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Wind Power Plant SCADA and Controls IEEE PES Wind Plant Collector System Design Working Group

Contributing Members: B. Badrzadeh, M. Bradt, N. Castillo, R. Janakiraman, R. Kennedy, S. Klein, T. Smith, L. Vargas

Abstract This paper discusses the range of application for SCADA and control systems in a wind power plant, the most important SCADA and control system considerations, and contractual requirements for SCADA and control systems. Index Terms SCADA, wind power plant, wind turbine control, data acquisition, supervisory control, plant control, security and reliability compliance. List of Acronyms

BOP Balance of Plant CIP Critical Infrastructure Protection DFAG Doubly Fed Asynchronous Generator FERC Federal Energy Regulatory Commission IA Interconnection Agreement IEC International Electrotechnical Commission IED Intelligent Electronic Device ISO Independent System Operator LGIA Large Generator Interconnection Agreement NERC North American Electric Reliability Corp. OEM Original Equipment Manufacturer OLE Object Linking & Embedding OPC OLE for Process Control PDD Presidential Decision Directive PLC Programmable Logic Controller POI Point of Interconnection PRC Protection and Control PRR Power Ramp Rate PSR Protection System Relaying RAS Remedial Action System RTO Regional Transmission System Operator SCADA Supervisory Control and Data Acquisition SPS Special Protection System WPP Wind Power Plant XML Extensible Markup Language

I. INTRODUCTION Modern wind power plants (WPPs) include an

amalgamation of Supervisory Control and Data Acquisition (SCADA) systems, control systems, and various other intelligent electronic devices (IEDs). SCADA and control systems are critical parts of all WPPs, regulating nearly every aspect from the individual turbine to the collection

substation. A WPPs ability to maximize efficiency is directly related to monitoring and control infrastructure.

SCADA and control requirements are contractually specified under interconnection agreements (IA), and are subject to security and reliability requirements under mandatory reliability compliance provisions. SCADA systems and options are integral to compliance with interconnection requirements including voltage and power factor control, curtailment, and ramp control. SCADA systems are also a medium for data transportation and communication with external sources, such as, providing meteorological data necessary for forecasting to Independent System Operators (ISOs). SCADA systems are integral to availability and performance measurement and warranty enforcement.

There are many different options for control and data acquisition in a WPP. This paper provides an overview of common applications, considerations, and requirements for WPP SCADA and control systems.

II. WIND TURBINE CONTROLS A wind turbines control system enables the safe,

reliable, and automated control necessary for continuous power production and shutdown, as required. A typical wind turbines control system consists of control hardware, supervisory controls, safety systems, and closed-loop controls that enable power production by controlling the blade pitch angle and the generator torque of the turbine [1].

A horizontal axis wind turbine control system will consist of several sensors, actuators, and a microprocessor controller. A list of common hardware elements include:

Nacelle mounted anemometer and wind vane; Rotor speed sensor; Electric power sensor; Pitch position sensors; Vibration sensors; Oil level and temperature indicators; Hydraulic pressure sensors; and Operator switches.

The microprocessor controller uses defined logic to

process inputs from the various sensors and generates outputs to operate the turbine. Commercially available programmable logic controllers (PLCs) are commonly used.

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There are several independent systems within the turbine that are controlled as a whole by the turbine controller. These include the pitch, yaw, generator, and supervisory control systems. A separate and independent safety system protects the turbine hardware from damage in the event of a controller failure.

A. Pitch There are typically two types of turbines, stall regulated

and pitch regulated. Stall regulated machines do not or only slightly vary the pitch angle of the blade and rely on the stall characteristics of the blade to limit the rotor speed and aerodynamic power.

Pitch regulated machines vary the aerodynamic power and rotor speed by changing the pitch of the turbines blades through electrical or mechanical linkages. Blades can be pitched collectively, independently, or individually. Collectively pitched systems move all of the blades at the same time to the same pitch angle. Independent pitch systems use separate (non-linked) systems for each blade although the blade angles are set to the same pitch angle. Individual systems, similar to independent systems, use separate systems for each blade although the pitch angle of each blade can be varied individually which will reduce aerodynamic loads.

