Meeting the IESO Interconnection Requirements for Ontarian Wind Farms

European Wind Energy Conference

Brussels, Belgium April 3, 2008

Written and presented by

Manisha Ghorai, American Superconductor Corporation

8401 Murphy Drive, Middleton, WI 53562 mghorai@amsc.com PH: +1 608 828 9127 FX: +1 608 831 5793

Narend Reddy, American Superconductor Corporation

8401 Murphy Drive, Middleton, WI 53362 nreddy@amsc.com PH: +1 608 828 9188 FX: +1 608 831 5793

I. SUMMARY

During the past few years, the province of Ontario in Canada, motivated by government incentives and renewable energy programs, has added an increasing number of wind farms to its transmission system. The integration of a wind farm to the transmission grid introduces a variety of issues that impact the grid and the wind farm itself. In order to ensure the reliability and safety of the rest of the grid, the Independent Electric System Operator (IESO) has issued interconnection requirements that need to be met by wind farms in Ontario. Often, wind turbines alone cannot provide the necessary compensation to meet these requirements. An American Superconductor D-VAR

® (Dynamic VAR) reactive compensation system can provide a cost-effective and reliable

solution to help the wind farm satisfy the interconnection requirements.

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II. INTRODUCTION

Wind energy is a clean, renewable form of energy gaining popularity all over the globe. Ontario,

Canada in particular continues to add wind farms to contribute generation to their grid as a result of

government incentives and renewable energy programs. The integration of a wind farm to the

transmission grid can cause a variety of issues that impact the grid and the wind farm itself.

The Independent Electricity System Operator (IESO) manages and operates the Ontario

transmission system, owned by Hydro One. The IESO has defined market rules that all wind farms

in Ontario must follow to ensure the safety and reliability of the grid, which are outlined in the

document Market Rules for the Ontario Electricity Market, Grid Connection Requirements [1]. The

IESO also performs System Impact Analyses (SIAs) for all proposed wind farms in Ontario to

confirm that the wind farm will in fact meet the Market Rules.

American Superconductor Corporation (AMSC) also studies wind farm interconnection concerns

and can provide a dynamic solution, a D-VAR® dynamic reactive compensation system installed at

the collector bus that will allow wind farms to meet the interconnection requirements.

III. WIND FARM INTEGRATION ISSUES

Introducing wind generation to a transmission system can be challenging as it can affect the

operation of the grid negatively. If a group of turbines trips offline for any reason, this could cause

voltage fluctuations also seen at the transmission level. A large amount of in-rush current is

required at the start-up of standard induction turbines which also causes voltage fluctuations. The

unpredictable nature of wind means that a variable wind speed could cause large and sudden real

power output changes. Finally, any voltage flicker and / or harmonics contributed by certain wind

turbines could magnify the existing flicker and harmonics on the grid to unacceptable levels.

Similarly, events that occur on the utility transmission grid can affect the operation and stability of

the wind farm as well. Often, as discussed in this paper, utilities require unique interconnection

requirements to address these kinds of issues. Variations on the transmission grid voltage could

cause the generators to trip. Voltage fluctuations and step changes on the grid due to capacitor

bank and reactor bank switching and a phase voltage imbalance or background harmonics on the

system could damage the turbine equipment as well [2].

IV. IESO REQUIREMENTS

The IESO has realized the interconnection concerns described above, and in response, has provided standards to prevent such issues. In order for a wind farm in Ontario to gain approval for its connection to the Hydro One

transmission system, it is required to meet the following key performance and capability standards for reactive power mandated by the IESO as stated in the Ontario Energy Market Rules [1]: 1. Supply continuous dynamic reactive power at all active power outputs in the range of 0.90

capacitive to 0.95 inductive power factor based on rated active power at its generator terminals [1].

2. Supply full active power continuously while operating at a generator terminal voltage

ranging from 0.95 to 1.05 pu of the generator’s rated terminal voltage [1].

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3. Voltage Ride Through (VRT) capability of the wind turbine generators (WTG) is expected to

be sufficient to allow the wind farm to remain connected to the IESO-controlled grid for

recognized system contingencies that do not remove the facility by configuration. The

reactive compensation must also be fast, dynamic and respond within 1 second [1].

These requirements ensure that the safety of the rest of the grid will not be compromised by the addition of wind generation.

V. MEETING INTERCONNECTION REQUIREMENTS

The figure below shows a 100-MW wind farm that is a typical connection configuration to the Hydro One transmission system in Ontario.

