Underfrequency Conditions Research Articles

Model-Based Algorithm for Flexible Power Point Tracking for Photovoltaic Participation in Primary Frequency Regulation

Grid-connected photovoltaic (PV) systems are commonly designed for maximum energy production. However, as their presence grows, revised grid regulations increasingly require these systems to partially adhere to the primary frequency regulation (PFR) by reducing their power production when the grid frequency exceeds the nominal value. Nevertheless, full participation in PFR of PV systems, without battery storage, would require operating with a dedicated active power reserve to be utilized in the event of underfrequency conditions. This paper presents a model-based (MB) flexible power point tracking (FPPT) algorithm for PV systems. Results of a microgrid simulation show that the proposed algorithm enables a PV system with a nominal power of 100 kW to maintain a 20 kW active power reserve under various irradiation and temperature conditions, with a relative error lower than 3%. Furthermore, a simulation of the PV system assisting in primary frequency regulation is presented. Thanks to the proposed strategy, when employing the algorithm, the frequency deviation is reduced both in the event of underfrequency and overfrequency conditions. By enabling operation with a constant active power reserve, as well as a quick adjustment of the produced active power, in the event of frequency imbalance, the proposed MB algorithm is an interesting candidate for achieving complete PFR capabilities of PV systems.

Mitigation of Cyber-Attacks on Wide-Area Under-Frequency Load-Shedding Schemes

This paper investigates the problem of cyber-attacks on WAUFLS, as these schemes are critical to maintaining power systems stability. First, we perform a detailed analysis on existing WAUFLS schemes and show that an adversary can launch a FDI cyberattack by manipulating the frequency measurements or Power Flow Measurements (PFMs), which may lead to system losses, unnecessary shedding of important loads, and system-wide blackout. Second, to address this vulnerability, we propose a novel UF-RMMS scheme to protect against FDI cyber-attacks. The disturbance calculation and load shedding process in UF-RMMS are based on reliable system states, obtained using a proposed data-classification method on the PFMs that secures the state estimation operation. These reliable states are then used to perform the power flow in order to calculate the power mismatch. The calculated magnitude of disturbance, as well as the obtained system states, are used to decide on the amount and locations of the load-shedding. We validate the effectiveness and accuracy of UF-RMMS by conducting extensive simulation on the IEEE-39 bus New England system using PSCAD/EMTDC. The results confirm the proposed scheme’s capabilities in evaluating system disturbance and performing load-shedding, thus protecting the system during under-frequency conditions, and demonstrate its robustness against FDI attacks.

An adaptive under-frequency optimal control strategy for power system combined pumped storage and under-frequency load shedding.

With the construction and development of ultra-high voltage (UHV) power grids, large-scale, long-distance power transmission has become common. A failure of the connecting line between the sending-end power grid and the receiving-end power grid will cause a large-scale power shortage and a frequency drop in the receiving-end power grid, which can result in the frequency collapse. Presently, under-frequency load shedding (UFLS) is adopted for solving the frequency control problem in emergency under-frequency conditions, which can easily cause large load losses. In this context, a frequency coordination optimal control strategy is proposed, which combines the mode transition of pumped storage units with UFLS to deal with emergency under-frequency problems. First, a mathematical model of the frequency dynamic response is established, which combines the mode transition of pumped storage units with UFLS based on a single-machine equivalent model. Then, an optimal model of the minimal area of the power system’s operation frequency trajectory is introduced, yielding the optimal frequency trajectory, and is used for obtaining the action frequency of the joint control strategy. A simulated annealing algorithm based on the perturbation analysis is proposed for solving the optimal model, and the optimal action frequency is obtained that satisfies the transient frequency offset safety constraint of the power system. Thus, the joint optimal control of the mode transition of the pumped storage units and UFLS is realized. Finally, the EPRI-36 bus system and China’s actual power grid are considered, for demonstrating the efficiency of the proposed strategy.

Two-Stage Load Control for Severe Under-Frequency Conditions

A two-stage load control scheme is presented to address severe under-frequency conditions. The first stage is event-based under-frequency load shedding that guarantees fast response to the detected high-consequence disturbances, for initial system protection. The control signals are obtained by solving mixed-integer programming problems. After the first stage, model predictive control is triggered in the second stage for step-by-step online closed-loop control of interruptible loads, the amount of which is identified via solving linear programming problems. Trajectory sensitivities are used in the optimization problems in both stages to facilitate the analysis. The proposed scheme has been tested on a 9-bus system and on the New England 39-bus system.

Design of a Priority-Based Load Shed Scheme and Operation Tests

Many large industrials operate internal cogeneration systems to provide a substantial portion of their own power and to use the resulting output heat as part of their industrial process. Depending on the amount of cogeneration, on loss of connection to the main grid or instability of the grid, these industrials may have the option of supplying all internal electrical loads from the cogeneration or rather shedding a portion of the load in order to maintain plant stability. In order to maintain and maximize plant operations during either an islanded condition, a severe (and dropping) underfrequency condition, or a prolonged undervoltage condition, a load shed scheme needs to dynamically monitor the load-generation unbalance in the plant and, upon detecting or determining any of the above conditions, needs to quickly initiate shedding of the optimal amount of load based on a user-determined priority schedule. Examples of an actual load shed test are shown and analyzed.

