60 research outputs found
Parameter estimation of squirrel-cage motors with parasitic torques in the torque–slip curve
This paper studies parasitic torques in steady-state torque–slip curves of squirrel-cage induction motors. The
curves of nine motors (small, medium and large size units), three of which were measured in the range s = 2 to 0, are
analysed. The torque–slip curves of eight of these nine motors differ significantly from the smooth curves predicted by the
classical single- and double-cage models: a torque dip at large slips in the motoring regime and a notable torque increase
in the braking regime occur. As parasitic torques have been traditionally associated with space harmonics, two singlecage
chain models (which consider the space harmonics) are tested to fit the measured torque and current of the three
measured motors: one neglects the skin effect, leading to the wrong torque prediction, whereas the other (the chain
model proposed in the early 60s in the literature) considers the skin effect, leading to an accurate torque prediction.Postprint (author's final draft
Power control strategies during voltage sags according to Spanish grid code
In order to connect any power converters into the
grid there are some grid requirements to insure the safe operation of the grid. So, the control of the converters especially during abnormal condition e.g. during voltage sags is a very important key to guarantee the good behavior of the distributed generation
system. In this paper four control strategies,
will be stated in the literature, are discussed in order to ensure their ability to match the grid requirements when unsymmetrical voltage sags are produced in the network. The Spanish grid code did not give any information about the
negative sequence, and it only represents the positive sequence components. Therefore, the main contribution of this paper is to verify the grid code with not only the positive sequence but also with the negative sequence.
Moreover, the system is tested by simulation to show that the results cope well with the analytical equations.Postprint (published version
Smart switching in single-phase grid-connected photovoltaic power systems for inrush current elimination
Grid-connected photovoltaic (PV) power systems are one of the most promising tech- nologies to address growing energy demand and ecological challenges. This paper proposes smart switching to mitigate inrush currents during the connection of single-phase transformers used in PV systems. An effective inrush current mitigation contributes to the reliability of PV systems. The inrush current severity is influenced by the pseudorandom residual flux at the transformer core and the energization point-on-wave. The most common approach to avoid inrush currents is controlled connection, which requires prior knowledge of the residual flux. However, the residual flux can differ in each case, and its measurement or estimation can be impractical. The proposed smart switching is based on a comprehensive analysis of the residual flux and the de-energization trajectories, and only requires two pieces of data (fRM and f0, flux values of the static and dynamic loops when the respective currents are null), calculated from two simple no-load tests. It has a clear advantage over common approaches: no need to estimate or measure the residual flux before each connection, avoiding the need for expensive equipment or complex setups. Smart switching can be easily im- plemented in practical settings, as it considers different circuit breakers with distinctive aperture features, making it cost-effective for PV systems.Peer ReviewedPostprint (published version
Voltage sag influence on controlled three-phase grid-connected inverters according to the Spanish grid code
© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksTo ensure the safe operation of the grid, there are some requirements to be taken into consideration to connect power converters. During abnormal conditions – e.g. during voltage sags –, the control of the converters is a very important key to guarantee power quality and good behaviour of the distributed generation system. The aim of this study is to employ two possible control strategies for a grid-connected inverter according to the Spanish grid code, and to analyse the behaviour of the output voltages during both symmetrical and unsymmetrical voltage sags. The analytical development shows the sag influence on currents, voltages, active and reactive powers. These influences are explained through Ku transformation in the synchronous reference frame, thus giving a representation for electrical variables easiest to analyse. The results show how control strategies affect the converter behaviour and how they can support the main grid during faults through the control of active and reactive power injection. Sags with different durations and depths have been taken into account, which can provoke critical values for electrical magnitudes and can lead to the violation of the grid code. The proposed control strategies study has been validated by means of both simulations in MatlabTM–Simulink and experimental resultsPeer ReviewedPostprint (author's final draft
Symmetrical and unsymmetrical voltage sag effects on the three-phase synchronous machine stability
This paper focuses on the effects of voltage sags, both symmetrical and unsymmetrical, on the three-phase Synchronous Machine (SM). Voltage sags on SM cause speed variations, current and torque peaks and hence may cause tripping and equipment damage. The consequences of voltage sags on the machine behaviour depend on different factors such as sag’s magnitude (or depth), duration, initial point-on-wave and the parameters of the electrical machine. In this study, three SMs of different rated power have been considered in order to simulate the voltage sag effects for specific conditions and analyze the machine stability.Postprint (published version
Control proposals for the operation of power converters in wind power systems
The wind turbines based on Doubly Fed Induction
Generators (DFIG) represent almost the 60% of the installed
windpower worldwide. Nowadays most of the existing algorithms,
used for regulating the production of active and reactive power in
such systems, are based on control techniques that use rotational
reference frames theories, where the dq0 is maybe the most
popular. Despite of its good operation under balanced conditions
its behaviour depends on a good phase synchronization, otherwise
the different transformations cannot be performed with enough
guarantees. Of course this kind of response is becomming
insuficient, as far as the windpower penetration is increasing.
