Integration of wind generation uncertainties into frequency dynamic constrained unit commitment considering reserve and plug in electric vehicles
Introduction
In the operation of a synchronous power system, both security and frequency dynamic constraints should be considered. Imbalanced supply and demand results in deviations from the nominal frequency. For example, excessive supply increases frequency while excessive demand can reduce frequency. The deviation from nominal frequency is also referred to as frequency dynamics, a term concerned with a temporary period of the frequency response in the power system. These dynamics can be alleviated by the natural system inertia and also primary frequency response (PFR).
Fossil, hydro, and nuclear power systems and also the conventional synchronous units provide abundant inertia in the power system. On the other hand, the inertia level in large power systems is not usually controlled by operators (Wen et al., 2016a, Wen et al., 2016b). At present, to operate the power system, the primary reserve level is simply adjusted as a percentage of the generator’s capacity for PFR. Nevertheless, this strategy cannot ensure frequency security during dynamic conditions when there is an abrupt loss of generation (Wei et al., 2016).
Nowadays, the clean energy generations, particularly from wind and solar technologies have either no or limited inherent inertia as they are based on inverted connected machines or small, light-weight, and often non-synchronous technologies. The remarkable contribution of these clean energy generations in the conventional power system has resulted in a considerable part of synchronous generation units being replaced with renewable units and consequently, the natural system inertia and PFR provided by synchronous generation units are reduced (Soares et al., 2016). Therefore, the frequency dynamic and RoCoF (Sigrist, 2015) management is a serious problem for power system operators (Fernandez-Bernal, Egido et al., 2015).
A very efficient method of enhancing frequency dynamics is to include constraints dealing with frequency in the UC and Economic Dispatch (ED). An optimal reserve scheduling in a PFR-constrained UC framework was presented in (Restrepo and Galiana, 2005) which was able to adjust the primary frequency by a quasi-steady-state frequency boundary. In (Ela et al., 2014), the application of PFR through an incentivized scheme was described in the ancillary service market, designed based on the scheduling, pricing, and settlement requirements. The studies reported in (Doherty et al., 2005) and (Ahmadi and Ghasemi, 2014) formulated the frequency related constraints, including the Rate-of-Change-of-Frequency (RoCoF) and minimum frequency limits. A dispatch instruction was used in (Chávez et al., 2014) to ensure the adequacy of frequency nadir and provide adequate and suitable constraint for an OPF considering a dynamic model of the PFR. Furthermore, the system inertia under stochastic patterns proposed in (Lee and Baldick, 2013) and (Teng et al., 2016) was associated with WTs generation and loss of synchronous generation units. The two-stage stochastic frequency ED formulated in (Lee and Baldick, 2013) took into account the loss of asynchronous generation unit. In (Teng et al., 2016), a UC based on stochastic frequency was employed that assured the frequency nadir, RoCoF, and quasi-steady-state frequency to meet the security requirements while minimizing the operational cost considering the stochastic behavior of the wind generation.
Improving frequency stability of power systems mainly depends on synchronous generation units with slow dynamic responses. If the committing of the conventional generation units is not satisfactory, the output power of clean generation units need to be curtailed in order for more conventional synchronous units to operate and ensure acceptable system inertia. Alternatively, automatic load-shedding should operate to guarantee the electrical power balance and to prevent a frequency collapse (Khazali and Kalantar, 2015).
Using the fast-acting ESSs in a power system can help with managing the dynamic frequency as they can discharge active power to restore system power imbalance (Zhao and Guan, 2016). ESSs generally come in a variety of groups, the main categories being compressed air energy storages (Xue and White, 2018), pumped hydro energy storages (Basu, 2019), super capacitors (Azizi and Radjeai, 2018), BESS (Azizi and Radjeai, 2018), flywheels (Gadelrab et al., 2015), hydrogen fuel cell storages (Das et al., 2019), and super magnetic energy storages (Bhowmik et al., 2018). BESSs used in PEVs are able to store energy in large capacities (Bioki et al., 2013) and can be used as fast response ESS to enhance the PFR.
