The conventional Under Frequency Load Shedding (UFLS) scheme could result in unacceptably low frequency nadirs or overshedding in power systems with volatile inertia. This paper proposes a novel UFLS scheme for modern power systems whose inertia may vary in a wide range due to high penetration of renewable energy sources (RESs). The proposed scheme estimates the rate of change of frequency (RoCoF) of the center of inertia (CoI), and consequently, the loss of generation (LoG) size, using local frequency measurements only. An innovative inflection point detector technique is presented to remove the effect of local frequency oscillations. This enables fast and accurate LoG size calculation, thereby more effective load shedding. The proposed UFLS scheme also accounts for the effect of the inertia change resulting from LoG events. The performance of the proposed scheme is validated by conducting extensive dynamic simulations on the IEEE 39-bus test system using Real Time Digital Simulator (RTDS). Simulation results confirm that the proposed UFLS scheme outperforms the conventional UFLS scheme in terms of both arresting frequency deviations and the amount of load shed.@en

Short-circuit faults close to either end of a transmission line, are normally cleared instantaneously by the distance relay at that end and after hundreds of milliseconds, i.e., in Zone 2 operating time, by the relay at the opposite end of the line. This sequential tripping can be accelerated on condition that a reliable communication link is available for signaling between the two line ends. This paper proposes a novel non-communication method providing high-speed distance relaying over the entire length of the protected transmission line. The inputs to the method are the protected line parameters and local voltage and current signals measured by the relay, similar to those to conventional distance relays. The proposed method accomplishes Accelerated Sequential Tripping (AST) within a couple of cycles after the opening of the remote-end circuit breaker (ORCB) of the line. To achieve this, an accurate closed-form solution is derived for the fault distance in terms of post-ORCB voltage and current phasors. For the detection of the ORCB instant, a set of proper indices are proposed. This is to verify the fault distance calculated by the relay, before issuing a trip command. The proposed method is successfully validated by conducting more than 20000 hardware-in-the-loop (HIL) tests, and also using real-life data.@en