B. Yaw The yaw system rotates the turbine into or out of the

wind using drive motors. The yaw action is essential to mitigate the turbine fatigue loads and maintain an optimal energy production. The turbine control system monitors the time-averaged difference between the turbine yaw angle and the wind direction and will adjust the turbine yaw angle into the wind once the difference becomes great enough over a set period of time. The yaw system also unwinds the power and control cabling that is run between the nacelle and equipment located at the base of the tower such as transformers and controllers. Different components of the yaw system can be placed in the tower, as well as inside and outside of the nacelle.

C. Generator Variable speed turbines are capable of controlling the

generator torque, which effectively controls the rotational speed of the turbine. Variable speed generators have several different topologies including full converter systems, Doubly-Fed Asynchronous Generators (DFAG), or variable slip induction generators. For a more detailed description of variable speed turbines, please see [4].

The turbine controller monitors the rotor speed and regulates the generator torque to maximize the power output and maintain the rotor speed below its rated rotational speed. Additionally, the generator torque control can be used to actively dampen drive train torsional vibrations by applying a small ripple torque close to the

drive train natural frequency and at an appropriate phase angle.

D. Closed loop design To maximize the power output and to minimize the

dynamic loading the turbine controller utilizes typical closed loop control algorithms. The usual method is to construct a proportional-integral (PI) or proportional-integral-derivative (PID) control loop to dynamically vary the pitch angle and generator torque for maximum power production and minimal dynamic loading.

E. Supervisory The turbines internal supervisory controls consist of the

logic and hardware necessary to operate the turbine autonomously from one operational state to another. These operational states consist of start-up, power production, shut-down, and stopped when faulted. Other functions of the turbine internal supervisory controls include operation of cooling equipment (gearbox, generator and power converters fans and pumps), heaters (for cold weather applications), and lubrication pumps (gearbox oil pumps and bearing grease pumps).

F. Safety The safety system is a highly reliable, independent, and

hardwired system separate from the microprocessor controlled system that is designed to shutdown the turbine during a serious problem. The control system is designed to operate the turbine in normal shutdown situations; the safety system is a backup to the turbine controller and functions as a fail-safe in the event the controller fails. Events that may trip the safety system include:

Rotor overspeed; Vibration sensor trip; Controller watchdog trip; Emergency stop button pressed by operator;

and Pitch system failure, i.e. stuck blade or large

pitch angle difference.

The turbine control system is a complex system consisting of several subsystems and components. Using different digital communication systems and hardwired inputs/outputs the wind turbine is controlled safely and reliably. Although not common there have been instances of external electromagnetic interference (radar systems) creating problems with control systems, specifically pitch control systems.

III. PLANT SCADA The SCADA system in a WPP provides real-time

visibility of the plant operations and also provides the ability to control the WPP assets centrally and remotely. Typically, a SCADA system is provided by the turbine

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original equipment manufacturer (OEM). For fleet wide monitoring and control across different OEMs turbines, a third-party SCADA solution can be implemented.

A. OEM SCADA The SCADA system provided by the turbine

manufacturers is typically a fully integrated system. OEM SCADA systems have variable functionality depending on the manufacturer. Some of the potential advantages of an OEM SCADA system include:

Tightly integrated with the turbine control system;

Turbine specific monitoring and control interface;

Advanced turbine details and diagnostics; Robust and integrated security model; Advanced troubleshooting and data analysis; Preconfigured displays and reports; Flexible and open system for data access Verification of contractual obligations

(production, availability guarantees, wind distribution, loss of production, power curves, etc);

Service and error correction; Data collection for statistical analysis both

long-term for product improvement and short-term for prediction of potential errors;

Reducing downtime and improving reliability and availability;

Ability to monitor practically all WPP equipment placed inside the substation;

Compliance with grid codes; and Reducing the number of service inspections.

System Overview

A SCADA system interfaces with the different devices such as turbines, meteorological (met) masts, substations, and other IEDs within the wind plant to acquire data and provide an aggregated view of the plant operation. Typically, the SCADA system architecture is designed to be scalable, to address different sizes of wind plants, as the constraints and needs are different. Also, the SCADA system should be capable of accepting additional monitoring control points to assist in troubleshooting and remediation of faults, errors, and other issues that may arise.

