Abstract
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.
Highlights
The structure of China’s power grid is undergoing profound changes [1, 2], with the power grid transitioning toward ultra-high voltage (UHV) AC/direct current (DC) connectivity
DPOL 1⁄4 DPD À DPG þ DPOL0 where Inertial time constant of the equivalent system (Ts) is the inertial time constant of the equivalent system; The variation of frequency (Δf) is the frequency variation; The overload or power (ΔPOL) is the overload or the power shortage of the system; TG is the comprehensive time constant of the system-wide generator set; ΔPG is the variation in the generator power; KG is the power-frequency static characteristic coefficient of the generator set; ΔPD is the load variation; KD is the load frequency coefficient; and the system’s initial overload or the initial power shortage ΔPOL0 can be expressed by the difference between the initial load PD0 and the initial generator power output PG0, ΔPOL0 = PD0−PG0
The under-frequency load shedding (UFLS) was carried out by five stages of proportional load shedding according to 20% of the power shortage DPOSÀLG after the mode transition process had completed, which was equal to the initial power shortage ΔPOL0 minus the power contribution of the mode transition process
Summary
The structure of China’s power grid is undergoing profound changes [1, 2], with the power grid transitioning toward ultra-high voltage (UHV) AC/DC connectivity. It is expected that the emergency frequency control of the system can be realized through the coordination strategy of the local pumped storage unit and UFLS, which can minimize the load loss while ensuring the frequency safety of the system. Based on the existing research on the emergency low-frequency problem of large power shortages in receiving-end power grids, this paper proposes a frequency-coordinated optimization control strategy, combining the mode conversion of pumped storage units and UFLS. To recommend a frequency-coordinated optimization control strategy that combines the mode conversion of pumped storage units and UFLS for treating large power shortages. 3. The entire process frequency dynamic response mathematical model that combines the mode conversion of pumped storage units and UFLS is established, and a control strategy based on dynamic frequency trajectory control is formulated.
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