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| Control of Voltage-Dependent Loads for Grid Frequency Response Based on Hierarchical Model Predictive Control |
| Zhang Yuanhang1,2, Kou Peng1,2, Mei Mingyang1,2, Tian Runze1,2, Liang Deliang1,2 |
1. State Key Laboratory of Electrical Insulation and Power Equipment School of Electrical Engineering Xi'an Jiaotong University Xi'an 710049 China;
2. Shaanxi Key Laboratory of Smart Grid School of Electrical Engineering Xi'an Jiaotong University Xi'an 710049 China |
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Abstract Driven by the national goal of “double carbon”, China's power system is accelerating the transition to a new kind of power system with high renewable energy penetration. However, the inherent intermittency and low inertia characteristics of renewable energy bring challenges to the frequency stability of the power grid. In this context, modern power systems require additional frequency regulation resources to ensure grid stability. Voltage-dependent loads are important frequency regulation resources, whose active power consumption is strongly correlated with bus voltage. For this reason, this paper proposes a control strategy for voltage-dependent loads. The proposed strategy regulates multiple reactive power devices, controlling the bus voltage of the load to change its active power consumption and contributing to frequency response.
Firstly, the proposed strategy has two layers. The upper layer, employing centralized model predictive control (MPC), calculates the active power references of loads and delivers the references into the lower layer. The lower layer employs decentralized MPC to calculate the reactive power of reactive power devices based on the active power references of the loads from the upper layer. Secondly, the control problem formulation of the lower layer includes the dynamic models of voltage-dependent loads and bus voltages, thus realizing effective load active power control without violating voltage constraints. Thirdly, the lower decentralized MPC strategy consists of multiple local controllers that do not communicate with each other, and the interactions between controllers are regarded as disturbances. A tube-based robust method is used to compensate for the communication deficiencies and achieve suboptimal control, thus significantly reducing the communication burden.
Simulation results on the modified IEEE 39-bus system show that, when a 522 MW load is suddenly disconnected, the grid frequency rises to 50.29 Hz at the highest level without the load support and is stabilized at 50.19 Hz in the end. If the conventional droop control is employed to regulate the voltage-dependent loads to participate in the frequency response, then the grid frequency rises to 50.22 Hz and eventually stabilizes at 50.16 Hz. In contrast, if the proposed hierarchical MPC strategy is adopted for voltage-dependent loads, the grid frequency rises to 50.18 Hz and is finally stabilized at 50.08 Hz. Based on the above analysis, it can be learned that, compared with the traditional droop control, the hierarchical MPC strategy for voltage-dependent loads reduces the peak and steady-state values of the frequency and significantly improves the frequency dynamics.
We can draw the following conclusions from the simulation results: (1) When an over-frequency event occurs, the proposed hierarchical MPC strategy makes the reactive power outputs of reactive devices increase rapidly. In this way, the voltage amplitude of the entire grid rises, and thus the active power consumption of voltage-dependent loads increases, effectively mitigating the severity of the over-frequency event. (2) To realize effective frequency control, the proposed hierarchical MPC strategy takes into account the dynamic model of voltage-dependent loads, grid frequency and bus voltage, and can dynamically adjust the outputs of reactive power devices according to the specific situation. (3) During the frequency response, the hierarchical MPC strategy can still effectively limit the bus voltages within the constraints even in the absence of communication between reactive power devices. In contrast, the bus voltages under the conventional droop control strategy suffer from voltage violations.
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Received: 23 December 2024
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2026, Vol. 41 
