Download EEE8054 Power Transformers – Taster Material Application of

EEE8054 Power Transformers – Taster Material
Application of Transformers in the System
Distribution transformers
These are used to distribute power to domestic or industrial premises. They may be single
phase or three phase, pole mounted or ground mounted, and they have ratings ranging from 16
kVA up to 2500 kVA.
The windings and core are immersed in mineral oil, with natural cooling, and there are two
windings per phase. The primary (high-voltage) winding has a highest voltage ranging from
3.6 kV to 36 kV; the secondary (low-voltage) winding voltage does not exceed 1.1 kV. The
high-voltage winding is usually provided with offcircuit tappings of ± 2.5 per cent, or +2 x
2.5 per cent, -3 x 2.5 per cent.
The preferred values of rated output are 16, 25, 50, 100, 160, 250, 400, 630, 1000, 1600 and 2500
kVA, and the preferred values of short-circuit impedance are 4 per cent or 6 per cent. Losses
are assigned from lists, for instance from HD 428 [1], or by using a loss-capitalisation formula.
Supply transformers
These are used to supply larger industrial premises or distribution substations. Ratings range from
4 MVA to 30 MVA, with primary windings rated up to 66 kV and secondary windings up to
36 kV.
Transformers in this class are generally fluid cooled. Most supply transformers use mineral oil,
but for applications in residential buildings, oil rigs and some factories the coolant may be
synthetic esters, silicone fluid, some other fluid with a higher fire point than mineral oil, or SF6
gas.
Transmission (or intertie) transformers
These are among the largest and highest voltage transformers in use. They are used to
transmit power between high-voltage networks. Ratings range from 60 MVA to 1000 MVA
and the windings are rated for the networks which they link, such as 33, 66, 132, 275 and 400
kV in the UK, or voltages up to 500 kV or 800 kV in other countries. The impedance of a
transmission transformer is usually 18 per cent in the UK, or 8 per cent in continental Europe,
but for some system conditions, an impedance of up to 30 per cent is used.
Many transmission transformers are autotransformers. In these transformers the 2-winding
approach is replaced by an arrangement where a common winding is shared between
primary and secondary circuits with a series winding connected in the higher voltage
circuit.
Transmission transformers are generally oil filled, and are usually fitted with oil pumps and
radiator fans to assist cooling of the windings and cores. They are usually fitted with on-load tap
changers (OLTCs), but some networks at 400 kV and 275 kV are linked by transformers without
regulating windings. In underground substations where the risk of fire is high, an SF6 gas
insulated design may be used.
Generator (or step-up) transformers
Power is usually generated in large power stations at typically 18-20 kV, and generator
transformers are used to step up this voltage to the system voltage level. These
transformers are usually rated at 400, 500, 630, 800 or 1000 MVA.
Base-load power stations usually operate continuously at full load, and the power is transformed
to the highest operating voltage in one step. Combined-cycle gas turbine stations may be
embedded in the transmission and distribution system at lower operating voltages such as 132
kV.
Generator transformers are usually fitted with regulating windings and OLTCs.
Phase-shifting transformers
Where power is transmitted along two or more parallel transmission lines, the power flow divides
between the lines in inverse proportion to the line impedances. Highest power is therefore
transmitted through the line with lowest impedance and this can result in overload on that
line, when the parallel line is only partly loaded. Phase- shifting transformers are used to link
two parallel lines and control power flow by injecting a voltage 90° out of phase (in
quadrature) with the system voltage into the line, at either leading or lagging power factor.
Where the transformer controls phase angle but not the voltage, the unit is known as a
quadrature booster. Where voltage is also controlled, the unit is known as a phase-shifting
transformer.
Converter transformers
Where power is transmitted through an HVDC system, a converter station is used to change ac
power to dc power using multiple rectifier bridges. DC power is converted back to ac power
using inverter bridges. Converter transformers handle ac power and dc power with mixed
ac/dc voltages by combining the power flow through 12 phases of rectifier inverter bridges
using dc valve windings.
The insulation structure must withstand all normal and abnormal conditions where ac voltage is
mixed with dc voltage of differing polarities over the operating temperature range. The presence of
dc currents may also cause saturation of the core, leading to abnormal magnetizing currents and
variations in sound.
A phase of a three-phase converter transformer bank typically comprises a high voltage
primary winding and two secondary ac/dc valve windings. Three such transformers
together form the two secondary three-phase systems; one is connected in delta and the other in star.
Each secondary system feeds a six-pulse bridge and the two bridges are connected in series to
form a 12-pulse arrangement. Two such transformer banks are used with the secondary circuits
connected in opposite polarity to form a high voltage dc transmission system.
An example of such a scheme is shown in schematic form in Figure 1, where two three phase
banks of single phase converter transformers are shown linking an AC transmission line to a DC
transmission line. Each single phase converter transformer has two dc secondary valve windings,
one is connected in a three phase delta system and one in a three phase star system to give the
necessary phase angle displacement.
Railway transformers
Transformers for railway applications may be trackside units to supply power to the track, or onboard transformers in the locomotive or under the coaches, to power the drive motors.
Trackside transformers are subjected to uneven loading depending upon the position of the train in
the railway system. On-board transformers are designed for the lowest possible weight, resulting
in a high-loss performance. Modern train control systems using thyristors or Gate Turn-Off
devices (GTOs) subject the transformers to severe harmonic currents that require special
design consideration.
+ ve
Filter
AC System
DC
System
earth
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DC
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Figure 1 Schematic diagram of ac/dc transmission scheme
Rectifier and furnace transformers
Special consideration is needed for transformers in industrial applications involving arc
furnaces or heavy-current dc loads in electrochemical plant. The primary windings in such
cases are usually rated at 33 kV or 132 kV in the UK, but the secondary windings carry
many thousands of amperes and are rated at less than 1 kV. Current sharing between parallel
paths in the transformer becomes important because of the magnetic fields created by the high
currents. These strong magnetic fields can cause excess heating in magnetic steels if these are used
in the structure of the transformer, because of the flow of proximity currents in the steel. To reduce
this excess heating, non-magnetic steel is often used to form part of the tank or the cover.
The OLTCs in furnace transformers are subject to a heavy duty; they may perform hundreds of
thousands of operating cycles a year, which is more than a lifetime's duty for many
transmission transformers.
Dry type transformers
A dry-type construction is possible where a higher-temperature class of insulation is required
than is offered by cellulose and mineral oil or a higher fire-point fluid. Dry-type
transformers use non-cellulosic solid insulation and the windings may be varnish dipped
to provide a higher temperature capability, or vacuum encapsulated in epoxy resin to form
a very high temperature system. Ratings can be up to 20 MVA at voltages up to 36 kV.
Overload performance can be provided by the use of fans.
This type of transformer is more expensive than a fluid-filled equivalent, and because of
the reduced fire risk they are used in special applications where the public are involved, such as
underground tunnels, residential blocks of flats or oil rigs.
Gas insulated transformers
For applications where low flammability is paramount, designs have been developed in
which the transformer is insulated and cooled with SF6 gas. This provides an alternative to
dry-type construction, but it has only been used on a limited scale. High-voltage SF6
transformers are available at high cost, but this may be justifiable in cases of high land values,
where the overall ‘footprint’ of the unit can be reduced by the elimination of fire-fighting
equipment.
Reference
1. HD 428, ‘Three phase oil-immersed distribution transformers, 50 Hz, 50 to 2500
kVA with highest voltage for equipment not exceeding 36 kV’.