Providing frequency support to offshore energy hubs using hydrogen and wind reserves

In a decentralized manner, providing Frequency Support (FS) to the AC integrated Multi-terminal DC (AC-MTDC) grids is requisite from most of the Transmission System Operators (TSO’s). Extracting this support from the Wind Farms (WFs) through the DC grid is engrossing researchers. In this paper, unlikely, P V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is proposed instead of P f droop control for Wind Farm Voltage Source Converters (WFVSCs) for effective FS. This support is achieved while operating the WFs in a derated mode of operation. Besides, this approach also enhances the DC-voltage support to the MTDC grid. In addition, wind variation is also considered to show the impact of this variation on the Frequency and DC-voltage Support (FDS). Further, the proposed approach is validated by considering two area power systems integrated into the five terminal mesh type DC grid with three various schemes simulated in PSCAD/EMTDC software.

One of the major concerns for integrated photovoltaic systems in power grid is frequency instability in the absence of rotating mass. Among possible solutions, PV deloading can provide support without any additional requirement or equipment inclusion. But as deloading depends on MPPT point of operation, environmental effects like change in irradiance or temperature value can hurt PV generation and frequency support. Setting deloading percentage considering a static condition may not provide expected support under such intermittent condition as frequency support will vary. Therefore, it is crucial to find out whether a deloading percentage is effective under changing environmental parameters. To this end, this paper investigates this dynamic behavior in frequency support and its relation with PV variability in deloaded mode. In addition, a methodology is proposed to estimate minimum irradiance and temperature up to which a specific deloading percentage can provide frequency support without any under frequency load shedding requirement. The approach utilizes linear regression analysis (LRA) algorithm which is subjected to varying frequency and generation support condition. The proposed method is implemented for several deloading cases on modified IEEE 39 bus test system in DIgSILENT PowerFactory. The findings indicate a specific PV deloading percentage results in a minimum PV power requirement to avoid load shedding. This minimum PV power corresponds to a specific range of irradiance and temperature in which this deloading can provide acceptable support. Furthermore, this paper also provides insights on the improvement rate of minimum PV power requirement with deloading. As a result, this methodology can be used to decide whether a certain deloading percentage can supply desired support in any situation. Thus, this paper will work as a guideline to grid operators to select proper PV deloading considering variability in PV power generation.

In the grid connection of an offshore wind farm, flexible low-frequency AC transmission technology has received special attention. With the increasing penetration of grid-connected offshore wind farms, it is of great importance that the offshore wind farm can provide frequency support to the bulk power grid to help solve its frequency stability problem. However, the offshore wind turbine is decoupled from the onshore bulk power system by the back-to-back converter, which plays the central role in the low-frequency power transmission technology. Therefore, the offshore wind farm cannot react to the change in frequency of the shore power grid. To this end, this paper proposes a frequency support strategy for a low-frequency transmission system integrated offshore wind farm, which has seldom been studied in existing literature. First, a mapping strategy is developed to establish the connection between the frequencies of the onshore bulk power system and the low-frequency transmission system. A dual loop based strategy is developed for the back-to-back converter. Finally, a droop control based strategy is developed to control the active power of the wind turbine, which will provide frequency support for the bulk power system. The simulation results show that with the method proposed in this paper, the offshore wind farm can provide effective frequency support in response to the frequency change of the onshore bulk power system.

Frequency droop control is widely used in permanent magnet synchronous generators (PMSGs) based wind turbines (WTs) for grid frequency support. However, under frequency deviations, significant DC-link voltage fluctuations may occur during the transient process due to sudden changes in real power of such WTs. To address this issue, a current feedforward control strategy is proposed for PMSG-based WTs to reduce DC-link voltage fluctuations when the WTs are providing frequency support under grid frequency deviations. Meanwhile, the desired frequency support capability of the PMSG-based WTs can be ensured. Simulation results verify the rationality of the analysis and the effectiveness of the proposed control method.

The proposal of the “carbon neutrality” target promotes the rapid development of variable renewable energy, which leads to the reduction of inertia and the deficiency of flexible reserve in power systems. Wind turbines (WTs) now are asked for providing frequency support and reserve through proper control strategy. This proposes a new issue on how to coordinate the operation of conventional units and WTs. This paper proposes a frequency security constrained scheduling approach considering wind farms providing frequency support and reserve. Two dynamic frequency indices: RoCoF and nadir, are derived and introduced into the scheduling model. The frequency support capability of wind farm is accurately quantified by considering the real grid-connected WT capacity and wake effect. A wind power reserve model considering uncertainty is established by linking the reserve under the forecasted scenario and uncertain scenarios through reserve auxiliary variables. The contribution of wind farms to power system frequency support and reserve is quantified in the robust scheduling model. Case studies show that the frequency support and reserve from wind farms can effectively reduce the whole operation cost and VRE curtailment. Moreover, the value of wind farms in CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> emission reduction is properly quantified.

