Transformer Physical Protection refers to the Protections which used the physical quantities to protect the Transformer. Apart from electrical protections which uses the electrical quantities to judge a fault and based upon the judgment, the electrical protections of Transformer isolates the Transformer. On contrary, physical protections continuously measure the physical quantities like oil / winding temperature, gas content in the Transformer Oil etc to judge a fault condition and isolate the Transformer from the fault. There are many physical protections provided in a Transformer, they are as follows:
- Transformer Winding Temperature Trip, WTI
- Transformer Oil Temperature Trip, OTI
- Buchholz Trip
- Pressure Relief Device, PRD
- Magnetic Oil Level Gauge, MOLG
All the above physical protections have already been discussed in earlier posts except Pressure Relief Device. Therefore we will focus here in this post on Pressure Relief Device or PRD.
Pressure Relief Device, PRD:
PRD is basically a last line of defense and it shall not normally operate if all other protection viz. electrical protections are working properly but if in case they fail then PRD shall operate to isolate the Transformer.
When it is required to limit the pressure rise inside a tank, in order to prevent an excessive mechanical stress of the walls, it is necessary to use a safety valve set at a precise overpressure value. Here Pressure Relief Devive, PRD works as a safety valve.
The tank of oil immersed Transformers is usually fit with this kind of protecting device; because in case of short-circuit due to an insulation failure, the dielectric arc between alive parts vaporizes the surrounding insulating fluid which generates a quick rise of the pressure inside the tank, with the risk of permanent deformations, or, failure of the tank walls with the consequent outflow of hot oil. Due to the rapidity of this event, it is necessary to mount on the Transformer an adequate protecting device which relieves quite suddenly the excess of pressure generated inside the tank by the failure mentioned above.
To understand the working principle of Pressure Relief Device, let us first understand the construction of simple Pressure Relief Device i.e. PRD.
A PRD has a shutter which can open and provide path for the outflow of Transformer Oil. The shutter is spring loaded due to which it tightly covers the Transformer Oil Tank. Apart from theses, normally two Normally Open (NO) contacts are also provided with the PRD. Under Normal operating condition these contact remain open but when PRD operates these electrical contact changes its status from NO to NC which can be used to provide alarm in the Control Room. A typical PRD is shown in figure below.
The way of working of Pressure Relief Device i.e. PRD is quite simple. If the pressure inside the Transformer tank reach the operating pressure of the PRD or if the pressure inside the Transformer tank reaches the set point for the operation of PRD, the shutter lift slowly from its rest position disjoining from the main tightening gasket; in this condition the excess of pressure can’t be released in the environment, being the shutter still in contact with the external gasket .Now the internal overpressure operates on the whole internal surface of the shutter, which is bigger than the initial surface: consequently, the strength acting on the shutter is much higher than the spring-load on the other side; this brings to a quick and high lifting of the shutter followed by an equally fast reclosure as the excess of pressure is released in a very short time through the big opening created by the shutter lifting.This way PRD helps to protect the Transformer from over-pressure.