Upgrading Legacy Backup Power Systems with a Modern High Efficiency UPS


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Control in Multi-Module Applications

The 9900 series UPS is capable of multiple module configurations (MMS), up to eight kVA matched units in parallel. For all multi-module 9900 series UPS systems, single modules are used without any changes. The same logic is used in each UPS in the multi-module configuration. For a multi-module system, redundant Cat 5e cables are used to connect the DSP and the FPGA of each UPS together. Therefore, the control logic will be looking at its own minor and major control loops, and also the information from the other UPS control loops, to match the output of each UPS on the critical bus. The result is a multi-module system which can instantaneously share load while maintaining clean, reliable, regulated and uninterrupted voltage on the critical bus (refer to figure 10). Since the 9900 series multi-module configuration is used with individual single module UPS systems, the control logic is completely redundant.

Virtual Neutral

Line-to-line noise produced by the high frequency switching inside the UPS system is easily filtered using the small input and output filter of the UPS. However, high frequency line-to-neutral components are not suppressed due to an absence of a common connection between the three phases (neutral connection).

To filter these components, the 9900 series UPS uses a virtual neutral as shown in figure 11, which is created by connecting the common point of each of the filter capacitors to a common point. By virtue of this connection, the common mode harmonics are passed through the virtual neutral of the UPS system.

In addition to the input connection, the virtual neutral is also tied to the common point of the output filter, where the common mode harmonics are cancelled by the output of the inverter, as shown in Figure 11. During battery operation, the input contactor for the UPS system (CB1) is opened and the common mode harmonics are eliminated from the equation.

The potential of the Virtual Neutral is derived from the three phases on the input. A capacitor is added between the system ground and the virtual neutral. This capacitor, under normal conditions, will have minimal potential across the terminals and minimal current, as the potential of the virtual neutral and the system ground is the same.

As shown in Figure 12 (including the following calculations), confirmation can be made that the output phase voltage referenced to ground will be the same as the output phase voltage referenced to the virtual neutral. The input common mode harmonics are introduced through the Virtual Neutral, but cancelled by the output common mode harmonics.

Fault Conditions

The 9900 UPS system is monitoring fault conditions in multiple ways (voltage differences, current flow and current limiters) and will detect these faults depending on the installation of the system, the configuration of the distribution system and the conditions of the fault.

As mentioned above, if a fault condition occurs on the output of the UPS which produces a large amount of current, the UPS system current minor loop, current limiter and the FPGA will detect the instantaneous over-current. The PWM will then stop firing the IGBTs in the inverter and transfer the system to bypass so the bypass over-current protection can clear the fault (Figure 16). In addition, during an output short circuit, the output voltage will try to collapse. The voltage major control loop with the DSP along with the FPGA will maintain the voltage at first. If the current from the short circuit extends beyond the capabilities of the inverter logic to maintain proper voltage regulation, the DSP and the major control loop will sense a collapse in the output voltage and the system will detect an instantaneous overload.

The same would apply to a fault on the utility side of the UPS system. The converter logic will sense an immediate drop in voltage and the FPGA will stop firing the IGBTs in the UPS converter going immediately to battery operation.

DC Ground Fault Conditions:

If a DC Ground fault occurs as in Figure 13, the potential of the system ground will be different than the potential of the Virtual Neutral. This potential difference will be detected by the UPS system and viewed as a safety hazard. The UPS will alarm on a DC Ground Fault condition. The circuit is also being monitored by the chopper circuit logic to detect improper voltages and current levels.

Ground Fault Detection

As shown in Figure 14, if the utility source for the UPS is not grounded (no neutral to-ground bond), the presence of a ground fault will present a potential difference between the UPS Virtual Neutral and the system ground. Therefore, the fault will be detected by the UPS.

As shown if Figure 15, if the utility source for the UPS is grounded (neutral-to ground bond is installed), the presence of a ground fault will result in a second connection to ground and will produce zero sequence currents. These zero sequence currents will be detected by the UPS.


The 9900 series UPS is a high efficiency True On-Line Double Conversion transformerless UPS designed specifically for large power applications. It uses an IGBT converter section with advanced control circuitry to eliminate common mode harmonics and leading power factor issues with the upstream generators. The minor and major control loop used with the Digital Signal Processor and the Field Programmable Gate Array for the IGBT inverter results in precise control of the output voltage for maximum coordination with downstream Power Distribution Units and Static Transfer Switches. The Virtual Neutral allows the UPS logic to monitor all types of faults including system ground faults to offer maximum protection for critical loads. The result is a UPS which can deliver reliable output power to your critical load with minimal footprint while saving the customer money through an efficiency of greater than 97% at 100% load. The 9900 is available in sizes ranging from 72 kW to 2000 kW (SMS) and up to 12,000 kW (MMS).

References: “The Power of Green: Mitsubishi 9900A Series High Efficiency True On-Line Double Conversion Uninterruptible Power Supply (UPS)” by Dean Richards and Junichiro Onishi.