Analysis of Power System Failure Occurred on 09 February 2025 in Sri Lanka’s Power System

In recent years several catastrophic power systems blackouts have occurred worldwide. Various reasons have been declared for these failures. Economical limitations due to power system restructuring restrictions, inadvertent operation of protective relays and inefficient design of conventional load shedding schemes are of the most important reasons causing these blackouts. In fact, due to both economical and technological restrictions, it is not possible to completely prevent these blackouts. However, with the aid of some protection and control strategies, frequency and intensity of these blackouts may be reduced. One of the important protection strategies used for this purpose is a class of protection schemes known as ‘System Protection Schemes’ or ‘Wide Area Protection Schemes’. One of the most commonly used types of system protection schemes, generally accepted after the north-eastern blackout of 1965, is Under Frequency Load Shedding (UFLS) scheme. Conventional under frequency load shedding scheme is designed to retrieve the balance of generation and consumption following a disturbance. In the conventional load shedding method frequency settings, time delay settings and the amount of load to be shed in each step are constant values. The loads to be shed by this scheme are also constant load feeders and are not selected adaptively. Using this constant non-adaptive load shedding algorithm is not the most efficient scheme for all power system disturbances. In some combinational disturbances, events causing frequency to drop are followed by other events causing voltage drop. In these cases, since loads are voltage dependent, total system load is reduced and system frequency might not decrease so much to activate UFLS relays. However, the system could eventually collapse due to voltage instability. In many such cases the system would survive if the load shedding relays operate adaptively and appropriately. For example if for large disturbances, higher frequency settings and lower time delays are used adaptively, a faster load shedding response is obtained and as a result system collapses may be prevented. In this paper a new UFLS algorithm is proposed. The purpose of this algorithm is to adaptively adjust speed of load shedding based on the magnitude of disturbance. In this method rate of frequency decline is used as a criterion to determine intensity of disturbance. Thereby, for large disturbances higher frequency settings and lower time delays are used. Application of the proposed algorithm to the simulated model of Khorasan network in Iran confirms its satisfactory performance. As the results of simulations show, several voltage collapse instabilities may be prevented by using the proposed adaptive UFLS method.

Under frequency load shedding (UFLS) scheme is a common practice for power system utilities for preventing frequency drop in power system after disturbances causing dangerous imbalance between the load and generation. The traditional UFLS scheme has a weak self-adaptive ability and set some certain permanent thresholds decided off-line, on the base of experience and simulations, which makes the act of loads shedding speed relatively slow. This paper gives a detailed new improved UFLS scheme based on the rate of change of frequency and applies it to a simulation on multi-machine model. Another simulation on a multi-region system shows that this new UFLS scheme will not cause overload problem on transmission lines. The new UFLS scheme overcomes all the disadvantages, mentioned above in traditional schemes and is especially suitable for modern large-scale complex power systems.

This paper analyzes the regulatory document "Continental Europe Operation Handbook Policy 5: Emergency Operations", which was prepared by experts of the European Network of Electricity Transmission System Operators (ENTSO - E) to focus national networks on a single conceptual model for creating (and fine tuning) an automatic under frequency load shedding (UFLS) scheme. The ENTSO-E proposed for the Regional Group Continental Europe a general under frequency load shedding scheme, which must be implemented taking into account the individual features (properties) of the national network. Given that Policy 5 is an important document, the implementation of which is a prerequisite of security and system stability not only of a separate national grid, but also adjacent power systems, the authors aimed to test perspectives for UFLS development according to Policy 5 binding requirements in a case study for Ukrainian power system.

In a power system, contingencies like the outage of an interconnection or a generator cause the system frequency to decline rapidly. It may lead to system wide collapse if appropriate frequency control measures are not adopted. During inadequate primary frequency response, Under Frequency Load Shedding (UFLS) schemes are deployed to restore system frequency followed by the contingency. To determine the appropriate location and amount of load shedding in such schemes, voltage stability indices of the load buses may serve as a reliable indicator since it helps in identifying the critical areas that are vulnerable to sudden collapse. In this context, a new under frequency load shedding scheme is proposed in this paper, which utilizes the Fast Voltage Stability Index (FVSI) of the load buses to divide the network into several zones. Then, a higher amount of load shedding is applied at a higher frequency threshold in relatively weaker zones. The concept of an equivalent zonal FVSI is developed that can be normalized to quantify the amount of load shedding in different zones. The proposed scheme is validated by applying it to the IEEE 39-bus system. The performances of the proposed scheme are analyzed at different load and generation levels. Simulation results reveal that the proposed method yields better frequency response and causes lower amount of load cut compared to the conventional scheme.

Under frequency load shedding (UFLS) is an effective way to prevent system blackout after a serious disturbance occurs in a power system. A novel centralized adaptive under frequency load shedding (AUFLS) scheme using the synchronous phase measurement unit (PMU) is proposed in this paper. Two main stages are consisted of in the developed technique. In the first stage, the active power deficit is estimated by using the simplest expression of the generator swing equation and static load model since the frequency, voltages and their rate of change can be obtained by means of measurements in real-time from various devices such as phase measurement units. In the second stage, the UFLS schemes are adapted to the estimated magnitude based on the presented model. The effectiveness of the proposed AUFLS scheme is investigated simulating different disturbance in IEEE 10-generator 39-bus New England test system.

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.

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.

