AckermannTh. (ed) Wind Power in Power Systems

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the active power levels of all wind turbines in a wind farm is reduced to 5 s and in the

affected group to 20 s this constitutes a value equivalent to the requirement presented

above regarding active power supply.

If individual wind turbine designs are unable to comply with the requirements, it may

be reasonable to switch such turbines selectively into backup voltage operation during a

brief period, thereby providing relief to the network voltage. In order to protect regional

supply structures and wind turbines during faults with voltage drops below 45 % of the

nominal network voltage at the PCC, it may also be reasonable in such cases to permit

a very brief disconnection of the affected turbines or farm followed by a fast return to

the pre-fault operation point.

This, however, requires a rapid resynchronisation of the wind generators to the

network after the fault is cleared and a fast return to the normal pre-fault operation

in order to ensure that the entire outage does not last longer than 10 s. In this case, the

turbines or farm must start supplying active power within 2 s after fault clearing. Active

power has to increase at a gradient of at least 10 % of nominal active power. In most

cases, the fast return to pre-f ault active power levels and frequency stability relating to

voltage quality must be given first priority. In cases of slow voltage recovery (e.g. when

the voltage remains below 10 % of the nominal voltage after the fault is cleared) voltage

stability and local voltage quality will be given first priority for the return of the wind

farm to normal operation.

Regarding the forecast for the year 2005, the results of the investigation show that the

stable turbine group will have to account for approximately 2 800 MW of installed

capacity, which can very rapidly return to its pre-fault operating point after a fault is

cleared. There are only about 200 MW in the affected group, which in the case of severe

network failures will be allowed a longer time to restore active capacity, because of the

behaviour of these turbi nes. This will not affect stationary frequency deviation, if the

stable group increases active power supply wi th a gradient of 20 % of nominal power.

Overloads on interconnecting feeders and the effect they have also need to be studied.

The settings for overcurrent time protection (I very large) have to be included as a

reserve protection for the remote protection of the transmission lines, because they can

cause a circuit switch-off in the few cases where the current in the protection-end stage

(3 s) reaches 1.4 times the nominal current value, unless the conductor current returns to

below 0.9 times the nominal current value.

Simulations for the year 2005 show that no trip of the overcurrent time protection on

the affected interconnecting lines is to be anticipated through compliance with the

described measures (Figur e 11.16). Further studies are required to study this phenom-

enon in Germany (Deutsche Energieagentur, 2004).

If all installed wind farms comply with the requirements presented here, a consider-

able improvement of the overall behaviour is to be expected in comparison with

the current situation. Auxiliary wiring configurations and changes to the regulating

design and/or the layout of the wind turbines may be necessary to comply with the

requirements.

Wind farms that do not have regulation capabilities will be able to comply with the

requirements if they use dynamic compensation facilities (controllable elements,

FACTS), or merge with wind turbines or wind farms that can be controlled. An

additional increase in wind power through repowering older wind turbines using

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modern wind technology, in combination with the above-mentioned requirement s

regarding the dynamic behaviour of wind farms, will allow a con siderable reduction in

the maximum expected win d power outage capacity. This means that, in turn, such

system-defined limits as voltage quality, voltage and frequency stability will continue to

be fulfilled in the future.

The requirements regarding the dynamic behaviour of wind turbines in the event of

network failures are a first step regarding regulations throughout the European

Communities as a whole. Over the next few years, large network studies with detailed

representations of wind turbines will be carried out to take into account the increas-

ing amount of onshore and offshore wind power and modify the requirements

accordingly.

11.9 Additional Dynamic Requirements of Wind Turbines

The dynamic requirements for wind turbines installed in the E.ON control area are

based on dynamic network studies. They describe the behaviour of wind turbines in the

event of network failures. For the current and future integration of large amounts of

wind power into the European transmission system, wind turbines have to comply with

the following regulations [14,15,16,17]:

Three-phase short circuits close to power stations shall not as a general rule lead to

generation unit instability nor to disconnection from the network above the lower solid

line in Figure 11.17. Active power capacity must begin directly after fault recognition

and be increased at a gradien t of at least 20 % of the nominal capacity per second. The

active power increase can take place at a rate of 5 % of the nominal capacity per second

within the hatched area in Figure 11.17.

