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Abstract Recently electricity generation from wind power

has been increasingly popular worldwide as one of the mostpromising renewable energy sources. This paper proposes anadaptive control strategy for interfacing distributed generations(DGs) from wind power to utility distribution grids. This paperpresents the voltage control requirements and protectionrequirements for wind-powered DGs according to IEEE-1547standards. This paper defines adaptive interfacing controller forthree common types of wind-powered DGs: doubly fed inductiongenerator, permanent-magnet synchronous generator, and

squirrel-cage induction generator. The design of the adaptivecontroller using state-of-the-art digital signal processingtechnology is presented. The key functions of the hardwarecomponents in the adaptive controller are provided.

Index Terms Wind power electricity generation, Distributed

generation, Adaptive control, Renewable energy, DFIG,Distribution system, IEEE1547 standards

I. INTRODUCTION

ODAY electricity generation from wind power is

increasingly popular, and wind power becomes the fastest

growing and most promising source of renewable energy

worldwide. There are three common ways of extracting powerfrom wind for electricity generation: 1. The power can be

obtained from wind by connecting a wind turbine through a

gearbox to the rotor of a squirrel-cage induction generator

(SCIG) and then interfacing the generator with acompensating capacitor to a utility electricity distribution grid.

2. The power can be extracted from wind by connecting awind turbine directly to a low speed multi-pole permanent

magnet synchronous generator (PMSG) and then interfacing

the generator through a back-to-back voltage source converter

to a distribution grid. 3. The power can be obtained from wind

by connecting a wind turbine through a gearbox to the rotor of

a doubly fed induction generator (DFIG). Currently the DFIGis probably the most popular way of wind-power electricity

generation [1].

In recent years, the cost of electricity generation from wind

power has decreased steadily and substantially, while offeringbenefits of environment friendly operation and no fuel price

volatility. However, a large increase of wind-powered

distributed generation (DGs) installations, connected on theelectricity utility distribution systems, has caused a number of

technical concerns. Common concerns are the impacts of

wind-powered DGs, particularly for those of large scale and

not-well-predictable instant-power generating capability, onthe distribution network voltage profile, frequency stability,

supply reliability, equipment control (capacitor switching,

transformer tap changing, etc.), and utility crew safety due to

A. Hamlyn ([email protected]), H. Cheung,, L. Wang, C. Yang,and R. Cheung are with Ryerson University, Canada.

978-1-4244-1583-0/07/$25.00 2007 IEEE

undetected DG islanding operations. These concerns can be

properly addressed with correct system protection and controloperations [2-4].

This paper presents an adaptive control strategy forinterfacing electricity generations from wind power to utility

distribution grids. A focus of this paper is to demonstrate the

significance of an adaptive interfacing control for common

types of wind-power electricity generations to utilitydistribution grids, to illustrate the interfacing requirements

from the utility power system standpoint, and to deal withcontrol issues caused by variability of wind farm power outputdue to wind fluctuating characteristics.

II. INTERFACING REQUIREMENTS FORWIND-POWERED DGS

In 2003, IEEE 1547 Standard for InterconnectingDistributed Resources with Electric Power Systems waspublished to establish criteria and requirements for

interconnection of distributed resource (DR) with electricpower systems (EPS). This standard provides requirements

relevant to the performance, operation, testing, safety

considerations, and maintenance of the interconnection. Fig.1

shows the relationship of the interconnections given in the

standard. The requirements shall be met at the point of

common coupling (PCC). The standard applies to

interconnection based on the aggregate rating of all the DR

units that are within the Local EPS. The functions of the

interconnection system that affect the Area EPS are required

to meet this standard regardless of their location on the EPS[5].

