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IEEEstd1094-1991

- IEEE Recommended Practice for the Electrical Design and Operation of Windfarm Generating Stations

- -

Standards Coordinating Committee 23

Sponsored by the IEEE Standards Coordinating Committee on Dispersed Storage and Generation

Published by the Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, NY 1001Z USA.

ApriI30.1991 SH13S37

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IEEE Recommended Practice for the Electrical Design and Operation of Windfarm Generating

Stations

1. Introduction

1.1 Scope. This recommended practice contains design information and procedures for the interconnection of multiple wind turbines (a windfarm generating station) with an electric utility. This document addresses is- sues relating to the interface and the electrical system between the utility and the individual wind turbines (an intraplant electrical system). It also provides recommended practices for monitoring systems, protection systems, and safe operations for personnel and equipment.

1.2 Scope Limitations. This document does not discuss protection of the windfarm-utility interface, which is covered by IEEE Std 1001-1988 [351’, or protection of the utility system. It also does not address the control or protection functions of individual wind turbines. Informa- tion on these functions is provided in AWEA Standard 3.1-1988 [612 and IEEE Std 1021-1988 [361. The unique aspects of wind-turbine generators using power electronics are not discussed in this document since they are not currently in widespread use in windfarms. However, this document does not preclude their use.

utility requirements, and sound engineering practices.

1.4 References. This standard shall be used in conjunction with the following publications. When the following standards are superseded by an approved revision, the revision shall apply.

[ l l ANSI C2-1990, National Electrical Safety Code.3

[21 ANSI C84.1-1989, American National Standard Electric Power Systems and Equip- ment Voltage Ratings (60 Hz).

[31 ANSI C93.1-1981, Requirements for Power Line Carrier Coupling Capacitors.

[41 ANSI C93.2-1976, Requirements for Power L ine Coup l ing Capac i to r Vo l t age transformer^.^

[51 ANSVNFPA 70-1990, National Electrical Code.5

[61 AWEA Standard 3.1-1988, Design Criteria Recommended Practices Wind Energy Con- version Systems.6

1.3 Significance and Use. This document is 3ANSI Dublications are available from t h e Sales intended to facilitate sound, economic en@- Department, American National Standards Institute, ,11

West 42nd Street, 13th Floor, New York, NY 10036, USA. 4ANSI C93.2-1976 has been withdrawn; however, copies

can be obtained from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA.

%T”FA publications are available from Publications Sales, National Fire Protection Association, Battery- march Park, Quincy, MA 02269, USA.

6AWEA publications are available from the American Wind Energy Association, Standards Program, 777 North Capital Street NE, #805, Washington, D.C. 20002, USA.

neering design and safe operations of a windfarm generating station. It should be utilized in conjunction with other standards, local

‘The numbers in brackets correspond to those of the

2AWEA is the acronym for the American Wind Energy references in 1.4.

Association.

7


IEEE std 1094-1991 IEEE RECOMMENDED PRACTICE FOR THE ELECTRICAL DESIGN

[71 AWEA Standard 5.1-1985, Wind Energy Conversion Systems Terminology.

181 IEEE C37.1-1987, IEEE Standard Defini- tion, Specification, and Analysis of Systems Used for Supervisory Control, Data Acquisi- tion, and Automatic Control.'

[93 IEEE C37.90-1989, IEEE Standard for Re- lays and Relay Systems Associated With Electric Power Apparatus (ANSI).

[lo1 IEEE C37.91-1985, IEEE Guide for Protec- tive Relay Applications to Power Transform- ers (ANSI).

[ l l l IEEE C37.95-1989, IEEE Guide for Protec- tive Relaying of Utilities-Consumer Inter- connections (ANSI).

[12] IEEE C37.97-1979 (Reaff. 1984), IEEE Guide for Protective . Relay Applications to Power System Buses (ANSI).

[131 IEEE C37.99-1990, IEEE Guide for Protec- tion of Shunt Capacitor Banks.

[141 IEEE C37.101-1985, IEEE Guide for Gen- erator Ground Protection (ANSI).

[151 IEEE C37.102-1987, IEEE Guide for AC Generator Protection.

[l6] IEEE C37.010-1979 (Re&. 1988), IEEE Ap- plication Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis (ANSI).

[17] IEEE C37.012-1979 ( R e d . 1988), IEEE Ap- plication Guide for Capacitance Current Switching for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis (ANSI).

[181 IEEE C57.12.00-1987, IEEE Standard Gen- eral Requirements for Liquid-Immersed Distribution, Power and Regulating Trans- formers (ANSI).

~~

'IEEE publications are available from the Institute of Electrical and Electronics Engineers, Service Center, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA.

[191 IEEE C57.12.01-1989, IEEE Standard Gen- eral Requirements for Dry-Type Distribution and Power Transformers Including Those with Solid Cast and/or Resin-Encapsulated Windings.

[203 IEEE C57.13-1978 (Reaff. 1986), IEEE Standard Requirements for Instrument Transformers (ANSI).

[211 IEEE C62.2-1987, IEEE Guide for the Appli- cation of Gapped Silicon-Carbide Surge Ar- resters for Alternating Current Systems (ANSI).

[221 IEEE C62.41-1980, IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits (ANSI).

[231 IEEE C62.45-1987, IEEE Guide on Surge Testing for Equipment Connected to Low- Voltage AC Power Circuits (ANSI).

[241 IEEE C62.92-1989, IEEE Guide for the Ap- plication of Neutral Grounding in Electrical Utility Systems, Part II-Grounding of Syn- chronous Generator Systems (ANSI).

[251 IEEE Std 18-1980, IEEE Standard for Shunt Power Capacitors (ANSI).

[261 IEEE Std 80-1986, IEEE Guide for Safety in AC Substation Grounding (ANSI).

[271 IEEE Std 81-1983, IEEE Guide for Measur- ing Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System.

[281 IEEE Std 100-1988, IEEE Dictionary of Electrical and Electronics Terms.

[291 IEEE Std 141-1986 (Red Book), IEEE Rec- ommended Practice for Electric Power Distri- bution for Industrial Plants (ANSI).

[301 IEEE Std 142-1982 (Green Book), IEEE Rec- ommended Practice for Grounding of Indus- trial and Commercial Power Systems (ANSI).

[311 IEEE Std 242-1986 (Buff Book), IEEE Rec- ommended Practice for Protection and Coor- dination of Industrial and Commercial Power Systems (ANSI).

8


AND OPERATION OF WlNDFARM GENERATING STATIONS IEm

SM 1094-1991

[321 IEEE Std 367-1987, IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault (ANSI).

[331 IEEE Std 399-1980 (Brown Book), IEEE Recommended Practice for Industrial and Commercial Power System Analysis (ANSI).

1343 IEEE Std 493-1990 (Gold Book), IEEE Rec- ommended Practice for the Design of Reliable Industrial and Commercial Power Systems.

[351 IEEE Std 1001-1988, IEEE Guide for Inter- facing Dispersed Storage and Generation Fa- cilities with Electric Utility Systems (ANSI).

[361 IEEE Std 1021-1988, IEEE Recommended Practice for Utility Interconnection of Small Wind Energy Conversion Systems (ANSI).

[371 Bassett, E. D. and Potter, F. M. “Capacitive Excitation for Induction Genera- tors.” AIEE Transactions. vol. 54, May 1935, pp. 540-545.

[381 Electrical Transmission and Distribution Reference Book. East Pittsburgh, PA: West- inghouse Electric Corp., 1950.

[391 Feero, W. E. and Gish, W. B. “Overvoltages Caused by DSG Operation: Synchronous and Induction Generators.” IEEE Transactions on Power Delivery. vol. 1, January 1986, pp. 258-264.

E401 Gish, W. B., Feero, W. E., and Gruel, S. “Ferroresonance and Loading Relationships for DSG Installations.’’ IEEE Transactions on Power Delivery. vol. 2, July 1987, pp. 953-959.

