1 Introduction
If the intermediate DC link of an AC-DC-AC frequency converter uses a large capacitor for smoothing, it is generally called a voltage source frequency converter. If referred to separately, its downstream inverter section is called a voltage source inverter (VSI), which is also the terminology used in GB and IEC standards. Its front-end rectifier section is a harmonic source for the power grid, hence it is called a voltage-source harmonic source. In contrast, if the intermediate DC link of an AC-DC-AC frequency converter uses a large inductor for smoothing, it is respectively called a current source frequency converter, a current source inverter (CSI), or a current source harmonic source. The reason for distinguishing between voltage source and current source frequency converters is that their AC input current waveforms and the waveforms and performance of the AC voltage and current output after frequency conversion are significantly different.
2. Voltage source inverter (VSI)
Almost all low-voltage frequency converters used in China are voltage source type, with intermediate DC voltage smoothed by capacitors. The DC voltage is relatively stable. The voltage waveform output by the inverter depends on the control and modulation method of the inverter, and can be broadly divided into two types of voltage waveforms.
2.1 Rectangular wave voltage output
If the output is dual, it can also be a "convex" shaped voltage wave. In short, it is far from a sinusoidal shape. That is to say, in addition to the fundamental wave, there are many harmonic voltages in the voltage waveform. The current generated under this voltage waveform depends on the impedance of the motor (which is connected in series with a branch cable) (fundamental impedance and harmonic impedance). The fundamental current can be obtained by dividing the output fundamental voltage component by the fundamental impedance, and the harmonic current can be obtained by dividing the output harmonic voltage component by the harmonic impedance. The fundamental impedance of the motor is inductive, so its harmonic reactance xh is h times the fundamental reactance x1 (h is the harmonic order of each harmonic). The harmonic voltage component of the rectangular wave voltage is 1/h of the fundamental component. Therefore, the harmonic current obtained by outputting a rectangular wave voltage is approximately 1/25 = 4% of the fundamental current for the 5th harmonic and approximately 1/49 ≈ 2% for the 7th harmonic. Although the harmonic current component is not large, it still has a certain negative effect on the motor. The harmonic components output by the frequency converter are seriously harmful in terms of harmonic voltage, which manifests as voltage peak value and voltage rise rate dv/dt. It threatens the phase-to-phase insulation, ground insulation and turn-to-turn insulation of the motor, mainly the first few turns at the motor inlet. This problem is more prominent for high-voltage motors, as has been discussed in the literature.
Inverters with rectangular or "convex" shaped voltage outputs are now rare.
2.2pwm modulated wave voltage output
This is the output voltage waveform of the most common frequency converters today (both low-voltage and high-voltage). Due to the use of sinusoidal modulation (SPWM) or other superior modulation methods, the output voltage waveform is close to a sine wave (referring to the envelope of the modulation wave). However, the dv/dt of each individual modulation wave is larger because the modulation frequency reaches thousands of Hz. To reduce losses and heat generation in power electronic devices, high-speed switching devices are used. Not only is the dv/dt larger each time, but it is also repeatedly increased. Due to the traveling wave phenomenon, the peak voltage applied to the motor terminals is also higher (not exceeding twice the DC intermediate voltage). As for the output current waveform, compared to the rectangular wave current output in the previous section, the harmonic current components are smaller, and the current waveform is relatively closer to a sine wave. This is the reason for using PWM modulation. However, the threats of du/dt and voltage peaks still exist, and are even more severe. In addition, there are many other adverse effects on the motor, such as shaft current.
2.3 Countermeasures
To reduce the severity of surges in the inverter output, under certain conditions, the following countermeasures (along with their effects) can be taken: (see IEC standards for details)
(1) Changing the length of the motor cable and grounding the cable will change the surge amplitude at the motor end, although this measure is often difficult or impractical.
