One Introduction

Usually connected to the output end of the generator, a step-down transformer is required due to the high output voltage of the generator and the low rated voltage of the excitation system.

The safe and stable operation of excitation transformers for generators is a prerequisite for the safe and stable operation of self-excited units, a prerequisite for stable and full load power generation of generators, and a key factor for the reliable operation of excitation systems.

The required electrical power for the excitation system is obtained from the output of the generator. The function of the excitation transformer is to reduce the output voltage of the generator (22kV) to the input voltage of the power thyristor (850 V), provide electrical isolation between the generator terminal and the excitation winding, and also serve as the rectification impedance of the power thyristor.

II Forms and characteristics of excitation transformers

Excitation transformers are mainly divided into four types according to insulation methods

(1) Epoxy resin cast dry-type transformer.

(2) Alkali free glass fiber winding immersed dry-type transformer.

(3) MORA type dry-type transformer.

(4) Oil immersed transformer.

Oil immersed transformers are traditional transformers that are gradually being replaced by dry-type transformers.

Dry type transformers have superior characteristics such as fire prevention, explosion prevention, and environmental protection, and have become the mainstream application of excitation transformers.

The world's first epoxy dry-type transformer was manufactured by the West German company AEG in 1964.

Characteristics of epoxy resin cast dry-type transformer:

(1) High insulation strength, epoxy resin for casting has an insulation breakdown field strength of 18-22kV/mm, and has approximately the same lightning impulse strength as oil immersed transformers of the same voltage level.

(2) Strong resistance to short circuits.

(3) Outstanding disaster prevention performance, epoxy resin is flame retardant and can self extinguish, without causing explosions.

(4) Superior environmental performance, epoxy resin is moisture-proof and dustproof, and can operate in harsh environmental conditions.

(5) The maintenance workload is small.

(6) Low operating losses, high operating efficiency, and low noise.

(7) Small size, light weight, easy installation and debugging

The characteristics of MORA dry-type transformer are as follows:

(1) MORA dry-type transformer is a new type of transformer developed by MORA Transformer Factory in Germany in the past decade to adapt to new environmental concepts and the application of new processes and materials.

(2) The high voltage winding of MORA dry-type transformer is layered and flattened on a ceramic insulation bracket with good insulation performance. The high and low voltage windings, as well as the longitudinal and transverse cooling air ducts between the windings, make the transformer have good short-term overload and short-circuit resistance capabilities.

(3) MORA dry-type transformers immerse the windings in composite insulation paint and then dry them under vacuum conditions, with a simple process.

(4) The insulation of transformer windings is made of glass fiber or NOMEX paper, achieving F or H level insulation.

(5) MORA type has good flame retardant properties.

(6) MORA type can be disassembled after failure. The winding material can be recycled and reused.

(7) MORA type does not require pouring equipment and molds, which can greatly save initial investment and provide greater flexibility in product design.

(8) MORA type has a slightly larger workload for operation and maintenance, and repairs are relatively easy.

At present, epoxy resin cast transformers are mostly used in Europe and Asia, while MORA type transformers are more commonly used in the United States.

The dry benchmark impact level of epoxy resin casting can reach 250kV, and the MORA type is 150kV.

Epoxy resin cast dry-type transformers have a larger capacity of up to 20MVA, while MORA type transformers can only reach 8-10 MVA. [1]

III General requirements for excitation transformers

The excitation power rectifier of a generator using self parallel excitation method is powered by an excitation transformer. The high voltage side of the excitation transformer is usually connected to the generator terminal busbar, and the low voltage side is connected to a thyristor three-phase fully controlled bridge rectifier. The load of the excitation power rectifier is a generator with high inductance and ground insulation. The characteristics of the load and wiring of the excitation transformer, as well as the specific requirements of the power grid and power plant for the excitation system of the generator, make the working conditions and technical requirements of the excitation transformer of the self parallel excited hydro generator not completely the same as those of the general application power transformer, mainly including the following aspects.

