DGC Brushless Excitation - Emerson Process Management

DGC Brushless Excitation 

System Description

Table of Contents 


System Description 

1.0 INTRODUCTION 4 

2.0 BRUSHLESS EXCITATION SYSTEM 4 

2.1 Permanent Magnet Generator 4 

2.2 AC Exciter 5 

2.3 Rectifier Wheel 5 

2.4 Voltage Regulator 5 

3.0 DGC VOLTAGE REGULATOR SYSTEM 5 

3.1 Exciter Field Breaker 6 

3.2 Digital Generator Controller 6 

3.3 VME Modules 6 

3.3.a Single Board Computer 7 

3.3.b Analog and Digital I/O Unit (ADIOU) 7 

3.3.c I/O Interface Control Unit (IOICU) 7 

3.3.d VME Power Supply 8 

3.3.e Network Hub Module 8 

3.4 Field Interface Panel 8 

3.5 Base Adjuster 10 

3.6 Power Amplifiers 11 

3.7 Field Breaker 12 

3.8 Control Switches and Indications 12 

3.8.a Field Breaker Control Switch 12 

3.8.b Regulator Mode Control Switch 12 

3.8.c Voltage Adjuster Control Switch 13 

3.8.d Base Adjuster Control Switch 13 

3.8.e Regulator Output Meter 13 

3.9 Feedback Signals 13 

3.9.a Generator Terminal Voltage 13 

3.9.b Generator Stator Current 14 

3.9.c Exciter Field Current 14 

4.0 CONTROL SOFTWARE 14 

4.1 Operating and Control Modes 14 

4.1.a OFF Mode - Base Control 14 

4.1.b Test Mode - Base Control 14 

4.1.c ON Mode – Automatic Voltage Control 15 

4.1.d FORCED Mode – Base Control 15 

© Emerson Process Management Power & Water Solutions. - 2 - 

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4.2 Basic Operation of the Voltage Regulator 16 

4.3 Automatic AC Voltage Control Software 17 

4.3.a Voltage Control 18 

4.3.b Setpoint 18 

4.3.c Voltage Feedback Signal 18 

4.3.d Load Compensation 18 

4.3.e Line Compensation 19 

4.4 Controller Characteristics 19 

4.4.a Proportional Action 19 

4.4.b High Initial Response 20 

4.4.c Integral Action 20 

4.4.d Derivative Action 20 

4.5 Limiters 20 

4.5.a Maximum Excitation Limiter 20 

4.5.b Minimum Excitation Limiter 21 

4.5.c Volts/Hz Limiter 22 

4.6 Protection Software 22 

4.6.a Over Excitation Protection 22 

4.6.b Volts/Hz Protection 24 

4.6.c Minimum Excitation Protection 24 

4.6.d Excitation Removal 25 

5.0 LEGAL NOTICE 26 


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1.0 Introduction 



Emerson’s Excitation System is a versatile combination of elements which can be adapted to 

provide finely controlled excitation to synchronous machines over a wide range of capacities. 

The heart of the excitation system is the Digital Generator Controller (DGC). The Digital Generator 

Controller is a VME-based computer system running Emerson’s proprietary DGC application 

software. The software provides all of the features required for precise control of the generator 

under normal and fault conditions. 

The DGC computer system is backed up by an independent manual control system which provides 

a high level of reliability for operation of the unit. The manual system is completely independent of 

the DGC computer, such that loss of the computers will not result in a loss of excitation. This 

document describes Emerson’s DGC as it is applied in a brushless excitation system. 

2.0 Brushless Excitation System 

On large generators, the field windings are mounted on the rotor so excitation current must be 

somehow transferred to the generator shaft. One way to accomplish this is to use slip rings and 

brushes; however, the disadvantage of that solution is that constant maintenance is required. 

The key to eliminating brushes is to generate the excitation current where it is to be used, on the 

turbine generator shaft. This task is accomplished by building a generator whose armature 

windings are mounted on the turbine generator shaft. In this way, the electrical energy for field 

excitation is transferred to the generator field windings by magnetic fields instead of brushes and 

slip rings. 

Figure 1: Brushless Exciter 

As shown in Figure 1, the Brushless Exciter is comprised of three components, all of which are 

contained in the Exciter housing. The Voltage Regulator is a separate component but is a vital part 

of the excitation system. 

2.1 Permanent Magnet Generator 

The Permanent Magnet Generator (PMG) is an AC generator mounted at the end of the exciter 

shaft. The PMG field is provided by permanent magnets which are mounted on the rotor. The 

armature windings are on the stator. It produces 3-Phase, 120 VAC at 420 Hz for use by the 

Voltage Regulator. The Voltage Regulator rectifies the AC and supplies DC current to the AC 

Exciter field windings. 


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2.2 AC Exciter 

The AC Exciter is also an AC Generator. It is mounted in the middle of the exciter shaft. This 

generator is constructed somewhat backwards, in that the armature windings are wound on the 

rotor, and the field windings are mounted on the stator. The current flow to the field windings of the 

AC Exciter is controlled by the voltage regulator. The output of the AC Exciter is multi-phase AC, 

which is carried through insulated conductors, along the exciter shaft to the Rectifier Wheel. 

2.3 Rectifier Wheel 

The Rectifier Wheel is a multi-phase full wave rectifier which converts the AC output of the AC 

Exciter into DC for application to the Main Generator Field. Each leg of the rectifier contains 

multiple rectification diodes with fuses. The fuses are designed to remove a leg of the rectifier from 

the circuit if the diode should short. 

The DC output of the Rectifier Wheel is wired to the generator shaft coupling insulated conductors 

inside the shaft. The exciter conductors are connected to the Main Generator field windings 

through bolted connections located inside the coupling between the Exciter shaft and the Main 

Generator shaft. Smaller units have the connection on the outside of the shaft. 

