Grid Substations

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.

There are basically two main categories for transformer protection

                                                  i.            Mechanical Protection

·         Over Temperature

·         Buchholz Protection

·         Pressure Relief Value

Figure : Bucholz Relay Figure : Pressure Relief Value

                                                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

Figure : CPC100 OMICRON

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. 

Figure 2.4: Kolonnava GIS