AC / DC Type Three-phase Power Transformer in Distribution Network

ac power transformer1

AC / DC type three-phase power electronic transformer applied in power system can not only realize energy transmission smoothly between various differentiated voltage levels, effectively isolate the differentiated voltage level system, maintain good control performance of the transformer, but also combine with the system demand, the actual output range of the transformer can be changed appropriately. The application of this equipment in the distribution network can significantly improve the operational efficiency and stability of the distribution network, which plays an important role in promoting the healthy development of the power system.

Based on this, this paper analyzes in detail the specific application of AC/DC three-phase power electronic transformers in the distribution network.

The AC / DC type three-phase power electronic transformer is a new type of transformer based on power electronics technology, which can effectively transmit electrical energy in the power system, and this equipment can effectively improve its control performance through the use of modern information technology and electronic components. Electricity service satisfaction.

In order to give full play to the important role of AC/DC three-phase power electronic transformers, this paper explores the application of these transformers in distribution networks in detail from their structure, mathematical model and simulation analysis.

Table of Contents

Structure of AC-DC three-phase power electronic transformer

The AC / DC type three-phase power electronic transformer combines a typical inverter circuit and rectifier circuit, while incorporating a high-frequency transformer to achieve the goal of transforming voltage levels and transferring energy based on the collaboration of all parts.

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This type of AC / DC type three-phase power electronic transformer structure has outstanding features, when the grid frequency voltage input to this power electronic transformer, after the rectification link to properly convert the frequency input to DC, and then again to convert the DC to high-frequency AC square wave pulse, the use of high-frequency transformer, the primary side of the corresponding square wave pulse for effective conversion, to obtain other amplitude types of high-frequency square wave The transformer is used to effectively convert the square wave pulses on the primary side to obtain other amplitude types of high frequency square wave pulses, and finally undergoes a rectification and an inversion to make the transformer output the required voltage level of the system corresponding to the frequency AC output.

This power electronic transformer has a high frequency transformer arranged in the middle to ensure the smooth passage of very high frequency types of AC voltage, but the overall size is much smaller compared to conventional transformers. In general the design work for transformers is carried out to determine the volume based on systematic consideration of core material selection, transformer winding temperature rise, core flux density and other factors.

Since the core material corresponding to the saturation flux density will gradually decrease with the increasing transformer operating frequency, so in order to obtain a smaller transformer volume, it is necessary to reasonably increase the operating frequency.

The switching frequency of power electronic transformers is usually kept in the range of tens of thousands to hundreds of kilohertz, which is a great improvement compared with the frequency of the power grid, so the volume of the equipment is effectively reduced compared with traditional transformers.

If the load on the output side changes or the voltage fluctuates on the primary bus, this type of transformer can be adjusted to ensure good stability of the output voltage by turning off and on the power electronics.

This type of AC / DC type three-phase power electronic transformer to achieve three-phase power input, to obtain twelve pulse wave DC voltage, it is necessary to achieve by three-phase full-bridge rectification circuit, and in the process of energy transfer, the use of high-voltage capacitors to achieve voltage stabilization purposes.

The primary side of the equipment corresponds to the post-stage structure, a single-phase full-bridge inverter, in order to convert the twelve-pulse DC voltage to high-frequency AC level.

The AC/DC three-phase power electronic transformer topology corresponds to a high frequency transformer that couples the primary side AC level to other voltage levels corresponding to the high frequency AC level, so that the voltage level can be changed smoothly.

According to the same principle, the secondary side of the high-frequency transformer can use the voltage regulator capacitor and the full-bridge rectifier module to obtain a stable DC voltage different from the primary side, and then realize the three-phase AC output through the three-phase inverter circuit, and then obtain the standardized sinusoidal output through the filter, so that the load in the grid has a different voltage support level.

This type of high-frequency voltage transformer is named as AC/DC/AC transformer because of the AC/DC/AC transition in the topology, both on the primary and secondary sides.

Although this type of transformer has more components in the structure and more complex circuit structure compared with traditional transformers, it is smaller in size and can effectively control the output voltage to ensure reliable and stable output, which can be applied to the distribution network to improve power quality based on its outstanding advantages.

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Mathematical model of AC-DC three-phase power electronic transformer

The most critical component of the AC / DC type three-phase power electronic transformer is the voltage source converter, which is mainly composed of three bridge arms, each of which has two fully controlled devices with relatively large power.

