Synchronous Power Controller Research Articles

Synchronous Power Control of Grid-Connected Power Converters under Asymmetrical Grid Fault

Control of grid-connected power converters is continuously developing to meet the grid codes, according to which the generation units should keep connected to the grid and further provide ancillary services, such as voltage and frequency support, negative sequence current injection, inertia emulation, etc. A virtual admittance controller is proposed in this paper for the objective of voltage support under asymmetrical grid faults. By using independent and selective admittances for positive and negative sequence current injection, the unbalanced voltage can be significantly compensated during asymmetrical faults. The controller is based on the generic control framework of the synchronous power controller (SPC), which is able to control a power converter with emulated and improved synchronous generator characteristics. Simulation and experimental results based on two paralleled 100 kW grid-connected power converters demonstrate the controller to be effective in supporting unbalanced voltage sags.

Damping torque analysis of VSC-based system utilizing power synchronization control

Power synchronization control is a new control strategy of VSC-HVDC for connecting a weak power system. Different from the vector control method, this control method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. So that the parameters of controllers in power synchronization control will change the electromechanical oscillation modes and make an impact on the transient stability of power system. This paper present a mathematical model for small-signal stability analysis of VSC station used power synchronization control and analyse the impact of the dynamic interactions by calculating the contribution of the damping torque from the power synchronization control, besides, the parameters of controllers which correspond to damping torque and synchronous torque in the power synchronization control is defined respectively. At the end of the paper, an example power system is presented to demonstrate and validate the theoretical analysis and associated conclusions are made.

Frequency support characteristics of grid‐interactive power converters based on the synchronous power controller

Grid-interactive converters with primary frequency control and inertia emulation have emerged and are promising for future renewable generation plants because of the contribution in power system stabilisation. This study gives a synchronous active power control solution for grid-interactive converters, as a way to emulate synchronous generators for inerita characteristics and load sharing. As design considerations, the virtual angle stability and transient response are both analysed, and the detailed implementation structure is also given without entailing any difficulty in practice. The analytical and experimental validation of frequency support characteristics differentiates the work from other publications on generator emulation control. The 10 kW simulation and experimental frequency sweep tests on a regenerative source test bed present good performance of the proposed control in showing inertia and droop characteristics, as well as the controllable transient response.

Equivalent Model of Large-Scale Synchronous Photovoltaic Power Plants

The impact that power plants employing renewable energy sources have on power systems is no longer negligible due to the ever-increasing portion of the power share that these generation systems represent. Therefore, the static and dynamic effects of these power plants on the grid must be analyzed in at least the same degree of detail as conventional generating units. For this purpose, it is necessary to develop models that are accurate and able to capture the dynamics of interest for the analysis, but also taking into account that they should be manageable and have a good computational performance. In the case of power plants based on photovoltaic (PV) or wind energy, formed by many power converters, an equivalent plant model that reduces the number of devices offers a good tradeoff between accuracy and complexity. Motivated by the analysis of an actual 100 MW PV power plant formed by 100 converters, each of them governed by a synchronous power controller, this paper presents a general method to derive a dynamic equivalent model of a group of converters that use this type of controller. The method is applied to this power plant in order to obtain several equivalent models with different degrees of detail that are compared through simulation. The results prove the accuracy of the equivalent aggregated models under different conditions and allow measuring the reduction in computation time that they achieve.

Synchronous Power Controller With Flexible Droop Characteristics for Renewable Power Generation Systems

The increasing amount of renewable power generation systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditional synchronous generators gets reduced. In this regard, the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronous power controller for grid-connected converters is proposed as a good solution for the renewable generation systems with energy storage. It provides inertia, damping, and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller.

Power Synchronization Control for Grid-Connected Current-Source Inverter-Based Photovoltaic Systems

Current-source converters (CSCs) have a promising potential to interface the large-scale photovoltaic (PV) generators to electric grids. In order to overcome several drawbacks associated with the conventional vector-controlled CSC-based PV systems, this paper presents for the first time, the power synchronization control (PSC) scheme for the single-stage grid-connected CSC-based PV generator. A detailed small-signal model for the complete CSC-based PV system with the PSC strategy is developed to investigate the system stability, characterize the converter interactions with the electric grid, and design the controller parameters. The PSC of the CSC-based PV system reflects a stable performance under different operating conditions and following sudden variations in the insolation level. Due to the inherent synchronizing feature of the PSC, the phase-locked loop is eliminated and, hence, the integration to the very weak grids is seamlessly achieved. Although there is no direct control on the injected ac current to the grid, the inherent self-current-limiting feature of the CSC through the modulation of the regulated dc-choke current offers an overcurrent protection. Detailed nonlinear time-domain simulations results validate the theoretical analysis and show the effectiveness of the PSC strategy.

Impedance and Damping Characteristics of Grid-Connected VSCs With Power Synchronization Control Strategy

Modeling and analysis of the output impedance of a grid-connected voltage-sourced converter (VSC)-based system under the recently-developed power synchronization control topology is presented in this paper. The impact of impedance passivity on subsynchronous damping profile of an electrically-nearby synchronous generator is then characterized. The impacts of key factors such as control modes and control parameter are addressed. A comparison with the standard vector control strategy is also presented. The results reveal that under this type of control, a VSC does not have negative impact on sub-synchronous dynamics; on the contrary, it has a positive impact, compared with commonly used vector current control where the latter has a potential to degrade the system damping. Time-domain simulation results are presented to validate the theoretical analysis.