This paper will present a new control topology for DFIG, that
are connected using back to back inverters, that is able to
behave better in case of faulty grids. Besides some proposals
for improving the operation of the DFIG will be presented as
well. The aim of that part is to extend the possibilities that the
back to back inverters are able to offer to the generator system
by means of including new functionalities, as the active filtering
capability or the reduction of the flicker emission that will be
later discussed.Postprint (published version
Artificial intelligence-based protection for smart grids
Lately, adequate protection strategies need to be developed when Microgrids (MGs) are connected to smart grids to prevent undesirable tripping. Conventional relay settings need to be adapted to changes in Distributed Generator (DG) penetrations or grid reconfigurations, which is a complicated task that can be solved efficiently using Artificial Intelligence (AI)-based protection. This paper compares and validates the difference between conventional protection (overcurrent and differential) strategies and a new strategy based on Artificial Neural Networks (ANNs), which have been shown as adequate protection, especially with reconfigurable smart grids. In addition, the limitations of the conventional protections are discussed. The AI protection is employed through the communication between all Protective Devices (PDs) in the grid, and a backup strategy that employs the communication among the PDs in the same line. This paper goes a step further to validate the protection strategies based on simulations using the MATLABTM platform and experimental results using a scaled grid. The AI-based protection method gave the best solution as it can be adapted for different grids with high accuracy and faster response than conventional protection, and without the need to change the protection settings. The scaled grid was designed for the smart grid to advocate the behavior of the protection strategies experimentally for both conventional and AI-based protections.This work is supported by Li Dak Sum Innovation Fellowship Funding (E06211200006) from the University of Nottingham Ningbo China.Peer ReviewedPostprint (published version
Testing of three-phase equipment under voltage sags
This paper provides insight into the testing of three-phase equipment exposed to voltage sags caused by faults.
The voltage sag recovers at the fault-current zeros, leading to a ‘discrete’ voltage recovery, that is, the fault is cleared in
different steps. In the literature, the most widespread classification divides ‘discrete’ sags into 14 types. The authors study shows that it is generally sufficient to consider only five sag types for three-phase equipment, here called ‘time-invariant
(TI)’ equipment. As the remaining nine sag types cause identical equipment behaviour in Park or Ku variables, the number of laboratory tests (or of extensive simulations) on equipment under sags is reduced by a ratio of 14/5. The study is validated by simulation of a three-phase induction generator and a three-phase inverter, which are ‘TI’, and a threephase
diode bridge rectifier, which is not ‘TI’. Both analytical study and simulation results are validated by testing a three-phase induction motor and a three-phase diode bridge rectifier.Postprint (author's final draft
Control of DFIG-WT under unbalanced grid voltage conditions
The voltage oriented control in the synchronous
reference frame (VOC-SRF) have been extensively used for
controlling wind turbines based on doubly fed induction
generators (DFIG-WTs) through the rotor side converter of a
back to back power processor. Although its performance is fast
and accurate under balanced conditions its behaviour is not
good enough when the voltage of the mains is unbalanced, unless
an independent control for the positive and the negative
sequence is implemented. This paper proposes a new control
system able to control the DFIG-WT under unbalanced
conditions using a simple algorithm, which does not need to be
implemented for both symmetrical components but on the static
αβ reference frame. The reliability of the presented system will
be tested by means of PSCAD simulations under balanced and
unbalanced grid conditions.Peer ReviewedPostprint (published version
Grid synchronization for advanced power processing and FACTS in wind power systems
The high penetration of wind power systems in the
electrical network has introduced new issues in the stability and
transient operation of the grid. By means of providing advanced
functionalities to the existing power converters of such power
plants it is possible to enhance their performance and also to
support the grid operation, as the new grid codes demand. The
connection of FACTS based on power converters, such as
STATCOMs, are also contributing to the integration of
renewable energies improving their behavior under
contingencies. However, in both cases it is needed to have a grid
voltage synchronization system, able to work under unbalanced
and distorted conditions. This paper presents the discrete
representation and performance of three PLL’s structures,
designed to work in that kind of situations. Their
synchronization capability will be tested in different scenarios
and their performance constraints will be established according
to the Grid Code Requirements (GCRs)Peer ReviewedPostprint (published version
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