Application of electronic load technology based on electric energy feedback in aviation power supply test

With the rapid development of modern science and aviation technology, modern aircrafts use a large amount of electrical equipment, and they are developing in the direction of multi-electric and all-electric aircraft. It has become a trend to replace mechanical energy, hydraulic energy, and pneumatic energy with electrical energy. This will cause the aircraft's power consumption to increase sharply. In order to ensure that the electrical equipment can work normally under various conditions, higher requirements are put forward for the reliability and performance of the aircraft's power supply. This requires more stringent tests on the power supply system before leaving the factory, such as reliability tests (aging discharge tests), output characteristic tests, etc. At present, the domestic aviation power supply factory test and the test test of the power supply by scientific research institutes all use the resistance box or the water resistance test bench as the load. This traditional test method has many shortcomings: the load adopts step-wise adjustment, the resistance power is small, the electric energy of the test is all consumed in the resistance, the load equipment is bulky and takes up a lot of space. With the development of semiconductor technology and the rapid development of power electronic converter technology, especially the continuous emergence and application of various current control technologies, electronic loads that can simulate traditional real impedance loads have emerged. Use an electronic load to test the power supply, and use effective current control technology to control the discharge current in a wide range. It can simulate loads of various impedance values, so that an electronic load can meet any impedance value test occasion; All of the energy is fed back to the grid to solve the problem of power waste; large-capacity power switching devices are used to complete the test of high-power power; because there is no high-power energy-consuming resistor, the load is small, which can greatly save installation space. Aiming at the shortcomings of the current domestic aviation power supply test, this article conducts an in-depth study of the electronic load, applies the electric energy feedback electronic load to the aviation power supply test, and solves many problems in the traditional aviation power supply test.

1 Overview of Avionics Load

1.1 Aircraft power system

Aircraft power supply system is an important part of the aircraft power supply system, including main power supply, auxiliary power supply, emergency power supply and secondary power supply. The main power supply of the aircraft is composed of a generator and its transmission, regulation, control and protection devices, which supply power to the electrical equipment of the aircraft in normal flight; the auxiliary power supply is a generator driven by an aviation battery or auxiliary power device, which works when the main power supply is not working. The emergency power supply includes aviation batteries and wind turbine generators. Once the main power supply fails during flight and cannot be supplied normally, the emergency power supply will be used; the secondary power supply is the conversion of one form of electrical energy from the main power supply into different voltages and different currents. And equipment of different quality to meet the requirements of different electrical equipment for different forms of electrical energy.

Avionics loads are equipment or conversion devices that output electrical energy in the aircraft power system, such as generators, AC/DC, DC/AC converters, aviation batteries, rectifiers, and the output characteristics and reliability of components such as inductors and capacitors ( Aging discharge test) for reliable and comprehensive testing.

1.2 Principle of electronic load

Electronic load is a kind of load that uses electronic components to absorb electrical energy and consume it. The electronic components are generally power semiconductor devices such as power field effect transistors (PowerMOS) and insulated gate bipolar transistors (IGBT). The electric energy feedback type electronic load is a saving-type electronic load that feeds the test energy back to the grid under the premise of realizing the power supply test, and realizes the recycling of electric energy. Figure 1 is a schematic diagram of the principle of an electric energy feedback electronic load.

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The input of the electronic load is the output of the test power supply, and the output is connected to the low-voltage power grid 220∕380V. On the input side, the electronic load must control the size and phase of the input current to make the impedance value or current value present to the power supply the set value; on the output side, the output current must be controlled to be in phase with the grid voltage to achieve the unit of test energy Power factor feedback.

The working modes of the electronic load are: constant current mode, constant resistance mode, constant voltage mode and dynamic test modes such as overvoltage, overcurrent, short circuit, etc. The constant current mode is used to test the load regulation rate of the voltage source and AC/DC power supply. The load regulation rate is the ability of the power supply to provide a stable output voltage when the load changes, and it is the percentage of the power supply output voltage deviation rate; the constant resistance mode is usually It is used to test the activation and current limiting characteristics of voltage or current source; the constant voltage mode is used to test the current limiting characteristics of the power supply. Because the voltage is a fixed value, the battery terminal voltage can be simulated to test the battery charger; the dynamic test mode is Through the control of the electronic load, various faults or sudden changes of some parameters in the power system are simulated to test the reliability of the system.