2.4 Voltage Regulator 

The Voltage Regulator controls the magnitude of the field current to the AC Exciter. Therefore, the 

voltage regulator controls the AC output voltage of the AC Exciter. The Rectifier Wheel simply 

converts the AC Exciter output into DC for application to the main generator field. Therefore, the 

voltage regulator directly controls the field current in the main generator. Having control of main 

generator field current means that the voltage regulator controls generator output voltage. 

3.0 DGC Voltage Regulator System 

The DGC hardware is normally supplied as a redundant computer system with an independent 

manual back-up system, providing high reliability and serviceability. Single channel systems are 

sometimes supplied for less critical applications. 

Figure 2: DGC Excitation System for Brushless Exciter 


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3.1 Exciter Field Breaker 

The Exciter Field Breaker controls the application of AC power to the system. AC power is typically 

supplied from a Permanent Magnet Generator which is part of the Brushless Exciter. Some 

systems may obtain AC power from a station service source. 

The Exciter Field Breaker is typically a molded case breaker mounted in a draw out assembly. 

Alternately, a latching contactor may be supplied for lower current applications. 

3.2 Digital Generator Controller 

The Digital Generator Controller is a customized compute r system which runs Emerson’s 

proprietary software to make it act as a voltage regulator. The components of the DGC computer 

conform to the VME Bus standard, which is an internationally recognized standard for the design 

and packaging of industrial electronic systems. The VME Bus standard is supported by an active 

syndication of manufacturers worldwide and is popular in the industrial, telecommunication and 

military marketplaces. VME Bus architecture was selected for use in the DGC based on its high 

reliability, flexibility and rugged performance. The wide support of the VME Bus standard ensures 

that the DGC will avoid the rapid obsolescence issues that naturally accompany the design and 

construction of proprietary computer systems. 

3.3 VME Modules 

Each channel of the DGC is comprised of four VME modules. In a redundant DGC, the two 

channels share an Ethernet hub module located in the center of the chassis. The following 

paragraphs describe the functions of each VME module. 

VME Modules, Redundant DGC 


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3.3.a Single Board Computer 

The computers used in each channel of the DGC are state-of-the art, industrial grade Single Board 

Computers (SBC), featuring a high-speed Pentium processor, a serial port, an Ethernet port and 

various other features. System operating software and calibration constants are stored in nonvolatile 

“Flash” memory so that all programming is retained on power loss. The SBC uses no 

moving parts to ensure rugged and reliable performance. 

The two processors share information via the Ethernet port. The video output is normally 

connected to a flat panel display. Keyboard and mouse connectors are available on the front of the 

module. 

All field I/O communication is accomplished across the bus connections on the VMEbus Backplane 

to the adjacent ADIOU module. 

3.3.b Analog and Digital I/O Unit (ADIOU) 

The Analog and Digital I/O Unit (ADIOU) module provides analog and digital input and output 

capabilities for the DGC using “Industry Pack” (IP) technology. IP modules are small circuit cards 

that provide specific I/O functions that can be mixed and matched to create a system with the 

desired types and quantities of I/O. The ADIOU circuit card is a “carrier card” that provides 

mechanical support and electrical interfaces to support four IP modules. 

The four IP modules utilized on the ADIOU are: 

• Slots A and B: 48-Point Programmable Digital I/O modules. Of the 96 available points, 

36 are configured as digital inputs and 60 are configured as digital outputs. The digital 

inputs provide the controllers with the ability to monitor switch positions, breaker 

positions and alarm conditions. The digital outputs enable the controllers to energize 

lamps, generate alarms, warnings and generator trips. Digital inputs and outputs 

operate at 5 VDC level. 

• Slot C: 16-channel A/D Converter which allows the controllers to monitor a variety of 

analog signals. The module supports monitoring of both AC and DC field signals in the 

range of + 10 Volts. 

• Slot D: 16-channel D/A Converter module which provides the controllers with the ability 

to generate + 10 VDC signals for control and indication. 

Communication with the single board computers is carried out across the VMEbus Backplane. 

Field input and output signals are connected through ribbon cables on the front of the card to the 

front edge of the I/O Interface Control Unit (IOICU). 

3.3.c I/O Interface Control Unit (IOICU) 

The IOICU module provides the circuitry to scale, buffer and filter the signal types associated with 

synchronous machine voltage regulators. In a single channel system, all of the field connections 

are made through a single IOICU, whereas a redundant unit has two IOICU modules whose outputs 

must be paralleled to fully implement the redundancy features. When configured as a redundant 

unit, the IOICU ultimately determines which DGC channel is in control of the system outputs. 

The IOICU receives and conditions analog input signals from the field in three ranges: 150 VAC, 6 

VAC and + 10 VDC, converting them to the range of + 10 Volts. 

The IOICU buffers the outputs and provides the capability to “tristate” each of the analog outputs as 

part of the redundancy features of the DGC. 


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The IOICU receives and conditions digital input signals from the field in three ranges: 125 VDC, 15 

VDC and 5 VDC and converts them to the 5 VDC logic states. Digital inputs are optically isolated. 

The IOICU converts the logic states to dry relay contacts and provides the capability to “tristate” 

each of the digital outputs as part of the redundancy features of the DGC. 

3.3.d VME Power Supply 

Each DGC channel is equipped with its own rack mounted, quad output, power supply module. 

Each Power Supply is fed by two independent power sources; Station 125 VDC system and an 

independent 120 VAC source. 

Each Power Supply provides dedicated power to its respective channel of the DGC at +5 VDC, +12 

VDC and -12 VDC. Each power supply also provides auctioneered +48 VDC to the IOICU 

Backplane for distribution to the Field Interface Panel and the cooling fans. 

3.3.e Network Hub Module 

The Network Hub Module provides a hub for Ethernet communication with the DGC channels. The 

two single board computers also use the network connection for inter-processor communications. 

The network connection is also used by the DGC configuration software and optional SCADA 

software. 

