Harmonics in Offshore Wind Power Plants Employing Power Electronic Devices in the Transmission System

Research output: Book/ReportPh.D. thesisResearch

Abstract

Introduction
The trend in power generation is to partly replace conventional power plants with renewable energy sources. Offshore wind power has been selected to take up a significant proportion of the renewable energy production. The grid codes have been updated to accommodate the rising share of wind power. The onshore as well as offshore wind power plants (OWPPs) therefore have to meet the same stringent requirement as the conventional power plants. This can be accommodated by employment of flexible alternating current transmission system (FACTS) devices, such as the static compensator (STATCOM).
Nowadays, OWPPs are connected through a widespread medium-voltage submarine cable system and grid connected using long high-voltage alternating current (HVAC) cables, representing challenges related to e.g. harmonics resonances. The trend is to locate the OWPPs even further from shore, which gives rise to a number of challenges to the wind power industry with regard to construction, installation as well as transmission of the generated energy. The STATCOM and the voltage-sourced converter high-voltage direct current (VSC-HVDC) are attractive solutions for grid connection of remotely located OWPPs.
The new system configuration requires in-depth knowledge of all relevant technical aspects, including e.g. the involved control systems performance and robustness for all possible operating conditions. The transmission system operator might impose new control requirements when the VSC-HVDC is going to be widely used for OWPP grid connection and the dynamic compliance specifications might change. This sets up a need to develop new strategies for ensuring robustness and adaptability, considering aspects such as fault-handling, ancillary services, congestion management, harmonics etc. It is therefore important to analyse and assess all possible control aspects related to the interaction between the OWPP and the power electronic devices (PEDs) in the transmission system (e.g. the STATCOM and the HVDC system).
OWPPs are susceptible to the harmonic instability, where the extensive sub-marine cabling and possible low available short-circuit power at the grid connection point may create resonance(s) within the bandwidth of the wind turbine generator (WTG) controller. The interaction between the controller and the electrical system external to the WTG may result in an unacceptable high harmonic distortion level. This reduces the efficiency of the generation and transmission of the electrical energy in the OWPP along with increasing the thermal stress of the PEDs and passive components such as the WTG transformer, causing accelerated component aging and may cause malfunctioning of the OWPP protection system. In addition, the harmonic instability may lead to excessive overvoltages, which will cause disconnection of the WTG(s) leading to loss of production, which, due to the size of nowadays OWPPs, will have a serious impact of the revenue to the owners.
The harmonic stability studies of grid-connected converters with the power system (i.e. the examination of the PED current controller’s susceptibility to resonances in the power system) have received academic focus in recent years due to the rapidly increasing penetration of renewable energy and distributed generation resources. Additionally, harmonic stability studies have also become an important part of the system design studies of a HVAC grid connected OWPP. Similarly, harmonic stability studies are needed for the planned VSC-HVDC grid connected OWPPs. Since the offshore electrical environment is significantly altered compared to the offshore network in an HVAC connected OWPP, there is a need to define the harmonic stability assessment procedure and its application for the HVDC grid connected OWPPs.
By achieving a better understanding of the complexity of how such systems (e.g. the WTG and the HVDC station) interact with each other, seen from a harmonic perspective; it will be possible to make a better and more cost-efficient design of e.g. harmonic filters and reactive compensation. Furthermore, it will also enable the OWPP developer, such as DONG Energy Wind Power (DEWP), to provide relevant input for the grid code compliance of an OWPP as well as define technical requirements to potential suppliers.
Purpose of the Industrial PhD Project
The purpose of this Industrial PhD project has been to investigate and address the interaction (from a harmonic perspective) between the OWPP, and the associated control systems in the WTGs and other PEDs in the transmission system with focus on the VSC-HVDC and the STATCOM. The main scope has been on the assessment of the frequency domain evaluation approach, commonly applied in the OWPP design phase. This has been accomplished by comparing results obtained from conventional and linearized frequency domain analysis methods such as the Nyquist stability criterion against the detailed electromagnetic transient (EMT) based model realised in PSCAD/EMTDC.