1.3 Classification of Avionics Load

Aviation power supply can be divided into main power supply, auxiliary power supply, emergency power supply and secondary power supply according to its function, which can be summarized into two types of power supply, namely DC power supply and AC power supply. Aviation DC power supplies such as: DC generators (rated voltages are 6V, 12V, 28.5V, 270V, etc.), aviation batteries, aviation transformer rectifiers, etc.; aviation AC power supplies such as AC generators (generally 115∕200V, 400Hz, three Phase), AC/DC converters, static converters, etc.

There are two types of avionics power supplies: DC and AC. Avionics loads are also divided into two types: DC electronic loads and AC electronic loads. The feedback type DC electronic load is used for the test of aviation DC power supply. Generally, the inverter is directly used to feed the electric energy of the DC power supply back to the AC grid; the feedback type AC electronic load is used for the aviation AC power supply test, and the AC/AC conversion is used to realize the power supply test. At the same time, the electric energy is fed back to the AC grid.

2. Avionics load topology

2.1 Topological structure of DC electronic load

At present, the research of DC electronic load has been relatively mature. Considering the aspects of energy saving, simplification of structure, and cost reduction, the main circuit topology of DC electronic load is determined to be DC/DC and DC/AC two-level structure, as shown in Figure 2. The main circuit mainly includes two parts: DC/DC and DC/AC converters. Among them, DC/DC adopts the Cuk converter with capacitor energy storage, as shown in the left dashed box in Figure 2, which can realize separate control of input and output currents. , And no other auxiliary circuit is needed to realize the soft turn-off of the switching tube to reduce the switching loss. At the same time, its structure is much simpler than the Boost-Buck converter; the DC/AC converter generates the inverter current synchronized with the grid, and the output filter Filter out the high-order harmonics in the inverter current and reduce the output current THD.

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2.2 Topological structure of AC electronic load

The input and output sides of the AC electronic load are both AC voltage, so in general, it is an AC/AC converter. According to the type of intermediate power transmission link, AC/AC converters can be divided into three categories: intermediate DC link (DC link) AC/AC converter, intermediate AC link (AC link) AC/AC converter and direct link AC∕ AC converter.

Considering that the AC/AC converter of the intermediate AC link is added to the high-frequency LC resonant circuit, since the energy provided to the load is completely passed through the resonant circuit, its inductance and capacitance components require a large rated capacity, and the bidirectional power flow and the control of the high-frequency bus are relatively Complex, the number of switching tubes is twice that of the intermediate DC link converter, and the cost is high; the direct AC/AC converter is also called the matrix AC-AC converter, which requires the use of bidirectional full-control switches, that is, two full-control devices are connected in reverse series Therefore, a total of 18 fully-controlled switching devices are required, the output voltage is 0.866 times the input voltage, the conversion efficiency is not high enough, and there are also problems in short-circuit protection. The intermediate DC link AC/AC converter has a simple structure and is easy to control. It is a common AC/AC conversion circuit at present. Although hard switching is adopted, the general 400Hz frequency of aviation AC power will not cause large switching losses. From the above analysis, this article adopts the AC-DC-AC structure of the intermediate DC link as the main circuit structure of the electronic load, as shown in Figure 3.

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3, avionics load control method

The control of the DC electronic load is relatively simple, mainly through the PWM wave generated by the integrated circuit IC to control the two transistors in the cuk circuit respectively, and change their conduction ratio to realize the control of the input and output current; the control of DC/AC will be adopted Control mode in AC electronic load.

