Basler DECS-250 Excitation System Debugging Guide

4.4 Reactive power regulation (Var)

Var mode adjusts excitation to track reactive power output to the set value (kvar). Activation conditions: Similar to PF, but PF_VAR_Select=1, set VARSetpt, gain to VAR_Ki, VAR_Kg.

4.5 Parallel compensation method

Droop: Reduce the voltage setting value proportionally based on the reactive current of the generator to avoid reactive power circulation between units. The Droop percentage is set in the Voltage tab, with V_DrooopEn=1 and Drooop_CCC_Select=0 enabled.

Cross current compensation: By using cross current CT to detect the difference in reactive current between each machine, adjust the excitation to evenly distribute the reactive power. Enable V_DrooopEn=1 and Drooop_CCC_Select=1, while configuring CrossCurrentGain.

Line Drop Compensation: Used for long-term power supply, it increases the terminal voltage as the load increases. Set LineDropComp (0-10%) in the Voltage tab, and disable Droop/Crossflow when enabled.

4.6 Limiter

Volt/Hertz (V/Hz): It automatically takes effect in AVR mode, and when the frequency is lower than the Upper Knee, the voltage drops according to the slope to protect the generator core from overheating.

Over excitation limit (OEL): Monitor the excitation current to prevent overheating of the rotor winding. Online OEL is divided into three stages: high, medium, and low, each allowing for different durations; Offline OEL is divided into high and low levels. Activated when OEL-En=1, and when the limit value is reached, OELActive=1 is used to intervene in PID regulation.

Underexcitation Limit (UEL): Monitor reactive power (absorption side) to prevent generator out of step or end core overheating. The user inputs 5 points (W, Var) to form a polyline, which is activated when UEL_En=1.

Synchronization and grid connection debugging

DECS-250 provides voltage, frequency, and phase error calculations required for synchronization, and outputs a closing permission signal (CloseBusA_Bkr). PLC needs to control the speed controller and closing circuit based on this.

5.1 Synchronization parameter configuration

In the Synch tab:

Set frequency matching window (e.g. ± 0.1Hz), voltage matching window (e.g. ± 2%), phase matching window (e.g. ± 5 °), and Acceptance Delay (e.g. 0.2 seconds).

For dual bus, select Bus A/B and set voltage multiples and phase offsets separately. Typical offset: Δ generator and Y bus require a 30 ° hysteresis.

Dead Bus Closing: Enable DeadBusClosure En and set the minimum frequency and voltage.

5.2 Synchronous startup process

PLC needs to perform the following operations:

Set InitiateSync=1 (keep it at 1 until closing is complete).

Choose one of the synchronization modes: AutoSyncEn (automatic, DECS-250 outputs Raise/Power voltage/frequency/phase recommendation), PermissiveSyncEn (check only), CheckSyncEn (test, does not output closing).

If AutoSyncEn is selected, the PLC should monitor labels such as Raise_V/Power_V and drive the governor/regulator to respond.

When CloseBusA_Bkr becomes 1, the PLC issues a closing command.

After closing, clear InitiateSync and switch to the appropriate operating mode (such as droop or cross flow).

Attention: The synchronization function must be backed up by an external synchronization check relay (such as ANSI 25) to avoid accidental closing in case of DECS-250 failure.

Redundant system debugging

DECS-250 supports dual machine hot standby, tracks the set values through RS-232 serial port, and the switching time is less than one power frequency cycle.

6.1 Hardware Connection

The excitation outputs of two DECS-250 are connected in parallel to the generator field winding via an external contactor (controlled by RD RLY).

The outer side of the magnetic field winding must be connected in series with 3 or 4 flywheel diodes (forward parallel) to ensure that the current path is not interrupted during switching.

The redundant communication cable is a null modem (2-3 crossover, 5 direct connection).

Control power supply PT/CT、 Cross flow CT is connected in parallel to two devices.

6.2 Redundant behavior

After power on, both devices are in Secondary state. When the primary PLC sends SoftwareExcEn=1 to the first machine, the machine detects no tracking data and automatically switches to primary mode, closes RD RLY, and outputs excitation.

The main machine periodically sends tracking data (excitation current, set values, etc.). The backup machine receives and adjusts internal tracking values.

If the main machine fails (power loss, ControlNet interruption, internal DSP failure), stop sending data, and the backup machine detects data loss and automatically switches to the main machine, closes its RD RLY, and outputs excitation.

After the host is restored, it will detect that there is already a primary use and maintain a standby state.

6.3 Redundant parameter setting

Redundance Tracking Rate: The primary and backup tracking speed (seconds/full-scale).

Redundance Tracking Delay: Tracking delay to prevent momentary disturbances from causing false switching.

In the configuration of the fault relay, "CGCM Internal Fault" can be enabled for PLC sensing.

Common troubleshooting