Analysis of sub-synchronous oscillation characteristics of PMSGs based on transient energy

Faced with the problem of sub-synchronous oscillation (SSO) caused by the interaction between permanent magnetic synchronous generator (PMSG)-based wind farms and weak AC grids, we construct a transient energy function model that follows the structure of a PMSG. The transient energy composition of a PMSG is analyzed, and the dissipation energy expression that can intuitively reflect the development trend of SSO is derived, reflecting the damping level of a grid-connected wind power system. Furthermore, in response to the problem of mutual coupling between the control links of the PMSG during the SSO process, which hinders oscillation characteristic assessment, a method based on oscillation energy is proposed to analyze the oscillation characteristics. Considering the dynamic changes in the output of the phase-locked loop during sub-synchronous oscillation, the transient energy dominated by various control links is derived, and the effects of the phase-locked loop, current inner loop, and voltage outer loop on transient energy and oscillation characteristics are analyzed. The simulation verifies the effectiveness of the analysis of the transient energy model

Comparisons of MVAC and MVDC systems in dynamic operation, fault protection and post-fault restoration

One of the most significant obstacles preventing the large-scale application of direct-current (DC) technology in medium voltage (MV) distribution networks is their fault protection. The existing AC relay protection needs to be changed or redesigned to protect the future overlay MVAC and MVDC distribution networks. Therefore, a comprehensive understanding of the dynamic and fault behavior and post-fault restoration strategies of MVAC and MVDC systems are critically important. Moreover, a comparison of MVAC and MVDC systems during a fault will also contribute to designing the protection systems of hybrid MV AC/DC systems. In this paper, the challenges of protecting DC faults of MVDC systems and possible solutions are first introduced. Then, the fault characteristics and post-fault restoration of MVDC and MVAC distribution systems are compared and investigated through case studies. Time-domain simulations have been conducted in PSCAD/EMTDC. The work in this paper will be valuable for the protection design for future hybrid MV AC/DC systems

Research on the mechanism of neutral-point voltage fluctuation and capacitor voltage balancing control strategy of three-phase three-level T-type inverter

In order to solve the neutral-point voltage fluctuation problem of three-phase threelevel T-type inverters (TPTLTIs), the unbalance characteristics of capacitor voltages under different switching states and the mechanism of neutral-point voltage fluctuation are revealed. Based on the mathematical model of a TPTLTI, a feed-forward voltage balancing control strategy of DC-link capacitor voltages error is proposed. The strategy generates a DC bias voltage using a capacitor voltage loop with a proportional integral (PI) controller. The proposed strategy can suppress the neutral-point voltage fluctuation effectively and improve the quality of output currents. The correctness of the theoretical analysis is verified through simulations. An experimental prototype of a TPTLTI based on Digital Signal Processor (DSP) is built. The feasibility and effectiveness of the proposed strategy is verified through experiment. The results from simulations and experiment match very well

储能友好型频率响应服务市场 : 英国视角

To counterbalance the reduction of system inertia with increasing renewable generation, the frequency response (FR) service markets in the UK have been reforming to enable procurement from diverse technologies including energy storage systems (ESSs). The Enhanced FR product was introduced in 2016 to allow ESSs to manage the state of energy (SOE) within two envelopes. A two-year weekly auction trial was launched in 2019 to test closer-to-real-time procurement and reduce the financial risk of ESSs by evaluating their performance and payments in each week separately instead of an entire month. In addition, an integrated suite of end-state services has been successively released since 2020 with the phase-out of existing FR services so as to increase the standardisation and transparency of FR markets. The end-end services consisting of Dynamic Containment (DC), Dynamic Moderation (DM) and Dynamic Regulation (DR) deal with different frequency deviation levels and have a limited requirement on full-response duration, which mitigates the barrier to entry for ESSs. The SOE rules are additionally specified to indicate the cases in which ESSs will not be penalised for the under-delivery of DC, DM or DR. Based on the latest technical requirements and procurement/payment mechanisms of DM, this paper simulates the techno-economic operation of a grid-scale lithium-ion battery ESS (BESS) that provides DM to the AC grid while following its operational baselines to restore the SOE. The SOE levels and FR errors of the BESS are calculated to determine its compliance with the SOE rules and under-delivery penalties. Then the DM payment is compared with BESS costs to estimate the net present value at the end of the energy throughput-based lifetime, indicating the profitability of the BESS under the latest energy storage-friendly FR markets. The BESS operating strategy designed here fully considers the specific market mechanisms of end-state FR services, allowing the closer-to-real-time SOE management and meeting the required FR delivery. The specific and novel strategy design and the resulting simulation offer BESS developers with an insight into potential operating scenarios especially at the early stage of end-state FR service markets

Optimization of Hybrid Energy Storage Systems at the Building Level with Combined Heat and Power Generation

The average daily benefit to cost ratio of a building energy storage system is mainly constrained by the battery lifetime. This paper aims to minimize the average daily cost of a hybrid energy storage system (HESS) (comprised of a battery and supercapacitor) by optimizing the battery capacity. A novel optimization model is proposed with the objective to find the minimum average daily investment cost of the HESS. The objective function has two parts: (1) the investment cost formula for the battery is derived as a function of the battery capacity, which has an interdependence with the minimum state of charge (SOC) and the maximum discharge current; (2) the investment cost formula for the supercapacitor is also established as a function of battery capacity by matching the maximum battery power with that of the supercapacitor. Case studies demonstrate several ways to increase the average daily benefit to cost ratio: (1) adopting a suitable control strategy to avoid capacity saturation; (2) reducing the battery SOC to increase the threshold for the maximum discharge current (MDC) saturation; and (3) increasing MDC to raise the threshold for the SOC saturation. Results show that the average daily benefit to cost ratio is doubled compared to previous work

