Design engineers have the luxury of several suitable topologies for Medium Voltage Drives (MVDs). Recently, one particular topology, the Cascade H Bridge, has come to the fore.
There are multiple topologies for higher voltage (≥2400 VAC) motor drives for designers to select from to suit the different technical or commercial requirements of the application. Recently, one particular topology, the Cascade H Bridge (CHB), has come to the fore. The basic operating principles of CHB-based drives will be explained in this article, along with a brief review of some suitable new power modules.
Design engineers have the luxury of several suitable topologies for Medium Voltage Drives (MVDs). These most common in use today include three level Neutral Point Clamp (NPC) type 1 using high voltage power modules, current source designs using reverse blocking semiconductors, five level T type and Modular Multi-Level (M2L), as well as others.
Although CHB was first developed in the early 1970s it was not until a decade later that Robicon USA commercialised the design. MVDs based upon the CHB topology are now quite common with multiple companies marketing drives based upon the technology. The Robicon brand survives in the product portfolio of Siemens Industry, which acquired Robicon in 2005.
The block diagram of one phase of a five-cell-per-phase CHB converter is shown in Figure 1. Figure 2 shows the detail of a single cell.
Figure 1: Overall schematic of one phase of a 5 cell per phase CHB converter.
Figure 2: Example of a typical CHB cell schematic.
At the heart of the cascade topology is a multiphase isolation transformer, as shown in Figure 1. This example has a medium voltage primary and a total of five isolated secondary windings, rated at 750VAC in this example. Each transformer secondary powers a single cell and all five cells are connected in series to create a complete phase. A complete three-phase drive requires two more phases with a total of 15 isolated secondary windings as shown in Figure 3.
Figure 3: Three phase system and showing unbalance with one phase being bypassed.
Each of the 15 cells includes:
- 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.
- 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.
- A capacitor bank to smooth out the rectified waveform and create a stable DC voltage source supply.
- The low voltage supplies required for the gate drivers and controls can be generated locally using a SMPS powered from the DC bus. Control signals to and from the drive central controller are via fibre 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 centre 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 deionised water to prevent leakage current and electro corrosion and deposition effects. An alternative is to use phase change cooling.
Figure 4a is a photograph of an 18-cell converter. Figure 4b is a photograph of an individual cell.
Figure 4a: MV3000 a 4160Vac 1500hp drive from WEG. Switchgear and transformer on the left and 18 cells in the centre.
Figure 4b: Air cooled cell showing rectifier modules (black) in the centre and 62mm modules(white) on the right and left.
- The power cell design is very similar to a normal AC drive except that for CHB operation, only two half bridges are used. This approach permits the use of high volume components intended for lower voltage converters.
- The output voltage can be increased by adding more cells and transformer windings through the modular approach.
- Redundancy is possible by bypassing cells, as shown in Figure 4. A bypass switch (either mechanical or solid state / thyristor) can short out any inoperable cells. The drive will function in an unbalanced condition as long as there is some voltage margin. If in a balanced condition is required, one cell on the other two phases can be shorted out.
- Very low input harmonics as the transformer secondary windings can be phase shifted. 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.
- As each cell is identical, spares inventory and cost is reduced. Maintenance is simplified as individual cells can be "pluggable."
- The transformer is relatively large and complex as shown in Figure 4a.
- Cells need galvanic isolation from each other and ground, thereby adding physical complexity and overall system size to meet relevant safety standards regarding separation.
- The single phase operation of the cell H Bridge requires a significant amount of smoothing capacitance.
- More complex liquid cooling.