Bakhtyar Hoseinzadeh

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.

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.

Intelligent load-frequency control contribution of wind turbine in power system stability

In future, with ultra high penetration of Wind Energy (WE) in power system, the burden of Load-Frequency Control (LFC) is switched to Wind Turbines (WT). In order to equip the WTs with LFC capability to support the grid during load-generation imbalance, sufficient reserve capacity should be procured. This paper addresses a novel fuzzy logic-based LFC scheme for inertial support offered by WTs, combined with pitch-controlled de-loading and improves transient response to achieve faster LFC action. Numerical simulations which are carried out in DigSilent PowerFactory, demonstrates the superiority of proposed methodology used in Permanent Magnet Synchronous Generator (PMSG) connected to 9-bus IEEE standard test system.

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.

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.

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.

Malfunction operation of LVRT capability of Wind Turbines under islanding conditions

The Low Voltage Ride Through (LVRT) capability of Wind Turbines (WTs) determines the connectivity of WT to the grid based on both voltage sag magnitude and duration locally measured at Point of Common Coupling (PCC), in order to protect the WT against overloading out of its tolerable apparent power. If the WT is still inside its safe and secure operating condition and there is still some standby capacity available in it to support the grid, the WT should remain connected to the power system regardless of voltage sag magnitude and duration until the WT apparent power exceeds its nominal value. In case of islanding, which often is accompanied with a low voltage drop, the WT may be improperly disconnected while operates with less than half of its nominal apparent power. This situation necessitates investigation or perhaps a revision of LVRT grid code to be efficient for all possible incidents, not only short circuit faults, but also cascading events and islanding.

Challenges with harmonic compensation at a remote bus in offshore wind power plant

This paper investigates the challenges associated with remote harmonic compensation in offshore wind power plants through long cables and transformers. The interaction between the grid network and the wind power plant network can lead to the amplification of certain harmonics and potentially resonant conditions. Hence, the plant developer is required to maintain the harmonic distortion at the point of common coupling within the planning level limits using harmonic compensation, which is usually done by static filters. In this paper an active damping compensation strategy with a STATCOM using emulation of using emulation of resistance at the harmonic frequencies of concern is analyzed. Finally the results are demonstrated using time domain simulations in PSCAD.