Method for reducing transformer and line losses based on active power electronic devices

Abstract

The present invention discloses an active power electronic device and method for reducing transformer and line losses. The device includes an AC-DC converter and a battery. The input end of the DC converter is connected to the load side of the transformer, and the output end is connected to the storage battery. A DC circuit breaker is arranged between the output end and the storage battery. The AC-DC converter is a T-type three-level topology or a two-level topology composed of power-type power electronic switches. By connecting a storage battery to the output end of the converter, when the load size changes, it can be adjusted through the storage battery. For example, when an intermittent load is added to the system, the storage battery can effectively cope with the intermittent load by discharging, thereby reducing the overload of the transformer and improving the power supply reliability.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and transformation, and particularly relates to an active power electronic device and method for reducing transformer and line losses. Background Art

[0002] At present, China has successively carried out electricity substitution work in many fields, such as industrial and agricultural manufacturing, transportation, heating technology and other fields. At the same time, the state has also introduced relevant policies to support electricity substitution work. However, large-scale electricity substitution has also brought new problems, which are mainly reflected in the following aspects: 1) A large number of intermittent loads (such as loads of induction cookers, rice cookers, electric heaters and electric water heaters, etc.) are connected to the power grid, especially in rural power grids. Since the previous round of rural power grid transformation has been basically completed, the distribution transformers, distribution switches and distribution lines in the substation area are designed according to the original expected power consumption. During the current peak power consumption period, the power supply capacity for users is slightly insufficient, and with the increase of load, the power supply pressure is getting greater and greater. 2) The daily fluctuation of the residential electricity load in the distribution network increases, and the peak-valley difference of the daily electricity consumption is getting larger and larger. Although the average load rate of the transformer has not increased significantly, the daily power loss of the transformer continues to increase. This not only increases the operating cost of the distribution substation area, but also the power supply efficiency will be affected. 3) Devices such as induction cookers and electric heaters are non-linear devices, and they will generate certain reactive current and harmonic current during operation, which will affect the stable operation of the substation area. If not effectively solved, it will not only reduce the power factor of the substation area, but also endanger the operation safety of distribution equipment such as transformers and switches. At the same time, it will increase the distribution loss of the substation area, reduce the power supply efficiency and increase the operation and maintenance cost. 4) Most of the residential electricity-consuming devices are single-phase loads, and it is inevitable to have unbalanced three-phase current during electricity consumption. With the promotion of electricity substitution, a large number of high-power single-phase loads are connected to the power grid, resulting in a continuous increase in the unbalanced degree of the three-phase current. The unbalanced three-phase current has a significant impact on the safe and efficient operation of the substation area. For example, it increases the eddy current loss of the transformer, reduces the load capacity and service life of the transformer; the distribution switch trips due to single-phase overcurrent, reducing the power supply reliability and increasing the maintenance cost; compared with the three-phase balanced load of the same power, the unbalanced three-phase load increases the distribution line loss and reduces the power supply efficiency.

[0003] If the above problems cannot be effectively controlled or improved, the economic benefits of electricity substitution will be difficult to manifest, and the substitution process will be difficult to deepen and continue. The distribution transformer is an important electrical equipment in the distribution network system, and its main function is to transform voltage and transmit power. Generally speaking, electricity needs to be stepped down several times from generation to use. The transformer will inevitably cause active and reactive power losses during operation, and its losses account for about half of the line losses in the power system, especially in rural power grid areas, accounting for about 60%. Therefore, reducing the comprehensive losses of distribution transformers and lines is of great significance for improving the safe and efficient operation of the distribution network.

[0004] At present, the following measures are taken to solve the above problems:

[0005] 1) Optimization of power grid structure

[0006] The power grid structure has a great impact on power line losses. Strengthening the reasonable planning and design of the power grid system is the main way to ensure the reduction of power grid losses. For example, in the rural line layout, it is necessary to conduct in-depth analysis, master the electricity consumption needs of the residents along the line, and strengthen the construction and operation on this basis to shorten the power supply radius as much as possible. However, this method has a long construction period, complex design, and high cost.