3.4 Field Interface Panel 

The Field Interface Panel (FIP) is a large printed circuit card with a purpose to provide simple and 

reliable connections between the DGC computer and the field. The FIP connects to the IOICU 

backplane using prefabricated ribbon cables. 

FIP2 for WTA Retrofit Applications / FIP3 for Stand Alone Applications 

The field terminations on the FIP are designed to accommodate the physical connections for two 

distinctly different DGC applications. For the WTA Retrofit application, the FIP connects to the field 

using four large AMP connectors, which are compatible with the existing WTA Regulator cable 

harnesses. For all other applications, the FIP connects to the field using “euro” style terminal blocks 

mounted on the periphery of the board. 

The FIP is largely a passive device. The only active components mounted on the FIP are 

interposing relays and LEDs that are assigned to some of the digital output circuits to accommodate 

higher contact rating requirements. 


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3.5 Base Adjuster 

The emergency manual adjuster or Base Adjuster is an independent modular PLC-based device 

which acts as the controller for manual operation of the excitation system. When in manual control, 

the adjuster maintains a constant exciter field current. The emergency manual adjuster will be 

controlled by the Base Adjuster control switch. 

OPTO 22 Programmable Adjuster Replacement (OPTO PAR) 

When the DGC is in automatic control the adjuster follows the automatic signal to ensure a 

bumpless transfer between auto and manual operations. 

The adjuster has an independently powered track/hold backup circuit. An alarm is sent to the DGC 

if the base adjuster senses an internal problem. 


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3.6 Power Amplifiers 

The Power Amplifiers come in either a drawer-mounted version or a panel-mounted version. Both 

versions consist of two basic components: a firing circuit and a rectifier bridge. 

The firing circuit generates the firing pulses to control the SCR’s in the rectifier bridge. The demand 

signal to the trigger circuit is the summation of the DC control signals from the DGC and the Base 

Adjuster. 

The rectifier bridge uses SCRs to convert 3-phase AC into DC current. The SCR’s pass current 

flow in response to the gating pulses from the firing circuitry. The output current can be controlled 

from zero to maximum amps. An alarm is provided to alert the operator of a problem with the 

rectification process. 

Integrated Power Amplifier 


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For WTA retrofits, the Integrated Power Amplifier incorporates the functionality of the Firing and 

Power drawers into one drawer. The Firing Drawers are removed and discarded and the Power 

Amplifiers are replaced with new Integrated Power Amplifiers. The Integrated Power Amplifier has 

the same form and fit as the existing WTA 

Power Drawer, utilizing the existing racking 

mechanism and bus-bar connections in the rear. 

A new cable and connector arrangement is 

installed to support the other needed 

connections. 

For all other applications, the Power Amplifier 

components are panel mounted. The panel 

dimensions and bolting patterns are designed to 

facilitate installation into a variety of existing 

cabinet configurations. The Power Amplifier 

Panel is typically constructed with redundant 

SCR Bridges; however, single channel panels 

are available. 

Panel Mounted Power Amplifier 

3.7 Field Breaker 

The DB25 (or DB15) Field Breaker is replaced with a Square D MasterPac breaker assembly which 

is configured as a form, fit and function retrofit. The new breaker assembly adapts to the existing 

rails with a carriage that becomes a permanent part of the cubicle. The MasterPac breaker carriage 

is permanently mounted inside the adapter carriage. The MasterPac breaker racks into its own 

carriage and provides the same functionality as the existing breaker. 

3.8 Control Switches and Indications 

The design of the DGC Excitation System is such that the operation of the new voltage regulator is 

largely unchanged from the old regulator. The control room interfaces are retained and their 

functionalities are nearly identical. 

3.8.a Field Breaker Control Switch 

The Field Breaker Control Switch (41CS) allows the operator to close the 41Breaker (contactor) to 

initiate excitation. The actual position of the 41 device is indicated with lamps. 

3.8.b Regulator Mode Control Switch 

The Regulator Mode Control Switch (90CS) is a three-position switch that allows the operator to 

select the operating mode of the regulator. The actual operating mode is indicated with three 

lamps. The state of the lamps is controlled by digital outputs from the DGC. 


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3.8.c Voltage Adjuster Control Switch 

The Voltage Adjuster Control Switch (90VCS) is used to establish the terminal voltage setpoint for 

the voltage regulator. The range of the switch is typically calibrated for 90% to 110% of the 

generator’s rated voltage. 

The Voltage Adjuster Control Switch position is indicated on an analog meter, driven by an analog 

output signal from the DGC. In the absence of a meter, the position may be indicated with a set of 

lamps. Digital outputs are available to drive indicating lamps or for status indication of the voltage 

adjuster position. 

The Voltage Adjuster Control Switch has “pre-position” capabilities that allow the adjuster to be 

forced to a specified position upon request. 

3.8.d Base Adjuster Control Switch 

The Base Adjuster (PAR) provides control of excitation when not in automatic voltage control. The 

Base Adjuster is typically calibrated for the full range of power amplifier conduction which results in 

zero to maximum amps of field current. 

The Base Adjuster position is indicated on an analog meter, driven by an analog output signal from 

the PAR. In the absence of a meter, the position may be indicated with a set of lamps. Digital 

outputs are available to drive indicating lamps or for status indication of the voltage adjuster 

position. 

The Base Adjuster has “pre-position” capabilities that allow the adjuster to be forced to a specified 

position upon request. 

The Base Adjuster operates in a “follower” mode when the DGC is in automatic control. This is a 

significant safety feature because the Base Adjuster will always be adjusting its output to bring the 

output of the regulator back to zero. The practical result is that if the regulator experiences a failure 

or is restricted from producing a regulator output, the Base Adjuster will maintain the generator 

voltage at the same level. 

The PAR alarm output is monitored by the DGC as a condition that will generate an alarm. This 

alarm is usually configured to generate the DGC Trouble Alarm output. 

3.8.e Regulator Output Meter 

The Regulator Output Meter in the control room provides indication of the control signal value when 

it is not in the OFF mode. 

