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Important Notes

Notes for power transformer: major components

1. Main body:including iron core, winding, insulating element and lead wire;

2. Voltage regulator: i.e., tap-switch, including non-excitation voltage regulation and on-load voltage regulation;

3. Oil tank and cooling device;

4. Protective device:inclusive of oil conservator, safety gas duct, moisture absorber, air relay, oil purifier and temperature measuring device;

5. Insulating bushing

What is the function of transformer oil?

Transformer oil is used for insulation and cooling;

And for arc blowout in on-load tap changer;

What is the function of oil conserver?

Oil conserver is used to regulate the oil level, so that the transformer oil tank is always fully filled;

And to reduce the area of the contact surface between oil and air, so that oil will not be damped or oxidized at a very fast speed.

What is the function of transformer bushing? what are the requirements for it?

The function of transformer bushing is as follows:

It leads the high-voltage and low-voltage wire leads inside the transformer to outside the oil tank; it insulates the leads against the ground and fixed the leads. Transformer bushing is one of the current-carrying components of transformer. It conducts load current for long in the transformer operation and conducts short-circuit current when short circuit occurs outside the transformer.

The requirements for transformer bushing are as follows:

1. Specific electrical strength and sufficient mechanical strength must be available.

2. Good thermal stability; withstand the instantaneous superheat when short circuit occurs.

3. Small size, light weight, good sealing performance and versatility, as well as easy maintenance.

What are the two types of transformer test items? What are their contents?

Transformer test is generally divided into insulation test and performance test.

Insulation test includes insulation resistance and absorption ratio test, dielectric loss tangent value test, leakage current test, transformer oil test, industrial frequency withstand voltage and induction withstand voltage test; local discharge test is required for a transformer with Um of no less than 220kV. Full wave and operating wave impact test is required for online terminal with Um of no less than 300kV.

Performance test includes tests of transformation ratio, wiring group, DC resistance, idle load, short circuit, temperature rise and sudden short circuit.

What is the purpose of DC resistance test of transformer?

The purpose of DC resistance test is to check whether there is short circuit, open circuit or wrong connection in winding loop, whether there are poor contacts in welding point of winding loop wire, lead wire bushing and tapping switch. Besides, it can be checked whether the lead wire used for winding meets the design requirements.

What is the purpose of idle load test of transformer?

The purpose of idle load test is to test the no-load current and no-load loss in iron core, find partial or total defects in the magnetic circuit, and check whether winding has turn-to-turn fault after induction withstand voltage test of transformer.

Why does transformer winding need to be dried?

The purpose of drying is to improve the insulation level of winding. Insulating cardboard can be compressed under certain pressure F, and the mechanical strength of the winding will be improved.

What are the commonly-used insulation materials of Grade A? What is the heat-resistant temperature?

Commonly-used insulation materials of Grade A for transformer are insulating cardboard, cable paper, yellow painted silk, phenolic cardboard, wood and transformer oil, etc.

Heat-resistant temperature is 105℃.

Why does vacuum drying transformer have better effect?

Under vacuum condition, the higher the vacuum degree is , the lower the boiling point of the water molecules will be; and heated water is easily evaporated. The water volatilized from the transformer is rapidly drawn out by vacuum pump, thereby speeding up the evaporation of water, so this method has a good effect.

What are the requirements for parallel running of transformers? What are the consequences if they are not met?

The parallel running of transformers should meet the following conditions:

1. The same connection group mark number (connection group);

2. Equal rated primary voltages and rated secondary voltages, i.e. equal transformation ratios.

3. Equal impedance voltage per unit (or percentage)

If they are not met, the consequences are:

1. If the connection group mark numbers (connection groups) are different, the phase difference between the secondary voltages will be very large, and there will be heavy current in the secondary circuit; the larger the phase difference is, the heavier the circulating current will be; and the transformer will be definitely burned out.

2. If the rated primary voltages and the rated secondary voltages are respectively different, i.e. different transformation ratio, there will be circulating current in secondary circuit; it will occupy the transformer capacity and increase the wear.

3. Different impedance voltages per unit (or percentages) will lead to unreasonable load distribution; and there will be the circumstance that one is fully loaded while the other is under-loaded or overloaded.

What measures should be taken for open core hanging of transformer?

Open core hanging should be conducted in fine weather and the relative air humidity should be lower than 75%. The part of core exposed in the air should be lower than 12h when the relative air humidity is lower than 75%. Core of no more than 16h can be exposed if the relative air humidity is lower than 65%. Effective measures should be taken to prevent dust and rain from the transformer. If the ambient temperature is higher than the core temperature, leave the core resting f the ambient temperature or a while before hanging it out, or heat the core to 10℃ higher than the ambient temperature when necessary.

What are the main equipment and materials used in the transformer core hanging?

