Critical Evaluation of slip synchronous wind generator technology
Date
2022-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Historically, the strength and stability of an electrical power grid depended on the
number of synchronous generators (SGs) connected to that grid. However, as traditional
SG-based electrical power sources are replaced with converter-tied renewable energy
sources (RESs), electrical power grids may become unstable following a grid disturbance.
In this dissertation, it is proposed that slip synchronous wind turbine systems (SS-WTSs)
be used to assist with grid stability by providing grid strength, inertia and reactive power
compensation in areas with a high concentration of converter-tied RESs.
An SS-WTS drivetrain fundamentally consists of a slip-permanent magnetic coupler
(S-PMC) and a directly grid-tied SG, i.e., without a power converter. However, the
literature regarding the design and modelling of the SG and S-PMC units in SS-WTSs
for grid-tied applications is not sufficient. Firstly, the designed SGs typically have low
synchronous reactance values, thus their performance is highly susceptible to grid voltage
variations. Secondly, the generators designed for SS-WTSs are not grid code compliant
and are thus ineligible for large-scale grid connected applications. Thirdly, the methods
used to model and analyse the S-PMC unit can yield inaccurate results when predicting
the maximum torque of the S-PMC, which can jeopardise the stability of the SS-WTS.
Finally, there are concerns regarding the added mass and cost of the S-PMC unit.
In this dissertation, optimal impedance matching for grid-tied SGs is investigated.
It is found that a synchronous reactance of 0.5 per unit yields the best grid-connected
performance. Furthermore, a rotor hybrid-excited design methodology for grid-tied synchronous
generators is proposed and evaluated. Accurate finite-element (FE) analysis
shows that the proposed hybrid-excitation method ensures that the grid-tied generator
has variable-flux and hence variable reactive power compensation capabilities to comply
with modern grid codes. Measured results from a novel 15 kW hybrid-excited SG verify
the FE results and the grid code compliance of the generator. Additionally, in this
dissertation, the FE-based, frozen permeability method is used to accurately determine
the modelling parameters and performance characteristics of the S-PMC over its entire
operating range. Measured results from an S-PMC prototype validate the FE analysis
results. A study that compares medium- and low-speed S-PMCs at a 3 MW power rating
indicates that the medium-speed S-PMC has a significantly lower cost-per-kW and active
mass compared to the low-speed S-PMC. Thus, S-PMCs in high-speed SS-WTSs should
be considered to reduce the overall mass and cost of the SS-WTSs.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD) -- Stellenbosch University, 2022.
Keywords
Synchronous generators, Wind turbines, Smart power grids, Photovoltaic power systems, Electric power distribution, UCTD