PCC PCCPCC

Area EPS

Local EPS 1 Local EPS 2 Local EPS 3

Load LoadDR DR

Fig.1: Relationship of interconnection stated in IEEE-1547

The following outlines the requirements from the IEEE-

1547 Standard specifically for the design of an adaptiveinterfacing control strategy for wind-powered DGs proposed

in this paper. Additional requirements will be added in this

paper to further improve the performance of the DG-utility

Alexander Hamlyn, Helen Cheung, Lin Wang, Cungang Yang, Richard Cheung

Adaptive Interfacing Control Strategy for Electricity

Generations from Wind Power to Distribution Grids

T


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interface. The specific requirements for the proposed design

can be grouped as follows:

A. Voltage Requirements

V1. The wind-powered DG shall not actively regulate the

voltage at the PCC.

V2. The DG shall not cause the Area EPS voltage at other

Local EPS to go outside the requirements of ANSI C84.1.V3. The DG shall parallel with the Area EPS without causing

a voltage fluctuation at the PCC greater than 5% of theprevailing voltage level of the Area EPS at the PCC.

V4. The DG shall not create objectionable flicker for other

customers on the Area EPS.

B. Protection Requirements

P1. The wind-powered DG shall not energize the Area EPS

when the Area EPS is de-energized.P2. The DG shall cease to energize the Area EPS for faults on

the Area EPS circuit to which it is connected.

P3. The DG shall cease to energize the Area EPS circuit towhich it is connected prior to reclosure by the Area EPS.

P4. When any voltage is in a range given in Table 1, the DG

shall cease to energize the Area EPS within the clearingtime as indicated. Clearing time is the time between the

start of the abnormal condition and the DG ceasing to

energize the Area EPS.

Table 1: Interconnection system response to abnormal voltages

(given in IEEE 1547)

Voltage range

(% of base voltage)

Clearing time

(s)

V < 50 0.16

50 V < 88 2.00

110 < V < 120 1.00

V 120 0.16

P5. When the system frequency is in a range given in Table 2,the DG shall cease to energize the Area EPS within the

clearing time as indicated.

Table 2: Interconnection system response to abnormal frequencies

(given in IEEE 1547)

Size (kW) Frequency range (Hz) Clearing time (s)

> 60.5 0.16 30

< 59.3 0.16

> 60.5 0.16

< {59.8-57.0}

Adjustable set point

Adjustable

0.16 to 3.00

> 30

< 57.0 0.16

P6. After an Area EPS disturbance, no DG reconnection shall

take place until the Area EPS voltage is within Range B

of ANSI C84.1, Table 1, and frequency range of 59.3Hz

to 60.5Hz.P7. For an unintentional island, the DG shall detect the island

and cease to energize the Area EPS within 2 seconds ofthe formation of an island.

III. ADAPTIVE INTERFACING FORWIND-POWERED DGS

This section describes the adaptive interfacing controlstrategy for wind-powered DGs proposed in this paper.

A. Commonly Used Configurations for Wind-powered DGs

The three common types of the wind-powered DGs are:

wound-rotor doubly fed induction generator (DFIG),

permanent magnet synchronous generator (PMSG), and

squirrel-cage induction generator (SCIG).Fig.2 shows the basic configuration for the DFIG. The

power of this DG is obtained from wind by connecting a wind

turbine through a gearbox to the rotor of the DFIG with the

use of a rectifier and an inverter. Currently the DFIG is

probably the most popular way of wind-power electricitygeneration.

Wound-rotorInduction

Generator

Utility Grid

Gear Box

Wind Turbine

RectifierInverter

DFIG

Fig.2: Basic configuration of DFIG

Fig.3 shows the basic configuration for the voltage-source

(VS) PMSG. The power of this DG is extracted from wind by

connecting a wind turbine directly to a low speed multi-pole

PMSG and then interfacing the generator to the distribution

grid through a rectifier and a voltage-source inverter.

Synchronous

Generator Utility GridRectifierVoltage-Source

Inverter

Wind Turbine

VS-PMSG

Fig.3: Basic configuration of VS-PMSG

Fig.4 shows the basic configuration for the current-source

(CS) PMSG. This DG is similar to the above one, except using

a current-source (instead of voltage-source) inverter.

