This article is a follow-up to "High-Pressure M2C Frequency Converter for New Oil and Gas Compressors (Part-1)". Several issues requiring attention in medium-voltage frequency converters, the principles, characteristics, and comparisons of H-bridge cascaded frequency converters (CHB) and modular multilevel frequency converters (M2C) have already been introduced there. This part introduces the low-frequency operation problems and countermeasures of M2C frequency converters, compares two other commonly used neutral-point clamped three-level frequency converters (3L-NPC) and five-level frequency converters based on three-level H-bridges (5L-HNPC) with CHB and M2C frequency converters, discusses their redundancy and reliability, and finally briefly introduces Siemens' GH150 product.
IV. Low-frequency operation problems and countermeasures of M2C frequency converters
M2C technology originated in 2001, and around 2006, Siemens pioneered the HVDCPLUS system based on this technology, which was successfully used in flexible DC transmission. The hope is to use M2C technology for variable frequency speed control, simplifying transformers and rectifier power supplies, but a technical obstacle remains—the low-frequency operation problem.
Depending on the device voltage, the 3L-NPC frequency converter has three voltage levels: 2.3kV, 3(3.3)kV, and 4.16kV. In China, only the 3(3.3)kV level is basically used. It uses 4.5kV devices, with a DC bus voltage of Vdc≈5kV and an output voltage step value of DVstep=Vdc/2≈2.5kV.
Advantages of the 3L-NPC frequency converter:
1) The circuit is the simplest and the number of components is the fewest.
2) It has a common DC bus, and the rectifier power supply and oil-immersed transformer are simple, making it easy to absorb or regenerate the braking energy of the motor. This is an advantage compared to CHB frequency converters, but not an advantage compared to M2C frequency converters, because both have a common DC bus.
3) The three-phase inverter uses a common DC energy storage capacitor, which is small in number (2). There is no low-frequency AC component in the DC bus current, and the total energy of the capacitor is also small (about 25% of CHB and 40% of M2C)[16]. It is a high-voltage film capacitor with a long service life and is centrally installed. Compared with the CHB inverter which uses electrolytic capacitors, it is an important advantage. The M2C inverter also uses film capacitors, but the number and total energy are large and they are installed in a decentralized manner.
The 3L-NPC frequency converter was one of the earliest medium-voltage PWM frequency converters on the market and was widely used abroad. my country imported a considerable number of these converters in its early days. After the introduction of the CHB frequency converter, this type of frequency converter was used less and less in my country for speed-regulating drives in oil-gas compressors and pumps where motor braking energy absorption is not required. The reasons for this are:
1) The output voltage does not reach 6kV, which does not meet my country's medium-voltage standard. Using a non-standard voltage without a specific reason is unlikely to be accepted by users.
2) The number of output voltage levels is small, and the PWM modulation frequency is low (the switching frequency of high voltage devices is <1kHz), resulting in a small number of PWM square waves and large harmonics in one fundamental cycle.
3) The output voltage step amplitude DVstep is high and the dv/dt is large, which leads to: ordinary medium-voltage motors cannot withstand it and need to strengthen the insulation (ABB requires [14] the minimum insulation level of 3.3kV motors to be 7.2kV, and the connection cable between the frequency converter and the motor is selected according to 6kV effective value/10kV peak value) and take anti-shaft current measures (insulated bearings and shaft grounding brushes); the additional current impact caused by the parasitic capacitance of the cable is large, and the allowable cable length is <300m. If an ordinary motor is used, a huge output filter needs to be installed at the output end of the frequency converter.
4) High-voltage switching devices are used. Although the number is small, the total chip area is larger than CHB (about 50%) and similar to M2C due to the thick chip, large on-state voltage drop and large chip area of the high-voltage devices[16].
Faced with the unfavorable situation of the 3L-NPC frequency converter in the Chinese market, ABB and Japan's TMEIC Corporation, in order to adapt to China's demand for 6kV, developed an improved product—the 5L-HNPC, a five-level frequency converter based on a three-level H-bridge—using their existing neutral-point clamped three-level technology and phase branch power components (PEBB).