Full active power capacity returns in 30 s

Full active power capacity returns in 10 s

200

160

120

100

Line current (%)

0.0

2.5

5.0

10.0

7.5

Time, t (s)

Line overloadings due to primary control

Critical Range

Tripping of the line protection

Figure 11.16 Overloading of coupled lines after a three-phase fault in the 380-kV transmission

system: line current as a percentage of nominal current as a function of time, t

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Short circuits not close to power stations shall not result in a disconnection of the

generation unit from the network, even with failure correction in the end stage of

network protection of up to 5 s. The stipulations concerning voltage support must be

complied with at the same time.

The generation units sh all provide voltage support in the event of three-phase short

circuits in the network. A switchover to voltage support is to take place in the event of a

voltage drop of more than 10 % of the momentary value of the generator terminal

voltage. Support of network voltage must take place within 20 ms following failure

recognition by supplying reactive current with a factor of 2 % of the nominal current for

each percent of the voltage drop. After 3 s, it is allowed to switch back from voltage

control to normal operation.

The transient compensation processes in relation to the reactive power consumption

following voltage return in quadrant II has to be completed within 400 ms, and the

demands regarding voltage support require the exchange of reactive power. Exceptions

to these basic principles are possible and ha ve to be agreed upon. The following

stipulation applies in such cases:

Within the broken-lined area of Figure 11.18, wind turbines may be disconnected

briefly from the transmission system if resynchronisation of the turbines following

failure correction does not exceed 2 s and the active power capacity is increased at a

gradient of at least 10 % of the installed nominal active capacity per second.

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.5

1.0 1.5 2.0 2.5

3.0

0.7

0.15

0.45

No Disconnection of wind turbines

above the solid line

Active power recovery directly after fault clearing:

Backup voltage operation

Fast active power return

Switching between normal

and back-up-voltage operation

- only short disconnection of the generator allowed

Time, t (s)

Voltage, U (p.u.)

Figure 11.17 Progression of the line-to-line voltage: above the solid line, wind turbines are not

allowed to disconnect automatically in the case of grid failures

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11.10 Conclusions

The political support for the use of renewable energies and the promotion of the use of

wind power has led to a substantial increase in wind power supply in Germany and

especially in the E.ON Netz control area. When transporting and distributing this

electricity, the grid reaches its technical limits, and bottlenecks are created on the grid.

For a transitional period, these bottlenecks can be handled by using local generation

management. In the medium and long term, however, the development of wind power

will require a considerable strengthening of the HV and EHV systems in order to ensure

an appropriate quality of supply in the future.

The marked growth in wind power supply increases the need for power station reserve

capacity, which is required in cases when the actual values of wind power supply differ

from predicted wind power supply. The provision and use of such power station reserve

capacity generates additional costs to the TSO.

In the future, wind turbines will have to be integrated to a larger extent into the

system control mechanism and they will have to comply with requirements that are

designed to guarantee secure operati on of the interconnected grids. Against this back-

ground, E.ON Netz has identified and set up specific requirements for wind turbines

and wind farms to provide system services. This has been done in close cooperation with

different manufacturers of wind turbines and with other TSOs.

However, further studies are necessary to follow developments in the wind sector.

These investigations may lead to a step–by-step upgrade of the requirements regarding

the capability of wind turbines and wind farms to support the overall system, especially

in the case of offshore wind power integration and its connection to the grid.

High voltage level

Reactive power generation

0.5

–1.0

Dead band

Normal operation

Switch to backup

operation

U/U

Low voltage level

Figure 11.18 Contribution to backup voltage operation in the case of network failures: plot of

voltage, U, is a percentage of nominal phase-to-phase voltage, U

, against reactive current, I

,asa

percentage of rated current, I

, for voltage drops caused by load flow variation, switching

operations and short-circuit faults

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References

[1] Bachmann, U., Erlich, I., Grebe, E. (1999) ‘Analysis of Interarea Oscillations in the European Electric

Power System in Synchronous Parallel Operation with the Central-European Networks’, paper

BPT99-070-12, presented at the IEEE Power Tech 9 Conference, Budapest, Hungary, 1999.