SynchronousGenerator Utility GridRectifier

Current-SourceInverter

Wind Turbine

CS-PMSG

Fig.4: Basic configuration of CS-PMSG


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Fig.5 shows the basic configuration for the SCIG. The

power of this DG is obtained from wind by connecting a wind

turbine through a gearbox to the rotor of the SCIG and then

interfacing the generator to a utility distribution grid with the

use of a static VAR compensator.

Squirrel-cage

induction

Generator

Utility Grid

Gear Box

Wind Turbine

Inverter

SCIG

Static

VARCompensator

Fig.5: Basic configuration of SCIG

B. Conventional (Non-Adaptive) Controls

The interface of the large-scale, wind-powered DGs toutility power distribution system requires power electronic

conversions as shown in Fig.2 to Fig.5. However, theconventional control for power electronic conversions is to

maintain either a constant voltage (CV) or a constant current

(CI) at the converter/inverter output. This conventional way of

power electronic control could not satisfy well the voltagerequirements of the IEEE 1547 standard. Specifically, either

the CV or CI output control could violate all voltage

requirements V1, V2, V3, and V4 given in Section II.

For example, V1 requires that the wind-powered DG shall

not actively regulate the voltage at the PCC. However, the CV

control will maintain the DG output voltage constant thatwould result in indirectly participating actively the regulation

of the voltage at the PCC. Similarly, V3 requires that the wind-

powered DG shall not cause a voltage fluctuation at the PCC

greater than 5% of the prevailing voltage level of the AreaEPS at the PCC. However, the CI control will maintain the

DG output current constant that would cause a voltagefluctuation greater than 5% of the prevailing voltage level at

the PCC, particularly during the high-wind maximum-power

output.

C. Basic Adaptive Control Unit for Wind-powered DGs

The conventional controls used for wind-powered DGs maynot be able to meet all the voltage and protection requirements

stated in IEEE 1547 standards particularly at the time of high-

power output operations together with events occurring in the

Area EPS. This paper proposes an adaptive control strategy to

satisfy the standards voltage and protection requirements,specifically those given in Section II.

The proposed adaptive controller uses state-of-the-art

digital signal processing technology and computer networking

technology. Fig.6 shows the basic unit of the proposed

adaptive controller. This unit consists of digital signal

processor (DSP), analog-to-digital signal converter (A/D),

data storage (Flash memory), network card, and field-programmable gate array (FPGA).

A/D Monitoring Signals

Flash

Network Card

Networking

SignalsDSP 1

DSP 2 FPGA

Control Signals

Protection

Signals

Fig.6: Basic adaptive control unit for wind-powered DGs

The A/D converts the electrical analog signals (voltages,

currents, etc.) monitored at the PCC into digital signals. The

DSPs carry out post-processing of the digital signals to

determine the adaptive control and protection signals. Thenetwork card communicates with the computer network in the

Area EPS. The FPGA carries out all logic coordination in thisunit and delivers protection commands. The design of this unit

will be discussed in Section IV.D. Adaptive Interfacing Controller for DFIG

Fig.7 shows the connection of the adaptive interfacing

controller for DFIG. This controller handles all requirements

for interconnecting the DFIG to the Area EPS. This controllermeasures the electrical data (voltages, currents, etc.) at the

PCC and communicates with other control circuits and the

control center in the Area EPS. Based on the measured data

and the communicated information, this controller determines

the control and protection commands, adaptively to the

operating states of the Area EPS in order to satisfy the IEEE

1547 voltage and protection requirements specifically given in

Section II.

Wound-rotorInduction

Generator

Gear Box

Wind Turbine

RectifierInverter

DFIG

Utility Grid

Original

Control

AdaptiveController

ProtectionSignals

ControlSignals

PCC

Monitoring

Signals

NetworkingSignals

Fig.7: Adaptive controller for DFIG

This DFIG interfacing controller shown in Fig.7 sends the

output reference command to the original control for theinverter that provides the reactive power to the wound rotor ofthe induction generator. The reference command from this

controller regulates the generator reactive power that

indirectly adjusts the output voltage of the DFIG to satisfy all

IEEE 1547 voltage requirements given in Section II-A. This

controller sends the protection command to close or open the

breaker to energize or cease to energize the Area EPS to

satisfy all IEEE 1547 protection requirements given in Section

II-B.


