1 Introduction
High-voltage frequency converters play a crucial role in energy and electrical energy conversion, and are widely used in high-power industrial fields such as oil and gas transportation, air compressors, and pumped storage. To improve efficiency, the rated voltage is typically increased to reduce conduction losses. Multilevel frequency converters can output higher voltage levels and use low-voltage power switching devices, resulting in low output voltage harmonic distortion and low electromagnetic interference, making them an effective solution for high-power AC drives.
There are two main ways to implement multilevel frequency converters: midpoint clamping topology and two-level H-bridge cascaded topology.
The advantages of midpoint clamped multilevel inverters are simple structure and easy energy feedback. However, in inverters with more than three levels, it is difficult to balance the DC capacitor voltage, which limits the increase in the number of levels. Two-level H-bridge cascaded multilevel inverters can easily obtain more levels through unit cascading. The device withstand voltage requirements are low, but the structure is complex, the dynamic performance of the system is difficult to improve, and energy feedback is difficult. It is only suitable for applications with low performance requirements [1 , 2].
To address the advantages and disadvantages of these two multilevel topologies, Lai et al. proposed the NPCH bridge multilevel structure. This structure simplifies the system to some extent and reduces the number of high-voltage isolated DC power supplies. Such products are already available on the market, such as ABB's ACS5000 inverter and TMEIC's TMdrive-XL85 series inverters.
This paper focuses on the NPCH bridge-type five-level frequency converter, studying its topology and modulation strategy. The analysis and verification are carried out by combining PSIM simulation and prototype experiments, and the practical application of the modulation wave phase-shifting modulation method is presented.
2NPCH Bridge Power Unit
Figure 1 shows the topology of phase A of the NPCH bridge five-level frequency converter. Each phase consists of two three-level bridge arms connected in an H-bridge configuration. Here, is a power switching device, and each switching device has an anti-parallel freewheeling diode. Each bridge arm has two clamping diodes, the DC-side voltage is 2E, and the single-phase output voltage of the frequency converter is [value missing].
Figure 1. Topology of NPCH bridge five-level frequency converter A phase
Compared to the NPC three-level inverter, the NPCH bridge five-level inverter has five phase voltage levels, reducing the output voltage and harmonic distortion (THD). Because there are no series-connected switching devices, the dynamic and static voltage equalization problems of power switching devices are eliminated. When using sinusoidal pulse width modulation with third harmonic injection, the effective value of each phase output voltage is...
This is the fundamental RMS value of the single-phase output voltage, where is the DC-side voltage, and M is the modulation coefficient. When the DC-side voltage is , it is the RMS value of the output line voltage. When the RMS value of the output current is 1.75kA , the output power of the NPCH bridge five-level inverter can reach 20MW.
Figure 2 shows the topology of a 20MW NPCH bridge-level frequency converter based on IGCT. Each phase of the frequency converter includes two identical 6-pulse diode rectifiers, which are powered by two three-phase symmetrical windings on the secondary side of the phase-shifting transformer, thereby reducing the harmonic distortion rate of the input current.
Figure 220MWNPCH bridge five-level frequency converter topology
3-carrier stacked modulation strategy
The basic principle of the carrier stacked pulse width modulation strategy is to compare a sinusoidal modulated wave with several triangular carriers. At the moment when the sinusoidal wave intersects with the triangular carriers, a corresponding switching is performed to achieve a specific voltage output level.
The carrier stacking control strategy adopted by the NPCH bridge five-level frequency converter in this paper can be divided into carrier in-phase stacking (PD) modulation method, positive-negative-phase stacking (POD) modulation method, and alternating-phase-phase stacking (APOD) modulation method according to the different carriers and modulation waves [4 , 5]. In addition, there is also the modulation wave phase shift (MPS) modulation method.
According to the analysis of relevant literature [4], among the three carrier stacking modulation methods, the output line voltage harmonic spectrum characteristics of carrier in-phase stacking PD modulation are the best. Define and as the frequency and amplitude of the modulating wave, and as the frequency and amplitude of the triangular carrier, respectively, the carrier ratio, the modulation coefficient, as the angular frequency of the modulating wave, as the angular frequency of the triangular carrier, E as the half-side DC voltage, and as the nth order Bessel function of the first kind.