[2] Bachmann, U., Breulmann, H., Glaunsinger, W., Hoiss, P., Lo

sing, M., Menze, A., Ro

melt,

S., Zimmermann, U. (2002) ‘Verbundstabilita

t nach dem Synchronanschluß der Netze der Elektrizita

ts-

unternehmen von Bulgarien und Ruma

nien an den UCTE/CENTREL-Verbund’, in ETG-Tage 1999,

ETG-Fachbericht 79, pp. 251–257.

[3] Bouillon, H. (2002) Auswirkungen des fluktuierenden Energieangebotes auf den Systembetrieb, VGB-

Tagung, Salzburg, Austria.

[4] Bouillon, H., Dany, G. (2003) ‘Ausgleichsleistungs- und energiebedarf in Regelzonen mit hohem

Windanteil’, Energiewirtschaftliche Tagesfragen 53 H9.

[5] Breulmann, H., Borgen, H., Ek, B., Hammerschmidt, T., Kling, W., Knudsen, H., Menze, A., Ross, H.,

Ring, H., Spaan, F., Winter, W., Witzmann R. (2000a) ‘New HVDC Power Links Between UCTE and

NORDEL – Analysis of AC/DC Interactions in the Time and Frequency Domains’, paper presented at

Cigre Conference 2000, Paris.

[6] Breulmann, H., Grebe, E., Lo

sing, M. Winter, W., Witzmann, R., Dupuis, P., Houry, M., Margotin, P.,

Zerenyi, J., Dudzik, J., Martin, L., Rodriguez, J. M. (2000b) ‘Analysis and Damping of Inter-area

Oscillations in the UCTE/CENTREL Power System’, paper presented at CIGRE Conference, Paris,

2000.

[7] Deutsche Energieagentur (2004) ‘Energiewirtschaftliche Planung fu

r die Netzintegration von Windenergie

in Deutschland an Land und Offshore bis zum Jahr 2020, dena-final-report. 2004’. www.deutsche-energie-

agentur.de/

[8] E.ON Netz GmbH (2001) ‘Erga

nzende Netzanschlussregeln fu

r Windenergieanlagen’, available at http://

www.eon-net2.com.

[9] E.ON Netz GmbH (2003) ‘Grid Code High and Extra High Voltage. Status 1. August 2003’, available at

http://www.eon-netz.com.

[10] Haubrich, H.-J. (2003) ‘Technische Grenzen der Einspeisung aus Windenergieanlagen’, expert opinion,

RWTH, Aachen.

[11] IEEE (Institute of Electrical and Electronic Engineers) (1987) ‘Committee Report: Computer Represent-

ation of Excitation Systems’, IEEE Transactions PAS-87, No. 6, 1460–1464.

[12] IEEE (Institute of Electrical and Electronic Engineers) (1992) ‘Committee Report: Dynamic Models for

Steam and Hydro Turbines in Power System Studies’, IEEE Transactions PAS-92, No. 6, 1904–1915.

[13] Lerch, E., Kulicke, B. Ruhle, O., Winter, W. (1999) ‘NETOMAC – Calculating, Analyzing and Optimiz-

ing the Dynamic of Electrical Systems in Time and Frequency Domain’, paper presented at 3rd IPST ’99,

Budapest, Hungary.

[14] Luther, M., Winter, W. (2003a) ‘Einbindung großer Windleistungen – Systemverhalten und Maßnahmen

zur Erhaltung der Versorgungsqualita

timU

bertragungsnetz’, ETG-Tagung Zuverla

ssigkeit in der Strom-

versorgung, Mannheim.

[15] Luther, M., Winter, W. (2003b) ‘Erweiterte Anforderungen an Windenergieanlagen zur Aufrechterhal-

tung der Systemstabilita

t’, paper presented at HUSUMWind 2003 – der Kongress, Husum, Germany.

[16] Luther, M., Winter, W. (2003c) ‘System Operation and Network Development with Large Scale Wind

Generation’, paper WG B4:38 presented at the CIGRE Conference on the Simulation of HVDC and

FACTS, Nu

rnberg, Germany.