Features of the 5L-HNPC frequency converter and its comparison with other frequency converters:
1) The 5L-HNPC can output 6 (6.9) kV, which meets the reserved voltage level in the national standard, but is lower than the 10 kV recommended by the national standard. For high-power devices with power close to or exceeding 10 MW, 6 kV is still too low.
2) Compared to the 3L-NPC, the 5L-HNPC increases the number of output phase voltage levels from 3 to 5, resulting in a reduction in harmonics. The 6kV M2C and CHB frequency converters (k=5) have 11 phase voltage levels, significantly higher than the 5L-HNPC. The National Electrical Control and Distribution Equipment Quality Supervision and Inspection Center, commissioned by the user, inspected the 5L-HNPC and concluded that the current output harmonic content was significantly higher, and the motor noise was considerably louder than during power frequency operation, which had a certain impact on the stable operation of the motor.
3) Compared to the 3L-NPC, the 5L-HNPC has an increased number of voltage levels, but because it still uses 4.5kV high-voltage devices, the output voltage step amplitude DVstep≈2.5kV remains unchanged, and the corresponding dv/dt also remains unchanged, only decreasing relative to the inverter's rated output voltage. Therefore, the motor and cables still require enhanced insulation. According to TMEIC's calculations: the inverter's maximum output voltage peak value = 4 × DVstep (see Figure 11), and the maximum overvoltage spike caused by dv/dt is 1 × DVstep. Therefore, the maximum peak voltage at the motor end = 5 × DVstep≈12.5kV, which translates to an effective value of ≈8.8kV. 11kV insulation is recommended.
4) Like the CHB, the energy storage capacitor of the 5L-HNPC is also connected to the DC input terminal of the single-phase inverter bridge. Both need to absorb the AC component of the DC bus current with a large amplitude of twice the output frequency. Their total energy per MVA of capacitor is about the same, much larger than that of the 3L-NPC. Unlike the CHB, the 5L-HNPC uses film capacitors due to its higher voltage.
5) Since the 5L-HNPC also uses high-voltage switching devices, its total chip area per MVA is similar to that of the 3L-NPC, larger than that of CHB, and similar to that of M2C[16].
6) The 5L-HNPC has six rectifier power supplies with 36-pulse rectification, resulting in very low grid-side current harmonics. However, due to phase shifts and amplitude errors between the multiple secondary windings of the rectifier transformer, the actual harmonic reduction effect is not significantly different from that of 30 or 24-pulse rectification. The large number of rectifier power supplies negates the advantage of the 3L-NPC in easily achieving braking energy absorption or feedback. The 5L-HNPC uses an oil-immersed transformer.
7) A review report by the American IEEE pointed out [2] that compared with the CHB frequency converter with two-stage series connection (k=2) using 4.5kV devices, the output voltage magnitude and waveform, number of levels, number of fully controlled devices, number of rectifier power supplies, transformer complexity and dv/dt of the 5L-HNPC are the same as those of CHB, but it uses 12 more clamping diodes, which is not an advantage. ABB and TMEIC chose this scheme because they already had mature 3L-NPC technology and phase branch power components (PEBB).
Although the 5L-HNPC is only an improvement on the 3L-NPC and has not completely solved its problems, it is still welcomed by many users in China and occupies a place in the field of high-power energy-saving speed regulation.
VI. Redundancy and Reliability
M2C and CHB are cascaded frequency converters, consisting of multiple power units connected in series. To improve reliability, one more unit (k+1 redundancy) or two more units (k+2 redundancy) can be added in series during the design phase. In the event of a unit failure, the unit is removed via a bypass switch, allowing the frequency converter to continue operating. This technology is called "unit redundancy and bypass." It requires minimal additional equipment and investment, yet yields significant results. It is particularly valuable for applications with high production continuity requirements where downtime due to failure would cause substantial losses (such as oil and gas compressors, metallurgical and mining fans).
