Substation Operation and Maintenance: Power Transformers Part 1 Flashcards
(148 cards)
1
Q
What is a transformer?
A
A device that transmits AC power from one circuit to another by electromagnetic induction.
2
Q
Why are transformers used in power systems?
A
To increase voltage for efficient transmission and reduce voltage for distribution to consumers.
3
Q
What are the three essential parts of a transformer?
A
Primary winding, secondary winding, and a core.
4
Q
What is the primary winding in a transformer?
A
A coil of insulated wire connected to a circuit that supplies AC power.
5
Q
What do the labels H1 and H2 represent?
A
The leads of the primary winding, typically indicating higher voltage connections.
6
Q
What is the secondary winding in a transformer?
A
A coil of insulated wire connected to a load, usually carrying lower voltage.
7
Q
What do the labels X1 and X2 represent?
A
The leads of the secondary winding, typically indicating lower voltage connections.
8
Q
What is a transformer?
A
A device that transmits AC power from one circuit to another by electromagnetic induction.
9
Q
Why are transformers used in power systems?
A
To increase voltage for efficient transmission and reduce voltage for distribution to consumers.
10
Q
What are the three essential parts of a transformer?
A
Primary winding, secondary winding, and a core.
11
Q
What is the primary winding in a transformer?
A
A coil of insulated wire connected to a circuit that supplies AC power.
12
Q
What do the labels H1 and H2 represent?
A
The leads of the primary winding, typically indicating higher voltage connections.
13
Q
What is the secondary winding in a transformer?
A
A coil of insulated wire connected to a load, usually carrying lower voltage.
14
Q
What do the labels X1 and X2 represent?
A
The leads of the secondary winding, typically indicating lower voltage connections.
15
Q
What is the core of a transformer made of?
A
The core is typically made of laminated metal layers that are tightly bound with insulation between them.
16
Q
Is there a direct electrical connection between the windings in a transformer?
A
No, the core and the windings are insulated from each other, and there is no direct electrical connection between the primary and secondary windings.
17
Q
How does a transformer transmit power between windings?
A
Power is transmitted by means of electromagnetic induction.
18
Q
What are the three requirements for electromagnetic induction?
A
- A magnetic field
- A conductor
- Relative motion between the magnetic field and the conductor
19
Q
How is a magnetic field created in a transformer?
A
By alternating current flowing through the primary winding.
20
Q
What role does the core play in a transformer?
A
The core concentrates the magnetic field and provides a path with low resistance, allowing the magnetic field to cut across the secondary winding.
21
Q
What does the secondary winding act as in electromagnetic induction?
A
It acts as the conductor where the changing magnetic field induces a voltage.
22
Q
What does the first part of a turns ratio represent?
A
The number of turns on the primary winding.
23
Q
What does the second part of a turns ratio represent?
A
The number of turns on the secondary winding.
24
Q
If the primary winding has 8 turns and the secondary has 2 turns, what is the turns ratio?
A
8:2, which simplifies to 4:1.
25
How does a transformer’s turns ratio relate to voltage?
The ratio between primary and secondary voltages is the same as the turns ratio between the windings.
26
If the turns ratio is 4:1 and the primary voltage is 480 volts, what is the secondary voltage?
120 volts.
27
What is a step-down transformer?
A transformer with fewer turns on the secondary winding, reducing the voltage from primary to secondary.
28
What is a step-up transformer?
A transformer with more turns on the secondary winding, increasing the voltage from primary to secondary.
29
If the turns ratio is 1:5 and the primary voltage is 100 volts, what is the secondary voltage?
500 volts.
30
What are the three types of transformers commonly found in substations and switchyards?
Power transformers, current transformers, and potential transformers.
31
What is the main function of a power transformer?
To transfer electrical power between circuits at different voltage levels, usually stepping voltage down for distribution or up for transmission.
32
What are current transformers (CTs) used for?
To reduce high current levels to a lower, safer level for measurement and protection equipment.
33
What are potential transformers (PTs) used for?
To reduce high voltage levels to lower levels suitable for metering and protective relays.
34
How do these transformer types differ from each other?
Each type is designed with unique features based on its function: power handling (power transformers), current sensing (CTs), and voltage sensing (PTs).
35
What distinguishes power transformers from other substation equipment?
Power transformers are large, stationary units designed to transfer electrical energy between circuits at different voltage levels.
36
What are bushings on a transformer?
