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2. The rated capacity and short-circuit voltage of three three-phase transformers with the same transformation ratio and connection group are respectively:

After running them in parallel, the load is 550OKVA,

Q: ① The load distributed by each transformer?

② How much maximum total load can three transformers bear without any overload?

③ What is the utilization rate of the total equipment capacity of the transformer?

Answer

Distribution ratio of each transformer:

After they are operated in parallel, the load is 5500KVA,

Q: ① The load distributed by each transformer?

② How much maximum total load can three transformers bear without any overload?

③ What is the utilization rate of the total equipment capacity of the transformer?

Answer



3. What are the differences between autotransformers and ordinary transformers?

Answer: The difference between autotransformer and ordinary transformer is:

(1) Its primary side and secondary side are not only connected by magnetism, but also by electricity, while ordinary transformers are only connected by magnetism.

(2) The capacity of the power supply through the transformer is composed of two parts: the electromagnetic induction power between the primary winding and the common winding, and the conduction power directly conducted by the primary winding.

(3) Since the autotransformer winding is composed of a primary winding and a common winding, the number of turns of the primary winding is correspondingly less than the number and height of turns of the primary winding of the ordinary transformer, the current of the common winding and the leakage reactance generated. The short-circuit reactance X of the autotransformer is (1-1/K) times of the short-circuit reactance X of the ordinary transformer, and K is the transformation ratio.

(4) If the autotransformer is equipped with a third winding, its third winding occupies the common winding capacity. It affects the operation mode and exchange capacity of the autotransformer.

(5) Because the neutral point of autotransformer must be grounded, the setting and configuration of relay protection are complicated.

(6) Autotransformer is small in size, light in weight, convenient for transportation and low in cost.

4. What problems should be paid attention to in the operation of autotransformer?

Answer: Problems to be noticed during the operation of autotransformer:

(1) Since the primary and secondary sides of the autotransformer have direct electrical connection, the neutral point of the autotransformer used in the power grid must be reliably and directly grounded to prevent the voltage rise on the low-voltage side caused by the single-phase grounding fault on the high-voltage side.

(2) Due to the direct electrical connection between the primary side and the secondary side, when the high-voltage side is subject to over-voltage, it will cause serious over-voltage on the low-voltage side. To avoid this danger, lightning arresters must be installed at both the primary and secondary sides.

(3) Since the short circuit impedance of autotransformer is small and its short circuit current is larger than that of ordinary transformer, it is necessary to take measures to limit the short circuit current when necessary.

(4) During operation, the current of the common winding shall be monitored to make it less than the load. If necessary, the operation mode of the third winding can be adjusted to increase the exchange capacity of the autotransformer.

5. Draw the autotransformer O - Y0 with the third winding/ Δ- Wiring diagram and vector diagram of 12-11

Answer: 

Wiring diagram

                                                                               Potential vector diagram






6. What are the types of transformer voltage regulation? Why are transformer taps on the high-voltage side?

Answer: There are two voltage regulation modes of transformer: on load voltage regulation and off load voltage regulation:

On load voltage regulation means that the transformer can adjust its tap position during operation, thereby changing the transformer ratio to achieve the purpose of voltage regulation. There are two modes of on load voltage regulating transformer: line end voltage regulating and neutral point voltage regulating, that is, the difference between the transformer tap at the line end side of the high-voltage winding or at the neutral point side of the high-voltage winding. Tapping at the neutral side can reduce the insulation level of the transformer tap, which has obvious advantages, but it requires that the neutral point of the transformer must be directly grounded during operation.

No load voltage regulation refers to regulating the tap position of the transformer in case of power failure and maintenance, so as to change the transformer ratio to achieve the purpose of voltage regulation.

The tap of transformer is generally taken from the high-voltage side, which mainly considers:

(1) The high-voltage winding of the transformer is generally on the outside, and the tap lead out connection is convenient;

(2) The current at the high-voltage side is smaller, and the conductor section of the current carrying part of the outgoing line and the tap changer is smaller. The impact of poor contact can be easily solved.

