A Power System Emergency Control Scheme in the Presence of High Wind Power Penetration

The main goal of the project is to improve existing protection technology by localizing the load shedding scheme in grids with high share of dispersed generation dominantly provided by renewable energy sources, i.e. wind, wave, solar, biomass, etc.

The higher complexity and lower predictability of grids with a high penetration of renewable energies makes it difficult to overseen and overcome widespread range of combinational and cascading events, just by relying on conventional protection systems, which generally do no coordinate the different grid variables in the respective protection schemes. Utilization of all of locally measurable variables, e.g. frequency, its rate of change, voltage drop, power flow direction under an integrated decentralized plan is done in this project, in order to improve the grid reliability.

The proposed scheme benefits from a decentralized strategy, which reduces the burden of central control by decreasing the amount of data processing and more important, by avoiding any control problems in the system due to loss of communication link resulting from accidents of Cyber Security attacks, which have been recently located in the center of attention. The algorithms developed in this project may also constitute the lower level of a hierarchical control strategy, which can be activated in case of losing the communication with the control center.

Modern power protection relays often provide several protection schemes inside of one common package. However, they normally operate without coordination even when they are implemented inside a common module. This project intends to benefit from the fact that the existing technology already uses, as input, all the required data, to coordinate distinct plans under an integrated and versatile scheme. The proposed scheme automatically updates the frequency set points, stage time delays used for load shedding as a function of fault location and severity of the disturbance/s. It is a comprehensive solution for all possible combination of contingencies; i.e., not only is efficient for conventional power systems, but also for the time-variant structure and dispatch of grids resulting from high share of renewable energy sources.

In order to achieve an efficient, fast and optimal load-generation balance following the under frequency contingencies in the power system, both load and generation sides should contribute in the efforts. The load-frequency control as an ancillary service provided by renewable energy sources and load shedding as an emergency control should be localized. It means that the active power deficit should be compensated locally at vicinity of event location in order to avoid transferring the demanded active power from distant units to the incidence place. In this thesis, localization of both load-frequency control an load shedding are fulfilled using locally measured voltage drop data in the decentralized control strategy.

The proposed load shedding scheme is coordinated with existing plant protection relays, which are normally installed on the conventional synchronous machines. Considering the frequency-time characteristic of plant protection relays in the load curtailing plan makes the proposed scheme preventive against successive outage of generation units by them, which worsen the stability of power system.

Moreover, preventive aspect of proper control decisions are further improved by considering early outage of renewable energy sources due to malfunction operation of LVRT grid code under the islanding situations, even though the wind turbine is not under stress neither electrically or mechanically.

Since the actual active power deficit may no longer be estimated properly due to outage of synchronous machines in cascading events or in the presence of renewable energy sources due to employed power electronic converters, decoupled inertia of renewable source and hence reduction of total inertia of the grid. The current inertia of the power system and therefore the actual active power deficit is approximated/updated at each load shedding stage.