The inertia of the power system changes when the generators in the system are disturbed or tripped. However, in some systems, there is no control center or communication device falling behind. It makes real-time inertia difficult to obtain and affects the adaptive under frequency load shedding (UFLS) scheme. This paper proposes an adaptive decentralized UFLS scheme based on load information to overcome this problem. First, a calculation method is derived according to the load information after the disturbance to identify the real-time of load equivalent virtual inertia. Then, the power deficit of load is estimated and the optimal adaptive load shedding scheme is obtained. Finally, compared with conventional UFLS schemes, the proposed scheme can be performed merely by load information. Hence, the requirement of communication conditions and the dependence on the control center are reduced. The simulation is carried out on the island utilizing the IEEE39-bus system. Furthermore, the accuracy of real-time identification of load equivalent virtual inertia and the validity of the load shedding scheme are verified.

This paper presents a long-term impact evaluation algorithm to assess advanced under frequency load shedding (UFLS) schemes on distribution systems with intentional islanding of distributed generation (DG). The algorithm is based on a combined discrete-continuous simulation model which is utilized to verify the effect of the schemes on reliability indices such as the system average interruption frequency index, system average interruption duration index, and energy not supplied. Moreover, a polynomial neural network-based approach to advanced load shedding is implemented to support DG islanding in order to illustrate the applicability of the evaluation. Simulation results highlight the long-term effect of employing UFLS to support intentional islanding of DG using an actual network from the South of Brazil.

Against the backdrop of continuously increasing renewable energy penetration, the large-scale integration of wind power has significantly altered the output characteristics of generation units. The growing volatility of wind power output exacerbates the complexity of frequency regulation, potentially rendering traditional under frequency load shedding (UFLS) strategies inadequate. Investigating methods that leverage the frequency regulation capabilities of doubly-fed induction generators (DFIGs) and coordinated demand-side load clusters to participate in UFLS holds significant research value for enhancing power system frequency stability, reducing load curtailment, and improving power supply reliability. Therefore, this paper proposes an optimized UFLS strategy considering large-scale wind power integration. The frequency response models of wind turbines and thermostatically controlled loads (TCLs) in grid frequency regulation are analyzed, and the configuration principles for implementing UFLS in power systems are elaborated. A simulation model incorporating both wind turbines and TCLs in frequency regulation is developed, and UFLS simulations are conducted based on the operational conditions of a regional power grid. The results demonstrate that the participation of wind power and TCLs in frequency regulation not only ensures grid security but also significantly reduces load curtailment.

Under frequency load shedding has been widely used to restore the power system frequency following a severe generation demand unbalance due to a disturbance. If system frequency is not counteracted properly system will be led to major blackouts. This frequency decline may be corrected by shedding certain amount of load so that system is back to stable state. This paper reports a case study on Sri Lankan power system. Existing under frequency load shedding (UFLS) scheme of Sri Lankan power system was reviewed. Then modification and improvement was made to match with the present network expansions and reduce the load shedding amount while maintaining system stability.

With the increase in world population and the advancement of industries, demand for power and energy has also sharply increased. Consequently, power systems are usually loaded very close to the steady state stability limit to meet this growing demand. This threatens the secure operation of power systems, therefore schemes that are more intelligent are needed to effectively handle power systems disturbances. In the event of a generation-demand imbalance caused by generator outage under frequency load shedding (UFLS) schemes are implemented. In this paper we propose an UFLS scheme based on the minimum predicted frequency and the extrapolated disturbance magnitude. Based on polynomial curve fitting techniques the future behavior of the system frequency can be predicted. A few data points soon after the disturbance are required to predict the minimum frequency the system will reach. The load to be shed can then be determined from the linear relationship between the frequency drops and load to be shed.

This paper presents a comprehensive under frequency load shedding (UFLS) model that can be implemented in a multi‐area power system with real network characteristics. Conventional single‐area UFLS models operate on the basis of an equivalent center‐of‐inertia (COI) model, which ignores the local dynamics of system frequency response (SFR) and the impacts of load shedding location. Unlike the single‐SFR model, that is commonly utilized in previous works, the suggested multi‐area or multi‐SFR UFLS plan of this research has the distinct benefit of taking into account the dynamics of power transfer across different electric areas. The proposed multi‐area UFLS design incorporates a flywheel energy storage system (FESS) to support the inertial system frequency response and alleviate more than 30% load shedding while improving the frequency nadir by 25%. In order to investigate the performance of the proposed method under high penetrations of inertia‐free renewables, the inertia of the power network is reduced by around 30%; therefore the proposed UFLS scheme is assessed under a low inertia scenario. The proposed multi‐stage UFLS scheme is formulated as a mixed‐integer linear programming problem, and the optimal settings, including frequency set‐points and load shedding, are then optimized. The efficiency of the proposed model is verified using the IEEE‐118 Bus dynamic test system.

In this paper a new Dynamic Security Assessment (DSA) based Under Frequency Load Shedding (UFLS) scheme that is adaptive to system inertia and disturbance size is proposed. The proposed Adaptive UFLS scheme achieves a more precise estimate of the minimum shed for a range of system conditions. This reduces the cost of the UFLS as well as alleviating the risk of over shedding that would expose an already stressed system to an increased risk of secondary contingencies, which might lead to system blackout. Various scenarios with different wind levels have been simulated with results verifying the effectiveness of the method compared to semi adaptive UFLS methods.

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.