The Regulator Output Meter signal is the final control signal of the DGC. It is supplied to the firing 

circuits to control the amount of excitation current supplied to the Generator. Since the reliability of 

the Regulator Control Output Meter is critical to the operation of the DGC, the signal is selfmonitored 

by the DGC to ensure that it is operable and accurate. 

3.9 Feedback Signals 

The DGC requires several Feedback Signals in order to provide control and protection for the 

generator. 

3.9.a Generator Terminal Voltage 

Generator Terminal Voltage is the principle feedback signal for controlling the generator. The signal 

is taken from the Regulator PT’s. It is also used as an input to the Limiters and Protection features. 


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3.9.b Generator Stator Current 

Generator Stator Current is taken from Generator CT’s. It is used to calculate VARs so that the 

DGC can compensate the generator output for changes in VAR loading. It is also used as an input 

to the Limiters and Protection features. 

3.9.c Exciter Field Current 

Exciter Field Current is taken from a shunt inside the voltage regulator cubicle. It is used in the DGC 

control software as a stabilizing signal to dampen the response of the regulator during transients. It 

is also used as an input to the Limiters and Protection features. 

4.0 Control Software 

The DGC is continuously running its Control Software. The Control Software performs four basic 

functions: it establishes the operating mode of the system, it monitors and controls the generator 

output voltage, it provides “limiting” of excitation under abnormal operating conditions and it 

provides generator protection under fault conditions. 

4.1 Operating and Control Modes 

The mode of the regulator determines how the operator controls the generator output. The 

operating mode is selected by the operator using the 90/CS but the actual mode is determined by 

the DGC software. 

4.1.a OFF Mode - Base Control 

In traditional OFF Mode, the DGC does not actively control excitation. Using the Base Adjuster, the 

operator manually raises or lowers excitation current as desired to adjust Machine Voltage, VARs 

and Power Factor. The DGC protection is still operating. 

Key points: 

• The operator uses the Base Adjuster to control the generator output voltage 

• The DGC output is held at zero 

• The Protection functions are operable 

4.1.b Test Mode - Base Control 

In TEST Mode, excitation current is still manually controlled by the operator using the Base 

Adjuster. 

TEST Mode was historically used by the operators to "null" the regulator output prior to placing the 

regulator into service. The DGC is normally configured to automatically null the regulator by moving 

the voltage adjuster to match the current generator voltage. 

If the Voltage Adjuster is moved, the regulator output meter will still respond to verify the operability 

of the regulator. However, when the voltage adjuster switch is release, the regulator output will 

automatically return to zero as the voltage adjuster tracks the current generator voltage. 


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Key points: 



• The operator uses the Base Adjuster to control the generator output voltage 

• The Voltage Adjuster tracks the generator terminal voltage (if so configured) 

• The DGC hardware output is held at 0 VDC 


All alarms are cleared by transitioning into TEST mode. 

4.1.c ON Mode – Automatic Voltage Control 

In AUTO (ON) mode, the DGC has full range control of excitation current. The operator establishes 

a setpoint using the Voltage Adjuster switch. The DGC monitors the actual generator terminal 

voltage and automatically adjusts excitation to maintain the generator output at the setpoint. 

Getting into ON mode requires four conditions to be met for the usual configuration: 

1. The field breaker must be closed 

2. The PT signals must be valid 

3. The regulator control switch must be in the ON position 

4. The automatic controller error signal must be less than the limit 

When in automatic, additional features are in place to prevent the generator from being operated 

outside its design limits. 

Key Points: 

• The operator uses the Voltage Adjuster to control the generator output variable 

• The Base Adjuster will only momentarily change generator output voltage because 

the automatic controller opposes any changes, so it should not be used 

• The generator terminal voltage is actively controlled by the DGC utilizing closed 

loop feedback control 

• The Limiters are operable 


4.1.d FORCED Mode – Base Control 

Under certain contingent circumstances, it is necessary and/or prudent to force the DGC out of 

automatic, back in to a manual mode. In the FORCED Mode, the operating mode is identical to the 

OFF Mode. 

The request to force manual operation may be generated internally by the protective features of the 

DGC or externally at the customer’s discretion. 


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The contingency conditions are: 

• Any Alarm that enforces the NO AUTO restriction i.e., Bad PT’s 

• Customer input requests a Regulator Trip 

• OXP requests a Regulator Trip 

• VHP requests a Regulator Trip 



In the FORCED Mode, the OFF lamp blinks at approximately 2 Hz until the fault condition is cleared 

and the Operator returns the control switch to TEST position. 

Figure 3: Simplified Voltage Regulator Control Loop 

4.2 Basic Operation of the Voltage Regulator 

The generator output voltage is directly proportional to the magnitude of the excitation current. 

Excitation current to the generator field is supplied by the exciter output, which in turn receives its 

stimulus from the power amplifiers. The power amplifiers and exciter are typically capable of 

producing more than 130% of the rated excitation current. The excess capacity is provided to 

support rapid transient response. 

The magnitude of the excitation current is directly proportional to the excitation demand. Excitation 

demand is the sum of two signals: the Base and the Regulator Output. 

The Base signal is controlled by the operator using the Base Adjuster control switch. It is capable 

of providing an excitation demand from 0% to 100%. The Base signal is also adjusted automatically 

when it receives the follower command. 


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When the DGC is in OFF or TEST modes, the regulator output signal is held at 0%, thus the 

excitation demand is entirely controlled by the Base signal. 

The Regulator Output signal is produced by the DGC. The DGC is usually configured as a 

proportional only controller, receiving a positive setpoint from the Voltage Adjuster (+) and a 

negative feedback signal from the generator. The two signals are added together to produce an 

“error signal” which can be positive or negative. Integral control can be utilized if desired, usually 

seen when a numeric setpoint is provided. In lieu of DGC integral control, the Base following action 

acts as an integrator. 

