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Posted on 20 November 2019

Modular Inverters for High Power Applications

SKYPER 32 PRO core driver

 

High power configurations to offer ratings from 320 to 1550A

Modular power systems must offer user benefits in terms of shorter lead times and lower costs. The modular SEMIKUBE B6CI is a compact, easy to maintain and flexible system for forced air-cooled inverters from 300A up to 1550A.

Built around the latest IGBT technology for industrial modules by Semikron, this inverter/converter platform is also the result of 45 years of experience which Semikron has in the power stack business. A global network of solution centres provides worldwide support for local service with development and production capabilities as close as possible to the customers’ market.

Designers of many high-power systems, particularly in developing market sectors such as distributed power generation, often act as systems integrators and in some cases may not have interest in the detailed knowledge of power electronics. The speed of development of silicon makes it necessary to adapt all interfacing components, such as heatsinks, drivers, capacitors, snubbers and busbars. These designers need reliable system building blocks to serve their power needs, but they also need versatile solutions, which can best be provided by a modular approach. Modular power systems can also offer user benefits in terms of shorter lead times and lower costs.

Semikron has used modular techniques to develop SEMIKUBE – an inverter/converter platform for applications up to 900kW with forced-air cooling. The modular approach reduces the design time and simplifies on-site installation. It also reduces the space required for spares storage, because a small number of standard parts can be kept in stock and used in various configurations as required. The new platform can accommodate cubes in compact or high-power configurations to offer ratings from 320 to 1550A. In the highpower configuration, a system fan is attached to each module, thus eliminating “thermal stacking” effects by providing a flow of ambient-temperature air to each module in the system. This approach offers a power/volume ratio previously only available from watercooled systems.

SEMIKUBE range

Figure 1. SEMIKUBE range

Model ratings of the SEMIKUBE range

Figure 2. Model ratings of the SEMIKUBE range

SKYPER 32 PRO core driver

Figure 3. SKYPER 32 PRO core driver is incorporated into each cube

An inside view of SEMIKUBE 1,2 und 3.

Figure 4. An inside view of SEMIKUBE 1,2 und 3

A SEMIKUBE usually comprises one, two or three cubes, each of which contains two, three or four blocks. Each block can contain diodes, thyristors or two half-bridge IGBT modules mounted on a high-performance, purpose-designed heatsink. A cube also incorporates its own bank of capacitors, which is separately fan-cooled to reduce size and maximise reliability. By using different component blocks, a range of current ratings can be offered.

A purpose-designed, optimised driver, based on Semikron’s SKYPER 32 PRO core driver, is incorporated into each cube. All functions and protection of SKYPER 32 PRO such as soft turn-off and external error input are maintained. The driver for each cube operates independently, but has a common user interface, which is accessible from the outside of the SEMIKUBE unit. Each driver is controlled by the user’s application controller and provides all necessary protection and monitoring features, including galvanic isolation, a safe extra low voltage (SELV) interface, temperature monitoring, short-circuit protection, current and DC voltage scaling and current balancing. In addition, for maintenance purpose, a set of LEDs has been added to detail the last fault detected by the driver. This is certainly helpful for the diagnosis and explanation of unattended shut down of the inverter.

Figure 5. Horizontal arrangement of SEMIKUBE

Figure 6. SEMIKUBE Size 1

current balancing. In addition, for maintenance purpose, a set of LEDs has been added to detail the last fault detected by the driver. This is certainly helpful for the diagnosis and explanation of unattended shut down of the inverter. Each block incorporates a high-speed, hall-effect current sensor. The monitoring of current balance between the modules is achieved by multiple current sensors whose signals are individually measured and compared. Any over-current or current imbalance would lead to the driver interrupting IGBT gate drive signals thus providing enhanced protection for the system. This principle offers triple protection against over currents:

• fast short-circuit via VceSat monitoring

• slow short circuit via current monitoring

• internal short circuit via current imbalance monitoring

Such a high level of protection guarantees a safe and reliable operation.

Blocks are interconnected by a modular, co-planar busbar system. This interconnection block achieves, in a couple of turns of a screw, a reliable and low inductive connection between the various DC blocks. SEMIKUBE™’s power connection system (patent applied for) is very easy to dismount and routes DC polarities together to reduce inductance and, therefore, oscillating currents. This results in improving the lifetime of the capacitors. In addition, the electrical connection is achieved by a system of spring bands to compensate the effects of thermal expansion, which guarantees a constant and long lasting connection. This configuration offers rapid and versatile assembly. The connection can be made from any side of each block, therefore many mechanical and electrical configurations are possible to suit almost any possible requirement in the stated power range. The higher power ranges of the SEMIKUBE system requires up to 8 individual half-bridge IGBT modules to be connected in parallel. Paralleling without de-rating is a must for efficient design; therefore novel DC and AC connecting busbars have been developed. The AC busbar utilises electrically symmetrical, tin plated copper designs.

These busbars are designed such that they have the same resistance and inductance between the load’s connection and each module’s AC terminal. The DC connecting busbars have again been designed for low inductance and allow interconnection between DC terminals for each and every variant in the SEMIKUBE range.

