Grid Substations
(1) Kelaniya Grid Substation
There is a 132 kV A.I.S. in Kleaniya grid substation. And
also while that time there was a 33 kV G.I.S installation due to the population
increases. It was done by ABB technology.
i.
Mechanical Protection
·
Over Temperature
·
Buchholz Protection
·
Pressure Relief Value
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ii.
Electrical Protection
·
Differential Protection
·
Restricted Earth Fault Relay
·
Directional O/C and Earth Fault
(2) Seethavaka Grid Substation
Steps taken before the
maintenance in Seethavaka,
1.
System
coordinate
2.
Circuit
breaker open (132 & 33)
3.
Isolator
open (132 & 33)
4.
Temporary
earth connection (132 & 33)
5.
External
cleaning
Some transformer testing can be observed in there which are
mention follow,
1 Megger Testing
Equipment- Foster oTS60SX
After keeping 5 min or 10 min for settling. It was taken 6
break down voltage values for every 2 min and get the average of the values as
the result. That value should be greater than 30 kV.
·
Last calibrating date of transformer 3 –
2013/07/04
·
Results – 33.7, 32.6, 34.0, 33.5, 34.5, 36.8 kV
·
Average Dielectric Strength – 34.18 kV
2 Tap changer winding resistance
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Applied 4 A current as input signal. Then measure the
voltage and calculate the resistance.
3 Dew Point testing
It’s about moisture content of gas (SF6) the
standard value is (-5 ºC) and the result was (-38.1 ºC)
4 Ratio Testing
Here checked the winding ration for the tap positions.
Standard ratio is 132:33 and input signal was 150 V.
5 Tan Delta testing
6 Contact resistance
7 Series
Frequency Response (SFR) Testing
3(3) Kolonnava Grid Substation
The kolonnawa GIS
consist of a 132 kv GIS and a 33 kv GIS. Thirteen 132 kv lines connected to the
132 kv bus. 3 of them are underground and 10 of them are overhead lines. Due to
the large population and the industrial zone there are five 132/33 kv 3 phase
transformes to supply power to 33 kV bus. Gas insulated substation was placed
instead of Air Insulated Substation in kolonnawa to utilize space otherwise due
to large numbers of lines and feeders the AIS consume a large piece of land.
We observed a GIS in
first time. So we identify the main parts of the GIS and main functions of
them.
1. Gas insulated bus bar.
HV bus bar is isolated
with an isolation gas. Mostly SF6 is used as the insulation gas.
This insulation is done to avoid flashes which can be happen due to small
spacing.
2. Isolators and disconnectors.
Isolators are used to
disconnect connection. Operation of this mast related to the circuit breaker
job. But the different we can physically see the operating status of the
isolator.
3. Circuit Breaker
It is impossible to
simply break a High voltage circuit by just disconnecting them. On load circuit
breaking causes arcs between disconnecting ends. This arcs happen until
terminals melt down or so on. To avoid this there is an insulation gas inside
the circuit breaker. To operate in fault situation CB has very small operation
time.
4. Current Transformer
Currents flow in power
lines can’t be directly measured. To measure this large currents current
transformers are used.
5. Voltage transformers
The reason of using VT
is same as the reason using CT. High voltage at the transmission lines cannot
be directly measure. Then VTs are used to measure those voltages.
6. Earth switches
Earth switches are
used to make voltages zero with respect to ground in bus bars, lines or
feeders. These components grounded at the occasions of maintenance.
7. Surge arrestors
Surge arresters are
used to bypass high voltages to the ground before entering to the grid. It has
the capable of protecting the line voltage during the bypassing.