If the generator voltage is too low, then the error signal is positive, calling for more excitation. If the 

generator voltage is too high, then the error signal is negative, calling for less excitation. If the 

generator voltage is equal to the setpoint, then the regulator output is zero. The Regulator Output is 

capable of providing an excitation demand from –100% to +100%. 

When the DGC operational mode implements the AC or DC control type, the Regulator Output is 

permitted to pass. If the regulator output signal is zero, then the excitation demand is equal to the 

Base signal. If the regulator output goes positive, then the excitation demand increases above the 

base. If the regulator output goes negative, then the excitation demand decreases to a value below 

the base. 

If the operator wishes to raise the output voltage of the generator, then he raises the voltage 

adjuster. The setpoint signal is greater than the feedback signal so the error signal is positive. The 

regulator output signal goes positive, adding to the base signal, causing the excitation demand to 

be increased. Excitation current increases and generator voltage goes up. 

If the operator wishes to lower the output voltage of the generator, then he lowers the voltage 

adjuster. The setpoint signal becomes less than the feedback signal so the error is negative. The 

regulator output signal goes negative, subtracting from the base signal, causing the excitation 

demand to be reduced. Excitation current decreases and generator voltage goes down. 

Due to the inherent voltage drop of the generator, as the generator is loaded, the output voltage will 

drop correspondingly. If the generator voltage drops below the setpoint, then the error signal is 

positive. If the generator is unloading the output voltage will rise correspondingly. If the generator 

voltage rises above the setpoint, then the error signal is negative. 

While in automatic, the Base Adjuster is not able to change terminal voltage because the regulator 

will fight against it. For example, if the operator raises the Base Adjuster, the excitation demand will 

increase. Excitation current increases and generator voltage goes up. The feedback goes up 

which makes the regulator output signal move in the negative direction. The reduction in regulator 

output causes the excitation demand to return to its initial starting point. Excitation current will drop 

back to its starting point and the generator output voltage will return to its starting point. 

4.3 Automatic AC Voltage Control Software 

The term AC control is applied when the controlled variable is measured from the generator 

terminals and is thus an AC quantity. This is most often the terminal voltage, but reactive power 

and power factor angle can also be the measured variable. The following paragraphs provide 

greater detail for each of the features of the software. 


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4.3.a Voltage Control 

The Control and Protection Software implements the voltage control function of the DGC. Many of 

the characteristics of the controller are tunable by the user. Tuning constants are entered and 

modified using the DGC Configuration software. The block diagram below will aid in understanding 

the operation of the voltage regulator controller software. 

4.3.b Setpoint 

The voltage Setpoint is expressed as a fractional percent, ranging from 0.9 to 1.1. The value is 

raised or lowered using the Voltage Adjuster control switch. The rate of change is typically set for 1 

minute from minimum to maximum. 

fld_i_pct 

gen_mvars_actual 

gen_mw_actual 

tgr_value 

in_va_up 

in_va_down 

in_vapre(n) 

vhz_actual 

tgr_value 

line_w_actual 

line_vars_actual 

Maximum 

Excitation 

Limiting 

Minimum 

Excitation 

Limiting 

Voltage 

Set Point 

Volts Per 

Hertz 

Limiting 

Line 

Compensation 

mel_out 

va_volts_value Σ auto_error 

vhl_out 

mxl_out 

+ - 

+ 

- 

- 

fb_comp 

Π 

pss_output 

- 

tgr_value 

Figure 4: Controller Block Diagram 

4.3.c Voltage Feedback Signal 

The Voltage Feedback Signal is a simple ratio of measured voltage to rated voltage. The Voltage 

Feedback Signal is passed through a dead band filter and modified by the compensators. The 

damping signal, the limiter outputs, and the power system stabilizer output (if installed) are summed 

with the f Voltage Feedback Signal and the setpoint to produce an error output. Each of the 

compensators and limiters are discussed in more detail in the following paragraphs. 

4.3.d Load Compensation 

Load Compensation is normally used when multiple generators of different types are operated in 

parallel on a common bus. The compensation is used to balance the reactive droop characteristics 

of the various machines to ensure that they share reactive load equally. The effect of the 

compensation is to make the apparent terminal voltage rise as the reactive loading of the machine 

increases in the over-excited direction. This causes the DGC to reduce the terminal voltage, 

transferring reactive load to the other machines. This function may also be used to compensate for 

a high impedance main transformer. 

Power 

System 

Stabilizer 

Proportional 

Action 

-10 

+10 

Integral 

Action 

Transient 

Gain Reduction 

gen_freq_actual 

Load 

gen_vars_actual 

Compensation 

Π gen_v_db Deadband Filter 

gen_v_pct 

i_output 

fld_i_pct 

p_output 

+ 

+ 

Σ 

-10 

+10 

Output 

ctlr_out 

Limits 

regulator_out 


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The Voltage Feedback Signal is modified by a multiplication factor. The multiplication factor is 

equal to 1.0 when there is no VAR loading on the generator, thus the value of the feedback signal is 

unchanged. 

As VAR loading increases in the VARS OUT direction, the compensator multiplication factor rises 

above unity, causing the perceived generator voltage to be higher than actual. As VAR loading 

increases in the VARS IN direction, the compensator multiplication factor drops below unity, 

causing the perceived generator voltage to be lower than actual. 

The compensation factor is applied proportionally based on the percentage of VAR loading and the 

Compensation setpoint. 

4.3.e Line Compensation 

Line Compensation is used when the voltage at the end of a long transmission-line is of concern. 

The compensation is used to overcome the voltage drop that occurs as the line is loaded with real 

(resistive) and reactive current. 

For Reactive load (MVAR) on the line, the effect of the compensation is to make the apparent 

terminal voltage lower as the reactive loading of the line increases in the VARS OUT direction. This 

causes the DGC to increase excitation, boosting the voltage at the end of the line. 

For Real load (MW) on the line, the effect of the compensation is to make the apparent terminal 

voltage lower as the megawatt loading of the line increases. This causes the DGC to increase 

excitation, boosting the voltage at the end of the line. 

