Basler Electric DECS-400 - Manual (Page 16)

4 9369700990 Rev R
Field Current Regulation Mode
When operating in FCR mode, the DECS-400 regulates the dc output current of the power bridge.
Because regulation of the field current is not dependent upon a generator voltage sensing input to the
DECS-400, FCR mode can provide backup excitation control when a loss of sensing is detected. In FCR
mode, as the generator load varies, the operator must manually vary the field current to maintain nominal
generator voltage.
Field Voltage Regulation Mode
When operating in FVR mode, the DECS-400 regulates the dc output voltage of the power bridge. FVR
enables the user to perform generator modeling and validation testing in accordance with WECC testing
requirements for bus-fed (shunt type) excitation systems. FVR mode can also be used to smooth the
transfer from the active exciter to a backup exciter.
Var/Power Factor Operating Mode
Var and Power Factor control modes are available when the generator is operating in parallel with the
utility power grid. In Var control mode, the DECS-400 regulates the generator’s var output at a user-
adjustable setting. In Power Factor control mode, the DECS-400 regulates the generator’s var output to
maintain a specific power factor as the kW load varies on the generator.
Overexcitation Limiters
Overexcitation limiters monitor the field current output of the voltage regulator or static exciter and act to
limit the field current to prevent field overheating. The Overexcitation Limiter (OEL) function includes a
cool-down feature to avoid damage to the rotor caused by repeated high forcing. The OEL is active in all
modes except FCR mode. In FCR mode, limiter action is optional. The DECS-400 provides a choice of
two types of overexcitation limiters: Summing Point and Takeover. The output of the Summing Point
limiter is applied to the summing junction of the AVR control loop in addition to the AVR controller output.
The output of the Takeover limiter overrides the normal AVR output.
Summing Point OEL
Three OEL current levels are defined for on-line operation: high, medium, and low. The generator can
operate continuously at the low OEL current level and for programmed times at the medium and high OEL
current levels. Two OEL current levels are defined for off-line (main breaker open) operation: high and
low. The generator can operate continuously at the low OEL current level and for a programmed time at
the high OEL current level.
Takeover OEL
The Takeover OEL determines the field current level at which limiting occurs by using an inverse time
characteristic. Two current levels and a time dial setting are defined for the Takeover OEL. 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. Selection of on-line or off-line OEL levels
and curves is determined by an OEL option selection.
Minimum Excitation Limiter
The Minimum Excitation Limiter prevents the excitation, being supplied to the generator field, from
decreasing below safe operating levels. This prevents pole slip and possible machine damage. This
action also limits the amount of vars being absorbed by the machine, based on user-defined settings. An
internally-generated Underexcitation Limiting (UEL) curve based on a permissible var level at 0 kW can
be utilized. Alternately, a five point UEL curve can be created to match specific generator characteristics.
UEL action is optional in FCR mode.
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 and when the DECS-400 is off-line (52 J/K and
General Information DECS-400