Offshore Wind Integration to a Weak Grid by VSC-HVDC Links Using Power-Synchronization Control: A Case Study

In this paper, the application of a recently invented power-synchronization control is proposed for integrating a doubly fed induction generator (DFIG)-based offshore wind farm to a weak ac grid through a voltage-source converter (VSC)-based high-voltage dc link. The control strategy, along with the antiwindup techniques and the bumpless transfer between two different control modes, is elaborately discussed. Two different fault cases, namely, onshore and offshore faults are considered and the fault-ride through techniques are presented. In case of the onshore fault, both with-chopper and without-chopper solutions are investigated. For an offshore fault, a coordinated fault-ridethrough scheme is proposed when the offshore HVDC converter and the wind farm are in voltage-control modes. The entire study is carried out in a real-time digital simulator (RTDS) platform.

POWER SYNCHRONIZATION CONTROL OF VSC-HVDC TRANSMISSION FOR WEAK AC SYSTEM

In this paper voltage source converter based HVDC transmission system is used for connecting two ac systems. The control method used is power synchronization control. This method is different from other control methods and it uses the internal synchronization mechanism in ac systems. It is applied for all grid connected VSC’s especially for HVDC application. This control method gives strong voltage support to a weak ac system. It shows that the proposed control allows 0.86 p.u power to be transferred from a system with short circuit ratio of 1.2 to a system with an SCR of 1.The result is compared with the vector current control for the same ac system where it can transfer only 0.4 p.u. The simulations in MATLAB/Simulink are done to demonstrate the system and observe the system behavior under three phase AC faults.

Modeling and Control of VSC-HVDC Links Connected to Island Systems

The recently proposed power-synchronization control for grid-connected voltage-source converters (VSCs) has been shown to be a feasible solution for high-voltage direct-current (HVDC) transmission connected to high-impedance weak ac systems. In this paper, power-synchronization control is investigated for VSC-HVDC links connected to another type of weak ac system, i.e., low-inertia or island systems. As an example, a linear model of a typical island system feeding by a VSC-HVDC link, including a synchronous generator, an induction motor, and some passive loads, is developed for tuning the control parameters of the VSC-HVDC link. Time simulations in PSCAD/EMTDC demonstrate that VSC-HVDC systems using power-synchronization control are flexible for various network conditions, such as large-ac-system connection, island systems, or passive networks. The time simulations also show that power-synchronization control can seamlessly handle transitions between operation modes, as well as ride through ac-system faults in all network conditions.

Analysis of Stability Limitations of a VSC-HVDC Link Using Power-Synchronization Control

In this paper, the stability limitations are analyzed for a voltage-source converter (VSC) based high-voltage direct-current (HVDC) link using power-synchronization control for two feasible operation modes: alternating-voltage control and reactive-power control. As a comparison, the analysis has been done by the phasor approach and the space-vector approach. While the phasor approach is straightforward to apply and gives intuitive analytical results, the space-vector approach reflects more dynamic insights of the system. This is achieved by using a unified dynamic modeling of both the ac system and the VSC by means of a new introduced modeling approach Jacobian transfer matrices. This approach shows that non-minimum-phase phenomena due to competing effects between alternating voltages and currents following phase-angle changes impose a fundamental limitation on the achievable bandwidth of the VSC-HVDC link. At moderate voltage levels, a VSC-HVDC link operating in alternating-voltage control mode is able to achieve higher bandwidth than in reactive-power control mode since the right-half plane (RHP) zero is located further from the origin in the former mode.

Interconnection of Two Very Weak AC Systems by VSC-HVDC Links Using Power-Synchronization Control

In this paper, voltage-source converter (VSC) based high-voltage dc (HVDC) transmission is investigated for interconnection of two very weak ac systems. By using the recently proposed power-synchronization control, the short-circuit capacities of the ac systems are no longer the limiting factors, but rather the load angles. For the analysis of the stability, the Jacobian transfer matrix concept has been introduced. The right-half plane (RHP) transmission zero of the ac Jacobian transfer matrix moves closer to the origin with larger load angles. The paper shows that, due to the bandwidth limitation imposed by the RHP zero on the direct-voltage control of the VSC, high dc-capacitance values are needed for such applications. In addition, the paper proposes a control structure particularly designed for weak-ac-system interconnections. As an example, it is shown that the proposed control structure enables a power transmission of 0.86 p.u. from a system with the short-circuit ratio (SCR) of 1.2 to a system with an SCR of 1.0. This should be compared to previous results for VSC based HVDC using vector current control. In this case, only 0.4 p.u. power transmission can be achieved for dc link where only one of the ac systems has an SCR of 1.0.

Power-Synchronization Control of Grid-Connected Voltage-Source Converters

In this paper, a novel control method of grid-connected voltage-source converters (VSCs) is proposed. The method can be generally applied for all grid-connected VSCs but may be of most importance in high-voltage dc (HVDC) applications. Different from the previous control methods, the proposed method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. By using this type of power-synchronization control, the VSC avoids the instability caused by a standard phase-locked loop in a weak AC-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a normal synchronous machine does. The control method is verified by both analytical models and time simulations.