Each of these calculations develops a compensation factor that modifies the feedback signal. 

The multiplication factor is equal to 1.0 when there is no load on the generator, thus the value of the 

feedback signal is unchanged. As loading of the line increases with real load or VARS Out reactive 

load, the compensator multiplication factor drops below unity causing the perceived generator 

voltage to be lower than actual. The compensation factor is applied proportionally based on the 

percentage of VAR loading and the compensation setpoints. 

4.4 Controller Characteristics 

The controller is a Proportional plus Integral controller with transient gain reduction (TGR) via rate 

feedback. The setpoint to the controller is the voltage adjuster value (va_volts_value) and the 

measured variable is the compensated feedback from the generator PT’s (fb_comp). The rate 

feedback signal, the limiter outputs, and the power system stabilizer output (if installed) contribute to 

the difference signal. The difference (auto_error) is applied to the proportional and integral sections 

of the controller. The proportional (p_output) and intregral (i_output) outputs are summed to 

produce the PID controller output (ctlr_out). 

If the Regulator Mode is OFF or TEST then the controller output is held at zero. 

4.4.a Proportional Action 

The Proportional Action section of the controller multiplies the error signal by a gain factor. The 

value of the gain factor can be dynamically adjusted as a function of the megawatt load on the 

generator, using a gain (gain_curve) vs. load curve (load_curve). Proportional control alone cannot 

reduce the error signal to zero. This is not usually an issue unless setpoint matching is needed. In 

most cases, the setpoint (va_volts_value) is adjusted by the operator to get the desired output 

without knowledge of the setpoint value. The fact that the controller error is not zero is of no 

concern to the operator. 


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4.4.b High Initial Response 

Proportional control is linear. To achieve High Initial Response (HIR) the controller needs to 

produce a large response from a large error. Proportional control cannot provide a High Initial 

Response for large errors without over-reacting to small errors. To achieve High Initial Response 

the DGC implements a non-linear gain function. This function supports a linear region around zero, 

a higher order response beyond that, and a high-value linear region at the high end. 

4.4.c Integral Action 

The Integral Action section of the controller generates a signal proportional to the sum of the errors 

over time. Integral Action can zero a steady state controller error and when setpoint matching is 

needed integral action should be applied. The gain of the integral action is tunable. The Integral 

Action section may be disabled, reverting the controller to proportional only action. 

The base adjuster follower function of the Base Adjuster (PAR) provides an integrator-like action to 

the control loop. The base follower function of the PAR is tuned with a long delay time and a slow 

ramp rate to minimize its interference with the transient response of the voltage controller. Base 

adjuster following is disabled if controller action moves the regulator into limit conditions. 

4.4.d Derivative Action 

The Derivative Action section calculates a damping factor based on the rate of change of exciter 

field current acting as a transient gain reduction function. Transient gain reduction is implemented 

with a tunable washout filter which has field current as its input. A washout filter "washes out" 

constant signals, but responds to transient signals, producing a response related to the rate of 

change of its input. The output signal then decays towards zero according to a time constant. 

The value of the Damping Factor opposes changes in the controller output. The tunable 

characteristics are time constant and gain. 

4.5 Limiters 

Three excitation limiter functions are incorporated into the DGC Control software. Each limiter is 

normally enabled but may be disabled at the customer’s discretion. The limiters act by modifying 

the controller’s error signal. The Maximum Excitation (MXL) and the Volts/Hz (VHL) Limiters 

subtract from the error signal, causing excitation and terminal voltage to be reduced. The Minimum 

Excitation Limiter acts by adding to the error signal, causing excitation and terminal voltage to be 

increased. The limiters (those enabled) are active in both AC and DC control. 

4.5.a Maximum Excitation Limiter 

The purpose of the Maximum Excitation Limiter (MXL) is to protect the generator rotor from damage 

due to overheating caused by excessive current flow in the field windings. The temperature limit of 

the rotor winding is fixed by OEM design. The rate at which the rotor heats up is a function of the 

magnitude of the excess current flow. 

Since moderately excessive current flow will not immediately cause damage to the rotor windings, 

the limiting action is time delayed, dependent on severity of the transient. Instantaneous action is 

taken on a severe transient. 

The MXL integrator is a software function that emulates the heating characteristics of the rotor. The 

integrator is started when the Exciter Field Current exceeds the setpoint, typically 105% of rated 

field current. The output value of the integrator increases with time, proportional to the difference 

between the measured field current and the setpoint. During this period, the MXL is said to be 

“timing”, and the output of the integrator is analogous to the heat buildup in the rotor winding. 


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When the integrator output value reaches the maximum allowable value, the MXL is said to have 

“timed out”. This is the equivalent of the rotor temperature having reached its thermal limit. 

The MXL is a proportional plus integral controller whose output is from the main controller error 

signal. The setpoint to the MXL controller is the calculated maximum allowable field current. The 

feedback is the measured exciter field current. The output of the controller is a function of the 

difference between actual measured field current and the setpoint, and the duration of the transient. 

The MXL controller setpoint is modified during the thermal integration phase. It starts out set to the 

ceiling current and stays there until reaching a thermal integration setpoint, which is configurable 

from 100% of the maximum allowable value down to 0%. Upon reaching the corner, the controller 

setpoint ramps from the instantaneous current setpoint towards the maximum allowable sustained 

setpoint. As the setpoint drops below the actual value of field current, the controller output will 

begin to act. As the setpoint continues to ramp lower, the control action will increase gradually, 

ensuring a smooth transition into the limiting condition. 

If the field current rises above the ceiling current, the time delay action of the integrator is bypassed 

and the limiter is activated instantly. 

The MXL calculation generates two alarms. The MXL TIMING alarm is generated when the 

integrator starts. The MXL LIMITING is generated when the limiter is actively limiting excitation. 

Both alarms have hysteresis to prevent relay chatter. 

