Basic Principles of Cascade Transformers
If a single transformer's voltage exceeds 500kV, it will cause a rapid increase in voltage costs and additional expenses, and it will greatly affect the insulation performance of the mechanical structure. In addition, it will also cause difficulties in installation and transportation. Based on this situation, the rated power of a single transformer currently used in China generally does not exceed 750kV. Therefore, in specific applications, when voltage requirements above 500kV or 750kV are needed, it is achieved through transformer cascade connection. Usually, several testers are connected in series by performing winding operations on several transformers to achieve voltage superposition, and then the insulation structure of a single transformer is optimally simplified as a whole. The autotransformer cascade transformer is the most common type of cascade transformer.
Measures to Reduce Short-Circuit Reactance of Test Transformers
In specific tests, to ensure that the test object does not have the problem of too small short-circuit current under flashover conditions, it is necessary to effectively reduce the short-circuit reactance of the test transformer to a certain extent. The voltage level of a single-high-ratio bushing test transformer is relatively low, so when reducing its short-circuit reactance, the low-voltage winding can be evenly wound on the left and right side rods of the iron core, and then the two low-voltage windings are connected in parallel to effectively strengthen the closure between the high and low voltage windings. The cascade operation of a double-high-voltage-tube transformer 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 valve, and if some reasonable and effective methods and measures are not taken, it will cause the short-circuit reactance of the transformer to increase. In response to this situation, balancing winding mounting operations can usually be performed on two iron core columns.
1. Number of turns
Assume that the number of groups of balancing windings on the left and right columns is NP0 and NP1 respectively, and the number of turns of the two 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 amount is N0. Based on this, we can obtain NP0=NP1=N0.
2. Connection
The balancing windings of the left and right columns can be connected through polarity terminals. If there is a problem that the winding directions of the two columns in the balancing windings are different, corrections should be made to achieve normal connection of head-to-head and tail-to-tail; 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
It may be due to some reason during operation that a certain current passes through the balancing windings. This is mainly because in the entire iron core circuit, the two windings have equal magnitudes but opposite directions in terms of magnetomotive force, 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 have unequal magnitudes. 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, 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, it will cause the output voltage to 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 excessively high voltage of a single transformer, and the production of the insulation structure is relatively simple and convenient, and the material cost is extremely cheap. In addition, since the single component is in a relatively light overall state, it will not cause the test transformer to be too heavy overall, and installation and transportation are extremely convenient.
(2) Wiring changes can be made to facilitate three-phase tests. Through the realization 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 is required to be in a low 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. And on the basis of reducing the number of cascade transformer units, the short-circuit reactance of the loop during the total test process can be effectively reduced.
(4) Due to the structural method, each transformer can also be used independently for its specific use, which will cause an increase in work locations and scope, and if one of them fails, it will not affect the normal operation of the whole, and can effectively achieve loss reduction.
2. Disadvantages of cascade test transformers
(1) The autotransformer is the most common device in cascade test transformers, so in its operating state, the power of the upper-level transformer needs to be supplied by the lower level, which will cause low overall utilization of the device. And during insulation operation and treatment, the excitation devices provided by the transformer to each level will also have low overall utilization due to insulation.
(2) The leakage reactance appearing in the low-voltage winding and the excitation winding is mostly caused by the phenomena in the insulating transformer. If the number of levels increases, the total reactance will continuously increase or even intensify, so generally the number of cascade levels should not exceed four, but in specific applications, it has not been effectively achieved.
(3) In a cascade high-voltage test transformer, if overvoltage occurs, the transient voltage between the total levels will be unevenly distributed, and under certain circumstances, insulation faults in the excitation winding or bushing flashover may even occur.
Usage Precautions
1) The test transformer should be used in conjunction with the operating box (console). For the usage of the operating box (console), please refer to the operating box (console) instruction manual in detail.
2) The transformer shell and the high-voltage tail must be grounded. To ensure safety, sufficient distance must be maintained between test personnel and other tested equipment and the test transformer.
3) The output of the test transformer should generally be connected in series with a current-limiting resistor to protect equipment safety.
