Wind Generations project development is accelerating along the Atlantic coast of the United States. Most transmission coastal service are remote, lack inertia, short circuit current and capacity to tolerate large amounts of Wind Generation. All of these grid characteristics we take for granted effect the reliability, availability and the ability to deliver power to the grid. This paper will discuss the study requirements and many of the design consideration decisions that will be required to successfully install and operate a large Wind Generation Facility.
Study and Considerations to Integrate and Manage Energy Storage Systems
Modernization and efforts for decarbonization of the electric grid is a worldwide movement and expectation to neutralize effects of climate change. The way forward is not universal or direct as there are many competing ideas and technologies. The most popular philosophy is to replace conventional carbon-based generation with Inverter Based Resource (IBR) generation. The current attractive technologies are solar and wind. A clear road map forward would be apparent if these new technologies were similar in their ability to provide the same functions or services that conventional electric generation has today. Unfortunately, IBR generation and applications are still evolving and lack services and abilities that make integration and interconnection challenging on a large scale.
IBR applications that are large grid scale have great replacement opportunity which are now being explored and implemented around the world. Each power technology has its benefits and complexity which are weighed by each electric utility, developer and regulator. The are many competing factors such as location, economics and environmental impact. Solutions are not easy and the larger the project the more engineering variables must be analyzed and considered. Simple designing and building grid scale inverter-based generation is a multivariant challenge.
Energy density in the form of magnitude vs footprint should be more environmental and practical. Solar farms while popular and dominant in installation require large tracks of land and are likely remote from the load centers. To reduce footprint environmental impact one of the more attractive and developing forms of generation are Wind Turbine Generators (WTG). The size and ratings of wind turbines are increasing each year making WTG more attractive and economical. What makes a project successful is the ability to recognize all the factors to consider and evaluate. Locating win offshore negates many of the social arguments and negative effects.
The hope and intent of this paper is to provide a codified list of studies and thoughts based on experience for the reader to consider in the process of developing large scale offshore wind. The most important take away from this paper is each project due to the nature of geography and the electrical characteristics of the grid interconnection point are different. This papers focus is on a conventional application of an AC connection due application less than 30 miles. A combination of Civil and Electrical study should be performed before major project decisions are made. Competing interest such as project schedule, regulatory contracts and supply chain logistics make each project different. Study, planning and organization allow for smooth project flow in the face of constant change.
A practical approach even before civil geotechnical study is performed is to understand the electrical connection impact and needs. For each project interconnection WTG size (rating) maters. How much generation can be injected before major investment in system upgrades and grid stabilization technology must be installed to operate. Each country and Regional Transmission authority has electric grid code requirements for stability as well as power quality that must be satisfied to generate. Investigating contractual interconnection requirements is a good starting point for planning.
In North America key documents such as FERC order 827 Reactive Power Requirements for Non-Synchronous Generation and 661a Interconnection for Wind Energy are basic starting points. Another standard that is underdevelopment is IEEE P2800 Draft Standard for Interconnection and Interoperability of Inverter-Based Resources Interconnecting with Associated Transmission Electric Power Systems. While P2800 standard is not approved at the time of this paper being presented it will be approved and published soon. Planners, developers and engineers are considering even now the impact of this standard draft on each project application.
IEEE P2800 establishes the recommended interconnection capability and performance criteria for inverter-based generation that is interconnected with transmission systems. The standard recommends performance criterion including, but not limited to, voltage and frequency ride-through, active power control, reactive power control, dynamic active power support under abnormal frequency conditions, dynamic voltage support under abnormal voltage conditions, power quality, negative sequence current injection, and system protection requirements. All of these services are mentioned in the standard are specifications to be studied and agreed on between the Wind Turbine supplier and owner. Many developers do not understand that the WTG service need to be prioritized and specified by the purchaser.