4.5.b Minimum Excitation Limiter 

The purpose of the Minimum Excitation Limiter (MEL) is twofold. It protects the generator end-iron 

from overheating when operating extremely under-excited and it prevents loss of synchronization 

which may result from insufficient excitation. These conditions occur when the generator is 

operating with excessive "VARs In". Both of these conditions are avoided by preventing the 

regulator output from going too low. 

The MEL is a proportional plus integral controller. Its output is added to the main controller's error 

signal calculation. The output of the MEL controller is clamped at 0 until VARs fall below the 

minimum allowable value. As VARs fall below the setpoint, the MEL controller output adds to the 

main controller error signal. The MEL setpoint is expressed in VARs and is calculated from a 

setpoint curve as a function of MW, described below. The measured variables are VARs and MW 

from the Machine PTs and CT. 

The setpoint is a value of minimum allowable VARs for a given megawatt load, which becomes 

more restrictive as the megawatt load increases. The setpoint is defined in a four-segment "XY" 

curve, where X is in Megawatts and Y is in VARs. The data points for the curve are selected such 

that the curve approximates the bottom segment of the generator capability curve. The values for 

the curve are unit specific and are extracted from the particular unit's capability curve. There are no 

default values. 

An additional limit can be generated by enabling the steady state stability limit calculation. This is 

calculated continuously when enabled. The MEL limit will be the most conservative of the two limit 

sources. 

The setpoint is modified by the Damping Factor such that the faster the excitation current 

decreases; the more the setpoint value is raised to account for the lag of the generator response to 

a change in excitation. 


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The MEL function generates two alarms. The MEL WARNING is generated when the VARs In is 

close to the allowable limit. The MEL LIMITING is generated when the limiter is actively limiting 

excitation. Both alarms have hysteresis to prevent relay chatter. 

4.5.c Volts/Hz Limiter 

The purpose of the Volts/Hz Limiter (VHL) is to protect the generator armature cores and the Main 

Transformer cores from damage due to overheating. Overheating results from iron saturation of the 

cores that occurs when operating above the rated voltage or below the rated frequency. In general, 

the main transformer has a higher tolerance for core saturation than the generator. However, in 

some cases, the transformer ratings may be more limiting than the generator. 

The V/Hz value is the ratio of voltage to frequency. Since the output voltage of generators, 

transformers and transmission-lines are monitored with PT’s; the nominal value of voltage is 120 

VAC. The nominal operating frequency of the North American power grid is 60 Hz, thus the 

nominal Volts/Hz ratio is 2.0. Operation above the ratio of 2.0 will result if the voltage goes too high 

or if the frequency goes too low. 

The VHL is a proportional plus integral controller. Its output is subtracted from the controller error 

signal. The output of the VHL controller is clamped at 0 until the ratio of Volts/Hz exceeds the 

setpoint. As the Volts/Hz ratio exceeds the setpoint, the VHL controller output adjusts the main 

controller error signal as a function of the amount of excess Volts/Hz. The measured variables are 

Machine Voltage and Generator Frequency from the Machine PT. 

The setpoint calculation takes into account the ratings of the main generator and the main 

transformer. The calculated setpoint is based on which of the two devices is most limiting. The 

setpoint is modified by the damping factor such that on increasing excitation current, the setpoint 

value is reduced to account for the lag of the generator response to a change in excitation. 

The VHL function generates two alarms. The VHL WARNING is generated when the Volts/Hz ratio 

is close to the limit. The VHL LIMITING is generated when the limiter is actively limiting excitation. 

Both alarms have hysteresis to prevent relay chatter. 

4.6 Protection Software 

The DGC is provided with three protection features: Over Excitation Protection (OXP), Volts/Hz 

Protection (VHP) and Minimum Excitation Protection (MEP). Each of the functions is capable of 

tripping the generator to protect it from damage. All three features are available but may they be 

individually disabled if not required or desired. 

4.6.a Over Excitation Protection 

The purpose of the Over Excitation Protection (OXP) is to protect the generator rotor from damage 

due to overheating caused by excessive current flow in the field windings. The temperature limit of 

the rotor winding is fixed by OEM design. The rate at which the rotor heats up is a function of the 

magnitude of the excess current flow. 

Since moderately excessive current flow will not immediately cause damage to the rotor windings, 

the Protective action is time delayed, dependent on severity of the transient. 

The OXP function operates similarly to the MXL function. The actions of the OXP are coordinated 

with the MXL to ensure that the MXL has an opportunity to work. 


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The design OXP sequence of actions is as follows: 

• Field Current exceeds the OXP setpoint: 

o Start OXP integrator 

o Disable the Base Adjuster Follower 

o Initiate a preposition request to the Base Adjuster (optional) 

• Wait for a while as: 

o Integrator integrates 

o Base Adjuster moves to the Full Load value 

• Integrator times out: 



o If the DGC is in AC control or DC control, OXP forces the DGC into Forced 

mode (Regulator Trip) and then waits for a timeout period (oxp_timeout1) 

o Base Adjuster is at the Full Load value of current 

• Final action: 

o If taking the DGC out of AC or DC control lowered the current below the OXP 

setpoint; nothing is done, the unit is safe 

o If not, OXP initiates Generator Trip contact output. Note that if the DGC was 

not implementing AC or DC control that OXP will initiate the Generator Trip 

contact output immediately. 

If at any time during the sequence the field current drops below the OXP setpoint, the sequence is 

terminated and the OXP functions are reset. 

The OXP integrator is a software function that emulates the heating characteristics of the rotor. The 

integrator is started when the Exciter Field Current exceeds the setpoint, typically 110% of rated 

field current. The thermal integrator increases with time, proportional to the difference between the 

measured field current and the setpoint. During this period, the OXP is said to be “timing” and the 

output of the integrator is analogous to the heat buildup in the rotor winding. 