3.1 Carrier In-Phase Stacking (PD)
Figure 3 shows the carrier in-phase superimposed modulation method applied to the NPCH bridge five-level frequency converter. The triangular carriers vc1, vc2, vc3, and vc4 have the same phase and amplitude, but are arranged and superimposed vertically with the same phase.
Figure 3 shows the PD modulation of the NPCH bridge five-level frequency converter.
When the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on. This generates 4 independent gate switching signals, and then based on the complementary relationship between the control signals of the switching devices, the gate control signals of 8 switching devices can be obtained.
The harmonic spectrum characteristics of PD modulation are analyzed using Fourier series analysis of bivariate control. The Fourier series expression of the bivariate control waveform is as follows:
The Fourier series expressions for the output phase voltage and line voltage of phase A of the frequency converter are shown in equations (3) and (4):
When an NPCH bridge five-level frequency converter uses PD modulation, the harmonic content of its output phase voltage mainly consists of sideband harmonics near the carrier frequency multiplication factor. Due to the asymmetry of the carrier frequencies above and below zero, the output phase voltage does not satisfy the half-cycle symmetry relationship, thus containing even-order harmonics. Its harmonic components are mainly even-order sideband harmonics near the odd-order carrier frequency multiplication factor and odd-order sideband harmonics near the even-order carrier frequency multiplication factor, such as... The harmonic distribution of the output line voltage is basically consistent with that of the phase voltage, and the third harmonic component in the phase voltage is eliminated.
3.2 Modulated Wave Phase Shift (MPS)
For the NPCH bridge five-level frequency converter, the left and right bridge arms of the three phases can be regarded as two NPC three-level frequency converters, which is equivalent to the cascading of two NPC three-level frequency converters. Figure 4 shows the applicable phase-shifting modulation method of the modulation wave. The modulation wave has the same frequency and amplitude, and the leading angle is θ. The triangular carrier wave has the same phase and amplitude, but is arranged and superimposed vertically with the same phase.
Figure 4 shows the MPS modulation of the NPC/H bridge five-level frequency converter.
When the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on; when the modulation wave is present, the switching device is turned on. This generates 4 independent gate switching signals. Based on the complementary relationship between the control signals of the switching devices, gate control signals for 8 switching devices can also be obtained.
With modulation index ma= 0.95 , when the phase shift angle θ=30°, the output phase voltage is three-level and the line voltage is five-level; when the phase shift angle θ=80°, the output phase voltage is five-level and the line voltage is seven-level; when the phase shift angle θ=180°, the output phase voltage is five-level and the line voltage is nine-level.
Let the phase shift angle θ = 180°. When ma = 0.4 , the output phase voltage is three-level and the line voltage is five-level; when ma = 0.6 , the output phase voltage is five-level and the line voltage is seven-level; when ma = 0.9 , the output phase voltage is five-level and the line voltage is nine-level.
This paper adopts a phase-shift modulation method with a phase-shift angle θ=180°. Through Fourier series analysis of the harmonic spectrum using dual-variable control, the Fourier series expressions for the output phase voltage and line voltage of the inverter's A phase are shown in equations (5) and (6):
As can be seen from equations (5) and (6), the output phase voltage spectrum of the five-level inverter does not contain even harmonics, and odd sideband harmonics exist near the even carrier frequency, such as, etc.; the third harmonic sideband harmonics in the line voltage are eliminated, and only the second harmonics are included, and even harmonics are not included.
4. Simulation Verification
A simulation model of an NPCH bridge five-level frequency converter was built in PSIM simulation software, with DC side voltage, modulation wave frequency, triangular carrier frequency, carrier ratio, modulation coefficient M=0.9, and the load being an RL resistive-inductive load.
The simulated output waveforms of phase voltage VAN and line voltage VAB are shown in Figure 5. FFT spectrum analysis was performed on the phase voltage VAN and line voltage VAB for the three modulation methods. Table 1 compares the harmonic orders of the output waveforms for different modulation methods.
(a) PD modulation
(b) MPS modulation
Figure 5 shows the output waveforms of phase voltage VAN and line voltage VAB.
