Basler DECS-250 Excitation System Debugging Guide

Basler DECS-250 Excitation System Debugging Guide

Product Overview and Technical Specifications

DECS-250 adopts a 24Vdc control power supply, with a continuous excitation power output of 15Vdc (forced 30Adc/10 seconds), and can provide 32/63/125Vdc excitation voltage according to the input voltage (60/120/240Vac). Its key functions include:

Adjustment modes: AVR (Automatic Voltage Regulation), FCR (Field Current Regulation), PF (Power Factor Regulation), Var (Reactive Power Regulation)

Protection functions: Generator over/under voltage (59/27), over/under frequency (81O/81U), over excitation voltage (59F), demagnetization (40), reverse reactive power (40Q), reverse power (32R), rotating diode monitoring, etc

Parallel compensation: Droop, Cross current compensation

Limiters: Voltage/Hertz (V/Hz), Over Excitation Limit (OEL), Underexcitation Limit (UEL)

Synchronization: single/double bus, single-phase/three-phase, with built-in dead bus closing judgment

Redundancy: Dual machine hot standby, tracking set values through RS-232 serial port to achieve undisturbed switching

All parameters are read and written through the Schedule/Unscheduled data table of Logix PLC, without the need for specialized debugging software.

Installation and wiring points

2.1 Environment and Mechanical Installation

DECS-250 must be installed vertically, leaving 63.5mm on the left and right, and 101.6mm on the top and bottom for heat dissipation. The ambient temperature is -40~+70 ℃, and the humidity is 5-95% without condensation. Suitable for Class I Division 2 and Zone 2 hazardous areas, it needs to be installed in cabinets with IP54 or higher.

2.2 Power Wiring

Control power supply: Terminal 47 (BAT+), 48 (BAT -), DC 18-32V, power consumption 30W.

Operating power supply (excitation power): terminals 49, 50, 51 (PMG A/B/C), can be connected to single-phase or three-phase, and the voltage range depends on the required excitation voltage (see Table 8-1). If self parallel excitation (Shun) is used, power is taken from the generator end through PT. Note: If a single-phase 300Hz PMG is used in a redundant system, only one DECS-250 can be connected at a time, otherwise a contactor needs to be added for switching.

2.3 Wiring of Voltage and Current Transformers

Generator voltage: terminals 13-16 (V GEN A/B/C/N), supporting single-phase, open delta, three-phase three wire, and three-phase four wire.

Bus voltage: terminals 17-20 (V BUS A/B/C/N), also supporting multiple wiring options. When using dual busbars, select the "Dual Breaker" configuration and only use single-phase.

Generator current: Terminals 36-41 (I1+/I1-, I2+/I2-, I3+/I3-), CT secondary 1A or 5A. Note: If only one CT is used, it must be connected to phase B.

Crossflow compensation CT: Terminal 42 (ID+), 43 (ID -), usually connected to phase B. The circuit resistance should be less than 0.1 Ω (internal load 1 Ω). If the resistance is too high, a resistor can be added in series at terminals 33 and 34 (ID RES+/-) to adjust the gain.

Remote voltage regulation input: terminals 24 (VREF -), 25 (VREF+), ± 10Vdc, used for power system stabilizers (PSS), etc.

2.4 Excitation output and relay

Excitation output: Terminals 54 (EXC+) and 55 (EXC -) require twisted pair shielded wires. In redundant systems, 3-4 flywheel diodes need to be connected in series outside the excitation winding on site to avoid measurement errors caused by intermittent current during switching.

Fault relay: Terminals 29 and 30 (FLT RLY), normally open contacts, can be configured to close when any protection action is taken.

Redundant relay: terminals 31 and 32 (RD RLY), with the main machine closed, used to control the external contactor to output excitation to the magnetic field.

Cross current relay: Terminals 27 and 28 (ID RLY), close when the cross current function is not activated, short-circuit the cross current CT input.

2.5 Load sharing and redundant communication

Active load sharing: Terminal 1 (LS+), 2 (LS -), 0-5VDC signal, all parallel units LS+and LS - are connected in parallel.

Redundant communication: DB9 (RS-232) uses a null modem cable to connect two DECS-250 devices for setting value tracking.

RSLogix 5000 configuration steps

DECS-250 exchanges data with Logix PLC through ControlNet or EtherNet/IP, and all configurations are completed in RSLogix 5000.

3.1 Adding Modules

Under I/O Configuration, right-click on 1756-CNB (R) (ControlNet Bridge) and select "New Module".

Select "1407-CGCM" (a compatible model of DECS-250) and click Create.

Enter Major Revision (host firmware major version, such as 4), which cannot be changed thereafter; Minor Revision corresponds to the firmware sub version (such as 25).

Set the module name, ControlNet node address (MAC ID), and select the electronic lock method (recommended Compatible Module or Exact Match).

3.2 Configuration Tab

Double click the module to open the properties dialog box, and configure the following tabs in sequence:

Generator tab

Rated Frequency (50/60Hz), Rated Voltage (line voltage) Rated Current、Rated Power（kVA）、Rated Field Voltage、Rated Field Current—— Fill in according to the generator nameplate.

