Fault detection and fault tolerance concerning electric drives are important
research fields that ensure robustness and increased safety of vehicles and other drive
systems. One of key components inside the permanent magnet synchronous motor
electric drive, for mobile systems, is it’s battery power supply. Fault inside the
drive’s power supply can cause a sudden interruption of the energy flow. In an event
of an improper response of the control system this interruption can result in a loss of
control over the electrical quantities of the drive, damage of the electronic
components, as a result of overvoltage condition, or an abrupt loss of supply feeding
the control systems.
The first step towards establishing operation in the presence of a fault is a
robust real-time detection of the power supply open circuit fault. This thesis proposes
a model-based approach of fault detection based on the lumped electric circuit model
of the power supply system. Using the power supply model, it is possible to
determine the DC-link voltage of the electric drive based on the calculated inverter
current and open circuit battery voltage. Since the open circuit battery voltage cannot
be measured directly, a sliding mode observer is proposed. The deviation between
the estimated and measured DC-link voltage is then used as a fault indicator. Value
of a diagnostic threshold, which enables elimination of false positive detection, is
based on the analytic calculation of measurement error and system parameter
variability.
Operation during the fault relies on the rotational energy of the electric drive,
which could provide the energy necessary for maintaining the level of a DC-link
voltage. Control over DC-link voltage enables operation in field weakening region,
prevents secondary damage due to failure of electric components and, in case of
drives supplied with single power source, provides the energy source for control
systems. A DC-link voltage regulator is proposed, with its internal control loop based
on the well-known field oriented control of the permanent magnet synchronous
motor. Parameters of the cascade control loop are crucial for the stability and rapid
transition to fault operation during an active open circuit fault. An analytic approach
based on the model of the system was developed for determining the parameters of
the DC-link voltage PI controller, using the method of reducing the order of system
transfer function.
System modelling and numerical calculations inside MATLAB/Simulink
environment were used for verification of the key approaches stated in the thesis.
Results comprise of comparisons between established lithium battery models and
that proposed in the thesis, impact analysis of the change in machine’s magnetic field
energy during the fault, and numerical results of error transformations that affect
successful fault detection.
Experimental results that confirm the possibility of deploying the proposed
principles on a real electric drive are performed by coupling two electric drives with
its respective battery power supplies. Firstly, the immunity to a false positive fault
detection in case of sudden changes of the drives operating point is verified, followed
by a successful presentation of fault detection in case of power supply circuit
interruption. The response of the DC-link voltage regulator in combination with
detection algorithm is verified for various operating conditions defined prior to the
occurrence of open circuit power supply fault.
Conclusion of the thesis contains an overview of novel contributions to the
related research fields, prospects for future research, and the possibilities of applying
proposed principles to the broader research field of electric drive systems.
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