Insulated devices that allow conductors to pass through the transformer tank without contact.
37
Why are transformer bushings important?
They maintain electrical insulation and mechanical support where conductors enter or exit the transformer.
38
What is the transformer tank filled with?
Usually filled with insulating oil that helps cool and insulate internal parts.
39
What are radiators on a transformer?
External components used to dissipate heat from the transformer oil to the surrounding air.
40
How does oil move through the radiators?
Either by natural convection or by using pumps to circulate the oil.
41
What is the function of cooling fans on transformers?
To increase the rate of heat transfer from radiators, improving cooling efficiency.
42
Why do power transformers need to be cooled?
Because they generate significant heat during operation, which must be managed to prevent overheating and equipment failure.
43
What are the two main cooling methods for transformers?
Natural cooling and forced cooling.
44
What is natural cooling?
A method where heat is dissipated without external assistance—typically using radiators and convection.
45
What is forced cooling?
A method where cooling is enhanced using fans or pumps to move air or oil and increase heat dissipation.
46
What are the two most common types of transformer cooling systems?
Oil-immersed self-cooled (OA) and oil-immersed forced-air cooled (OA/FA).
47
How does OA (Oil-immersed Self-cooled) work?
Heat generated in the transformer is carried by oil to the radiators, where it is dissipated naturally into the surrounding air.
48
How does OA/FA (Oil-immersed Forced-air cooled) work?
It uses fans to blow air over the radiators to enhance cooling.
49
Why do power transformers need to be cooled?
Because they generate significant heat during operation, which must be managed to prevent overheating and equipment failure.
50
What are the two main cooling methods for transformers?
Natural cooling and forced cooling.
51
What is natural cooling?
A method where heat is dissipated without external assistance—typically using radiators and convection.
52
What is forced cooling?
A method where cooling is enhanced using fans or pumps to move air or oil and increase heat dissipation.
53
What are the two most common types of transformer cooling systems?
Oil-immersed self-cooled (OA) and oil-immersed forced-air cooled (OA/FA).
54
How does OA (Oil-immersed Self-cooled) work?
Heat generated in the transformer is carried by oil to the radiators, where it is dissipated naturally into the surrounding air.
55
How does OA/FA (Oil-immersed Forced-air cooled) work?
It uses fans to blow air over the radiators to enhance cooling.
56
What type of oil is typically used in transformers?
Mineral oil is the most common, though some use silicone or other less flammable alternatives.
57
What is the function of insulating oil in transformers?
It provides both electrical insulation and cooling for internal components.
58
What does a conservator tank do in a transformer?
It allows for oil expansion and contraction due to temperature changes without exposing the oil to the outside atmosphere.
59
What is the purpose of a breather in a conservator system?
It filters and dries air entering the conservator to prevent moisture contamination of the oil.
60
How does the conservator system reduce oxidation?
By minimizing the oil’s exposure to air, it helps reduce oxidation and contamination.
61
What is a bladder or diaphragm used for in some conservator systems?
To separate the oil from the air completely, offering better protection against moisture and oxidation.
62
What is the purpose of a sealed-tank transformer?
It prevents contact between the insulating oil and outside air, reducing contamination and moisture absorption.
63
How do sealed-tank transformers accommodate oil expansion?
By using flexible diaphragms or nitrogen gas cushions to handle internal pressure changes.
64
What role does nitrogen gas play in some sealed transformers?
It provides a pressure cushion and prevents air (and moisture) from contacting the oil.
65
Why are pressure relief devices important in sealed transformers?
To vent excess internal pressure safely, protecting the tank from rupture.
66
What is the function of a pressure/vacuum gauge on a sealed transformer?
It monitors internal pressure and alerts operators to abnormal pressure conditions.
67
How is oil condition typically monitored in a sealed transformer?
Through sight glasses and sample ports for inspecting oil levels and testing quality.
68
What is the purpose of a sealed-tank transformer?
It prevents contact between the insulating oil and outside air, reducing contamination and moisture absorption.
69
How do sealed-tank transformers accommodate oil expansion?
By using flexible diaphragms or nitrogen gas cushions to handle internal pressure changes.
70
What role does nitrogen gas play in some sealed transformers?
It provides a pressure cushion and prevents air (and moisture) from contacting the oil.
71
Why are pressure relief devices important in sealed transformers?
To vent excess internal pressure safely, protecting the tank from rupture.
72
What is the function of a pressure/vacuum gauge on a sealed transformer?