Transformers tab

Generator VT Configuration (single-phase/open delta/three wire star/four wire star), VT Primary/Secondary voltage.

Bus VT Configuration (similar, with Dual Breaker option).

Generator CT Primary/Secondary current (1A or 5A).

Cross current CT primary/secondary current.

Excitation tab

Soft Start Initial Voltage (%), Soft Start Time (seconds).

Overexcitation Voltage Setpoint and Time Delay.

Loss of Excitation Current Setpoint and Delay.

Rotating Diode Fault parameters (number of main poles, number of excitation poles, detection level and delay of on/off diodes).

Excitation Select (PMG or Shun), PMG Phase Select (single-phase or three-phase).

Volts/Hz tab

Upper/Lower Knee Frequency and Upper/Lower Slope can be viewed by clicking on "Validate&Graph".

OEL tab

Check 'Overexcitation Limiting Enable' and set the high, medium, and low current settings and time limits for online/offline OEL. Please note that there are three levels of online OEL and two levels of offline OEL.

UEL tab

Check 'Underexcitation Limiting Enable' and enter 5 (power, reactive power) points to construct a polyline limit curve.

Gain tab

PID parameters (Kp, Ki, Kd, Td) and Overall Gain Kg for AVR/FCR.

Proportional gain (Ki) and total gain (Kg) controlled by PF and Var.

Integral gain (Ki) and total gain (Kg) of OEL and UEL.

Cross Current Gain, AVR/FCR Auxiliary Gain PF/Var Auxiliary Gain。

These gain parameters are stored in the Unscheduled Write data table and need to be written to DECS-250 using the MSG instruction.

Tracking tab

Internal Tracking Enable, Tracking Rate, Tracking Delay - used for undisturbed tracking during mode switching.

Redundant Tracking Rate, Redundant Tracking Delay - Redundant Hot Spare Tracking.

Traverse Rates (AVR, PF, Var, FCR) - Set the transition time from the tracking value to the target value after switching modes.

Synch tab

The high and low limits and acceptance delay of Frequency/Voltage/Phase Match.

Voltage Multiplier and Phase Offset for Bus A/B are used to compensate for PT ratio and wiring phase differences (e.g. 30 ° offset required for Δ - Y conversion).

Dead Bus Limits (frequency, voltage upper and lower limits).

Generator/Bus Rotation (ABC or ACB).

Load Share tab

Full Scale Voltage (LS output voltage at full load, usually 5V), Limit (maximum load sharing error), Rate (error change rate).

Voltage tab

Overvoltage Setpoint/Delay、Undervoltage Setpoint/Delay。

Droop Percentage (reactive power droop coefficient, 0-30%).

Line Drop Compensation (0-10%).

Current tab

Overcurrent Setpoint (overcurrent setting,% rated current), Overcurrent Curve (16 inverse time curves+time limit), Time Dial, Voltage Braking Setpoint (voltage braking coefficient).

Frequency tab

Overfrequency/Underfrequency Setpoint and Delay.

Power tab

Reverse kW Setpoint and Delay, Reverse kvar Setpoint and Delay.

Fault Relay tab

Check the type of fault that needs to trigger the output of the fault relay (overvoltage, undervoltage, overclocking, reverse power, etc.).

3.3 Configure data writing mechanism

The Configuration data table (including most of the above parameters) is automatically written to DECS-250 in the following two situations: ① when the connection is first established; ② After clicking Apply/OK (excitation must be disabled).

The Gain tab and Line Drop Compensation belong to the Unscheduled Write data table and must be sent by the user program through the MSG command. The example cascade is shown in Figure 6-1 (MSG can be triggered using XIC UnschdWriteRcvd, with the target element being "Module Name: C. UnschWrite").

When the Configuration reception is completed, the position of Config Rcvd is 1; When the Unscheduled Write reception is completed, UnschdWriteRcvd position 1. After both bits are set to 1, DECS-250 accepts the Scheduled Write data and allows excitation.

Detailed explanation of operation mode

The adjustment mode of DECS-250 is selected by the label in the output table, and all modes must meet the following requirements: hardware excitation enable (EX-D+terminal has 24V), software excitation enable (SoftwareExcEn=1), and ControlNet connection is normal.

4.1 Field Current Regulation (FCR)

FCR is in manual mode and directly controls the excitation current. Applicable scenarios: Temporary voltage maintenance during machine testing and AVR faults. Activation condition: AVR-FCR_Select=1 (FCR select), FCRSetpt writes the target current (Adc). In FCR mode, AVR, PF, Var, and all limiters (OEL/UEL) do not work.

4.2 Automatic Voltage Regulation (AVR)

AVR is the default mode, maintaining a constant voltage at the generator terminal. Activation conditions: AVR-FCR_Select=0, V_Droop En=0 (droop disabled), set AVRSetpt (voltage value). The soft start function takes effect in AVR mode: after excitation, the voltage rises from SoftStart_SnitLevel to the set value in a ramp manner according to SoftStartTime.