(1) The excitation transformer winding current is a non sinusoidal current, and the design of the transformer needs to consider the influence of harmonic currents in the winding. Due to the fact that the time constant of the generator rotor is usually in the order of several seconds, the thyristor current of the excitation power rectifier device and the line current on the AC side (i.e. the low-voltage side of the excitation transformer) are considered as rectangular waves, with fundamental and harmonic components. Harmonic currents will increase the copper and iron losses of the transformer and distort the voltage waveform at the generator end. Therefore, when designing and manufacturing excitation transformers, it is necessary to consider the impact of harmonic currents in the transformer windings, including the magnetic density, capacity, overload capacity, etc. of the transformer core, which all need to be taken into account. Harmonic currents may cause harmonic noise during transformer operation, so measures to reduce harmonic noise need to be considered in the structure and mechanical strength of the iron core and winding.

(2) As an excitation transformer connected to the generator end, it needs to be designed according to the technical requirements of the electrical equipment at the generator end. According to the requirements of GB 1094.1 "Power Transformers Part 1 General Principles", when the generator is loaded, the terminals connected to the transformer and the generator should be able to withstand 1.4 times the rated voltage for 5 seconds. It is usually required to operate for 60 seconds under an overvoltage of 1.3 times the rated voltage at the generator terminal. The excitation transformer should be able to operate continuously for a long time at 110% of the rated voltage.

(3) The rated voltage of the low-voltage winding of the excitation transformer should be designed and selected according to the excitation peak voltage requirements when the generator is strongly excited. When the generator is strongly excited, there is a high requirement for the output voltage of the excitation power rectifier, which is the excitation peak voltage of the generator. The excitation peak voltage is selected according to the requirements of the power system where the generator is located.

(4) The transformer capacity should be able to meet the excitation capability required for long-term continuous operation of the generator. When the excitation current and voltage of the generator are 1.1 times the rated load excitation current and voltage, it can operate continuously for a long time.

(5) The overload capacity of the excitation transformer should be able to meet the requirements of the excitation capacity and duration of the generator's strong excitation. When the excitation transformer is strongly excited by the generator, the generator operates at the excitation peak voltage, and the steady-state value of the excitation current is also the excitation peak current. At this time, the excitation power has high requirements for the load capacity of the excitation transformer.

(6) Static isolation shielding and grounding should be installed between the high and low voltage windings of the excitation transformer. When the transformer is put into operation and transient overvoltage occurs on the high-voltage side, overvoltage will be generated on the low-voltage winding of the excitation transformer through the distributed capacitance between the high-voltage and low-voltage windings of the excitation transformer. To reduce the overvoltage on the low-voltage side of the excitation transformer at this time, electrostatic shielding should be installed between the high-voltage and low-voltage windings of the excitation transformer and grounded together with the transformer core to avoid overvoltage threats to the safety of the excitation power rectifier. Static shielding can also reduce the impact of high-order harmonics and overvoltage on the high-voltage winding and power grid of the transformer's low-voltage winding, and improve excitation

4 Electromagnetic compatibility of magnetic transformers.

In addition, as an application category of power transformers, excitation transformers still need to meet the technical requirements of general power transformers. It mainly includes the following aspects:

(1) Operating temperature rise and insulation heat resistance level.

(2) Ability to withstand short circuits.

(3) Insulation level.

(4) Requirements for auxiliary equipment, including current transformers, temperature monitoring devices, etc.

(5) Other factors include noise level, partial discharge level, and three-phase symmetry.

Five There are still some engineering related technical requirements for excitation transformers in practical engineering applications, such as:

(1) Type and structure of excitation transformer.

(2) Assembly method and protection level.

(3) The installation method and requirements at the power plant site, including the connection with the generator busbar.

For the convenience of transportation or suitable connection with the isolated phase enclosed busbar of the generator, large generator excitation transformers usually adopt a structure of single-phase transformers forming a three-phase transformer group, and require single-phase transformers to have the same structure and good interchangeability.