Measurement Campaigns
Detailed, yet generic models of the involved PEDs have been developed in order to meet the project requirements of analysing the harmonic stability phenomena. The generic EMT models of the PEDs have been validated based on comparison of measurement data, in order to ensure their trustworthiness. Test and field measurements have therefore been conducted on commercial PEDs such as ±50 MVar Siemens SVC Plus, the 7 MW ABB SVC Light (modified back-to-back configuration) and a commercial multi-megawatt sized type 4 WTG.
Measurements constitute a core part in industry-oriented research. Due to this fact, the research project owes its uniqueness and contributes new insight to the academia. A long term harmonic measurement campaign has therefore been prepared and conducted within the PhD project at Clevehill substation, UK, where four STATCOMs are installed. The Cleveill substation serves as grid connection for the 600 MW sized London Array OWPP. The STATCOMs employ the modular multi-levels cascaded converter (MMCC) technology, which is considered state-of-the-art within the industry. The measurement data obtained on the STATCOM has allowed the author to acquire detailed information preferred in order to develop a generic, yet detailed, EMT based model of the MMCC STATCOM, including the switching devices and distributed sub-module (SM) dynamics.
Additionally, the author has, together with colleagues at DEWP, established a Cooperative Research and Development Agreement (CRADA) between DEWP and the National Renewable Energy Laboratory (NREL) in Boulder, Colorado, USA. NREL has recently commissioned an advanced multi-megawatt sized power electronic grid simulator test system. Test results obtained during the author’s stay at NREL are used to develop and evaluate generic, yet detailed EMT models of the grid simulator (referred to as controllable grid interface, CGI) and a commercial multi-megawatt sized type 4 WTG.
PED Modelling and Validation
The relatively high number of non-linear semi-conductors in the MMCCs possesses some challenges in the EMT programs, such as PSCAD/EMTDC, as the semi-conductors are triggered by relatively high-frequency signals and as the electrical system’s admittance matrix is altered at each switching instant. A significant computational effort is required for re-triangularisation of the electrical network subsystem’s admittance matrix. The computational burden is considerably increased for the large number of semi-conductors, making the simulation of the MMCC in some occasions impractical when using a conventional modelling approach. Previously, a detailed equivalent model of the MMCC VSC-HVDC has been devised based on the “Nested Fast and Simultaneous Solution” procedure. However, there are some limitations to the previous model as it is specifically intended for the MMCC employing half-bridge converters in the SMs. The somewhat more complex structure of the full-bridge converter employed in the MMCC STATCOM makes it complex to apply the existing modelling approach for the STATCOM. A universal modelling technique is proposed, which is able to represent both the half- and full-bridge based SMs. The modelling technique is simple as the derivation of the SM’s Norton equivalent is merely based on circuit inspection. The measurement data from the London Array OWPP measurement campaign has successfully been applied in the evaluation of the generic model of the STATCOM implemented in PSCAD/EMTDC. In general it can be said that the proposed generic model is highly capable of replicating the measured waveforms even with no information available of the control and modulation technique applied in the commercial STATCOM.
The evaluated STATCOM model has enabled the development of a trustworthy, full model of the MMCC HVDC, which has been implemented in PSCAD/EMTDC. Simulation studies on the HVDC system has demonstrated the converter's ability to successfully control the distributed SM voltages, dynamically and for steady-state operating conditions.
The model evaluation of the commercial WTG and the CGI has been done for both normal and abnormal operating conditions, such as fast and slow balanced and unbalanced faults realised at the test facility at NREL. The evaluation demonstrated that it is possible to accurately simulate the measured waveforms with little or even no information available applied PED control methodology as well as the control system parameters, which are influencing the characteristics of both the CGI and the WTG. The successful validation gives credibility to the time domain modelling of the involved PEDs for the harmonic stability assessment in OWPPs.