As the finite nature of non-renewable energy resources is realised and climate change concerns become more prevalent, the need to shift to more sustainable forms of energy such as the adoption of renewable energy has seen an increase. More specifically, wind energy conversion systems (WECS) have become increasingly important as a contribution to grid frequency support, to maintain power at the nominal frequency and mitigate power failures or supply shortages against demand. Therefore, limiting deviations in frequency is imperative and, thus, the control methods of WECS are called to be investigated. The systematic literature review methodology was used and aimed at investigating these control methods used by WECS, more specifically variable-speed wind turbines (VSWT), in supporting grid frequency as well as the limitations of such methods. The paper identifies these to be de-loading, energy storage systems and emulated inertial response. Further classification of these is presented regarding these control methods, which are supported by literature within period of 2015–2022. The literature indicated a persistent interest in this field; however, a few limitations of VSWTS were identified. The emulated inertial response, specifically using a droop control-based frequency support scheme, was the primary means of providing frequency support. This systematic literature review may be limited by the number of papers selected for the study. Results and conclusions will not only be useful for WECS development but also in assisting with the security of the transmission grid’s frequency stability. Future work will focus on further studying the limitations of WECS providing frequency support.

The flexible control characteristic of energy storage system makes it have an advantage in participating in grid frequency regulation. The combination of wind power and energy storage has the effect of synergistic enhancement in providing frequency support. However, traditional PID controllers are difficult to achieve coordinated control of wind farms and energy storage. To address this issue, a model predictive control (MPC) based scheme of wind farm with energy storage system for frequency support is proposed. The MPC controller optimizes the power reference for each offshore wind turbine as well as the energy storage system, with the objective of minimizing the grid frequency deviation. In the MPC design, power constraints for wind turbines that adapt various wind speeds are considered, thereby ensuring the stable operation of wind turbines during the frequency support. Meanwhile, state of charge (SoC) constraints for the energy storage system is also proposed to extend its life. What's more, the moving horizon estimation (MHE) is introduced to estimate the grid power imbalance. The results show that the proposed scheme can effectively improve the performance of the grid frequency.

An optimal control and sizing strategy for a coordinated WTG-ES system to provide frequency support

This paper proposes a frequency control strategy for voltage source converter based multi-terminal HVDC systems (VSC-MTDC) to facilitate the exchange of primary frequency reserves among asynchronous AC systems, thus providing frequency support from each AC system to the others. The proposed frequency control utilizes a reference signal calculated from global measurements which reflects the overall frequency dynamics of all connected asynchronous AC systems. The proposed control outperforms the traditional frequency droop scheme by reducing impact on the DC voltage profile and improving frequency nadir. The performance of the designed frequency control with different DC voltage droop controls is evaluated. The adaptation of the proposed frequency control for converter outages is analyzed. The robustness of the proposed control to communication latency and the sensitivity of frequency support to power limits are also investigated. The VSC-MTDC model and the proposed control are implemented in the commercial grade software PSS/E and thus is suitable to study large-scale realistic systems and can be incorporated into the power system planning process at utilities and ISOs. The effectiveness of the proposed control is illustrated on a developed AC-MTDC test system and also on a large-scale realistic model combining the North American Western Interconnection (WI) and Eastern Interconnection (EI) with continental HVDC interconnections.

Provision of inertial and primary frequency support in a controlled manner through multiterminal direct current (MTDC) grid connecting asynchronous AC areas and offshore wind farms (OWFs) is explored. Under nominal condition, only voltage droop is considered to allow the AC systems operate asynchronously as they are meant to. Following an AC-side disturbance, the corresponding converter transmits basic information via a distress signal through the DC lines or existing communication channels used for common DC voltage-droop control. Upon receiving the distress signal, a new decentralized inertial-droop-based controller and the traditional decentralized frequency-droop controller are activated in preselected participating converters with values that are predetermined based on our proposed design procedure. The procedure ensures a new ratio-based performance while providing frequency support. The proposed scheme is based on an Nth-order model of N-asynchronousarea MTDC system. Furthermore, to extract frequency support from an OWF connected to an AC-MTDC system, its power reference is modified such that the whole of the AC-MTDC system with OWF is emulated as an N-asynchronous-area MTDC system. The proposed strategies are first implemented in the Nth-order model for theoretical validation and later, in full-order models of study systems (with and without OWF) for a more rigorous verification.