For AC electronic loads, the control is more complicated. It can be seen from the topological structure of the electronic load that the DC/AC stage of the DC electronic load, the DC/AC and AC/DC stages of the AC electronic load all use voltage-type reversible PWM rectifiers (VSR). The conventional VSR control system generally adopts double dead loop control, that is, voltage outer loop and current inner loop control. Because the voltage control is also indirectly realized through the current control, the dynamic performance of the current control directly affects the performance of the entire system, so the current control of the inner loop is the key, and the current control is adopted here. The direct current control of VSR takes the fast feedback grid-side current as the control object for endless loop control, which can obtain higher quality current response. Among the direct current control methods of VSR, hysteresis PWM current control has faster current Response, and the current tracking dynamic deviation is determined by the hysteresis width, and does not fluctuate with the current change rate. So this article adopts hysteresis PWM current control as the control method of AC electronic load.

Figure 4 is a schematic diagram of hysteresis current control. The control object is input current i, and its control target value is i*. Detect the actual input current i and compare it with the current reference value i*. When the deviation between i and i* does not exceed the loop width, the state of the switch tube remains unchanged; when the deviation between i and i* does not exceed the loop width, The corresponding switch tube is controlled to be turned on or off, so that the current i changes in the direction where the deviation from i* decreases.

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4, avionics load control strategy

The control strategy of the DC electronic load is relatively simple. As shown in Figure 5, the WM IC controller is used to control the input current. If the reference current value liref changes, the conduction ratio σi of Ti will change to obtain the corresponding value. Among them, in the steady state, σi is a constant. As shown in Figure 6, the voltage across the capacitor Cb must be equal to the reference voltage Vref. If they are not equal, a voltage error will occur. The product of the voltage error and the correction value of the output voltage is used as the reference voltage flow. The reference current is compared with i0 to generate a PWM control. signal.

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Because the AC electronic load adopts the circuit structure of alternating and direct, so the input (rectifier) and output (inverter) can be controlled separately. The rectifier link realizes the constant current or constant impedance working mode of the electronic load by controlling the input current (including amplitude and phase); the inverter link realizes unit power factor inversion by controlling the output current to be in phase with the grid voltage, that is, the feedback of electric energy . As shown in Figure 7, if i1 and i5 are directly used as input and output hysteresis control objects, then i1 and i5 will contain high-frequency fluctuations with the amplitude of the loop width, and the input and output filter capacitors will not play a filtering role. Here i2 and i4 are used as the hysteresis control object, so that some high frequency harmonics in the hysteresis control current are filtered out by the filter capacitor, so as to obtain a more ideal input and output current waveform. At the same time, when calculating the reference current value, the fundamental reactive currents i3 and i6 flowing in the filter capacitor are taken into consideration to achieve the purpose of indirectly controlling i1 and i5 by directly controlling i2 and i4. It is worth noting that the use of hysteresis current control must first meet the condition that the DC side voltage is greater than the AC side voltage amplitude; secondly, whether the electronic load can simulate the set positive reactance or current, the key is whether the calculation of the reference current accurate.

5. Avionics load system structure

The structure of the avionics load system is shown in Figure 8. The hardware system includes a microprocessor circuit, an electronic load main circuit, a detection circuit, a trigger drive circuit, a protection circuit and a filter circuit. The microprocessor circuit is composed of a microprocessor and its power supply circuit. It adopts the TMS320LF2407A of Ti's TMS320C2000 series. It has powerful data processing capabilities, rich on-chip resources and peripheral resources, and especially provides programmable PWM waveform control. The main circuit of the electronic load is mainly composed of four groups of switch bridge arms that input and output two VSRs. The system detects the input and output current and voltage signals, performs filtering and bias processing, and sends them to the microprocessor. The protection circuit signals are also sent to the microprocessor. The microprocessor completes the calculation of reference current (voltage), the control method and the realization of strategy programming. , The generation of drive signal, so as to realize the control of the electronic load.

Under the background that the current civil DC electronic load is mature and the AC electronic load is being studied in depth, in view of the shortcomings of the current domestic aviation power supply test, the electric energy feedback electronic load is applied to the aviation power supply test, from the topological structure of the avionic load, Demonstration of its feasibility in terms of control methods and control strategies. Avionics loads based on electric energy feedback have the advantages of small size, energy saving, and high automation, which will bring huge economic benefits to the defense industry.