Vassilis C. Nikolaidis

Postmortem analysis and data validation in the wake of the 2004 Athens blackout

On July 12, 2004 at 12:39 local time, the south part of the Hellenic Interconnected Transmission System (including Athens) was split from the rest of the system and collapsed, driving consumers with a total load of 4500 MW into blackout. Technical information about the causes, the evolution, and the consequences of the blackout are presented in this paper. Some qualitative conclusions drawn immediately after the blackout are presented, as well as the results of further studies, including detailed simulations with corrected data, generator adjustments, and protection considerations

A Communication-Assisted Overcurrent Protection Scheme for Radial Distribution Systems With Distributed Generation

Conventional overcurrent protection schemes for radial distribution systems usually attempt to coordinate a recloser at the beginning of the feeder with the fuses on the laterals. The integration of distributed generation in distribution systems leads to problems related to protection coordination that are difficult to be solved by applying conventional protection techniques. This paper proposes an efficient communication-based protection scheme that implements common directional overcurrent relays instead of reclosers at the line, assisted by intertripping and blocking transfer functions. The proposed protection strategy guarantees selectivity regardless of whether the generating units are connected to the network or not, and can be designed retaining either the fuse-blowing or fuse-saving philosophy. Meaningful conclusions are derived from the application of the scheme on a test distribution system.

Emergency Zone 3 Modification as a Local Response-Driven Protection Measure Against System Collapse

This paper proposes a protection scheme that applies two supervisory zones on each distance relay in the transmission system and timers in order to avoid undesirable Zone 3 operations under voltage-stressed system conditions. The speed at which the apparent impedance trajectory crosses the two supervisory zones is measured by the relay in order to determine the rate of change of the apparent impedance. Zone 3 modifications based on the normalized rate of change of the apparent impedance are applied: 1) Zone 3 operating area is reduced by using circle or straight line blinders when the instability is clearly recognized; 2) Zone 3 is blocked when there is no enough time to identify the cause of the apparent impedance transition; and 3) no measure is applied for a fault or a power swing. Simulations performed in a test power system show the effectiveness of this method in mitigating system blackouts due to cascaded outages.

Online parameter identification and generic modeling derivation of a dynamic load model in distribution grids

The advent of smart grids and the installation of phasor measurement units in the distribution network have renewed the interest on the measurement-based load modeling approach. In this paper, a real-time load modeling and identification procedure of the well-known exponential recovery dynamic load model using synchrophasor data is presented. The performance of the proposed method is evaluated using measurements recorded in a low-voltage laboratory scale test rig. Several parameters of the procedure are investigated to evaluate the applicability of the method under real world conditions, including the impact of filtering techniques, outlier rejection, model optimization algorithms, etc. The findings of this paper verify the validity of the proposed method for realtime applications.

Adjusting third zone distance protection to avoid voltage collapse

Undesirable distance relay operations are possible to be initiated under heavy stressed system conditions leading to voltage instability. This can result in cascaded outages in the transmission system that may cause severe system deterioration. A number of recent blackouts have been reported to be initiated from undesired distance protection operations. This paper proposes an adaptive distance relaying scheme that avoids undesirable third zone tripping during the evolution of voltage instability phenomena. The relaying scheme is based on local measurements and provides the necessary time for other emergency controls like undervoltage load-shedding to be applied, if needed, in order to mitigate system collapse.

Setting Zero-Sequence Compensation Factor in Distance Relays Protecting Distribution Systems

Reliable protection of modern distribution systems is challenging due to various technical difficulties, mainly related to the need for more efficient use of existing networks (e.g., shorter interruption durations and fewer affected customers, integration of distributed generation, meshed network operation, etc.). Such challenges require, among others, the reconsideration of conventional protection techniques. A potential solution under examination concerns the adoption of distance protection for distribution feeders, due to its advantages compared to overcurrent protection. However, as distance relays are mainly designed for transmission networks, there are several issues to deal with in distribution applications, such as the proper setting of the zero-sequence compensation factor (K 0). The latter issue is critical in order to guarantee correct relay operation during single-line ground faults. This paper examines the effect of K0 on the operation accuracy of distance relays protecting inhomogeneous distribution feeders. Theoretical analysis, as well as investigation of various influencing factors, result in the determination of a pattern, which relates relay accuracy to K 0. Based on this analysis, a simple methodology for setting K0 properly is proposed, which is implementable with commercially available relays. The methodology is applied on a test distribution feeder and meaningful conclusions are derived.

Influence of fault-ride-through requirements for distributed generators on the protection coordination of an actual distribution system with reclosers

This paper analyses the existing protection scheme of a real distribution system with distributed generators, in Greece. Network protection utilizes three successive reclosers at the main trunk and fuses at the laterals. The generating units are protected by overcurrent and voltage/frequency relays. The analysis focuses on the fault-ride-through capability of the generating units and proposes the resetting of the generators and network protection relays so as to conform to the requirements imposed by distribution system operators and international standards. The proposed protection system guarantees selectivity for any short-circuits occurring inside or outside the distribution system, irrelative if the generating units are connected to the network or not. Meaningful conclusions are derived from the application of the proposed protection coordination principle.

A distance based protection scheme for distribution systems with distributed generators

This paper investigates the application of distance relays in distribution systems with distributed generators. An analysis of the parameters that influence the effectiveness of distance relaying in distribution networks is performed. Protection coordination between the distance relays and the existing protection means on the line is studied with the help of a realistic distribution network. Coordination problems are identified and a simple distance-based protection scheme is proposed.