A grid inductance detection method based on the oscillation characteristic of inverter terminal voltage

In grid-connected photovoltaic (PV) systems, grid inductance greatly influences the performance of grid-connected inverters. However, the grid inductance usually varies with the changes of the grid operation conditions. Therefore, accurate grid inductance detection is one of the key technologies to achieve an adaptive control of the grid-connected inverters under different operation conditions. In this paper, an equivalent circuit model of a grid-connected PV system which includes the controller, filter, and grid impedance is established. The oscillation characteristic of the inverter terminal voltage caused by the step change of the inverter output power is analyzed. The oscillation mechanism of the inverter terminal voltage caused by the interaction of the controller, filter, and grid impedance is investigated. A grid inductance detection method based on the oscillation characteristic of the inverter terminal voltage is proposed. The main advantage of the proposed method is that it does not need complicated signal processing and calculation procedures. A 10 kW grid-connected PV system was built for both simulations and the experimental setup. Finally, simulation and experimental results validate the correctness and effectiveness of the proposed method

Equivalent Modeling of Dfig-Based Wind Farms for Sub-Synchronous Resonance Analysis

The order of the detailed model of doubly-fed induction generator (DFIG) wind farms are too high and the simulation is difficult. Most of the existing research has used a single-machine equivalent model and clustering aggregation model for equivalence and few papers have explored the principles and equivalent conditions of the single-machine equivalent model under sub-synchronous resonance (SSR). Due to this reason, this paper equates DFIG wind farms connected with series compensated transmission network to two separate units based on the principle of matrix similarity transformation and the mathematical model and physical model of each unit has been studied. Then, the DFIG wind farm equivalent model’s validity is analyzed in number difference and collecting line difference based on linearization analysis. Finally, the system model is built in EMTDC/PSCAD, the damping analysis method is used to test the equivalent model’s validity and further reveal the mechanism of the system’s unstable operation. The results show that: the equivalent model can effectively reduce the system dimension and accurately reflect the dominant oscillation characteristics of DFIG wind farm under SSR; when SSR occurs, the damping coefficient of DFIG wind farm is negative under the oscillation frequency

Low Voltage Ride-through Scheme of the PMSG Wind Power System Based on Coordinated Instantaneous Active Power Control

Fast control DC-link voltage is the key to enhance Low-Voltage Ride-Through (LVRT) for the Permanent Magnet Synchronous Generator (PMSG). When grid voltage dips deeply, by the constraint of the capacity of the grid-side converter and reactive power support by the grid-code required, the active power capacity of the grid-side converter will be reduced. Therefore, it cannot stabilize the DC-link voltage relying solely on the grid-side converter. This paper proposes an improved control strategy to combine generator-side and grid-side converter active power. Reactive power support and negative sequence feed-forward compensation are also considered in the paper. The effectiveness of the control strategy is verified by both simulation and the experimental results

Novel adapted de-loading control strategy for PV generation participating in grid frequency regulation

Existence PV generation mostly operates under the Maximum Power Point Tracking (MPPT) mode and has no ability to increase or decrease active power, accordingly, PV generation does not participate in grid frequency regulation. Due to an increasing penetration of grid-connected PV generation, when a frequency contingency event occurs, grid may have neither enough inertia response nor governor support and frequency deviation may have a risk of exceeding the limit. Therefore, a novel adapted de-loading control strategy without MPPT for grid-connected inverters is proposed here. This control strategy provides a reserve power in PV system to meet the frequency control demand and an online output Active Power-Voltage Matching (APVM) technique, which is generated as a control flowchart, is implemented in this control strategy to make it possible to adjust PV output voltage performance matching different active power demand with different operation modes. The proposed control strategy of PVs is simulated using PSCAD/EMTDC Software. Results verify the correctness and effectiveness of the proposed adapted de-loading control strategy under several operating conditions

Stability analysis of grid-connected PV generation with an adapted reactive power control strategy

The static VAR compensator is widely applied in large-scale grid-connected photovoltaic (PV) generation to participate in voltage regulation of power system, which ignores the reactive power regulation capability of PV inverter. In this study, firstly, the constraints of maximum operating current and output voltage's amplitude of PV inverter are considered, and the reactive power regulation capability of the PV generation is evaluated; secondly, an active participation in voltage regulation and control strategy of PV generation is proposed considering reactive power demand of system. This control method through the real-time detection and comparison with reference value of point of common coupling (PCC) voltage, automatically obtain the reactive demand to maintain PCC voltage by PI controller and realises the dynamic adjustment; finally, a time-domain small-signal model for PV generation is established, and based on this small-signal model, eigenvalue analysis is employed to study the influence of the grid strength, operating condition, and the reactive power control strategy on operating stability. Based on the time-domain simulation example in EMTDC/PSCAD, the validity of the theoretical analysis and the feasibility of the control strategy are verified