In the AC / DC type three-phase power electronic transformer, the switching tubes belong to the control switch, which controls the output voltage based on the turn-on time and turn-off time.

A-phase bridge arm in its equivalent circuit, the switch tube as an ideal switch, maintaining a series connection with the resistor, on the basis of which the switch Sa1 and switch Sa2 on the switch tube with the control role, the switch tube in the actual opening and closing operation of the loss occurred by the resistor Rsm to replace.

In order to clarify the specific state of switch Sa1 and switch Sa2, 1 is used to represent the switch state when the switch is open and 0 is used to represent the switch state when the switch is open. When the circuit remains in normal operation, the upper and lower bridge arms are in complementary conduction states.

Because the three-phase circuit has a symmetrical relationship, so through the calculation and analysis of one phase of the circuit, the other two corresponding dynamic equations can be obtained in the same way, and the relevant equations can be integrated to obtain the final mathematical model formula representing the three-phase circuit and the mathematical formula of the voltage regulator-capacitor relationship.

This type of power electronic transformer in the actual control operation stage, usually to control the output stage and input stage separately, and the most widely used method is through the current loop and voltage loop to achieve double closed-loop control, in this strategy to ensure that the output voltage has good stability performance.

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Simulation analysis

To verify the output characteristics and operating conditions of such ac power transformers, scientific simulation analysis is focused on the three-phase PET. First, a simulation model is built and constructed for a 10 kV/380 V step-down type power electronic transformer. Afterwards, the parameters involved in the constructed model are scientifically designed.

Among them, the bus phase voltage is designed to be 10kV rms, the voltage frequency is designed to be 40Hz, the high frequency transformer is designed to contain a capacity of 5000kVA, the operating frequency is designed to be 10kHz, the corresponding positive phase conduction voltage drop of the IGBT is designed to be 1.5V, and the output side inductance is designed to be 2.5mH.

In the specific simulation experiments, the specific output state of the power electronic transformer is observed to change under various load conditions by making differential changes to the load.

By comparing the specific output voltage waveforms of the ac power transformer under no-load and full-load operation, it is found that the transformer can ensure the output voltage is stable at the rated part to achieve effective output and obtain a standard sine wave regardless of no-load or full-load operation. Comparing the output voltage and current waveforms under inductive load condition, it is found that the transformer can maintain the same phase between current and output voltage.

This state can lead to an effective reduction of reactive power losses in the distribution line and an overall increase in energy efficiency.

After the observation of the steady-state output of the power electronic transformer, the transient change process should be reasonably analyzed.

Specific simulation operation, from pure resistive load to power factor 0.7 liabilities for transformation, according to the corresponding waveform can be found, electronic transformer corresponding output waveform in the moment of load change, began to occur jitter situation, the specific waveform and the standard sine wave deviation, but after holding 2 ~ 3ms on the gradual return to normal.

Therefore, based on appropriate closed-loop feedback and control strategies, such transformers can eliminate the effects of load fluctuations or voltage changes in the distribution network and ensure that the output value remains highly stable.

Experimental results of AC/DC power electronic transformers

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Through the use of modern components and new power electronics technology, AC/DC power electronic transformers can be used in distribution networks not only to achieve energy transfer and voltage conversion, but also to dynamically monitor the output and effectively control the switching components, so that the voltages can maintain a stable output state in actual operation and ensure the standard sinusoidal output waveform.

At the same time, it can also achieve a certain degree of compensation for the output power, promote a significant increase in power factor, so that the amount of reactive power loss generated in the system is greatly reduced.

The reasonable application of power electronic transformers in the distribution network can effectively reduce the space cost and economic cost, and achieve comprehensive optimization of the comprehensive power quality of the distribution network.

Therefore, in the future development of the distribution network, we should strengthen the application of ac power transformer, and strengthen the research on this equipment, so that it can be applied in more occasions, and cooperate with the introduction of intelligent customer service, more fully play the advantages and role of power electronic transformer itself, and promote the healthy and sustainable development of the distribution network.

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The reasonable application of AC / DC type three-phase power electronic transformer in the distribution network can not only play the normal function of the transformer, but also make up for the defects of the traditional transformer, ensure the high stability of the output state, reduce the reactive power loss in the distribution network, improve the comprehensive economic efficiency of the power enterprise, and promote the long-term healthy development of the power grid business.

In order to give full play to the value of such transformers, the relevant application areas need to strengthen research to make it more effective and widely used in the field, and to promote the further development of the relevant application areas on the basis of a more comprehensive play of the transformer function.

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