Specifications

39
EATON Pulsar Series and Powerware Series Product Catalogue
Powerware Hot Sync Technology
Figure 2. Well-balanced load share is achieved by adjusting output
frequencies;thusthephasedifferencebetweenparallelUPSoutput
voltages is forced to zero.
Voltages out of phase
Outputs are synchronised
V
UPS 1
V
UPS 2
UPS 1
f
1
UPS 2
f
2
P
1
P
2
P
1+2
P
1
P
2
Power level difference diminishes
Frequencies are continuously
adjusted based on P
1
and P
2
The secret here is a patented built-in digital signal processor (DSP)
algorithm, running continuously in each unit. It drives the UPS
outputs toward synchronisation and takes care of load sharing. If
there is a common bypass available, it is used as valid synchro-
nisation source for output. In the absence of a common bypass,
the processor makes subtle adjustments to the inverter frequency
on the basis of output power level measurement in order to find
a common frequency and load balance among the units. There
exists, as shown in Figure 2, a relationship between the power
imbalance and the voltage phase difference.
90
deg
V
diff
0 deg
V
1
V
2
I
diff
Compute
output power p
n
Integrate Dp
n
into
inverter input frequency
f
n
= f
n-1
- K1*p
n
- K2*p
n
Compute
Dp
n
- p
n-1
Adjust frequency
of inverter to f
n
Fn = F
n-1
– K1(Pn) – K2(DPn)
Where:
Fn = frequency
F
n-1
= previous frequency
Pn = power to load
K1 = frequency reduction factor
K2 = power change rate factor
Figure 3. A phase displacement between parallel connected UPS volt-
ages (V1 and V2) causes current flow between the units thus imbal-
ances load share.
Figure 4. With HotSync algorithm, inverter phase angel is adjusted by
output power and its change rate.
The internal output impedance of a UPS is inherently mainly induc-
tive, i.e. it looks as a small inductor in series with a stiff alternating
voltage source. So, if there is any difference between the output
voltage phases, it means that there is a power flow from unit to
unit, resulting in unequal load sharing. In the Figure 3, two units
have equal output voltages with phase angle displacement.
The voltage Vdiff and current Idiff between units exhibit a 90
degrees phase shift due to the inductive source impedance. The
main voltage (V1 and V2) and the current between units Idiff are in
phase resulting in active power flow.
The greater the phase shift, the heavier the power imbalance. If
we now introduce a controller to adjust the voltage phase by the
output power, the phase difference can be forced to decrease.
To adjust the phase difference to zero and to achieve accurate
load sharing, we may integrate the measured phase thus arriving
at power-controlled frequency. For the purpose of fast frequency
locking and to enable synchronisation to external bypass, a term
containing the power level change rate is added.
The flow diagram (Figure 4) shows how the load sharing proceeds.
The output power is monitored and the new frequency calculated
at 3000 times per second. The measurements are also used for
fast identification of a failed module. This feature is based on the
computation of instantaneous output power. A negative value,
even for a single instant, is an indication of an internal failure, e.g.
a shorted inverter IGBT. In a response the UPS trips immediately
off-line, causing minimal voltage disturbance. This feature is known
as ‘selective tripping’.
Accurate, equal load share is the number one characteristic to
determine the integral quality and reliability of the parallel UPS
system providing redundancy or increased capacity. With HotSync
technology this is achieved without need for additional communi-
cations line between UPSs thus no single point of failure is added
when introducing parallel modules to a system. From operational
and also economical viewpoint, the achieved “close to perfect”
reliability returns clear savings in the long run as every downtime
incident is costly and might lead to unpredictable consequences.
Hot Sync technology allows full maintenance to be performed
one-by-one on redundant UPS modules without an external main-
tenance bypass switch. The critical load does not need to be dis-
connected from the conditioned power. Scheduled or unscheduled
maintenance can be performed with the load supported continu-
ously by the UPS-grade clean power.