Each generation project needs to contribute to the betterment of the grid and supply power in a seamless manner. Injection rights are determined by the regional or governmental transmission authority. In order to achieve successful integration basic studies and questions need to be answered. The following study items should be addressed to start the project:
Task 01 – Data Gathering/Review – Short-Circuit Analysis
Task 02 – Power Flow Case Development
Task 03 – Reactive Power Capability (WTG or On Shore Interconnection)
Task 04 – Steady-State Contingency Analysis
Task 05 – Dynamic Model Review and Ride-Through/Small-Signal Stability Testing
Task 06 – Dynamic Stability Analysis
Task 07 – Dynamic Performance, Coordination, and Interaction Analysis
Task 08 – Harmonic Model Development of the Wind Farm
Task 09 – Frequency Scan of the Wind Turbine and Project
Task 10 – Harmonic Model Development of the Grid
Task 11 – Harmonic Sectors
Task 12 – Incremental Harmonic Emissions Analysis
Task 13 – Background Harmonic Amplification Analysis Task
Task 14 – Harmonic Performance Assessment
Task 15 – Harmonic Filter Design
Task 16 – Harmonic Interaction Screening Analysis
Task 17 – Harmonic Field Measurement Benchmarking Analysis at POI
Task 18 – Electromagnetic Transients (EMT) System Model Development
Task 19 – EMT Model Review and Ride-Through/Small-Signal Stability Testing
Task 20 – RMS-EMT Wind Farm Model Benchmarking Analysis
Task 21 – EMT Switching and Internal Fault/Clear Analysis
Task 22 – EMT Dynamic Performance Analysis
Task 23 – EMT Project Stability Threshold Analysis
Task 24 – EMT Short-Circuit Current Verification
Task 25 – Wind Farm Flicker Analysis
Task 26 – Multi-Frequency Stability Analysis
Task 27 – SSTI and SSCI Screening Analysis
Task 28 – Countermeasure and Mitigation Solutions If Required
The electrical characteristics and studies are not trivial and should be examined by an experienced study engineer or firm. Multiple simulation tools will need to be used to determine accurate data and information. Not every Task needs to be studied to determine the scope of the project but at least the first 7 Tasks should be considered to prevent unintended project surprises. Unanticipated cost overruns due to need for additional reactive support, stability mitigation or congestion due to limited equipment ratings can disrupt or cancel a project. Developers want to install as much generation as economically possible but understanding how much generation can be added before upgrade and uprating costs can be an initial factor as well.
The initial Tasks require data collection which can be a difficult task. Often the starting data such as underwater cable, transformer impedance and reactor ratings are an educated guess. Once a Point Of Interconnection (POI) is chosen a basic Thevenin equivalent for short circuit analysis can be obtained from the utility or Regional Transmission Authority (RTO). Understanding the short circuit magnitude impact on the project success is a significant step. Often developers or planners focus on load flow and sizing of a wind farm. The developer or consultant wants to install as many wind turbines in the project area as possible to maximize the investment without considering project needs and equipment ratings to make the project work. The place to start evaluating the Short Circuit Ration (SCR) compared to the WTG rating. An SCR rating greater than 3 typically means that the wind turbine operation is stable and will not require any additional electrical technologies to operate successfully. Typical contingency study is required for N-1 and N-1-1 to obtain a worst-case value for design. Why is this important? The regulatory requirements for ability to generate requires stable operation of the wind farm under contingency conditions. In North America a remedial action scheme is not an acceptable solution since wind farm and system stability can be controlled by limiting generation. Manufactures confirm stable operation of a WTG for SCR greater than 3. If the SCR is less than 3 some manufactures wind turbines can operate in a stable manner. Detailed analysis is required to determine what SCR level is possible. The engineering take away is to determine minimum and maximum short circuit levels which have a profound impact on project success.
For connection to the grid, power transfer will also influence stability, beyond short-circuit capacity. the effects of full or maximum power production should also be considered when evaluating the magnitude and rate-of-change of the system quantities. Considering only short circuit current at a single point is only an initial screening task. Influence on power transfer will also be required to ensure operating stability. Week grids characterized by low short circuit and high system impedance will influence the WTG converter’s ability to provide watts or VARS and effect angular stability. The active and reactive contributions form a WTG are a multivariant control problem and significant evaluation is necessary to determine the right mix between on shore and offshore control and reactive contributions.
Determining an initial WTG start up need part of the study analysis is tricky because several design factors can come in to play. Initially work is done with generic models such as those chosen in the popular planning software PSSE. Using the generic wind turbine models stability will never be achieved at low SCR levels. Electro Magnetic Transient software for example such as PSCAD must be used. The next requirement is a WTG vendor specific models must be obtained. During initial studies the WTG vendor may be unknown so understanding that Low SCR impacts the project is a critical success factor. Either the generation size will be required to be reduced or upgrades to the interconnection grid will be required to generate at full capacity. These Transmission upgrades may include but are not limited to Transmission lines, Step up Transformers, Synchronous Condensers and or STATCOMS. The final analysis will require specific WTG models and as detailed equipment data as possible to specify not only the equipment procurement but also the wind farm operation control.