In 2000, Maratbon Asbland Petroleum Company in the United States used this technology in the motor drive system of an oil and gas compressor[15]. The project used CHB frequency converters with a power of 5500Hp/voltage of 4.16kV. Originally, only 4 stages were needed in series. In order to achieve the goal of continuous operation of the frequency conversion speed regulation system for 5 years, it added 2 more stages, for a total of 6 stages. When the number of phase fault units is ≤2, the frequency converter can still output the rated voltage; when the number of phase fault units is >3, the frequency converter reduces the maximum output voltage. In order to maintain the three-phase output line voltage balance after removing the fault units, there are two bypass control methods:
1) Symmetrical bypass: Bypass the same number of fault-free units in the other two fault-free phases, and maintain the phase voltage difference between the three phases at 120°. This method is simple and suitable for when the number of phase fault units is ≤2. If the number of phase fault units is >3, this method is still used. Although the three-phase voltage is balanced, the maximum output voltage of the frequency converter drops significantly.
2) If the midpoint offset only bypasses the faulty unit, the phase relationship between the three phase voltages can still be maintained by appropriately adjusting the phase relationship between the three phase voltages.
The phase output line voltage is balanced, and the voltage drop is reduced[15].
The bypass process waveform of the CHB frequency converter is shown in Figure 12. The current-free time is about 250ms. During this period, the motor maintains free operation by mechanical inertia, and the speed drops by about 10%. It recovers after the bypass process ends.
Figure 12 shows the bypass process waveform of CHB.
To achieve "unit redundancy and bypass," Siemens also connects a bypass switch in parallel across each power unit of its M2C frequency converter (Figure 13a), and developed a dedicated fast bypass contactor for this purpose (Figure 13b). This contactor remains in an active state throughout the frequency converter's operation, with an operating time of approximately 625ms, and the entire bypass operation time is <1ms. Due to the extremely short operation time, the current remains stable and unchanged during bypass operation, resulting in no torque or speed reduction ("seamless" bypass operation), as shown in Figure 14.
a) Frequency converter b) Bypass contactor
Figure 13 shows an M2C frequency converter with a bypass switch and its bypass contactor.
Figure 14 shows the inverter output voltage and current waveforms during bypass operation.
It should be noted that unit bypass technology is used not only when there is power unit redundancy, but also when there is no power unit redundancy. If there is no redundancy, even if only one unit fails and is bypassed, the maximum output voltage of the frequency converter will drop. Using midpoint offset control can reduce the degree of voltage drop.
3L-NPC and 5L-HNPC inverters are not composed of multiple smaller power units connected in series or parallel, making redundancy difficult to achieve. To achieve k+1 redundancy in a 3L-NPC inverter, 12 additional 4.5kV high-voltage IGBTs and diodes are required; to achieve k+2 redundancy, 24 additional 4.5kV high-voltage IGBTs and diodes are needed, resulting in significant costs and operational complexities. Although some lower-cost fault-tolerant operating solutions exist, they have not been implemented due to control complexity and significant output voltage drops. Redundancy in the 5L-HNPC inverter is even more difficult to achieve and has not yet appeared on the market.
3L-NPC and 5L-HNPC frequency converters use 4.5kV high-voltage components in smaller quantities, while cascaded M2C and CHB frequency converters use 1.7kV low-voltage components in larger quantities. Generally, frequency converters with fewer components are more reliable. However, from the perspective of the components themselves, low-voltage components are more technologically mature, widely used, have more experience, and are more reliable than high-voltage components. So, which type of frequency converter is more reliable?
To compare the reliability of commonly used frequency converters in the oil and gas field and to understand the redundancy effect, reference [16] provides quantitative analysis results of the mean time between failures (MTBF) of four 6.6kV medium-voltage frequency converters: M2C, CHB, 3L-NPC and 5L-HNPC.
Mean Time Between Failures (MTBF) = 1/l
In the formula, l represents the total failure rate, which equals the total number of failures of all components over 109 hours. The calculation of the failure rate includes the grid-side transformer, grid-side rectifier, motor-side inverter, cooling unit, and inverter control system; the motor itself is not considered. To make the calculation results more intuitive, this literature uses relative MTBF when presenting the comparison results, with the MTBF value of the non-redundant 3L-NPC inverter set at 100%, and other inverters compared to it. The comparison results are as follows:
1) The relative mean time between failures (MTBF) of the four types of frequency converters without redundancy is shown in Figure 15. They are not much different from each other and are all relatively reliable.