It monitors internal pressure and alerts operators to abnormal pressure conditions.
73
How is oil condition typically monitored in a sealed transformer?
Through sight glasses and sample ports for inspecting oil levels and testing quality.
74
What is a load tap changer (LTC)?
A device used to adjust the transformer’s output voltage under load without interrupting power.
75
Where is a load tap changer usually located?
Inside a separate compartment attached to the main transformer tank.
76
What does the LTC do when system voltage changes?
It adds or removes winding taps to raise or lower the output voltage accordingly.
77
What is the benefit of using an LTC?
It helps maintain steady voltage levels to customers despite changes in system load.
78
What is an important maintenance consideration for LTCs?
Regular inspection and servicing are crucial due to their moving parts and arcing contacts.
79
What kind of fluid is often used in LTC compartments?
Insulating oil, similar to or the same as in the main transformer tank.
80
What is a no-load tap changer (NLTC)?
A device used to change the transformer voltage ratio, but only when the transformer is de-energized.
81
How does an NLTC differ from a load tap changer (LTC)?
An NLTC can only be adjusted when the transformer is offline, whereas an LTC operates under load.
82
When is an NLTC typically used?
During installation or scheduled maintenance, to match system voltage requirements.
83
How many tap positions are usually available in an NLTC?
Typically 5 positions: 2 above nominal, 2 below, and 1 at nominal voltage.
84
Why must the transformer be de-energized for NLTC adjustment?
To prevent arcing and equipment damage, as the device is not built for live switching.
85
What is the main purpose of both LTCs and NLTCs?
To regulate and optimize voltage for system stability and customer service quality.
86
What is a forced-oil/forced-air-cooled power transformer?
It is a transformer with enhanced cooling using both forced circulation of oil and air.
87
What components are typically included in these transformers?
Heat exchangers, pumps, and flow gauges.
88
What does the heat exchanger typically consist of?
Fans and a radiator with finned tubes housed in a metal shroud.
89
What do the fans and shroud do?
Fans circulate air around the radiator tubes, and the shroud channels airflow and protects the equipment.
90
What does the flow gauge indicate?
Whether or not the pump is circulating oil through the system.
91
Can the appearance and layout of cooling components vary?
Yes, depending on the manufacturer.
## Footnote
For example, fans may be mounted at different angles.
92
Can cooling equipment be separate from the transformer?
Yes, sometimes heat exchangers, pumps, and gauges are mounted near the transformer but not directly on it.
93
What does ‘FOA’ on a transformer nameplate stand for?
Forced Oil to Air.
94
How does the forced-oil/forced-air cooling system work during operation?
A pump circulates oil from the transformer through a heat exchanger and back. Fans cool the oil using increased surface area of finned tubes.
95
Why do fans and finned tubes improve heat dissipation?
Finned tubes provide more surface area and fans circulate air, both enhancing cooling performance.
96
What is the purpose of temperature and level gauges on a power transformer?
To monitor the transformer’s oil and winding temperatures and oil level, helping ensure safe operation.
97
What does the gauge in Figure 3-31 measure?
The temperature of the oil at the top of the transformer.
98
What happens when the oil temperature becomes too high?
A switch is triggered, sending an alarm to a control center.
99
What does the light (white) pointer on a winding temperature gauge indicate?
The current winding temperature.
100
What does the dark (red) pointer on a winding temperature gauge indicate?
The highest temperature reached since the last reset.
101
What does the first switch on the winding temperature gauge do?
It starts additional cooling fans and pumps when a preset temperature is reached.
102
What does the second switch do if temperature keeps rising?
It triggers an alarm at the control center.
103
What does the third switch do?
It activates a relay to disconnect the transformer, preventing damage.
104
How is oil level affected by temperature?
It rises as the oil expands (heats up) and drops as the oil contracts (cools down).
105
Why is oil important in a transformer?
It keeps the transformer cool and provides electrical insulation.
106
What can reduce the oil’s insulating quality?
Air and moisture contamination.
107
Why do power transformers have sealing systems?
To prevent air and moisture from mixing with the oil and degrading its quality.
108
What are two common types of transformer sealing systems?
Gas-sealed and conservator-type sealed systems.
109
How does a gas-sealed system protect transformer oil?
By pressurizing the space above the oil with dry nitrogen gas, preventing air and moisture from entering.
110
What gas is commonly used in gas-sealed transformers?