An overriding complication to design is the balance of the underwater cable design and the reactive power needs to balance both voltage rise and reactive power contribution to power factor. To the inexperienced designer cable specification might seems basic and straight forward. To the contrary cable sizing and routing can be one of the most difficult aspects of an offshore wind project. The selection of cable effects the ability to deliver not only real watts but also the steady state power factor requirements at the POI. Designers always try to select the minimum cable rating in order to minimize the underwater cable expense which can be significant. The key is not to overlook all of the electrical phenomena that effect long cable ratings. Underwater cable is typical a 3-phase bundled cable with optical fiber for protection, control and communications. An example is shown in figure #1. To the power system the cable itself looks like a transmission capacitor bank and in many respects at the POI is treated as such. What is often neglected is the impact of system voltage has on capacitance and the charging current will add to the current generated by the wind turbines. As the voltage increases the capacitance of the cable increases and so does the charging current. Failure to take cable charging to account can constrain the ability of the wind farm to deliver its designed maximum rating at full output. To help balance the reactive power cable needs shunt reactors are placed on both ends of the cable.
Charging current in addition to generation current will impact the ability of the WTG to supply reactive power necessary to correct power factor at the POI. If the WTG cannot supply the needed reactive power for compliance, then reactive resources must be supplied at the interconnection substation. Depending on the size and complexity of the proposed wind farm. The tool many designers use today is a Static Synchronous Condenser (STATCOM) due to its size and flexibility of operation. Sizing the STATCOM at the start of a project is not straight forward because the impact of cable rating and shunt reactor sizing to offset cable capacitance.
Figure #2 below is an example of voltage impact on cable rating and capability to deliver power from the WTG to the OSS. Figure #2 points more to the complexity of plant control design (can’t be arbitrary as to prior practice), than cable rating alone in general. A single voltage reference or single power-dependent trend would be unable to satisfy cable ratings.
The ideal sizing of underground cable between the POI and the Offshore Substation (OSS) would be to install equal size shunt reactors at both ends of the cable. For relatively small wind farms this should be possible as the OSS physical size can accommodate the weight of the transformers and shunt reactors. Cost of the OSS is greatly influenced by the weight of the substation platform. There is a physical limitation to weight that can be lifted and installed on the support members (Jackets). This limitation depends completely on the construction and transport ship that procured for the project. The good news is ship size is increasing year to year and will benefit the designers. If weight is an issue, then reactor size can be decreased or additional OSS substations can be added but economic analysis will drive this part of the project design. Choosing the right shunt reactor for the project depends on the utility design requirements and asset management practice. European designers often chose a variable shunt reactor for the application. There are advantages and disadvantages to this approach. Variable reactors can reduce the need for a large STATCOM to make up the Power factor difference but due to the constant fluctuation of WTG the Tap Changer in the shunt reactor will need to be maintained frequently. If a cable is out of service to maintain a reactor tap changer, then there is a loss of generation that will impact the project capacity factor. The other consideration cable lengths between multiple OSS and the POI are different lengths. Some designers will install different size reactors to minimize reactive power needs, but then additional spare reactors will be required.
Each developer will need to decide the best practice in selection of shunt reactors. Regardless of choice leaving the cable partially capacitive is a reasonable practice to mitigate switching transients. Controlled switch is a must for switching the underground cable to prevent over voltage transients. Synchronous switching or pre insertion resistors are required for the cable breakers.
The addition of FACTs devices to the project is typical and an example of configuration is in figure #3.
Most Developers chose a STATCOM over a Static VAR Compensator (SVC) or Synchronous Condenser (SYNCON) due to reduction in size and lower losses and response time. Depending on how week the system SCR it is possible that both a STATCOM and SYNCON would be necessary during certain times of the year to operate stable. Reduced footprint at the POI is beneficial and reduced losses depending on the project greatly increases economic benefit. Normally a fault or a perturbation leads to a low voltage drop. The technical advantage of a STATCOM’s is it emulate a constant current source even during low voltage conditions. An SVC is voltage dependent and provides lower compensation proportionally at lower than nominal voltage. An SVC requires Harmonic filters and a STATCOM typically does not. The design of harmonic filters for an SVC to support a wind farm is complex and a multivariant problem. The SVC and Wind Turbines can generate significant harmonics that can add to the site or background harmonic distortion. There are also possible resonant points in the wind turbine array and system that have to be evaluated to comply with grid code. Most State utilities follow IEEE 519 guidelines in North America, so it is a good reference for the project study work. The interconnection harmonics based on the project model and analysis is evaluated in Task 10 -17th. If gride code is not meet the wind farm will be curtailed until compliant.