Figure 15 Relative MTBF of a Non-Redundant Frequency Inverter
2) Cascaded M2C and CHB frequency converters with k+1 and k+2 redundancy show significantly improved MTBF, with remarkable effects and reliability far exceeding that of 3L-NPC and 5L-HNPC frequency converters. Redundancy in the 3L-NPC frequency converter is costly and its effects are not significant; k+2 is actually inferior to k+1 due to the complexity of implementing redundancy. The 5L-HNPC frequency converter is not suitable for redundancy.
Figure 16 shows the relative MTBF of frequency converters with and without redundancy (k+1 and k+2).
3) If the control system also has redundancy, the MTBF can be increased by 16-20%.
VII. Siemens GH150 series M2C frequency converters
Siemens' GH150 series frequency converters were the first M2C frequency converters to enter the speed control market. Currently, only research reports on similar frequency converters from other companies have been seen, but no products have been produced. Because the low-speed torque of M2C speed control systems is less than the rated torque, Siemens has limited the application of the GH150 to motor drives of high-power oil and gas compressors, pumps, and fans, which have a limited speed range and low starting torque.
Both the M2C and CHB frequency converters are based on cascaded power units and share the same excellent output performance: high output voltage, low harmonics and dv/dt, compatibility with common motors, suitability for long cable operation, and support for power unit redundancy. Siemens already has the CHB frequency converter series GH180, which is well-known and widely used. The reason for launching the M2C is to achieve simplification and flexibility in the grid-side transformer and rectifier power supply at the cost of approximately 40% increase in the total area of the device chips. The M2C has a common DC bus, which can absorb the motor's braking energy by adding a braking unit and resistors, thus achieving rapid braking. This performance is crucial for fans with high mechanical inertia.
The current specifications of the GH150 inverter are as follows: power 4-13.3MVA, expandable to 30MVA; voltage 4.16-7.2kV; uses cost-effective and reliable 1.7kV low-voltage IGBTs; 6-7.2kV inverter cell (dual module) count = 36, cascade count k = 6; output voltage levels = 13 (phase) 25 (line); water cooling, unpurified water inlet temperature 35℃, maximum 47℃. Planned specifications (single unit): power 52MVA; voltage 12kV. The grid-side transformer and rectifier power supply of the GH150 can be flexibly configured according to user needs: the transformer can be dry-type or oil-immersed, indoor or outdoor; rectifier power supply pulse count 12-36. The appearance of the GH150 inverter cabinet and power unit (cell) is shown in Figure 17.
a) Variable frequency cabinet
b)cell
Figure 17 shows the appearance of the GH150 frequency converter cabinet and cell.
Some application examples of GH150:
Part-2 Conclusion
Siemens has launched the GH150 series speed inverter based on M2C technology. While achieving the same excellent performance as CHB, it simplifies and increases the flexibility of the grid-side transformer and rectifier power supply by increasing the total area of the device chips by approximately 40%. The principles, characteristics, and comparisons of CHB and M2C have been introduced in Part 1 of this paper. This Part discusses the following issues:
The A.M2C has a low-frequency operation problem, which can be solved by injecting harmonics. However, its low-speed continuous operating torque is less than the rated torque, making it suitable for speed control applications such as compressors, pumps, and fans.
B. The article introduces two other commonly used medium-voltage frequency converters, 3L-NPC and 5L-HNPC, and compares them with M2C and CHB. 3L-NPC is unsuitable for equipment such as compressors, pumps, and fans due to its output voltage not meeting national voltage standards, high harmonics and dv/dt, and the need for special motors. 5L-HNPC is merely an improvement on 3L-NPC and does not completely solve its problems.
C.M2C and CHB are cascaded frequency converters, and their reliability can be significantly improved through "unit redundancy and bypass" technology. To this end, Siemens has developed a dedicated fast bypass contactor to achieve "seamless" bypass operation of the M2C without speed or torque reduction. This paper presents quantitative analysis results of the mean time between failures (MTBF) for four medium-voltage frequency converters.
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