Claus Leth Bak

A Simple Adaptive Overcurrent Protection of Distribution Systems With Distributed Generation

A significant increase in the penetration of distributed generation has resulted in a possibility of operating distribution systems with distributed generation in islanded mode. However, overcurrent protection of an islanded distribution system is still an issue due to the difference in fault current when the distribution system is connected to the grid and when it is islanded. This paper proposes the use of adaptive protection, using local information, to overcome the challenges of the overcurrent protection in distribution systems with distributed generation. The trip characteristics of the relays are updated by detecting operating states (grid connected or island) and the faulted section. The paper also proposes faulted section detection using time overcurrent characteristics of the protective relays. Simulation results show that the operating state and faulted section can be correctly identified and the protection system settings can be updated to clear the faults faster.

A Review of Passive Power Filters for Three-Phase Grid-Connected Voltage-Source Converters

In order to reduce size and cost, high-order passive filters are generally preferred in power converters to cancel out high-frequency harmonics caused by pulsewidth modulation. However, the filter resonance peaks may require the use of passive dampers to stabilize the interactions between the load and source impedances. Furthermore, the stabilizing effect is more difficult to be guaranteed for cost-optimized filters, which are characterized by low-inductance and high-capacitance passive components. In this paper, several passive filter topologies used to interface voltage-source converters with the utility grid are reviewed and evaluated in terms of damping capability, stored energy in the passive components, and power loss in the damping circuit. In addition, the influences of different switching frequencies of power converters on the passive filter design are discussed in the range 1-15 kHz. Illustrative design examples of the passive filters and experimental data are also provided.

A Systematic Approach for Dynamic Security Assessment and the Corresponding Preventive Control Scheme Based on Decision Trees

This paper proposes a decision tree (DT)-based systematic approach for cooperative online power system dynamic security assessment (DSA) and preventive control. This approach adopts a new methodology that trains two contingency-oriented DTs on a daily basis by the databases generated from power system simulations. Fed with real-time wide-area measurements, one DT of measurable variables is employed for online DSA to identify potential security issues, and the other DT of controllable variables provides online decision support on preventive control strategies against those issues. A cost-effective algorithm is adopted in this proposed approach to optimize the trajectory of preventive control. The paper also proposes an importance sampling algorithm on database preparation for efficient DT training for power systems with high penetration of wind power and distributed generation. The performance of the approach is demonstrated on a 400-bus, 200-line operational model of western Danish power system.

Optimal Design of High-Order Passive-Damped Filters for Grid-Connected Applications

Harmonic stability problems caused by the resonance of high-order filters in power electronic systems are ever increasing. The use of passive damping does provide a robust solution to address these issues, but at the price of reduced efficiency due to the presence of additional passive components. Hence, a new method is proposed in this paper to optimally design the passive damping circuit for the LCL filters and LCL with multituned LC traps. In short, the optimization problem reduces to the proper choice of the multisplit capacitors or inductors in the high-order filter. Compared to existing design procedures, the proposed method simplifies the iterative design of the overall filter while ensuring the minimum resonance peak with a lower damping capacitor and a lower rated resistor. It is shown that there is only one optimal value of the damping resistor or quality factor to achieve a minimum filter resonance. The passive filters are designed, built, and validated both analytically and experimentally for verification.

A review of passive filters for grid-connected voltage source converters

LCL filter is the common interface between the Pulse Width Modulated Voltage Source Converter (PWM VSC) and the utility grid due to high harmonic attenuation capability and reduced size of the passive elements. The present paper investigates the most promising passive damping methods for the LCL topology but also propose an overview of high order filters capable to offer even more attenuation than the LCL filter at a reduced size. This is the case of more recently introduced LCL topology with tuned traps. However, it is shown that by decreasing the size of the passive elements the robustness of the filter also is decreased. Thus, a comparison of the filter features including the robustness towards grid impedance variation, harmonic attenuation capability and losses in the damping circuit became interesting from a practical implementation point of view. The design of the proposed filters is validated by simulation and experimental results and covers two scenarios: high and low/medium power applications.