[411 “Intertie Protection of Consumer-Owned Sources of Generation, 3 MVA or Less.” IEEE Power Engineering Society Special Publica- tion 88 TH0224-6-PWR.

[421 Smith, D., Swanson, S., and Borst, J. “Overvoltages with Remotely-Switched Cable- Fed Grounded Wye-Wye Transformers.” IEEE Transactions. vol. PAS-94, 1975, pp. 1843-1853.

[431 Wagner, C. F. “Self-Excitation of Induc- tion Motors.” AIEE Transactions. vol. 58, February 1938, pp. 47-51.

[441 Working Group on Fast Transfer of Mo- tors, IAS-PSPC. “Source Transfer and Reclos- ing Transients in Motors: A Preliminary Working Group Report.” IEEE 1982 Industrial and Commercial Power Systems Technical Conference. pp. 43-50.

1.5 Bibliography

ASME PTC42-1988, Wind Turbines.8

UL 347-1985, The Standard for High-Voltage Industrial Control E q ~ i p m e n t . ~

UL 493-1988, The Standard for Thermoplastic Insulated Underground Feeder and Branch Circuit Cables.

UL 508-1989, The Standard for Industrial Control Equipment.

UL 810-1981, The Standard for Capacitors.

UL 891-1984, The Standard for Dead-Front Switchboards.

UL 1062-1983, The Standard for Unit Substations.

UL 1072-1986, The Standard for Medium- Voltage Power Cables.

UL 1558-1988, The Standard for Metal Enclosed, Low-Voltage Power Circuit Breaker S wi tchgear .

UL 1561-1986, The Standard for Dry-Type General Purpose and Power Transformers.

1.6 Definitions. Terms other than those defined below have standard definitions as listed in IEEE Std 100-1988 [281 or AWEA Standard 5.1-1985 [71.

islanding. Operation of non-utility electric generation equipment, with or without a portion of an electric utility system, isolated from the remainder of the utility system.

BASME publications are available from the American Society of Mechanical Engineers, 345 East 47th Street, New York, NY 10017, USA.

’UL publications are available from Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook, IL 60062-2096, USA.

9


IEEE std 1094-1991 IEEE RECOMMENDED PRACTICE FOR THE EUCTRICAL DESIGN

micrositing. Of, or related to, the characteristics of a particular wind-turbine site, as con- trasted to those characteristics tha t prevail over the entire windfarm.

self-excitation. A condition in which an induction generator, operating in an isolated power system, derives i ts excitation from shunt capacitors or the natural capacitance of the power lines. Applies only to induction machines.

2. Intrapbt Electrical system

2.1 Collection System Design. An electrical system must be provided to collect the generator outputs and transmit the aggregate net electrical output to the electric utility interconnection point, described in Section 3. This system is referred to as the “collection system.” The collection system is also the source of start-up/auxiliary power t o each individual generator or site electrical load. Fig 1 shows a typical windfarm interconnection with collection system.

Disconnecting means should be provided for each collection system element in accordance with the requirements in ANSINFPA 70-1990 [51 (the National Electrical Code). In addition, overload and short-circuit protection should be provided to protect conductors and equipment by mitigating the consequences of electrical faults and to minimize the disruption to the remaining system. 2.1.1 Voltage Levels. The collection system

usually consists of distinct low-voltage and medium-voltage systems. The low-voltage system connects one or more individual wind generators to low-voltage/medium-voltage step-up transformers. The medium-voltage system connects the step-up transformer outputs to the utility interconnection substation, which may further step up the voltage to a transmission voltage level.

The voltage levels of the collection system should be chosen by an economic analysis t o minimize the total cost of equipment and losses over the expected lifetime of the facility. Voltage levels should normally be chosen from those listed as “preferred” in ANSI C84.1- 1989 [21. Equipment with the preferred ratings will generally cost less and be more readily available than nonstandard equip-

ment. Use of a nonstandard voltage may be justified, however, if that voltage is supplied by the interfacing utility.

When carrying power, the actual voltage level of the collection system will deviate from the nominal voltage (see 2.5.1). The changing voltage may affect the efficiency of the wind- turbine generators. The transformer ratios and generator voltage ratings should be chosen to maximize the total energy production of the windfarm for the expected wind regime. Conversely, equipment ratings should not be exceeded under any expected wind conditions. 2.1.2 Conductors. Underground cable is typ-

ically used for the low-voltage collection system. Medium-voltage systems typically are a combination of underground and overhead conductors. When overhead conductors are used, special consideration should be given to safe working distances for cranes and other equipment.

Underground cable systems can be either direct-buried, installed in conduit, or installed in concrete-encased duct banks. A concentric-neutral cable or dedicated-neutral return cable should be installed with each col- lector system feeder cable. This neutral-return conductor(s) provides a fault return path and can assist in alleviating certain ferroresonance conditions (see 2.6.2).

2.2 Power Flow and Equipment Loading 2.2.1 Power Flow. When the turbines are op-

erational, real power will flow from the generators to the utility when the electrical output exceeds the turbine and site service loads. When turbines are not operating, or a re in start-up mode, utility power will be used for electronics, motors, transformer losses, and site support. Some types of turbines “motor” to start, i.e., they draw power from the collection system to accelerate the turbine to operating speed. This can cause an inrush of power from the utility or from other operating turbines. Other types of wind turbines are driven to operating speed by the wind. Reactive power flow will be determined by the type of generator (synchronous or induction), any capacitors in the system, and the excitation level of synchronous generators. Generally, most wind turbines use induction generators with a moderate quantity of local capacitance. The var demand of an induction generator changes with its power output, whereas the var supply of

10


AND OPERATION OF WINDFARM GENERATING STATIONS IEEE

std 1094-1991

umTy SYSTEM uwTy TRANSMISSION LINE T

STEP-UP SUBSTATION HIGH-VOLTAGE SECTION SEE 3.3.8 FOR PROTECTION DETAILS

STEP-UP SUBSTATION MEDIUM-VOLTAGE SECTION SEE 3.3.8 FOR PROTECTION DETAILS

M ED1 U M-VOLTAGE OVERHEAD COLLECTION LINE

MEDIUM-VOLTAGE UNDERGROUND CABLE

PADMOUNTED SWITCHGEAR/ STEP-UP TRANSFORMER

LOW-VOLTAGE COLLECTON SYSTEM

L.V. UNDERGROUND CABLE

/ I

HIGH-VOLTAGE BREAKER

MAIN STEP-UP TRANSFORM ER

MEDIUM-VOLTAGE P.F.C.

POWER- FACTOR CORRECTION BREAKER

OTHER COLLECTION LINES

3-POLE GANG-OPERATED SWITCH

M.V. U.G. CABLE

OTHER PADMOUNT TRANSFORMERS OPER.

FUSES

PADMOUNT TRANSFORMER

) MAIN L.V. BREAKER I

FROM -+- , L.V. FEEDER BREAKER

OTHER TURBINES -n r

-

WIND TURBINE CONTROL

LOW-VOLTAGE U.G. CABLE --LL--

:> WIND TURBINE 61 &] Ft$:R 6 GENERATOR

CORRECTION

Fig 1 Typical Single-Line Diagram

11



a capacitor is constant, assuming constant voltage. Their combination may result in reactive power (var) flow from the utility to the windfarm a t high generator output levels and flow from the windfarm to the utility during periods of low power output. Capacitors are often switched off and on with various schemes (see 2.5.4).

2.2.2 Equipment Loading Considerations. Several important factors should be considered in the specification of equipment loading capability. First, the maximum simultaneous output of all wind turbines should be estimated from the wind-turbine ratings (or actual output, if higher) to provide a basis for determining electrical equipment loading. Secondly, ambient temperatures and prevailing winds should be taken into consideration for the loading, placement, and orientation of equipment. This may allow increased loading due to wind cooling of transformers, generators, and overhead conductors. Likewise, de-rating due to high ambient temperature and solar ra- diation may be required. Transformers should be oriented to allow prevailing winds to pass freely through the cooling radiators. Cooling fans should be directed so as not t o compete with prevailing wind patterns. Pro- ject capital costs and loss economics should also be considered in the loading and selection of equipment. For recommended practices for system design and analysis, see IEEE Std 141- 1986 [291, IEEE Std 399-1980 [331, and ANSUNFPA 70-1990 [51.

