Pressure Relief Devices for oil filled electric Transformers - TERMAN by fjzhangweiyun


									                 Pressure Relief Devices
            for oil filled electric Transformers

Description and general specifications             page   02
Operating instructions and maintenance             page   03
Finished product quality control tests             page   09

Description and general specifications.
Aim of the safety valves on electric transformers

The transformer tank filled with cooling liquid is a container subject to internal pressure and then has to be
provided with one or more safety valves suitably calibrated for the maximum allowed pressure, so that
overpressure caused by internal faults can be instantaneously relieved through the valves, thus avoiding
greater damages such as the deformation or even the burst of the tank and the spraying of hot oil with
subsequent fire risks. It is necessary to protect the transformer tank with a suitable equipment capable of
almost instantaneously discharging overpressure already at the development, before the structure integrity is
damaged and the safety valves has just this function.

General features

The safety valves consist schematically of:
    a valve base comprising the valve opening venting area with its specially profiled gasket and a seat
       for an O-ring gasket on the flanged end towards the transformer’s tank;
    a valve cap pressed against the profiled gasket by
       calibrated helical spring, thus making the valve completely tight up to the rated pressure;
    a splash diverter to avoid damages caused by hot oil sprinkles (on request);
    a single or a double electrical contact (on request).

Mechanical protection degree : IP 65                       Microswitch breaking capacity :
                                                           10 A 250V AC
Insulation : 2000V 50Hz between terminals and
                                                            1 A 125V DC
earth for a 60 seconds time
Cable gland : PG 13,5

                                           Safety valves - types

The safety valves are built with different Major Diameters and rated pressure to satisfy the requirements of
the various applications.

       TYPE             MAJOR DIAMETER             RATED PRESSURE                  PREVAILING USE

   T 125 - VS 150            125mm.                    0,3  1   bar            big power transformers
   VS 100                    100mm.                    0,3  1   bar          medium power transformers
   T80 - VS 80                 80mm.                   0,3  1   bar           small power transformers
   T 50 – VS 50                50mm.                   0,3  1   bar           cable boxes - small tanks

Safety valves series VS : the advantage of safety valves serie VS consists in the total absence of projecting
parts in the transformers tank making the mounting point choose easier. These valves can be mounted, with
regards to their base plane, both horizontally on the cover and vertically on the transformer walls at the
points where their safety action is presumed to be more necessary.

Safety valves series T : the advantage of safety valves series T is that, showing a good effectiveness and
reliability of valves serie VS, their simplicity consent to obtain very competitive prices.

Operating instruction and maintenance.
Instruction for mounting safety valves

The data about the transformer point where a short circuit is most likely to occur, the preferential direction the
resulting shock wave may have, the intensity this one can reach, all depend on the transformer power, its
transformation ratio, its construction characteristic and the behaviour of the other installed protection
Therefore, is not possible to give strict rules about safety valves application. It is the manufacturer who must
decide each time and on his own experience the valve type and its position.

It is important to take these general suggestions into due consideration for a correct choice :

       the safety valves must be mounted near                           Therefore, before operating the
        the points where a failure is most likely to                     transformer, it is important to let possibly
        occur. When a single valve is mounted                            accumulated air come out through the
        the barycentric position with regards to                         suitable breather screw and above all to
        these points shall be chosen;                                    mount the valve so that the possible gas-
       the exhaust rated diameter shall be                              bubbles developed during operation can
        connected with the transformer oil                               not accumulate on the valve;
        quantity and with the number of mounted                         the operating pressure must be definitely
        valves;                                                          lower than the maximum allowed
       the presence of gas or air-bubbles under                         pressure in the tank, but higher than
        the valve cap may negatively affect valve                        possible pressure peak reached during
        opening speed.                                                   operation.

Mounting and maintenance

The safety valves mounting is carried out by means of the suitable fastening holes of the flange, after the
splash diverter removal and after the insertion of the O-ring gasket supplied with the valve.
After the transformer filling, the air developed under the valve must be breathed by unscrewing the suitable
breathing screw. This breathing screw shall be tightened again as soon as the oil starts to come out.
During operation the safety valves do not need a particular maintenance. Nevertheless, it is convenient to
regularly check the electric contact good operation and to verify if there is no gas accumulation.

Instruction for ordering safety valves

The exhaust rated diameter shall be connected with the transformer oil quantity and with the number of
mounted valves. When a single valve is mounted the barycentric position, with regards to the points where a
failure is most likely to occur, must be chosen.
To help the Technical Department to state types and number of valves to be mounted on a transformer, is
possible to study the following indicative table :

                     VALVE TYPE                             OIL QUANTITY
                         T   50                    up to 1.000 dm (900Kgs.)
                         VS 50                     up to 1500 dm (1300Kgs.)
                         T   80                    up to 4.000 dm (3.600Kgs.)
                         VS 80                     up to 6.000 dm (5.400Kgs.)
                         VS 100                    up to 20.000 dm (18.000Kgs.)
                         T 125                     up to 20.000 dm (18.000Kgs.)
                         VS 150                    up to 35.000 dm (31.500Kgs.)

In case of tanks containing larger quantities of oil must be mounted 2 or more valves of suitable dimensions
(i.e.: if oil quantity is 95.000dm must be mounted 3 valves type VS 150).

               SAFETY VALVES type 50T – 80T – 125 T


    Safety Valve type VS 50


         Safety Valve type VS 80
   equipped with 1or 2 change-over contacts

       SAFETY VALVE type VS 100

               88.0                                                           100.0   100.0






                                                                             102.5    102.5
25.0   170.0

                           Safety Valve type VS 100
                      equipped with 1 change-over contact

          SAFETY VALVE type VS150





                         Ø 18

                                  4                                               294.0

50.0   190.0                    16.

                                      Safety Valve type VS 150
                       equipped with 1 or 2 change-over contacts

Finished product quality control tests.
Hydraulic test for tightness for the body :
the aluminium alloy case is mounted on the wall of a tank filled with oil at the temperature of 90°C and at the
pressure of 300 kpa. After 8 hours no leakages must be observed.

Dimensional check.

General function test :
the safety valve is mounted on a testing pressure circuit at the atmospheric pressure value. The pressure,
controlled by a microprocessor based monitoring unit and recorded by a special recording instrument,
slowly increases up to the Operating Pressure value - 5 kpa. The valve remains at this pressure value for 6
hours. During this period the valve must be completely sealed.
Following three different gradients the pressure increases again to the Operating Pressure value. It’s verified
that the effective pressure of opening of the valve is contained in the following interval : O.P. -5% < O.P. <
O.P. +8%.

Check of contacts actuation : this test is performed by means of visual indicators mounted on a panel
board. Commutation tolerance, commutation differential and electrical circuits are verified.

Check of electrical contact unit mechanical protection degree : IP 65.

Insulation test : carried out by means of a microprocessor controlled testing unit.

                                                                                                                               I EDITION AUGUST 2006

Terman '90 S.r.l., Via Ghisalba, 13-20/21, 20021 Bollate (MI) – Italy. Tel: +39 02 38 30 37 12, Fax: +39 02 38 30 37 19,
C.F./P. IVA 09970270154 – C.C.I.A.A. 1332904 – Trib. Milano Reg. Soc. 302729 – Cap. Soc. € 119.000


To top