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E. Adaptive Interfacing Controller for PMSG


controller for VS-PMSG. This controller handles the

interconnection of the VS-PMSG to the Area EPS and satisfies

the IEEE 1547 voltage and protection requirements

specifically given in Section II.

Utility Grid

Adaptive

Controller

Protection

Signals

Control

Signals

PCC

Monitoring

Signals

Synchronous

Generator

Rectifier

VS-PMSGWind Turbine

Voltage-SourceInverter

Original

Control

NetworkingSignals

Fig.8: Adaptive controller for VS-PMSG

Based on the measured data and the communicated

information, this VS-PMSG interfacing controller determines

the control and protection commands, adaptively to the

operating states of the Area EPS. Then this controller sendsthe output reference command to the original control that

directly regulates the output voltage of the PMSG to satisfy all

IEEE 1547 voltage requirements given in Section II-A. This

controller sends the protection command to close or open the

breaker to energize or cease to energize the Area EPS tosatisfy all IEEE 1547 protection requirements of Section II-B.

The connection and performance of the adaptive interfacing

controller for CS-PMSG is similar to that of Fig.8.

F. Adaptive Interfacing Controller for SCIG


controller for SCIG. This controller handles the connection ofSCIG to the Area EPS and satisfies the IEEE 1547 voltage and

protection requirements specifically given in Section II.

Squirrel-cageinductionGenerator

Gear Box

Wind Turbine

Inverter

SCIG

StaticVAR

Compensator OriginalControl

ControlSignals

Utility Grid

Adaptive

Controller

Protection

Signals

PCC

Monitoring

Signals

NetworkingSignals

Fig.9: Adaptive controller for SCIG

Based on the measured data and the communicated

information, this SCIG interfacing controller determines the

control and protection commands, adaptively to the operating

states of the Area EPS. Then this controller sends the outputreference command to the original control of the static VAR

compensator that directly regulates the output voltage of the

SCIG to satisfy all IEEE 1547 voltage requirements given in

Section II-A. This controller sends the protection command to

close or open the breaker to energize or cease to energize theArea EPS to satisfy all IEEE 1547 protection requirements of

Section II-B.

IV. DESIGN FORADAPTIVE INTERFACING CONTROL FOR

WIND-POWERED DGS

This section describes the hardware design of the adaptive

interfacing controller for wind-powered DGs, proposed in this

paper. Fig.10 shows the block diagram of the hardware design

for the controller. The main components of this controller are:

two digital signal processors (DSPs), one analog-to-digital

converter (A/D), one data storage (Flash memory), one

network card, and one and field-programmable gate array(FPGA). A description of these components is given below.

A. DSP-1: Core Processor for Adaptive Control Computation

Hardware features: DSP-1 of this design is the core processor

for executing the proposed adaptive interfacing control. A 32-bit, 225MHz, floating-point digital signal processor shown in

Fig.10 is selected for DSP-1. This DSP is C-friendly

processor. Its CPU can fetch very long instruction words to

supply up to eight 32-bit in instructions to the eight functional

units during every clock cycle. Its memory architecture has

4kB program cache, 4kB data cache, 64kB unified

cache/mapped RAM, and 192kB mapped RAM. Additionalmemory is provided with 2Mx4x16-bit SDRAM and 2Mx16-

bit Flash. Its enhanced DMA handles 16 channels that greatlyrelieves its CPU from bulk data movement and preserves its

bandwidth for application-specific code. This DSP is

supported by a set of industry benchmark development toolsincluding optimizing C/C++ compiler, emulation, real-time

debugging, DSP/BIOS real-time kernel, etc.

