AckermannTh. (ed) Wind Power in Power Systems

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Table 7.3 Overview of protection issues

Requirement Source

Fault tolerance:

3-phase faults on a random line or transformer with definitive

disconnection without any attempt at reclosing

Eltra

2-phase fault on a random line with unsuccessful reclosing.

3-phase fault in transmission network during 100 ms Eltra and Eltkraft

2-phase faults and 2-phase to ground faults for 100 ms

followed by another fault in 300–500 ms with a duration of

100 ms

WT has to have enough capacity to fulfil these requirements at

least in the case of:

two 2-phase or 3-phase short circuits during 2 min

six 2-phase or 3-phase short circuits with a 5 min interval

During asymmetrical faults in the system, the WF has to be

disconnected faster than the fault is cleared:

If the connection point is within 50 m of the WT transformer,

the cable between transformer and connection point has to be

equipped with its own current-measuring fault protection in

the connection point. If the length of the cable is substantial

compared with the outer network, this protection has to be

directional. Settings of the protection have to be coordinated

with the protection of the respective network.

If there is a risk of islanding, the fault protection that

measures neutral point voltage in the network has to be

installed at the connection point. The recommended setting

for the disconnection is 5 seconds.

Overcurrent protection:

This should take into account selectivity and provide

maximum protection and at the same time avoid disconnection

during normal operation. The following should be taken into

account:

Sintef, DEFU 111,

AMP

maximum in-rush current (see Table 7.2 on voltage quality)

maximum current at continuous operation (see Table 7.1 on

power control)

Tolerance of over frequency:

See Figures 7.1 and 7.2 All

Tolerance of undervoltages (during and after the fault); see

also Figure 7.5:

U ¼ 0 %, WF disconnection after > 0:25 s SvK (P

> 100 MW)

U < 90 %, WF disconnection after > 0:75 s SvK

U ¼ 25 %, WF disconnection after > 0:25 s SvK (P

< 100 MW)

U ¼ 0 %, WF disconnection after > 0:1 s Eltra

U < 60 %, ..., 80 %, WF disconnection after 2, ...,20s

U ¼ 20 %, WF disconnection after > 0:7 s E.ON

U < 80 %, WF disconnection after > 3s

(continued overleaf)

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Table 7.3 (continued)

Requirement Source

0% U < 90 %, WF (installed after January 2004)

disconnection after > 0:14 s

Scotland

15 %  U < 90 %, WF (installed before January 2004)

disconnection after > 0:14 s

U ¼ 25 %, WF disconnection after > 0:1 s Eltra and Elkraft

U < 75 %, WF disconnection after > 0:75 s

U < 90 %, WF disconnection after > 10 s

50 %  U < 90 %, WF disconnection after > 0:6 s ESBNG

Tolerance of undervoltages (general):

U < 90 %, WF disconnection after 500 ms Scotland

U < 80 %, WF disconnection after 3–5 s E.ON

U < 94 %, WF disconnection after 60 s

U < 80 %, WF disconnection after 0.2 s

U < 85 %, ..., 95 %, WF disconnection after 60 s

DEFU 111

U < 90 %, WF disconnection after 60 s Sintef

U < 90 %, WF disconnection after 10, . . . , 60 s Eltra and Eltkraft

U < 70 %, WF disconnection VDEW

Tolerance of overvoltages (general):

WF should not trip within normal operating voltage range,

including the 15-minute overvoltage (defined in grid code)

Scotland

For overvoltages (not specified) a protection time of 30–60

seconds

Eltra

U > 106 %, WF disconnection after 60 s Eltra and Elkraft

U > 110 %, WF disconnection after 0.2 s

U > 110 %, WF disconnection after 60 s AMP, Sintef

U > 120 %, WF disconnection after 0.2 s AMP, Sintef

U > 95 %, ..., 110 %, WF disconnection after 50 s

DEFU 111

U > 115 %, WF disconnection VDEW

Tolerance of overvoltages (island conditions):

U > 115 % (at 275 kV) or U > 120 % (at 132 kV), WF

disconnection after 250 ms

Scotland

U > 120 %, < 100 ms, voltage reduction Eltra

U > 120 %, disconnection after maximum of 100 ms (if

equipped with capacitor banks)

DEFU 111

U > 120 %, disconnection after maximum of 200 ms (if not

equipped with capacitor banks)

Power control:

Reduction to  20 % of maximum power in 2 s (by individual

control of each WT), after fault clearance

Eltra

Motor mode protection:

Disconnection after 5 s of operation in motor mode AMP

Settings for undervoltage and overvoltage protection required by DEFU 111 (DEFU, 1998)

must be adjustable within a given range.

