Secondary protective control for mitigation of protection misoperations in electric power systems

Abstract

This paper investigates the feasibility of a selective secondary protective control strategy proposed to maximize the likelihood of recovery from misoperations of the existing (primary) protection in a power system. A scalable stochastic discrete-state model is established, taking into consideration of the processes of protection misoperations and their mitigations. Such misoperations have been a main culprit of cascading failures in modern power systems. The likelihood of recovery from protection misoperations is quantified by a set of security indices that formally incorporate the uncertain knowledge of the continuous-state of rotor angles/speed deviations of synchronous generators and that of the discretestate of equipment faults and primary protection misoperations. The proposed secondary protection leverages on the ever more available time-synchronized samples of networked sensors for diagnosis and fault-tolerant control to cost effectively improving power system reliability without altering the existing protection system. The technology readiness for implementing the secondary protective control is examined through a three-area test system.

Transient stability assessment of large lossy power systems

“…There are many reasons for online transient stability assessment, such as the need to inform operators of the expected behaviours of a system under a list of contingencies and on the available transfer limits at key interfaces [4] associated with its current operating condition. This study is mainly motivated by the more time critical needs, such as the need to recover from primary protection misoperations [5] and the need to call upon special protection schemes (SPS) [6] when the opportunities to recover have been lost. A primary protection system, consisting of mainly relays and breakers, is deployed to remove (or trip) faulty equipment and keep the remaining power system secure.…”

“…SPS [6,11] use generation tripping, or load shedding, or network separation, etc., as a last resort to prevent large-scale blackouts. The authors of this paper have proposed a secondary protection scheme that exploits modern technologies to avoid invoking an SPS [5,12]. Online transient stability assessment plays an essential role in both secondary protection and SPS.…”

“…This assessment approach involves (i) online diagnosis to select the protective control action/ inaction that establishes the sought post-fault RoA, or retains the pre-fault RoA [28], (ii) online tracking of the electromechanical state trajectory [12], (iii) offline computation of the RoAs of prefault and anticipated post-fault circuit topologies [25], and (iv) online evaluation of fault coverage to conclude the likelihood a transiently stable post-fault system. A 9-bus system was used for proof of concept summarised in (i)-(iv) [5]. Resolving computation and scalability issues for (ii), (iii), and (iv) is the focus of this paper.…”

“…iv. A fault coverage profile [5,26], the probability of post-fault system stability as a function of the time of a protective control action, is presented for one simulated event sequence of the 68-bus system. The profile is generated using an offline EEA-computed RoA, and individually tracked electromechanical state trajectories based on the 16 QSSS measurement models for the 16 generators.…”

“…The terms in (2) that do not directly contribute to the terminal circuit variables, such as those containing excitation current and damper currents, as well as the circuit parameters directly affected by the motion of rotor angle θ(t) = ω 0 t + δ(t) are moved to the lefthand side of (4). They collectively define a fictitious voltage source, which corresponds to the modified QSSS measurement model (5). The terms in (2) that contain terminal variables (v a , i a ) attached to known and fixed circuit parameters are moved to the right-hand side of (4).…”