A single cube can be used in a standalone application rated between 320 and 390 A, including the rectifier and the inverter. If more power is required, two cubes can be connected in two different layouts: one that uses a single system fan to save space or one that uses two system fans to maximise current handling. Ratings for these configurations range from 520 to 770A. Similarly, three cubes can be arranged in either of these formations, with one or three system fans, to offer current ratings from 830 to 1550A. Another arrangement is also possible, featuring three inverter cubes, plus a fourth cube containing a rectifier and additional capacitance. This configuration uses two system fans, providing the best of all worlds in terms of size and current handling. In fact, the system platform can accommodate any number of cubes, offering a very high degree of versatility, scalability and configurability. The SEMIKUBE system allows very compact inverters, converters and other topologies to be constructed. The cubes themselves are small and, in addition, clever design of the interconnect busbars allows three cubes to be mounted very close to each other. Off-the-shelf cubes and off-the-shelf platforms allow rapid assembly of a very wide range of solutions.

The SEMIKUBE design, with its high standardisation of parts allows for easy adaptation to many different topologies. Even though some standard configurations are proposed, there are no limits to the combination of possible arrangements. Four quadrant inverters, single phase inverters, diode rectifiers with brake choppers, polypropylene or electrolytic capacitors, all of these variants can be included in the design to give the most optimized solution. In the standard, converter/inverter SEMIKUBE, the following component parts are employed: The rectifier section can be configured with the following topologies; uncontrolled (B6U), half-controlled (B6HK) and controlled (B6C). The semiconductor devices used are the latest IGBT generation of SEMIPACK® 1600V isolated (3kVac) modules, incorporating the proven ruggedness and thermally efficient copper base plate technology. In addition to trigger modules, RC snubbers are standard and line fuses optional. The input voltage should be in the range 380 to 500Vac (-15%/ +10%), 45 to 66Hz. The maximum rectified DC voltage is 707V.

The DC link capacitors fitted as standard, are long-life, electrolytic, screw terminal types. The types selected have been chosen for costeffectiveness, without compromising performance and longevity. The sizing of the capacitors is decided by the nominal current of the inverter and will have an operating life time expectancy (LOP) > 60kHrs. The DC working voltage is carefully chosen to allow for sudden IGBT switch off which would result in a significant rise in DC link voltage. The capacitor bank is housed in the top half of a very robust frame for easy handling and mechanical protection. The top half of this frame, containing the capacitor bank is seated into its corresponding lower half of the SEMIKUBE frame, which has slotted fixing points. This allows, where volumes make it a cost-effective option, the top half to be lifted to accommodate larger capacitors without changing any other aspects of the mechanical design. The lower half of the SEMIKUBE frame is fixed firmly to the heatsink.

The heatsink used in all SEMIKUBE designs are highly efficient types but present a relatively high air pressure drop. Consequently, a high pressure centrifugal type fan is employed. The fan is a 230V, 50/60Hz type with noise levels that do not exceed 72dBA. Thermoswitch protection is incorporated and the fans have zero maintenance, long life bearings. Centrifugal fans may also be fitted to the capacitor bank frame as an option.

The inverter topology is a standard three phase (B6CI) and uses the latest 1200V soft punch through (SPT) silicon in the proven, cost effective SEMITRANS dual (half bridge) IGBT modules. SPT silicon offers efficient operation across a wide range of switching frequencies (up to 20kHz) with typical combined switching losses around 21mJ (@125°C, per 100A). The conduction losses are also low, being typically 2.0V @ 25°C. The standard inverter is optimised for a switching frequency of 3kHz, giving a system efficiency (conduction losses + switching losses + fan losses) of around 98%. The gate charge required for these IGBT modules is typically 1uC (VGE -8/+15V 100A) which is easily accommodated by the new SKYPER 32 gate drivers employed.

By using common diode, thyristor or IGBT modules, capacitors, heatsinks, fans, bus-busbars and current sensors across the range, Semikron can offer short lead times and easy maintenance with a reduced reference list of spares. All component parts of the SEMIKUBE system are fully qualified in areas such as temperature, humidity, EMC and safety. SEMIKUBE has been designed with the RoHS directive in mind.

SEMIKUBE has been developed for medium-sized customers for whom the benefits of an off-the-shelf, fully expandable, completely tested and characterised product is very attractive. In markets such as motor drives, solar inverters, wind-related power generation, uninterruptible power supplies and fuel cells, these new units offer designers the widest range of off-the-shelf power blocks in the industry. Because the component blocks can be manufactured in high volumes to meet many different mediumvolume applications, prices are very competitive. Users will have the benefit of a single user interface across the range, the only difference for different power levels will be the number of boxes.

Semikron International Solutions Centres are located in Australia, Brazil, France, India, Korea, Slovenia, South Africa, the United Kingdom and the USA. The network designs, develops and manufactures custom power assemblies for a wide range of markets.

 

For more information, please read:

Shunt Current Measuring up to 800A in an Inverter

Compact Inverters

More Efficiency for 3-Level Inverters

Main Advantages of 3-level Inverters

 

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