2.3 System Protection. The protection of the intraplant power collection system and the wind-turbine generator connection is similar in protection concepts to those of utility and industrial systems involving the same voltages. The IEEE color book series L29-31, 33-341, the National Electrical Safety Code (ANSI C2-1990 1111, ANSI/NFPA 70-1990 [51, and the IEEE Protection Guides [9-153 can be applied in most cases. However, the windfarm application of these documents does require some special consideration. Fig 1 shows typical protection for t he windfarm collection lines and transformers.

2.3.1 Fault Current Flow. Sources of fault current include both the utility system and the wind-turbine generators themselves. In most cases, the utility network will supply the majority of the fault current. This is particularly

true in the case of induction generators. Even when equipped with power-factor correction capacitors, the fault contribution from induction generators will be moderate in magnitude and short in duration. Fault current contribution will be similar to motor contribution values used in s t anda rd shor t -c i rcu i t calculations. Most induction generators operate ungrounded and, as a result, have no zero-sequence contribution to the fault current.

Synchronous generators will normally be wye-connected and, if grounded, are capable of supplying positive, negative, and zero-sequence currents. Like any other synchronous generator, they will be able to maintain these fault currents based on the particular characteristics of the generator and the excitation system. Recommended techniques for fault current analysis will be found in IEEE Std

2.3.2 Fault Interruption and Momentary Ratings. A system fault study should be performed early in the project. All electrical equipment should be rated to withstand and, if required, interrupt calculated system fault currents. Applicable guides and standards are IEEE C37.010-1979 (Re&. 1988) [l61, IEEE Std

2.3.3 Protective Device Coordination. A protective device coordination study should be performed on the entire intraplant electrical system, including the utility interface. Relay settings should be coordinated with the local utility for proper operation. Settings mcst be calculated and relays set and tested to specified values. Relays should be retested and their calibration and tripping verified on a two- t o three-year schedule. The utility may have other requirements for relays a t the interconnection station.

2.3.4 Phase Sequence and Single Phasing. Particular care should be taken to connect and check the phase sequence a t each wind turbine. Some wind turbines may sustain damage if the phase sequence is incorrect. For some applications a phase-sequence relay may be advisable.

Single phasing can result in damage to three-phase generators. Each wind turbine should employ a protective device capable of detecting loss of one phase. It may be necessary to use a detection scheme that monitors current or both voltage and current. Single- pole switching should be avoided to minimize

399-1980 [331.

141-1986 [29], and IEEE Std 242-1986 [31].


AND OPERATION OF WINDFARM GENERATING STATIONS IEm

std 1094-1991

the possibility of single-phase operation of a three-phase system.

223.5 Wind Turbine Shutdown. Shutdown is typically required for maintenance, overspeed, mechanical failure, and electrical fault conditions. Wind turbines should be able to shut down safely without utility power. I t is also recommended that the site communication system not be required for safe shutdown.

2.3.6 Reclosing and Torque Transients. As a result of very short duration power outages or islanding, very high mechanical torque transients can be experienced by the rotating equipment. When an outage occurs and the electrical interconnection is opened, the rotating equipment accelerates and becomes out-of-phase with the utility system. Upon re- connection, the electrical transients can be very high, up to twice the locked rotor current, and the mechanical drive train torques can reach up to 20 times rated values. For more information, see [441. This transient can be caused by site collection-line reclosing or by high-speed reclosing on the utility system.

2.4 Insulation, Grounding, and Surge Pmtection

2.4.1 Insulation Ratings. Standard insulation guidelines may be applied in most cases (see the Electrical Transmission and Distri- bution Reference Book [381). Local utility practice may also be used as a guide. Special considerations may, however, need to be given to overhead lines, outdoor substations, and metal-clad switchgear due t o blowing dust, sand, salt spray, agricultural chemicals, and other contamination in some windfarm areas.

2.4.2 Grounding. Generally, effectively grounded collection systems are preferred for their simplicity and freedom from transient overvoltage and simultaneous fault problems. Effectively grounded systems are usually obtained by a wye-connected, low-voltage winding on the interface station transformer. The high-voltage windings of tha t transformer may be wye or delta, depending upon the voltage level and utility requirements. Wye-wye connected transformers may require a delta tertiary winding to provide a path for zero-sequence and triplen harmonic current flow.

If the collection system is tied directly t o utility lines without an intervening transformer, the collection system grounding must

be coordinated with the utility. The utility should be consulted as to the types of transformers permitted to be connected to such a system. Recommended practice for grounding is given in IEEE Std 142-1982 [301. For additional information on system neutral grounding, refer to IEEE C62.92-1989 [241.

Regardless of the method of system grounding, i t is recommended that the entire windfarm installation have a continuous metallic ground system connecting all equipment. See Fig 2 for a typical arrangement. This should include, but not be limited to, the substation, transformers, towers, wind turbine, generators, and electronic equipment. This system will consist of ground conductors, rods, mats, and connectors. This system will serve to

Minimize shock hazards to personnel Establish a preferred path of return current for fault currents to prevent damage to on-site electrical systems Provide a multigrounded neutral to assist in ferroresonance suppression in the site collection system Improve reliability and consistency in sensing faults and operation of overcurrent relays Improve protection from lightning

2.4.3 Surge Protection. Surge protection for lightning and, particularly, switching surges should be part of the system. Guidelines for surge protection of medium-voltage collection systems can be found in IEEE C62.2-1987 [211. Information on typical surge environments for low-voltage systems is provided in IEEE C62.41-1980 [22] and IEEE C62.45-1987 [231.

25 Voltage Contam1 and Reactive Power Supply 2.5.1 Voltage Considerations. Careful man-

agement of the system voltage is very important for several reasons.

Changes in voltage have an effect on the slip, torque,and power output characteristics of an induction generator. Induction generator installations may reduce the line voltage when loaded because of their reactive power (var) demand. On-site electronic equipment may be sensitive to variations in voltage.


lEEE std 1094-1991 IEEE RECOMMENDED PRACTICE FOR THE ELECTRICAL DESIGN

--- LOWYOLTACE FEEDER

_---- GROUND HlRE

COMMUNICAllON CABLE MlH SHIELD

Fig2 Grounding Arrangement

14



std 1094-1991 ( 4 ) Power-factor correction capacitors tend

to raise the voltage. (5) Low voltage may increase collection

system loading and losses because of higher current levels.

2.5.2 Reactive Power Supply. Induction generators, like induction motors, demand reactive power (var) when operating. If this demand is not supplied by a reactive power source a t the windfarm, the reactive power will be drawn from the utility. The utility and windfarm operator must come to an agreement as to who will supply the reactive power needs of the windfarm. Technical and economic factors, described in the succeeding sections, need to be considered in reaching this agreement.

Synchronous generators or power condition- ers may be designed or adjusted to require no reactive power or even t o produce reactive power. However, even with these types of equipment, the transformers and lines of the collection system will demand reactive power while carrying the windfarm output.

The most common method of supplying reactive power is the use of shunt capacitors, either singly or in banks of multiple units. Capacitors supply the reactive demand by drawing a current that counteracts the out-of- phase component of current required by induction motors or generators.

