Excitation Control for a Synchronous Generator

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Excitation Control for a Synchronous Generator
Sponsor: Schweitzer Engineering Laboratories
Team Name: Three Phase
Duration: Fall 2014 - Spring 2015
Faculty Advisor:
Technical Advisor:
Students:
  • Carlos Valdez
  • Jacob Nelson
  • Kerri McGinty
  • Chien-Kai Wang
  • The goal of the project is to establish a static excitation system for the synchronous generator in the University of Idaho Model Power System (BEL Power) Lab. Students will need to specify, install, and commission the exciter, including user documentation and a verified RTDS system model.

    Background

    Exciter Overview

    An exciter is a control system that adjusts the magnetic field of a generator. It is capable of monitoring the AC output of a generator and feeding a DC current back into the generator's field. The exciter uses system measurements and control logic to determine if the generator’s AC output is deviating from its desired rating. If a deviation is found, the exciter readjusts the DC current being fed into the field. This process ensures that the generator's output remains stable and at the desired value.

    Depending on how the generator is connected to the system it may either control the bus voltage or it may inject reactive power into the system. It is common for modern exciters to be able take system measurements and regulate either the reactive power or voltage output of the generator based on a user dictated regulation mode.

    There are two types of exciters, static and rotating. A static exciter is physically independent of a generator and requires a power source (can be the generator's output or some other external source). A Modern static exciter uses a rectifier and controller to convert the AC source into a DC output for the field. The static exciter contains no moving parts and receives its name from the stationary nature of the system. The second type of exciter is the rotating exciter. The rotating exciter uses a DC machine that is connected to the rotor shaft to generate a DC voltage that can be amplified or attenuated for use in the field. The rotating exciter receives its name due to the fact that is has components in motion by rotating.

    The excitation system of a synchronous generator is the main equipment of operation and control of generators and the power system. According to experiments (Li, L., Caixin, S., & Daohuai, M. 2005), more than fifty percent of all the faults of the generator are because of the excitation system. Finding the tiny fault of the excitation system in time and making adjustments is important to ensure the safety of the generator group and power system.

    Generator Overview

    Synchronous generators have an additional coil on the rotor that will help pull the rotor and generator into synchronization. Also they only produce torque when the rotor is not turning at synchronous speed. The amount of current depends on the frequency difference between the stator and the rotor. The equation to get the voltage terminal of the generator is the following: Vt=Ra*Ia+j*Xs*Ia+Ea. To calculate the current on the field, a no load measurement of the current needs to be done first. Since at no load Ea = Vt Then solving for the constant kw = Ea(no load)/If(no load) using the equation to calculate the voltage terminal we can find Ea and using the following equation If = Ea/kw we could find the current of the field.

    Problem Definition

    In this project it is desired that Three Phase either design, assemble or purchase a static exciter. The Exciter must be capable of fast field forcing to temporarily increase the generator's AC output voltage during voltage sags caused by simulated faults. The exciter should also provide closed loop regulation of the generator's AC output voltage during normal operating conditions.

    Specifications

    Specific Requirement Description Target Acceptable Performance
    Size

    The exciter must be capable
    of resting on the red carts
    in the BEL Power Lab.

    18" x 27" x 32"

    All dimensions
    smaller than Target.

    "Normal"

    Operating Range

    The output of the exciter
    with no load on the generator
    at rated conditions.

    3-6 Amps DC

    Can sustain 6 Amps or Better

    Generator Output

    The exciter should adjust
    the field so that the generator
    remains at rated voltage.

    220 Volts AC +/- 1 Volt of Target
    Forcing Output

    The exciter must be capable
    of supplying a short term
    "burst" of power to the generator.

    Up to 24 Amp DC
    Lasts more than 3 seconds.

    Can sustain 24 Amp DC
    for 3 seconds or better

    Generator Starting

    The exciter needs be able
    to start the generator.

    A battery can be used
    to start the generator.

    The exciter can start the
    generator using an external
    power source.

    Display

    The exciter should be capable
    of displaying relevant information
    to the operation of the generator.

    Field Current
    Field Voltage
    Generator Output Voltage

    Field Current
    Field Voltage


    Design Decision Comparison

    2014 ExcitaterControl Buildbuy.PNG

    Exciter "Buy" Options

    Product Comparison Matrix

    The UNITROL-1020 was selected for this project. Since all products have greater capabilities than necessary for this project, ABB was chosen to remain consistent with the VFD being used in the lab.

    Features ABB
    Unitrol - 1020
    Basler
    DECS-250
    Reivax
    RTX Energy DRV01
    Maximum Input
    Voltage
    300 Vac 277 Vac 220 Vac
    Exciter Output Up to 20 Adc Up to 15 Adc Up to 20 Adc
    Short Term
    Overload
    Up to 40 Adc
    Up to 10 seconds
    Up to 30 Adc
    Up to 10 seconds
    Up to 40 Adc
    Up to 30 seconds
    VR accuracy +/- 0.20% +/- 0.25% +/- 0.10%
    Operating Temperature -40 to 70 degC -40 to 70 degC 0 to 70 degC
    Interface

    USB
    Ethernet
    RS485
    HMI
    CAN bus

    USB
    Ethernet
    RS485
    HMI
    CAN bus
    RS232
    IRIG-B

    Ethernet
    RS485
    HMI
    RS232

    Size 6.69" x 6.69" x 11.89" 8.08" x 8.62" x 12" 11.8" x 23.6" x 31.5"

    Digital Simulation Model

    Models

    The models and default parameters in this section are directly from ABB's document ZAB-3BHS223093-E62 for use in an RTDS simulation. The models provided by ABB were translated into RSCAD software for use in future projects and research.