[0007] 2) Selection of energy-saving distribution transformers

[0008] Distribution transformers also generate corresponding losses during actual operation. Different transformers have great differences in their losses. Therefore, energy-saving transformers should be used as much as possible to reduce the line loss level of the power system. In addition, the load rate of distribution equipment also affects the loss, and it is necessary to ensure a scientific and reasonable load range. Understand the actual situation of the distribution load, and on the basis of ensuring stable power supply, increase the load rate to improve the power factor and reduce the loss of the distribution transformer. Using this method, when the selected transformer capacity is too large, it can only cope with the peak electricity consumption period (not exceeding 4 hours on average per day), and the utilization rate is low and the working efficiency is low at other times, and the line loss is not reduced; when the selected transformer capacity is medium, when intermittent loads are put in, it is easy to have overload and large loss phenomena.

[0009] 3) Optimization of reactive power compensation

[0010] Voltage constancy is the basis for ensuring the safe operation of the power grid, and usually requires the support of reactive power and active power. If the reactive power in a certain regional power grid is insufficient, reactive power compensation needs to be implemented and corresponding reactive power compensation devices need to be set up, otherwise phenomena such as low power factor, large line losses, and low voltage will occur. Reactive power compensation technology can not only reduce the power grid losses, but also improve the power quality. Reactive power compensation needs to be comprehensively planned and reasonably arranged according to requirements. However, this method cannot solve the problem of large losses of transformers and lines caused by the increase in active power. Summary of the Invention

[0011] In view of the problem that the increase in intermittent and non-linear loads leads to an increase in the losses of transformers and lines in low-voltage distribution substations, the present invention proposes an active power electronic device and method for reducing the losses of transformers and lines.

[0012] To achieve the above object, an active power electronic device for reducing transformer and line losses according to the present invention includes an AC-DC converter and a battery. The input end of the DC converter is connected to the load side of the transformer, and the output end is connected to the storage battery. A DC circuit breaker is provided between the output end and the storage battery. The AC-DC converter is a T-type three-level topology or a two-level topology composed of power-type power electronic switches.

[0013] Further, an overload protector is connected to the input end of the AC-DC converter.

[0014] Further, a soft start circuit is connected to the input end of the AC-DC converter.

[0015] Further, the soft start circuit includes a first AC circuit breaker, a second AC circuit breaker, and a resistor. A branch formed by the series connection of the second AC circuit breaker and the resistor is connected in parallel with the first AC circuit breaker.

[0016] Further, a series reactor is connected to the input end of the AC-DC converter.

[0017] Further, a filter capacitor is connected to the input end of the AC-DC converter.

[0018] A method for reducing transformer and line losses based on the above active power electronic device includes the following steps:

[0019] S1. Set the optimal economic operation range of the transformer;

[0020] S2. Detect the load current, separate the reactive current, harmonic current, and unbalanced current in the load current, and generate a reference command for the output current of the active power electronic device;

[0021] S3. The active power electronic device responds to the current reference command output in S2, outputs a compensation current, compensates the reactive, harmonic, and unbalanced currents of the load, so that the current output by the active power electronic device and the reactive current, harmonic current, and unbalanced current of the load are completely offset;

[0022] S4. Detect the transformer current:

[0023] If it contains reactive, harmonic, and unbalanced currents, separate the reactive current, harmonic current, and unbalanced current in the transformer current, and generate a reference command for the output current of the active power electronic device; the active power electronic device responds to the current reference command output in S2, outputs a compensation current, compensates the reactive, harmonic, and unbalanced currents of the load, so that the current output by the active power electronic device and the reactive current, harmonic current, and unbalanced current of the load are completely offset;

[0024] Otherwise, execute S5;

[0025] S5. Calculate the current load rate of the transformer and determine whether it is within the optimal economic operation range:

[0026] If the judgment is true, the process ends; otherwise, control the battery to discharge or charge to make the transformer enter the optimal economic operation mode, and the process ends.

