36000kVA Hvdc High Voltage Direct Current Converter Transformers

ZSFPZ-36MVA/35KV converter transfomer for +- 50kv DC transmission
project

Smoothing reactor for +-50KV DC transmission project
Converter transformer refers to the power transformer connected between the converter bridge and the AC system. The converter transformer is used to realize the connection between the converter bridge and AC bus, and provide a three-phase commutation voltage with ungrounded neutral for the converter bridge. Converter transformer and converter bridge are the main parts of converter unit.

The functions of converter transformer in DC transmission system include:
1. Transmission of electricity;
2. Converting the AC system voltage to the commutation voltage required by the converter;
3. Using different connection methods of transformer windings, two groups of three-phase symmetrical commutation voltages with equal amplitude and 30 ° phase difference (fundamental wave electrical angle) are provided for the two converters connected in series to realize twelve pulse commutation;
4. Isolate the DC part from the AC system to avoid direct short circuit caused by the neutral point grounding of AC system and the neutral point grounding of DC part, so that the commutation cannot be carried out;
5. The leakage reactance of converter transformer can limit the fault current;
6. It can buffer and suppress the lightning impulse overvoltage wave intruded into the converter station along the AC line.
Because the operation of converter transformer is closely related to the nonlinearity caused by converter commutation, it has different characteristics and requirements from ordinary power transformer in leakage reactance, insulation, harmonic, DC bias, on load voltage regulation and test.
Leakage reactance
In the past, due to the limited rated current and overload capacity of thyristor, in order to limit the fault current of valve arm short circuit and DC bus short circuit, the leakage reactance of converter transformer is generally larger than that of ordinary power transformer, generally 15-20%, and even more than 20% in some projects. With the improvement of the rated current of the thyristor and its ability to withstand surge current, the leakage reactance of the converter transformer can be reasonably selected according to the corresponding capacity and insulation level, and the impedance can be reduced accordingly, usually 12-18%. Therefore, the main parameters of the equipment, insulation level, reactive power consumption and energy consumption of the converter can be reduced accordingly. At the same time, the operation performance of the converter is also improved.
In order to reduce non characteristic harmonics, the three-phase leakage reactance balance of converter transformer is required to be higher than that of ordinary power transformer, and the leakage reactance tolerance is usually no more than 2%. If the transportation conditions permit, the single-phase three winding converter transformer structure is mostly adopted in the project to further reduce the difference of six impedance values of converter transformer in twelve pulse converter unit.
insulation
The valve side winding and bushing of converter transformer work under the joint action of AC and DC voltage. Under the action of this voltage, because the ratio of conductivity coefficient to dielectric coefficient of oil and paper insulation materials is very different, the DC field strength in oil paper composite insulation is distributed according to conductivity coefficient, and the AC field strength is distributed according to dielectric coefficient. When the polarity of DC voltage changes rapidly, the oil gap insulation will be subjected to great electrical stress. In the connection part between casing and base, this problem is the most serious due to the complex insulation structure.
The closer the valve side winding to the DC poles, the higher the voltage to the ground. In the design, the distance between the winding end and the iron core yoke must be increased, so that the radial magnetic leakage and local loss at the winding end increase, and the loss caused by harmonic magnetic leakage increases more.
As the bushing with external insulation of valve side winding, its creepage distance shall take into account the component of DC voltage. In order to avoid flashover fault caused by uneven wet flashover under the action of DC voltage in rainy days, generally, the bushing at valve side extends into the valve hall. Dry synthetic casing has been applied in practice. In order to resist earthquake, a vibration damping device is generally installed at the casing flange.
harmonic
The harmonic component of magnetic flux leakage of converter transformer will increase the stray loss of transformer, and sometimes cause local overheating of some metal parts and oil tank. Non magnetic materials or magnetic shielding measures shall be used for parts with strong magnetic leakage. The magnetostrictive noise caused by harmonic magnetic flux is in the frequency band with more sensitive hearing, and more effective sound insulation measures should be taken when necessary.
DC bias
The interval of converter triggering time is different, the positive sequence second harmonic voltage of AC bus and the induction of AC line parallel to DC line will produce DC component in the winding current at the valve side of converter transformer; The change of ground potential caused by grounding electrode grounding current will produce DC component in the winding current on the AC side, which together cause DC magnetic bias in the converter transformer. Make the detour track of the operating point on the B-H curve of the iron core deviate from the symmetrical state, and some enter the saturation section on one side, and the excitation current component has a half wave peak waveform, which significantly increases the loss, temperature rise and 50Hz noise of the transformer (the fundamental noise frequency is 100Hz under normal conditions), which should be fully considered in the design of the converter transformer.
On load voltage regulation
The converter transformer shall have more on load voltage regulating switches. The use of voltage regulating switches can make the DC transmission system operate close to the best state and the trigger angle of the converter operate within an appropriate range, so as to take into account the safety and economy of operation. The voltage regulation range of tap changer is generally 20-30%, and the regulation amount of each gear is 1% - 2%, so as to achieve the joint work of tap changer regulation and converter bridge trigger control, so that there is no obvious regulation dead zone and frequent round-trip action can be avoided.



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