    ST5C AVR MODEL
    This model includes

    • V/Hz limiter
    • Under excitation limiter
    • Over excitation limiter
    • Stator current limiter
    • Reactive power limiter
    • Power System Stabilizers (not included in UNITROL so forced off by the model)
    2014 ExciterControl AVRmodel.jpg

    ST5C SUPPLY MODEL
    This model includes

    • The output from the AVR model
    • The field input for the generator (in P.U.)
    • The possibility of being modeled as a rotating exciter (not used, the control for static excitation system is always enabled).
    2014 ExciterControl Supplymodel.jpg

    URST5T MODEL
    This model is a simplified version of the ST5C digital model (AVR and SUPPLY) that has been programmed into the RSCAD master library. Although this is generally the same model some assumptions were made so it is not an exact match.

    2014 ExciterControl URST5T.jpg

    RTDS Testing

    • To verify the Digital Simulation Model behaves as an exciter it was compared to a similar RSCAD library exciter model.
    • In this system the two dynamic loads and the two generators are identical.
    • The speed deviation of the system is assumed constant at the rated speed(thus rated frequency)
    • By varying each dynamic load equally and monitoring the terminal voltage of the generator the exciter's responses can be compared.
    2014 ExciterControl testsystem.jpg

    In general we can see the same response from the models. The ST5C (left):

    • has a faster response
    • has smaller peaks

    (Note: The Waveform on the right was varied a moment before the waveform on the left)

    2014 ExciterControl results.jpg

    Further Verification

    The digital simulation model has a similar response to the library exciter model, but it has yet to be compared the the physical UNITROL-1020. To fully verify the digital simulation model the PID controller of the UNITROL-1020 will need to be tuned for the generator. The new values, after tuning, will be needed to calculate the actual time constants and gains for digital simulation model.

    Team Information

    Picture Bio Discipline
    2014 ExciterControl KerriPhoto.jpg
    Kerri McGinty:

    Kerri is a senior in Electrical Engineering emphasizing in power. Her fields of interest are power distribution and generation.

    Electrical Engineering
    2014 ExciterControl CarlosPhoto.jpg
    Carlos Valdez:

    Carlos Valdez is a senior in Electrical Engineering who was raised in Mexico City. He came from a family of engineers and decided to study Electrical Engineering after taking a High School course on electronics. His emphasis is in Power Engineering and Power Electronics.

    Electrical Engineering
    2014 ExciterControl JacobPhoto.jpg
    Jacob Nelson:

    Jacob is a senior in Electrical Engineering who was raised in Battle Mountain, Nevada. He has been interested in electricity ever since he came into contact with an electric fence during his childhood. His hobbies include fly fishing, computer games and playing the great highland bagpipes.

    Electrical Engineering


    2014 ExciterControl GaryPhoto.jpg
    Chien-Kai Wang:

    Place Holder Text

    Electrical Engineering

    References

    ​Li, L., Caixin, S., & Daohuai, M. (2005, January 1). Study on the Excitation Protection and Control of Synchronous Generator Based on the Delta and S. Retrieved December 12, 2014, from http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1546872

    Document Archive

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    • Reports

    File:2014 ExciterControl RTDSquickguide.pdf

    • Meeting Minutes

    File:2014 ExciterControl Minutes 9 16.pdf
    File:2014 ExciterControl Minutes 9 19.pdf
    File:2014 ExciterControl Minutes 9 23.pdf
    File:2014 ExciterControl Minutes 9 24.pdf
    File:2014 ExciterControl Minutes 9 30.pdf
    File:2014 ExciterControl Minutes 10 07.pdf
    File:2014 ExciterControl Minutes 10 14.pdf
    File:2014 ExciterControl Minutes 10 21.pdf
    File:2014 ExciterControl Minutes 10 28.pdf
    File:2014 ExciterControl Minutes 11 04.pdf

    File:2014 ExciterControl Minutes 11 11.pdf
    File:2014 ExciterControl Minutes 12 02.pdf
    File:2014 ExciterControl Minutes 12 09.pdf
    File:2014 ExciterControl Minutes 01 20.pdf
    File:2014 ExciterControl Minutes 01 27.pdf
    File:2014 ExciterControl Minutes 02 10.pdf
    File:2014 ExciterControl Minutes 02 24.pdf
    File:2014 ExciterControl Minutes 03 03.pdf
    File:2014 ExciterControl Minutes 03 31.pdf
    File:2014 ExciterControl Minutes 04 14.pdf

    • Design Review Power Point Slides

    File:2014 ExciterControl DesignReviewPPT.pdf
    File:2014 ExciterControl DesignReview2.pdf

    • Technical Presentation Power Point Slides

    File:2014 ExciterControl TechPres.pdf