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The insulating system of a transformer includes various materials, list five:
Kraft paper, Enamels, Oil, Pressboard, vulcanized rubber, etc.
What four main functions does transformer oil provide?
- Provides dielectric strength
- Provides cooling
- Insulation, by filling voids in cellulose insuilation.
- Displaces oxygen
What is the flash point of transformer oil in degrees Celcius?
145 to 150 degrees Celcius
Is all oxygen content inside a transformer eliminated during vacuum filling?
Which has the greatest affinity for oxygen, oil or air?
What material has an even greater affinity for water?
Kraft paper insuluation
What affect does transformer temperature have on the moisture in a transformer?
As temperature increases, moisture is pulled out of the paper as it evaporates and absorbed into the paper insulation as temperature decreases.
A transformer oil sample must not be handled in dirty containers or in contact with dirty test equipment. It is especially important that a transformer oil sample be protected from what other elements?
Light, Air, and Moisture
The significance of a transformer oil dielectric test is that it will only detect what?
- Conductive particles
- Free water
What two other elements that detect deterioration, cannot be detected by the oil dielectric test?
Acids and Sludges
The most frequent contaminent in oil is what?
The most difficult to detect is what?
Sludge comes about when the ___________ attacking the iron, copper, varnishes, paints, etc. and these materials (iron, copper, etc.) come into solution and combine together.
What are the ASTM test numbers of the following oil tests?
1. Interfacial Tension
2. VDE Spherical Electrodes Dielectric Test
3. Flat-Disk Dielectric Test
4. Acid Number
The flat disk cup uses a (a)_____ gap and a voltage rise of (b)_______ volts per second.
(a) .1 inch
Which dielectric test method uses a nylon impellor stirrer immersed in the oil?
Describe the relationship between the IFT test and the neutralization number.
They are indirectly proportional: the lower the Neutralization number, the higher the IFT.
What is the upper limit for each test listed below?
1. Dielectric Strength ASTM D1816 (.04 gap)
2. Interfacial Tension
3. Acid Number
4. Water - ASTM D1533
5. % Power Factor at 25 deg. C
6. % Power Factor at 100 deg. C
7. Oxidation Inhibitor ASTM D2668
- 1. 28 KV minimum
- 2. 30 dynes/cm minimum
- 3. .15 mg KOH/g
- 4. 25 ppm (% Saturation < 30%)
- 5. .5%
- 6. 5%
- 7. .3% by weight
List five general types of elements that a relay can use to operate.
- a. Plunger
- b. Clapper
- c. Induction disk
- d. Induction cup
- e. Solid-state
(True / False) The basic funtion of an auxiliary relay is to detect and isolate a fault.
In general, a __________ relay has no moving parts.
Match each relay with the condition that will cause it to operate:
1. Auxiliary relay
2. Overcurrent relay
3. Voltage relay
4. Directional Overcurrent relay
5. Reclosing relay
3 a. To little voltage
4 b. To much current flowing in an undesirable direction
5 c. A circuit breaker opening
1 d. A signal from a protective relay
2 e. To much current
3 f. To much voltage
(True / False) An overcurrent relay with time-delayed operation gives other protective equipment time to isolate a fault before sending a trip signal.
The contacts for the (a)_________ unit of a single-phase directional overcurrent relay must close before the contacts of the (b)__________ unit can close.
(True / False) An auxiliary relay operates only when it receives a signal from a protective relay.
In an induction disc differential relay, two (a) _________ coils work to keep the relay's contacts open during operation, while one (b) ___________ coil works to close the contacts.
Which is the wrong answer, When a fault occurs on a transmission line that is protected by a transfer tripping system?
a) A relay in a substation at one end of the faulted line trips breakers in its substation.
b) A relay in a substation at one end of the faulted line provides a trip signal that is sent to a substation at the other end of the faulted line.
c) A relay in a substation at one end of the faulted line sends a trip signal to a transmitter.
d) A relay in a substation at one end of the faulted line gets a signal from a receiver.
e) None of the above
a) is wrong
Name 4 communication channels used for transfer tripping.
1) Transmission line (powerline carrier)
4) Fiber optics
Name the relay that correctly completes each of the following statements.
a) A __________ relay protects substation transformers and buses by comparing the current going to the protected device with the current leaving the device.
b) A __________ relay provides high-speed protection for transmission lines by responding to the ratio of voltage to the current on the line.
c) A __________ relay protects short transmission lines by comparing three-phase current at one end of the line with three phase current at the other end of the line.
d) A __________ relay is part of the system that detects when a circuit breaker fails to operate, and opens other breakers to isolate a fault.
c) Pilot wire
d) Breaker failure
How much upward pressure is on a round manhole cover that is 20" in diameter and has 1//2 psi pressure on it?
Pi (3.14)x r 2 x .5= 157 lbs.
What three things can be accomplished by processing an oil-filled transformer?
a) Remove excess water in insulation
b) Remove gasses and free water from the oil
c) Filter out contaminents
Water is (a)_____ to remove from thick pieces of core material and is (b)_____ to remove from thin cellulose tape or wraps?
Based on vacuum decay and temperature of the winding, a Piper chart can be used to find what?
Moisture content in %
___________ can be calculated from the amount of resistance in the windings.
Winding / core temperature
What are four methods for determining whether a transformer is dry enough?
1) Vapor pressure (using a piper chart)
2) Rate of water collection in a cold trap
3) Rate of Insulation temperature change
4) Dew point
(True / False) You should use only one of the methods mentioned in the previous question and ignore the rest.
What are the basic steps to processing a transformer?
1) Circulate hot oil until the core is at max. teperature
2) Remove all the oil
- 3) Apply vacuum until dry ( < .5% )
- *Reheating the core may be necessary
4) Vacuum fill the transformer