Upon starting to integrate, the OXP software disables the Base Adjuster Follower and initiates a 

digital output to move the Base Adjuster to a known safe value in preparation for the regulator trip. 

The recommended known safe value is the equivalent of 100% rated field current. 

When the integrator output value reaches the maximum allowable value, the OXP is said to have 

“timed out”. This is the equivalent of the rotor temperature having reached its thermal limit. Upon 

timeout OXP takes one of two paths: 

1. If the regulator was in AC or DC control, it initiates a regulator trip changing the 

DGC mode to “Forced”, leaving the DGC in manual operation with the Base 

Adjuster in control of excitation. This is followed by a configurable time delay 

(oxp_timeout1) (usually in the range of 3 to 5 seconds). The reasoning is that 

the automatic controller may be contributing to the problem and by forcing 

manual control the problem situation would then be alleviated. The time delay 

provides a small time period for the correction to take place. 


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2. If the regulator was not in AC or DC control, or if it has gone through the time 

delay specified above, then all other options have been exhausted and the 

generator must be protected by taking it off-line; the Generator Trip contact 

output is initiated. 

The OXP calculation generates three alarms. The OXP TIMING alarm is generated when the 

integrator starts. The OXP TRIPPED REGULATOR alarm is generated when the integrator times 

out. The OXP TRIPPED UNIT alarm is initiated in conjunction with the Unit trip contact. 

4.6.b Volts/Hz Protection 

The purpose of the Volts/Hz Protection (VHP) is to protect the generator armature cores and the 

Main Transformer cores from damage due to overheating. Overheating results from iron saturation 

of the cores that occurs when the ratio of voltage to frequency becomes too high. This is typically 

seen as operating above the rated voltage while at rated frequency, or below the rated frequency at 

rate voltage. In general, the main transformer has a higher tolerance for core saturation than the 

generator. However, in some cases, the transformer ratings may be more limiting than the 

generator. 

Since over voltage or under frequency operation will not cause immediate damage, the Volts/Hz 

Protective action is time delayed. Two separate setpoints and time delays are provided. A lower 

setpoint is coupled with a long time delay to accommodate a less severe V/Hz condition. The 

higher setpoint is coupled with a short time delay to protect against a more severe V/Hz condition. 

Upon timeout VHP takes one of two paths. 

1. If the regulator was in AC or DC control, it initiates a regulator trip changing the 

DGC mode to “Forced”, leaving the DGC in manual operation with the Base 

Adjuster in control of excitation. This is followed by a configurable time delay 

(vhp_reg_timeout) (usually in the range of 3 to 5 seconds). The reasoning is 

that the automatic controller may be contributing to the problem and by forcing 

manual control the problem situation would then be alleviated. The time delay 

provides a small time period for the correction to take place. 

2. If the regulator was not in AC or DC control, or if it has gone through the time 

delay specified above, then all other options have been exhausted and the 

generator must be protected by taking it off-line; the Generator Trip contact 

output is initiated. 

4.6.c Minimum Excitation Protection 

Minimum Excitation Protection (MEP) is the protective version of MEL. It implements a reactive 

versus real power curve, based on the MEL curve. For MEP, the reactive limits are offset from the 

MEL values by a constant value (mep_mel_offset). As reactive power becomes more negative, 

approaching the MEP limit, a warning is issued based on a warning offset (mep_warn_offset) by an 

alarm and a digital output. If the reactive power continues to fall and reaches the MEP setpoint, an 

alarm is generated and another digital output is set. This output can be used to trip the generator. 

The MEL, MEP, and KLH curves should be coordinated to provide a comprehensive minimum 

excitation protective strategy. 


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4.6.d Excitation Removal 

The purpose of Excitation Removal (also called phaseback) is to reduce excitation current to zero 

prior to opening the Field Breaker (41). This minimizes the circuit interruption requirements of the 

breaker, allowing a smaller breaker to be used on high current static excitation systems. In general, 

the circuit is not needed on brushless excitation systems as the breaker is already oversized for the 

application. 

The basic sequence is as follows: 

Upon receipt of the 86 Lock Out Set signal, the following actions occur simultaneously: 

• The voltage controller output is overridden, taking the hardware output to Full Buck, causing 

the firing circuitry to reduce excitation to zero as fast as physically possible 

• A contact is closed to supply the hardware phase back signal to the firing circuits as a back 

up to the Regulator output action 

• The Base Adjuster is prepositioned to minimum (0%) 

• The DGC closes an alarm contact for annunciation 

Five seconds after receipt of the 86 Lock Out Set, the Field Breaker is commanded to open. The 

excitation removal functions are reset when the field breaker status indicates it is open. 


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5.0 Legal Notice 



The document is the property of and contains proprietary information owned by Emerson Process 

Management Power & Water Solutions, Inc. and/or its affiliates, subcontractors and suppliers 

(collectively, “Emerson”). It is transmitted in confidence and trust, and the user agrees to treat this 

document in strict accordance with the terms and conditions of the agreement under which it was 

provided. 

The text, illustrations, and images included in this overview are intended solely to explain the retrofit 

overview solution. Due to the many variables associated with specific uses or applications, 

Emerson cannot assume responsibility or liability for actual use based upon the data provided in 

this document. 

No patent or other intellectual property liability is assumed by Emerson with respect to the use of 

circuits, information, equipment, or software described in this document. 

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any 

form or by any means, including electronic, mechanical, photocopying, recording or otherwise 

without the prior express written permission of Emerson Process Management Power & Water 

Solutions, Inc. 

The Emerson logo is a trademark of Emerson Electric Co. Ovation is a trademark of Emerson 

Process Management Power & Water Solutions, Inc. All other marks are properties of their 

respective owners. 

Copyright 2011 © Emerson Process Management Power & Water Solutions, Inc. All rights reserved. 

Emerson Process Management 

Power & Water Solutions, Inc. 

200 Beta Drive 

Pittsburgh, PA 15238 

USA 


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DGC Brushless Excitation - Emerson Process Management

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