Conventional PQ Research Articles

Research on adaptive optimal control strategy of virtual synchronous generator inertia and damping parameters

AbstractAs the ratio of newly installed power generation utilizing power electronic converters increases, it results in a decrease in the overall inertia and damping of the power system, as well as the decrease in its ability to resist interference. Aiming to address this issue, the authors propose an adaptive optimal control approach for the parameter in the virtual synchronous generator (VSG). This strategy considers the power angle curve and analyzes the principles of inertia and damping within the oscillation period and its range of values. By coordinating the values of parameters, it achieves the optimization of these parameters. At the same time, in order to take into account the demand for fast regulation response under normal operation control conditions and the demand for synchronization support under disturbance fault conditions, a multi‐mode switching control strategy including virtual synchronization control and conventional PQ control is further proposed. Based on MATLAB/Simulink software, the strategies mentioned above have been proven through simulation. The findings testify the proposed method can significantly enhance the regulation response characteristics of the VSG by optimizing its parameters. Additionally, it enables smooth switching of the control mode independently, based on the specific requirements of different control scenarios. This approach fully leverages the virtual synchronous support and fast regulation characteristics of the power electronic converter, and has practical engineering value.

Research on Fast Frequency Response Control of PV Power Stations

In this paper, the frequency support capacity of power system integrated by A large number of PV generation is considered. Based on the conventional inverter PQ control strategy, virtual droop control and inertia control are introduced, and a fast frequency response control strategy of PV inverter similar to the synchronous generator is proposed. Meanwhile, in simulation software of power system, a three-machine nine-node simulation model including the PV power station is established, and the simulation verification is carried out. The simulation results show that when the system is disturbed, PV station can actively provide frequency regulation and enhance frequency stability of the power system.

An Advanced Shunt Active Power Filter (SAPF) for Non-ideal Grid Using Predictive DPC

The usage of power electronic converters and other non-linear devices is very common in this modern era because of their flexibility in control. However, they inject harmonics into the power system. To deal with the problem, the use of filters has become almost inevitable. Active power filters controlled by direct power control (DPC) have overtaken this responsibility from the passive filters because of their numerous advantages. To deal with the real-world power system grid which is far from the idealized character of being balanced and sinusoidal, the proposed method uses the extended pq theory integrated with the dual second-order generalized integrator (D-SOGI) for the correct estimation of the all-important instantaneous powers. In contrast, the conventional pq theory is used in the existing works, which doesn’t hold in a non-ideal grid. In addition to that, here, for the selection of the switching vector, an advanced and more accurate predictive DPC method which is specifically designed to operate in a non-ideal grid, replaces the static look-up tables used in the previous works. Further, to make the filter designing easier, the proposed method focuses on constant switching frequency operation eliminating the problems arising from variable switching frequency used in literature. The proposed technique is tested in MATLAB/Simulink environment and the real-time system to validate its feasibility.

Improved Instantaneous Reactive Power (PQ) Theory Based Control of DVR for Compensating Extreme Sag and Swell

In today’s power system, power quality is a critical topic having several impacts on customers and utilities. In the current electric power system, the integration of renewable energy sources, smart grid technologies, and significant usage of power electronics equipment has generated a slew of issues. The sensitive equipment might be damaged by harmonics, voltage sag, and swell. These devices are vulnerable to Interference with other elements of the system resulting in input voltage changes. As a result, in the contemporary period, Power quality is becoming more important as the number of sensitive and costly electronic devices grows. To overcome the challenges of non-standard voltage, the Dynamic Voltage restorer (DVR) device has been extensively utilized to keep the load voltage stable. To have a dynamic and fast response of the DVR a modified instantaneous reactive power (PQ) theory is proposed to control DVR under extreme transient voltage circumstances. The proposed technique is based on the extraction of the positive sequence component of grid voltage and the negative sequence component of load current for generating a voltage reference signal. The power system network with the proposed PQ control scheme is investigated and assessed under various scenarios to compensate for severe balanced, unbalanced (voltage sags and swells), and load change. MATLAB/Simulink is used to verify the mathematical models of the conventional PQ and proposed PQ control system for DVR. The complete system is implemented experimentally using a dSPACE 1104 based laboratory system to validate the presented control scheme. The simulation and experimental results are correlated, demonstrating the efficacy of the suggested modified PQ control technique.

Distributed neural network enhanced power generation strategy of large-scale wind power plant for power expansion

Power generation is an important energy conversion process. A prominent feature of wind power plant compared with traditional power plants is that the equipment utilization has great intermittency and uncertainty. Fortunately, with the popularity of converter-based wind turbines, doubly-fed induction generator wind power plants are able to not only employ wind turbines in converting wind power to electrical power, but also use the converter capacity to produce reactive power. Traditionally, PQ curves considering current constraints are used for active and reactive power distribution, and therein the fixed maximum slip in power control is conservative for the utility of converters. To fully extract the power generation capacity, a novel power control realizing adaptive variation of maximum slip is proposed for each wind turbine to dynamically expand the reactive power capacity in the high active power region. In the meanwhile, the dispatch scheme of the wind power plant based on the conventional PQ curve has to be upgraded accordingly. Given that the unfixed maximum slip makes it impossible to obtain the PQ curve with explicit expression, an online learning method based on neural network is adopted and further developed in a distributed architecture to be consistent with natural distributed characteristics of the large-scale wind power plant. With the help of communication among agents through consensus protocol, the algorithm convergence can be guaranteed even under distinct information fragments. Case studies demonstrate that the proposed scheme is able to achieve up to 30% expansion of reactive power capacity and 16.3% voltage sag alleviation under certain conditions.

