Vertiv
™
| Liebert® EXL™ 625-1100kVA Installation Manual | Rev. 3 | 02/17 7
1.4.5 Special Considerations for 1+N Parallel Systems
Design the grounding configuration of the system before finalizing module placement See 2.7 - Grounding.
The cabling impedance must be carefully controlled to ensure good load-sharing. Impedance mismatch may
cause an overload on one module in a 1+N system, triggering a shutdown and loss of power to the connected
load. Mismatched cable impedance is amplified when a 1+N system is operating on bypass because the power on
the bypass path is not controlled.
The Liebert 1+N UPS module is supplied with a sharing inductor to minimize the impact of cable impedance
mismatch. An impedance mismatch may be further minimized by controlling the wiring length of each unit. The
design and the layout of the UPS system and associated panels and cabling must be designed to ensure that
cable lengths and impedances are closely matched.
Table 1 shows bypass load sharing when the cabling length is varied for three 800kVA UPS modules powering
the load via their static bypass switches.
For further information about matching cable impedances or refer to “Performance Improvements with Sharing
Inductors in Distributed Static Switch UPS Systems” found at the Liebert EXL section of Liebert’s Web site,
www.liebert.com; or contact Vertiv
®
customer support.
Table 1 Load distribution for various load cable differences for systems with and without sharing inductor, 800kVA UPS
With Share Inductor, 100% System Load
(4) 600kcmil per phase, 0.9 PF
Without Share Inductor, 100% System Load
(4) 600kcmil per phase, 0.9 PF
% cable
Difference
% Current
UPS #1
% Current
UPS #2
% Current
UPS #3
% cable
Difference
% Current
UPS #1
% Current
UPS #2
% Current
UPS #3
Bypass Cable Length 50ft. (15.3m)
01001001000100100100
10 99.5 99.5 101.1 10 96.4 96.4 107.2
25 98.6 98.6 102.8 25 90 90 120 *
50 97.2 97.2 105.8 50 75 75 150.1 *
Bypass Cable Length 75ft. (23m)
01001001000100100100
10 99.2 99.2 101.6 10 96.4 96.4 107.1
25 98 98 104 25 90 90 120 *
50 96 96 108.3 50 75 75 150.1 *
Bypass cable length 100ft. (30.5m)
01001001000100100100
10 99 99 102 10 96.4 96.4 107.2
25 97.5 97.5 105.1 25 90 90 120 *
50 94.9 94.9 110.6 * 50 75.1 75.1 150.1 *
Bypass Cable Length 125ft. (38m)
01001001000100100100
10 98.9 98.9 102.3 10 96.4 96.4 107.2
25 97.1 97.1 106 25 90 90 120 *
50 93.8 93.8 112.8 * 50 75.1 75.1 150.1 *
* Overloaded unit, causing to breaker trip and load shutdown.
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