In principle, the tap can be located on any side, and economic and technical comparison shall be made. For example, the tap of 500kV large step-down transformer is drawn from the 220kV side, while the 500kV side is fixed.

7. What is transformer overexcitation? How does overexcitation of transformer occur?

Answer: When the voltage of the transformer increases or the frequency decreases, the working magnetic flux density will increase. The saturation of the transformer core is called transformer overexcitation.

After the power system is disconnected due to an accident, the load rejection overvoltage, ferromagnetic resonance overvoltage, improper adjustment of transformer tap connection, no-load transformer at the end of a long line or other misoperation, premature increase of excitation current before the generator frequency reaches the rated value, self excitation of the generator, and other conditions of some systems may generate higher voltage to cause transformer overexcitation.

8. What are the possible consequences of transformer overexcitation? How to avoid it?

Answer: When the transformer voltage exceeds 10% of the rated voltage, the transformer core will be saturated and the iron loss will increase. Magnetic leakage increases the eddy current loss of metal components such as the box shell, causing transformer overheating, insulation aging, affecting the service life of the transformer and even burning the transformer.

Avoidance:

(1) Prevent operation with excessive voltage. Generally, the higher the voltage is, the more serious the overexcitation is, and the shorter the allowable operation time is.

(2) Add overexcitation protection: send alarm signal or cut off the transformer according to the transformer characteristic curve and different allowable overexcitation multiples.

9. What safety protection facilities are provided for the transformer body structure? What is its main function?

Answer: The protection facilities in the transformer body structure include:

(1) Conservator

Its capacity is about 8-10% of transformer oil. Its function is to accommodate the change of transformer oil volume due to temperature change, limit the contact between transformer oil and air, and reduce the degree of oil moisture and oxidation. A moisture absorber is installed on the conservator to prevent air from entering the transformer.

(2) Moisture absorber and oil purifier

The moisture absorber, also known as respirator, is filled with adsorbent, which is silica gel type activated alumina. A part of discolored silica gel is often put in it. When the blue turns red, it indicates that the adsorbent has been affected with moisture and must be dried or replaced.

The oil purifier is also called a filter. The clean oil cylinder is filled with adsorbent, which is silica gel activated alumina. When the oil passes through the oil purifier and contacts the adsorbent, the water, acid and oxide in the oil purifier are absorbed, which makes the oil clean and extends the service life of the oil.

(3) Explosion proof pipe (safety airway)

The explosion-proof pipe is installed on the cover of the transformer tank as a relief protection against high pressure in the oil tank in case of internal failure of the transformer.

The pressure relief valve has been used in modern large transformers to replace the safety air passage. When the internal fault pressure of the transformer rises, the pressure relief valve acts and contacts are connected for alarm or tripping.

In addition, the transformer also has gas protection, thermometer, oil meter and other safety protection devices.

10. What is the difference between voltage transformer and current transformer in their working principles?

Answer: Voltage transformer is mainly used to measure voltage, while current transformer is used to measure current.

(1) The secondary side of current transformer can be short circuited, but cannot be open circuited; The secondary side of voltage transformer can be open circuit, but not short circuit.

(2) Compared with the load at the secondary side, the primary internal impedance of the voltage transformer is so small that it can be ignored, and the voltage transformer can be considered as a voltage source; The primary internal resistance of current transformer is so large that it is considered as a current source with infinite internal resistance.

(3) When the voltage transformer works normally, the magnetic flux density is close to the saturation value, and the voltage drops when the system fails; The magnetic flux density decreases, and the magnetic flux density is very low when the current transformer works normally. When the system is short circuited, the current at the primary side increases, which greatly increases the magnetic flux density, and sometimes even exceeds the saturation value, causing an increase in the error of the secondary output current. Therefore, try to select the current transformer that is not easy to saturate.


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