Direct Power Control of PWM Rectifier With Feedforward Compensation of DC-Bus Voltage Ripple Under Unbalanced Grid Conditions

Three-phase voltage source pulsewidth modulation (PWM) rectifiers can achieve constant dc voltage and sinusoidal grid currents under ideal grid voltages. However, under unbalanced grid voltage conditions, there are ripples at twice the grid frequency in the dc voltage, and the grid currents become highly distorted. To address the aforementioned problems, control methods developed under ideal grid voltage conditions should be modified. Various techniques have been proposed in the literature to calculate the new current reference or power reference. However, most methods use conventional pq theory and only consider the grid-side power control. Hence, the dc voltage ripples are not completely eliminated. Furthermore, the derived mathematical expressions for reference calculations are usually very complicated. In contrast to current methods, this paper proposes a very simple yet effective direct power control (DPC) method based on extended pq theory. By analyzing the instantaneous extended power of the line inductance, a simple and concise power compensation is analytically derived and added to the original power references. As a result, the dc voltage ripples are significantly reduced, and the grid currents remain sinusoidal even under unbalanced grid voltage conditions. The proposed method is compared to conventional DPC methods, and its effectiveness is confirmed by the presented experimental results.

A distributed coordinated economic droop control scheme for islanded AC microgrid considering communication system

The conventional PQ droop control method is easily affected by load changes and line impedance in the islanded AC microgrid and always results in the problems in dynamic performance. Therefore, the parallel Distributed Generators (DGs) have uneven power distribution and poor economy. To solve the problem of inaccurate power distribution of DGs and low operating costs, we propose an improved economic droop control strategy based on distributed algorithm, in which the second-level optimization control is added to iteratively modify the reference voltage and frequency in the droop control. In this process, the system optimal economic operating point will also be achieved. Considering the advantages of centralized and decentralized control, the control architecture based on sparse multi-agent communication network improves the robustness and flexibility of the system because there is no central leader. Finally, simulation analysis of the islanded AC microgrid is carried out by using PSCAD/EMTDC simulation software to verify that this strategy can achieve the precise control of the tie-line power according to the load demand response, and the optimal economic operation for each DG.

Direct neural method for harmonic currents estimation using adaptive linear element

The use of nonlinear loads has increased in power systems consequently the harmonic currents have also increased causing detrimental effects to the supply system and user equipment. The aim of the present paper is to identify harmonics in order to obtain a perfect compensation by active power filter (APF). A study of harmonic currents identification by two different methods is conducted in this work. The instantaneous power (PQ) theory method requires two low-pass filters for the extraction of direct power components from total power components. However the direct neural method based on ADALINE neural network method estimates total harmonic current as well as harmonic components separately. Moreover, the identification of each component separately enables the selective compensation of harmonics by the active filter if the objective is to minimize the cost. The method is easy to implement in real time compared to PQ method.In present paper two algorithms based on conventional PQ method and direct neural method were developed in order to identify the harmonic currents. These developed algorithms are confirmed by experimental tests by implementing these techniques in a dSPACE controller in order to show their effectiveness. The obtained results are compared, discussed and analyzed.

Line outage identification-based state estimation in a power system with multiple line outages

Line outages in power systems cause data centers to miss some measurements thus making parts of networks unobservable. They also change the network topology and, consequently, classical state estimators do not converge. This paper proposes a new state estimation methodology based on the concepts of sparse vector recovery and line outage modeling. The proposed method first uses the pre- and post-outage voltage angle measurements to find the changes in network topology. Then it uses either the post-contingency data from PMUs and updated line flows or post-contingency measurements from PMUs and conventional PQ meters, as well as the updated topology, to provide an estimate of the power system state. The performance and accuracy of the proposed method are evaluated for the IEEE 24-, 57- and 118-bus systems. The simulation results show that the method provides good results for the three networks.

Power angle control of grid‐connected doubly fed induction generator wind turbines for fault ride‐through

This study proposes a power angle control strategy of grid-connected doubly fed induction generators (DFIGs) to improve the transient performance during grid fault ride-through (FRT). The proposed power angle control can efficiently restrict the rotor-side current under a limited converter rating, support the terminal voltage of DFIGs and hence improve system stability during the fault period. In this study, the power angle of DFIGs in a multi-machine power system is defined firstly and then the relation between the defined power angle and rotor-side current surge and terminal voltage dip of DFIGs are analysed, respectively. Based on the analysis, control of DFIG power angle is implemented expediently on the existing flux magnitude and angle control (FMAC) scheme of DFIGs as an improved FMAC scheme. In this study, two test examples are presented to demonstrate and validate the effectiveness of improved FMAC with power angle control and to compare it with the conventional PQ dq control and FMAC. Simulation results show that it can significantly enhance the FRT capability of grid-connected DFIGs.

Modeling of wind farms in the load flow analysis

Two methods are proposed, for the simulation of wind farms with asynchronous generators in the load flow analysis. Both methods are based on the steady-state model of the induction machine. The first involves improving the conventional PQ bus, and the second involves modeling the generators in steady-state in the bus where the wind farm is located. The two sets of results are then compared.