Analysis of reactive power strategies in HVDC-connected wind power plant clusters

© 2017 John Wiley & Sons, Ltd. Offshore wind power plants (WPPs) built near each other but far from shore usually connect to the main grid by a common high-voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, the wind turbine converters and the high-voltage DC voltage-source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC-connected WPP clusters when applying 5 different reactive power control strategies. The case study is made for a 1.2-GW-rated cluster comprising 3 WPP and is implemented in a combined load flow and converter loss model. A large set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of the WPPs with unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization-based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high-voltage DC voltage-source converter. Reactive power control in HVDC-connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster.Postprint (author's final draft

Online Estimation of Dynamic Capacity of VSC-HVdc Systems –Power System Use Cases

The dynamic capacity describes the capability of high voltage direct current (HVdc) systems to operate temporarily beyond their guaranteed active and reactive power (P/Q) limitations under specific conditions. In this work, the dynamic capacity is intended to be applied in various power system use cases to ensure a more efficient and secure grid operation. In contrast to previous works, the dynamic capacity is considered with a holistic view on the HVdc system’s components. Moreover, to overcome existing limitations considering only the HVdc system design, it is introduced to estimate the dynamic capacity based on real-time operational data. In principle, dynamic capacity could help for any power system use case where temporarily additional capacity is required. The article details five use cases, including congestion management, voltage support, frequency response, offshore wind overplanting and grid planning to be of high interest for such a feature. The main HVdc applications, embedded systems, interconnectors and offshore grid connection, and anticipated time frames for dynamic capacity are highlighted from power system perspective. Also, the time-criticality of the remedial actions is outlined

Handling of unbalanced faults in HVDC-connected wind power plants

High-voltage DC (HVDC) connections enable integration of wind power plants located very far from shore. The decoupled AC offshore grid comprises multiple WT converters, and the voltage magnitude and frequency is primarily controlled by the offshore high-voltage DC voltage-source converter (VSC-HVDC). Faults in the offshore grid challenge the connected converters to provide an adequate response improving the overall fault behavior. Of special interest are asymmetrical faults due to the resulting unbalanced voltage conditions. This article addresses such conditions in the offshore grid and analyzes the impact on the offshore grid behavior for different converter contributions. Four fault ride-through strategies are studied for the WT converters. The effect of over-modulation of the converter voltages during such voltage conditions is highlighted. A test system is defined to analyze the fault and post-fault behavior. It is found that voltage support from the WT converters in both positive and negative sequence shows the best performance compared to controlled negative sequence current suppression. This scheme helps additionally the VSC-HVDC AC voltage control to return quickly to normal operation. To validate this statement simulations are performed for line-to-line (LL) and single line-to-ground (SLG) faults in immediate vicinity of the VSC-HVDC.Peer ReviewedPostprint (author's final draft

Optimization-based reactive power control in HVDC-connected wind power plants

One application of high–voltage dc (HVdc) systems is the connection of remotely located offshore wind power plants (WPPs). In these systems, the offshore WPP grid and the synchronous main grid operate in decoupled mode, and the onshore HVdc converter fulfills the grid code requirements of the main grid. Thus, the offshore grid can be operated independently during normal conditions by the offshore HVdc converter and the connected wind turbines. In general, it is well known that optimized reactive power allocation might lower the component loading and power losses. This paper aims to propose and assess a reactive power allocation optimization within HVdc–connected WPPs. For these systems, the offshore converter operates the adjoining grid by imposing frequency and voltage. The reference voltage magnitude is used as additional control variable for the optimization algorithm. The loss function incorporates both the collection grid and the converter losses. The use of the proposed strategy results in an effective reduction of losses compared to conventional reactive power dispatch strategies alongside with improvements of the voltage profile. A case study for a 500 MW–sized WPP demonstrates an additional annual energy production of 6819 MWh or an economical benefit of 886 k€yr-1 when using the proposed strategy.Postprint (author's final draft

Droop Control Design of Multi-VSC Systems for Offshore Networks to Integrate Wind Energy

