Excitation Control for a Synchronous Generator

The objective of this project is to develop a static exciter for a generator used in the BEL power Lab at the University of Idaho Campus. The exciter should support a continuous operating mode and a three to four second burst for fault correction and testing. The exciter will consist of three main modules: a rectifier, a converter and a controller.

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.

Build
The First option Three Phase could have used for this project would have been to design each of the main components. This option would have had Three Phase building an exciter from scratch. The option was given some consideration; however, it was decided that there was too much work involved for the time permitted. Three Phase confirmed this assumption when they obtained the portfolio for a similar rectifier and controller that performed closed loop regulation only, a 1996 design project. <>

Assemble
The second option Three Phase could have used for this project would have been to purchase the main components and complete the necessary assembly and programming. After the "Build" option was eliminated Three Phase gave this option some consideration. However, the client recommended that this option was only to be explored if the "Buy" option could not satisfy the desired specifications and budget.

Buy
The "Buy" option is to purchase an exciter as an individual unit. This option requires Three Phase to develop a list of required specifications and locate a product that will meet all of the specs. If the exciter meets all of the specs and is under the budget cap Three Phase will need to purchase and install the exciter. Installing the exciter will include programming the exciter to perform the functions desired by the client.

Selected Exciter Benefits


Overview of Fault Testing for a Generator
One way to conduct fault testing for a generator is to conduct an Automatic Test Pattern Generation of ATPG. This method is used to find an input sequence that, when applied to a digital circuit, enables automatic test equipment to distinguish between the correct circuit behavior and the faulty circuit behavior caused by defects. This can also help determine the cause of the failures.

ATPG A fault is said to be detected by a test pattern if the output of that test pattern, when testing a device that has only that one fault, is different than the expected output. The ATPG process for a targeted fault consists of two phases: fault activation and fault propagation. Fault activation establishes a signal value at the fault model site that is opposite of the value produced by the fault model. Fault propagation moves the resulting signal value, or fault effect, forward by sensitizing a path from the fault site to a primary output.

Overview of Fault Testing for a Power System
Types of potential faults in a power system include: 

Transient faults Persistent faults Symmetric faults Asymmetric faults Bolted faults Realistic faults Arcing faults

In power systems, protective devices detect fault conditions and operate cirucuit breakers and other devices to limit the loss of service due to a failure.

Document Archive



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