Field Test and Simulation of a 400-kV Cross-Bonded Cable System

This paper discusses cable modeling for long high-voltage ac underground cables. In investigating the possibility of using long cables instead of overhead lines, the simulation results must be trustworthy. Therefore, model validation is of great importance. This paper gives a benchmark case for measurements on a 400-kV cable system with cross-bonded sheaths. This paper describes in detail the modeling procedure for the cable system and compares simulation results with the transient field test results. It is shown that although the main characteristics of the waveforms are well reproduced in the initial transient, there are significant deviations between the simulation and measurement results. An analysis indicates that the main cause for the deviation is inadequate representation of the current distribution on conductors since the modeling approach does not take proximity effects into account. The measurement results can be received by contacting the first author of this paper.

A theoretical bilevel control scheme for power networks with large-scale penetration of distributed renewable resources

In this paper, we present a bilevel control framework to achieve a highly-reliable smart distribution network with large-scale penetration of distributed renewable resources (DRRs). We assume that the power distribution network consists of several residential/commercial communities. In the first level of the proposed control scheme, distributed community-level controllers are designed based on the stochastic model of demand and generation. These controllers utilize the local storage units and DRRs to maintain a certain level of reliability for the community. In order to formulate the residential demand and DRRs, we use the Gaussian white noise added to some periodic signals, formulated as a stochastic process. In the second level of the proposed control framework, we take the advantage of bulk generation units to improve the reliability by a global flow controller. In other words, we get help from a few number of bulk power plants in the grid to improve its reliability in the context of satisfying the residential demand with high probability. The global controller dispatches the available non-renewable power plants between communities to enhance the reliability of each community. Using our stochastic model, we obtain a theoretical low-threshold for the certainty of the demand satisfaction in the smart power distribution network.

Adaptive Tuning of Frequency Thresholds Using Voltage Drop Data in Decentralized Load Shedding

Load shedding (LS) is the last firewall and the most expensive control action against power system blackout. In the conventional under frequency LS (UFLS) schemes, the load drop locations are already determined independently of the event location. Furthermore, the frequency thresholds of LS relays are prespecified and constant values which may not be a comprehensive solution for widespread range of possible events. This paper addresses the decentralized LS in which the instantaneous voltage deviation of load buses is used to determine the frequency thresholds of LS relays. The higher frequency thresholds are assigned to the loads with larger voltage decay which are often located in the vicinity of disturbance location. The proposed method simultaneously benefits from individual UFLS and under voltage LS (UVLS) features which operate in the power system without coordination. Numerical simulations in DigSilent PowerFactory software confirm the efficiency of proposed methodology in the stabilization of the power system after various severe contingencies.

Couplings in Phase Domain Impedance Modeling of Grid-Connected Converters

The output impedance of a power converter plays an important role in the stability assessment of the converter. The impedance can be expressed in different frames such as the stationary frame (phase domain) or in the synchronous frame (dq domain). To treat the three-phase system like a single-phase system, the system can be divided into positive and negative sequences in the phase domain. This paper demonstrates that there exist couplings between the positive and negative sequences, even in a balanced system due to the PLL, which is important for synchronization. Further it will be shown that even though these couplings are very small in magnitude, they are important in the stability of the converter.

Comparative evaluation of passive damping topologies for parallel grid-connected converters with LCL filters

In this paper a comprehensive analysis of three passive damping methods is done under parallel operation of multiple current controlled voltage source converters. One could argue that a well damped LCL filter with no peaking in the output impedance and stable designed controllers will turn into stable status under the parallel operation with other converters that share similar configuration. However, it is shown that this is not always the case, especially because of the coupling between converters and the grid impedance. For the considered ratings, under grid impedance variation, it is found that with grid-side current feedback the stability may be improved in parallel operation while for converter-side feedback, the stability of the current controller is always decreased compared with the single converter case. The proposed stability analysis and experimental tests demonstrates the theoretical analysis.