Nitrogen that has had most of its moisture removed.
111
How can a gas-sealed transformer be identified?
By the presence of a gas cylinder with regulators, gauges, and a pressure relief device.
112
Where are these components typically housed?
In a weatherproof cabinet mounted near the transformer.
113
What components make up the sealing system in a gas-sealed transformer?
A gas cylinder, two pressure regulators, two gauges, and a pressure relief device.
114
What surrounds the windings in a gas-sealed power transformer?
Oil. The rest of the enclosure is filled with gas (usually nitrogen).
115
What do the regulators in a gas-sealed system do?
They ensure gas is maintained at slightly above atmospheric pressure to prevent air and moisture from entering.
116
What happens when the transformer heats up?
The oil expands and compresses the gas. If pressure gets too high, the relief valve releases gas.
117
What happens when the transformer cools down?
The oil contracts, reducing pressure, and the regulator adds gas from the cylinder to restore proper pressure.
118
How is gas pressure monitored?
Gauges on the gas cylinder indicate the pressure; low readings may mean gas needs to be replenished.
119
What does a pressure relief device do in this system?
It vents excess pressure to atmosphere to protect the transformer.
120
What is a pressure-vacuum gauge (compound gauge) used for?
To measure pressure inside the transformer enclosure—alerting operators if pressure is too high or low.
121
What do the regulators in a gas-sealed system control?
The flow of gas from the cylinder to the transformer and from the transformer to the atmosphere.
122
What happens when pressure exceeds a safe limit?
The pressure relief device vents excess gas from the enclosure to the atmosphere.
123
What is a common issue with gas-sealed transformers?
Gas can be lost over time due to small leaks or through the relief device, requiring periodic replenishment.
124
What is a conservator-type sealed transformer?
A transformer that uses an external expansion tank (conservator) to isolate the oil from outside air and moisture.
125
Where is the conservator tank mounted?
At a level higher than the top of the transformer.
126
What is the main function of the conservator tank?
To allow oil to expand and contract with temperature changes without mixing with air inside the main enclosure.
127
What feature helps keep air and oil separate in the conservator?
A breather and piping system that control air flow and oil transfer between the conservator and transformer.
128
What is the function of the air space in the conservator tank?
To accommodate oil expansion and contraction without exposing oil to external air.
129
What is a breather in a conservator system?
A device that dries and filters incoming air before it enters the conservator tank.
130
What is the breather filled with?
Desiccant crystals (often silica gel) that absorb moisture from incoming air.
131
What happens when desiccant crystals in a breather change color?
It indicates the crystals are saturated with moisture and need replacement.
132
Why is it important to keep the desiccant in good condition?
To ensure air entering the transformer is dry and free from contaminants.
133
What type of fluid might also be used in the breather to trap dust and dirt?
Mineral oil, placed in a small container inside the breather.
134
What happens if the breather is neglected?
Moisture and contaminants can enter the transformer, degrading the insulating oil.
135
How is the breather connected to the transformer?
It’s connected to the air space in the conservator tank via a pipe.
136
What happens when the oil level in the conservator rises?
Air in the conservator is displaced and pushed out through the breather.
137
What happens when the oil level drops?
Air is drawn into the conservator tank through the breather.
138
What do some transformers use to completely separate air from oil?
A rubber diaphragm or bladder inside the conservator tank.
139
What is the function of the diaphragm or bladder?
It expands and contracts to accommodate oil volume changes without allowing oil to contact air.
140
Why is a diaphragm or bladder system better than just a breather?
It offers superior protection against moisture and oxygen entering the oil.
141
What happens if the bladder in the conservator is damaged?
Moisture and air may enter the oil, risking contamination and insulation failure.
142
What is a sight gauge used for in a conservator tank?
To visually monitor the oil level in the conservator.
143
Why is it important to monitor the oil level in the conservator?
To ensure there is enough oil for expansion and to detect leaks or other issues.
144
What maintenance should be done on a conservator system?
Regularly check the sight gauge, inspect the breather, and test the oil quality.
145
What does a sudden drop in oil level indicate?
A possible oil leak or a bladder failure.
146
How is the oil sampled in a conservator system?
Using an oil sample valve or port to draw fluid for testing.
147
Why is oil sampling important?
To detect contamination, moisture, or degradation of insulating properties.
148
What should you do if the oil sample indicates high moisture?
Replace or dry the oil and investigate the sealing system for failures.
