Просмотр полной версии : Pulling vacuum on power transformers

16.07.2012, 16:11
Transformers are vacuumized prior to filling transformer oil to evacuate air from the tank, as well to eject moisture and gas, accumulated by solid insulation after breach of sealing. Transformers of 150 – 750 kV must be vacuumized.

To evacuate vapor and gas from the transformer tank, positive displacement pumps with oil seals are used; these move gas by changing volume of the chamber.

The primary specifications of mechanical pumps are residual pressure and speed of action. A pump’s residual pressure is the lowest pressure reached by the pump without load, i.e. with closed inlet. Since in this mode, when residual pressure is reached, apart from residual gases there are also condensed vapors at the pump’s inlet, the two parameters measured are full residual pressure (the sum of partial pressure of residual vapor and gas) and the residual gas pressure. The vacuum pump’s speed of action at a certain inlet pressure is the volume of gas entering the pump in a given time at this pressure. Speed of action is the definition of the pump’s capacity.

A — mechanical oil-sealed displacement pump; B — Roots blower

The residual pressure created by vacuum oil-sealed pumps is largely defined by performance of the oil in the pump, specifically its viscosity, and the pressure of the oil’s saturated vapor. As the oil ages and absorbs condensed vapor and gas, the lowest residual pressure created by the pump may increase significantly.

During operation of the vacuum pump, some of the oil are exhausted into the atmosphere in the form of fine mist with the evacuated gas, therefore, it is important to keep track of the oil level at all times and add more oil as required; after a certain time, the oil must be changed.

To prevent condensation of evacuated moisture in the oil, mechanical vacuum pumps are equipped with a gas ballast device, which injects some air into the pumps chamber before compression. Mixed with the evacuated gas, the atmospheric air reduces condensation of water vapor in the gas. The higher the gas concentration, the more ballast gas is required. The flow of ballast gas is regulated with a portioning valve. It should be noted, that the use of gas ballast device, the lowest residual pressure is increased.

Roots blowers are often used for large transformers; these pumps have high capacity at low input pressures.

The design of this pump does not allow creation of low pressure in the evacuated volume with exhaust into the atmosphere, therefore a booster vacuum pump is connected to the exhaust of the Roots pump; the booster creates the vacuum required to start the Roots vacuum pump.

Beside the speed of action and minimum residual pressure, Roots pumps are characterized by the highest inlet and outlet pressures.

The highest inlet pressure is the highest pressure at the entry section of the vacuum pump at which the pump can start operation.

The highest outlet pressure is the highest pressure in the outlet section, at which the pump can still operate.

Roots blower inlet pressure changes gradually depending on the outlet pressure, so the highest outlet pressure is rarely considered.

Oil-sealed mechanical displacement vacuum pumps are used as boosters for the Roots blowers. The preliminary evacuation of the vacuumized volume is done via the through chamber of the Roots blower. To accelerate the evacuation, and in such cases when excessive formation of water vapor is expected to cause condensation in the Roots blower, it is recommended to build a bypass vacuum line connecting the booster pump with the evacuated volume circumventing the Roots blower.

Capacity of the booster pump at the pressure equal to that of the Roots blower should be no less than capacity of the Roots blower at inlet pressure.

Vacuum pump capacity in Pa·m3/second is calculated using the following formula:


Where P is the residual pressure in Pa;
S is the pump’s speed of action in m3/second.

During pump operation, inlet pressure, oil levels in the gear housing and sealing ring, supply of cooling water and nature of noise must be observed. Starting the Roots blower is not allowed, if the intake pressure exceeds its inlet pressure specification, as this may lead to quick heating and jamming of the rotors. Therefore, the Roots blower must be stopped before the booster pump.

Residual pressure in the transformer tank is measured using vacuum meters which operate on the principle of correlation of gas thermal conductivity to the pressure. These devices consist of a sensor directly connected to the vacuum system to measure the pressure in, and the measuring system to which the sensor is connected electrically.

The manometric transducers used in such vacuum meters are built as follows: glass or metal case with plastic or nickel heater installed on two terminals; the other pair of terminals connect a thermal couple. The thermal couple and the heater are welded via a bridge. The heater’s temperature is increase by a current which can be adjusted and measured by the measurement device. The heated thermal junction is the source of the electromotive force, which is controlled by a device in the measuring section.

At low pressures, when molecule collisions are non-existent and therefore cause no heat loss, thermal conductivity is in direct correlation with the gas’s pressure. If the current passing through the heater is constant, the temperature of the heater, controlled by the values of the electromotive force, will be a function of the residual pressure in the volume. Electromotive force can be obtained from the measuring unit scales, and then converted to pressure valued with the help of the calibration curve.

Pressure up to 13.3 Pa is measured with constant current. As the pressure rises above 13.3 Pa, gas thermal conductivity is no longer proportional to its pressure, and the measurements are made in constant temperature mode. In this mode the current required to maintain constant temperature of the heater is proportional to the pressure. Measurable pressure range in this mode is 13.3 to 667 Pa. Current value is taken from the unit’s scale, and converted to pressure using calibration curve.

Pressures from atmospheric to the limit of the manometers are measured with regular elastic element pressure gauges. When using these devices, ambient atmospheric pressure is a factor and should be taken into consideration.

The figure below shows an arrangement for vacuumizing a transformer.


1 — oil dryer; 2 — oil heater; 3 — vacuum meter; 4 — vacuum measuring sensor; 5 — oil level sight glass; 6 — stop valve; 7 — vacuum pump; 8 — vacuum meter to control oil level in the tank

The vacuum hose used to connect equipment must be as short as possible, and its bore should be at least equal to or greater than the inlet diameter of the connected vacuum pump. Vacuum hose connecting Roots blower and booster pump should be equipped with compensators to reduce vibration.

Before vacuumizing, all connections should be thoroughly sealed; transformer sealing must be checked.

Transformer air-tightness test is as follows: a residual pressure equal to the required vacuumizing pressure is created in the tank, the vacuum line is closed and the transformer is kept for 1 houts. For 150 – 500 kV transformers the maximum residual pressure for this test is 665 Pa; 750 kV and 400 – 750 kV DC the max residual pressure is 200 Pa. Transformer is considered sealed if the pressure if the increase of the pressure in the tank is no more than 665 Pa within 1 hour..

Prior to oil filling, 150 – 500 kV transformers must be vacuumed for at least 20 hours at residual pressure of no more than 665 Pa, while 750 kV and 400 – 750 kV DC require 72 hours of vacuum pulling below 200 Pa or 48 hours at 133 Pa or below.

The vacuuming arrangement must prevent damage to various components of the transformer due to vacuum. This requires that transformer OLTC tanks, contactor vessels etc must be connected to the transformer tanks via vacuum lines. Components not design to withstand vacuum must be securely disconnected from transformer tank.