Reactive power compensation applied in a windfarm generating station has several effects, one or more of which may be the reason for the application:

(1) Increases voltage level (2) Improves voltage regulation if the ca-

pacitors are properly switched (3) Reduces electrical losses in the collec-

tion system due to a reduction in current (4) Decreases loading on utility generation

and circuits (5) Reduces investment in windfarm fa-

cilities per kilowatt of load supplied

2.5.3 Power-Factor Correction Economics. Since many electric utility companies include low power-factor penalties or power-factor in- centives in their rate schedules, i t may be economical for windfarm generating stations to install equipment for power-factor improve-

ment. An economic study should be made to determine the optimum amount of reactive power compensation or “var compensation.” The best point a t which to connect capacitors to the collection system of the windfarm depends upon economic and technical considerations.

Relatively small capacitor units can be connected a t the individual wind turbines, or the total capacitive requirement can be grouped a t one or several points throughout the windfarm. Each windfarm must be individu- ally evaluated to determine the cost versus the benefits of power-factor correction capacitors.

2.5.4 Capacitors Applied to Induction Gen- erators. In windfarm applications tha t include induction generators, the use of capacitors warrants special consideration. The amount of capacitance connected must be limited to values that do not cause excessive voltage at the generator due to self-excitation when the generator/capacitor combination is disconnected from the utility source. Capacitance exceeding this value should be switched by a suitable control algorithm. Failure to observe this precaution could lead to generator and equipment damage.

Frequently, shunt capacitors are connected directly in parallel with induction generators by means of a generator switching device. Overvoltage may result, due to self-excitation, when the generator and capacitor combination is islanded o r disconnected from the utility source, since wind and inertia of the turbine will keep the turbine rotating and the generator operating.

When the generator can be rapidly de-energized and re-energized, the possibility of high transient torques should be considered. Ca- pacitors switched with the generator switching device prolong the duration of residual voltage in the generator as i t comes to rest after shutdown. The generator manufacturer should be consulted regarding the impact of utilizing capacitors in parallel with the generator.

2.5.5 Capacitor Characteristics. Capacitor characteristics should be carefully checked for voltage and temperature ratings. Current- limiting reactors may be required for groups of switched capacitors to minimize transients during switching. Most capacitors are manu- factured with a tolerance of -O%, +15% of rated capacitance and usually provide 5-10% more capacitance than specified. Refer to IEEE Std 18-1980 [251.


mm s&d 1094-1991 IEEE RECOMMENDED PRACTICE FOR THE ELEXTRICAL DESIGN

2.5.6 Drainage of Stored Charge. When capacitors are disconnected from the windfarm collection system, they are typically in a charged state. Under this condition, consider- able energy is stored in the capacitors, and there is a voltage present between the terminals. If the capacitors were lefi in this charged state, a person servicing the equipment might receive a dangerous shock or the equipment might be damaged by an accidental short circuit. As a result, all capacitors should be provided with a means of draining the stored charge. Typically, these means are provided by the capacitor manufacturer. However, proper shorting and grounding techniques should still be followed.

2.5.7 Protection of Shunt Capacitor Banks. Shunt capacitor bank design requirements necessitate an increase in minimum bank size with system voltage. As the system voltage increases, the capacitor bank investment and thus the risk of costly damage increases. Capacitors of larger kvar ratings reduce the investment, but they may also reduce the choice of different capacitor combinations. Protection begins with the design of the capacitor bank.

Bank protection equipment must guard against the following conditions:

(1) Overcurrents due to capacitor bank bus faults

(2) System voltage surges (3) Overcurrent due to individual capacitor

unit failure (4) Continuous capacitor unit overvoltages ( 5 ) Discharge current from parallel capaci-

tor units (6) Inrush current due to switching

In all applications, the windfarm designer should consult Article 460 of ANSVNFPA 70- 1990 [51 for guidance in sizing the protection and interconnecting equipment associated with capacitors. Guidance on the protection of capacitor banks is provided by IEEE C37.99- 1990 [131. In addition, the manufacturer and the interconnecting utility may be consulted regarding capacitor protection practices.

2.5.8 System Considerations. A shunt capacitor bank may form a resonant circuit with system inductive elements. The resonant frequency may be excited during the switching of a remote capacitor bank, which may result in

excessive voltages and currents and the possible associated failure of equipment such as other capacitors, surge arresters, instrument transformers, and fuses. These undesirable resonance effects are more likely to occur if the capacitor bank switching device has a long arcing time and multiple restrike characteristics. A switching device should be chosen that is rated for switching capacitors (see IEEE C37.012-1979 { R e d . 1988) [171).

The capacitor bank, in combination with system inductances, may cause resonance with harmonics produced elsewhere, such as at remote loads. There is extensive and growing use of thyristors in industry to derive variable potentials from an ac source. Such phase-controlled thyristors generate harmonics, particularly 3rd, 5th, 7th and 11th.

Problems associated with resonance may usually be resolved by the application of the proper capacitor switching device, the addition of appropriately rated reactors (or reactors

and resistors in parallel) in series with the switched capacitor bank, or the relocation or change in size of the switched capacitor bank.

Capacitor banks can also cause or aggravate ferroresonance conditions. This is discussed in 2.6.

2.6 Self-Excitation and Ferroresonance 2.6.1 Self-Excitation. Self-excitation can oc-

cur if the windfarm or a portion of it loses its connection to the utility. Unstable voltages and frequencies can be developed during self- excitation. Due to the potential for equipment damage, steps should be taken to prevent this occurrence.

One of the first indications of self-excitation or islanding within the system will be a voltage and frequency deviation. This deviation will usually appear as a rapid turbine accel- eration and increase in frequency. When adding power-factor correction capacitors, a careful study of self-excitation potential is recommended (1371, 1431). Switching of the capacitors may be required to eliminate self- excitation.

2.6.2 Ferroresonance. Ferroresonance is a special type of electric resonance that can occur when a nonlinear inductive reactance is connected in series with o r parallel to a capacitive reactance. The inductive reactance is usually the magnetizing reactance of a sin-

16



std 1094-1991

SERIES L & C ELEMENTS ,-,

DisTRiwTioN TRANSFORMER

Fig3 Series Ferroresonance

gle-phase or three-phase transformer. The capacitive reactance is usually the conductor-to- sheath capacitance of primary cable and/or any shunt capacitors used on the system.

One form of ferroresonance is associated with switching and can occur as the result of opening one of the phases of a source supplying a single-phase o r three-phase transformer (1421). For this condition, the capacitance of the open phase is energized through the magnetizing reactance of the transformer. Since in this case the reactance and capacitance are in series, this type of ferroresonance is sometimes referred to as series ferroresonance (see Fig 3).

This unsymmetrical opening may be due to switching with single-pole devices such as cutouts, or by conductor breakage, or by fuse operation. Since ferroresonance disappears or does not occur when all phases are open or

closed, the probability of occurrence will be minimized when gang-operated devices are used for switching. Note tha t the overvoltage takes some period of time to build up, which is the reason that switching with a gang-operated device will usually be satisfactory even though the pole openings may not be simultaneous.

A second type of ferroresonance can occur during an islanding condition (see 1.6). Un- der normal operation, in parallel with the utility, the voltage applied to the interface and distribution transformers is less than the saturation voltage level of the transformers. Dur- ing an islanding condition, however, there may not be sufficient voltage control in the island to hold the voltages below their saturation point. If the transformer(s) saturates, there will be an interchange of energy between the system capacitance and the highly nonlinear

17



magnetizing reactance of the transformerW. The rapid changes in transformer flux during this period can produce high system overvoltages. Since in this case the reactance and capacitance are in parallel, this second type of ferroresonance is sometimes referred t o as parallel ferroresonance (1391, [401; see Fig 4).

This type of ferroresonance, as well as the series type, may be accompanied by an abnormal voltage, low or high, across the transformer terminals and from terminal t o ground. A high abnormal voltage due to ferroresonance will manifest itself by abnormal transformer sound and, if sufficiently high, by equipment damage.

