The function of the converter transformer is to provide rectified power for the converter valve. The converter transformer and the converter valve together form a converter to realize the mutual conversion of AC and DC, and realize the mutual electrical isolation of AC and DC. TBEA has significant advantages in the converter market.
1. Advantages of converter transformers
TBEA has significant advantages in the converter market. Send the electric power of the AC power system at the sending end to the rectifier or receive power from the inverter to the AC power system at the receiving end. It uses the magnetic coupling of the windings on both sides to transmit power, realizes the electrical insulation and isolation of the AC system and the DC system, and avoids the short circuit of some components caused by the grounding of the neutral point of the AC power network and the grounding of the DC part. On the other hand, the transformation of voltage is realized, so that the AC bus voltage on the side of the converter transformer network and the voltage on the DC side of the converter bridge can meet the rated voltage and allowable voltage offset on both sides respectively. In addition, it also suppresses the overvoltage that invades the converter from the AC grid.
2. Characteristics of converter transformers
The converter transformer is the main equipment of the DC transmission system, and its main parameters are determined according to the special requirements of the DC system. The function of the converter transformer is to supply AC power to the converter or receive AC power from the converter, and convert the grid-side AC voltage into the voltage required by the valve side.
In the rectifier station, a converter transformer is used to isolate the AC system from the DC system, and the electric energy of the AC network is converted into high-voltage DC power through the converter device, which is transmitted through the high-voltage DC transmission line; at the inverter station, the DC power is converted into It is converted into AC power, and then sent to the AC grid through the converter transformer; thus realizing the connection between the AC transmission network and the high-voltage DC transmission network. The converter transformer provides a 12-pulse AC voltage with a phase difference of 30° to reduce the harmonic current on the AC side, especially the 5th and 7th harmonic current; as an electrical isolation between the AC system and the DC system, it weakens the intrusion into the DC system. Overvoltage on the AC side; limit the short-circuit current of the DC system from entering the AC system through the impedance of the converter transformer; through the converter transformer, a relatively large step-by-step adjustment of the DC voltage can be realized.
3. Differences between converter transformers
Characteristics Compared with Ordinary Power Transformers
3.1 There is a DC bias problem. DC bias not only causes periodic saturation of the iron core and emits low-frequency noise, but also greatly increases the loss and temperature rise of the converter transformer;
3.2 Higher insulation margins are required. During operation, the converter transformer has to bear both the AC stress and the larger DC stress. It is required that the converter transformer insulation, especially the valve side insulation, has sufficient tolerance margin for the working field strength during operation. The insulation problem very prominent. Insulation accidents during operation of converter transformers account for about 50% of all accidents.
3.3 Large-scale on-load voltage regulation capability When the bridge arm of the converter transformer is short-circuited, in order to limit the excessive short-circuit current from damaging the converter valve, the converter transformer should have a large enough short-circuit impedance, that is, a large leakage reactance. At the same time, in order to meet the requirement that the voltage on the valve side changes frequently with load changes, the converter transformer also has a wide range of on-load voltage regulation capabilities, making its on-load tap position far more than that of ordinary power transformers.
3.4 Harmonic problem During the operation of the converter transformer, characteristic harmonic and non-characteristic harmonic currents will flow. These harmonics act on the magnetic flux leakage of the converter transformer to increase the stray loss of the converter transformer, and sometimes cause local overheating of some metal parts and oil tanks. Harmonic flux with a large value will cause hysteresis stretching noise, and it is in the acoustically sensitive frequency band, so effective sound insulation measures must be taken.
3.5 Due to the requirement of step-down operation in the system, the tapping range of the network side is large (about 30%) and the number of stages is large. And the diversity of operation modes increases the complexity of converter rheology design.
3.6 In terms of structure, since the valve side bushing should go deep into the valve hall, in order to prevent the valve body from being damaged in the event of an accident in the converter, the valve side bushing adopts a dry bushing.
3.7 The most common faults in converter transformers are coil insulation damage, oil-paper insulation strength reduction, tap changer, bushing and cooling system (pump) faults, and the fault rate of converter transformers is about twice that of AC transformers. For UHV transformers, attention should be paid to the main DC insulation structure between the valve winding and the ground-AC winding. Although some online detection systems used can avoid some possible failures, but because the design of the system is not mature enough, it cannot detect possible catastrophic damage as early as possible and take corrective measures. For the UHV converter transformer developed for the first time, its design and production reliability must be verified by simulation tests. At the same time, these test results are also the basis for fault diagnosis after equipment operation.
The polarity of the DC voltage may reverse rapidly, and these problems make its internal insulation potential distribution very different from that of ordinary power transformers. In the voltage distribution of different insulating materials, different considerations will be made for AC voltage, transient impulse overvoltage and DC voltage. The distribution of the AC voltage depends on the ratio of the permittivity of the materials, and the distribution of the DC voltage depends on the ratio of the resistivities of the various materials. The internal insulation of converter transformers is mostly made of cellulose solid materials such as converter transformer oil, paper, and pressure plates. The ratio of their dielectric constants does not exceed 1:3. However, the size of material resistivity is affected by many external factors, such as temperature, humidity, field strength, time, aging, etc., and the ratio of resistivity may exceed 1:1000. Therefore, due to the influence of these factors, the potential distribution in the insulation of the converter transformer under the action of a DC electric field is very different from that of the AC. Therefore, UHV converter transformers must be considered in terms of structure design, insulation configuration and testing.