Note:WT¼ wind turbine; WF ¼ wind farm; P

¼ rated power of wind farm; U ¼ voltage.

Sources: See Table 7.1.

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7.3.6 Modelling information and verification

The interaction between wind farm and power system during faults in the power system

is usually verified through simulations. In order to make such simulations possible, wind

farm owners have to provide the system operator with the necessary models.

In order to verify the wind farm model and the response of the farm to faults in the

power system, registration equipment has to be installed (see Table 7.4).

7.3.7 Communication and external control

In most regulations, the wind farm owner is required to provide the signals necessary

for the operation of the power system. Unlike the other aspects of regulations dis-

cussed above, the requirements on communication are quite similar in all considered

documents. Table 7.5 summarises the requirements.

–10

110

130

0.01 0.1 1 10 100 1000

Voltage (%)

Time (s)

SvK (power > 100 MW) Eltra SvK (power < 100 MW)

E.ON

Scotland, after

January 2004

Scotland, before

January 2004

Eltra and Elkraft ESBNG Vestas

Figure 7.5 Requirements for tolerance of undervoltages during and after a fault in the system as

well as technical capability of a standard Vestas V80 2 MW turbine and modified Vestas V80

2 MW turbine

Sources: See Table 7.1.

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7.3.8 Discussion of interconnection regulations

The brief overview of the interconnection regulations presented above shows that the

regulations vary considerably and that it is often difficult to find a general technical

justification for the different technical regulations that are currently in use worldwide.

This applies particularly to power quality regulations such as flicker and harmonic limits.

Many of the differences in the technical regulations are caused by different wind power

penetration levels in the national power systems and different power system robustness.

For instance, countries with a rather weak power system, such as Scotland or Ireland,

have to consider the impact of wind power on network stability issues, which means that

they require fault ride-through capabilities for wind turbines already at a lower wind

power penetration level compared with countries that have very robust systems.

Discussions with wind turbine manufacturers show that they would prefer a greater

harmonisation whenever possible, but they generally also say that they are able to

comply with the different technical regulations. They are, however, much more con-

cerned about the continuous changes in technical standards worldwide, the very short

notice given for updates and changes and the little influence distributed resource

manufacturers have on these aspects. Large wind turbine manufacturers, for instance,

employ 4–5 or even more expert s to keep track of the ongoing changes in technical

regulations and to document the technical capabilities of their wind turbines. Smaller

wind turbine manufacturers, which cannot employ as many experts in this area, often

stay out of certain national markets because they cannot follow the changes in regula-

tions and the corresponding required technical documentation and validation of the

technical capabilities of their product.

Table 7.4 Requirements regarding modelling information

Requirement Source

Modelling information (dynamic parameters):

The models have to be well documented and agree with tests on

corresponding WT prototypes.

Eltra, Scotland

If the WF consists of several WT types, models for each individual WT type

must be presented.

Eltra

Technical data of WF have to be documented. SvK, E.ON

Modelling information (fault contribution):

A detailed calculation of the fault infeed has to be provided to specify

3-phase short-circuit infeed from the system and how the value has been

determined.

Scotland

Fault recorder (recorded variables):

Voltage, active and reactive power, frequency and current in the WF

connection point; voltage, active and reactive power, rotating speed for a

single WT of each type within a WF.

Eltra

3-phase currents, 3-phase voltages and wind speeds. Scotland

Note:WT¼ wind turbine; WF ¼ wind farm.

Sources: See Table 7.1.

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In addition, new network interconnection regulations increase the costs of wind

turbines. According to wind turbine manufacturers, allowing wind turbines with doubly

fed induction generat ors to ‘ride through a fault’, as defined in many European regula-

tions for wind turbines, increases the total costs of a wind turbi ne by up to 5 %.

Hence, it can be summarised that interconnection regulations should be harmonised

in areas that have little impact on the overall costs of wind turbines (i.e. power quality

regulations). In other areas, interconnection regulations should take into account the

specific power system robustness, the penetration level and/or the generation techno-

logy. Therefore, interconnection standards of different countries may also vary in

future. It is important that national regula tions should aim at an overall economically

efficient solution; that is, costly technical requiremen ts such as a ‘fault ride-through’

capability for wind turbines should be included only if they are technically required for

reliable and stable power system operation.

