At present, flexible DC transmission technology is moving toward higher voltage and larger capacity. Due to the limitation of capacitance and capacity of insulated gate bipolar transistors (IGBT), IGBT series technology is often needed. In actual engineering, IGBT The reliability and efficiency of the series valve are particularly important. For the reliability and high efficiency of the IGBT series valve, this paper proposes an IGBT series active adaptive voltage balance control technology. Firstly, the IGBT and diode series voltage unbalance mechanism is analyzed. The IGBT series active adaptive voltage balance control technology is proposed. The basic principle and method of the control technology are analyzed. Finally, it is verified by Saber simulation and test. It shows that IGBT series active adaptive voltage balance control technology can effectively realize IGBT series voltage balance and loss optimal control.
Key words: flexible direct current transmission; IGBT; series voltage equalization; active adaptive control;
0 Preface
With the advancement and development of power electronics technology, flexible DC transmission will show its unique advantages in solving long-distance, large-capacity transmission, new energy distributed power supply access, and many problems faced by large-scale AC-DC hybrid power grids. . As a new generation of direct current transmission technology, flexible direct current transmission provides an effective solution for the transformation of power transmission mode and the construction of future power grids, and will play an important role in improving the overall economic efficiency of the power grid and promoting the sustainable development of the economy and society [1] ].
With the introduction of the global energy Internet concept, flexible DC transmission will also move toward higher voltage levels and greater capacity. In order to meet the application requirements of the high voltage field of the power system, the series or cascade technology of devices or integrated components is often used.
The converter valve technology with IGBT directly connected in series has the advantages of compact structure, low cost, small floor space and simple control [2]; modular multi-level converter valve technology with cascaded converter unit has high modularity The advantages of convenient installation and maintenance; and the use of the converter valve cascaded in series with the converter unit is the development direction of the current high-voltage large-capacity converter, which solves the problem of increasing the number of series connection of IGBT devices under high-voltage environment. The high stress, the pressure equalization, the large number of subunits connected in series, and the complicated control. Therefore, IGBT series technology is still one of the core technologies of ultra-high voltage direct current transmission in the future.
In the thyristor-based ultra-high DC transmission, the RC damping circuit is often used to solve the voltage balance problem of the thyristor series connection, but the IGBT switching speed is fast, the working frequency is high, and the RC damping equalization scheme is low in efficiency, which is more feasible in practical engineering. Poor, so the IGBT series voltage balance needs to be achieved through advanced gate control [3].
The literature [4,5] proposed a synchronous control technology. The instantaneous voltage balance controller makes the voltages of the tubes equal by delaying the opening and closing of the switch tubes in advance. This controller can be digitally discretized to ensure that each switching signal is synchronized. Literature [6] proposed a method of realizing dynamic voltage equalization by enhancing the Miller effect. If a switch of a series branch is turned off in advance, the pre-charged capacitor will inject a positive pulse into the switch. Allow the voltage to be balanced on each switch. The active voltage control method proposed in [7] makes the collector-emitter voltage of each switch in series in the dynamic process follow the same reference signal, so the change of collector-emitter voltage does not depend on the device. It depends on the reference waveform. At present, the above methods can effectively realize the IGBT series voltage balance, but the optimization of IGBT switching loss is also very important in practical engineering. The active adaptive voltage balance control strategy proposed in this paper is based on minimizing the IGBT switching loss. Voltage balance.
1 IGBT series voltage imbalance analysis
IGBT series voltage imbalance is divided into static voltage imbalance and dynamic voltage imbalance. Because IGBT switching speed is faster, dynamic voltage imbalance is the core problem that IGBT series needs to solve, and the voltage of freewheeling diode in anti-parallel with IGBT Balance control is also an important issue to consider in tandem. The factors that cause the IGBT series voltage imbalance are mainly divided into the following five categories:
(1) IGBT leakage current is inconsistent
The difference in leakage current of the IGBT will lead to inconsistency in the off-state impedance of the IGBT. After the IGBT is turned off, since the leakage current flowing through the series device is the same, different off-state impedances will cause the quiescent voltage of the IGBT to be unbalanced. The junction temperature also affects the static voltage equalization [8].