(VAN/(kV/division); VAB/(kV/division); t/(5ms/division))
Table 1 Comparison of harmonic orders under different modulation methods
As shown in Table 1, when the frequency converter adopts PD modulation and MPS modulation respectively, the spectrum distribution of output phase voltage and line voltage is consistent with the calculation results of theoretical formula, which verifies the correctness of the Fourier series analysis of bivariate control.
The most prominent harmonic in the phase voltage spectrum of PD modulation is the first carrier component, with a high content of lower harmonics; the most prominent harmonic in the phase voltage spectrum of MPS modulation is the sideband harmonic near twice the carrier frequency, which is easy to filter out; the THD of the output phase voltage using PD modulation is 32.60 %, and the THD of the output phase voltage using MPS modulation is 32.92 %, with little difference between the two.
When the system is operating at low power, in the application of PD modulation, only the right bridge arm switches within one cycle per phase, while the left bridge arm does not switch. This causes an imbalance in the switching frequencies of the left and right bridge arm devices, affecting the thermal distribution of the power devices and limiting the inverter's output capacity. Switching the control signals between the left and right bridge arms at a fixed cycle to achieve thermal balance of the power devices increases the complexity of the control method and reduces the reliability of system operation. MPS modulation, on the other hand, can automatically achieve a balanced distribution of the equivalent switching frequencies of the power devices in the left and right bridge arms, which is beneficial to the safe and reliable operation of the system.
Experiments and Applications of 5HD8000 Frequency Inverter
Based on the modulation strategy studied in this paper, Hopewind Electric successfully developed the HD8000 series frequency converter, as shown in Figure 6. The main circuit structure of the system is shown in Figure 2. The system adopts a modular structure design, with 4.5kV /4kAIGCT power switching devices, a half-side rated DC voltage of 2.5kV , and an output frequency of 0-80Hz. The control system is implemented using the TMS320F28346 digital signal processing chip combined with a field-programmable gate array (FPGA).
Figure 6. Appearance of HD8000 series frequency inverter
Fig6TheappearanceofHD8000inverter
Figure 7(a) shows the no-load output voltage and current waveforms of the HD8000 series frequency converter (the line voltage is measured by resistance voltage divider, and the displayed value is 1/4 of the actual value). Figure 7(b) shows the full-current test waveform of the HD8000 series frequency converter under inductive load.
(a) Waveform of no-load full-voltage test
(1: Line voltage waveform; 2: Load current waveform)
(Voltage/(2kV/division); Current/(100A/division); t/(10ms/division))
(b) Waveform of full current test with inductive load
(1: Device terminal voltage; 3: Phase A load current; 4: Phase B load current)
(Voltage/(1kV/division); Current/(1kA/division); t/(5ms/division))
Figure 7 shows the experimental waveforms of the HD8000 series frequency converter.
Fig7ExperimentresultsofHD8000inverter
Figure 8 shows the output voltage waveforms of the left and right bridge arms of the inverter under different modulation coefficients. It can be seen that under low-frequency operating conditions, the phase-shift modulation method can still ensure the switching frequency balance of the left and right bridge arms.
(a) M = 0.99
(b) M = 0.3
Figure 8 shows the voltage waveforms of the left and right bridge arms of phase A under different modulation schemes.
(Voltage / (20V/division); t / (5ms/division))
Fig8ExperimentresultsofvoltagewaveformsfortwolegsofphaseAwithdifferentmodulationindex
The HD8000 series frequency converters have a rated output voltage of 6.6kV , a rated output current of 1.75kA , and an output capacity of up to 20MW. They can be widely used in high-power applications such as large fans and water pumps that do not require energy feedback. Currently, the HD8000 series frequency converters have been successfully applied in the oil and gas transportation field.
6 Conclusions
This paper analyzes and studies the topology and modulation strategy of the NPCH bridge five-level frequency converter. Fourier series analysis and simulation results show that the carrier in-phase cascade modulation method has high low-order harmonic content and is not suitable for low-level operating conditions, while the modulation wave phase-shifting method has better performance and adaptability. Load experiments on the Hopewind HD8000 series frequency converters verify the correctness and effectiveness of this modulation strategy, and this series of frequency converters has been successfully applied in the oil and gas transportation field.
