A unified intelligent control framework for UFLS relay failure mitigation, load frequency regulation, and demand forecasting in solar-PV systems

This paper analyzes some major factors for the impact of wind power integration on power system dynamic frequency characteristics in theory. Combining with an actual grid, test scenarios with various scales of doubly fed induction generator (DFIG)-based wind power integration are formulated. Through simulation in the scenarios with FASTEST software, the impacts on the action of under frequency load shedding (UFLS) scheme are displayed in two aspects, the rate of change of frequency (ROCOF) and wind turbine generator own protection. Simulation result shows that the ROCOF will not affect the lock action of UFLS devices in the test system of this paper. Wind turbine generator protection may worsen frequency responses, so it is proposed to be in harmony with the design of UFLS scheme.

The fast growth of residential and industrial demands, combined with the incorporation of renewable energy sources (RESs) like wind and solar, is placing an increasing amount of strain on modern interconnected power systems (IPSs). System stability and dependability may be jeopardized by these additions since they cause notable variations in tie-line power, terminal voltage, and system frequency. Loops for automatic voltage regulation (AVR) and load frequency control (LFC) are essential for delivering high-quality electricity with the least amount of variance. For improved dynamic regulation in a two-area IPS, where one area is fueled by a traditional thermal generator and the other combines photovoltaic (PV) and wind energy sources, this research proposes a proportional integral derivative (PID) controller. Both domains incorporate battery energy storage systems (BESS) to facilitate frequency regulation. This work's unique contribution is the combination of a perturb and observe (P&O) MPPT-controlled permanent magnet synchronous generator (PMSG)-based wind energy system and a fuzzy-based maximum power point tracking (MPPT) PV system with BESS. A gradient-based optimizer (GBO), a meta-heuristic technique, is used to optimize the PID controller. The fitness function for assessing performance is the integral of time times the squared error (ITSE). A 5% step load perturbation (SLP) is used to compare the frequency, voltage, and tie-line power responses of the GBO-tuned PID controller to those of other GBO-based variants, such as integral–proportional–derivative (GBO-IPD), tilt integral derivative (GBO-TID), and integral–proportional (GBO-I-P) controllers. The suggested GBO-PID controller performs better in this hybrid power system setup, according to extensive simulations. The robustness and efficacy of the suggested controller are further confirmed by sensitivity analysis conducted under varied load fluctuations and parameter adjustments of ±25%. Findings show that the GBO-PID controller is a viable option for contemporary, RES-integrated power networks since it consistently stabilizes frequency, voltage, and tie-line power variations with quicker settling times.

The rate of change of frequency (ROCOF) of the grid network is greatly influenced by the renewable energy (RE) penetration level, total grid inertia, and the effectiveness of the existing governing systems. However, the intermittency of photovoltaic (PV) systems may lead to a power mismatch to create a significant frequency change or severe ROCOF. This phenomenon is more severe in Malaysia due to its location near the equatorial region where there is a large number of passing clouds. The control mechanism for frequency regulation is not discussed in this paper due to page limitations. In this project, the effects of the intermittent power output of PV systems on the system frequency and ROCOF are studied. An emulator grid with two PV systems and controllable loads is set up to evaluate the relationship between high intermittent PV power output with the system frequency and ROCOF.

Nowadays many of the power systems are facing serious problems because of the lack of know-how to utilize the available renewable energy resources (RER) so as to balance between the power supply and demand sides. As the consequence of the power unbalancing into their distribution networks, under frequency load shedding (UFLS) which leads to life span reduction of various expensive equipment and deteriorating production in general are of much concerns. Thus, proper control system for the load flow in a system like microgrids (MG) with RER in general is the first thing to carry out the assessment with the aim to solve the power balancing problem within the power system networks. Actually, the major problems which many utilities are facing all over the world are how to utilize the available and future energy resource reserves in order to balance between the supply and demand sides within their power distribution networks. Moreover, because of the quick, improvised and unforeseen increasing number of consumers’ power demands and lack of additional macro energy resources plants which can favorably respond to the instantaneous consumer requirements, optimal control strategy (OCS) is inevitable. The OCS is required to maintain the steady-state operations and ensure the reliability of the entire distribution system over a long period. For that case, the OCS is required to principally stabilize parameters such as voltage, frequency, and limit the injection of reactive power into the MG system under stress. Therefore, in this chapter, the OCS is proposed as an approach to be applied in an intelligent way to solve the UFLS and blackout problems (BP) in a typical MG with RER. The proposed control solution is analyzed using emergency power supply reserves integrated with RER. These typical energy resources can be wind and photovoltaic (solar PV) systems associated with the battery energy storage system (BESS), hydro pump storage, biomass power plant and fuel cell systems.