(2) Use cables with higher dielectric loss (e.g., butyl rubber or paper insulation). Special cables with iron shielding are also acceptable. These methods will reduce oscillations and improve electromagnetic compatibility (EMC) performance.
(3) If a problem occurs between phase and ground, the grounding configuration can be changed.
(4) Installing an output reactor can increase the peak rise time. Its combined effect with the cable capacitance will reduce the peak voltage of the traveling wave. At this time, the increased voltage drop across the reactor should be considered.
(5) Installing an output dv/dt filter can significantly increase the peak rise time. This measure can increase the cable length.
(6) Installing an output sine wave filter can increase the peak rise time. The feasibility of this approach depends on the required characteristics of the object, especially the speed range and dynamic performance. There are two types: Type i can reduce phase-to-phase and phase-to-ground voltage stress simultaneously; while Type ii can only reduce phase-to-phase voltage stress. In addition, this filter can reduce EMC interference and additional losses and noise of the motor, and standard unshielded cables can be used after using a Type i filter.
(7) Installing a terminal unit near the motor end can suppress overvoltage at the motor port.
(8) Reduce the voltage amplitude of each pulse step, for example, by using a three-level or multi-level converter.
3 Current Source Inverter (CSI)
Among the products available in the domestic market, only AB's high-voltage frequency converters use this CSI solution. Other brands' high-voltage frequency converters and all low-voltage frequency converters do not use this CSI solution. A newly published book in China discusses this solution extensively. This solution has unique technical principles. In order to understand its inner essence, it is worthwhile to explore it in order to compare its performance with that of voltage source inverters.
The difference in the construction of CSI is that a large inductor is used to smooth the DC link after rectification, so the DC current is more stable, hence the name current source type (but not constant current).
3.1 Rectangular Wave Current Output
The earliest circuit schemes used thyristors in series with diodes, employing forced commutation. Another method, driving synchronous motors, used load commutation. Since these are rarely used today, their circuit principles will not be discussed further; only their external characteristics will be addressed below. Among scientific books, the literature that most frequently introduces the characteristics of CSI is the following. The main characteristics of CSI are as follows:
(1) The intermediate DC current is basically pulsating, and the DC circuit exhibits high impedance;
(2) The AC output current is a rectangular wave and is independent of the load impedance angle;
(3) The output voltage waveform and phase on the AC side are determined by the load impedance;
(4) When the AC side is a resistive-inductive load, reactive power needs to be provided. The current does not reverse to provide feedback reactive power. Therefore, it is not necessary to provide anti-parallel diodes to the switching devices like a voltage-type inverter. The DC side inductor can store and release reactive power.
(5) Similarly, functional quantities can be fed back to the AC power grid through the controllable thyristor bridge, so there is no need to set up a separate inverter bridge circuit for feedback to the power grid.
(6) Requirements for triggering signals: For DC links, there must always be a current flow path and no open circuit; for AC links, there must be no short circuit path.
Why is the output AC current a rectangular wave? Because there is a large inductor on the DC side, which can stabilize the DC current (but not a constant current). Why is the output AC voltage waveform determined by the load impedance? This is because v=iz, where i is a rectangular wave with a width of 120° in both forward and reverse directions (or possibly a convex wave with a width of 120°), and z is the load inductive reactance, which can be decomposed into the fundamental frequency and characteristic harmonics. The load on the AC current side is a motor, and its load characteristics are resistive-inductive load. For each harmonic, the harmonic reactance is h times the fundamental reactance, where h is the characteristic harmonic order, such as 5, 7, etc. However, it should be noted that the large inductor on the DC side is equivalent to a very large internal reactance of the power supply for each harmonic. There will be a large harmonic voltage drop across this large inductor. As a result, although the output AC voltage waveform is not a sine wave, it is definitely not a rectangular wave either, but is closer to a sine wave. The reason for this is that most of the harmonic voltage is removed by the large DC inductor.
