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 single-high-ratio bushing test transformers, the voltage level is relatively low, so when reducing their short-circuit reactance, the low-voltage windings can be evenly wound on the left and right side limbs of the iron core, 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 double-high-voltage tube transformers, 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 iron core limb, and if reasonable and effective methods and measures are not taken, it will cause an increase in the short-circuit reactance of the transformer. In response to this state, balancing winding mounting operations can usually be performed on two iron core limbs.
1. Number of turns
Assume that the number of turns of the balancing windings on the left and right limbs are 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 that the same number is N0. Based on this, we can obtain NP0=NP1=N0.
2. Connection
The balancing windings on the left and right limbs can be connected through polarity terminals. If there is a problem of different winding directions in the balancing 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, head-to-tail connection can be achieved. To ensure overall standardization and reasonable effectiveness.
3. Current
Due to certain operational reasons, a certain current may pass through the balancing windings. This is mainly because in the entire iron core circuit, the two windings have magnetomotive forces that are equal in magnitude but opposite in direction, which makes it difficult to generate magnetic flux in the entire iron core circuit.
4. Magnetic flux
When the magnetic flux received by the balancing windings on the left and right sides through interlinking is in an unequal state, the electromotive forces induced by the two will be unequal in magnitude. Under such basic conditions, current will flow through.
Short-Circuit Reactance of Autotransformer Cascade Test 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, which in turn will affect the test results of the 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 structures 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 rewiring for three-phase test connections is achieved, 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 convenient and simple. 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 in their operating state, the power of the upper-level transformer needs to be supplied by the lower level, which causes the overall utilization rate of the device to be low. And during insulation operation and treatment, the devices provided by the transformer for excitation at each level will also have a low overall utilization rate due to the effect of insulation.
(2) The leakage reactance in the low-voltage winding and the excitation winding is mostly caused by phenomena occurring 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 has not been effectively achieved.
(3) In cascade high-voltage test transformers, if overvoltage phenomena occur, 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