By increasing the number of severe frequency events in the network, which might lead to an islanding incident, the need for using an adaptive special protection scheme has been highlighted. In this paper, an efficient method for obtaining the optimal values of under frequency load shedding (UFLS) relay settings is presented to minimize the amount of load removal and maintain network security. In this method, the rate of change of frequency (ROCOF) is used as a decent criterion for detecting the severity of the incident and optimizing the relay's adjustable parameters. Finally, this model has been studied and evaluated in a test system.

A Significant change to power systems’ dynamic behavior, especially frequency responses, following a contingency event is a major concern due to the high penetrations of low/inertia-less renewable energy sources. Power system inertia can be getting weaker with the integrations of renewable energy into the grid. As a result, sometimes the under frequency load shedding (UFLS) schemes fail to protect the frequency decline below the threshold limits with conventional settings. This paper addresses this problem and analyse the impacts of penetration of renewable energies into the power systems. Furthermore, a modified load-shedding method is proposed by considering the rate of change of frequency (ROCOF) and the total system’s damping factor. Then a comparison study between proposed method and other methods (conventional and MILP) is presented. A 13-bus real power system is considered as test bus and several case studies are conducted using the Python based PSS/E simulation software platform. From the simulation results it is found that, the proposed load shedding method successfully restricts the frequency decline within a safe limits and thereby, avoids the possibility of major blackouts.

Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro‐resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme.

Renewable Energy (RE) units especially grid connected Wind and Solar PV which have no rotational inertia are effectively displacing the conventional generators and their rotating systems. This reduces both the cost of generation and environmental effects. However, this has implications on the frequency stability in that frequency dynamics become faster with low inertia. This makes frequency control complex and difficult and thus frequency stability becomes challenging. The frequency deviation should be kept small as Damaging Vibrations (DV) and Under Frequency Load Shedding (UFLS) occur for large deviations in the worst case, leading to total black out. For example, in Kenya, there has been increased penetration of RE especially wind and solar into the grid. On Tuesday 7th June 2016 at 1130Hrs, a nationwide black out hit the country for almost three hours when a monkey tripped at transformer at Gitaru Hydroelectric Power Station, leading to a loss of more than 180MW from the grid. This paper revisits Frequency Stability, UFLS and proposes a Combined Frequency with Renewable Energy Storage Cost (CFS) approach for mitigating frequency instability with RE. A brief outline of the Kenyan Case is also provided.

Under frequency load shedding for low inertia grids utilizing smart loads

Under Frequency Load Shedding (UFLS) is an important scheme to prevent the collapse of the power system. However, this method is not able to stabilize the system all by itself, as well as frequency. Voltage also affects the stability of the network. Shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve, inject too much reactive power and thus voltage will be increased. So UFLS will not work alone and a scheme should be considered to remove the capacitor proportional to load shedding. In this paper two methods proposed for coordinated under frequency load and capacitor shedding (UFCS) and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and UFCS schemes for a power network and has been simulated with PSAT toolbox of MATLAB. In addition, the proposed scheme has been combined with automatic switching of shunt reactors to improve the performance of the scheme.

Under frequency load shedding is implemented to restore power system frequency stability if system frequency drops below the operational set point during major disturbance such as lost of generation. Different countries/utility companies have their own philosophies in implementing the under frequency load shedding scheme. Generally, it is based on country/utility requirements, e.g. the overall power system network and the country's demographic. This paper presents the principles and implementation of the under frequency load shedding (UFLS) and presented using simulations of 56 test bus-system. The performance of the developed schemes under various conditions of disturbance were compared and analyzed. All the simulation works were performed using Siemens PTI software PSS®E.