Key Operations: The features of DSP-1 are utilized for

high-speed computations that are required in the adaptive

interfacing control algorithms. For example, this DSP carries

out computations for forecasting the wind-powered DG outputin the immediate term and the near term. Then this DSP

carries out computations for determining the rate of change of

the output reference to maximize the DG output without

exceeding the IEEE 1547 voltage requirements stated inSection II-A. The maximum rate of change of output shall not

cause the Area EPS voltage at other Local EPS to go outsidethe requirements of ANSI C84.1. The maximum output

change shall not cause a voltage fluctuation at the PCC greater

than 5% of the prevailing voltage level of the Area EPS at

the PCC, and it shall not create objectionable flicker for other

customers on the Area EPS.

A/D

ADS7866

200KSPSMonitoring Signals

Flash

2Mx16bit

MBM29PL3200TE

Network CardNetworking

Signals

DSK91C111

EthernetLAN Card

DSP 1

TMS320C6713

32-bit

floating point

DSP 2

TMS320C641632-bit

fixed point

FPGA

XC3S400400k Gates

264 Outputs

Control Signals

Protection

Signals

Fig.10: Adaptive controller for wind-powered DGs


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B. DSP-2: Processor for Adaptive Protection Determination

Hardware Features: This DSP is responsible for monitoring

the complete domain and collecting data from all cell units

inside the domain. A 32-bit, 720MHz, fixed-point digital

signal processor shown in Fig.13 is selected for DSP-2. This

DSP can execute over 5700 million instructions per second

and is an excellent choice for multi-channel and multi-

function applications. This DSP possess the operational

flexibility of high-speed controllers and the numericalcapability of array processors. This DSP has 64 general-

purpose registers of 32-bit word length and eight highly

independent functional units and two high-performanceembedded coprocessors for speeding up channel-decoding

operations on-chip. This DSP has a two-level cache-based

architecture and has a powerful and diverse set of peripherals.

This DSP has a complete set of development tools including

an advanced C compiler, an assembly optimizer to simplify

programming, and a debugger interface for visibility into

source code execution.

Key Operations: The features of this DSP are used to

determine the adaptive protection for the wind-powered DG.

This DSP obtains the measured data directly from the A/D andthe Area EPS information from the network. Based on these

data, this DSP determines the appropriate protection to satisfy

the IEEE 1547 protection requirements given in Section II-B.

The protection determination requires high-speed processing

but does not have involved mathematical computation, and

therefore a fixed-point high-speed DSP is sufficient comparing

with DSP-1. This DSP generates protection commands toensure that:

- The DG shall not energize the Area EPS when the

Area EPS is de-energized.

- The DG shall cease to energize the Area EPS for

faults on the Area EPS circuit.- The DG shall cease to energize the Area EPS prior to

reclosure by the Area EPS.- The DG shall cease to energize the Area EPS within

the clearing time as indicated in Table 1.

- The DG shall cease to energize the Area EPS within

the clearing time as indicated in Table 2.

- The DG shall detect for unintentional islandingoperation and cease to energize the Area EPS within

2 seconds of the formation of an island.

C. Data Acquisition and Storage

Analog-to-Digital Converter: A/D of this design converts the

electrical measurements at the PCC into real-time digital dataand then supplies to DSPs for determining the correct

operation of the wind-powered DG. A 12-bit, 200kSPS, serialanalog-to-digital converter shown in Fig.12 is selected forA/D. This A/D is a low power, miniature converter with a

unipolar, 3.6V max, single-ended input. The serial clock is

used for controlling the conversion rate and shifting data out

of the converter. This provides a mechanism to allow DSP-2

to synchronize with the converter. The converter interfaces

with DSP-2 through a high-speed SPI compatible serial

interface. There are no pipeline delays associated with the

device.Data Storage: Flash of this design stores the original

measured digital data or values after post-processed by DSP-1

and DSP-2. A 2Mx16-bit page mode flash memory shown in

Fig.10 is selected for data storage. This Flash offers fast page

access time of 25ns and random access time of 70ns, allowing

operation of high-speed processors without wait states.

