Grid fault ride-through in matrix converters for adjustable speed drives

University of Minnesota Ph.D. disseration. December 2014. Major: Electrical Engineering. Advisor: Ned Mohan. 1 computer file (PDF); vii, 81 pages, appendices A-B.A novel ride-through approach for matrix converters in adjustable speed drives is presented, utilizing the input filter capacitors as an energy transfer mechanism to support motor flux during grid fault events. The addition of three bi-directional switches is required to isolate the input filter capacitors from the collapsed grid voltages. The additional input switches, a new ride-through vector control strategy, and the post fault reconnection logic are shown to enable ride-through of many cycle faults without the use of an additional energy storage devices. The proposed architecture is verified in theory, simulation, and hardware.The architecture is valid for both indirect and direct matrix converters, provides full bi-directional power flow, and requires no additional reactive components. Additional benefits include reduced in-rush current, reduced transient voltage overshoot at plug-in, reduced damping losses, and potential harvesting of energy from remaining active grid phases.To support the work, a review of power quality assessments is included. Through this review it is shown that the proposed architecture allows matrix converter-based adjustable speed drives to successfully operate in >95% of grid fault events

Time-Delay Switch Attack on Networked Control Systems, Effects and Countermeasures

In recent years, the security of networked control systems (NCSs) has been an important challenge for many researchers. Although the security schemes for networked control systems have advanced in the past several years, there have been many acknowledged cyber attacks. As a result, this dissertation proposes the use of a novel time-delay switch (TDS) attack by introducing time delays into the dynamics of NCSs. Such an attack has devastating effects on NCSs if prevention techniques and countermeasures are not considered in the design of these systems. To overcome the stability issue caused by TDS attacks, this dissertation proposes a new detector to track TDS attacks in real time. This method relies on an estimator that will estimate and track time delays introduced by a hacker. Once a detector obtains the maximum tolerable time delay of a plant’s optimal controller (for which the plant remains secure and stable), it issues an alarm signal and directs the system to its alarm state. In the alarm state, the plant operates under the control of an emergency controller that can be local or networked to the plant and remains in this stable mode until the networked control system state is restored. In another effort, this dissertation evaluates different control methods to find out which one is more stable when under a TDS attack than others. Also, a novel, simple and effective controller is proposed to thwart TDS attacks on the sensing loop (SL). The modified controller controls the system under a TDS attack. Also, the time-delay estimator will track time delays introduced by a hacker using a modified model reference-based control with an indirect supervisor and a modified least mean square (LMS) minimization technique. Furthermore, here, the demonstration proves that the cryptographic solutions are ineffective in the recovery from TDS attacks. A cryptography-free TDS recovery (CF-TDSR) communication protocol enhancement is introduced to leverage the adaptive channel redundancy techniques, along with a novel state estimator to detect and assist in the recovery of the destabilizing effects of TDS attacks. The conclusion shows how the CF-TDSR ensures the control stability of linear time invariant systems