This research envisages the droop control design of multi voltage source converter systems for offshore networks to integrate wind power plant with the grids. An offshore AC network is formulated by connecting several nearby wind power plants together with AC cables. The net energy in the network is transferred to onshore using voltage source high voltage direct current (VSC-HVDC) transmissionsystems. In the proposed configuration, an offshore network is energized by more than one VSC-HVDC system, hereby providing redundancy to continue operation in case of failure in one of the HVDC transmission lines. The power distribution between VSC-HVDC systems is done using a droop control scheme. Frequency droop is implemented to share active power, and voltage droop is implemented to share reactive power. Furthermore, a method of calculating droop gains according to the contribution factor of each converter is presented. The system has been analyzed to evaluate the voltage profile of the network affected by the droop control. Nonlinear dynamic simulation has been performed for the verification of the control principle

Droop control design of multi-VSC systems for offshore networks to integrate wind energy

This research envisages the droop control design of multi voltage source converter systems for offshore networks to integrate wind power plant with the grids. An offshore AC network is formulated by connecting several nearby wind power plants together with AC cables. The net energy in the network is transferred to onshore using voltage source high voltage direct current (VSC-HVDC) transmissionsystems. In the proposed configuration, an offshore network is energized by more than one VSC-HVDC system, hereby providing redundancy to continue operation in case of failure in one of the HVDC transmission lines. The power distribution between VSC-HVDC systems is done using a droop control scheme. Frequency droop is implemented to share active power, and voltage droop is implemented to share reactive power. Furthermore, a method of calculating droop gains according to the contribution factor of each converter is presented. The system has been analyzed to evaluate the voltage profile of the network affected by the droop control. Nonlinear dynamic simulation has been performed for the verification of the control principle.Peer Reviewe

Analysis of reactive power strategies in HVDC-connected wind power plant clusters

© 2017 John Wiley & Sons, Ltd. Offshore wind power plants (WPPs) built near each other but far from shore usually connect to the main grid by a common high-voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, the wind turbine converters and the high-voltage DC voltage-source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC-connected WPP clusters when applying 5 different reactive power control strategies. The case study is made for a 1.2-GW-rated cluster comprising 3 WPP and is implemented in a combined load flow and converter loss model. A large set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of the WPPs with unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization-based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high-voltage DC voltage-source converter. Reactive power control in HVDC-connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster

Extended current limitation for unbalanced faults in MMC–HVDC–connected wind power plants

IEEE Modular multilevel converters (MMCs) are currently used in a number of high–voltage dc (HVdc) projects connecting very remote offshore wind power plants (WPPs) to shore. For this application the offshore high–voltage dc voltage– source converter (VSC–HVdc) provides the reference for voltage and frequency, i.e. grid–forming operating mode. In fault ride through (FRT) situations, however, the converter reactive current increases and has to be limited to protect the equipment. At the same time, the injection of maximum currents into the offshore grid is advantageous for fault detection and isolation by circuit breakers and to maintain the voltage level. State–of–the– art current limitation might underuse the converter capability in the grid–forming operating mode in specific unbalanced conditions. This paper proposes an extended limitation method for the converter current references for these conditions. It is demonstrated that the proposed method improves the voltage and current profile. Additionally, the impact on the internals of the MMC is outlined. Dynamic simulations are conducted to validate the proposed extended limitation for two operation scenarios. The results demonstrate an enhanced utilization of the MMC capability, a better voltage profile, and an improved transient behavior of the MMC for the proposed extended limitation compared to the normal limitation during specific unbalanced conditions

Handling of unbalanced faults in HVDC-connected wind power plants

High-voltage DC (HVDC) connections enable integration of wind power plants located very far from shore. The decoupled AC offshore grid comprises multiple WT converters, and the voltage magnitude and frequency is primarily controlled by the offshore high-voltage DC voltage-source converter (VSC-HVDC). Faults in the offshore grid challenge the connected converters to provide an adequate response improving the overall fault behavior. Of special interest are asymmetrical faults due to the resulting unbalanced voltage conditions. This article addresses such conditions in the offshore grid and analyzes the impact on the offshore grid behavior for different converter contributions. Four fault ride-through strategies are studied for the WT converters. The effect of over-modulation of the converter voltages during such voltage conditions is highlighted. A test system is defined to analyze the fault and post-fault behavior. It is found that voltage support from the WT converters in both positive and negative sequence shows the best performance compared to controlled negative sequence current suppression. This scheme helps additionally the VSC-HVDC AC voltage control to return quickly to normal operation. To validate this statement simulations are performed for line-to-line (LL) and single line-to-ground (SLG) faults in immediate vicinity of the VSC-HVDC.Peer Reviewe