Power Electronics

Modular Megawatt Control

The modular system of power conversion components employs a flexible design concept that consists of OEM components for thermal management, electrical, and mechanical interconnection and interfaces between power unit and control.

Representing a fast-growing field is the application of power electronic converters in the 0.1MW to 5MW range.

Decentralized power solutions such as UPS, active filtering, the implementation of decentralized generation of electric power and heat, and the use of wind energy for generation of electric power are driving development in this application field. Each application requires special expertise concerning systems design, control, and power management.

Basic topologies for power conversion are similar, so it makes sense to separate electric power engineering from control and systems engineering. Therefore, a modular power conversion component system based on a flexible design concept enables manufacturers to focus on their own core competence. Answering this need is the ModSTACK design kit for power converter solutions, which consists of OEM components for thermal management, electrical, and mechanical interconnection and interfaces between power unit and control. The designer can use them as building blocks for special-purpose systems.

Many power conversion applications in the multi kilowatt range have the basic structure shown in Fig. 1. Ac-dc, dc-ac and dc-dc conversion control the power flow at various points in the conversion chain. Dc buffers in the chain provide decoupling.

Systems like Fig. 1(a) are found in battery storage systems and fuel cell systems for heat and power. Variable speed power generators, for example, in wind energy converters, micro turbines, and diesel generators are mostly equipped with power conversion systems, as shown in Fig. 1(b).

The application determines a unidirectional or bidirectional energy flow. Fig. 2(a) shows bidirectional dc-dc converters and Fig. 2(b) shows dc-ac converters using half-bridge branches of IGBTs with freewheeling diodes.

Converter circuits shown in Fig. 2 contain only the semiconductor elements. But the “art” of power electronic circuits is the implementation of their elements into real electric circuits. Every bus bar, copper layer, and capacitor represents itself as a reactive component. ModSTACK design kits offer optimized inductances for commutation, including operation of all IGBTs in the SOAR (safe operating area rating). The design of ModSTACK components provides the flexibility for implementing different converter topologies for extending the power range and designing expandable power conversion systems.

Modular design is the key concept for minimizing the cost of IGBT ModSTACKs. This idea of modularity concerns all levels of these subsystems — mechanical design, control architecture, and power electronic layout.

The usual inverter topologies in the higher power range are attainable with high-quality IGBT modules. The dc link/buffer employs the appropriate switchmode, high-voltage electrolytic capacitors.

Depending on the application, flexible monitoring, and control units with integrated IGBT drivers are available. The supervising and controlling unit generates signals for control and monitoring. Fig. 3 is an overview of the functional components of a single back-to-back converter.

By combining the basic building blocks in different ways, you can obtain various topologies, including some resonant power conversion circuits. Designers can use a tested and approved power unit with signal interface, implemented according to international standards. This allows designers to focus on the special control and systems design problems of the application. To decrease costs, we divided the design of ModSTACKs in classes of power and voltage. Table 1 shows the main differences between the two voltage classes. The mechanical design of each class is almost fixed; however, variations are possible by using elements with different ratings and identical mechanical interfaces.

Mechanical Aspects

The modular stack system is for industrial-approved cabinets. Width of Size 1 is 600 mm and Size 2 is 800 mm, as is the one shown Fig. 4. This saves design and manufacturing costs. However, the internal components of the standard IGBT stacks can combine in different configurations for other customized solutions. The stacks are available with forced air cooling and water cooling. By using maximum extension for a 690V application, you can handle a maximum current of 1500A for air-cooled systems and 1800A for water-cooled systems with 800 mm cabinets. Table 2 lists the maximum ratings of the IGBT stacks.

To achieve necessary electrical balance for the system with parallel power conversion units, all electric connections, bus bars, and mechanical interfaces are prepared.

Monitoring and control circuits support the modular power electronic design. You can connect up to four identical units in parallel. Each unit contains an IGBT driver with short-circuit protection and monitoring signals that include:

  • DC link voltage
  • Output current
  • Heatsink temperature
  • Chip temperature (calculated)
  • Overvoltage (dc)
  • Overcurrent
  • Overtemperature
  • Auxiliary voltages OK

These signals are available at the internal control board of a power stack (Fig. 5).

Exceeding the preset limits for a power unit shuts it down and initiates a fault signal. To accelerate the reaction to overtemperature faults, an analog circuit calculates the virtual chip temperature. This takes into account the measured heatsink temperature, output current, dc link voltage, and switching frequency. Parameters of the calculation circuit include specific conduction and switching losses and the cooling system's equivalent thermal resistance.

Paralleling of units requires an additional control/supervising board to split the control signals and summarize the monitoring and measurement signals. For temperature monitoring, the systems control unit only receives the detected maximum. Fault signals are sent to the systems control if one unit detects violations of set limits. Additionally, a fault signal is sent when the nonsymmetry of the output current exceeds a specific tolerance.

Supply voltage for the auxiliary electronic boards for measurement and monitoring has a range 18Vdc to 30Vdc. A dc-dc converter delivers stabilized voltages to all subsystems, including voltage and current converters. The maximum rating for a unit is 2.5A at 24Vdc.

The standard interface for a control unit includes filters and sophisticated grounding. Grounding is an issue in every power electronic system. The grounding philosophy for interconnected components are compatible with each other, which minimizes any interference between power units and control units. To expand the signal-to-noise level, the system employs optocouplers with snap-in connectors for extreme attenuation of disturbances, small signal delay, high reliability of transmission, and long-term reliability for special applications.

To ensure bidirectional signal flow from systems control to parallel-connected power units, these units are designated by power unit identifiers. This enables a straight diagnosis and addressing of the units.

Paralleling of IGBTs or diodes is present within the IGBT modules. The influence of derivations in v-i-characteristics is negligible when a module consists of IGBTs and diodes of the same wafer lots. Variations of parameters inside single lots are very small, and all silicon chips are thermally close-coupled. This leads to minute differences in current splitting between the chips and modules.

ModSTACKs have additional inductors to balance the currents during switching of paralleled IGBT modules. The rated inductance is small but it supports a uniform spreading of switching losses to all units.
For more information on this article, CIRCLE 331 on Reader Service Card

For more information on this article, CIRCLE 331 on Reader Service Card

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