Download Slide 1

White paper: Isolation in UPS systems
In PowerWAVE UPS, the addition of an EDCP system
provides effective protection from the DC component
There are three distinct types of isolation 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. It is important to understand the distinction between these types of isolation in order to
avoid misinterpretation of specifications.
Galvanic isolation input/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 transformerbased UPS systems, the neutral line passes through the bypass line and therefore no galvanic isolation
between UPS input and output is provided. If a true and total galvanic isolation is required in
transformer-based or transformerless UPS, an additional transformer is necessary at the output of the
UPS, so that a galvanic isolation from the load is provided for both the inverter and bypass.
Input isolation between mains and battery
In the early 60s, when only open lead-acid batteries were available, galvanic isolation was required for
safety reasons. Since the late 80s, when the maintenance-free lead-acid or nickel-cadmium batteries
came into use, input galvanic isolation was abandoned. Today this isolation is very rare.
DC-component output isolation
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, the transformer isolates DC components, and therefore the inverter transformer isolates
the DC circuit from the output load. Figure 1 shows a block diagram of a transformer-based, doubleconversion UPS system. It can be seen that the transformer is on the output of the inverter and not on
the output of the UPS.
Figure 1 Transformer-based UPS
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.
UPS 085-01-00
Figure 2 The effect of an IGBT fault in a transformer-based UPS
In the event of an inverter IGBT fault – if, for example, 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 (Figure 2).
Figure 3 The effect of a bypass thyristor fault in a transformer-based UPS
In the event of a bypass thyristor fault – if, for example, one of the thyristors does not conduct – a
considerable DC component will feed the load as the transformer does not isolate the bypass. The
transformer-based UPS does not control this DC component (Figure 3).
Transformerless technology
As transformerless UPS technology (Figure 4) does not provide an inverter output transformer, the DCcomponent issue must be handled differently. The DC component is blocked at the output by hardware
and software regulation and control so that it cannot be fed to the load. The transformerless UPS
behaves as follows in the two cases.
Figure 4 Transformerless UPS
In the case of an inverter IGBT fault – if, for example, IGBT 2 of the inverter does not conduct – a DC
component will be generated. Transformerless UPS technology handles the DC component by means of
a fully-redundant EDCP (electronic dc protection) system, so that the probability of a DC component
appearing at the inverter output is practically zero (Figure 5). How does the EDCP system in
PowerWAVE transformerless UPSs work?
UPS 085-01-00
Figure 5 The effect of an inverter IGBT fault in a transformerless UPS
The PowerWAVE UPS is provided with a fully-redundant EDCP system on the inverter side consisting of
three parts. Firstly, redundant DC-component regulation continuously detects (double, redundant
detection) and regulates (double, redundant regulation) the DC component within a tolerance of ±10mV.
Note that a normal mains supply to which all non-protected equipment is exposed has a DC-component
tolerance of ±300mV.
Secondly, redundant DC-component control continuously detects (double, redundant detection) the DC
component, and if it is higher than 4V the DC-component control circuit (double, redundant control) will
automatically and instantly transfer the load to bypass. The inverter, rectifier and booster will be
switched off, and the battery will be disconnected. The alarm DC-COMPONENT FAULT will appear. 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 mode. Bearing in mind that the DC-component detection, regulation
and control circuits are redundant, this makes the EDCP system very safe and secure.
Thirdly, a DC component may appear on the output if one IGBT fuse blows and the other IGBT continues
to conduct. The PowerWAVE inverter bridges are designed in such a way that if one of two vertical
fuses (F1 or F2) blows the other fuse will also automatically blow, preventing the DC component flowing
to the load.
Using advanced electronic technology, the EDCP system is extremely reliable. The probability of a DC
component passing through a transformerless EDCP system is no higher than the probability of a
transformer going short circuit (and allowing the DC component to pass). Furthermore over 6000
PowerWAVE three-phase transformerless UPS systems are operating and being protected by the EDCP
system, without a single case of a DC component appearing on the load.
Figure 6 The effect of a bypass thyristor fault in a transformerless UPS
In the case of a bypass thyristor fault, PowerWAVE UPS are provided with an additional EDCP system
on the bypass side, which detects if one of the static bypass SCRs is not conducting. In this event, the
load will be automatically transferred to inverter within 2 to 5ms in order to avoid a DC component on the
load side.
UPS 085-01-00
High reliability
In the PowerWAVE transformerless UPS, the addition of the EDCP system on the bypass side provides
more effective overall protection from the DC component than simply having a transformer on the
inverter side.
Contact
Uninterruptible Power Supplies Ltd
Bacchus House
Calleva Park
Aldermaston
Berkshire
RG7 8EN
Phone:
Email:
Web:
UPS 085-01-00
0118 981 5151
[email protected]
www.upspower.co.uk