Studies carried out in the field of fast-response storage systems to improve PFRs have drawn the researchers’ attention to BESSs. In (Aghamohammadi and Abdolahinia, 2014), a BESS was used to offer primary frequency reserves. Fast-response batteries have been used to improve PFRs (Hsieh and Johnson, 2012). In (Wen et al., 2016), the operation of BESS was optimized for regulation of dynamic frequency as a component of Frequency Dynamics-constrained Unit Commitment (FDUC). The UC problem is expressed as an interval-based optimization, or Interval Unit Commitment (IUC), by taking into account the way stochastic behavior of WTs influence frequency dynamics and system inertia. It formulates the uncertainties of wind generation considering the ranges within upper and lower limits defined for WTs. An extra constraint is considered for transitions between upper to the lower limits. However, this type of modeling is conservative involving a lot of computation and increasing the total operational cost. The dynamic frequency control strategy proposed in (Sedighizadeh et al., 2019) employed BESSs taking into account uncertainties of WTs generation. Also, the system inertia, primary reserves, and fast-response compressed air energy storage were considered in the FDUC formulated in (Sedighizadeh et al., 2019). Ref (Daneshi and Srivastava, 2012). proposes a deterministic method for security constrained unit commitment with integration of CAES and wind generation with considering environmental and fuel constraints. Literature on PEVs and power system operation with considering renewable power generation is just starting to raise (Hu et al., 2016). Ref (Yang et al., 2015). reviews scheduling methods to integrate PEVs based on their computational techniques. In (Imani et al., 2014), the load uncertainty for solving Security constrained unit commitment (SCUC) problem in presence of PEVs is studied. Ref (Haddadian et al., 2015). analyzes the impact of integration of distributed ESSs with high penetration of variable renewable sources on the security, emission reduction, and the economic operation of electric power systems. In (Hosseini Imani, Jabbari Ghadi et al., 2018), the employment of a PEV in the SCUC problem is considered. In (Ban et al., 2017) this study, an energy hub with both a electrolyzer and a hydrogen gas turbine is proposed to accommodate a high penetration of wind power into a SCUC problem.
The studies that are performed for FDUC problem can be classified from different perspectives including type of formulation, considering energy storage technologies, considering renewable energy technologies, and considering DR. Table 1 lists the recent references regarding the above-mentioned perspectives.
Review of the extant literature suggests that using PEV as fast-response storage has been an under researched issue in attempts to enhance PFR. This paper proposes a less conservative model with reduced computing efforts for wind generations as an alternative to the FDUC model introduced in (Wen et al., 2016). The maximum hourly power generated by WTs in the proposed FDUC changes with respect to the wind uncertainty formulation as presented in (Pandžić et al., 2016) based on a limited number of three non-probabilistic scenarios. An alternative to the solution proposed in (Wen et al., 2016), PEVs were used instead of conventional BESSs as the representative of ESSs. The other contribution of the study has to do with considering the DR program for conserving the frequency dynamic security.
The main contributions of this paper are summarized as follows:
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Presenting a strategy for dynamic frequency control using PEVs to ensure RoCof and frequency nadir adequacies,
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Proposing an FDUC taking into account system inertia, primary reserves, and fast-response PEVs,
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Comparing the uncertainties of WTs involving only three non-probabilistic scenarios proposed in (Pandžić et al., 2016) with the modelling proposed in (Wen et al., 2016), and
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Incorporating the DR program in the FDUC problem.
The rest of this paper is arranged as follows. Section 2 explains the issues of frequency dynamics and their control regarding PEVs. The problem formulated for the study is stated in Section 3. This is followed by the presentation of numerical results and the conclusions in Sections 4 Numerical results, 5 Conclusions, respectively.
Section snippets
Frequency dynamics
In this study, the generation and reserve scheduling are combined with the stability of dynamics of system frequency. Whenever a synchronous generation unit is separated from the power system, a frequency drop is observed in the power system due to a contingency and the system experiences such dynamics. The first-order swing equation can be formulated to express these dynamics as follow (Wen et al., 2016):
A typical short-term frequency response is
Problem formulation
The formulation proposed in this paper considers several key factors in power system operation including the practical technological limits in the normal operation of the system, the need for the initial reserve of synchronous generation units, the corrective measures of PEVs for post-contingency inertia response, controlling the primary frequency, and the DR program to secure the dynamic frequency. Although employing PEVs imposes extra costs on the power system operator, the expenditure is
Numerical results
The formulation of the proposed model as brought in Section 3 is not Case specific. However, to decrease the burden time, the proposed model was tested in a reference 6-bus test system shown in Fig. 8 for five case studies. The necessary data for numerical simulation was obtained from (Wen et al., 2016). Fig. 9 illustrates the aggregated electrical demands for each time period of the day ahead. Table 2 presents the characteristics of the synchronous generation units. The active power generated
Conclusions
A new model was proposed in this work for simultaneous scheduling of generation and reserve in synchronous generation units of power systems with high penetration of WTs incorporating PEVs and considering the DR program. The model deals with system frequency dynamics and identifies the required generation and reserve for minimizing operation costs in normal operation to meet the frequency nadir and RoCoF indices in post-contingency operation. The simulated Case studies showed that controlling
Author contribution
Dear Prof. Jiří Jaromír Klemeš, The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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