When we talk about UPS isolation, it is necessary to be very precise in what we understand under isolation in order to avoid misunderstandings and misinterpretations. There are three distinct types of isolations in a UPS System: the Galvanic Isolation between Input and Output, the input isolation between Mains and Battery and the Isolation between the DC-circuit and the UPS Output.
In transformer-based UPS-systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. A common misconception is that the transformer is also used to provide galvanic isolation, which is not the case. In transformer-based UPS-systems the neutral line passes through bypass line and the therefore the galvanic isolation between UPS input and output is not provided. If a true and total galvanic isolation is requested in transformer-based and in transformer-less UPS an additional transformer would be necessary at the very output of the UPS so that a galvanic isolation is provided for both the inverter and bypass from the load.
A. TRANSFORMER-BASED TECHNOLOGY
As mentioned above in transformer-based UPS-systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. Furthermore it is in the nature of the transformer to isolate DC–components and therefore the inverter transformer isolates the DC-circuit from the output load.
There are two possibilities for the DC-component to pass from the UPS to the load when there is an inverter IGBT-fault or a bypass THYRISTOR fault.
The transformer-based UPS behaves as follows in the two cases:
A. Inverter IGBT-Fault: If for example the IGBT 2 of the inverter does not conduct, a DC-component will be generated and in the transformer-based UPS the output inverter transformer will isolate the inverter DC-component from the load.
B. Bypass THYRISTOR Fault: If for example one of the THYRISTORS does not conduct, a considerable DC-component will be feeding the load as the transformer does not isolate the bypass. The transformer-based UPS does NOT control this DC-component.
B. TRANSFORMER-LESS TECHNOLOGY
As the transformer-less UPS-technology does not house an inverter output transformer the DC-component issue must be handled differently. The DC-component is blocked out at output by hardware and software regulation and control so that it can not be fed to the load.
The transformer-less UPS behaves as follows in the two cases:
A. Inverter IGBT-Fault: If for example the IGBT 2 of the inverter does not conduct, a DC-component will be generated. The transformer-less UPS-technology handles the DC-component by means of a fully redundant EDCP-SYSTEM (ELECTRONIC DC-PROTECTION SYSTEM) so that probability of a DC-component appearing at the inverter output is practically ZERO!
B. Bypass THYRISTOR Fault: Furthermore the CONCEPTPOWER is provided with an additional EDCP-SYSTEM on the bypass side, which consists of a SCR-non-conducting-detection-circuit. If one of the static bypass SCR’s does not conduct the load will automatically be transferred to inverter within 2-5msec in order to avoid the DC-component on the load side.
C. ELECTRONIC DC-PROTECTION SYSTEM
The CONCEPTPOWER EDCP-SYSTEM on the inverter side consists of 3 parts:
A. Redundant DC-COMPONENT REGULATION
The DC-Component is continuously detected (double, redundant detection) and regulated (double, redundant regulation) within a tolerance of +/- 10mV. Note: A normal mains supply to which all non protected equipment is exposed to has a DC-component tolerance of +/- 300mV!
B. Redundant DC-COMPONENT CONTROL
The DC-Component is continuously detected (double, redundant detection) and if it is higher than 4V the DC-component Control Circuit (double, redundant control) will automatically and instantly transfer the load to bypass and the inverter, rectifier/booster will be switched–off and the battery will be disconnected. The Alarm “DC-COMPONENT FAULT” will appear. Note: To make sure the DC-component does not appear on the load side the EDCP-SYSTEM operates at all times, even if the UPS is in LOAD-OFF-Modus!
Conclusion: If we bare in mind that the DC-Component detection, regulation and control circuits are redundant this makes the EDCP-SYSTEM very safe and secure.
C. FUSE PROTECTION of Inverter Bridge
A DC-Component may appear on the output if one IGBT fuse blows and the other IGBT continues to conduct. The CONCEPTPOWER Inverter bridges are designed in such a manner so that if one of two vertical fuses (F1 or F2) blows the other fuse will also automatically blow, avoiding the DC-Component from flowing to the load.
With the advanced electronic technology the EDCP-SYSTEM is extremely reliable. The probability of a DC-component passing through a transformer-less EDCP-SYSTEM is not higher than a transformer going in short-circuit (and allowing the DC-component to pass).