Monitoring the First Frequency Derivative to Improve Adaptive Underfrequency Load-Shedding Schemes

The actual activation of underfrequency load shedding is rarely needed in a power system. However, this does not diminish its significance, as it is the last barrier before a system blackout if underfrequency conditions occur. Like in all other areas of application, a tendency also exists to maximize the efficiency of the underfrequency load-shedding scheme and to bring its operation to the limits. By default, the scheme should be able to stop a frequency fall under any circumstances. However, trying to lower the quantity of the disconnected load in the same process is the purpose of this paper. After the theoretical background is summarized, in terms of the reaction of individual generators in the islanded system to a certain power imbalance, a procedure is described showing how it is possible to adjust any of the adaptive load-shedding schemes to utilize as much primary frequency control as possible. As a reference, a theoretically optimal underfrequency load-shedding scheme is presented.

A shot in the dark

Investigations of recent blackouts indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960s and 1970s. The operation of today's power system with generation frequently being remote from the load centers has made the power system very dependent on the utility transmission system. When transmission lines trip, voltage drops at the load center, whereas frequency may remain normal until a complete system collapse occurs. Utilities have begun to recognize this problem and are installing UVLS programs. Also the industrial customers that have in-house generation that operates in parallel with the utility need to recognize the problem. These industrial cogeneration customers should consider the use of undervoltage separation schemes in addition to their existing underfrequency schemes to address the voltage collapse scenario. This article proposes undervoltage separation logic schemes that can be easily installed within digital relays to enhance the security of an undervoltage separation to prevent false operation due to slow-clearing system faults. The article also indicates the required point of installation of these relays so that they properly measure system voltage.

Fuzzy rule-based methodology for residential load behaviour forecasting during power systems restoration

Inadequate load pickup during power system restoration can lead to overload and underfrequency conditions, and even restart the blackout process, due to thermal energy losses. Thus, load behaviour estimation during restoration is desirable to avoid inadequate pickups. This work describes an artificial intelligence method to aid the operator in taking decisions during system restoration by estimating residential load behaviour parameters such as overload in buses and the necessary time to recover steady-state power consumption. This method uses a fuzzy rule-based system to forecast the residential load, obtaining correct estimates with low computational cost. Test results using actual substation data are presented.

Revised load shedding schedule for power system incorporating the effect of transmission line performance

There is a growing trend towards the use of sophisticated relays like frequency trend relays for the purpose of automatic load shedding to avert the major system collapses due to integrated operation of modern power system. Accurate load shedding saves loss of revenue to power utility in addition to avoiding unnecessary disruption of vital feeders. Face values of MW capacities of generation outages are normally used for estimating the system overloading and consequent quantum of load shed to recover the system to safe workable frequency. Inherent dependence of transmission line parameters upon the system frequency affects the line performance in terms of its reduced output during under-frequency condition. The paper has attempted to present a modified load shedding scheme incorporating the relieving effect of transmission network and showed the advantages over the conventional method.

Adaptive approach to load shedding including pumped-storage units during underfrequency conditions

An adaptive approach is proposed to load shedding including the consideration of pumped-storage units during underfrequency conditions. Based on the idea of average or uniform frequency behaviour, a load-frequency control model is utilised in the scheme design. At this stage, Taiwan Power Company (Taipower) owns ten pumped-storage units that amount to about 13% of the total rating capacity. These units can be operated in generating mode during peak period, to reduce generation cost, or operated in pumping mode during off-peak period, to consume the surplus power. As the difference between the peak load and off-peak load of Taipower is considerable, pumped-storage units are playing a critical role in Taipower operations. Once disturbances take place in off-peak periods, pumped-storage units can be thus shed with a high priority. As they are useful in both restoring system frequency and decreasing the degree of service interruptions, these pumped-storage units have been included in the proposed load-shedding scheme design and tested through the utility data. From test results and comparisons with the existent utility shedding scheme, they have demonstrated the feasibility and practicality of the method for such applications.

An adaptive approach to load shedding and spinning reserve control during underfrequency conditions

Emergency conditions arising due to generating power deficiency and the consequent drop in power system frequency can lead to system collapse and a large scale loss of load. Underfrequency load shedding (UFLS) is a globally accepted practice to handle this situation. Most UFLS schemes use pre-specified step sizes based upon frequency measurements. This paper presents an adaptive scheme which uses both frequency and rate-of-change of frequency measurements to dynamically set UFLS relays if more than one generation outage is experienced. It then proposes a technique for coordinating UFLS and the activation of spinning reserves through localized governor control. Recent developments in computer based frequency and rate-of-change of frequency relays makes this scheme both feasible and attractive.

Electrical studies for an industrial gas turbine cogeneration facility

Electrical studies are required to assure the proper integration of a gas-turbine cogeneration facility into an existing industrial-plant electrical system and the connected utility grid. Details of such a study are presented, including boundary-limit definition for the system model, individual component modeling, load-flow and short-circuit studies, stability studies, and simulation of online isolation from the electric utility during system undervoltage or underfrequency conditions. The impact of such studies on the design process and plant system reliability is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A status report on methods used for system preservation during underfrequency conditions

A status report on methods used for system preservation during underfrequency conditions