1. Lifting equipment: crane, U-rings, steel wire slings, ropes, and wood, etc.;

2. Oil containers and oil filtering equipment;

3. Electric welding equipment;

4. General tools and special tools;

5. Cloth belt, dry insulating cardboard, cable paper, etc.;

6. Ladder;

7. Fire-fighting equipment;

8. Megohm meter, double-bridge resistance meter and thermometer, etc.

Maintenance of circuit breaker (at least once half a year):

1. Ensure that the ambient environment of a circuit breaker meets general provisions;

2. All frictional and rotary components shall be lubricated regularly;

3. Check whether the bolt at the connection between circuit breaker and busbar is screwed tightly and whether they are properly contacted;

4. Check whether the terminal of the circuit breaker’s secondary loop is properly connected;

5. Check whether the circuit breaker’s intelligent controller is properly displayed;

6. Check whether the setting value for protective features of intelligent controller is correct;

7. Check whether the on/off indication of circuit breaker is correct and reliable.

Inspection and repair of circuit breaker (at least once a year):

1. Check whether all parts of the circuit breaker are complete and correct, for example, shell, chassis and other insulating parts;

2. Check whether the circuit breaker’s base is securely connected with the baseboard, and no vibration in operation.

3. Manual on-off mechanism should be flexible and free of obstruction, and auxiliary switch changeover of secondary circuit shall be reliable and correct;

4. Shake-in and shake-out the drawer seat manual; separation, test, position for connection shall be correct, interlock shall act reliably;

5. Actions of shunt tripper, closing electromagnet and undervoltage tripper shall meet technical regulations of products, and the power operating mechanism should act normally, after secondary circuit is energized,

6. For contact system of arc extinguish chamber: complete contact finger, correct position, sound silver coating, clean inside the chamber (note: don’t switch on/off the mechanism when cleaning the arc extinguishing chamber);

7. Circuit breaker and connecting busbar are reliably connected, and the bolt shall be screwed tightly;

8. Check whether the contact surface between the circuit breaker and the drawer seat is clean and correct; if not, clean the surface to ensure that there is no surface dusk or oxide, and the connection is reliable;

9. Once overhauled, use a 500V megohmmeter to test the circuit breaker’s insulating resistance, and the resistance shall not be smaller than 20 megohms under ambient medium temperature of 20℃±5℃ and relative humidity of 50% to 70%.

Use and maintenance: (only persons with corresponding electrical professional qualifications permitted)

1. Check the operation status of a circuit breaker regularly to clear away dusts on shell surface and maintain good insulating property;

2. The circuit breaker’s tripping features are preset by the manufacturer and no need to make adjustment generally. To adjust the features as per requirements, customer shall cut off the power at first and then adjust the overload protection knob and the short-circuit protection knob until the requirements are met;

3. If the circuit breaker is opened for overload, short-circuit and undervoltage protection, it’s necessary to identify causes and solve the problem before closing the circuit breaker;

4. Be sure to prevent rainwater and falling when a circuit breaker is in use, storage and transport.

Problem Shooting

Drawer-type circuit breaker cannot be rocked to the "connection" position; what is the cause?

1. Foreign matter has fallen into the drawer base and stuck the rocking mechanism or the rocking mechanism has jumping chain;

2. The rated current of circuit breaker does not match that of drawer base;

3. The operation is not in place; no "click" from two sides in the drawer base.

What are the common causes for contactor noise?

1. Power voltage is too low;

2. Magnetic system is skewed or mechanically stuck, which causes that the iron core cannot be pulled level;

3. Iron core polar surface is rusted or has foreign matter (such as grease or dust) on it;

4. Short-circuit ring of iron core is fractured.

What are the common causes for damage to the contactor coil?

1. Low power voltage causes that iron core cannot be pulled normally, and the current in the coil will rise sharply, resulting in overheated and burnt coil.

2. High power voltage (> 110% Us) leads to overheated and burnt coil.

3. Foreign matter has fallen into the contactor, which has demagnetized contactor coil and caused too large gap; the magnetic circuit is not closed, the  coil is overheated and burnt.

What are the common causes for thermal relay malfunction?

1. Setting current value is set too small;

2. The starting time of motor is too long;

3. The cross-sectional area of connection wire is too small;

4. Strong shock and vibration;

5. Frequent reversible operation and jogging.

Main causes for and solution to the malfunction of residual current circuit breaker:

1. Operating current is selected too small;

When selecting operating current, it should be higher than 2-4 times normal leakage current in circuit. If normal leakage current of the electronic equipment is larger, the equipment on each circuit cannot be too many, and the total leakage current should be lower than the rated non-operating current of RCD. For example, the leakage current of a desktop computer is 3-4mA, so the number of computers connected to 30mA RCD circuit should not be more than five.

2. Improper use of residual current circuit breaker may cause malfunction;

When a three-pole earth residual current circuit breaker is used in a three-phase four-wire circuit, as the normal operating current in neutral line does not go through zero sequence current transformer, residual current circuit breaker will action once single-phase load is started.

Solution: A three-pole four-wire or four-pole residual current circuit breaker must be used for a three-phase four-wire circuit.

3. Grounding of load-side neutral line behind the residual current circuit breaker causes malfunction;

Grounding of load-side neutral line of the residual current circuit breaker will lead normal operating current to the ground through the grounding point, causing malfunction of residual current circuit breaker.

Solution: Connect the grounding wire to the neutral line on the power side of residual current circuit breaker.

4. Leakage current and wire-to-ground capacitive current cause malfunction.

The load-side wire of residual current circuit breaker is long and is laid closely to the ground, and there will be heavy wire-to-ground capacitive current, which may cause malfunction. Or the earth leakage current may be heavy due to decreased insulation of load-side wire, causing malfunction.

Solution: Use residual current circuit breaker with high leakage operating current or raise the wires overhead.