Figure 1: 100-MW Ontario Wind Farm

This 100-MW wind farm has three feeders of wind turbines which all connect to a 34.5-kV

collector bus. A 60-MVA base rated power transformer with a typical impedance of 9% on the base rating steps the voltage at the 34.5-kV collector bus to the transmission voltage of 230 kV, where the wind farm connects to the Hydro One transmission grid. The high voltage side is typically known as the Point of Interconnection (POI), where the IESO requires voltage regulation and power factor regulation capabilities to be met. A wide selection of wind turbines is available with a variety of capabilities for developers as they

contemplate installing a wind farm. Depending on the manufacturer and model of the wind turbine, the turbine could have the ability to operate with a fixed power factor or with a variable power factor over a range. These wind turbines could also have Low-Voltage-Ride-Through (LVRT) or High-Voltage-Ride-Through (HVRT) capabilities built in. The level of LVRT and HVRT capability also could vary depending on the options purchased. These wind turbines can also be configured to provide voltage and power factor regulation at a defined connection point. To meet the IESO capacitive power factor requirement of 0.90, the Market Rules state that 13%

of losses will be allowed by utility. Therefore, the minimum reactive power required to be injected at the POI is:

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tan(acos(0.9))-0.13 = 0.35 pu (of rated active power)

For a 100-MW wind farm, this translates to a capacitive reactive power target at the POI of:

0.35*100 = 35 MVAR

The minimum reactive power required to be absorbed (inductive) at the POI is:

tan(acos(0.95)) = 0.33 pu (of rated active power)

For a 100-MW wind farm, this translates to an inductive reactive power target at the POI of:

0.33*100 = 33 MVAR

The reactive power targets are summarized in the chart below:

Table 1: IESO PF Requirement Targets at POI

Typical losses that can be considered for this size wind farm include about 7% of generator step up (GSU) transformer losses, 4% of collector grid losses, 2% of collector grid charging, and 9% of power transformer losses. The following one-line diagram also shows the loss breakdown at each level:

Figure 2: 100-MW Wind Farm Loss Breakdown

IESO Target Requirements Capacitive

(MVAR)

Inductive

(MVAR)

+90%/-95% PF at 230 kV POI 35 -33

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After the losses are taken into consideration, the total reactive compensation required to meet the power factor requirements at the POI are shown below:

The above calculations assume that the wind turbines are operating at a fixed power factor of

100%. If the wind turbines had the capability to operate with a variable power factor range (95% capacitive to 95% inductive, for example), the wind turbines would be able to inject and absorb up to 33 MVAR at the wind turbine generator terminals. With the additional compensation provided by the wind turbines, the total reactive compensation required is reduced to 20 MVAR capacitive only. With the inductive power capability of the turbines the need for any additional inductive compensation at the collector level is eliminated. This paper explores the application of the AMSC D-VAR system to provide the necessary reactive compensation requirements at the collector bus level.

VI. AMSC D-VAR SYSTEM

The AMSC D-VAR system could consist of a dynamic reactive compensation system (the D-VAR device) as well as switched shunt capacitors and reactors, if required. The AMSC D-VAR unit, a STATCOM device, is an ideal choice to address the issues of voltage

regulation as it can provide continuous dynamic VARs [2]. It makes use of the AMSC proprietary PowerModule

TM power electronic converters, which provide continuous reactive compensation over

an engineered inductive and capacitive range. The D-VAR unit also has short-term overload capability to provide 2.67x the continuous rating for 2 seconds, which is a valuable benefit to address transient voltage excursions. The D-VAR unit has a response time in the order of cycles and is modular and relocatable [3]. The D-VAR unit is available in 4-MVA modules which can be combined to provide whatever

dynamic compensation required. Each unit is contained in an 8’x8’x8’ NEMA Type 4 outdoor enclosure, fit to be installed directly at a substation without any additional housing. The D-VAR unit can also be combined with shunt capacitors and reactors, thus expanding the complete continuous reactive capability of the D-VAR system significantly. A typical D-VAR based reactive compensation system configuration placed at the wind farm collector bus is illustrated in the figure below:

Capacitive

(MVAR)

Inductive

(MVAR) Total Reactive

Compensation Required 53 -15

Table 2: Total Reactive Compensation Range Required

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Figure 3: AMSC D-VAR System Reactive Compensation Solution Configuration

As shown in Figure 3 above, the D-VAR device’s control system monitors the voltage and current at the POI and adjusts its output depending on the required compensation. The D-VAR device’s control system manages the switching of the shunt capacitors and reactors to provide additional steady state or dynamic compensation as needed. If the wind turbines have a variable power factor capability, the control system can also communicate with the wind turbine controls to leverage the available VARs from the wind turbines as necessary. The D-VAR control system has the capability to manage these various reactive compensation sources as needed to provide the performance expectation of the wind farm. The D-VAR system control operates with a dead band and droop approach. A typical control approach for the D-VAR device is shown below:

Figure 4: Typical D-VAR Control Approach

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The D-VAR system will either inject VARs or absorb VARs to boost or buck the voltage depending on what is necessary to bring the controlled voltage back within the specified range. Normally the D-VAR system will operate in slow-regulation mode to limit voltage excursions to within the above defined parameters. However, the D-VAR system will also operate in fast-regulation mode with a subset of parameters specifically used for this purpose, utilizing the D-VAR unit’s overload capability to mitigate any transient voltage events that may occur on the transmission grid or that may be caused by the wind farm itself [2]. The D-VAR system control parameters are tested and tuned using dynamic studies undertaken using PSS/e. The PSS/e model for the D-VAR system utilizes the exact same algorithm as the actual device, which allows engineers to accurately determine the response of the complete system to different types of transient events. The AMSC D-VAR system has been installed at over 30 wind farms throughout the world to

address power factor correction, voltage regulation, Low-Voltage and High-Voltage-Ride-Through concerns and requirements. The following picture shows a D-VAR system installation at a North American Wind Farm facility.

Figure 5: A North American Wind Farm D-VAR Installation

VII. SYSTEM STUDIES

Engineers at AMSC usually undertake variety of studies to confirm that the size and configuration of the D-VAR system needed for a particular wind farm to meet the interconnection requirements.

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Some of these studies include load flow analyses, dynamic stability and transient analyses, and harmonic and flicker analyses. A load flow analysis considers the steady state continuous capabilities of the solution. This

analysis can be used to confirm that the entire power factor range required is achieved as well as the required voltage regulation [3]. A harmonic analysis determines if and how the existing system harmonics may affect the solution

equipment, and if the reactive compensation solution may impact the overall system harmonics. A dynamic and transient analysis will determine whether the turbines do achieve low and high

voltage ride through for given contingencies. The parameters that the D-VAR system uses in the field are also configured and tested these dynamic stability analyses to confirm its operation in the field. Presented below are some results of a dynamic stability analysis for a wind farm project

performed by AMSC. These results show the real power output of the wind farm, without additional compensation, as a 600-ms fault is applied on the utility transmission grid. In this particular case the wind turbines utilized cannot withstand a voltage below 0.75 pu for more than 5 cycles, resulting in tripping.

Figure 6: Wind Farm Power Output, without Additional Compensation, for a Fault on

Transmission Grid

The figure below shows the results of the same fault analysis, but with the AMSC D-VAR system

modeled as well. In this simulation, the D-VAR unit detects the low voltage on the transmission system due to the fault, and provides enough reactive compensation to quickly rebuild the voltage at the wind turbine generator terminal above 0.75 pu. This allows the wind turbines to stay online during the actual fault period and continue to operate normally as the voltage returns back to normal upon clearing.

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Figure 7: Wind Farm Power Output, with AMSC D-VAR Reactive Compensation System, for

a Fault on Transmission Grid

VIII. CONCLUSIONS

Wind farm installations can introduce a variety of issues when connected to the rest of the transmission grid, including transient voltage events and steady state voltage regulation problems. As a result, transmission system operators such as the IESO of Ontario have initiated strict interconnection requirements and grid codes to protect the transmission grid. Often, wind turbines alone cannot provide the necessary compensation to meet these requirements. An American Superconductor D-VAR reactive compensation system can provide a cost-effective and reliable solution to help the wind farm satisfy the interconnection requirements.

IX. REFERENCES

1. IESO. Market Rules for the Ontario Electricity Market, Grid Connection Requirements, September 14 2005. 2. Kehrli B, Ross M. Utility-Connected Power Electronic Compensators in Wind Power Applications. Feb 2007. 3. Saylors S, Diaz de Leon J. How the HAWI Wind Farm Met HECO’s Rigorous Interconnecting Requirements June 2007.

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X. BIOGRAPHIES

Manisha Ghorai joined American Superconductor in 2006 after graduating from the University of Wisconsin with her degree in Electrical Engineering. She currently works on the Transmission and Distribution Planning Team. She performs studies where she analyzes transmission and distribution system problems such as for voltage collapse, transfer capability, wind farm interconnection, harmonic and power quality problems.

Narend Reddy is currently the Manager of Transmission and Distribution Planning at the Power Systems business unit of American Superconductor Corporation (AMSC). He joined AMSC in 2001 as a Transmission and Distribution Planning Engineer working on planning studies to analyze systems for voltage, capacity, stability, transfer capability, power quality and wind interconnection problems. This work also included application of STATCOM devices to solve voltage and stability related problems as well as developing solutions for voltage regulations, power factor control and voltage ride-through problems associated with wind farms.