[0027] Further, in S5, the process of controlling the battery to discharge or charge to make the transformer enter the optimal economic operation mode includes the following steps:

[0028] Judge the relationship between the transformer load rate and the optimal economic operation range of the transformer:

[0029] If the transformer load rate is greater than the upper limit of its optimal economic operation range and the battery power meets the discharge condition, the battery enters the discharge mode, and the magnitude of the battery discharge current is determined by the total load and the upper limit of the transformer's optimal economic operation range, ultimately making the transformer enter the optimal economic operation mode, and the process ends;

[0030] If the transformer load rate is less than the lower limit of its optimal economic operation range and the battery power meets the charging condition, the battery enters the charging mode, and the magnitude of the charging current is determined by the total load and the lower limit of the transformer's optimal economic operation range, ultimately making the transformer enter the optimal economic operation mode, and the process ends.

[0031] Further, the calculation formula for the battery discharge current is: battery discharge current = (total current load power value - power value corresponding to the upper limit of the transformer's optimal economic operation range) / current battery voltage; the calculation formula for the battery charging current is: battery charging current = (power value corresponding to the lower limit of the transformer's optimal economic operation range - total current load power value) / current battery voltage.

[0032] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0033] In the device of the present invention, a storage battery is connected to the output end of the converter. When the load size changes, it can be adjusted through the storage battery. For example, when an intermittent load is added to the system, the battery discharge can effectively cope with the intermittent load, thereby reducing the transformer overload and improving the power supply reliability.

[0034] Further, an overload protector is connected to the input end of the AC-DC converter to prevent current overload and damage the active power electronic device.

[0035] Further, a soft start circuit is connected to the input end of the AC-DC converter. When the active power electronic device is put into the system each time, it limits the transient current, making the power electronic device smooth without impact during the startup process and improving the service life of the active power electronic device.

[0036] Furthermore, a series reactor is connected to the input end of the AC-DC converter to eliminate the ripple generated by the power electronic switches in the AC-DC converter.

[0037] Furthermore, a filter capacitor is connected to the input end of the AC-DC converter, and the filter capacitor is used to stabilize the DC bus voltage.

[0038] The method of the present invention collects the transformer and load currents, separates the active, reactive, harmonic and unbalanced currents therefrom, calculates and synthesizes the modulation signals of the active power electronic device, adopts the SPWM modulation technology, controls the on-off of the IGBT, so that the device outputs the compensation current, which can supplement the reactive, harmonic and unbalanced currents in the load, and at the same time controls the battery charge and discharge current, so that the transformer enters the optimal economic operation mode, effectively reducing the power loss of the transformer and the line, and the daily average reduced loss amount is greater than the loss of the device itself; finally, the purpose of reducing the losses of the transformer and the line is achieved.

[0039] The method of the present invention first adopts the reactive, harmonic and unbalanced priority compensation technology for power energy management. When the transformer has entered the optimal economic operation range, the battery in the active power electronic device is not put into operation, so as to minimize the device and battery losses and improve the battery service life at the same time. When the transformer deviates from the optimal economic operation range, the battery is put into operation, and through battery charging or discharging, the transformer enters the optimal economic operation mode, and finally the purpose of reducing the losses of the transformer and the line is achieved. Description of the Drawings

[0040] Figure 1 Schematic diagram of the installation position of the active power electronic device;

[0041] Figure 2 System structure diagram of the active power electronic device;

[0042] Figure 3 Control flow chart. Detailed Embodiments

[0043] In order to make the purpose and technical solutions of the present invention clearer and easier to understand. The following further details the present invention with reference to the drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0044] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more. In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected", and "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] The active power electronic device is installed between the load side of the transformer T and the load, and the best governance effect is achieved when it is installed close to the load side, as Figure 1 shown. This device can reduce the power loss of the transformer and the line before the installation point, and improve the power supply efficiency and reliability of the power supply area.

[0046] The active power electronic device consists of an AC circuit breaker, a series reactor, a power electronic switch, a filter capacitor, a DC relay, a storage battery, a controller and other components, as Figure 2 shown.

