1 Introduction
The development of high-power power electronic devices [8][9] and large-scale integrated circuit technology has made it possible to realize high-voltage (6kV, 10kV) variable frequency speed control devices using high-to-high direct conversion. Compared with high-low-high conversion high-voltage frequency converters, high-high frequency converters have advantages such as small size, light weight, high efficiency, and high performance-price ratio, and are therefore increasingly used. Several domestic companies have also launched high-voltage frequency converter products using a unit series single-phase bridge main circuit structure based on IGBT devices. This main circuit structure has poor overall reliability and low driving capability due to the large number of IGBT devices and complex signal modulation. Its output power is also limited by the limited capacity of a single IGBT tube. In contrast, the IGBT (integrated gate commutated thyristor) is a new type of device developed in the 1990s based on thyristor technology, combined with mature technologies such as IGBT [5] and GTO. Therefore, it is more suitable for high-voltage and high-capacity applications than IGBT. Meanwhile, IGCT has been redesigned based on GTO, thus offering advantages such as lower switching losses, simpler gate control, faster turn-off speed, and simpler main circuit wiring compared to GTO. Currently, the highest withstand voltage level of IGCT components is 5.5kV, making them suitable for high-capacity frequency converters. Given that the integrity of IGCT components is crucial to the safe operation of frequency converter equipment, frequency conversion modification of AC motor power supplies can improve starting performance, thereby extending the service life of motors and reducing production costs for enterprises, which is of paramount importance [1].
Establishment of the 2igct sub-circuit model
2.1 IGCT Structure and Working Principle [6]
IGCT is a general term for integrated gate drive circuits and gate commutated thyristors (GCTs). In terms of manufacturing process, it adopts a unit structure integration method. Its cathode is subdivided into many unit cells, which are surrounded by gates to form a so-called multi-cathode structure. The outside of the entire chip is a fast diode connected in anti-parallel.
The IGCT's anode PNP transistor is a high-voltage, high-current transistor with a thick n-base region. At the start of turn-off, this n-base region stores a large amount of charge, requiring a certain amount of time (1-2 μs) for the anode current to remove this charge. Because its cathode NPN transistor has a large ANPN value (i.e., is highly sensitive), the cathode transistor can switch out of the operating region within this time. Thus, when the anode voltage rises, there is no cathode current; in other words, the IGCT is an open-base PNP transistor during turn-off. During the turn-on phase, a strong gate current pulse of several hundred amperes rapidly and effectively brings the cathode NPN transistor into the saturation region before the thyristor switches. Even with very high di/dt, the turn-on loss is almost negligible.
2.2 Modeling and Simulation of IGCCT
In this paper, the purpose of establishing the IGCT model [2] is to study the dynamic behavior characteristics of the inverter, so a sub-circuit model is used for its simulation. Furthermore, due to the low inductance of the IGCT (negligible unit delay), a single 2T-3R simulation circuit [9] is used for the IGCT. Figure 1 shows the IGCT simulation circuit of model 5SHX04D4502 manufactured by ABB, with VDRM=4500V and ITGQM=630A.
The simulation results are shown in Figure 2(a), which are in good agreement with the measured turn-off waveform of IGCT (as shown in Figure 2(b)), thus completing the simulation of IGCT quite well. However, some differences still exist: the simulation results are relatively linear; and the oscillation transition process after the anode peak voltage is not given in more detail. Using this model to simulate the inverter unit will have sufficient accuracy.
2.3 Inverter Unit Based on igct
The parameters of the high-voltage frequency converter simulated in this paper are determined based on the fan system of a thermal power plant. The voltage is set to 6kV and the capacity to be 2mVA. The principle wiring of the inverter circuit in each power unit is shown in Figure 3. Compared with the circuit diagram of a single rectifier-inverter power unit, the yoke circuit and stray inductor in the actual circuit are added. In order to observe the dynamic behavior characteristics of igct in the inverter, a detailed model is used in the inverter simulation, as shown in Figure 3. S1~S4 in the figure are all replaced by the single 2t-3r simulation circuit shown in Figure 1 [7], and a fast diode is connected in anti-parallel to form the PSpice model of reverse-conducting igct. RSSnubber, dclamp, lsnubber, and cclamp are the absorption resistor, clamping diode, yoke inductor, and clamping capacitor, respectively. They form the turn-on absorption circuit of the inverter, that is, the yoke circuit. ls is the stray inductor in the circuit.