4.3 Power factor regulation (PF)

PF mode adjusts the excitation to track the power factor to the set value. Activation conditions: AVR-FCR_Select=0, V_DrooopEn=1, Drooop_CCC_Select=0 (select droop), PF_VAR_ En=1, PF_VAR_Select=0 (select PF), set PFSetpt (-0.5~+0.5). PF_Ki and PF_Kg gains need to be configured.

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

7.1 Excitation cannot be established

Possible causes of the phenomenon and inspection methods

No excitation current output, excitation not enabled. Check if there is 24V at EX-D+terminal, SoftwareExcEn=1, and ControlNet connection is normal

Is the PMG/Shunt power supply functioning properly when measuring the voltage at terminals 49-51 due to a missing power supply

Configuration not received. Check if the Configuring Rcvd and UnschdWriteRcvd bits are available. If they are 0, check if the RSLogix configuration has been downloaded

Excitation output short circuit protection activates Spare2 bit=1, check if the resistance of the magnetic field winding is too low, reset by disconnecting the control power or setting SoftwareExcEn=0

7.2 Unstable voltage or power factor control

Improper gain: Gradually increase AVR_Kp until there is slight oscillation in the response, and then adjust it back by 20%; Ki affects steady-state error, while Kd improves dynamics. The RSLogix waveform monitoring function can be used to capture step responses.

Limiter activation: If OEL-Active or UEL-Active=1, it indicates that the regulator has been taken over by the limiter and the OEL/UEL setting value needs to be increased or the load needs to be reduced.

PT/CT wiring error: Check whether the voltage and current readings of the generator are consistent with the actual meter, and pay special attention to whether the phase rotation (GenRot_ABC-ACB) is consistent with the actual meter.

7.3 Uneven reactive power in parallel operation

Droop/Crossflow not enabled: Confirm that V_DrooopEn=1 and Drooop_CCC_Select is selected correctly.

Cross current CT polarity reversal: Measure the voltage between ID+and ID -. When the generator outputs inductive reactive power, this voltage should be positive (relative to the B-phase voltage). If it is negative, switch the CT polarity.

Cross current gain too low: gradually increase CrossCurrentGain and observe reactive power distribution.

Inconsistent CT ratio: All units should have the same CT ratio and load resistance for cross flow.

7.4 Synchronization Failure or No Closing Command

The matching window is too narrow: temporarily relax the frequency/voltage/phase limits and observe if the Match bit changes to 1.

Phase offset error: If the generator is Δ and the bus is Y, BusA_PhOffset=30 ° (or -30 °, depending on the wiring) needs to be set.

Dead bus closing does not work: Check that DeadBusClosure En=1, and the generator voltage/frequency is within the DeadBus limit, while the bus voltage should be less than 10% of the rated value.

The PLC did not process the Raise/Power command correctly: In AutoSync mode, DECS-250 outputs Raise_V/Power_V and other bits, and the PLC must adjust the AVR setpoint or governor accordingly.

7.5 Alarm and Fault Code Localization

DECS-250 reports specific faults through each fault bit in the Input table (Scheduled Read). Common fault location meanings:

Common reasons for tag meanings

LossExcFit demagnetization (40): If the excitation current is lower than LossExc_Sept, it may be due to CT disconnection, magnetic field open circuit, or incomplete soft start

OvrExcFit Overexcitation Voltage (59F) The excitation voltage exceeds the set value, usually due to an open circuit in the rotor circuit or high PMG voltage

Ovr_V_Fit Generator Overvoltage (59) Voltage regulator loss of control or sudden load shedding

Undr_V_Fit Generator Undervoltage (27): The prime mover speed is too low, the excitation is insufficient, or there is a soft start

Loss Sensing Fit (60FL) PT disconnection, secondary circuit open circuit, or PT fuse blown

LossPMGFit PMG loses power and PMG output disappears (this function should be disabled for self excitation systems)

RevVARFit reverse reactive power (40Q) phase advance is too deep, usually related to improper UEL setting or high system voltage

RevPwrFit reverse power (32R) generator is dragged, and the prime mover has no output

RotDiodeFit rotating diode fault diode open/short circuit, can be confirmed by observing the excitation current ripple with an oscilloscope

PhRotFit phase sequence error (47) generator PT wiring phase sequence does not match configuration

Fault reset: After clearing the fault source, the PLC sets FitReset=1 (pulse), DECS-250 clears the non locking fault and responds with FitResetAck.

Maintenance and firmware upgrade

The DECS-250 firmware can be upgraded online through ControlNet or EtherNet/IP, and requires the use of ControlFlash software. Before upgrading, be sure to save the current configuration (export module properties in RSLogix). Attention: Firmware upgrade may cause module restart and excitation output interruption.

Regular inspection items:

Control the power supply and PMG input voltage within the specified range.

The cooling fan (if any) is running normally, and the ventilation port of the chassis is not blocked.

The voltage of the redundant battery (when using an absolute encoder) is normal (DECS-250 itself does not have a battery, but if an external absolute encoder is connected, it needs to be checked).

The resistance of the cross current circuit shall not exceed 1.1 Ω (including external resistance).