[17] Winter, W. (2003) ‘Grid Code Requirements to Wind Generators’, paper WG C6 presented at the CIGRE

Conference on Dispersed Generation, Montreal, Canada.

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Wind Power on Weak Grids in

California and the US Midwest

H. M. Romanowitz

12.1 Introduction

The first large-scale deployment of wind energy in wind farms worldwide occurred in

California during the period 1982–86. A unique combination of Federal and State incentives,

combined with standard offer power purchase agreements (PPAs) created an environment

that fostered rapid development with early technology, much of it unproven. The perceived

abuses such rapid development fostered brought these incentives to an end by late 1985 and

wind energy development stalled. These pionee ring efforts demonstrated critical business

and technology lessons on a massive scale and laid the foundation for future success.

This chapter looks at the historical practical evolution of wind energy from the

perspective of an operator and developer struggling through early problems and uncer-

tainties arising from integrating wind energy into weak rural grids on a large scale. It is

important to recognize the early state of grid technology, compared with what is

available today. Grid protection assumed load and no generation. Monitoring and

communication was poor and slow, leaving g rid operators largely unaware of critical

operating circumstances in real time. Grid models were primitive and did not accurately

reflect the impact of a large proportion of induction machines on the grid. Adequate

VAR support was feared, and VAR transport relationships were not well understood.

Last, those responsible for grid investment questioned that wind energy would ever be

anything other than a passing fad, to be tolerated but certainly not accommodated with

investment in adequate facilities.

California has three major wind energy resource areas. The Altamont and San

Gorgonio areas are located within the two major California load centres, and adjacent

to major 500 kV substations (see Figure 12.1). Tehachapi, the most developed wind

Wind Power in Power Systems Edited by T. Ackermann

Ó 2005 John Wiley & Sons, Ltd ISBN: 0-470-85508-8 (HB)

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ALTAMONT

TEHACHAPI

ANTELOPE VALLEY

Lancaster

Coram

Gold Town

Rosamond

Del Sur

Antelope

Vincent

230 kv

230 kv Sagebrush

230 kv

Big Creek

Path 26 (3) 500 kv

SANGORGONIO

Southern California

Load Centre

Northern

California

Load Centre

Malin

Olinda

Round Mt.

Table Mt.

Tracy

Telsa

Captain Jack

Vaca

Dixon

Metcalf

Los

Banos

Wind Area

Gates

Midway

Diablo

Canyon

Magunden

Lorraine

Monolith

Tehachapi

Westwind

Cal Cement

Skyriver

Valley

NGila

Miguel

Devers

Imperial

Valley

Serrano

Rinaldi

Pardee

Sylmar

Loma

Lugo

Victorville

t p

ullough

Eldorado

Mohave

alo

Verde

Mira

PATH 15

PATH 26

Adelanto

ntelope

TEHACHAPI AREA

(2)

Private Line

500 kVAC

Proposed 500 kVAC

500 kVDC

Moss

Landing

rivate Line

Figure 12.1 California simplified one-line 500 kV grid, major wind resource areas and load

centres; Tehachapi grid, including 66 kV subtransmission spread across mountains and desert

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resource area, is also the one with by far the greatest potential for additional develop-

ment. While not remote, the area has not been well integrated into the main grid and is

connected by a weak 66 kV system.

Tehachapi presents several interesting and instructive lessons. From necessity, the

wind industry has worked closely with the local utilit y to maximize transfer capability of

the existing 66 kV grid with modest upgrades, and may have achieved the greatest

transfer capability as a percentage of short-circuit duty of the grid, largely with Type A

wind turbines and a small early flexible alternating-current transmission system

(FACTS).

(1)

A small group of Tehachapi developers built a private 230 kV transmission

line that has proven to be extremely successful, and presents an expansion opportunity.

Tehachapi sits closely adjacent to the major north–south transmission grid of the state

(see Figure 12.1) and is an opportunity for substantial effective future integ ration, if

planning and seams interzonal issues can be resolved.