Wind energy is becoming a major source of generation in many countries because of its zero fuel cost and no air pollution. Due to the integration of large-scale wind power in conventional grids, synchronous generators are being economically replaced. Modern wind turbine generators (WTGs) are based on power electronic interfaces. Hence, unlike synchronous generators, they do not usually provide inertia and governor response. Therefore, from a power system security point of view, wind penetration can be limited by frequency response criteria. Up to now, several methodologies have been proposed to estimate the maximum wind penetration level. However, none of them suggest how a system operator could be immediately informed about a secured wind penetration limit as soon as a generation profile is known. Therefore, a tool to estimate the maximum wind penetration level by maintaining an adequate frequency response is proposed in this thesis. The proposed tool is able to instantly provide the highest margin of wind power that could be dispatched at a particular load condition without violating frequency response constraints. In some power systems, the combined generation capacity of wind and distributed photovoltaic (PV) sources is close to reaching the level of average demands (e.g. South Australia). Under high availability of wind and PV, a few synchronous machines may be required to be committed for frequency regulation and hence the system can be viewed as a low inertia grid. Such a grid could be potentially at risk of experiencing an excessive rate of change of frequency (ROCOF) after a contingency. A high ROCOF may initiate tripping of other synchronous generators. As a result, network frequency response may become more vulnerable and the system may be subject to significant under frequency load shedding (UFLS) or even at risk of a blackout. Furthermore, some existing distributed PV units may have a default under frequency protection setting, which is higher than the UFLS threshold. Hence, a contingency may cause a subsequent trip of those PV units. It may result in an unacceptable low frequency, triggering further load shedding. To explore the above issues, the effects of cascading contingencies triggered by high ROCOF and PV tripping on the frequency response of a low inertia grid are investigated in this thesis. Remedial measures using synchronous condenser and WTG frequency support to prevent the cascading contingencies are also suggested. Any mismatch between load and generation is intended to be balanced by contingency reserve, which is also known as contingency Frequency Control Ancillary Services (FCAS) requirement. Load frequency relief (LFR), which represents the effect of frequency dependent loads on power system frequency excursion, is crucial for correctly evaluating contingency FCAS requirement during a generation dispatch to ensure an adequate frequency response. Conventionally, the LFR is considered as a fixed quantity during the estimation of frequency response and FCAS requirement. However, recent experience in the Australian power system suggests that such an assumption may lead to an inaccurate outcome. To explore the above issue, the LFR is investigated based on field measurement data. Data were captured from different locations in the Southern states of Australia. An approach is developed to identify the predominating factors affecting the LFR. A technique is then proposed to appropriately determine the LFR and contingency FCAS requirement at any operating conditions. Along with frequency response, short-circuit performance is another vital index regarding power system security. Modern WTGs have limited fault current contributions, which may cause unacceptable short-circuit ratios at the grid connection point of those wind power plants. Traditionally, frequency response and short-circuit performance are individually improved. However, both of them are simultaneously affected by high wind penetration. Hence, a common approach to concurrently enhance both indices is essential. Therefore, an idea of operating some of the retired synchronous generators in the synchronous condenser mode, which is termed as ‘post-retirement scheme’, is introduced in this thesis. Such a second use of the retired synchronous generators can jointly improve frequency response and short-circuit strength during high wind penetration. A methodology is proposed to determine when and how much post-retirement scheme should be deployed for ensuring adequate security performances in a wind dominated power system.

The conventional Under Frequency Load Shedding (UFLS) scheme could result in unacceptably low frequency nadirs or overshedding in power systems with volatile inertia. This paper proposes a novel UFLS scheme for modern power systems whose inertia may vary in a wide range due to high penetration of renewable energy sources (RESs). The proposed scheme estimates the rate of change of frequency (RoCoF) of the center of inertia (CoI), and consequently, the loss of generation (LoG) size, using local frequency measurements only. An innovative inflection point detector technique is presented to remove the effect of local frequency oscillations. This enables fast and accurate LoG size calculation, thereby more effective load shedding. The proposed UFLS scheme also accounts for the effect of the inertia change resulting from LoG events. The performance of the proposed scheme is validated by conducting extensive dynamic simulations on the IEEE 39-bus test system using Real Time Digital Simulator (RTDS). Simulation results confirm that the proposed UFLS scheme outperforms the conventional UFLS scheme in terms of both arresting frequency deviations and the amount of load shed.

Wind power penetration in power system is increasing rapidly in many countries due to its zero fuel cost and zero air pollution. Unlike the conventional synchronous generation, wind generation has different dynamic characteristics and hence influences the frequency response of a power system. This paper investigates the frequency response of a realistic power system with reasonable integration of wind power. The investigation is carried out on six different load scenarios, with three different wind power penetrations. The frequency sensitivity index and the rate of change of frequency (ROCOF) of the system are estimated. It is observed that the system may become more vulnerable in terms of frequency response as the penetration level of wind power increases. Based on different case studies, this paper also attempts to estimate the minimum synchronous generation requirement of the system after a synchronous generator trips to prevent possible under frequency load shedding (UFLS).

The increasing dependence on renewable energy particularly solar Photovoltaic (PV) to supply energy consumption needs in Jordan has placed operational challenges on the power system operator to cope with the significant drop in the system’s net-demand and the reduction in synchronous inertia. These challenges were not expected to become critical until the penetration of renewables increases to meet future national energy targets in the forthcoming years. However, the adoption of lockdowns to restrict the outbreak of COVID-19 combined with PV injections reduced the system’s net-demand particularly during daytime in spring 2020 like expected levels in the future with high PV penetration. Thus, the implications of future significant penetration of renewables on system security could be better understood based on the operating conditions during lockdowns. In particular, it is important to assess the system’s frequency adequacy during emergency events that might be occurred whilst running a low-inertia power system. To do so, this paper provides a detailed dynamic frequency analysis of the Jordanian power system during lockdowns using Power Factory software. The results highlight the importance of energy curtailment of renewables to maintain adequate level of synchronous inertia to maintain security when the system is islanded without interconnections to neighboring countries. However, deciding the proper level of curtailment requires performing dynamic analysis to ensure that both the Rate of Change of Frequency (RoCoF) and the minimum frequency level during generation contingency events will not trigger the Under Frequency Load Shedding (UFLS) relays.