Since simulating the characteristics of the control system requires a long time, Figure 4 shows a simplified inverter control circuit and modulation signal. However, this paper focuses on the characteristics of the inverter device. Therefore, without loss of generality, the behavior of the control loop can be replaced by a fixed signal source as shown in Figure 5(a). The comparator is modeled using table functions (etable, e1 and e2 in the figure) to compare the magnitudes of the triangular wave carrier signal and the sine wave reference signal at the two input terminals (the signals are shown in Figure 5(b)). Based on this, PWM control signals are generated to control s1 and s2 (IGCT units) respectively. The output signals are reversed by the control terminals of a voltage-controlled voltage source (e3 and e4), and their output signals control s3 and s4 (IGCT units). The control signals of s1 and s3 are shown in Figures 5(b) and 5(c). It can be seen that they are complementary. The control signals of s2 and s4 lag s1 and s3 by 180° respectively.
3igct Simulation and Analysis
In the simulation, in Figure 5(a), vr1 is the sinusoidal modulation signal voltage, vr2 is the triangular carrier signal voltage, and the amplitudes are 8 and 10 respectively. The amplitude of vc is 10, that is, the modulation ratio m is 0.8; the frequency of vc is 800 Hz, that is, the carrier frequency is 800 Hz. The following three cases were simulated respectively:
(1) Ideally, without considering the stray inductance of the line, a resistive load rload=20ω is applied. The simulation waveform is shown in Figure 6.
(2) Consider the large stray inductance. Figure 8
Considering the simulation waveform under stray inductance conditions, the stray inductance in the bridge arm has a significant impact on the magnitude and shape of the overvoltage generated during the turn-off of the IGCT. A schematic diagram of the stray inductance in the inverter bridge arm simulation circuit is shown in Figure 7. Here, ls1 is the stray inductance of the RSNUBBER branch of the snubber resistor in the turn-on snubber circuit; its value is closely related to the selection of the snubber resistor, generally between 100 and 500 nH. The stray inductance of the commutation circuit is also shown; its value is closely related to the mechanical structure design of the main circuit, generally between 100 and 1000 nH. ls3 is the stray inductance of the capacitor branch in the turn-on snubber circuit; its value is closely related to the selection of the snubber capacitor, generally between 100 and 500 nH. In the simulation, ls1 = 100 nH, ls2 = 400 nH, and ls3 = 1000 nH are taken respectively. Simultaneously, an inductive load is applied with rload=20ω and rload=10mh. Other parameters are the same as in the first case. The simulation waveforms are shown in Figure 8(a) and Figure 8(b).
(3) Consider a fault condition where the neutral point on the AC power input side and the capacitor in the lg filter on the AC output side are grounded to the same grounding grid. When the capacitance of the capacitor is large, for example, 9000μF, the other parameters are the same as in the first case. Figure 8(c) shows the simulated waveform of the igct terminal voltage under fault condition, and Figure 8(d) shows the igct bridge arm current under fault condition.
The simulation results show that for case (1), the ideal case, glitches appear in the waveform compared to the theoretical results. This is due to the presence of a yoke current inductor in the circuit, which is consistent with the actual situation. Case (2) is closest to the actual situation. The parameters used describe a relatively loose structure with a large stray inductance in the cable. The simulation results are basically consistent with the measured results. When the load is relatively large, the simulation results show that the terminal voltage of igct has exceeded vdrm (4500V) at some points, and igct may break down. During the debugging, a large rise voltage was also measured, and the simulation results are consistent with the actual measurements.
Situation (3) is a serious fault condition, with the current being much greater than itgqm=630a. At the same time, the voltage that igct bears has exceeded vdrm (4500V) at some points, and igct will inevitably break down or even burn out.
4. Conclusion
Analysis of the experimental results after simulation leads to the following conclusions:
(1) There are conditions for not installing a turn-off snubber circuit in igct: the circuit structure must be compact and the stray inductance must be small. When the stray inductance of the main commutator is large, a turn-off snubber circuit must be added.
(2) Through PSpice simulation, fault conditions can be reproduced and fault conditions can be artificially simulated. During the research and development stage of power electronic products, relevant operating data can be found to provide a basis for product debugging. At the same time, the development cycle can be shortened and development costs can be saved.