Table 7.5 Requirements regarding communication and external control

Requirement Source

Variables required from wind farm:

Voltage SvK, Eltra and Elkraft, Eltra, ESBNG,

Scotland, E.ON

Active power SvK, Eltra, Scotland, AMP, Eltra and Elkraft;

ESBNG, E.ON

Reactive power SvK, Eltra, Scotland, AMP, Eltra and Elkraft,

ESBNG, E.ON

Operating status SvK, Eltra, Scotland, Eltra and Elkraft,

ESBNG

Wind speed Eltra, Scotland, ESBNG

Wind direction Scotland, ESBNG

Ambient temperature and pressure ESBNG

Generator transformer tap position ESBNG, E.ON

Regulation capability SvK

Frequency control status (enabled or disabled) Scotland

Abnormalities resulting in WF tripping or

startup in 15 min

External control possibilities

WF > 20 MW, manual, local or remote

control within 15 mins after the fault to allow:

disconnection from the network, connection to

the network and regulation of active and

reactive power output

SvK

It must be possible to connect or disconnect

the wind turbine generator externally

Eltra and Elkraft, Eltra, E.ON

Some of the requirements on external control possibilities have already been mentioned in the

preceding subsections in text.

Sources: See Table 7.1.

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7.4 Technical Solutions for New Interconnection Rules

In the following example, the control system of the recently installed Horns Rev off-

shore wind farm is briefly presented. The wind farm is the first wind farm that had to

comply with the requir ements outlined by Eltra (2000), the TSO in Western Denmark.

At the time of writing, the control system was still being tested, therefore, there were no

details available of practical experience.

The offshore wind farm Horns Rev is located approximately 15 km into the Nort h

Sea. The wind farm has an installed power of 160 MW and comprises 80 wind turbines

laid out in a square pattern. The turbines are arranged in 10 lines with 8 turbines each.

Two lines constitute a cluster of 32 MW, with the turbines connected in series. Each

cluster is connected to the offshore transformer substation where the 34/165 kV trans-

former is located (Christiansen and Kristoffersen, 2003).

From an electrical perspective, the wind farm had to comply with new specifications and

requirements for connecting large-scale wind farms to the transmission network. As dis-

cussed before, the TSO (Eltra) had designed requirements for power control, frequency,

voltage, protection, communication, verification and tests (Eltra, 2000). According to those

requirements, the wind farm has to be able to contribute to control tasks on the same level as

conventional power plants, constrained only by the limitations imposed at any time by the

existing wind conditions. For example, during periods with reduced transmission capacity in

the grid (e.g. as a result of service or replacement of components in the main grid) the wind

farm may be required to operate at reduced power levels with all turbines running. The wind

farm must also be able to participate in regional balance control (secondary control).

The general control principle of the farm has to take into account that the control

range of the farm depends on the actual wind speed. Furthermore, as the wind speed

cannot be controlled, the power output of the farm can only be downregulated. For

instance, if the wind speed is around 11 m/s, the power output from the farm can be

regulated to any value between 0 MW and approximatly 125 MW. In the following,

some of the key elements of the overall control strategy are presented:

7.4.1 Absolute power constraint

This control approach limits the total power output of the wind farm to a predefined

setpoint (see Figure 7.6).

7.4.2 Balance control

The balance control approach allows a reduction in the power production of the overall

wind farm at a predefined rate and later an increase in the overall power output, also at

a predefined ramp rate.

7.4.3 Power rate limitation control approach

This approach limits the increase in power production to a predefined setpoint

(e.g. maximum increase in power production 2 MW per minute, see Figure 7.7). It is

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important to emphasise that this approach cannot limit the speed of power reduction,as

the decrease in wind cannot be controlled. In some cases, however, this can be achieved

when combined with the delta control approach.

7.4.4 Delta control

This reduces the amount of total power production of the wind farm by a predefined

setpoint (e.g. 50 MW). Hence, if delta control is now combined with a balance control

approach, the production of the farm can be briefly increased and decreased according

to the power system requirements (see Figure 7.8). A wind farm equipped with such a

control approach can be used to supply automatic secondary control to a power system.

Figure 7.6 Absolute power control (Reproduced by permission of Vestas Wind Systems A/S,

Denmark.)

Figure 7.7 Power rate limitation (Reproduced by permission of Vestas Wind Systems A/S,

Denmark.)

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7.5 Interconnection Practice

While interconnection standards usually define the minimum requirements to be ful-

filled by the wind turbin es to be connected, they usually say little or nothing about the

approach used for interconnection analysis (e.g. how much capacity can be connected at

a given point, and how long this analysis may take).

