Digitally controlled IGBT converters:
IGBT converters offer many benefits over SCR type rectifiers if applied correctly (see figure 4). The IGBT converter can switch at speeds in the kilo-hertz range as opposed to the slower SCR rectifier, which fires pulses in the hundreds-hertz range. The reason the SCR rectifiers can not be switched faster is because the thyristors are turned on by a gate signal, but turned off by natural commutation (zero crossing of the AC input sine wave) or by a snubber circuit. If used properly with a digital signal processor and a Field Programmable Gate Array, the IGBT switching can be controlled to minimize the harmonics normally produced by a converter, thus eliminating the input harmonic filter.
As stated above, the problem exists with the UPS efficiency. If the IGBT converter is turning on and off in the kilo-hertz range, and the IGBT inverter is turning on and off in the kilo-hertz range, the switching losses will quickly add up creating a less efficient UPS.
Inverter Section:
Most manufacturers have recognized the importance of using IGBTs in the UPS inverter. The faster switching capabilities of the IGBT, if controlled properly, will result in a less distorted sine wave and will offer better response to various steps in the output loads and improved compatibility with downstream static transfer switches.
The IGBTs are fired in a series of pulses, which are used to invert the DC from the converter (or batteries) into a clean sine wave (refer to figure 7). Since the pulses are not continuous, high frequency noise is generated from the inverter. The high frequency noise and harmonic content on the output side of the UPS is filtered using an output filter and an output isolation transformer. In addition, the isolation transformer on the output of the UPS provides a means to re-establish a neutral ground bond for the downstream distribution.
The Mitsubishi 9900 Series Transformerless UPS Systems
As mentioned earlier, there are many different factors that need to be considered when designing a transformerless UPS system. Ideally, the UPS should be designed with minimal losses throughout the conversion processes (high efficient UPS). This was one of the design goals for the 9900 series UPS. After researching the available power conversion technologies, it was concluded that the three-level topology offered the most benefits while improving the reliability of the system.
The three-level topology reduces the switching losses associated with IGBTs. The result is a more efficient True On-Line Double Conversion UPS system that maintains this higher level of efficiency at loads as low as 10% on the UPS system (reference “The Power of Green: Mitsubishi 9900A Series High Efficiency True On Line Double Conversion Uninterruptible Power Supply (UPS)”).
Adding to the efficiency improvements of the 9900 series is the reduction of the filtering requirements. The input current harmonics of the 9900 is controlled to less than 3% at 100% load without the use of a low frequency harmonic filter. Only a small high frequency filter is required for the input of the UPS. Generator compatibility issues due to harmonics and leading power factor are eliminated by reducing the overall input capacitance by eliminating the low frequency harmonic filter.
On the output, since the Three-Level Topology generates pulses that more closely resemble a sine-wave (see figure 7), the output filter is also minimized. The combination creates a highly reliable and efficient True On-Line Double Conversion UPS system. Reducing the size of the UPS filters and eliminating the transformer will also reduce the size and weight of the UPS system and increase the efficiency.
To further enhance the improvements in technology, the 9900 series UPS systems are designed with the newest, most advanced generation of Mitsubishi designed IGBTs. Mitsubishi UPS systems are using fifth and seventh generation IGBTs which offer better efficiencies, improved reliability and a lower gate voltage. Although these are all great benefits for the customer, the UPS control techniques and the UPS response to different types of faults that can occur must now be considered.
Control System
Switching the IGBTs faster is of great benefit, but more important are the control techniques used for the gate signal. The Mitsubishi 9900 series UPS utilizes a high speed Digital Signal Processor (DSP), Application Specific Integrated Circuit (ASIC) and Field Programmable Gate Array (FPGA) for control. As shown in the block diagram of the control circuit, Figure 8, the 9900 series UPS uses a minor current control loop with a major voltage control loop to provide a precise response to various conditions imposed on the UPS.
Data from the voltage major control loop is collected and evaluated in the digital signal processor (DSP). The field programmable gate array (FPGA) is used to collect information provided by the DSP in addition to comparing the currents from the current minor control loops. Application specific programming in the FPGA processes the information and provides a switching sequence to compensate for the specific scenarios the load presents. The multiple feedback control loop allows the UPS logic to quickly detect changes in current in addition to detecting deviations to the voltage allowing for more precise control of the IGBT switching. By using the DSP and the FPGA, which have samplings rate greater than 100kHz (1,667 times per cycle), the switching sequence of the IGBTs can be manipulated to provide complete control of the power conversion processes.
In the event a fault occurs (including short circuits) which produces currents exceeding the current limits of the UPS (input or output), the control circuit will immediately stop firing the IGBTs and initiate the opening and closing of the contactors. On the converter section, the input contactor will open and the UPS will operate from the battery source. On the inverter, the UPS will initiate a transfer to bypass to allow the bypass current protection to clear the fault.
With minimal voltage distortion, the UPS will support any currents that do not exceed the limits of the system, including downstream inrush currents. For example, the UPS inverter can support 0% to 100% step loads on the UPS output while maintaining a voltage with less than 2% deviation (See Figure 9). Since the converter section is also controlled using the same logic, the converter can support the same step loads to the input of the inverter. Therefore, the UPS does not require power from the battery system to perform this step load.