The probability of either type of ferroresonance is somewhat unpredictable, as both de- pend on such factors as the cable lengths, the amount of system capacitance, the connection and saturation characteristics of the transformers, the amount of load, etc. In addition to the use of gang-operated switching devices mentioned previously, the probability of occurrence of series ferroresonance can be minimized by wye-connecting all transformer windings to the primary neutral. Parallel ferroresonance should be controlled by system design and by the use of high-speed relay protection t o shut down the windfarm for an islanding condition.

DISTRIBUTION TRANSFORMER

CAPACITOR BANK

W

SMALL GENERATOR

Fig4 Parallel Ferroresonance



std 1094-1991

3. Utility Inkmnndon

This section describes general substation, transmissioddistribution, and related facility requirements appropriate to interconnect a commercial windfarm generating station with an electric utility. I t provides a ”checklist” and selected guidance for areas of coordination that must be considered by the station owner and the utility.

3.1 System Interconnection Considerations. It is essential that all parties involved understand the basis for designing the interconnection. Care should be taken t o ensure that each party of the interconnection uses a compatible basis for ratings (e.g., temperature rise), the same formulas for calculations (e.g., strength of bus conductors), and the same philosophy of loading (e.g., transformer loss-of-life or no loss-of-life for overloading; plant generation cycle/peak generation considerations) t o avoid wasteful overbuilding on the one hand or destructive overloading on the other. The following significant design factors should be considered for the high-voltage interface.

3.1.1 Voltage, Voltage Range, and Fre- quency. The interconnection facility must match the utility nominal voltage and frequency. Further, the design of the interconnection facility must consider the anticipated minimum and maximum voltage ranges below and above normal, respectively.

3.1.2 Phase Rotation and Phase Position. The phase designation (A, B, C; 1, 2, 3; etc.) should be coordinated, as well as the phase r o t a t i o n d i r ec t ion (c lockwise o r counterclockwise).

The phase position must be known if the station has interconnections to different utility voltage levels or different utility locations that may have a relative phase shift by virtue of geography o r sys t em t r ans fo rmer configuration.

Accommodating different phase positions (phasing) within the windfarm collection system is usually accomplished by means of appropriate transformer connections or by main ta in ing isolation of out-of-phase facilities.

3.1.3 Fault Interrupting and Momentary Ratings. Power generating and distribution equipment directly connected to an electric utility will be subjected to fault levels that are a

function of the utility system and station characteristics. Therefore, the station and interconnection facilities should possess sufficient fault interrupting and momentary withstand ratings t o meet the maximum calculated levels, with appropriate margins for future system growth. Induction generators are prin- cipally a source of momentary fault current, whereas utilities and synchronous machines are sources of sustained fault current.

3.1.4 Continuous Ratings. The continuous current ratings of the electrical power apparatus associated with the interconnection must be sufficient to accommodate the maximum generation (or load) of the station, as well as con- tributions resulting from system exchanges, such as power from the utility system being passed through the facilities of the station.

3.1.5 Type of Service. The overall interconnection of a station can be generally classified a s either “radial” or “loop” service. Most windfarm services are radial. Radial service usually consists of one line, in effect a branch of the utility system, with no utility load current passing “through” the interconnection station (see Fig 5).

Loop service provides two (or more) utility lines to the windfarm (see Fig 6). Other configurations can also be used to provide multiple-line service. For additional information, see the Electrical Transmission and Distribu- tion Reference Book [381.

3.1.6 LoadPower-Factor Considerations. The magnitude, reliability, duration (load factor), and time-of-day availability of the generation (or load) of the station will be of interest to both parties. In addition, total real power (kW) and reactive power (kvar) to be accommodated can have significant impact on ratings and configuration of facilities.

The demand for (or generation of) reactive power will warrant special attention. Any large power producer can usually be expected to maintain the overall facility power factor a t or above a given level that will be specified by the agreement with the connecting utility (see 2.5 for additional information). Conversely, customer-owned synchronous generating facilities may be expected to produce reactive power and make it available to the system, as is discussed in 2.5.2. This is a capability that cannot be achieved, however, with induction generators.


IEEE std 1094-1991

TOUTIUTY 4 LINE 1

IEEE RECOMMENDED PRACTICE FOR THE ELECTRICAL DESIGN

b TOUTIUTY LINE 2

TO UTILITY LINE

t TO WINDFARM

Fig5 Radialservice

TO WINDFARM



std 1094-1991

3.1.7 Additional Information. Other items neering analysis of reliability and cost, and it t h a t should be considered for the util- must be acceptable t o the windfarm operator itylwindfarm interconnection facility in- and to the utility. clude the following: 3.2.2 Transformer Connections. The wind-

farm and utility should agree on the configu- (l) Drawings and Data: A ration for power and metering (instrument)

be Prepared for the submissions and re- transformers (wye-delta; delta-wye; wye- view Of appropriate drawings and data wye; zig-zag; autotransformer, etc.). This se- by all Parties during the faci1- lection will have significant effects on i tY phase and during ‘YS- reliability, relay selection, fault current tem modification activities. levels, overvoltage conditions, equipment cost,

and other technical (2) Color Codes: For cable, wiring, race- way, signs, etc.

(3) Nameplates: For major equipment. (4) Equipment Numbers: For operations

and maintenance coordination. (5) Signs: To indicate ownership limits

and for safety. (6) Accessibility: Access of utility em-

ployees to the owner’s substation. 3.2 Substation Configuration. Normally, a substation facility is needed to provide for the high-voltage termination of the utility interconnection line(s). This substation contains the necessary switching and protective device required t o isolate and mitigate the effects of faults and to permit routine maintenance and switching. The substation is generally the location for any required power transformers.

A visible-break disconnecting means is usually required between the windfarm and the utility. To comply with some safety requirements a lockable disconnect device may be needed.

3.2.1 Circuit Breaker Arrangement. Power circuit breakers, and their associated bypass and/or isolating disconnecting switches, where present, usually constitute the most con- spicuous components of the substation.

Circuit breakers permit power circuit switching for synchronizing, if necessary, and interruption of fault current.

The simplest circuit breaker arrangement is a single circuit breaker in a simple radial arrangement (see Fig 7). However, any scheme requiring more than one connection each for the utility and customer, or requiring loop service, will necessitate a more complex circuit breaker arrangement. Several of the most common alternatives are: modified radial; “H”-tie; ring bus; main and transfer bus; breaker and one-half; and double-bus.

The final selection of a particular circuit breaker arrangement depends on an engi-

When choosing transformer connections, the phase shifts must be considered to ensure proper phase relationships if there are multiple interconnections from the windfarm to the utility.

3.2.3 S t r u m u s Construction. Substation facilities are usually air insulated. The con- ventional outdoor air-insulated substation frequently utilizes aluminum cable or tubing for conductors because of economic advan- tages. Copper is sometimes encountered in older stations or where environmental conditions preclude the use of aluminum. Ener- gized parts are supported by porcelain, glass, or epoxy insulators and maintained at safe vertical and horizontal clearances, which are dictated by industry and/or utility standards and/or code requirements.

Supporting structures for electrical equipment are usually galvanized steel. Other ma- terials t ha t can be used a re aluminum, concrete, and wood.

3.2.4 Interconnecting Lines Location and Orientation. The location and orientation of the substation facility must consider the direction and ultimate destination of the overhead transmission lines connecting to either the utility or the station facilities. Provision must be made for appropriate rights- of-way and avoidance of structures and other existing facilities, turning towers, and conductor phase transpositions. The use of underground power cables is a feasible alternative to overhead lines, bu t it is relatively expensive, particularly for high- voltage lines.

3.2.5 Future Expansion Consideration. Pro- vision should be included in any substation for future expansion as required to accommodate additional utility interconnections, station interconnections, and/or windfarm development.

21


IEEE std 1094-1991

V.T.'S 7fT

\

PROTECTIVE RELAYING '

IEEE RECOMMENDED PRACTICE FOR THE EWCTRICAL DESIGN

V.T.'S 7 c REVENUE

/ METERING

I: -3 C.T. (TYP.)

i

HIGH-VOLTAGE CONNECTION TO UTILITY

LEGEND: C.T. -CU RRENT XFM R. V.T.-VOLTAGE XFMR. L.A.-LIGHTNING ARRESTER

A

0 POWER CIRCUIT BREAKER

\ 3 L.A.