Synchronverter-based frequency control technique applied in wind energy conversion systems based on the doubly-fed induction generator

The increase of renewable energy use in a power generation decreases the flexibility of power supply. The intermittent and variable nature of renewable resources necessitates the use of a transmission system operator to prepare reserve generating power in case of power imbalance. Unfortunately, the generation and reserve margins of renewable energy generators are often insufficient. To ensure a balanced and resilient power system, surplus power generation from renewable resources must provide frequency support with the essential number of synchronous generators being remained in the network. Assessment of the amount of frequency support power from wind farm became available through forecast and real‐time operation planning. However, the exact amount of back‐up power that expected to be provided by individual wind farm can be barely determined due to the uncertainty and variation of renewable resources generation. A stochastic method for assessing the required frequency support operation of specific wind power plants is thus proposed in this paper. The stochastic correlation characteristics between a single wind farm and power system stability are applied in this method. The essential metrics for deloading operation and inertia response are calculated using the stochastic correlation model based on copula function.

The frequency stability of modern power systems is challenged due to widespread application of large-scale renewable energy systems, of which the offshore wind farm (OWF) is one of the dominant resources. The OWFs are usually integrated into the grid by multi-terminal direct current (MTDC) transmission systems, which makes the energy flow complicated and the frequency control design challenging. A frequency support control method of MTDC system integrated OWFs (referred to as the OWF-MTDC system) is proposed in this paper. First, the wind turbine generation system (WTGS) is controlled to reserve a certain amount of available power according to the real-time wind speed for more comprehensive frequency regulation. Then, the frequency support control of OWFs is designed, and they can release the rotor kinetic energy and reserved power to support the onshore grid frequency. In addition, the virtual inertia control of a modular multi-level converter (MMC) is designed, which can also provide frequency support in an emergency by use of the DC capacitor. To ensure that the frequency control of the OWF-MTDC system does not degrade the stability of the system, a detailed DC impedance model of the MMC-based MTDC systems is developed, considering the constant power control and DC voltage control. Based on the impedance model, the impact of the frequency control coefficients on the DC side stability of the MTDC system is analyzed. Simulation results validate the stability analysis and verify the proposed frequency control method, which can effectively provide frequency support to the onshore power grid.

This paper mainly focuses on how to provide frequency supports by the doubly fed induction generator (DFIG) during system disturbances. Two coordinated controls that enable system frequency supports by DFIG-based wind turbines (WTs) are proposed in this paper. The first control scheme seeks to render system support via simultaneously utilizing the energy from the installed super-capacitor between the back-to-back converter of DFIG, and WT rotational kinetic energy (KE). The second one stabilizes system frequency by firstly exerting the installed super-capacitor energy and then WT rotational KE via a unique cascading control. Both proposed coordinated control schemes jointly utilize two virtual inertia sources, namely super-capacitor in the DFIG and rotor rotational mass in the WT to fast provide system frequency support. However, the second proposed one stands itself out by reducing its impaired impacts on the overall wind energy production. Two proposed controls on rapidly providing frequency support are effectively verified and compared in detail by different system disturbances in the DIgSILENT/Powerfactory software.

Securing a future sustainable decarbonised economy involves moving towards a system with rising penetration levels of distributed photovoltaics (PVs) within the low voltage distribution network (LVDN). This power system evolution is displacing conventional generators and has resulted in a decline in inertia that is essential for frequency stability. Emerging network codes require PV generators to maintain a scheduled curtailed active power (CAP) reserve for under-frequency contingencies. In this paper, the development, verification and application of an enhanced, two-stage grid-connected, state-space, linear parameter varying (LPV) PV system model is presented. The LPV model provides accurate and efficient modelling for PV systems over the wide range of operating points associated with curtailed active power and is suitable for power systems with large numbers of distributed PV systems capable of frequency support in the LVDN, to be simulated within reasonable simulation times. In addition, the LPV model can be used to investigate voltage rise due to reverse active power. The model performance is evaluated using recorded experimental data with step changes in irradiation and active power curtailment. The measured data is generated from a power hardware in the loop (PHIL) testbed. The model’s performance is investigated on an adapted radial European LVDN benchmark with several distributed PV systems to present some of the challenges, opportunities and benefits. Step changes in solar irradiation are used to evaluate the dynamic behaviour of the LPV model compared to a discrete-time electromagnetic transient (EMT) model. A frequency droop control characteristic for frequency support is demonstrated. The results show a computational burden reduction of 132:1 compared to the EMT model and demonstrate the voltage rise due to reverse active power from providing frequency support during under-frequency contingencies.