Turn to Turn Fault Detection for Air Core Reactors

The goal of this project is to create a small scale model of a 3 phase air core reactor in order to test for inter-turn faults, as well as an RTDS model of the system to further test more in depth fault scenarios.

Background
Avista deployed a system of air core reactors at their Noxon Rapids Dam in Montana in order to drop the bus voltage at the dam to help keep voltage levels even across the grid. Although the reactors have common protection schemes deployed already, there is currently no way for a protective relay to detect an inter-turn fault within a single phase of an air core reactor; a situation which could lead to complete failure and destruction of a reactor.

Deliverables
The goal of this project is to develop a protective relaying algorithm which will enable an IED, specifically an SEL 487E, to detect inter-turn faults within a single phase of an air core reactor.

Specifications





 * Project Specifications (As of 2/20/2015)
 * Length: 6-7 inches
 * Width: 4-5 inches
 * Weight: Approximately 6 oz.
 * Power Capacity: 5.33-7.2 W/H
 * Capacitance: Approximately 1.82 Farads at 120 Vrms
 * Needs to carry a charge for twice the time it take to charge the phone. (About 3 hours)
 * Discharge Time: 80 minutes
 * Two minutes to charge device
 * Create a USB interface for the charger based on USB specifications

Client Interview

 * Wrote a list of technical, budget, and miscellaneous questions for client to make sure we approached this project fitting our client's needs
 * From the answers we obtained, we were able to build a better foundation for starting this project

Lead Instructor and Technical Advisor Meetings

 * Every week we meet with our lead instructor and our technical advisor to go over the progress of the project
 * Topics such as due dates, technical progress, budgets, and brainstorming are all discussed during these meetings

Buck Converter
In order to create a rapid, charging circuit, we would need to rely on a familiar layout that would rectify ac voltage to a dc value and then convert the dc value to a particular dc value needed for a supercapacitor bank. Upon recommendation from our mentor, Professor Hess, we decided upon using a buck converter.

This buck converter would then be intended to be used a current source; this means that we wanted to sense the current and have a current swing so that the average current would be of a sufficient value to allow for a rapid charge of the supercapacitors.

Isolated Gate Driver
Due to the high input DC voltage to the buck converter, a special type of gate driver, the isolated gate driver, would need to be incorporated into the project design. This gate driver is considered a charge pump, charge elevator or level shifter and works by means of charging a capacitor, the bootstrap capacitor, and switching the leads of the capacitor to turn the N channel power mosfet off and on.

Microcontroller
In order to control the current and to allow the buck converter amperage to swing above or below our average current, we would need a microcontroller that would sense the current. This would be the brain of our feedback circuitry.

Sensing the current would be achievable via a resistor. With the current passing through the resistor, a voltage would develop across the resistor. The microcontroller then would read the voltage on one side of the resistor and subtract the voltage from the other side of the resistor, to get a voltage difference. The microcontroller would compare this value with code and either open or close the N channel mosfet switch.

Looking around for different microcontrollers we settled upon the Arduino Uno. This microcontroller has the ability to sense analog voltages and has a decent processing speed of 16 MHz. This microcontroller also had headers that allowed for solderless connections.

With the microcontroller selected, we then needed to drop down voltages for feedback. This was done via two voltage divider networks. In addition, we were needing amplification of our input signal; from the microcontroller to the N channel mosfet. This was achieved by an operation amplifier (CD4007 – see product data sheet). The isolated gate driver required a minimum of 10 volts for switching and the output of the microcontroller was only 5 volts. We needed a gain of 2 Volts/Volts. This was achieved by using a noninverting operational amplifier layout and an Rf and Rin value of 1 Kohm.

Future Work
The following are problems that need to be addressed in the future:
 * Getting the isolated gate driver to work properly – Under voltage lockout? Incorrect size of Bootstrap Capacitor? Circulating currents from charging supercapacitors? Interference with power supplies? Do we need a power supply with enough DC voltage and amperage to test circuit (170 V dc with up to 27 Amps)?
 * Miniaturization – need smaller components (inductors especially - some have up to a 24 week lead time)
 * Assembly of 2nd prototype – printed circuit board and surface mount components picked and assembled. Install of Vishay supercapacitors.
 * Possible thermal issues with surface mount components (especially the N channel power mosfet)
 * Feedback isolation for microcontroller. Needs op amps or other isolation on voltage divider input signals.
 * Feedback code or circuit for shutting microcontroller off when fully charged
 * Self powered. Right now requires a 20Vdc source in addition to a USB cable hooked up to the microcontroller.

Minutes

 * [[File:2015 AirCap Meeting Minutes.pdf]]

Client Interview

 * [[File:2015 AirCap Client Interview.pdf]]

Design Review

 * [[File:2015 AirCap Design Review.pdf]]
 * [[File:2015 AirCap Design Review ppt.pdf]]

Final Report

 * [[File:2015 AirCap Final Report.pdf]]