1- WIl

vvvvv POWER TRANSFORMER

\

1 M ED I U M -VO LTAG E B U S

TO TO VOLTAGE W I N D FAR M WIN D FAR M

INTRAPLANT AUXILIARY & RELAYING COLLECTION LOADS

IN DICATIO N

SYSTEM

E

Fig 7 mid Collection Station Arrangement

3

22



std 1094-1991

3.3 Substation Facilities Design. Proper design of an interconnection substation must include the following significant areas for consideration in the design criteria. 3.3.1 Grounding. A buried grounding grid

consisting of bare conductors and/or elec- trodes (ground rods) is necessary for personnel safety and to provide an effective ground for transformer and other equipment connections. Proper design requires consideration of local soil conditions.

The grounding design must be coordinated with adjacent utility, plant, or other facilities to ensure that transferred potentials will not present personnel hazards or damage protection, control, or communication circuits. The utility should be consulted on whether or not to interconnect the station grounding system with the ground grid or neutral conductor of the utility. Guidance on the design of substation grounding systems is provided in IEEE

Std 367-1987 [321. See 2.4.2 for information on grounding within the windfarm.

The effect of a generation facility or substation grounding system on existing or future cathodic protection systems (e.g., pipelines) requires careful study and field testing. 3.3.2 Insulation Levels. For any given

electric power interconnection, the basic im- pulse insulation level (BIL) must be coordinated for power distribution equipment and transmission lines. Industry references or the local utility are the appropriate sources for existing insulation levels, which are frequently different for overhead line, cable, and transformer insulation systems at the same voltage level (see ANSI C93.1-1981 [31, ANSI C93.2 [41, IEEE C37.010-1979 (Reaff. 1984) [161, IEEE

Std 80-1986 [261, IEEE Std 81-1983 [27], and IEEE

C37.012-1979 (haff . 1988) [171, IEEE C57.12.00- 1987 [l8], IEEE C57.12.01-1989 [19], IEEE C57.13-1978 [201, IEEE C62.2-1987 [211, IEEE Std 18-1980 [251, and IEEE Std 493-1990 [341). Con- sideration should be given to the prevailing wind direction and sources of insulation contamination from manufacturing effluents or a marine environment. 3.3.3 Clearance and Access. Appropriate

normal and minimum electrical clearances from energized parts; above walkways, roads, and/or railroads; and in other special cir- cumstances are specified in ANSINFPA 70- 1990 [51, ANSI C2-1990 [ll, and local codes and standards. Overhead high-voltage lines can

require significant right-of-way area, and the presence of energized overhead conductors significantly reduces the potential use of prop- erty located under the conductors. 3.3.4 Surge (Lightning) Protection. Light-

ning protection for outdoor substation facilities should be provided as indicated by the isokeraunic level (thunderstorm daysjyear) for the particular site location. The local utility is a good source of information on local lightning frequency, outage records, and application of particular lightning protection methods (shield wires, masts, arrays, arresters, BIL levels, reclosing, etc.) to the local system. Procedures for the selection and application of silicon-carbide high-voltage surge arresters are provided in IEEE C62.2-1987 1213. Information about the application and selection of metal-oxide surge arresters is available from the manufacturers. A new guide for the application of metal-oxide arresters is being developed by the IEEE Surge Protective Devices Committee. 3.3.5 Transmission (or Distribution) Line

Termination. High-voltage transmission lines or medium-voltage distribution lines, whether overhead ox- underground, will usually constitute the most significant physical interface between an interconnection substation and electric utility. Particular attention is required to the interfacing span between the last utility tower and the substation termination structure for overhead lines.

For underground lines, the presence of existing underground utilities in the substation vicinity must be identified, which may impact the underground cable location and/or rating. For both overhead and underground lines, orientation of the substation with respect to the utility and windfarm lines should be ar- ranged to minimize line angles, turning towers, crossings, etc., all of which will affect the complexity and cost of the transmission facilities. 3.3.6 Switching Equipment. The final selec-

tion and arrangement of particular types of substation switching devices is closely related to the substation circuit breaker configuration, discussed in 3.2.1. Power circuit breakers are required for full-range current switching and fault interruption (see IEEE C37.010-1979 (Reaff. 1984) [161 and IEEE C37.012-1979 [Reaff. 1988) 1171). Circuit switchers are capable of load switching and low-level fault

23


lEEE std 1094-1993 IEEE RECOMMENDED PRACTICE FOR THE EWCTRICAL DESIGN

interruption and may be suitable for interrupting line-charging current and switching capacitors. Disconnecting switches are also suitable for interrupting line-charging current, although to a lesser degree, and for switching transformer magnetizing current when quick break (high-speed) interrupting devices are added to the switch. However, they are normally used only to isolate power transformers, switching apparatus, lines, and buses and do not typically serve as a principal switching device. 3.3.7 Instrument Transformers. Instru-

ment transformers are devices that produce a reference low-voltage potential or current source as required for protective relaying, synchronizing, metering, control, and indication functions (see IEEE C57.13-1978 (Redf. 1986) [201).

Potential sources can be oil-filled or dry- type (epoxy or rubber insulated) voltage transformers, coupling capacitor voltage transformers (CCVTs) (see ANSI C93.1-1981 [31 and ANSI C93.2-1976 [41), or bushing potential devices. Voltage transformers are the most accurate devices and are usually required for revenue metering. Bushing potential devices, transformer or breaker mounted, are usually suitable only for indicating the presence of voltage, but they are not adequate for major relaying or metering functions.

Current sources can be separately mounted oil-filled or dry-type (epoxy or rubber insulated) current transformers o r bushing current transformers (BCTs). Again, separately mounted current transformers are normally the most accurate type and are usually required for revenue metering. BCTs are mounted in circuit breakers and power transformers and are suitable for vast majority of relaying, control, and indication functions.

Protection, instrumentation, and control circuits should be designed to minimize the possibility of open-circuiting the secondary of a current transformer. An open secondary may develop hazardous voltages, resulting in equipment damage and danger of fire. 3.3.8 Protective Relaying. Protective relay-

ing is necessary to detect system disturbances and initiate or supervise proper response from power switching equipment. Protective relaying application is probably the most complex aspect of power systems design. Successful operation of a windfarm requires close coordi-

nation of utility and windfarm protective relaying facilities and practices. Information on protective relaying for consumer-utility interconnections is given in IEEE C37.95-1989 [ l l l and the Power Engineering Society Spe- cial Publication “Intertie Protection of Con- sumer-Owned Sources of Generation, 3 MVA or Less” [411. A brief description of the principal protective relaying categories will be found in the following subsections.

3.3.8.1 Line Protection. Line-protective relaying monitors the conditions of a transmission line and initiates isolation of the line if a fault or other disturbance is detected. Transmission and distribution line disturbances are commonly caused by lightning, fog, trees, animals, motor vehicle accidents, etc.

3.3.8.2 Transformer Protection. Trans- former-protective relaying monitors the internal condition of a power transformer and initiates i ts isolation in the event abnormal conditions are detected. These can indicate overloading or an internal fault. For further information, refer to IEEE C37.91-1985 [lo].

3.3.8.3 Bus Protection. Bus-protective relaying monitors the integrity of a power bus. Detection of a bus fault will initiate isolation of the bus. Complete information on bus protection application is given in IEEE C37.97-1979 ( R e a . 1984) [121.

3.3.8.4 Synchronizing/Synchro Check. Synchronizing or synchro-check relays are necessary to verify the phase relationship of interactive power systems o r synchronous generators before permitting the interconnection of two energized (interactive) systems.

