There are many different accessories used to monitor and
protect power transformers, some of which are considered standard features, and
others of which are used based on miscellaneous requirements. A few of the
basic accessories are briefly discussed here.
Liquid-Level Indicator
A liquid-level indicator is a standard feature on
liquid-filled transformer tanks, since the liquid medium is critical for
cooling and insulation. This indicator is typically a round-faced gauge on the
side of the tank, with a float and float arm that moves a dial pointer as the
liquid level changes.
Pressure-Relief Devices
Pressure-relief devices are mounted on transformer tanks to
relieve excess internal pressures that might build up during operating
conditions. These devices are intended to avoid damage to the tank. On larger
transformers, several pressure-relief devices may be required due to the large quantities
of oil.
Liquid-Temperature Indicator
Liquid-temperature indicators measure the temperature of the
internal liquid at a point near the top of the liquid using a probe inserted in
a well and mounted through the side of the transformer tank.
Winding-Temperature Indicator
A winding-temperature simulation method is used to
approximate the hottest spot in the winding. An
approximation is needed because of the difficulties involved
in directly measuring winding temperature.
The method applied to power transformers involves a current
transformer, which is located to incur a current proportional to the load
current through the transformer. The current transformer feeds a circuit that
essentially adds heat to the top liquid-temperature reading, which approximates
a reading that models the winding temperature. This method relies on design or
test data of the temperature differential between the liquid and the windings,
called the winding gradient.
Sudden-Pressure Relay
A sudden- (or rapid-) pressure relay is intended to indicate
a quick increase in internal pressure that can occur when there is an internal
fault. These relays can be mounted on the top or side of the transformer, or
they can operate in liquid or gas space.
Desiccant (Dehydrating) Breathers
Desiccant breathers use a material such as silica gel to
allow air to enter and exit the tank, removing moisture as the air passes
through. Most tanks are somewhat free breathing, and such a device, if properly
maintained, allows a degree of control over the quality of air entering the
transformer.
Liquid-Preservation Systems
There are several methods to preserve the properties of the
transformer liquid and associated insulation structures that it penetrates.
Preservation systems attempt to isolate the transformer’s internal environment
from the external environment (atmosphere) while understanding that a certain
degree of interaction, or “breathing,” is required to accommodate variations in
pressure that occur under operational conditions, such as expansion and
contraction of liquid with temperature.
Free-breathing systems, where the liquid is exposed to the
atmosphere, are no longer used. The most commonly used methods are outlined as
follows:
• Sealed-tank systems have the tank interior sealed from the
atmosphere and maintain a layer of gas — a gas space or cushion — that sits
above the liquid. The gas-plus-liquid volume remains constant. Negative
internal pressures can exist in sealed-tank systems at lower loads or
temperatures with positive pressures as load and temperatures increase.
• Positive-pressure systems involve the use of inert gases
to maintain a positive pressure in the gas space. An inert gas, typically from
a bottle of compressed nitrogen, is incrementally injected into the gas space
when the internal pressure falls out of range.
• Conservator (expansion tank) systems are used both with
and without air bags, also called bladders or diaphragms, and involve the use
of a separate auxiliary tank. The main transformer tank is completely filled
with liquid; the auxiliary tank is partially filled; and the liquid expands and
contracts within the auxiliary tank. The auxiliary tank is allowed to
“breathe,” usually through a dehydrating breather. The use of an air bag in the
auxiliary tank can provide further separation from the atmosphere.
“Buchholz” Relay
On power transformers using a conservator
liquid-preservation system, a “Buchholz” relay can be installed in the piping
between the main transformer tank and the conservator. The purpose of the Buchholz
relay is to detect faults that may occur in the transformer.
One mode of operation is based on the generation of gases in
the transformer during certain minor internal faults. Gases accumulate in the
relay, displacing the liquid in the relay, until a specified volume is
collected, at which time a float actuates a contact or switch.
Another mode of operation involves sudden increases in
pressure in the main transformer tank, a sign of a major fault in the
transformer. Such an increase in pressure forces the liquid to surge through
the piping between the main tank and the conservator, through the “Buchholz”
relay, which actuates another contact or switch.
Gas-Accumulator Relay
Another gas-detection device uses a system of piping from
the top of the transformer to a gas accumulator relay. Gases generated in the
transformer are routed to the gas-accumulator relay, where they accumulate
until a specified volume is collected, actuating a contact or switch.
No comments:
Post a Comment