voltage dips characterization at wind projects

7/29/2019 Voltage Dips Characterization at Wind projects

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Characterization of Measured Voltage Dips in Wind

Farms in the Light of the New Grid Codes

Emilio Gomez-Lazaro and Miguel CanasRenewable Energy Research Institute

Dept. of Electrical, Electronic

and Control Eng. EPSA

Universidad de Castilla-La Mancha

02071 Albacete (SPAIN)

Email: [email protected], [email protected]

Juan Alvaro Fuentes and Angel Molina-GarcaDept. of Electrical Eng.

Universidad Politecnica de Cartagena

30202 Cartagena (SPAIN)

Email: [email protected], [email protected]

Abstract Voltage dips are a major concern for some transmis-sion system operators. With increasing wind farm penetration inpower systems, many national grid codes are imposing require-ments for uninterrupted generation throughout power systemdisturbances.

The aim of this paper is to present the results of measuredvoltage dips using two power quality analyzers installed formonths in a Spanish wind farm. From the measured voltagewaveforms, voltage dips are characterized by extracting dip typeand characteristics.

I. INTRODUCTION

Voltage dips are short duration reductions in rms voltage

(between 10% and 90%) of the voltage at a point in the

electrical system, which lasts for half a cycle to 1 min, [1]. The

duration of voltage dips is described as the total time interval

between the point on wave of sag initiation and recovery,

[2]. They are caused by faults in the electric supply system

and the starting of large loads, such as motors. Voltage dipsare generally considered a power quality problem of equal

importance as long and short interruptions in the supply. The

interests in voltage dips are increasing because they cause

the detrimental effects on several sensitive equipments such

as adjustable-speed drives, process-control equipments, and

computers. Such equipments can be tripped when the voltage

drops below 90% of the rated voltage for longer than a

few cycles, [3], [4]. In accordance with the practice in wind

farms, wind turbines are disconnected from the grid when the

terminal voltage fell below 80-90%, [5], [6]. Even relays and

contactors in motor starters can be sensitive to voltage dips,

resulting in shutdown of a process when they drop out, [7].

Voltage dips are mainly due to short-circuits and earthfaults in the grid, [8]. These faults in the power system, even

far away from the location of the wind farm, or any other

power installation, can generate a voltage dip at the connection

point of the wind turbines. Factors governing the magnitude

and duration of voltage dips include the fault impedance and

location, the configuration of the power network, and the

system protective relay design and operation, [7]. This last

aspect is important since voltage dip condition lasts until the

fault is cleared by a protective device. Solutions to the voltage

dip effects must be implemented in the customer facility, since

although it is possible for the utility to reduce the number of

faults through design practices and specific equipment, it is

impossible to avoid faults on the power system, [7]. Therefore,

in wind farms, the solution must be installed at the wind farminterconnection point with the electrical grid or at the wind

turbine level.

The installed capacity of wind power generation has grown

very fast in the past years, increasing dramatically the level of

wind power generation into existing utilities network. On the

other hand, the rating of large wind farms reaches rating of

conventional generating units and this development has carried

out to requirements of how to connect wind farms to the grid.

Until now, these grid codes specified by the transmission

system operators mainly dealed with how a wind farm

should operate in steady state while requirements recently

imposed in some countries dealed with how a wind installation

response to grid faults must be addressed, [9]. These arecommonly referred to as the fault ride-through specifications.

Specifically, national grid codes are requiring uninterrupted

generation throughout power system disturbances supporting

the network voltage and frequency, and therefore, extending

characteristics such as low voltage ride through, or reactive and

active power capabilities, [9], [10]. In [9][13] grid connection

requirements in Spain, Denmark, Germany, Ireland, Sweden

and Scotland are studied. When the wind power penetration

level is high, the protective disconnection of a large amount of

wind power is an unacceptable consequence that may threaten

the power system stability. This is due to conventional power

plants will be not be able to support the voltage and the

frequency of the grid during and immediately following thegrid failure.

In Spanish case, REE the transmission system operator

Red Electrica de Espana grid code, recently approved, spec-

ifies that the wind farm must support voltage dips, at the point

of interconnection with the transmission network, without

tripping. The voltage-time curve that limits the magnitude

and duration of the voltage dips, produced by single-phase-to-

ground, two-phase-to-ground and three-phase short-circuits, is

shown in figure 1. For non-earthed two-phase short-circuits,


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Voltage(pu)

1

0.2

0.5 1 Time (sec.)

Beginning of

perturbation

0.8 0.95 pu

0 15

Fault clearance

Duration

of fault

Fig. 1. Voltage-time curve that the generation facility must support

the voltage limit is chosen at 0.6 p.u. instead of 0.2 p.u.