A number of empirical studies (e.g. Alderfer Eldridge and Starrs, 2000; DOE, 2000;

rß et al., 2003) include a varie ty of case studies showing that wind farm project

developers often have long, fairly expensive, discussions with network companies about

the maximum capacity that can be co nnected at a proposed connection point. Typically,

network companies do not publish netwo rk data, hence it is difficult independently to

verify the capacity limit ations defined by network companies. If there is a considerable

lobby for a wind farm project , developers in some countries use public media to put

pressure on network companies to reconsider initial capacity limitations. In situations

with less public support, network companies may even consider not to react to any

network interconnection application. In Sweden, for instance, a local distribution

company did not react for 15 months to an interconnection application. After contin-

uous complaints to the network company, the network company finally rejected the

application. At the same time, however, the company informed the ap plicant in a

personal discussion that the interconnection is technically possible but that the network

company did not have any economic interest in an interconnection. Interestingly

enough, the network company directly suggested that the applicant should sue the

network company, because the network company meant that a legal clarification of

such cases would be required, because the Swedish regulator did not provide the

relevant clarification.

A possible solution to avoid such conflicts is a clear outline of the method to be

used for defining the maximum interconnection capacity at a given network point as

well as a definition of the maximum time for the network company to perform the

relevant studies. Ideally, the method as well as the maximum time of a response to an

interconnection application should be defined by a neutral authority, for instance a

regulator.

Figure 7.8 Delta control (Reproduced by permission of Vestas Wind Systems A/S, Denmark.)

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The regulator in Texas, for instance, has clearly defined methods of how to analyse

different interconnection requests. Figure 7.9, for instance, describes the methods for an

interconnection request for distributed resources on the customer side of the meter. The

regulations force the network companies to answer to any interconnection application

within 6 weeks. If the application requires a network integration study, an additional

6 weeks can be added to this deadline (PUCT, 1999).

In addition, the regulator should consider setting up an independent body to help to

clear up network interconnection disputes. Such a body could be approached by

Yes

Proposed system on

nonnetwork feeder ?

Is equipment precertified?

Is export <15 %

of Load?

No network study

No study fee allowed

Approve application

Network study allowed

Study fee allowed

Recommendation

Is distributed

generation < 25 %

of maximum short circuit?

Is distributed

generation <

500 kW?

Figure 7.9 Method for interconnection requests for distributed resources on the customer side of

the meter as outlined in regulations in Texas (PUCT, 1999)

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network companies as well as by project developers. In Germany, for instance, a

clearing office for network interconnection issues related to wind power has been

operating for many years now. For many years, the office was part of the University

of Aachen. It was recently relocated to the German Ministry for Environment, which is

the ministry in charge of renewable energy.

7.6 Conclusions

This chapter has presented a comparison of the existing regulations for the interconnec-

tion of wind farms to the power system. Most of the analys ed documents are still under

revision and will probably undergo some changes in the future. The comparison

revealed that the regulations differ significantly between countries. This depends on

the propert ies of each power system as well as the experience, knowledge and policy of

the TSOs.

In general, new interconnection regulations for wind turbines or wind farms tend to

add the following requirements:

to maintain operation of the turbine during a fault on the grid, know as ‘fault ride-

through’ capability;

to operate the wind turbine in the range of 47–52 Hz (for European networks);

to control the active power during frequency variations (active power control);

to limit the power increase to a certain rate (power ramp rate control);

to supply or consume reactive power depending on power system requirements

(reactive power control);

to support voltage control by adjusting the reactive power, based on grid measure-

ments (voltage control).

In general, the wind energy industry is able to comply with the increased requirements

outlined in new interconnection standards. However, in some cases this can increase the

total costs of a wind turbine or wind farm significantly. Hence, interconnection regula-

tions should increase requirements only if needed for secure operation of the power

system. In other words, countries with a very low wind power penetration do not

necessarily need to adopt similar interconnection requirements as countries that have

significant wind power penetration.

References

[1] Alderfer, B. R., Eldridge, M. M., Starrs, T. J. (2000) Market Connections: Case Studies of Interconnection

Barriers and their Impact on Distributed Power Projects, National Renewable Energy Laboratory, Golden,

USA, May 2000, available at http://www.nrel.gov/docs/fy00osti/28053.pdf.

[2] Bolik, S. M., Birk, J., Andresen, B., Nielsen, J. G. (2003) Vestas Handles Grid Requirements: Advanced

Control Strategy for Wind Turbines, European Wind Energy Conference, Madrid, Spain, 2003.

[3] California Energy Commission (2000) ‘Distributed Generation Interconnection Rules’, Committee Report

(Draft), Sacramento, CA, USA, September 2000.

[4] Christiansen, P., Kristoffersen, J. R. (2003) ‘The Wind Farm Main Controller and the Remote Control

System of the Horns Rev Offshore Wind Farm’, in Proceedings of the Fourth International Workshop on

140 Technical Regulations