Filipe Miguel Faria da Silva

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.

Power system stability using decentralized under frequency and voltage load shedding

Load shedding (LS) is the last emergency control action against voltage or frequency instability or even system blackout. Conventional LS relays, often rely on only frequency data and due to equality of the frequency throughout the entire power system, there is the possibility of coincidence operation of relays and perhaps over load shedding. Besides, the LS locations are already determined regardless of disturbance location and may not be adaptive to the event scale and place. This paper addresses the decentralized coordination of Under Frequency Load Shedding (UFLS) and Under Voltage Load Shedding (UVLS) schemes in the LS relays instead of independent methods. The independent and constant frequency and voltage stage thresholds are merged to achieve a united and new elliptical thresholds. Since in addition to frequency, the voltage drop information is also employed in the new method, the load curtailment is started from the vicinity of failure point and radially propagates in the network until not only the frequency collapse is completely prevented, but also the frequency settles down in the permissible range. Numerical simulations which are carried out in DigSilent PowerFactory software confirm the efficiency of proposed methodology to stabilize the power system after a severe contingency.

Decentralized Coordination of Load Shedding and Plant Protection Considering High Share of RESs

This paper estimates the average Rate of Change of Frequency (ROCOF) following islanding and/or cascading event/s using inflection points of frequency profile. Moreover, a frequency collapse barrier scheme is set up by tuning the frequency set points of Load Shedding (LS) relays as a dynamic variable using voltage drop data coordinated with plant protection scheme. A frequency anti stalling scheme is developed to interrupt more load feeders in case of frequency stall between consecutive set points. This time-based approach adjusts the time delay of the relay stages to disconnect the feeders with more voltage drop first, in order to bring the frequency back to the permissible range before the tolerable time period of plant protection relays is over. The load curtailment is stopped to avoid over LS, if the proposed algorithm recognizes that the frequency reaches the safe region in time based on estimated average ROCOF. Tripping of feeders, which inject active or reactive power to the grid due to penetration of dispersed generations into the load feeders is also prevented by considering the power flow direction of feeders.

Methods to Minimize Zero-Missing Phenomenon

With the increasing use of high-voltage ac cables at transmission levels, phenomena, such as current zero missing, start to appear more often in transmission systems. Zero-missing phenomenon can occur when energizing cable lines with shunt reactors. This could considerably delay the opening of the circuit breaker (CB), leaving the system unprotected and vulnerable to failures. Methods to prevent zero-missing phenomenon are still being studied and compared in order to identify effective countermeasures. This paper contributes to these efforts by presenting several countermeasures that can be applied to reduce the hazards of zero-missing phenomenon. The authors discovered that this phenomenon can be eliminated, merely by using an extra CB or a preinsertion resistor.

Use of a pre-insertion resistor to minimize zero-missing phenomenon and switching overvoltages

With the increasing use of High-Voltage Cables, which have different electric characteristics from Overhead Lines, phenomenon like current zero-missing start to appear more often on the transmission systems. Methods to prevent zero-missing phenomenon are still being studied and compared to see which countermeasure works the best. Technically the best way to avoid zero-missing phenomenon produces very high switching overvoltages, making the operator to choose to either avoid the zero-missing phenomenon or to minimize the switching transients. This paper presents a method of determining an optimal value of the resistance of the pre-insertion resistor that results in minimizing both the zero-missing phenomenon and switching overvoltages simultaneously.

Harmonic stability assessment for multi-paralleled, grid-connected inverters

This paper investigates the dynamic interactions of current controllers for multi-paralleled, grid-connected inverters. The consequent harmonics instability phenomena, which features with oscillations above the fundamental frequency, are evaluated by the impedance-based stability criterion. The frequency range of effective impedance-based stability analysis is first identified. The effect of each inverter on the system harmonic instability is then identified by case studies on different groups of inverters. Lastly, the PSCAD/EMTDC simulations on a system with five passively-damped, LCL-filtered inverters are performed to verify theoretical analysis. It shows that the impedance-based stability analysis results agree with the time-domain simulations provided that the frequency of concerns are around the half of the Nyquist sampling frequency.

Coordination of voltage and frequency feedback in load-frequency control capability of wind turbine

In close future, with high Wind Power (WP) penetration in the power system, the burden of Load-Frequency Control (LFC) is gradually shifted to Variable Speed Wind Turbines (VSWTs). In order to equip the VSWT with LFC capability to support the grid during sudden load-generation imbalance, sufficient reserve capacity should be procured. This paper addresses the LFC scheme offered by VSWT. Feedback loop of locally measured voltage and frequency data is employed to improve transient and permanent response to achieve faster and more efficient LFC action and voltage regulation. The proposed scheme demonstrates remarkable improvement in transient state of both voltage and frequency profiles in comparison with conventional LFC designs provided by Central Power Plants (CPP) or Wind Power Plants (WPP). Numerical simulations carried out in DigSilent PowerFactory confirm the superiority of proposed methodology used in Permanent Magnet Synchronous Machine (PMSG) connected to 9-bus IEEE standard test system.

Harmonic modelling, propagation and mitigation for large wind power plants connected via long HVAC cables: Review and outlook of current research

This paper presents a state-of-the-art review on grid connection of large offshore wind power plants (OWPPs) using extra-long high voltage AC (HVAC) cables. The paper describes research by DONG Energy Wind Power in close collaboration with Aalborg University addressing related challenges through an industrial PhD project. The overall goal is to gain a better understanding of extra-long HVAC cable connected OWPPs, in order to ensure reliability and availability of OWPPs. This will reduce the cost of energy, as the risk of costly delays and modifications after the project has been commissioned can be reduced and operational availability and the reliability can be improved. Challenges hereto include lower resonance frequencies due to the long cables, frequency dependent modelling of long submarine cables, passive filtering, and PLL dynamics in the control loops (from a harmonic stability point of view).

Active power deficit estimation in presence of Renewable Energy Sources

The inertia of the power system is reduced in the presence of Renewable Energy Sources (RESs) due to their low or even no contribution in the inertial response as it is inherently available in the Synchronous Machines (SMs). The total inertia of the grid becomes unknown or at least uncertain following outage of SMs during cascading events. Therefore, the active power deficit following the disturbance/s may not be properly estimated by existing conventional System Frequency Response (SFR) methods in which the total inertia of the power system is required to be known. In this paper, the actual active power deficit is estimated independent of grid inertia, type and number of occurred cascading events after each Load Shedding (LS) stage using shed load amount, pre-shed and post-shed Rate of Change of Frequency (ROCOF). Numerical simulations conducted on IEEE 39 bus standard test system in DigSilent PowerFactory software demonstrates the accuracy of the proposed method.

Decentralized power system emergency control in the presence of high wind power penetration

The frequency thresholds in the classical Under Frequency Load Shedding (UFLS) are constant and prespecified independent of event scale and place. Furthermore, almost all of recent LS schemes are based on System Frequency Response method to estimate the active power deficit using initial frequency gradient following the event; a method that may no longer be valid in the presence of renewable energy sources. The Adaptive Under Frequency and Voltage LS (AUFVLS) scheme which is a decentralized and automatic LS plan and benefits from locally measured frequency and voltage is proposed to tune the frequency thresholds of LS relays online based on voltage deviation from pre-disturbance value. Instead of active power deficit estimation, the load feeders with more voltage decline and hence higher assigned frequency thresholds are shed first i.e. the feeders often located in the neighborhood of event point. The method efficiency is confirmed by numerical simulations conducted in DigSilent PowerFactory software with cascading events and islanding contingencies with different level of wind power share and active power deficit and compared to the classical UFLS method.