[0047] The active power electronic device includes an input part, an AC-DC converter and a charging circuit connected in sequence.

[0048] The input part includes three phases A, B, and C. The input part of phase A includes an overload protector FU1, a soft start circuit, and a series reactor L1 connected in series. The soft start circuit includes an AC circuit breaker K1, an AC circuit breaker K11, and a resistor R1. The branch formed by the series connection of the AC circuit breaker K11 and the resistor R1 is connected in parallel with the AC circuit breaker K1. The input part of phase B includes an overload protector FU2, a soft start circuit, and a series reactor L2 connected in series. The soft start circuit includes an AC circuit breaker K2, an AC circuit breaker K21, and a resistor R2. The branch formed by the series connection of the AC circuit breaker K21 and the resistor R2 is connected in parallel with the AC circuit breaker K2. The input part of phase C includes an overload protector FU3, a soft start circuit, and a series reactor L3 connected in series. The soft start circuit includes an AC circuit breaker K3, an AC circuit breaker K31, and a resistor R3. The branch formed by the series connection of the AC circuit breaker K31 and the resistor R3 is connected in parallel with the AC circuit breaker K3.

[0049] The AC-DC converter is a T-type three-level topology or a two-level topology composed of IGBTs or MOS transistors.

[0050] The charging circuit includes a filter capacitor C1, a filter capacitor C2, a DC circuit breaker DCK1, a DC circuit breaker DCK2, and a battery Bat. The DC circuit breaker DCK1, the battery BAT, and the DC circuit breaker DCK2 are connected in series in sequence. The first end of the filter capacitor C1 is connected to the DC circuit breaker DCK1, the second end is connected to the first end of the filter capacitor C2, and the second end of the filter capacitor C2 is connected to the DC circuit breaker DCK2.

[0051] Refer to Figure 3 , based on the above active power electronic device, a method for reducing transformer and line losses includes the following steps:

[0052] 1) Set the upper and lower limits of the load rate of the optimal economic operation range of the transformer;

[0053] 2) Detect the load current, separate the reactive current, harmonic current, and unbalanced current in the load current, and generate a reference instruction for the output current of the active power electronic device;

[0054] 3) The active power electronic device responds to the current reference instruction output in 2), outputs a compensation current, compensates the reactive, harmonic, and unbalanced currents of the load, so that the current output by the active power electronic device and the reactive current, harmonic current, and unbalanced current of the load are completely offset. At this time, the transformer current only contains the active component and is three-phase balanced;

[0055] 4) Detect the transformer current. If it contains reactive, harmonic, and unbalanced currents:

[0056] If it contains reactive power, harmonics and unbalanced current, separate the reactive current, harmonic current and unbalanced current from the transformer current, and generate a reference instruction for the output current of the active power electronic device; the active power electronic device responds to the current reference instruction output by S2, outputs a compensation current, compensates the reactive power, harmonics and unbalanced current of the load, so that the current output by the active power electronic device and the reactive current, harmonic current and unbalanced current are completely cancelled out;

[0057] Otherwise, go to 5);

[0058] 5) Calculate the current load rate of the transformer and determine whether it enters the optimal economic operation range:

[0059] If the judgment is true, the primary process ends; otherwise, go to 6);

[0060] 6) Judge the relationship between the transformer load rate and the optimal economic operation range of the transformer:

[0061] If the transformer load rate is greater than the upper limit of its optimal economic operation range and the battery Bat power meets the discharge condition, for example, the state of charge SOC > 20%, then the battery enters the discharge mode, otherwise the battery enters the standby mode and the device does not regulate the transformer load rate. Finally, the transformer enters the optimal economic operation mode and the process ends; the magnitude of the battery discharge current is determined by the total load and the upper limit of the optimal economic operation range of the transformer, and the battery discharge current = (the total power value of the current load - the power value corresponding to the upper limit of the optimal economic operation range of the transformer) / the current battery voltage;

[0062] If the transformer load rate is less than the lower limit of its optimal economic operation range and the battery power meets the charging condition (for example, the state of charge SOC < 100%), then the battery enters the charging mode, otherwise the battery enters the standby mode and the device does not regulate the transformer load rate. The magnitude of the battery charging current is determined by the total load and the lower limit of the optimal economic operation range of the transformer. Finally, the transformer enters the optimal economic operation mode and the process ends. The battery charging current = (the power value corresponding to the lower limit of the optimal economic operation range of the transformer - the total power value of the current load) / the current battery voltage.

