Oil Transformer Electrical and mechenical
Safety or protection with their functions.
1 - Need of transformer safety or protection.:
Transformer is a electrical static device which is used for high voltage transformation as 11KV to 400KV. It is very costly device and after occurred fault the maintenance procedure is very costly and lengthy process.
Two type transformer are mostly used one is oil type and other is dry type. Transformer has two windings primary and secondary windings which is important and costly part. Many safety devices are used for protection of both windings.
2- Types of oil transformer protection or safety :
1- Electrical Protection:
Tank is a main body part of transformer. Both winding is kept in tank and oil is filled in it for cooling of winding. It is also used as base for installing other accessories on this tank body just like as terminal, bushing, conservator, breather vent pipe diagrammed, remote tap changer, Buchholze relay etc.
- Over current and earth fault relay
- Buchholz relay or gas relay
- Oil temp indicator device
- Winding temp indicator device
- Earthing:
2- Mechanical Protection:
Tank is a main body part of transformer. Both winding is kept in tank and oil is filled in it for cooling of winding. It is also used as base for installing other accessories on this tank body just like as terminal, bushing, conservator, breather vent pipe diagrammed, remote tap changer, Buchholze relay etc.
- Explosion vent
- Silica gel breather
- Oil level indicator
- Terminal bushing
3- Electrical protection :
1- Over current and earth fault relay:
Over current and earth fault relay is used in transformer for over load, short circuit and earth fault. It is installed on transformer incomer panel or HT feeder panel.
- Over load
- Short circuit
- Earth fault
- Restricted earth fault
2- Buchholz relay or gas relay:
Buchholz relay is known as oil and gas actuated protection relay. It is safeties device which is used above 500 KVA capacity for transformers. It is installed between the conservator and main tank. It is not required for small rating transformers below 500 KVA. When the faults occur inside the transformer than the gas will generate in oil tank and it will travel to the conservator side. Buchholz relay is installed
1. Major fault occurred in transformer tank and generating gas in tank than it will provide alarm and trip protection.
2. Oil level going to down than it will provide protection alarm and trip condition.
3- Oil temp indicator device:
Oil temp indicator is used for measuring the temp of transformer oil tank oil. If the temp of oil in main tank is crossed the permissible limit due to any fault like as short circuit, loss of insulation, then alarm and trip circuit will be activated. Resultant the breaker will be tripped by oil temp indicator (OTI) device.
Alarm and tripping limit of OTI can be set as per site requirements.
OTI default limit setting:
- Oil temp alarm: 70℃
- Oil temp tripping alarm: 75℃
4- Winding temp indicator device:
temp indicator is used for measuring the temp of transformer winding. If the temp of winding is crossed the permissible limit due to any fault like as short circuit, loss of insulation, then alarm and trip circuit will be activated. Resultant the breaker will be tripped by winding temp indicator (WTI) device.
Alarm and tripping limit of WTI can be set as per site requirements. The standard limit is below mentioned.
WTI default limit setting:
- Winding temp alarm: 80℃
- Winding temp tripping alarm: 85℃
5- Earthing:
Earthing is the safety protection system which is used for discharging fault current in ground and also discharging the neutral reverse current into ground.
- Body Earthing: Two nos earthing is connected with body for leakage current protection.
- Neutral Earthing: Two nos earthing is connected with transformer secondary star connection junction point which is connected with neutral. Any harmonic current is received at this point then it is grounded.
4- Mechanical protection :
1- Explosion vent:
It is known as safety device for protection to the oil type transformer. It is a vent pipe diaphragm which is made by aluminum. On upper end a thin mesh is provided so that during heavy fault in tank than it will be break. It works against internal fault and fumes and smoke generate into a lot of amounts. We can say it another name pressure relief vent during a heavy fault. it is installed above tank.
2- Silica gel breather:
Breather is installed on conservator for removing moisture and dust particle from entering air through breather. Breather is used in transformer for breathing air in and out in body of transformer during thermal expansion of oil.
During the time of operating when is loaded by power. Same time the ambient temp of out is also more than tank oil temp will increase and oil label also will be increased. Due to oil label increasing the tank oil will move towards conservator side than conservator air will move outside through silica gel of breather.
When unloaded or decrease the label of tank oil than conservator oil will move to downside than conservator will suck air through silica gel breather. This process is called transformer breathing.
Silica gel :
It is used inside of breather bottle when the air enters in the breather through silica gel than moisture and dust will be absorb by silica gel. After absorbing the air moisture, the silica gel color will be changed blue to pink. This type silica gel should be changed ass on urgent basis.
Color off Silica gel:
- Dark blue: Dry type silica gel.
- Light blue: Dry type silica gel.
- Pink color: 100% silica gel moisturized- Should be changed on urgent basis.
3- Oil label indicator:
It is mounted on conservator tank vertical side for measuring oil level in conservator. the oil level should be minimum 70% of conservator tank. by this we can check visibly and the level is low than the level can be top-up.
4- Terminal bushing:
Terminal and bushing are installed on transformer for connecting power supply cable to input side and output side.
Terminal:
it is used for connecting the Power supply with winding. Two types terminal are used as input and output terminal. Terminal are mounted upon bushing.
Bushing:
It is installed on tank as insulator on tank and terminal. It is made by porcelain or epoxy resin or ebonite.it is installed over tank.
- High voltage bushing: It is used in primary side as 11 KV and 33 KV voltage.
- Low voltage bushing: It is used in outside low voltage supply like as 433 voltages.