3.3.8.5 Reclosing. Reclosing relays acti- vate power circuit breakers t o re-energize transmission lines that have been isolated due to the occurrence of a fault. Reclosing is sometimes desirable t o support continuity of service and to maintain stability of the interconnected system. The concept of rapid reclosing is based on the transient nature of most line faults. Particular care must be used in applying reclosing t o generator interconnections (see 2.3.6.).

3.3.8.6 Islanding. Windfarm generating stations normally operate interconnected with an electric utility system. Abnormal conditions within the utility system may cause por- tions of the utility system to become separated from the main network, but remain connected

24


AND OPERATION OF WINDFARM GENERATING STATIONS EEE

std 1094-1991

to the windfarm. The isolated system thus cre- ated is frequently termed an “island” (see 1.6).

Except in rare system configurations, islanding of a windfarm generating station is generally considered undesirable and should be avoided. The frequency or voltage in the island may become abnormal, in some cases reaching levels that could cause damage to the windfarm facilities or the equipment of the utility customer.

To avoid such possible damage, it is recommended that protective relays measuring voltage and frequency be used at windfarms t o establish a band of acceptable voltage and frequency. Deviations outside th i s band (frequently termed a voltagelfrequency win- dow) should result in disconnecting the windfarm from the utility system or in the shutdown of the wind turbines.

The frequency and voltage relay settings to be used should be agreed upon between the windfarm and the utility. If there is concern that utility reclosing may suddenly reparallel the utility with the island without synchronizing, tight tolerances such as k0.5 Hz may be desirable to try to avoid any sustained islanding. The disadvantage to such tight tolerances is that other utility system disturbances may cause unnecessary tripping of the windfarm generation.

3.3.9 Metering. The following forms of metering will generally be provided as part of any windfarm generating facility.

3.3.9.1 Interconnection Revenue Meter- ing. Utility revenue metering should be located at the interconnection facility. Typically, this consists of a single metering installation for each interconnection facility. The revenue parameteds) to be metered depends on the specific interconnection agreement and could include kilowatt-hours (kWh), kilovar-hours (kvarh), kilowatts (kW), kilovars (kvar), kilovolt-amperes (kVA), or power factor, in almost any combination. Since bidirectional power flow is possible, separate detented meters to monitor flow in each direction may be required. Also, if time-of-day (also called time-of-use) pricing is included in the rate structure, meters that record power flow as a function of time may be needed. Revenue metering requires accurate metering equipment, including current and voltage transformers rated for metering ser-

vice. Consideration should be given to possible metering errors introduced by harmonics in the current or voltage.

Most utilities have well-defined requirements for the configuration, type, and location of revenue metering equipment. Revenue metering should normally be located in a n area accessible to both owner and utility repre- sentatives. In most instances, the utility owns, monitors, a n d maintains the revenue metering.

3.3.9.2 Monitoring of Parameters, En- ergy, and Status. Parameter monitoring is provided to indicate the presence and magnitude of a given parameter on specific lines, buses, and equipment. Parameters that could be measured are voltage, current, kilowatts, kilovars, kilovolt-amperes, and power factor. Energy monitoring should include kilowatt- hours and kilovar-hours or kilovolt-ampere hours. Status monitoring includes the discrete (open-closed) status of equipment such as power circuit breakers, electrically-operated load break, and disconnecting switches.

3.3.9.3 Telemetering. The transmission of parameters, energy measurements, or status indications t o a remote location is called telemetering. The parameters that could be transmitted to a remote location are those identified in 3.3.9.1 and 3.3.9.2.

The remote location that is likely to be the destination for the data would be a utility dispatch center or the remote controVmonitoring facility of the windfarm. Transmission of data can be accomplished by dedicated communications lines, power line carrier equipment , radio t ransmission, microwave facilities, or optical fiber circuits.

3.3.9.4 Production Metering. It may be necessary to know the energy production of individual wind turbines for administration of the windfarm. This can be accomplished by the installation of kilowatt-hour meters at each turbine or by the use of watthour trans- ducer outputs transmitted to a central data collection point.

3.3.10 Control and Communications. Con- trol facilities are necessary to provide the hu- man interface for the operation of windfarm interconnecting facilities. Utility companies may require remote metering, monitoring, and control capabilities for windfarm generating facilities in order t o satisfy overall system safety and operations needs.

25



3.3.10.1 Local Control. Local control is usually accomplished by a direct electrical, mechanical, pneumatic, or hydraulic device and is initiated right a t the controlled equipment or a t some control facility in the imme- diate vicinity of the equipment. I t is the least sophisticated and most reliable control mode. “Remote control” is an extension of local control that implies a greater distance between the controlled device and the controlling point.

3.3.10.2 Supervisory Control. Supervisory control, also generically referred to as SCADA (Supervisory Control and Data Acquisition), is the long-distance control and data transmission methodology for distances much greater than tha t possible with direct localhemote control. As opposed to hard-wired direct localhemote control, which utilizes a dedicated wire pair o r similar dedicated communication medium for each control or data signal, supervisory control utilizes mul- tiplexing technology to superimpose multiple signals on a single communication channel. This requires an RTU (remote terminal unit) a t each terminus to collect, process, and transmit the appropriate input and output signals. Supervisory control is generally required for a utility interconnection that includes remote utility control of selected customer-owned equipment, such as high-voltage power circuit breakers.

3.3.10.3 Remote Dispatch. Remote dispatch is the control of generating equipment in accordance with the instructions received from a remote central control location, such as a utility dispatching center. Remote dispatch can reasonably be considered as part of the operations functions of any large generator connected to an operating utility system.

3.3.11 Lighting. Appropriate minimum lighting levels for various indoor, outdoor, and roadway areas a re given in IES (Illuminating Engineering Society) standards, as well as other industry sources. Emergency lighting must be provided in at- tended areas, in accordance with local codes and regulations.

4. Automatic Control and Monitoring systems

Wind turbines installed in a windfarm must be controlled so that the generators can

produce electrical power when wind conditions are favorable and can be disconnected from the electric utility system when the wind conditions are not favorable. The operator of a windfarm needs t o monitor the turbines to determine which turbines are operating properly and which require attention. Many of these functions can be accomplished manually, but as the number of wind turbines in a windfarm increases, the task of performing these functions becomes very large. A central automatic control and monitoring system in which some functions are performed a t the windfarm control building and other functions are dis- tributed a t the individual wind turbines may give more efficient and flexible control. An automatic monitoring system also allows performance evaluations of individual and groups of wind turbines.

4.1 Wind-Turbine Control System. The most common purposes of a wind-turbine control system are t o optimize the energy captured by the wind turbine, to insure a safe turbine shutdown when required, and to regulate the power output of the generator. The following is a general list of functions that may be included in a wind-turbine control system to maintain the wind turbine within its allowable operating parameters.

(1)

(2)

(3) Rotor overspeed detection (4) Turbine startlstop control ( 5 ) Turbine vibration detection (6) Generator overpower/current detection (7) Single phasing and phase reversal

detection (8) Yaw control (for horizontal axis

turbines) (9) Blade pitch control (for variable pitch

turbines) (10) Generator synchronizing control (for

synchronous generators) (11) Turbine brake control (12) Generator voltagehar control

Measurement of wind speed a t turbine site Detection of interconnection with col- lector system or loss of load

The specific control requirements for a particular wind turbine are not within the scope of this document. The manufacturer of the wind

26


AND OPERATION OF WINDFARM GENERATING STATrONS IEEE

std 1094-1991

turbine should be consulted about the specific control requirements of the wind turbine.

There are several items to consider in the control of wind turbines. These are start/stop algorithms to optimize energy capture, real and reactive power regulation, simultaneous starting of multiple turbines, energizing a de- energized feeder, and operation in an islanding condition.

