9440300990 Rev J
29
DECS-250
Regulation
The DECS-250 can be used to control the excitation level of two or more generators operating in parallel
so that the generators share the reactive load. The DECS-250 can employ either droop compensation or
cross-current compensation (reactive differential) schemes for reactive load sharing. A separate load
sharing function enables each machine to share the load proportionally without incurring a voltage and
frequency droop.
Paralleled generator settings are illustrated in Figure 19 and described in the following paragraphs.
Reactive Droop Compensation
Droop compensation serves as a method of controlling reactive current when the generator is connected
in parallel with another energy source. Droop compensation utilizes the B-phase CT in single-phase
applications. When droop compensation is enabled
A
, the generator voltage is adjusted in proportion to the
measured generator reactive power. The reactive droop compensation setting
B
is expressed as a
percentage of the generator rated terminal voltage.
Note
For droop compensation to operate, the PARALLEL_EN_LM logic
block must be set true in BESTlogic™
Plus
programmable logic.
Cross-Current Compensation
Cross-current compensation (reactive differential) mode serves as a method of connecting multiple
generators in parallel to share reactive load. When reactive load is shared properly, no current is fed into
the DECS-250 cross-current compensation input (which is connected to the B-phase transformer).
Improper sharing of reactive load causes a differential current to be fed into the cross-current
compensation input. When cross-current compensation is enabled
C
, this input causes the DECS-250 to
respond with the proper level of regulation. The response of the DECS-250 is controlled by the cross-
current compensation gain setting
D
which is expressed as a percentage of the system nominal CT setting.
Application information about cross-current compensation is available in the
Voltage and Current
chapter
of this manual.
Network Load Sharing
In a multiple-generator application, the load sharing function ensures equal generator reactive-power
sharing. It operates in a similar manner to cross-current compensation but without the external hardware
requirements and distance limitations. Instead of sharing load based on the CT ratio, load is shared on a
per-unit basis calculated from the generator rated data. Sharing of load information between DECS-250
controllers is accomplished through the Ethernet port of each DECS-250 communicating over a peer-to-
peer network dedicated for the load sharing function. Each DECS-250 measures the reactive current of its
associated generator and broadcasts its measurement to all other DECS-250 controllers on the network.
Each DECS-250 compares its level of reactive current to the sum of all measured currents and adjusts its
excitation level accordingly.
Network load sharing implements a fading droop function based on the droop setting, washout filter gain,
and washout filter time constant. During transients, load sharing will droop according to the droop
percentage and washout filter gain settings. The droop characteristic will fade with a time constant
according to the washout filter time constant.
A Load Share ID setting identifies the DECS-250 as a load sharing unit in the network. Checking a Load
Sharing Unit number box allows any DECS-250 load sharing units on the network with that Load Share ID
number to share load with the currently connected DECS-250. It is not necessary for the Load Share ID to
be unique for each unit. This allows for load sharing units to be grouped.
Load sharing settings consist of an Enable checkbox
E
and Droop
F
, Gain
G
, Washout Filter Time Constant
H
,
Washout Filter Gain
I
and Load Share ID
J
settings.
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