Operation of Power Silicon for Cascade Cell Based MV Motor Drives
For standard low voltage (≤690Vac) motor drives the IGBT based voltage source 2 level topology dominates the landscape. However at higher voltages (≥2400Vac)
For standard low voltage (≤690Vac) motor drives the IGBT based voltage source 2 level topology dominates the landscape. However at higher voltages (≥2400Vac), the situation changes and there is a plethora of different topologies, each one with unique technical and/or economic advantages. One of these topologies, often referred to as cascade, or Cascade H Bridge (CHB), has increased in popularity in recent years. This article will explain the basic operating principles of the CHB drive and showcase some new power modules matched to this type of converter.
For Medium Voltage Drives (MVD’s), there are several competing technologies: three-level Neutral Point Clamp (NPC) type 1 using high voltage power modules (1), current source designs using reverse blocking semiconductors (2), five-level T type (3), and Modular Multi-Level (M2L) (4), to name some of the more common types available today in the market. The history of the CHB topology is that it was invented in the early 1970’s but first commercially brought to the market by Robicon USA (now owned by Siemens) in the early 1980’s. In recent years numerous companies have introduced CHB based MVD’s. The overall topology of one phase is shown in Figure 1 and a more detailed schematic for an individual cell in Figure 2.
The key to the cascade topology is a multiphase isolation transformer, see Figure 1, here shown with a medium voltage primary and 5 isolated secondary windings, in this example each rated at 750 Vac. Each transformer secondary powers a single cell and the 5 cells are “daisy chained” together in series to make a complete phase. Two other phases are used to build a complete three-phase drive with a total of 15 isolated secondary windings. See Figure 4.
Figure 1: Overall schematic of one phase of a 5 cell per phase CHB converter.
Figure 2: Example of a typical CHB cell schematic.
The detailed schematic of a typical cell is shown in Figure 2. It comprises of:
- A three-phase input uncontrolled rectifier with a circuit to provide “soft” charging of the capacitor bank when the main power is applied. Note this soft charging can be performed with a separate winding on the main transformer, this allows the drive to be “control powered” without the grid three-phase medium voltage being applied to the primary of the transformer
- A capacitor bank to smooth out the rectified waveform and create a stable DC voltage source supply.
- An “H bridge” using 4 IGBT’s and anti-parallel diodes. These can be switched in sequence to apply either positive, zero or negative voltage to the output terminals and the total phase voltage is the sum of the voltage of all 5 cells.
- The low voltage supplies required for the gate drivers and controls can be generated locally using an SMPS powered from the DC bus. Control signals to and from the drive central controller are via fiber optic cables for voltage isolation. This allows the complete cell electronics assembly to “float” with respect to earth potential.
- The heat sink cannot be connected to earth potential as the IGBT modules and other components do not have the required isolation rating. Typically the heat sink is connected to either to an artificial center point of the DC bus or to the DC negative bus, often using a low ohmic resistor to damp out any potential high-frequency oscillations. For liquid cooling, it is required to use deionized water to prevent leakage current and electro corrosion and deposition effects. An alternative is to use phase change cooling (5).
Figure 3a shows a typical converter with a transformer and 18 cells. Figure 3b shows an individual cell.
Figure 3a: MV3000 a 4160Vac 1500hp drive from WEG. Switchgear and transformer on the left and 18 cells (shown in detail in Figure 3b) in the center.
Figure 3b: Air-cooled cell showing rectifier modules (black) in the center and 62mm modules(white) on the right and left.
- From Figure 2, it can be seen that the power cell design is very similar to a standard AC drive except that only two half bridges are used. This allows for the use of components that are manufactured in high volume for lower voltage converters.
- Modular design allows for flexibility, as the output voltage can be increased by adding more cells and transformer windings.
- Cells can be bypassed to allow for redundancy. See Figure 4. If one cell is inoperable for any reason, it can be shorted out using a bypass switch, see Figure 2. This switch can be mechanical or semiconductor-based using a pressure contact technology-based thyristor module. As long as there is some voltage margin, the drive can still operate either in an unbalanced condition, or by also shorting out one cell on the other two phases, in a balanced condition.
