Measures to Reduce Short-Circuit Reactance of Test Transformers
In specific tests, to ensure that the test object does not experience insufficient short-circuit current under flashover conditions, it is necessary to effectively reduce the short-circuit reactance of the test transformer to a certain extent. For bushing test transformers with a high ratio, the voltage level is relatively low, so when reducing their short-circuit reactance, the low-voltage windings can be evenly wound around the left and right core limbs, and then the two low-voltage windings can be connected in parallel to effectively strengthen the closure between the high and low voltage windings. For transformers with double high-voltage bushings, cascade operation is mainly to effectively save insulation, so that the secondary low-voltage winding and the secondary voltage winding are not mounted on the same core limb. If reasonable and effective methods and measures are not taken, the short-circuit reactance of the transformer will increase. In response to this situation, it is usually possible to perform balanced winding mounting on two core limbs.
1. Number of turns
Assume that the number of groups of balanced windings on the left and right limbs is NP0 and NP1 respectively, and the number of turns of both is in the same state, and in specific operations, the number of turns generated during the winding process of the primary low-voltage side is the same, assuming this same number is N0. Based on this, we can obtain NP0=NP1=N0.
2. Connection
The balanced windings on the left and right limbs can be connected through polarity terminals. If there is a problem of different winding directions in the balanced windings of the two limbs, corrections should be made to achieve normal head-to-head and tail-to-tail connection; but if the winding directions are in a consistent state, then head-to-tail connection can be achieved. To ensure overall standardization and reasonable effectiveness.
3. Current
Due to some operational reasons, a certain current may pass through the balanced windings. This is mainly because in the entire core circuit, the magnetomotive forces of these two windings are equal in magnitude but opposite in direction, which makes it difficult to generate magnetic flux in the entire core circuit.
4. Magnetic flux
When the magnetic flux linked by the balanced windings on the left and right sides is in an unequal state, the electromotive forces induced by them will be unequal in magnitude. Under such basic conditions, current will flow through.
Short-Circuit Reactance of Autotransformer Cascade Transformers
If there is a phenomenon of large short-circuit reactance in the test transformer, it will cause a serious reduction in the short-circuit capacity of the test equipment, thereby affecting the test results of pollution flashover or wet flashover voltage of insulators. In addition, if the test transformer frequently contacts capacitive loads, when the capacitive current flows through the test voltage regulator and the short-circuit reactance of the transformer, the output voltage will exceed the rated values on both sides of the transformer. Therefore, the short-circuit reactance value of the test transformer should not be too large.
Advantages and Disadvantages of Cascade Test Transformers
1. Advantages of cascade test transformers
(1) There will be no phenomenon of a single transformer having too high voltage, and the production of insulation structure is relatively simple and convenient, with extremely cheap material costs. In addition, since the single-piece structure is in a relatively light overall state, it will not cause the test transformer to be too heavy overall, making installation and transportation extremely convenient.
(2) Wiring changes can be made to facilitate three-phase testing. Through the implementation of rewiring, transformers can be connected in parallel with each other, thereby supplying large load currents. When implementing the rewiring for three-phase test connections, the corresponding voltage can be reduced.
(3) If the voltage of the test transformer needs to be in a lower state, only one or two of the transformers can be selected for use, which can effectively reduce the voltage without causing the problem of insufficient excitation of the power generator, and the operation is relatively simple and convenient. Moreover, based on the reduction in the number of cascade transformers, the short-circuit reactance of the loop during the total test process can be effectively reduced.
(4) Due to structural issues, each transformer can also be used independently for its specific purposes, which will increase the number of work locations and scope, and if one of them fails, it will not affect the normal operation of the overall system, effectively achieving loss reduction.
2. Disadvantages of cascade test transformers
(1) Autotransformers are the most common device in cascade test transformers, so during their operation, the power of the upper-level transformer needs to be supplied by the lower level, which results in low overall utilization of the device. Moreover, during insulation operation and treatment, the devices provided by the transformer for excitation at each level will also have low overall utilization due to insulation effects.
(2) The leakage reactance in the low-voltage winding and excitation winding is mostly caused by phenomena in the insulating transformer. If the number of stages increases, the total reactance will continuously increase or even intensify, so generally the number of cascade stages should not exceed four, but in specific applications, this is not effectively achieved.
(3) In cascade high-voltage test transformers, if overvoltage occurs, the transient voltage between stages will be unevenly distributed, and in certain cases, insulation faults in the excitation winding or bushing flashover may even occur. [1
