Basler Electric DIGITAL EXCITATION CONTROL SYSTEM DECS-300

DIGITAL EXCITATION CONTROL SYSTEM 

DECS-300 

GENERAL 

Digital Excitation Control Systems (DECS-300) are microprocessor based devices intended for generator 

power management. These devices provide analog control signals for SCR bridges manufactured by Basler 

Electric and other manufacturers. Programmability of system parameters and regulation settings allows the 

DECS-300 to be used in a wide range of applications and provides greater flexibility in excitation system 

optimization. DECS-300 units are designed to provide control for generators of any size, and can be used 

for both exciter field and main field applications.  

FEATURES 

DECS-300 units have the following features and capabilities. 

• Four control modes automatic voltage regulation (AVR), manual or field current regulation (FCR), 

power factor (PF) regulation, and reactive power (var) regulation). 

• Two programmable stability setting (PID) groups. 

• Two pre-position setpoints for each mode. 

• Soft start and voltage buildup control with an adjustable ramp in AVR and FCR control modes. 

• Takeover-style (on-line and off-line) overexcitation limiting (OEL), underexcitation limiting (UEL), and 

stator current limiting (SCL) in AVR, var, and PF control modes. 

• Underfrequency or volts/hertz ratio limiting. 

• Three-phase or single-phase generator voltage (rms) sensing/regulation in AVR mode. 

• Single-phase bus voltage (rms ) sensing. 

• Single-phase generator current sensing for metering and regulation purposes. 

• Field current and field voltage sensing. 

• Fine voltage point regulation in AVR (–30 to +10%) over the voltage range. 

• One analog input for proportional remote control of the setpoint. 

• Programmable analog control output: 4 to 20 milliamperes, 0 to +10 Vdc, or –10 to +10 Vdc. 

• Autotracking between modes within the DECS-300 (internal tracking). 

• Autotracking between two DECS-300 units (external tracking). 

• Autotransfer (bumpless) between modes and between units. 

• Thirteen contact sensing inputs for system interface. 

• Eight output relays for system control and/or annunciation with four of the eight outputs being 

programmable. 

• Seven protection functions (field overvoltage, field overcurrent, generator overvoltage, generator 

undervoltage, loss of sensing, field overtemperature, and loss of field). 

• Real-time metering.  

• Data logging and event recording. 

• Generator paralleling with reactive droop compensation and reactive differential compensation.  

• Front RS-232 communication port for personal computer communication using BESTCOMS 

Windows®-based software for fast, user-friendly, accurate setup, and control.  

• Rear RS-232 port for dedicated communication with a redundant DECS-300.  

• Rear RS-485 port supporting the Modbus communication protocol.  

• Operates from redundant power sources (ac and/or dc). 

APPLICATIONS 

Introduction 

In this typical SSE-N application (Figure 1-1), the DECS-300 controls the exciter field in a synchronous 

generator. Front panel operation and serial communication links using PC software makes the system user 

friendly and easy to operate from local or remote locations. The user should study the operation, setting, 

and safety setup procedures in this manual before attempting to operate a similar application. For detailed 

application assistance, please contact Basler Electric or your local sales representative.

Based on the system operating state, the DECS-300 provides an analog control signal to the firing circuit

chassis. This analog control signal (4 to 20 milliamperes, 0 to 10 Vdc, or ±10 Vdc) controls the phase angle

of the SCR firing pulses generated in the firing circuit chassis.

DECS-300 can control power bridges that have various output current capabilities ranging from 20 to 5.000

amperes dc at nominal voltage levels of 32 to 375 Vdc. These power bridges may be semicontrolled or fully

controlled. Full controlled bridges provide faster de-excitation of the generator or exciter field to obtain

faster, load off, transient recovery.

The isolation module (transducer) senses the field voltage and current and develops analog voltages for

the DECS-300. It also isolates the DECS-300 from the field. These analog voltage signals are applied to

the DECS-300 through the cable connected between the two connectors (P1 on the DECS-300 and J1 on

the isolation module). Operating voltage for the sensing circuits in the isolation module comes from the

DECS-300 through the same cable.

Each DECS-300 has integrated over/underexcitation limiters. Overexcitation limiters are present for both

on-line and off-line excitation levels. This feature provides maximum overexcitation protection by having

different settings for off-line operation. During off-line operation, lower limiter settings are required to

properly protect the generator.

The DECS-300 has many customizing features to meet the various power generation system operating

requirements. Var/power factor controllers are available for operating the generator in parallel with a utility.

Another feature is the user selectable, 20 standard pre-programmed stability settings for exciter field

applications, 20 standard stability settings for main field applications, and one user programmable stability

setting. It has programmable output contacts for annunciating the various DECS-300 operating and

protection features. DECS-300 can be programmed to operate safely in the event of loss of sensing.

For critical applications where the failure of the excitation system keeps the generator from operating, a

redundant DECS-300 can be used to provide back-up excitation control. Redundant excitation systems

need to be properly designed to allow for proper removal of the failed system and also for the proper

enabling of the back-up system. Provisions also have to be made for the periodic checkout of the back-up

system to insure it is operational and can be put into service without warning. DECS-300 allows for

excitation system redundancy by providing automatic tracking and transfer between control modules. In a

single DECS-300 application, the DECS-300 can be selected to allow the inactive operating modes of the

DECS-300 to track the active operating mode. If the excitation system is normally operating on-line in the

automatic mode and a loss of sensing occurs, the unit could be changed to the manual mode of operation

where the loss of sensing has no impact on the exciters ability to provide proper levels of excitation. While

performing a routine maintenance checkout of the DECS-300 in back-up mode, the internal tracking feature

allows a transfer to an inactive mode that will result in no disturbance to the system. DECS-300 also

provides for tracking between DECS-300 units. The back up DECS-300 unit can be programmed to

designate any of the operating modes to track the primary DECS-300 operating mode.

DECS-300 is Modbus™ compatible via the RS-485 port. For more information about Modbus™, see

Section 7 in this manual. The front panel RS-232 communications port supports the BESTCOMS PC

software. BESTCOMS allows for user-friendly programming of setpoints and ranges, and allows for step

changes to facilitate proper stability programming. BESTCOMS provides easy start/stop control and

operator adjustment of the excitation system with real-time metering. The software catalog number is

BESTCOMS-DECS300-32. BESTCOMS is provided with the DECS-300 as part of the software/manual

package.