(2) Inconsistent drive signal and difference in drive circuit parameters
The inconsistency of the drive signal and the difference in drive circuit parameters (such as gate resistance) will cause the IGBT gate drive signal to be out of sync, thereby greatly affecting the IGBT collector-emitter voltage balance. When turned off, the device that is turned off first will generate a high overvoltage, and the device with the hysteresis turned on during the turn-on will also withstand higher overvoltage [9].
(3) Discreteness of parasitic parameters of IGBT itself
Inconsistent characteristics such as parasitic inductance and parasitic capacitance of the device may result in different switching characteristics and voltage spikes. In the process of turning off the series IGBT, the device with faster turn-off speed is subject to high overvoltage and is turned on slowly during turn-on. Devices will also withstand higher overvoltages [10].
(4) IGBT series valve stray parameters
The IGBT drive and the stray capacitance of the device itself to the ground and between them will cause significant difference in switching delay and dv/dt of the IGBT, resulting in dynamic voltage imbalance of the IGBT.
(5) Difference in reverse diode recovery characteristics
A fast recovery diode is usually connected in parallel with the IGBT. In the case of inductive load, there is a commutation process between the turn-on of the IGBT and the diode of the inductor. Due to the reverse recovery of the diode, an overvoltage is generated across the freewheeling diode at the instant the IGBT is turned on. At the same time, due to the difference in charge recovery of the diode reverse, a difference in voltage will occur when the series diode is turned off, which will also cause diode overvoltage. The overvoltage across the diode is the overvoltage of the IGBT.
2 Active adaptive voltage balance control strategy
According to the literature [], the delay time td and the voltage change rate dv/dt during the IGBT switching process are as shown in equation (1)(2).
By adjusting the IGBT gate voltage, the delay time td and the voltage change rate dv/dt during the IGBT switching process can be effectively controlled, and the IGBT series voltage balance can be realized. Active voltage control proposed in the literature enables the IGBT collector voltage VCE to quickly follow the reference voltage Vref through closed-loop control. When the IGBT terminal voltage is higher than a given voltage, a positive gate voltage signal is generated to turn on the IGBT; when the IGBT terminal voltage is lower than a given voltage, a negative gate voltage signal is generated to turn off the IGBT, and the closed-loop control is used to make the IGBT The stage voltage can quickly follow Vref.
Vref is divided into six stages: pre-shutdown, main-off, off-state, pre-opening, main-on and on-state. According to the formula, the design of Vref phase parameters will directly affect the IGBT series voltage balance and switching loss. The longer the pre-switching and main switching time, the higher the IGBT voltage balance control, but the IGBT switching loss increases accordingly; if the pre-switching and main switching time are too short, the IGBT voltage balance is poor. Therefore, it is necessary to find the optimal Vref in the switching loss and the IGBT series voltage balance.
In this paper, an active adaptive voltage balance control strategy is proposed. Based on the active voltage control, Vref is optimized according to the IGBT series voltage balance. As shown in the figure, the active adaptive voltage balance control is mainly composed of two closed loop feedback loops: IGBT collector The stage voltage closed-loop control IGBT collector-emitter voltage Vce quickly follows the reference voltage Vref; the IGBT collector-emitter voltage closed-loop optimization reference waveform Vref pre-switch and main switching time, the optimization diagram is shown in the figure.
3 simulation and experimental verification
3.1 Simulation verification
For the active adaptive voltage balance control, two IGBT series simulation studies are carried out on the BOOST circuit. Simulation conditions: IGBT turn-off voltage 400V, IGBT on-state current 180A, series valve arm stray inductance 100nH, IGBT switching frequency 1kHz, Vref pre-off time initial value 1μs, Vref main turn-off time initial value 1.5μs.
As shown in the figure, in order to optimize the voltage and current waveforms of the two IGBTs under Vref, the pre-off time of the IGBT is close to 1μs due to the pre-shutdown of Vref and the main off-time is long, and the dv/dt is 200V/μs. Therefore, the IGBT series voltage balance is very high, but the IGBT turn-off loss is large, and the single turn-off loss is 198 mJ.
