Excitation Control for a Synchronous Generator

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===
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==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)	
 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).	
 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.	
===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.	
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)	
===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
 	Kerri McGinty:
Kerri is a senior in Electrical Engineering emphasizing in power. Her fields of interest are power distribution and generation.	Electrical Engineering
 	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
 	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
 	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
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Category: Capstone Design