Kohler EKOZD Series Service page 260

Figure 14-7 Load Shed Timing Based on Generator
Frequency
(Underfrequency Shed Level = 0.50 Hz,
Base Underfrequency Shed Time = 5 sec.)
If the generator was overloaded to 110% of
capacity and this caused the generator frequency
to dip to 58 Hz, Underfrequency Shed would cause
priority 6 to shed in 2 seconds (Overload Shed
would also be active, but would require 5 seconds
to shed, so it would only be 40% shed).
When removing priority 6 decreases the load to
105% and causes the frequency to recover to
58.5 Hz, the curve in Figure 14-7 would indicate
that priority 5 would shed in 5 seconds, but
Underfrequency Shed has acceleration of 0.56,
meaning that each priority sheds in half as much
time as the previous priority if the condition is not
cleared, hence priority 5 would actually shed in
2.5 seconds (Overload Shed would have a total
time of 7.5 seconds, meaning that another 33%
would be added to the 40% shed condition, making
priority 4 73% shed due to overload).
When removing priority 5 decreases the load to
93% and allows the frequency to recover to 59.2 Hz
in the next two seconds, the curve in Figure 14-7
would indicate that the time to shed would be about
60 seconds, but the acceleration drops it to
0.25* 60 = 15 seconds. Even at that load, the
generator is allowed to recover to rated frequency
by the underfrequency shed logic, but the overload
shed logic will still cause the priority 4 to shed in
another 15 seconds (45 sec time to shed at 93%
load, and shed already 67% complete).
260 Section 14 Load Management
14.2.2 Sequence of Operation—
Paralleling System
This sequence follows the operation of a normal
paralleling system which uses Load Management. The
Load Management logic uses the paralleling bus
frequency and the paralleling bus percentage of rated
capacity to determine a need to add or shed load. A
generator does not have to be running to support Load
Management.
When a start signal is applied to any of the generators in
the paralleling system, all the generators that are in auto
and have no shutdown faults will start and all the load
priorities will shed. This shed is intended to remove
excess load from the system so that a single generator
can support it.
Figure 14-8 illustrates the system after the first
generator has reached rated voltage and frequency and
has closed to the paralleling bus (any of the generators
can be the first to close, depending on which generator
reaches rated voltage and frequency first).
25kW
Generator #1 Generator #2 Generator #3
(50kW rating) (50kW rating) (50kW rating)
Load
Shed
Panel
Figure 14-8 One Generator Energizes the Paralleling
Bus
The load on the first generator to connect to the
paralleling bus is merely that of the critical loads.
The time to Add Priority 1 can be determined from
Figure 14-9 based on a 50% generator load. The curve
indicates approximately 1500 seconds (25 minutes)
before priority 1 will add.
After synchronizing to the paralleling bus, generators #1
and #3 will close their paralleling breakers, providing a
total of 150 kW of capacity to the paralleling bus. The
25 kW load is only 17% of the capacity of the bus with all
three generators supplying it, hence the time to add a
priority drops to 26 seconds.
When priority 1 adds, the output from generator #1 that
drives the load shed priority will de-energize.
Load
Shed
Panel
25kW 18kW 8kW 3kW 10kW 12kW 5kW
Critical Most Least
loads important important
non- non-
Additional
critical non-critical critical
loads loads
loads
TP-6953 7/19