In the modern power systems operating at lower stability margins, conventional non-adaptive schemes cannot offer adequate protection for securing the power system, especially against combinational events. For major disturbances, the active power deficit is usually accompanied by reactive power deficit and frequency stability and voltage stability of the system are jeopardized simultaneously. In this paper, three adaptive combinational load shedding methods are proposed to improve operation of the conventional underfrequency load shedding scheme in order to enhance power system stability following severe disturbances. The proposed methods use locally measured frequency and voltage signals to counteract such events. In the proposed algorithms, load shedding is started from the locations which have higher voltage decay for longer period of time. The speed, location, and amount of load shedding are changed adaptively depending on the disturbance location, voltage status of the system, and the rate of frequency decline. Operation of the conventional and the proposed load shedding methods have been simulated in an actual large network. Obtained simulation results confirm that the proposed methods provide considerable enhancement in the power system voltage stability margin, and by using the proposed algorithms, various power system blackouts could be prevented.

The open-phase fault in vector controlled drives is known as one of the most prevalent failures that require immediate detection. It is still challenging to develop a trustable fault diagnosis scheme at light load, transients, and low-speed regions. This paper presents a novel open-phase fault diagnosis strategy for three-phase induction motor drives over the entire speed range and for different load conditions. By taking advantage of the second-order rotational park transformation (SORP), new fault indices are proposed to identify the open-phase location without using any additional sensor or electric equipment. Comparing with root mean square (RMS) calculation based methods, the proposed method detects the fault at least twice faster, with minimal system resources. The presented scheme may be implemented simply and is robust against the operating point changes. Proof of the proposed fault detection and localization method has been shown through experimental tests on a 250-W three-phase induction machine which is controlled by TI-DSP F28335.

In the recent years, operation of power systems at lower stability margins has increased the importance of system protection methods that protect the system stability against various disturbances. Among these methods, the load shedding serves as an effective and last-resort tool to prevent system frequency/voltage instability. For major combinational disturbances, the active power deficit is usually accompanied by reactive power deficit. Therefore frequency stability and voltage stability of the system are jeopardised simultaneously. For such cases, the conventional non-adaptive underfrequency load-shedding scheme cannot offer adequate protection to prevent system collapse. In this study a new combinational load shedding method is proposed to increase the security of power system during large disturbances. The proposed method uses locally measured frequency and voltage signals and does not need any communication link. Performance of the conventional scheme and the proposed load-shedding method using different load shedding criterions have been simulated in two test systems: an actual large network and a standard test system. Obtained simulation results confirm that the proposed schemes provide considerable enhancement in the power system voltage stability margin and by using the proposed method various power system blackouts could be prevented.