The site network, which forms the backbone of the SCADA system, is a very critical component of the system architecture. The internal SCADA network connecting all wind plant assets typically uses fibre optics for speed and high bandwidth. Possible network configurations include bus, star, and ring. To enable high availability and high data integrity the network is typically designed to have redundancy. The wind plant size, layout, and cost

constraints will typically dictate the type of fibre optic implementation. Examples of design considerations include:

Distances: Multimode has shorter distance capabilities than single mode, splices may be necessary,

Costs: Single mode cable cost less than multimode, but single mode transmitters cost more,

Mixed Modeusing single mode for long home runs and multimode inter-turbine: This increases inventory and construction complexity.

Additional information on network, SCADA, and control design considerations is available in [5]. Information Management

The SCADA system typically provides the ability to manage the wind plant remotely and locally. The robustness of the security model is extremely critical to provide the appropriate level of access control. To avoid unintentional start or stop of the wind turbines, the SCADA system is generally equipped with hardware token based or username-password based authentication.

The SCADA system also consists of databases to manage both real-time and historical information. The real-time data update from the turbine is typically done once every second, while the SCADA system aggregates and compiles the raw data into meaningful information. The real-time data server also feeds the graphical interfaces and displays, referred to as mimics, to provide visibility into wind plant operation. Turbine status and performance metrics such as production, wind speed, availability, capacity factor, and fault notifications are examples of key information that the user would typically visualize via the graphical interface. Mimics can also be used for control purposes, for example manually stopping or starting individual turbines or a group of turbines, opening and closing circuit breakers, changing transformer tap position, and various other functions. The graphical interface module typically provides preconfigured displays, which may include a plant level view, turbine specific details, control user interface, and auxiliary device user interface. Most mimics are standard mimics and appear in all WPPs. A small number of mimics might be implemented for specific projects.

The turbine data is normally sufficient for detailed reporting but sometimes data is insufficient or missing, for example when turbines are without power, or when turbine computer is shut down. In such cases, the SCADA system makes use of complex methods to estimate loss of production based on the data available from neighbouring turbines or meteorological stations.

The historical database typically retains the ten-minute average data from all turbines, high voltage substations,

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meteorology stations, and the event/fault data for at least 12 months. The ten-minute data is gathered from more than 100 sensors and counters on each wind turbine. Older data are usually archived and stored in external hardware such as tape. The historical database, along with a reporting module, is used for generating reports and data analysis. Typically, the reporting module of the SCADA systems consists of pre-configured reports, which may include standard reports such as performance, power curve, fault analysis, and wind rose.

Typically, SCADA systems offered by the turbine OEM are divided into two categories: The first solution is intended for small WPPs in range of 10-20 MW, whereas the second solution allows the operator to utilize the WPP in the same fashion as a conventional power plant in terms of meeting the grid code requirements. The turbine itself can meet some of the grid code requirements, but more stringent requirements often necessitate the use of an additional centralized intelligence. A SCADA system can be used for this purpose.

Both solutions are designed as a server-client system and generally utilize the same software platform but with different level of capability. The main options included in the second solution are remote monitoring of meteorological data, grid monitoring system, and power plant controller. The metrological data gathered include wind speed, wind direction, ambient temperature, atmospheric pressure, relative humidity, rain direction, and other meteorological information. The grid monitoring system measures quantities, such as, harmonics and flickers, grid voltages and currents, and grid frequency and power factor at the point of common coupling. The power plant controller is employed to control the output of the WPP, and generally utilized in counties with stringent grid code requirements. Its commands are generated by site specific control algorithms depending on the particular grid code requirements. To achieve a very fast response the power plant controller is normally run on dedicated hardware instead of the SCADA server. System Interface of Plant SCADA

The SCADA system typically uses a controller specific protocol to communicate with the turbine controller and uses an industry standard protocol, such as, Modbus for data exchange with auxiliary devices. Besides the internal communication interface, the SCADA system is designed typically to support data exchange with external systems such as enterprise SCADA, weather forecasting system, and historian systems. The list of such interfaces may include OPC, web services, and XML.

B. Third-Party SCADA A third-part SCADA system is often implemented for

manufacturers that do not offer OEM SCADA, to overlay or supplement the OEM SCADA, to manage operation in a

plant which has assets (turbines) from different OEMs, or to have an enterprise level view of many wind plants. The SCADA system commonly provides standard monitoring and control capabilities. Some of the potential advantages of a third-party SCADA system are listed below.