There are a couple of considerations if specifying a STATCOM. The first is to understand if there are any Sub Synchronous Control Interactions that could be possible between the STATCOM, any nearby generator and the WTG. This topic is too large to detail in this paper, but the wind farm study will need to evaluate to make sure there are no concerns. If a control interaction is identified the STATCOM can be designed to eliminate the concern in its control. The effectiveness of the design can be proven during factory commissioning with a hardware in the loop test using a Real Time Digital Simulator (RTDS). Interaction of a wind farm with a Shunt FACTS device is rare but is a necessary part of the evaluation. A quick scanning study will show if more detailed investigation is necessary.
A STATCOM has features that may be beneficial to the project such as Harmonic Cancelation, Power Oscillation Dampening and Automatic Restart with Re-Insertion. Negative Sequence balancing while available is not generally necessary since the power train cables are 3 phase and included in a single cable. Voltage stays pretty well balanced and negative sequency is minimized. A STATCOM also has the ability to control additional Shunt Reactors or Capacitors to help with voltage and power factor management. Remote management of voltage and VAR set points are available and can be integrated as part of the wind farm operator control thorough SCADA.
The Wind Farm controller (Park Controller) is a design combination of controls based on needed capacity factor, ability of the WTG to regulate voltage and power factor based on compliance. The Wind Turbine control needs to work in concert with the Park Controller, so communications and prioritization of Wind Turbine Control features are essential. Modern wind turbines have multiple service features and have the ability to provide significant reactive power, negative sequency, voltage and droop control.
Modern wind turbine technology is evolving rapidly on the market is a double feed induction generator which is not mechanically coupled to the system. The advantage is expanded ability to operate in constantly changing weather condition. A must have service is the No Load Wind feature which allows a wind turbine to provide reactive power to grid when there is no wind. Basically, when the turbine blades are not spinning. The capacitance in the cable array can cause significant high voltage similar to a capacitor bank. The ability for the WTG to provide reactance similar to a shunt reactor with no wind is a significant operating advantage. The addition of No-Load Wind feature allows for power factor and voltage compensation control can limit the need for some shunt reactors on the OSS. Anything to reduce OSS weight and size provide economic benefit the project. The construction ships have a maximum weight limit that the OSS can be lifted. OSS platform weight is a critical design factor a design limitation. As the weight limit approaches maximum additional substation platforms may need to be installed.
A second feature to consider is if available is High Wind Ride Through. US costal utilities are subjected to climate conditions such as North Easters and Hurricanes. The ability to generate in abnormal weather condonations is a decided advantage even if capacity is reduced. At some point winds may exceed the safe operating magnitude and will require lock down. The generation support to the utility system during poor or abnormal weather can prove invaluable if there is significant solar penetration in the grid. All resources will be needed to ride through future climate events. The importance of an experienced proper study cannot be emphasized enough. The specification of the wind turbine requires the owner to choose operating features and parametrize the control operation. The initial study will save time and change orders during the project. Changes can delay the project as well as increase purchase costs.
System protection is straight forward and not complicated. The substation at the POI can be built to local utility standards. Differential relays of the utility choice can be used for Substation Buss, transformers, shunt reactors, harmonic filter banks, SATCOMS or SYNCONS. Typical applications at the transmission level require redundant sets of protection primary one and two. It is important to establish and operating one line and protection online as soon as possible to identify all of the current transformers (CTs)needed for the project. Depending on the utility requirement or practice metering separate CTs are used for each set of protection relays as well as metering and sensing for the wind farm control system. It is important to know CT locations and number if Gas Insulated Switchgear (GIS) will be used so the proper configuration and equipment size is specified. Air Insulated Substations (AIS) are more forgiving but take up more space at the POI. GIS substations have the advantage of reduced size and exposure to salt contamination near the coast. Both technologies are commonly used depending on location, economic viability and environmental exposure. Transmission underwater cable uses both differential and overcorrect relays for high-speed detection and interrupting operation. The underwater cables have a built-in fiber used for communication and control but also can be used for differential protection. The number of fibers combinations should be established so the cable is properly specified as it is a long lead item to obtain. Spare fiber and redundancy are common but microwave back up can be used as back up depending on the distance from shore.
This paper cannot possible detail all of the civil and electric design decisions that have to be made. Each project is a compromise of equipment ratings to successfully operate. The key factor is to determine the maximum amount of generation that can be operated before system upgrades and costly mitigation. Cable design and rating have a significant role to determine the ability of the WTG to provide steady state or transient stability. Preliminary study and detailed studies are the key to a successful project design and trouble-free operation. Many decisions will be juggled integrate a project to the grid. There is not a good substitute for experience.