[0063] The above content is only to illustrate the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for reducing transformer and line losses based on an active power electronic device, characterized in that It includes the following steps: S1. Set the optimal economic operation range of the transformer; S2. Detect the load current, separate the reactive current, harmonic current and unbalanced current in the load current, and generate the output current reference instruction of the active power electronic device; S3. The active power electronic device responds to the current reference instruction output by S2, outputs the compensation current, compensates the reactive, harmonic and unbalanced currents of the load, so that the current output by the active power electronic device and the reactive current, harmonic current and unbalanced current of the load are completely offset; S4. Detect the transformer current: If it contains reactive, harmonic and unbalanced currents, separate the reactive current, harmonic current and unbalanced current in the transformer current, and generate the output current reference instruction of the active power electronic device; the active power electronic device responds to the current reference instruction output by S2, outputs the compensation current, compensates the reactive, harmonic and unbalanced currents of the load, so that the current output by the active power electronic device and the reactive current, harmonic current and unbalanced current of the load are completely offset; Otherwise, execute S5; S5. Calculate the current load rate of the transformer and judge whether it is within the optimal economic operation range: If the judgment is true, the process ends; otherwise, control the battery to discharge or charge to make the transformer enter the optimal economic operation mode, and the process ends; The active power electronic device includes an AC-DC converter and a battery. The input end of the DC converter is connected to the load side of the transformer, and the output end is connected to the storage battery. A DC circuit breaker is arranged between the output end and the storage battery. The AC-DC converter is a T-type three-level topology or a two-level topology composed of power-type power electronic switches.

2. The method for reducing transformer and line losses according to claim 1, characterized in that In S5, the process of controlling the battery to discharge or charge to make the transformer enter the optimal economic operation mode includes the following steps: Judge the relationship between the transformer load rate and the optimal economic operation range of the transformer: If the transformer load rate is greater than the upper limit of its optimal economic operation range and the battery power meets the discharge condition, the battery enters the discharge mode. The magnitude of the battery discharge current is determined by the total load and the upper limit of the transformer's optimal economic operation range, and finally makes the transformer enter the optimal economic operation mode, and the process ends; If the transformer load rate is less than the lower limit of its optimal economic operation range and the battery power meets the charging condition, the battery enters the charging mode. The magnitude of the charging current is determined by the total load and the lower limit of the transformer's optimal economic operation range, and finally makes the transformer enter the optimal economic operation mode, and the process ends.

3. The method for reducing transformer and line losses according to claim 2, characterized in that The calculation formula for the battery discharge current is: battery discharge current = (current total load power value - power value corresponding to the upper limit of the transformer's optimal economic operation range) / current battery voltage; the calculation formula for the battery charging current is: battery charging current = (power value corresponding to the lower limit of the transformer's optimal economic operation range - current total load power value) / current battery voltage.

4. The method for reducing transformer and line losses according to claim 1, characterized in that, An overload protector is connected to the input end of the AC-DC converter.

5. The method for reducing transformer and line losses according to claim 1, characterized in that, A soft start circuit is connected to the input end of the AC-DC converter.

6. The method for reducing transformer and line losses according to claim 5, characterized in that, The soft start circuit includes a first AC circuit breaker, a second AC circuit breaker and a resistor. The branch formed by the series connection of the second AC circuit breaker and the resistor is connected in parallel with the first AC circuit breaker.

7. The method for reducing transformer and line losses according to claim 1, characterized in that The input end of the AC-DC converter is connected with a series reactor.

8. The method for reducing transformer and line losses according to claim 1, characterized in that, The input end of the AC-DC converter is connected with a filter capacitor.

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