Consolidated view of all assets; Common monitoring interface across various

OEMs; Common control interface across OEMs; Platform to have consolidated and standard

reporting module; Single interface for communication with

system operators; and Flexibility in incorporating substation SCADA.

IV. PLANT CONTROL

In general, WPPs will have a SCADA system for control and acquisition of data from each of the individual wind turbines. To enable the wind plant to behave like a conventional power plant and meet the specific electric grid requirements, advanced plant controls are typically required at the point of interconnection (POI). The advanced plant controls may include the provisions that general plant information be provided in a format compatible with the interconnecting utilitys or system operators SCADA system. The information allows the balancing authority to communicate system and stability information and make necessary adjustments between entities. It is important to coordinate the turbine operation and provide a stable response to the grid requirements. Some of the advanced plant control features are listed below.

A. Voltage and Power Factor Regulation

Voltage and power factor regulation can be accomplished by closed loop control of the reactive power capability of the individual wind turbines, through a plant control system. This allows the wind plant to provide regulation services much like a conventional generating plant. The plant control system is an integral part of the voltage regulation strategy within a WPP, in order to transmit all needed decision making data between controllers within a given time period, typically in the millisecond range. More coarse voltage regulation can be accomplished using static capacitor and reactor banks located at the substation or individual wind turbines. Regulation to a remote point such as an interconnection switchyard connected to the WPP requires input from remote instruments or interface to remote switchyard SCADA systems.

B. Capacitor/Reactor Banks and Dynamic Var Devices

Reactive compensation equipment such as capacitor banks, reactor banks, switched static reactive

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compensators, and dynamic reactive compensation equipment are typically located at the wind plant substation(s), switchyard(s), and wind turbine generator(s). The plant control system will coordinate the operation of these BOP (Balance of plant) equipment, in order to achieve the desired reactive power behaviour at the POI. Further discussion of WPP reactive power compensation is presented in companion paper [6].

C. Ramp Rate Control System operators increasingly require the ability to

control the power removed or inserted at any given point of time, in both up and down directions. The plant control systems ability to enforce a MW per minute ramp rate maximum is required for the wind plant to accomplish this. Often referred to as the PRR (power ramp rate), PRR is typically calculated in ten minute or less intervals. Power fluctuations, which are caused by variations in wind speed, can be compensated quickly by adjusting the power output of the individual turbines in order to provide a wind plant level ramp rate control.

D. Frequency Droop Control Conventional power plants typically have frequency

droop capability (i.e. varying power output as a function of grid frequency). Depending on the turbine manufacturer, a WPPs plant control system may be able to provide similar governor response capability by adjusting turbine power output in response to grid frequency variation.

E. Power Curtailment System operators often require the ability to control the

power output of a WPP at any given point of time to deal with grid stability and transmission constraints. The power curtailment feature of the plant control system ensures that the WPP power output is capped to the desired limit. The SCADA system may utilize a simple turbine shutdown curtailment algorithm, offer global turbine power setpoint curtailment, or utilize combined algorithms to optimize efficiency and consider other turbine constraints. Curtailment rotation may be employed to balance curtailment time across the WPP.

F. Auxiliary (i.e. battery banks, alarms etc) There are many auxiliary SCADA points that provide

normal and critical alarms for the wind plant and interconnecting substations. These alarms are for equipment such as: wind turbines (over speed safety system, controllers, auxiliary power, batteries, battery chargers, protection, reactive equipment) and substation (battery chargers, batteries, system over/under voltage alarms, protection lockout alarms, trip coil failure alarms, low SF6 gas pressure, transformer alarms, substation entry).

System Interface of Plant Control The plant control system is typically integrated with the

wind plant SCADA system to provide real-time visibility and the ability to provide control set points. The plant control system can accept set-points from multiple systems via different methods. For example, system operators can provide control set-points as an analog signal using a RTU or a user can provide set-points via plant control user interface. The plant control system can interface with other auxiliary devices in the substation and also inputs from CTs/PTs at the POI.

V. SECURITY AND RELIABILITY COMPLIANCE Implementation of SCADA and control systems in a

WPP is not just prudent engineering. WPPs must comply with contractual obligation under an IA, which often has specific SCADA and control provisions necessary for maintaining the security and reliability of the grid. Failure to comply with IA requirements can result in default and termination. IA provisions vary regionally, and are dependent on project specific parameters such as size, interconnection point, and expected impact to the grid.

