Basic Principles of Cascade Transformers
If the voltage of a single transformer exceeds 500kV, it will cause a rapid increase in voltage costs, significantly impact the insulation performance of mechanical structures, and also create difficulties in installation and transportation. Based on this situation, the rated power of single transformers currently used in our country generally does not exceed 750kV. Therefore, in specific applications, when voltage requirements above 500kV or 750kV are needed, transformer cascade connection is used to achieve this. Typically, connecting several test transformers in series involves winding operations on multiple transformers to achieve voltage superposition, and then optimally simplifying the insulation structure of each individual transformer. The autotransformer type cascade transformer is the most common cascade transformer.
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 during flashover, 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 to reduce their short-circuit reactance, the low-voltage winding can be evenly wound around the left and right core legs, and then the two low-voltage windings can be connected in parallel to effectively strengthen the closure between the high and low voltage windings. Cascade operation of double-high-voltage-tube transformers 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 leg. If reasonable and effective methods and measures are not taken, the short-circuit reactance of the transformer will increase. To address this situation, balancing windings can typically be installed on the two core legs.
1. Number of Turns
Assume the number of turns of the balancing windings on the left and right legs are NP0 and NP1 respectively, and both have the same number of turns, and in specific operations, they are the same as the number of turns of the primary low-voltage winding during the winding process. Assume this same number is N0. Based on this, we can obtain NP0=NP1=N0.
2. Connection
The balancing windings on the left and right legs can be connected through polarity terminals. If there is a problem with different winding directions in the balancing windings of the two legs, corrections should be made to achieve proper head-to-head and tail-to-tail connections; however, if the winding directions are consistent, head-to-tail connections can be achieved. This ensures 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 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 balancing windings on the left and right sides is not equal, the electromotive forces induced in them will be unequal in magnitude. Under such basic conditions, current will flow.
Short-Circuit Reactance of Autotransformer Type 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 capacitive current flows through the test voltage regulator and the transformer's short-circuit reactance, the output voltage may 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 excessive voltage in a single transformer, 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 become overly heavy overall, making installation and transportation extremely convenient.
（2）Wiring changes can be made to facilitate three-phase testing. Through the implementation of reconnection, transformers can be connected in parallel with each other, thereby supplying large load currents. When implementing the reconnection for three-phase test wiring, the corresponding voltage can be reduced.
（3）If the voltage of the test transformer is required to be at 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 the structural design, each transformer can also be used independently for its specific purposes, which increases the number of work locations and scope. Moreover, 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）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 results in low overall utilization of the device. Moreover, during insulation operation and treatment, the excitation devices provided by the transformer to each level 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 may 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 control box (console). For the usage of the control box (console), please refer to the instruction manual of the control box (console) in detail.
2) The transformer casing and 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.