System Studies
Harmonic stability studies have been realised in OWPPs employing PEDs in the transmission system. The harmonic stability in an HVDC grid connected OWPP has been investigated in both the frequency and time domain using the validated models of the PEDs. The application of the frequency domain in the stability analysis raises the conceptual challenge of recognising the source and the load (i.e. the plant), as either the WTG or the HVDC system can be treated as the source in the analysis, yielding different results. In this case both the load and the source are actively controlled power system devices. The time domain approach on the other hand provides a holistic approach, without the need to assign the source in the analysis.
Based on a number of study cases, it is demonstrated that a good correlation exists between the time and frequency domain methods for the stability analysis. However, limitations of the frequency domain were observed when resonances at higher frequencies exist (i.e. near the switching frequency and above the bandwidth of the current controller). Therefore, it is proposed to conduct the analysis primarily in the frequency domain. Once all the considered operating scenarios are covered (typically in the excess of 1000) a few cases should be selected and repeated in the time domain, which is both more challenging and more time consuming than the frequency domain method.
Time domain studies in an HVAC grid connected OWPP with a STATCOM located at the grid connection point have shown that the undesirable controller interaction between the WTGs and the STATCOM deteriorates the controllability of the internal dynamics of the MMCC STATCOM.
It was found that the total inter-harmonic distortion index according to IEC Standard 61000-4-7 is useful to assess possible and undesired control interaction between the PEDs in OWPPs. The total harmonic distortion index, on the other hand, is found to contain very little information on possible PED controller interaction.
It was shown that an application of active filtering in the WTGs by means of a band rejection filter (i.e. notch filter) in the main control chain can potentially reduce the harmonic emission and improve the overall stability in OWPPs without interfering with the OWPP overall design. This can be achieved by providing additional damping or shifting the resonance frequencies. Improving the PED’s controllers’ rejection capability called active damping is a certain type of active filtering. The converter may be controlled adaptively or tuned to suppress selected harmonic components. The application of active filtering was demonstrated for OWPPs with a STATCOM or an HVDC located in the transmission system. The active filtering was found to improve the internal dynamics of the MMCC STATCOM.
Based on the findings of the research project, a best practice for the evaluation of the harmonic stability in OWPPs employing PEDs in the transmission system has been formulated and is enclosed in the dissertation.
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Details

Introduction
The trend in power generation is to partly replace conventional power plants with renewable energy sources. Offshore wind power has been selected to take up a significant proportion of the renewable energy production. The grid codes have been updated to accommodate the rising share of wind power. The onshore as well as offshore wind power plants (OWPPs) therefore have to meet the same stringent requirement as the conventional power plants. This can be accommodated by employment of flexible alternating current transmission system (FACTS) devices, such as the static compensator (STATCOM).
Nowadays, OWPPs are connected through a widespread medium-voltage submarine cable system and grid connected using long high-voltage alternating current (HVAC) cables, representing challenges related to e.g. harmonics resonances. The trend is to locate the OWPPs even further from shore, which gives rise to a number of challenges to the wind power industry with regard to construction, installation as well as transmission of the generated energy. The STATCOM and the voltage-sourced converter high-voltage direct current (VSC-HVDC) are attractive solutions for grid connection of remotely located OWPPs.
The new system configuration requires in-depth knowledge of all relevant technical aspects, including e.g. the involved control systems performance and robustness for all possible operating conditions. The transmission system operator might impose new control requirements when the VSC-HVDC is going to be widely used for OWPP grid connection and the dynamic compliance specifications might change. This sets up a need to develop new strategies for ensuring robustness and adaptability, considering aspects such as fault-handling, ancillary services, congestion management, harmonics etc. It is therefore important to analyse and assess all possible control aspects related to the interaction between the OWPP and the power electronic devices (PEDs) in the transmission system (e.g. the STATCOM and the HVDC system).
OWPPs are susceptible to the harmonic instability, where the extensive sub-marine cabling and possible low available short-circuit power at the grid connection point may create resonance(s) within the bandwidth of the wind turbine generator (WTG) controller. The interaction between the controller and the electrical system external to the WTG may result in an unacceptable high harmonic distortion level. This reduces the efficiency of the generation and transmission of the electrical energy in the OWPP along with increasing the thermal stress of the PEDs and passive components such as the WTG transformer, causing accelerated component aging and may cause malfunctioning of the OWPP protection system. In addition, the harmonic instability may lead to excessive overvoltages, which will cause disconnection of the WTG(s) leading to loss of production, which, due to the size of nowadays OWPPs, will have a serious impact of the revenue to the owners.
