Mostafa Malekpour

The present paper devises a second‐order system frequency response (SFR) model for power systems integrating virtual synchronous variable speed wind turbines (VSWTs). The inertial response model of full converter VSWTs, in which grid‐side converters are controlled using a virtual synchronous generator concept, is developed. The derived model is incorporated into the traditional SFR model deployed to estimate the frequency behaviour of the synchronous generators following infeed loss contingencies. The proposed SFR model formulates interaction between the VSWTs rotor speed reduction and the system frequency drop. The system loads inertia is also included in the SFR model. Additionally, an analytical approach to derive the suggested SFR model for the large power system including VSWTs with different capacities and operating points is presented. Solving the proposed model gives the analytical functions for estimating deviations in the system frequency as well as VSWTs rotor speed variations. Finally, the accuracy of the devised SFR model is verified through the time‐domain simulations of a modified IEEE 39‐bus system. Particularly, impact of the VSWTs virtual inertia on the efficiency of the proposed SFR model is studied.

This paper proposes a hybrid adaptive strategy to control virtual inertia of virtual synchronous generators (VSGs). The proposed technique can provide low frequency oscillation (LFO) damping and simultaneously improve primary frequency control (PFC) performance metrics. To this end, a control variable is defined to control extremum values of the VSG inertia during PFC process. The control variable uses an estimation of the grid frequency gradient to identify the PFC process stages following a power deficit event. The extremum of the VSG inertia to provide LFO damping is governed based on the control variable. In particular, the minimum value of the VSG inertia is restricted to its base value during the frequency arresting process in order to moderate the frequency rate of change. In contrast, the maximum value of the VSG inertia is limited to its base value in frequency recovery period to achieve a faster frequency recovery. The efficiency of the suggested method is validated through a power system with VSG penetration level as 50% of the system generation rating. The achieved results verify the superiority of the proposed scheme in the PFC improving and the LFO damping in comparison to the conventional strategy. Moreover, the suggested approach can efficiently decrease the required size of the VSG energy storage by about 50%.

During the power system restoration there is a high possibility that the load demand is greater than the normal system operation for thermostatic loads. This demand increment, called Cold Load Pick‐up (CLPU) has been a critical concern to utilities and will slow down the restoration process. Thus, it is imperative to predict the CLPU demand to perform restoration process more precisely. The use of new loads such as electric vehicles (EVs) has made new problems. In this paper, first the negative impact of EVs after an outage, on CLPU demand is investigated and then it is shown that the behavior of EVs during restoration service is the same as CLPU phenomenon. In doing so, the proposed approach uses Monte Carlo simulation method to predict EVs recharging demand. Since there will be some other new loads in the future that may have CLPU performance, a new approach is proposed to predict the CLPU demand of any load, without the need to have its exact model. ANFIS method is used to estimate CLPU demand based on previous outage cases in the feeder. The proposed approach is implemented in MATLAB ^® and simulation results confirm its ability in load prediction for restoration service.

Increasing the penetration of converter‐interfaced renewable energy sources like variable speed wind turbines (VSWTs) leads to drastic challenges in keeping the system frequency stability due to system inertia decrement. In this study, a novel variable virtual inertia control strategy for VSWTs is developed which considers their mechanical restriction. It also makes the implication of VSWTs virtual inertial response straightforward to be analytically considered in the system frequency dynamic constraints like rate of change of frequency and frequency nadir. Furthermore, these constraints are formulated and incorporated into a chance frequency constrained unit commitment in order to minimize system operation cost while co‐optimizing both inertial response from VSWTs/conventional units and governor‐based primary frequency response. The proposed stochastic methods can curtail the load and wind power to satisfy a predefined level of operation risk which can coordinate the unit commitment problem with wind turbines. The comparison analysis on a 10 units standard test system and 4 case studies illustrates that increasing the penetration level of VSWTs from 17% to 36% can provide important savings up to 22.5% while the frequency constraints are also met by VSWTs virtual inertia provision. This helps increasing the penetration of renewables from the view point of frequency stability.

This paper proposes an induction motor variable frequency drive system to investigate the demand response potential in handling smart grid's frequency. This can consequently mitigate the grid reliance on expensive power plants. To this end, a primary frequency controller is presented enabling the drive to reduce its consumption power in proportion to grid frequency drop. In this context, dynamic limitation of the drive due to load's inertia is considered through a motor's speed rate limiter. Moreover, an appropriate inertia emulator is proposed for the induction motor's smart drive by inspiring the inertial response of a direct-on-line motor. In the model validation study, the impacts of speed rate, maximum reserve power and motor driven load's inertia on dynamic behavior of smart motor load during the frequency support are addressed. Furthermore, the effectiveness of the smart load's contribution in primary frequency regulation of IEEE 39-bus test system is explored. In this regard, a critical droop coefficient that guarantees the maximum reserve power delivery for smart drives is analytically derived. Finally, the proposed approach is extended to include the critical droop derivation for a set of smart drives with different sizes and priorities to rank their participation in primary frequency regulation process.

A torque limit-based (TLB) method was proposed in literature in order to emulate inertial response of variable speed wind turbines (VSWTs). In this paper, this conventional TLB scheme is firstly modified by considering a finite ramp rate for inertial power of the VSWT. It is exposed that the maximum values of the VSWT's inertial power and kinetic energy released by its rotor have a non-linear relationship with its operation point. Then, a linear TLB scheme is proposed to make the inertia emulation more flexible by customizing its key parameters based on the VSWT's operating point. Accordingly, the released kinetic energy and power ramp rate can be selected in proportion of the VSWT's power, rotor speed and/or its reserved kinetic energy. The derived scheme offers a significant reduction of the mechanical tensions on the turbine compared to the conventional one. In addition, when the parameters of the proposed strategy is designed according to the VSWT's power, the inertial response of the corresponding wind farm can be exactly estimated only by deploying its total generation, regardless of its wind turbines' installed capacities and operating points. Furthermore, a new approach is projected to estimate the VSWT's inertial response during the deceleration period using an analytical closed-form function. This facilitates large scale system studies. Finally, the efficiency of derived linear inertia emulation is evaluated through a typical grid with various levels of the wind power penetration.

Using their fast power reduction capabilities, the induction motor variable speed drives can successfully participate in the power system primary frequency control. A new control strategy for fast frequency support from the diode front-end induction motor variable speed drive is presented in this paper. By this, practical restrictions are considered. In this context, an existing conventional scheme presented in literature is firstly modified by deploying an estimated motor's power losses for a more exactly control design. Next, a novel control scheme is proposed. It attains the fastest frequency support from the drive and simultaneously restricts its minimum power consumption to a given positive value to prevent regeneration mode during the frequency support process. The frequency of applied voltage to the motor is adjusted through a proportional-integrator compensator with deviation of the drive's consumption power from its reference power as input. The reference power is synthesized by permanent and temporary components, which are determined based on deviation and time derivative of the power system's frequency, respectively. This method improves system frequency response in terms of the rate of change of frequency and the frequency nadir. Furthermore, the new control scheme can contribute to quickly damping inter-area oscillations in multi-machine power system.