DECS-200 Digital Excitation Control System
FEATURES/FUNCTIONS, continued
Takeover Type OEL
The field current level at which limiting occurs is determined by an
inverse time characteristic. Two current levels and a time dial
setting are defined for the takeover-style OEL limiter. Separate
curves may be selected for on-line and off-line operation. If the
system enters an overexcitation condition, the field current is
limited and made to follow the selected curve. The selection of
on-line or off-line OEL levels/curves is determined by an OEL
option selection.
Minimum Excitation Limiter
The minimum excitation limiter limits the amount of excitation
supplied to the field of the generator from dropping below unsafe
operating levels. This prevents the machine from possibly slipping
poles and from damaging the machine. It limits the amount of
vars being absorbed by the machine, based on user-definable
settings. An internally generated Underexcitation Limiting (UEL)
curve can be utilized based on a var level at 0kW, or a
customizable 5 point UEL curve can be selected to match specific
generator characteristics.
Stator Current Limiter
The stator current limiter (SCL) senses the level of stator current
and limits it to prevent stator overheating. The SCL operates in all
modes except FCR. In FCR mode, the DECS-200 only announces
that a stator overcurrent condition exists; it does not provide
current limiting. Two SCL current levels are provided: high and
low. The generator can operate continuously at the low SCL level
but only for a programmed time at the high SCL level.
Internal Autotracking Between DECS-200 Operating
Modes
DECS-200 is an intelligent device that can provide autotracking
(autofollowing) of the controlling mode by the non-controlling
modes. This allows the operator to initiate a controlled, bumpless
transfer of the DECS-200 operating modes, causing minimum
amounts of line disturbance for the power system. This feature
can be used in conjunction with a set of protective relays to
initiate a transfer to a backup mode of operation, such as FCR
mode, upon the detection of a system failure or fault, i.e., loss of
sensing.
External Autotracking between Dual DECS-200 Units
(Optional)
A DECS-200 can also follow (autotrack) a second DECS-200 unit.
The second DECS-200 is put into a specific operating mode and
follows the excitation level of the first. In the unlikely event of a
failure of the first DECS-200, protective relays can initiate a
transfer of control from the first to the second DECS-200.
Protective Functions
There are several protection functions built into the DECS-200
unit. These functions may be used as backup to the primary
protection relays and can be assigned programmable output
contacts via the PC software. The protection features offer fully
adjustable tripping levels and time delays. The protective features
are as follows:
• Generator Overvoltage
• Watchdog Timer
• Generator Undervoltage
• Loss of Sensing
• Field Overvoltage
• Loss of field
• Field Overcurrent
• EDM Exciter Diode Monitor
Sequence of Events Recording (SER)
A sequence of event report (SER) is a very powerful tool when
reconstructing the exact timing of an event or disturbance. The
DECS-200 monitors its contact inputs and outputs for a change of
state, system operation changes, and alarm conditions. If any of
these events occurs, the DECS-200 will log that event with a date
and time stamp. Date and time stamping of the event allows the
user to recreate a chain of events in the sequence in which they
occurred. The DECS-200 can store 127 events in volatile
memory, and those events are retrievable using BESTCOMS.
Oscillography (See Figure 6)
The data recording feature can record up to eight (8) oscillo-
graphic records stored in volatile memory. The user can select up
to six (6) variables to be monitored when triggered by the DECS-
200 BESTCOMS, a Logic Trigger, or a Level Trigger. Variables
that can be selected are: generator voltage, current (single
phase), frequency, kW, Power Factor, exciter field voltage, and
current.
The user can utilize the DECS-200 BESTCOMS to trigger and
save a record of a voltage step response during commissioning.
Once commissioned, a logic trigger or level trigger can be used
to activate the data recorder to capture the occurrence for review
at a later time. DECS-200 alarms can also be used to start the
data recorder. When an alarm condition occurs, an oscillographic
record can be stored. A level trigger will initiate a record to be
saved when a variable exceeds a predetermined setting. An
example of this is when the exciter field current exceeds a
predetermined setting.
The oscillographic records are recorded in accordance with the
IEEE Standard Common Format for Transient Data Exchange
(COMTRADE). Basler Electric can provide BESTWAVE, a
COMTRADE viewer, which is a program that will allow the user to
view the oscillography records saved by the DECS-200.
Communications
DECS-200 comes complete with Windows
®
based PC software.
This software makes the programming and customization of the
DECS-200 easy and fast. The software comes with a PID
selection program that allows the user to select stability settings
quickly and easily in a user-friendly format. The PC software has a
special monitoring function that allows the user to view all
settings, a metering screen for viewing all machine parameters,
and a control screen for remote control of the excitation system.
The RS-485 port supports Modbus™ communications protocol.
This is an open protocol with all registers and operating instruc-
tions available in the instruction manual, to make it simple for the
user to develop custom communications-based interface.
Password Protection
All DECS-200 parameters are viewable via the front panel LCD
display, the PC software or via Modbus™ without the need of a
password. If the user wishes to change a setting, the proper
password must be entered to allow access to the parameter. Two
levels of password protection exist, one for global access of all
parameters and one for a limited amount of access to parameters
normally associated with operator control.
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