The harmonic stability studies of grid-connected converters with the power system (i.e. the examination of the PED current controller’s susceptibility to resonances in the power system) have received academic focus in recent years due to the rapidly increasing penetration of renewable energy and distributed generation resources. Additionally, harmonic stability studies have also become an important part of the system design studies of a HVAC grid connected OWPP. Similarly, harmonic stability studies are needed for the planned VSC-HVDC grid connected OWPPs. Since the offshore electrical environment is significantly altered compared to the offshore network in an HVAC connected OWPP, there is a need to define the harmonic stability assessment procedure and its application for the HVDC grid connected OWPPs.
By achieving a better understanding of the complexity of how such systems (e.g. the WTG and the HVDC station) interact with each other, seen from a harmonic perspective; it will be possible to make a better and more cost-efficient design of e.g. harmonic filters and reactive compensation. Furthermore, it will also enable the OWPP developer, such as DONG Energy Wind Power (DEWP), to provide relevant input for the grid code compliance of an OWPP as well as define technical requirements to potential suppliers.
Purpose of the Industrial PhD Project
The purpose of this Industrial PhD project has been to investigate and address the interaction (from a harmonic perspective) between the OWPP, and the associated control systems in the WTGs and other PEDs in the transmission system with focus on the VSC-HVDC and the STATCOM. The main scope has been on the assessment of the frequency domain evaluation approach, commonly applied in the OWPP design phase. This has been accomplished by comparing results obtained from conventional and linearized frequency domain analysis methods such as the Nyquist stability criterion against the detailed electromagnetic transient (EMT) based model realised in PSCAD/EMTDC.
Measurement Campaigns
Detailed, yet generic models of the involved PEDs have been developed in order to meet the project requirements of analysing the harmonic stability phenomena. The generic EMT models of the PEDs have been validated based on comparison of measurement data, in order to ensure their trustworthiness. Test and field measurements have therefore been conducted on commercial PEDs such as ±50 MVar Siemens SVC Plus, the 7 MW ABB SVC Light (modified back-to-back configuration) and a commercial multi-megawatt sized type 4 WTG.
Measurements constitute a core part in industry-oriented research. Due to this fact, the research project owes its uniqueness and contributes new insight to the academia. A long term harmonic measurement campaign has therefore been prepared and conducted within the PhD project at Clevehill substation, UK, where four STATCOMs are installed. The Cleveill substation serves as grid connection for the 600 MW sized London Array OWPP. The STATCOMs employ the modular multi-levels cascaded converter (MMCC) technology, which is considered state-of-the-art within the industry. The measurement data obtained on the STATCOM has allowed the author to acquire detailed information preferred in order to develop a generic, yet detailed, EMT based model of the MMCC STATCOM, including the switching devices and distributed sub-module (SM) dynamics.
Additionally, the author has, together with colleagues at DEWP, established a Cooperative Research and Development Agreement (CRADA) between DEWP and the National Renewable Energy Laboratory (NREL) in Boulder, Colorado, USA. NREL has recently commissioned an advanced multi-megawatt sized power electronic grid simulator test system. Test results obtained during the author’s stay at NREL are used to develop and evaluate generic, yet detailed EMT models of the grid simulator (referred to as controllable grid interface, CGI) and a commercial multi-megawatt sized type 4 WTG.