Articles 7, 8, and 9 of the Federal Energy Regulatory Commission (FERC) Standard Large Generator Interconnection Agreement (LGIA), used by many electric reliability entities, contain common requirements governing metering, communications, and operational capabilities, such as those discussed in Plant Controls above. Following are other common IA requirements, standards, and considerations, which influence WPP SCADA and control system capabilities.

A. Remedial Action Scheme (RAS) An RAS, also known as a Special Protection Scheme

(SPS), is an automatic protection system specifically designed to detect abnormal or in some cases, predetermined system conditions to take corrective actions. The corrective actions from an RAS would take place, in place of or in addition to the isolation of faulted components, to maintain system stability and reliability. The actions of an RAS can include changes in:

Demand; Generation (MW and Mvar); System configuration to maintain system

stability; Acceptable voltage; Power flows; or Frequency or rate of change of frequency.

The use of SPS is generally justified for loss of network

integrity characterized by one or more of the following phenomena [7]:

Transient angle instability; Small signal angle instability;

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Frequency instability; Short-term voltage instability, Long-term voltage instability, and Cascaded tripping.

Some of the most commonly used RAS implemented in

conventional power systems include [7]: Under frequency or under voltage load

shedding; Generator rejection and fast valving of steam

turbines; Automatic Generation Control (AGC); Dynamic braking or braking resistor; HVDC fast power change; Tap changers blocking; Controlled opening of an interconnection; Automatic shunt reactor/capacitor switching;

and Fast increase in the generator voltage set-point.

An RAS does not typically include the functions needed

for the following: Fault conditions that must be isolated or Out-of-step relaying (not designed as an

integral part of RAS). The entity responsible for regional system impact,

stability, and reliability generally develops standards for the remedial action schemes and their design, operation, and testing.

B. Protection System Relaying (PSR) The protective scheme needs to be interoperable with

existing systems and technology in the immediate interconnection area. Given the need to interact with a system greater than the generator (beyond the interconnection point) it will be necessary to provide the interconnecting entity with balance control capabilities. Allowing this control will improve safety and help protect equipment at the generator and within the interconnecting entitys system.

The PSR control scheme will be governed by the interconnection agreement executed between the interconnecting entities and the WPP. Parties to the agreement may also include third party entities like a system balancing authority. The control process is generally facilitated through the provision of SCADA summary information to the interconnecting entity. The information requested may include current and forecasted WPP output. The information is then used to perform dynamic system analysis and provide feedback to the WPP about the ability of the utility to accept the energy produced and any limitations that must be placed upon it. Signals are transmitted through the WPP SCADA system to each individual unit for total system adjustments to minimize impacts caused by system disturbances.

C. Data Telemetry IAs typically requires WPPs to provide a variety of data

to external sources such as a Utility, ISO, and/or balancing authority control centers. In order to better manage the schedule and generation of the grid, interconnecting entities will often require ongoing forecasts for availability of power (net output) from the generator. The balancing authority may place restrictions on output from the generator depending on forecasted system loads and the expected availability of other generating resources.

WPPs commonly have meteorological towers onsite which require connection to the site communications network or SCADA system to transmit data to the other systems or entities to help with forecasting. Increasingly, transmission owners and service providers are requiring live meteorological SCADA points to optimize wind power forecasting. Accurate wind forecasting is critical to reliable and economic system operation, especially as wind penetration increases in certain regions.

D. NERC Reliability Standards In 1998, Presidential Decision Directive (PDD) 63 was

issued with the intent of protecting critical infrastructure in the U.S. PDD 63 was general and applied to a wide range of industries including electric generators. Compliance with PDD 63 was explicitly required for electric generators under Appendix D of the standard LGIA, which lacked specifics.

The North American Electric Reliability Corporation (NERC), whose mission is to insure the reliability of the bulk power system within North America, eventually established a specific series of reliability standards covering such areas as communications, transmission, critical infrastructure protection (CIP), and protection and control (PRC). The standards are enforced after approval by FERC under provisions of the 2005 Energy Policy Act. Specifically, NERC CIP, governing cyber security, was approved by the FERC under Order 706 in January 2008, thus making the standard mandatory in the U.S., and applied at a regional level with monitoring and enforcement by the local reliability councils, corporations, and organizations. Consequently, WPP SCADA and control system are required to comply with cyber security provisions of NERC CIP, as well as, many other standards such as PRC-012-0 regarding RAS/SPS procedures. The need for further improvements in communications between wind plants and balancing area operators was identified in [8].