3.2pwm modulated wave output
The fundamental current waveform of the modulated wave is a rectangular wave because it is a current source. After PWM modulation, the envelope of the current waveform is initially close to a sine wave, but there will still be high-frequency current waves generated by the modulation frequency. These high-frequency current waves will also be suppressed by the large inductance of the intermediate DC link. Due to the high frequency, the suppression effect is stronger. Therefore, the AC output, whether it is a current wave or a voltage wave, is close to a sine wave. The basic reason should be the result of the large inductance suppressing characteristic harmonic components and high-frequency components.
In high-voltage frequency converters, besides the output voltage amplitude, the most serious threat to the motor is the dv/dt of the output AC voltage. This high dv/dt value is essentially a high-frequency voltage component. As analyzed above, due to the suppression effect of the large DC inductor, the dv/dt value is greatly reduced.
3.3 Filtering effect of output and input capacitors
A set of parallel capacitors is provided at the output of the pulse width modulation (CSI-PWM) current source inverter. This capacitor is designed to provide a current path during the commutation process (because the DC circuit inductance is very large, the current cannot be turned off and an alternative path should be found). This bypass capacitor has a smaller impedance to the harmonics and high-frequency components of the current, respectively (at the same time, a small fundamental frequency component also flows through the parallel capacitor), thus also playing a certain filtering role, making the current flowing to the motor closer to a sine wave. Similarly, a set of parallel capacitors is also required at the input of the AC power supply, but it is easy to generate lc series resonance with the inductor in the power grid system. In order to avoid resonance, the product manufacturer must take suppression measures. Reference [7] introduces a low-loss active damping scheme.
4. Harmonic current on the grid side of the frequency converter
This harmonic current is independent of the inverter circuit; it depends only on whether the input rectifier circuit before the frequency converter uses capacitors or inductors for smoothing the intermediate DC current. PWM rectification will not be discussed here. PWM rectification has excellent performance, can operate in four quadrants, has high cosφ, and low harmonics, but it generates high-frequency interference to the grid (related to the modulation frequency). The main problem is its higher price. This discussion will only cover the harmonics output to the grid by commonly used three-phase or multi-phase rectifiers.
4.1 Harmonics of Voltage Source Inverters
The intermediate DC link uses a large capacitor for smoothing, which can only stabilize the DC voltage. However, this large capacitor has a low impedance to the fluctuating input, resulting in a large harmonic component in the input current. The IEC standard has listed the data for this harmonic component in a table as shown in the attached table.
The following characteristics can be observed from the attached table:
(1) Harmonics are characteristic harmonics, which are only related to the number of rectifier pulses. For example, a three-phase symmetrical bridge rectifier has 6 pulses and the lowest harmonic order is 5. If it has 18 pulses, the lowest harmonic order is 17 (theoretically there are no low-order harmonics such as 5, 7, 11, 13). Therefore, high-power rectifiers often use multi-phase rectification, that is, the transformer has multiple auxiliary windings with phase angles shifted. Moreover, the higher the harmonic order, the smaller the relative value of the harmonics.
(2) The magnitude of each harmonic is positively correlated with the short-circuit capacity of the system at the inverter input. The smaller the short-circuit capacity, the smaller the harmonic quantity. Therefore, it is required to connect an input reactor with a relative reactance value of 4% (x%) before the inverter input. For low-voltage inverters, manufacturers generally provide this as a complete set. The same principle applies to high-voltage inverters, and the values in the appendix are also applicable. x% cannot be too large or too small.
(3) Compared with the current-source inverter below, the harmonic current of the voltage source inverter is much larger under the same conditions. This point will be further analyzed in Section 4.3 below.
4.2 Harmonics of Current Source Frequency Converters
The intermediate DC link uses a large inductor, which has a large internal reactance for fluctuating current. Therefore, the harmonic components in the inverter input current are relatively low, and it has the following characteristics:
(1) ih/i1=1/h
In the above formula: i1 - fundamental current, determined by the load size; ih - the h-th harmonic current in the characteristic harmonics.
