Week 1 Flashcards
What are 5 reasons we transformers are used?
- Transmission
- Isolation
- Metering/Relaying and instrumentation
- System Matching
- HV DC
What are 2 reasons we step up voltage from the generator?
- Reduce the power loss
- Reduce the conductor size
Transformers move power by stepping ______ or stepping ________
Up/Down
Power transmitted by the transformer is a function of :
Voltage x Current
S = VI where V is the:
System Voltage
S = VI where I is the:
Current determined by the load
If we were to double the transmission voltage but maintain the same power flow, what would happen to I
and PLoss assuming the line impedance stayed the same?
Current decreases and Ploss decreases
Using the equations below, explain how power losses decreases when voltage is increased:
Inital: S1 = V1I1, Ploss1 = I1^2R
New: S2 = V2I2, Ploss2 = I2^2R
Keeping the power the same and doubling the voltage, the formula for power can be rearranged into I2 = S/2V1. This is where I2 is equal to 1/2I1. Power loss is therefore reduced by 1/4.
Increasing the voltage of a transformer decreases the Ploss exponentially, linearly, or non-linearly?
Exponentially
Isolation prevents high or low voltage from coming through?
High
Isolation can filter high frequency, voltage or current? How does it manage to do this?
- High frequency
- With an electrostatic shield
Isolation transformers have at least _____ windings that are electrically insulated from each other.
2
Isolation transformers help limit the amount of _________ available
Fault current
A transformer has a primary winding of 13800V and a secondary winding of 600V. The secondary is connected to a load. Half of the windings are non-insulated. If the circuit opens, what will happen to the load?
The windings would become shorted and the load would be damaged.
PT (VT) stands for:
Potential transformer/Voltage transformer
CT stands for:
Current transformer
What is the purpose of a voltage transformer?
Bring voltage down toa safe value for the use of meters or protective relays
What is the purpose of a current transformer?
Bring current down to a safe level so that it can be used for meters or protective relays.
CT transformers are made of a _______ core.
Torroid
The primary winding of the PT is connected in parallel or series with the line?
Parallel
THe primary winding of the CT is connected in parallel or series with the line?
In series
What type of transformer is used to transmit power from Canada to the US?
A phase shifting transformer
What is the purpose of a phase shifting transformer?
To match the voltage of 2 different systems
How does a phase shifting transformer control power flow?
By shifting the phase angle of the voltage
A phase shifting transformer must do 4 things. What are these 4 things?
- Match the amplitude
- Match the phase shift
- Match the frequency
- Match the phase sequence
Lenz’s Law
An induced voltage will cause a current to flow in a closed circuit in such a direction that the magnetic field which is caused by the current will oppose the change that produced the current.
At no load, the induced voltage in a transformer is caused by:
Self inductance
When a voltage is applied to the primary winding at no load, is the generated voltage the same or opposite of the source voltage? What is the law that determines this?
Opposite
Lenz’s Law
At no load the net voltage across the input is:
Vp - V1
Explain the operation of a transformer at No Load
When a voltage is supplied to the primary winding, an exciting current is produced in the winding. The exciting current generates an exiting flux in the core. Since the flux is common to both windings, the voltage induced on the secondary is by mutual inductance.
The assumptions for an ideal transformer are:
- Winding resistance is 0
- Winding reactance is 0
- Power losses in the core are 0
- Reactive losses in the core are 0
At load, exciting flux induces a voltage in the primary and secondary winding. Is this true or false?
True
At load, V2 will lag exciting flux by _______. Why is this?
- 90 degrees
- Inductance and V1 and V2 are in phase because of the same flux.
Why are V2 and I2 in phase for a transformer at load?
Rload is purely resistive
V2 is directly proportional or inversely proportional to V1 at load? Why?
- Directly proportional
- Due to turns ratio
At load, I2 is determined by ______
The load
At load, I1 is determined by ______
The load in the secondary
At load, when I2 increases what happens to I1?
I1 increases
