down the structure that connects to the station ground.
Conductor Systems
The most suitable material for a conductor system is copper or
aluminium. Steel may be used but has limitations due to poor conductivity and high
susceptibility to corrosion. An ideal conductor may be flat surfaced, stranded or tubular.
An ideal conductor should fulfil the following requirements:
· Capable of carrying specified load currents and short time currents
· Able to withstand forces on it due to its situation. These forces comprise self weight,
and weight of other conductors and equipment, short circuit forces and atmospheric
forces such as wind and ice loading
· Corona free at rated voltage
· Have minimum number of joints
· Need minimum number of supporting insulators
· Be cost effective
Insulation
insulation security is of high importance in a well designed substation.
Extensive research is done on improving flashover characteristics. Increased creepage length,
resistance glazing, insulation greasing and line washing have been used with varying degrees
of success.
A standoff insulator is required to keep high voltage overhead conductors in position and at a
certain distance from conductors in other phases or neighbouring equipment. It also insulates
between a conductor and ground. An insulator is always mounted on an earthed support and
contains a porcelain section that is mounted between a base plate and a top mounting plate.
The base plate is fixed on the grounded support and the top mounting plate is equipped with
bus support hardware, on which the overhead conductor is clamped.
Power Transformers
Transformers are large, box-shaped structures connected to multiple
wires and are usually the largest single item in a substation. Transformers are usually located
on one side of a substation, and the connection to switchgear is by bare conductors. Because
of the large quantity of oil, it is essential to take precaution against fire hazards. Hence, a
transformer is usually located around a sump used to collect excess oil.
Power from a generating station is sent at a much higher voltage than required for home
appliances. Step-down transformers decrease voltage of transmission lines en route to
neighbourhoods. Auto transformers can offer advantage of smaller physical size and reduced
losses.
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 low voltage signal, for measurement or to operate a protective 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 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 burden of the connected metering or protection devices. [1] 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.
Low voltage circuit breakers
Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial
application, and include:
· MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip
characteristics normally not adjustable. Thermal or thermal-magnetic operation.
Breakers illustrated above are in this category.
· MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or
thermal-magnetic operation. Trip current may be adjustable in larger ratings.
· Low voltage power circuit breakers can be mounted in multi-tiers in low-voltage
switchboards or switchgear cabinets.
The characteristics of Low Voltage circuit breakers are given by international standards such
as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow
removal and interchange without dismantling the switchgear.
Large low-voltage molded case and power circuit breakers may have electric motor operators
so they can trip (open) and close under remote control. These may form part of an automatic
transfer switch system for standby power.
Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with
the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid.
The circuit breaker contacts are held closed by a latch. As the current in the solenoid
increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which
lets the contacts open by spring action. Some magnetic breakers incorporate a hydraulic time
delay feature using a viscous fluid. A spring restrains the core until the current exceeds the
breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid.
The delay permits brief current surges beyond normal running current for motor starting,
energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release
the latch regardless of core position thus bypassing the delay feature. Ambient temperature
affects the time delay but does not affect the current rating of a magnetic breaker