Starthtop control algorithms must determine when to start or synchronize a wind turbine so that the turbine can generate power. Also, the algorithm controls when to stop or separate the wind turbine from the utility system. The control system must attempt to determine the mean wind speed even when the wind is gusting. The control system must determine whether the wind energy available is sufficient to produce power or if too much wind energy is present to operate the wind turbine. This decision is a trade-off between start-up cost, energy production revenues, and fatigue life consumption. S t ads top algorithms may have to take into account the wind direction and the effects of shadowing of the wind turbine or i ts control anemometer by another turbine.

Since the energy available in the wind is proportional to the cube of the wind speed, variations in the wind can have large effects on the output power and var requirements of a windfarm. Var and voltage control may be required or may be economical due to reduc- tions in collection system losses or to incen- tives that the utility may impose (see 2.5).

Some turbines s ta r t aerodynamically, others start by motoring the generator. When a wind turbine starts by motoring the generator, the generator can draw several times the normal full load current for several seconds. This large current can cause the voltage on the line feeding the turbine to drop because of the resistance and reactance of the line. If several of the wind turbines in a windfarm start a t the same time, the voltage could drop to unaccept- able levels, with possible damage t o the generators.

A utility or windfarm feeder may be out of service due to the operation of a protective device (i.e., fuse, circuit breaker, recloser), a line failure, or an interruption. The energizing of a feeder while a repairperson is working on the line could cause serious injury or death. If a wind turbine is capable of starting

without auxiliary power from the feeder, the controller must prevent the turbine from connecting to the line when the feeder is de- energized.

If one or more operating wind turbines become separated from the utility system and do not shut down, the turbines will be operating without the voltage and frequency regulation that the utility system provides. This situation is called islanding (see 1.6). In a windfarm, islanding may be difficult t o detect with simple relaying because the operating generators may mimic the presence of the utility generating system. The wind-turbine control or protection system must be designed to detect islanding and shut down the affected turbine (see 3.3.8.6).

4 2 Central Control of Multiple Units. Some or all wind-turbine control functions of two or more machines can be combined to reduce the cost of individual controllers and sensors and to reduce the impact on the utility system due to variations in wind and turbine starting requirements (see Figs 8 and 9.)

For wind turbines utilizing anemometers in their startistop control algorithms, one anemometer may be utilized for control inputs for two or more turbines, reducing the cost over individual anemometer control inputs. Mi- crositing considerations will influence the number and locations of wind turbines that may share anemometer control inputs. For turbines exposed to the same wind conditions, one wind turbine can sometimes be utilized as a “scout” to determine if other turbines should be started.

4.3 Central Monitoring System. A windfarm may utilize a centralized monitoring system to collect data on the status of the turbines in the windfarm. This information may be used by a central control system, performance tracking system, or a maintenance coordinating system.

4.4 Communication System. There are several options available for communicating between the different wind-turbine sites on a windfarm. These include utilizing dedicated metallic conductors extending between the turbines and a central site, microwave trans- mitters and receivers, fiber-optic cables, and carrier wave signals on the electrical power

27


lEEE std 1w-1991 JEEE RECOMMENDED PRACTICE FOR "HE ELECTRICAL DESIGN

ANEMOMETER

TURBINE

J

WINDFARM COLLECTION

SYSTEM ANEMOMETER

I A

~

L- TURBINE 1 CONTROLLER e

Fig 8 Local Conhl

lines. Each of these options has its own advan- tages and disadvantages, depending on the requirements for t he system and the environment.

5. Operations and &&+Work Procedures

6.1 Communication With Utility. Good communication and a good working relationship with the interconnecting utility will facilitate the efficient operation of the windfarm. The windfarm operator should develop and maintain contacts with the utility operating personnel and supervisors responsible for service to the windfarm. Several different departments within the utility (e.g., distribution, transmission, substations) may be involved, depending upon the voltage level and complexity of the interconnection.

The utility personnel may request information from the windfarm operator about faults

within the windfarm that were noticed by utility monitoring equipment. They may also wish to know about scheduled equipment outages that affect the windfarm production. Likewise, the windfarm operator may desire information about the cause and expected duration of interruptions to the utility interconnection. If agreeable to the utility, it may be desirable for the windfarm operator to have a telephone number for the central or regional dispatching center of the utility in order to have 24 h communication of operating information.

5.2 Coordination of Maintenance and Opera- tion. In addition to the design coordination necessary to realize the physical and electrical interfaces between the customer facilities and the utility, it is necessary to coordinate normal maintenance and operation functions. The details of such coordination are usually outlined in an operational agreement to which all parties are obligated.


AND OPERATION OF WINDFARM GENERATING STATIONS Em

std 1044-1991

WIN DFARM

SYSTEM COLLECTION -

1

TURBINE CONTROLLER \

v \ ANEMOMETER r

1

TURBINE CONTROLLER

Fig9 Centtal Control

Particular items that need to be covered by an operational agreement are:

(1) Coordination of Switching: In order to avoid disruption of station and utility service and/or equipment damage.

(2) Tagging/Lockout: Clearly defined procedures for isolation of equipment and circuits as required for utility, owner, and contractor maintenance work.

(3) Personnel Safety and Training: State- ment of the minimum level of compe- tence required for operat ions, ma in tenance , a n d construct ion personnel.

(4) Access: Provision for both owner and utility access in all weather conditions.

5.3 Utility Clearance Procedures. When preparing to do work on de-energized equipment that is normally energized at 600 V or higher, utilities follow a highly formal proce-

dure. This procedure, often called a “clearance,” is intended to guarantee that the equipment is, and will remain, safe to work on. The clearance is issued by the central or regional dispatching center to the person who will work on the equipment or who will directly supervise the work. The procedure itself is usually written and the various steps checked off by both the dispatching center and the individual who will receive the clearance. The principal steps are

(1) Identifying the equipment to be cleared and the switches that must be opened to de-energize it; switches are often num- bered to facilitate identification.

(2) Checking every possible source of energy that might inadvertently energize the cleared apparatus, to be sure it is disconnected; the utility will consider the windfarm to be a possible source of e n e r gy .

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IEEE std 1094-1991

(3) De-energizing the equipment by opening all the identified switches, locking them open, and placing signs t o indicate that the switches must not be closed.

(4) Verifying tha t the equipment is de- energized by measuring the voltage with a suitable voltmeter or indicator.

( 5 ) Grounding all normally energized conductors through the use of grounding switches or portable grounding conductors; this is done to eliminate capacitively o r inductively coupled voltages as well as to provide an additional safeguard against inadvertent re-energization.

Depending upon the arrangement of the utility-windfarm interconnection, it may be necessary to open switches belonging to the windfarm in order to clear utility equipment or vice versa. In such cases, it will be necessary for the two parties t o agree upon who will operate the necessary switches. Regardless of who does the actual switching, it is vitally important that the utility and the windfarm understand and respect each other’s clearance procedures.

6.4 Intraplant Safe-Work Procedures. The windfarm operator should be concerned with the safety and health of each employee, contractor, and visitor entering windfarm facili

ties or field work locations. The operator should make every effort to do his or her best to identify and eliminate potential and existing hazards and t o reduce the number of employee injuries resulting from on-the-job accidents to an absolute minimum.

Employees should be responsible for their personal safety and the well being of co-work- ers through strict adherence to written, estab- lished safe-work procedures. Management should be responsible for establishing and monitoring these procedures. Additionally, employee groups should meet regularly to discuss and refine these procedures.

The turbine supplier or manufacturer may have special procedures for operation and maintenance of the turbine and its acces- sories. Additionally, the suppliers of other major electrical and mechanical equipment may have recommended safe-work procedures. These procedures should be carefully inte- grated with the windfarm’s own operating and maintenance procedures.

Depending on state and local jurisdiction, the installation may be required to conform to and use ANSI C2-1990 [ll. This code addresses design, maintenance, and work rules as applied t o high-voltagehigh-power electrical systems. Other local and national codes may also apply to the installation; local code en- forcement authorities should be consulted as to applicable codes and standards.

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ieee recommended practice for the electrical design and ... · pdf file[261 ieee std 80-1986,...

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