In addition to the NERC CIP requirements, the Smart Grid efforts have produced NISTIR-7628 V1.0 that addresses Smart Grid cyber security strategy and requirements.

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E. IEC Standards The International Electrotechnical Commission (IEC)

Standard 61400-25 (communications for monitoring and control of wind power plants) provides uniform information exchange for monitoring and control of wind power plants. It deals with communications between wind power plant components such as wind turbines and actors such as SCADA systems. It is designed for a communication environment supported by a client-server model. The application area of IEC 61400-25 covers all components required for the operation of wind power plants, not only the wind turbine, but also the meteorological system, the electrical system, and the wind power plant management system.

IEC 61400-25 extends the technology of the IEC 61850 utility automation standard to address needs particular to wind power. The core of IEC 61850 is an abstract data model that includes data objects that describe power system equipment and services that provide data communications functions. Examples of the services include report-by-exception and definition/management of device logs. The data model is mapped onto communications technologies by specifying how particular data types and services are to be communicated using the technology. There is also an XML-based language for defining the configuration of the facility and setting the values of pre-defined parameters.

IEC 61400-25 provides two major areas of extension. One is an extension to the data model to cover equipment found in wind plants. For equipment such as switches, breakers, protective relays, and transformers, the 61850 objects can be used. The other extension area is mapping to additional communications technologies beyond those found in 61850. For example, 61400-25 adds XML-based web services communications that offers improved compatibility with communications in enterprise systems.

The data objects in 61850 and 61400-25 are named rather than numbered. Part of the name is defined in the standard and part is defined by the using organization. Object naming removes any limit on the number and scope of objects that can be handled thus, new power system equipment technologies or data requirements can be accommodated by simply adding the relevant objects to the data model. This approach is being used by Smart Grid efforts focused on ensuring that 61400-25 can support wind power data needs particular to North American practices.

VI. CONCLUSIONS In the past SCADA systems were almost or literally

afterthoughts, put in as needed to get the job done. SCADA in modern WPPs is recognized as integral to optimize WPP performance and financial return, and necessary to meet contractual obligations including strict security requirements.

Beyond meeting requirements and obligations, sophisticated developers and owners recognize the solid return on investment that proper SCADA implementation provides. More and more data isnt the whole answer. The right data must be acted on to implement operational, maintenance or other changes.

The publication of this paper was the result of two years of concerted effort by the authors and the IEEE PES Wind and Solar Plant Collector System Design working group. The authors sincerely hope that this and other working group papers are found to be valuable to those who will plan, design, analyze, construct, and operate wind power plants. Recognition is given to the authors and their employers for contributing the resources for the preparation of this work.

For more information on available materials, or to find out how to participate in this working groups activities, please see: http://grouper.ieee.org/groups/td/wind

VII. REFERENCES

[1] T. Burton, D. Sharpe, N. Jenkins, E. Bossanyi, Wind Energy Handbook, West Sussex, UK: John Wiley & Sons Ltd, 2001.

[2] G. Smith, Development of a Generic Wind Farm SCADA System, DTI Publishing 2001.

[3] Wotruba, Bill. The Essentials of Ethernet Equipment in SCADA Systems. North American Windpower, June 2010.

[4] Wind Plant Collector Design WG, Characteristics of Wind Turbine Generators for Wind Power Plants, Proceedings of 2009 IEEE Power and Energy Society General Meeting, Calgary, Canada, July 2009.

[5] IEEE Std. C37.1-2007, IEEE Standard for SCADA and Automation Systems.

[6] Wind Plant Collector Design WG, Reactive Power Compensation for Wind Power Plants, Proceedings of 2009 IEEE Power and Energy Society General Meeting, Calgary, Canada, July 2009.

[7] System Protection Schemes in Power Networks, CIGRE Technical Brochure 187, Task Force 38.02.19, June 2001.

[8] North American Electric Reliability Corporation (NERC), Special Report on Accommodating High Levels of Variable Generation, 2009