It can be seen that the higher the harmonic order h, the smaller the current, which is inversely proportional to h. For example, the 5th harmonic is only 20% of the fundamental current.
(2) Similar to point (1) of the voltage-type harmonic source, the harmonics are also characteristic harmonics. If multiphase rectification is used, such as 18 pulses, the lowest harmonic order is 17th, and there are no harmonics below the 13th order.
(3) When the short-circuit capacity of the inverter input terminal is reduced, the harmonic current is reduced slightly, but the change is not significant.
4.3 Comparison of harmonics between voltage sources and current sources
The above analysis shows that for ordinary rectification, both types of harmonics are characteristic harmonics. Multiphase rectification can eliminate lower-order characteristic harmonics. The higher the order of the harmonic, the smaller its value. However, for the same harmonic, the harmonic current from a voltage source is much larger. Taking the 5th harmonic as an example, the relative value of the harmonic current from a current source is 1/5, approximately 0.2, while the harmonic current from a voltage source is 0.3. This is conditional: RSC = 20, meaning an input reactor needs to be connected in series before the inverter input, and its relative reactance plus the reactance of the power supply system (mainly the transformer reactance) must equal 5%. Current source inverters do not require an input reactor to limit characteristic harmonics.
Comparison of dynamic performance of output current of 5 frequency converters
Some argue that current source inverters have better output current speed, but I disagree. Their speed is definitely inferior to voltage source inverters for the following reasons: If you need to instantaneously increase the output current:
(1) Change the modulation law of PWM on the inverter side to improve the DC voltage utilization rate. If the output is a square wave, then there is nothing that can be done.
(2) Increase the intermediate DC voltage value from the input AC side, for example when the input rectifier bridge uses a controllable or semi-controllable device (thyristor).
However, even if both measures are used at the same time, the current rise rate is greatly suppressed due to the large reactor in the intermediate DC link. The voltage source inverter is the opposite. The large capacitor in the middle parallel is a low impedance. Whether it is receiving energy from the grid or outputting energy to the inverter and motor, it has almost no obstruction effect. As long as there are control measures, it can respond quickly.
Based on this analysis, current source frequency converters are not suitable for machinery with high dynamic performance requirements, such as rolling mills and hoists. However, the low current rise rate also has advantages, namely, in case of a short circuit, electronic overcurrent protection is easy to take effect, and the inherent property of low current rise rate can be fully and appropriately utilized.
6. Overall Performance Comparison
Currently, voltage source inverters are the dominant mainstream in low-voltage products and also in high-voltage products above 1kV. This is an indisputable fact, and the situation is not expected to change in the future. This is because voltage source inverters have strong performance versatility, are suitable for various loads with different requirements, and their design and production technologies are relatively mature and generally mastered by most manufacturers. However, high-voltage inverter products are still under development. The main problems that still exist include: the need for further development of fully shut-off power electronic devices for high voltage and high current, and the limited ability of motors to withstand high dv/dt. Therefore, three-level or multi-level voltage source high-voltage inverters are a realistic and feasible solution. In order to obtain a reliable and economical three-level or multi-level solution, different topologies are still under research and development.
Current source inverters are not suitable for loads with rapidly changing load conditions. Their advantage is that the two-level scheme has dv/dt output that does not harm the motor. If high-voltage, high-current fully shut-off devices can become widely available at a reasonable price in the future, their development momentum may increase.
7. Conclusion
As can be seen from the above analysis, voltage source and current source high-voltage frequency converters each have their own development space and are currently in a competitive situation regarding cost-effectiveness. The purpose of this article is only to conduct some preliminary analysis and comparative discussion on the technical performance of the two types of high-voltage frequency converters, which may be helpful for a deeper understanding. Criticism and corrections are welcome.