PED Modelling and Validation
The relatively high number of non-linear semi-conductors in the MMCCs possesses some challenges in the EMT programs, such as PSCAD/EMTDC, as the semi-conductors are triggered by relatively high-frequency signals and as the electrical system’s admittance matrix is altered at each switching instant. A significant computational effort is required for re-triangularisation of the electrical network subsystem’s admittance matrix. The computational burden is considerably increased for the large number of semi-conductors, making the simulation of the MMCC in some occasions impractical when using a conventional modelling approach. Previously, a detailed equivalent model of the MMCC VSC-HVDC has been devised based on the “Nested Fast and Simultaneous Solution” procedure. However, there are some limitations to the previous model as it is specifically intended for the MMCC employing half-bridge converters in the SMs. The somewhat more complex structure of the full-bridge converter employed in the MMCC STATCOM makes it complex to apply the existing modelling approach for the STATCOM. A universal modelling technique is proposed, which is able to represent both the half- and full-bridge based SMs. The modelling technique is simple as the derivation of the SM’s Norton equivalent is merely based on circuit inspection. The measurement data from the London Array OWPP measurement campaign has successfully been applied in the evaluation of the generic model of the STATCOM implemented in PSCAD/EMTDC. In general it can be said that the proposed generic model is highly capable of replicating the measured waveforms even with no information available of the control and modulation technique applied in the commercial STATCOM.
The evaluated STATCOM model has enabled the development of a trustworthy, full model of the MMCC HVDC, which has been implemented in PSCAD/EMTDC. Simulation studies on the HVDC system has demonstrated the converter's ability to successfully control the distributed SM voltages, dynamically and for steady-state operating conditions.
The model evaluation of the commercial WTG and the CGI has been done for both normal and abnormal operating conditions, such as fast and slow balanced and unbalanced faults realised at the test facility at NREL. The evaluation demonstrated that it is possible to accurately simulate the measured waveforms with little or even no information available applied PED control methodology as well as the control system parameters, which are influencing the characteristics of both the CGI and the WTG. The successful validation gives credibility to the time domain modelling of the involved PEDs for the harmonic stability assessment in OWPPs.
System Studies
Harmonic stability studies have been realised in OWPPs employing PEDs in the transmission system. The harmonic stability in an HVDC grid connected OWPP has been investigated in both the frequency and time domain using the validated models of the PEDs. The application of the frequency domain in the stability analysis raises the conceptual challenge of recognising the source and the load (i.e. the plant), as either the WTG or the HVDC system can be treated as the source in the analysis, yielding different results. In this case both the load and the source are actively controlled power system devices. The time domain approach on the other hand provides a holistic approach, without the need to assign the source in the analysis.
Based on a number of study cases, it is demonstrated that a good correlation exists between the time and frequency domain methods for the stability analysis. However, limitations of the frequency domain were observed when resonances at higher frequencies exist (i.e. near the switching frequency and above the bandwidth of the current controller). Therefore, it is proposed to conduct the analysis primarily in the frequency domain. Once all the considered operating scenarios are covered (typically in the excess of 1000) a few cases should be selected and repeated in the time domain, which is both more challenging and more time consuming than the frequency domain method.
Time domain studies in an HVAC grid connected OWPP with a STATCOM located at the grid connection point have shown that the undesirable controller interaction between the WTGs and the STATCOM deteriorates the controllability of the internal dynamics of the MMCC STATCOM.
It was found that the total inter-harmonic distortion index according to IEC Standard 61000-4-7 is useful to assess possible and undesired control interaction between the PEDs in OWPPs. The total harmonic distortion index, on the other hand, is found to contain very little information on possible PED controller interaction.
It was shown that an application of active filtering in the WTGs by means of a band rejection filter (i.e. notch filter) in the main control chain can potentially reduce the harmonic emission and improve the overall stability in OWPPs without interfering with the OWPP overall design. This can be achieved by providing additional damping or shifting the resonance frequencies. Improving the PED’s controllers’ rejection capability called active damping is a certain type of active filtering. The converter may be controlled adaptively or tuned to suppress selected harmonic components. The application of active filtering was demonstrated for OWPPs with a STATCOM or an HVDC located in the transmission system. The active filtering was found to improve the internal dynamics of the MMCC STATCOM.
Based on the findings of the research project, a best practice for the evaluation of the harmonic stability in OWPPs employing PEDs in the transmission system has been formulated and is enclosed in the dissertation.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
StatePublished - Mar 2015
Publication categoryResearch

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