True/False
At generating stations, the generators produce power at voltages between 12,000 and 25,000 volts
True
The transformers on our system fall into three classes
Power Transformers
Transformers that handle energy in large quantities.
Distribution Transformers
Transformers that furnish energy at service voltages.
Instrument Transformers Transformers that reduce operating voltages and currents to levels
suitable for instrumentation or protective relays.
True/False
There are two types of instrument transformers: potential transformers and current transformers
True
True/False
A transformer is basically two sets of coils on a magnetic core of some kind. It has no moving parts and works on the principle of mutual induction.
True
True/False
Turns-ratio is the number of turns of wire in the primary winding compared to the number of turns in the secondary winding
True
True/False
When the number of turns or voltage on the secondary is less than that of the primary, the transformer is known as a Step-down Transformer.
True
True/False
When a load is applied to the secondary windings, load current will appear in both windings. These currents are inversely related to the turns-ratio of the windings.
True
The flow of current in the primary circuit is governed by three factors
Characteristics of the source
voltage and frequency
Impedance of the winding
Strength and direction of the magnetic field in the iron core
The voltage induced in the secondary coil is also governed by three factors:
Strength and direction of the magnetic field in the iron core
Rate of change in the magnetic field
Number of turns in the coil
Core loss is the sum of and losses
eddy-currents, hysteresis
True/False
Hysteresis losses are the result of the molecules of the core material being magnetized and de-magnetized
True
True/False
Copper losses are an additional transformer loss. They are due to the resistance of a transformer winding to the magnetizing current
True
True/False
Transformer losses dissipate power and produce unwanted heat that must be removed
True
The purpose of the tank is to contain the and , and the oil they are immersed in
core, windings
Oil in a transformer has two purposes:
Insulating the transformer
Cooling the transformer
A combination of power transformers used to energize three-phase loads. Banks may be made up of three single-phase or a single unit. Mobile unit banks can be transported to stations for emergency replacement.
Banks
banks are large transformers used at switching and receiving stations to change the voltage between sections of a bus
Bus
banks are large transformers used at switching and receiving stations to change the voltage between a line and a bus
Line
are instrument transformers that
measure voltage
Potential transformers (PT)
are instrument transformers that
measure current.
Current transformers (CT)
transformers are power transformers used on the
distribution system. These transformers may be on poles
overhead, on ground level pads, or in underground structures
Distribution
are potential devices connected to the system through capacitors. CCPDs are used for metering
Coupling Capacitor Potential Devices (CCPD)
A transformer is basically two sets of coils on a magnetic core of some kind. It has no moving parts and works on the principle of .
mutual induction
True/False
The insulating ability of transformer oil depends on its being
completely free of contaminants, water and oxygen in particular.
True
True/False
In order to prevent pressure build up in sealed tank units, some transformers have breathing regulators.
True
Both the approach and the allow atmosphere into the space above the oil. The regulator allows both water vapor and oxygen in.
breathing regulator, desiccant breather
is used in the space above the oil. An automatic pressure relief valve is connected to the gas space so that when the internal pressure reaches 5 psi, gas is vented to reduce the pressure.
Nitrogen (N2)
True/False
In a Transformer, Pressurized nitrogen is connected to the gas space. This maintains a positive pressure in the gas space that mitigates against entry of air through leaks
True
Oil heated in and around the windings and core rises to the top of the tank. This oil is replaced by cooler oil from the bottom of the tank in a circular motion
convection process
The of the transformer is determined by the rate of heat flow from the coils and core
MVA rating
The maximum top oil temperature is for normal transformer operation
95°C
What are three general methods for increasing cooling rate seen in DWP transformers:
1. Increase Surface Area of Tank.
2. Increase the Rate of Air Movement.
3. Increase the Rate of Oil Circulation.
A transformer is considered when it is equipped with radiators accept oil from the top of the tank and separate it into several paths. This exposes the oil to a large surface through which its heat can move to the surrounding air.
self-cooled
As heat is transferred from the metal to the air it is in contact with, the air expands, becomes less dense, and rises.
Cooler air from the surroundings flows in to take its place. This air, in turn, is heated and rises. Cooler air from the surroundings flows in to take its place
natural convection
True/False
A self-cooled transformer can be identified on its nameplate by the letters OA signifying Oil to Air.
True
A transformer with thermostatically controlled fans has two ratings:
1. Self-cooled rating
2. Forced convection rating.
How are these ratings identified on a Nameplate
OA/FA
The letters are used on the nameplate to signify forced oil to air heat transfer capability
FOA
True/False
The power banks high side might include a no load tap changer with a five-position range. If the high voltage side is 230kV the range would be from 220kV to 240kV in 5kV increments
True
True/False
The low side of the same power bank might include a load tap changer with a 33 position range. If the low voltage side is 34.5kV the range would be from 32.4kV to 39.6kV in approximately 120V increments
False - 220V
Due to variations on the primary side of distribution transformers, taps are changed to maintain a constant on the secondary
voltage
If the current in the low voltage winding is very high or if the voltages of both windings are above 33kV the use of a is desirable.
series transformer
Transformers having a single coil configuration are called
autotransformers
True/Fasle
Autotransformers are much cheaper to produce than two coil
transformers of the same rating. The disadvantage is that the primary and secondary circuits are electrically connected and therefore cannot be safely used to stepping down voltages for consumer use.
True
The application of wye connected transformers led to the use of a third transformer winding, referred to as a winding
tertiary
are defined as electric currents that alternate at a frequency other than 60 Hz
Harmonics
True/False
Tertiary windings also furnish a convenient means of connecting
synchronous condensers to the transmission system, thus eliminating the need of a separate transformer for this purpose
True
When transformers are connected delta the line current will be 1.732 times the phase current and thevoltage will be the same as the voltage
line, phase
When transformers are connected wye, the line voltage will be 1.732 times the phase voltage and the current will be the same as the current
line, phase
Regarding a Closed Delta circuit, The secondary phase wire farthest removed from the grounded neutral is called the
"power leg" (N to a)
True/False
Regarding a Closed Delta circuit, The voltage from the power leg to ground is approximately 208 volts. The voltage from phase to phase is 220 volts.
False - 240
The use of two single phase transformers
Open Delta
Pros and Cons of Open Delta
The initial cost is reduced because one less transformer is needed
only 58% of the power of an equivalent closed delta arrangement.
True/False
An opened delta connection can not be used if one transformer in a closed delta system fails.
False
True/False
In a three-phase circuit, the phase to ground voltage times the square root of 3 equals the line to line voltage
True
True/False
The secondary three-phase voltage can be used for either three-phase load or stepped down to supply 120/240 volts.
True
are used to control the flow of power between parallel circuits emanating from the station.
Phase-shifting banks
Direction of current flow in a closed circuit
Polarity
are used to transform a constant potential system to a constant current system. Street light circuits, connected in series, require a constant current
Constant current transformers
scale down values of current and voltage for use by meters and relays.
Instrument transformers
Instrument transformers used to step the voltage down
potential transformers
Instrument transformers used to step the current down
current transformers
Provide low voltage power for the stations internal operating needs and provide single phase power for the lighting circuits
and receptacles
Station Service Transformers
Continually evaluate the combustible gas content of
the insulating oil.
Hydran Analyzers
True/False
The Hydran Analyzer reads:
100% of the Hydrogen present 15% of the Carbon Monoxide present 8% of the Acetylene present 1% of the Ethylene present