D. Adaptive Controller Networking and Logic Coordination

Network Card: The network card of this design

communicates with the Domain unit. A 10/100 MBit Ethernet

LAN daughter card shown in Fig.10 is selected for connection

to the DSP-1. This card has integrated IEEE 802.3/802.3u100Base-TX / 10Base-T physical layer, auto-negotiation10/100, full/half duplex, 32-bit data bus interface, memory

mapped to CSA or CSB daughter card address range. Thiscard supports interrupt driven, busy-polling or DMAoperation, and optimized TCP/IP protocol stack. It can run as

a DSP/BIOS task and requires no DSP resources (timer,

interrupts, etc.). This card transmits data between this unit and

the Domain unit, for monitoring the operating states of the

circuits supplied by a load transformer tapped along the feeder

length.Programmable Logic: FPGA of this design is interfaced with

the 32-bit external memory interface and two synchronous

serial ports of DSP-1 and DSP-2. A 400k-gates, 264-outputsfield-programmable gate array FPGA shown in Fig.10 isselected. This FPGA has 56k-bit distributed RAM, 288k-bitblock RAM, 16 dedicated multipliers, etc. This FPGA

provides logic controls of all peripherals in the unit.

V. CONCLUSIONS

This paper has presented an adaptive control strategy for

interconnecting the wind-powered DGs to utility distributiongrids. The voltage control requirements and protection

requirements for wind-powered DGs have been specifiedaccording to IEEE-1547 standards. This paper has defined

adaptive interfacing controller for three common types of

wind-powered DGs: doubly fed induction generator,permanent-magnet synchronous generator, and squirrel-cage

induction generator. The design of the adaptive controller

using state-of-the-art digital signal processing technology has

been presented. The key functions of the hardware

components in the adaptive controller have been provided.

VI. REFERENCES

[1] M. Yin, G. Li, M. Zhou, G. Liu, C. Zhao, Study on the Control of

DFIG and its Responses to Grid Distributions, IEEE PES GeneralMeeting, 2006.

[2] O. Samuelsson, N. Strath, Islanding Detection and Connection

Requirements, IEEE PES General Meeting, Tampa, Florida, USA, June24-28, 2007.

[3] X. Wang, W. Freitas, W. Xu, V. Dinavahi, Impact of Interface Controlson the Steady-State Stability of Inverter-Based Distributed Generators,IEEE PES General Meeting, Tampa, Florida, USA, June 24-28, 2007.

[4] A. Uchida, S. Watanabe, S. Iwamoto, A Voltage Control Strategy for

Distribution Networks with Dispersed Generations, IEEE PES GeneralMeeting, Tampa, Florida, USA, June 24-28, 2007.

[5] IEEE Standards 1547, IEEE Standard for Interconnecting Distributed

Resources with Electric Power Systems, July, 2003.

VII. BIOGRAPHIES

Alexander Hamlyn received his B.Eng from Ryerson University, Canada inJune 2007 and is currently pursuing his M.A.Sc degree. He has worked asan NSERC USRA in the WAN lab, and as a research assistant in the WAN

and LEDAR labs, all at Ryerson University.


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Helen Cheung received her B.Eng. from Ryerson University and is currentlya M.A.Sc. student at Ryerson. She has worked as Research Assistant in

Ryerson LEDAR Lab and Engineer in RC Power Conversions Inc.Lin Wang received her B.Eng., M.Eng., and Ph.D. degrees from Huazhong

University of Science and Technology, where she was an Associate

Professor. She is currently conducting research at Ryerson University.

Cungang Yang received his Ph.D. degree from University of Regina. He iscurrently an Assistant Professor at Ryerson University. His research areas

include security and privacy, enhanced role-based access control model,

information flow control, web security, and multimedia security.Richard Cheung received his B.A.Sc., M.A.Sc., and Ph.D. degrees from the

University of Toronto. He was a Research Engineer in Ontario Hydro.

Currently he is a Professor at Ryerson University, and he is an activePower Engineering consultant and is the President of RC PowerConversions Inc.

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