- Very low input harmonics as the transformer secondary windings can be phase-shifted using, for example, a zig-zag transformer design.
- Motor current harmonics are very low and the dv/dt applied to the motor windings is reduced. The H bridge operates at low switching frequencies typically 500Hz – 1kHz as the number of cells multiplies the effective switching frequency seen by the motor windings.
- Simplified maintenance as the individual cells can be light enough to be handled by two people and made pluggable, see Figure 3b. Also as each cell is identical it reduces spare parts inventory.
Figure 4: Three phase system and showing unbalance with one phase being bypassed.
- The transformer is large and complex. See Figure 3a.
- Cells need to be isolated from each other and ground. This adds mechanical complexity and size to meet clearance standards.
- Requires a high value of smoothing capacitance due to the single-phase operation of the cell H Bridge.
- More complex liquid cooling.
The H bridge can be switched to generate three different voltages at the output, for a positive voltage, S1 and S4, see Figure 2, are turned on, for a negative voltage S2 and S3 are turned on. For a zero voltage state, either S1 and S2 or S3 and S4 can be turned on.
In the simplest mode of operation the output sine wave can be built up by, for example, firing each cell on in sequence with Cell E first, then D all the way up to A with each step increasing the output voltage by ≈1000 VDC and PWM switching used at each level to shape the waveform to a sine wave. This is referred to as a level shift modulation, but it has the drawback that the power semiconductors in each cell do not see equal losses and the transformer windings do not draw equal current which does not minimize the input harmonic currents.
Another method, referred to as angle or phase shift modulation, is shown in Figure 5, and does create equal losses across the cells. The pattern uses a standard common sine wave reference for all cells, but phase shifts the PWM reference by a factor of the number of cells divided by 180°, in this case with 5 cells, 36°. This staggers the PWM patters and prevents two cells switching at the same time, it also shares the zero voltage states equally between the two possible choices. Figure 5 shows the phase shift between cells A and D at 3 x 36 = 108°. Now as with regular drives there are numerous nuances of PWM pattern generation, such a space vector modulation, all with targeted technical advantages.
Figure 5: Angle or phase shift cell switching pattern for a half positive sine wave.
The losses are dependent on the type of switching pattern used. However, a simple approximation to the losses can be made using an online tool such as the Infineon’s Iposim and estimating the losses assuming a single-phase operation. A rough “rule of thumb” is that at a 60Hz fundamental output frequency, 1 kHz switching frequency on an air-cooled heat sink the IGBT modules should be capable of an RMS output of 0.6 - 0.7 x DC current rating of the module. So for a 300A module, the RMS current can be ≈ 180 – 210 ARMS.
Power Silicon Options
For the three-phase rectifier standard 20mm, 34mm and 50mm dual diode modules are available in both solder bond and pressure contact technology. See Figure 6.
Figure 6: Infineon 20mm, 34mm and 50mm Rectifier modules.
For the H bridge, 1700V IGBT’s devices are often selected as this allows a higher cell voltage, up to a maximum of ≈1200Vdc enabling fewer cells in series. At the same time, 1700V silicon and peripheral components such as rectifiers and bus capacitors are readily available due to their use in 690Vac rated drives. Infineon has increased its portfolio of IGBT modules in industry-standard packages to match the requirements of CHB drives as shown in Figure 7. Dual modules in the 62mm or EconoDUAL™3 packages are available up to 600A and complete H bridges, for a more compact design, are available in the EconoDUAL™3 and Econo 3 packages. The later suitable for PCB based designs using press-fit technology.
Figure 7: Modules options offered by Infineon tailored for a cascade cell H bridge.
For the power electronics design engineer, the use of a CHB topology has provided a lower technical entry point into the MVD market by utilizing the basic power converter components manufactured in higher volume for the lower voltage converter market. This and the performance advantages listed have fueled the growth of this topology in the MVD market.
About the Company
Infineon Technologies AG is a Germany-based designer, developer, and manufacturer of semiconductors and related system solutions. The Company operates through four segments: Automotive, Industrial Power Control, Power Management & Multimarket, and Chip Card & Security. The company is a world leader in semiconductor solutions that make life easier, safer, and greener.
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This article originally appeared in the Bodo’s Power Systems magazine.