During the later part of the 1990s, wind energy development again began to flourish

in the USA, with interesting and important developments in the US Midwest, and with

repowers in California. Projects in West Texas and Western Minnesota were de veloped

on weak grids utilizing Type C and D wind turbines with effective results from the

enhanced VAR capability of that technology. Projects in Wyoming and North Dakota

have utilized high-performance FACTS technology to enhance the weak grids asso-

ciated with those developments. Repowers in Tehachapi further increased utilization of

its weak grid, with Type A and Type B wind turbines, plus added VAR support internal

to the projects. The successful achievements from these projects give practi cal insight

into the necessity for and benefit of effective VAR control associated with wind energy

projects as they become increasingly large.

12.2 The Early Weak Grid: Background

12.2.1 Tehachapi 66 kV transmission

The Tehachapi Area is served by the Southern California Edison (SCE) company, being

on the northern edge of its service territory and immediately adjacent to the Pacific

Gas & Electric (PG&E) company, which serves most of Northern California. Service to

wind development has been provided by SCE on an existing 66 kV subtransmission

and distribution network that covers thousands of square kilometres immediately north

of Los Angeles (Figure 12.1).

The Tehachapi area 66 kV system gateway Cal Cement is 40 km from the source

Antelope Substation, a 230 kV substation 30 km from the 500 kV Vincent Substation.

A network of 66 kV lines winds a web across the desert and mountains of the area. Planned

expansion of the grid at 230 kV did not happen for a number of reasons, and currently

three 500 kV lines are being planned to serve the area, California’s richest wind resource.

SCE was obliged by the state to accommodate renewable energy suppliers, and from

1982 to 1985 signed over 900 MW of standard offer contrac ts for wind projects in the

Tehachapi area, without regard for transmission needs. The prim ary technical concern

(1)

For definitions of turbine Types A–D, see Section 4.2.3.

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covered rigidly in PPAs was with islanding, causing wind-farm-provided internal VAR

support to be severely restricted. Contractual VAR restrictions were aggressively

enforced, with the predominant Type A turbines being limited to supplying not more

than no-load VARs. In the early years, the interconnection agreement was an appendix

to the PPA, with all transmission rights firm.

12.2.2 VARs

VARs are reactive or imaginary power and do no useful work but are essential to properly

magnetize and control induction machines, transformers and other inductive devices.

VARs, effectively controlled, can cause induction machines to behave much like self-

excited synchronous machines. Effective VAR supply can control grid voltage, stabilize

the grid and improve grid quality. Chapter 19 has a detailed discussion of VARs.

12.2.3 FACTS devices

FACTS is emerging as a potentially very powerful tool that can materially improve the

performance of transmission and distribution systems. It can provide an alternative to

‘wire in the air’ transmission expansi on by unlocking thermal capacity in a line that

cannot be used with conventional expansion planning because of the impacts of

embedded impedance that is not accommodated conventionally.

Static VAR compensators are at the low en d of FACTS technology and have been

evolving for ne arly 30 years, but only recently have they become widely used. The

simplest SVC uses transient free thyristor switching of capacitors to provide automatic

VAR support. Such devices can be turned on, off and back on every other cy cle and can

provide responsive VAR support, including voltage control, to improve grid stability

very effectively while supplying the VARs needed by the system. A full-range SVC

includes a phase-controlled inductor and can thus also consume VAR, which may be

needed where lightly loaded transmission lines are highly capacitive and create

unwanted high voltages. The inductor makes the SVC stepless, while the capacitor-only

type utilizes one or more steps with granularity dependent on the number of steps.

Hybrid SVC is another variation where, in addition, conventional breaker-switched

capacitors are added but are controlled in harmony with the fast-switched capacitors to

extend the effective capacity at low er cost.

High-end FACTS employ an integrated gate bipolar transistor (IGBT) inverter to

couple a capacitor or other energy-storage device to the grid and provide two quadrant

stepless control of the VARs, so that they can be full range producing or consuming VARs.

An advantage of the IGBT type is that VAR current can be independent of grid voltage.

Thus the supplied VARs drop only as the first power of grid voltage instead of as the

square of voltage, as with directly connected capacitors or inductors. D-VAR

is the

trade name of one such FACTS device that has had notable success, (see Section 12.3.12).

(2)

D-VAR

is a trademark of American Super Conductor.

260 Wind Power on Weak Grids