VI Structure and Design of Excitation Transformer

The following is an example of epoxy resin cast dry-type transformer.

iron core

The iron core is the magnetic circuit of a transformer, consisting of silicon steel sheets and clamping devices. The calcium core material is made of high-quality cold-rolled grain oriented silicon steel sheets, with a 45 ° fully inclined joint structure. The core rod is wrapped with insulation tape and sealed with special resin on the surface. The iron core must be grounded at a point, otherwise it will form a circulating current and increase losses. The no-load loss of transformers is mainly due to the loss of the iron core.

The main measures to reduce the no-load loss of transformers are:

① Reduce the magnetic flux density of transformer iron cores;

② Select high-quality iron core silicon steel sheet materials;

③ Reduce the thickness of the iron core sheet

④ Adopting a fully inclined seam structure.

winding

The winding is an important component of dry-type transformers, mainly composed of wires (zinc wires) and insulation structures (resin).

The structure of the winding determines the rated capacity, rated voltage, and operating conditions.

The load loss of a transformer is composed of resistance loss and additional losses in the winding wires. The winding calculation should meet the following requirements:

(1) Electrical strength. The winding insulation must meet the power frequency and lightning impulse test voltage requirements specified by mainland standards or user requirements, and leave a certain margin.

(2) Heat resistance strength. Under load operation, the temperature rise of the winding is not allowed to exceed the temperature rise limit specified by the insulation material's heat resistance level.

(3) Mechanical strength. The electric force generated by the windings of dry-type transformers under the action of short-circuit current will cause displacement of the windings and changes in short-circuit impedance, both of which should meet the requirements of mainland standards.

For casting dry-type transformers. The high-voltage winding is poured with resin in the mold, and the low-voltage winding end is encapsulated with resin.

The winding materials are mainly copper and aluminum. According to the physical properties of the resin system and conductive material itself, the thermal expansion coefficient of the resin system filled with glass fiber wire is similar to that of copper. Therefore, dry-type transformers filled with glass fiber wire often use copper conductors. The thermal expansion coefficient of the resin system filled with silicon micro powder is similar to that of aluminum, so aluminum conductors are commonly used in dry-type transformers filled with silicon micro powder. Aluminum winding dry-type transformers have shortcomings such as poor mechanical strength and high requirements for welding quality.

There are two main types of conductors used for transformer windings: linear and foil shaped.

The winding types mainly include layered winding and foil winding.

The high-voltage winding technology is mature, the insulation quality is reliable, the degree of automation is high, and the utilization rate is over 70%.

Low voltage foil winding has high efficiency, saves materials, has less magnetic leakage, strong short-circuit resistance, and a usage rate of over 90%.

7 Selection of excitation transformer

In terms of design and structure, the excitation transformer, like ordinary distribution transformers, has a short-circuit voltage of 4% to 8%. Considering that the excitation transformer must be reliable, it must have a certain overload capacity during strong excitation. And the excitation power supply is generally not designed as a backup power supply, so it is advisable to choose a dry-type transformer with simple maintenance and strong overload capacity. If we consider reducing the cost of the excitation system, using oil immersed transformers is also feasible.

When the excitation transformer is installed outdoors, the feeder between the transformer side and the rectifier bridge should not be too long due to the reactance voltage drop, especially in the case of high excitation current, which must be considered. It is not advisable to use single core armored cables. Instead, rubber cables should be selected. When a single core armored cable is supplied with alternating current, it will induce high voltage and significant current in the steel armor, causing interference to the communication cable.

① Performance and wiring of excitation transformer. The performance and wiring requirements of the excitation transformer should be clearly defined, such as type, rated capacity (meeting the requirements of the excitation system), temperature rise, insulation withstand voltage requirements, three-phase wiring group of the transformer, insulation level, noise level, and partial discharge level.

② Technical requirements. Clarify the detailed technical requirements for excitation transformers, and in the selection process, some hydropower plants require the use of excitation transformers from well-known manufacturers in mainland China.

③ For units that use electrical braking shutdown, it is necessary to clarify whether the excitation transformer also serves as the braking transformer.