VSC Fault Protection

VSC Fault Protection The objective is to design and test a power system protection scheme that combines voltage and possible measurements from different locations in or near a wind farm to locate faults. A brand new method to detect the fault before it occures.

Project Statement
The objective is to design and test a power system protection scheme that combines voltage and possible measurements from different locations in or near a wind farm to locate faults. A brand new method to detect the fault before it occures.

Project Learning
Some of the shelf technologies that we will use are the following:
 * Real Time Digital Simulator (RTDS) -- The Real Time Digital Simulator (RTDS) is a special purpose computer that does time domain simulation solving the differential equations for a power system at discrete instants in time, 50 microseconds apart, and doing analog or digital I/O to external protection and control devices in real time. Such that the external devices don't know they are talking to a simulation instead of a real system.
 * RSCAD -- A graphical interface intended to be used in conjunction with RTDS.
 * PSCAD -- A non-real time graphical interface program that is the forerunner to RSCAD.

Project Goal

 * Design and simulate a power system protection scheme that uses voltage measurements from different locations in or near a wind farm to determine fault locations.



Project background
Type 4 WTG & Type 3 WTG √Excitation Permanent Magnets √The Prime mover --- torque production
 * Permanent Magnet Synchronous Machine

√Active Machine Side Converter √Firing controls the real and reactive power
 * Squirrel Cage Induction Machine''

Faults
 * What are Faults?
 * 1) An electrical fault is an abnormal defect in an electrical system caused by equipment failures, human errors, and/or environmental incidents.
 * 2) Faults occur when two or more lines or the neutral point are shorted together causing a disturbance in the phase and or magnitude of the voltage, current, and/or impedance.

There are two main types of faults: symmetrical ones and asymmetrical ones.
 * Types of Faultssymmetrical_fault_compressor.jpg
 * 1.Symmetrical Faults
 * Also known as a balanced faults
 * Affect all three phases of the system and become short circuited together


 * 2.Asymmetrical faults
 * Known as unbalanced faults
 * Do not affect all three phases and usually are faulted between each other or between a phase and neutral.
 * The three most common types are Line to Line, Line to Ground, and Double Line to Ground.

2017 Tasks

 * State space averaged model of Converter
 * Controller for converter
 * Full working model of Type 4 wind turbine

2018 Tasks

 * Test faults with Wind Turbine Model in PSCAD
 * Build same model in RSCAD
 * Simulate in RTDS
 * Determine algorithm to locate faults on transmission lines

System Diagrams

 * System diagrams used from Master's Thesis located in the appendix

The system diagram of type 4 wind turbine generator shows it is providing mechanical power to a permanent magnet synchronous machine (PMSM). This then provides electrical power to a diode bridge rectifier to convert AC to Dc. Then the power will flow through a DO link and then convert DC back to AC through the active PWM VSC. This output will go through a transformer and then to the AC grid. A squirrel cage induction machine can also be used in place of the PMSM.

Engineering Modeling
The projects model was built in RTDS for both the diode bridge rectifier and the Neutral Point Clamped Converter (NPC).

The diode bridge rectifier is used to rectify the AC input from the generator to a DC output. To convert DC back to a source of AC that the power grid can use, the active grid side voltage converter (NPC) is used.

On top of the power electronics models implemented above, a controller was designed, by Shashidhar Reddy Sathu, to generate the modulating functions for the three phase legs based on real and reactive power set points. Below are the outer and inner control loops.

Outer control Loop



 * The outer control loop was created for determining Id and Iq reference values from the set point values for real and reactive power. In the case of a wind turbine, the output power from the generator is hard to determine correctly and the Id reference value needs to follow the generator output, otherwise the energy gets dumped into the capacitor and raises the DC bus voltage and could damage the power electronic devices. In another case, if the power input falls and the inverter supplies more AC power than is coming into the DC link, the voltage will fall will fall as the capacitor gets drained and the system will not be able to maintain the voltage at the inverter terminals. This control loop was implemented to vary the exported ac power with the change in input and is based on regulating the DC link voltage.

Inner control Loop



 * The inner control loop generates modulating waves for all three phases for the use in switching functions. The Id and Iq reference values are compared with the processed measured values. The controlled determines the voltage that has to be supplied to the terminal to drive the current corresponding to the reference values. The control is done in the dq reference frame and has to be transformed back into the ABC reference frame to get modulating waved for the three phases.

The schematic of the overall system with the control loops, diode bridge rectifier and NPC is shown below.

PSCAD Work
Firstly, we do the single phase circuit simulation to find out what will happen when faults occur. Here is the circuit we built in PSCAD: When we use time fault logic gate to generate the single phase fault, we will see the output graph of voltage and current as show below: As we can see, when the fault occur, the voltage we measure at the point of fault branch, the voltage turns to nearly zero, because the fault is a line-to-ground fault, meanwhile, the branch will flow with a sinusoidal current, and the outside branch current will goes to zero.
 * Single Phase Simulation

Next, we do the three phase circuit simulation. The circuit we built for the WTG system is show: Inside the full power converter, the circuit will be:
 * Three Phase Simulation

Also, we use the output Graph Pane to output the waveform, then we will get the fault occur waveform, and the fault type we use is A-G fault. Here is the voltage relation under three phase fault case. We will easily to find that the when fault occur at PhaseA, here is very similar to the same case under the single-phase circuit. But one thing is different is the PhaseB and PhaseC. As the voltage in PhaseA goes to zero, the other two phases goes up slightly. We can explain this situation as the compensation of voltage, to keep the whole circuit balanced. Meanwhile, if we do the same graph output to the current, we will have that: Here is the current relation under this case. As we can see the compensation effect also occur in the current case. As the PhaseA current goes down, PhaseB may not change so clearly, but the PhaseC current does change to keep the circuit balance.