Brushless Excitation System

DGC Brushless Excitation System Description

Table of Contents 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 3.3.b 3.3.c 3.3.d 3.3.e

Single Board Computer Analog and Digital I/O Unit (ADIOU) I/O Interface Control Unit (IOICU) VME Power Supply Network Hub Module

7 7 7 8 8

3.4

Field Interface Panel

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 3.8.b 3.8.c 3.8.d 3.8.e 3.9

8

Field Breaker Control Switch Regulator Mode Control Switch Voltage Adjuster Control Switch Base Adjuster Control Switch Regulator Output Meter

12 12 13 13 13

Feedback Signals

3.9.a 3.9.b 3.9.c

13

Generator Terminal Voltage Generator Stator Current Exciter Field Current

13 14 14

4.0

CONTROL SOFTWARE

14

4.1

Operating and Control Modes

14

4.1.a 4.1.b 4.1.c 4.1.d

OFF Mode - Base Control Test Mode - Base Control ON Mode – Automatic Voltage Control FORCED Mode – Base Control

© Emerson Process Management Management Power & Water Solutions. PWS_005075 [3]

14 14 15 15

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4.2

Basic Operation of the Voltage Regulator

16

4.3

Automatic AC Voltage Control Software

17

4.3.a 4.3.b 4.3.c 4.3.d 4.3.e 4.4

Proportional Action High Initial Response Integral Action Derivative Action

19 20 20 20 20

Maximum Excitation Limiter Minimum Excitation Limiter Volts/Hz Limiter

20 21 22

Protection Software

4.6.a 4.6.b 4.6.c 4.6.d 5.0

19

Limiters

4.5.a 4.5.b 4.5.c 4.6

18 18 18 18 19

Controller Characteristics

4.4.a 4.4.b 4.4.c 4.4.d 4.5

Voltage Control Setpoint Voltage Feedback Signal Load Compensation Line Compensation

22

Over Excitation Protection Volts/Hz Protection Minimum Excitation Protection Excitation Removal

22 24 24 25

LEGAL NOTICE

© Emerson Process Management Power & Water Solutions. PWS_005075 [3]

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

Maximum Excitation Limiting

Minimum Excitation Limiting

Power System Stabilizer

gen_freq_actual

mxl_out mel_out

pss_output

-

+ in_va_up in_va_down

Voltage Set Point

+

Σ

va_volts_value

-

in_vapre(n)

auto_error

-

Proportional Action

vhz_actual

tgr_value

Volts Per Hertz Limiting

p_output vhl_out

+10

+10

fb_comp

Integral Action

i_output

+

+ Σ

Output Limits

ctlr_out

regulator_out

tgr_value

-10

-10 line_w_actual

Line Compensation

Π

Load Compensation

Π

Transient GainReduction

line_vars_actual

gen_vars_actual

gen_v_db

Deadband Filter

fld_i_pct

gen_v_pct

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.


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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 m ain 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

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) Base Adjuster is at the Full Load value of current

Final action: o

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 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

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Brushless Excitation System

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