As shown in the figure, for the Vref of two IGBTs under active adaptive voltage balance control, it can be seen from the figure that the Vref parameter is optimized, the Vref pre-off time is optimized from 1μs to around 0.5μs, and the Vref main turn-off The time is optimized from 1.5μs to around 0.4μs. Due to the different characteristics of the two IGBTs, the optimized Vref of the respective IGBTs is different.
As shown in the figure, in order to control the voltage and current waveforms of the two IGBTs under active adaptive voltage balance control, the IGBT turn-off speed is significantly improved compared with the figure. The IGBT pre-shutdown time is reduced from 1μs to 0.5μs, and the dv is turned off. /dt is increased from 200V/μs to 500V/μs, and the single turn-off loss is reduced from 198mJ to 95mJ, which is very close to the hard turn-off loss of IGBT. Compared to the figure, the IGBT series voltage balance is reduced, but it is still within the preset range. Through simulation, the active adaptive voltage balance control can optimize Vref, and the IGBT series voltage balance can increase the IGBT switching speed and reduce the IGBT switching loss within the allowable range.
3.2 Experimental verification
According to the active adaptive voltage balance control strategy described above, the IGBT intelligent driver board is developed as shown in the figure. IGBT intelligent driver board mainly includes active adaptive voltage balance control, fault protection, communication coding, high-level energy-taking and other functions.
Design the reference voltage waveform using FPGA programming as shown. The initial value of the pre-shutdown time is 4μs, the amplitude of the pre-shutdown platform is 1.5V, the initial value of the main turn-off time is 1μs, the clamp voltage amplitude is 7V, the initial value of the pre-turn-on time is 3μs, and the pre-opening platform amplitude is 4V. The initial turn-off time is 2μs.
The two IGBTs are tested in series. The experimental circuit uses a passive inverter circuit. The specific parameters are as follows: DC voltage 1000V, load current 400A, IGBT switching frequency 1050Hz. Reference voltage parameters: pre-shutdown platform time 4μs, pre-shutdown platform amplitude 1.5V, main shutdown time 1μs, clamp voltage amplitude 7V, pre-opening platform time 3μs, pre-opening platform amplitude 4V, main turn-on time 1μs .
The experimental results are shown in Figure 4-22. After the 2μs turn-off delay of the IGBT in the off-phase, the two IGBTs can quickly follow the reference voltage waveform. The voltage balance of the series IGBT is very good, but at this time, due to the long pre-shutdown time, Therefore, the IGBT turn-off loss is also large, 390 mJ.
According to the previous analysis, the pre-switching platform is used to compensate for the voltage imbalance caused by the inconsistent IGBT switching delay, while the excessive platform time will lead to an increase in the IGBT switching loss, but the IGBT series voltage equalization is meaningless. When the active adaptive voltage balance control strategy is adopted, the Vref pre-off time is significantly reduced from 4μs to 3μs, and the IGBT turn-off loss is reduced from 390mJ to 350mJ.
At the same time, the active adaptive voltage balance control strategy adjusts dv/dt according to the IGBT voltage following condition in the main turn-off phase. The main turn-off time is reduced from the preset 1μs to 0.4μs, and the IGBT turn-off dv/dt is from 500V/μs. It is boosted to 850V/μs, so the IGBT turn-off loss is reduced to 240mJ on the basis of ensuring the IGBT voltage balance. Through experiments, the active adaptive voltage balance control can optimize Vref, and the IGBT series voltage balance can increase the IGBT switching speed and reduce the IGBT switching loss within the allowable range.
4 Conclusion
In flexible DC transmission engineering, the reliability and loss of the converter valve is its most important indicator. In this paper, based on active voltage control, the active adaptive voltage balance control strategy is proposed. On the one hand, the IGBT voltage quickly follows Vref, controls the IGBT series voltage balance, reduces the IGBT switching stress, and improves the reliability of the converter valve. On the one hand, through closed-loop control, Vref is optimized according to the voltage balance, and the IGBT loss is minimized. The IGBT series converter valve using this technology can be widely applied to high-voltage fields such as flexible direct current transmission and flexible alternating current transmission with high reliability and efficiency.
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