A Grid Substation is the part of the electricity grid system -i.e. a part of the transmission
system. In Bangladesh this involves voltage levels of 400 kV, 230 kV, 132KV, 33KV, 11KV,
400V, 230V.Transmission of power occurs through the power distribution grid. The latest
technology that is used in power distribution is the ‘smart grid’. It is nothing but digital
technology which helps to assess the amount of power consumed and required in the
future.To supply electrical power from existing 50 MW and upcoming 100 MW power
plants, amnura sub-station is playing a vital role in the Chapainawabganj area.
Incoming and Outgoing Voltage
The incoming voltage of Amnura grid sub-station is 132 KV and outgoing voltage is 33 KV.
132/33 KV transformer of Amnura sub-station.
Bus Bar System of amnura sub-station
When a number of lines are operating at the same voltage and are directly connected
electrically, then it is called bus bar. Bus bars are made of copper or aluminum bars and
operate at a constant voltage. The incoming and outgoing lines in a substation are connected
to the bus bars. Basically, a bus bar links all the equipment of a substation. At Amnura sub-station
we have seen double bus bar system.
Double bus bar system of Amnura grid sub-station.
Protection on the transmission and distribution serves two functions. First one is protection of
the plant and 2nd one is protection of the public and employees. At the basic level, protection
looks to disconnect equipment which experiences an overload or a short to earth. Some items
in sub-stations such as transformers might require additional protection based on temperature
or gas pressure etc. Different kinds of protection mechanism are described below-
Circuit Breakers at amnura sub-station
A circuit breaker is an automatically operated electrical switch designed to protect an
electrical circuit from damages caused by overload or short circuit. Its basic function is to
detect a fault condition and to immediately discontinue electrical flow by interrupting
continuity. It also performs the breaking operation automatically when fault current is
generated. When the fault current occurs, relay detects the leakage current and gives a trip
signal. After receiving the trip signal, the circuit breaker operates.
At Amnura Sub-station, there are used SF6 circuit breakers are used. In SF6 circuit breaker
the current carrying contacts operate in sulfur hexafluoride or SF6 gas. The working of SF6
CB is quite similar to air blast circuit breaker. Here SF6 gas is compressed and stored in a
high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is
released through the arc in breaker and collected in a relatively low pressure reservoir and
then it is pumped back to the high pressure reservoir for re utilization. This type of circuit
breaker is quite popular now-a-days.
A relay is an automatic device which senses an abnormal condition of electrical circuit and
closes its contacts. Relay operates a circuit breaker. Relay makes the circuit breaker tripped
for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.
There are various types of protective relays in Amnura Sub-station. Some major types of
relays are described below.
This type of relay responds to vector (phase or magnitude) difference between two or more
similar electrical quantities. Most differential relay are current differential relay in which
vector difference between the current entering the winding and current leaving the winding is
used for sensing relay operation. At Amnura, differential relay is used to protect generators,
transformers, buses, and transmission lines from the effects of internal faults.
Over Current Relay
Over current relay senses the current flow and trips the circuit breaker if over current flow is
detected. In an over current relay, there is a current coil. When normal current flows through
this coil, the magnetic effect generated by the coil is not sufficient to move the moving
element of the relay, as in this condition the restraining force is greater than deflecting force.
But when the current through the coil increases, the magnetic effect increases, and after
certain level of current, the deflecting force generated by the magnetic effect of the coil,
crosses the restraining force, as a result, the moving element starts moving to change the
contact position in the relay. At Amnura, over current relay is used to prevent extra current
flow of current transformer.
The operation of such relay depends on the value of voltage to current ratio. This ratio is
nothing but impedance. The relay will only operate when this voltage to current ratio
becomes less than its predetermined value. Hence, it can be said that the relay will only
operate when the impedance of the line becomes less than predetermined impedance (voltage
/ current). As the impedance of a transmission line is directly proportional to its length, it can
easily be concluded that a distance relay can only operate if fault is occurred within a
predetermined distance or length of line.
An isolator is a mechanical switch used to isolate a section from any energized conductors.
So, it is a manually or automatically operated mechanical switch which separates a part of the
electrical power system usually at off load conditions.