I I . VOLTAGE DIP CHARACTERIZATION

Voltage-dip characterization concerns the quantification of

voltage-dip events through a limited number of parameters,

[14]. Most methods for voltage dip characterization use two

parameters to quantify the severity of a voltage dip

magnitude (or remaining voltage) and duration, [3], [15]

[17]. However, voltage dips can be far more complicated than

this type of characterization can show:

Usually, methods are based on the lowest remaining

voltage and the longest duration of all the three voltages.

This causes some problems. For example, a voltage drop

in one phase is not distinguished from other in three

phases, or voltage dips due to an earth fault in a high-

impedance grounded network will be seen as equallysevere or even more severe than the dip due to a

short-circuit fault, [15]. Therefore, it is an appropriate

approximation for balanced dips, whereas the majority

of dips are unbalanced, [16].

Dip depth and phase-angle jump information can be

required along with start and end times, [15].

Therefore, some authors are researching other methods

to characterize unbalanced dips, being based on measure-

ments or applying the basic circuit theory in the faults,

[14].

It is usually assumed that the voltage profile during

voltage dip is rectangular, failing in the characterization

of non-rectangular dip, overestimating it, [3], [14]. Thiscan be important to many industrial customers with large

induction motor loads or in the case or wind farms.

This methodology is not adequate to multistage voltage

dips events, in which the fault can evolve to a different

type.

Basically, the input data to the characterization methods can

be fitted in:

Monitoring the lowest remaining voltage and the longest

duration of all the three voltages.

Monitoring all the voltages waveforms with an adequate

sample rate.

Monitoring of

V2d + V2q in a vector controller, [4].

This method is derived from space vector control usually

employed in the control of induction machinery. The

three-phase voltages are converted into one phasor with

two orthogonal components and synchronous reference

frame, which is locked via a phase-locked loop PLLIn [15] two methods to obtain three-phase voltage dip char-

acterization ABC classification and symmetrical com-

ponents classification were exposed and compared. It is

concluded that ABC classification due to its simplicity is

more used than the symmetrical components, being also more

intuitive and giving a good approximation about the evolution

of the dips along the different voltage levels of the network.

The ABC classification should not be viewed as a different

classification, since it can be considered as a special case

of the symmetrical components classification. However, this

classification is based on a simplified model of the network,

being not recommended in [15] for the classification of voltage

dips obtained from measured instantaneous voltages. ABCclassification considers seven types of voltage dips, by defining

complex voltages and phasor diagrams.

In [18], theoretical relations between the minimum phase-

to-neutral voltage VPN and the minimum phase-to-phase

voltage VPP are presented for the seven types of dips,

defined in [15]:

Type A: all phases experience the same retained voltage

and phase-angle jump.

VPP = VPN (1)

Type B: It is not common because it is seen only when

a line to ground fault occurs at the same voltage level orat a location connected by star-star transformer grounded

at both sides.

VPP =

1

2+ VPN

2+ 3

43

(2)

Type C: It is a reduction of the voltage in two phases. It

is caused by a line to line fault or by a propagation of

dip type B through a delta-star connected transformer.

V2

PP =4

3V

2

PN1

3(3)

Type D: It is caused by a propagation of a type C dip

through a delta-star windings connected transformer. It is

a voltage drop in one phase.

V2

PP =1

4+

3

4V

2

PN (4)

Type E: It shows a symmetrical relation between PP and

PN voltage like type A. This dip is rare as type B by the

same reasons.

Type F: It is a reduction of the voltage in one phase,

caused by the propagation of a line to line to ground


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Transformer

690/20 kV

Power converter

Crowbar

Dfig

Transformer

20kV/66 kV

Power quality analyzer Power quality analyzer

Wifi RouterWifi Router

GPRS Ethernet

Modem

GPRS Network

Internet

GPS Time

Synchronization

Fig. 2. Power quality analyzer installation scheme

fault through a delta-star connected transformer.

3V2

PP = (2 +1

3 )V2

PN +1

3VPN +

1

3 (5)

Type G: These dips are obtained from the propagation of

a dip type F through a delta-star connected transformer.

VPP = 0.0707 +

3.112 V2PN 0.3271.556

(6)

In this classification, voltage dip types D and F C and

G, are similar, making difficult their distinction from the

measurements without knowing the fault types that caused

them, [19].

III. RESULTS

Two power quality analyzers fulfilling IEC 61000-4-30class A accuracy, frequency synchronization, and absolute time

requirements have been installed in a Spanish wind farm.

These analyzers, with a 10MHz sample rate, are useful to

capture detailed voltage and current waveforms during the

voltage dip and the clearance of the fault, together with

powerful trigger options to obtain the entire transient. Figure

2 presents the power quality analyzer installation scheme,

showing a power quality analyzer connected in the nacelle

Gamesa G90 2,0 MW between the doubly fed induction


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generator and the 690/20.000V power transformer. This power

quality analyzer measures three stator voltages and currents,

together with a rotor line current and the DC bus voltage in

the rotor power converter. The other power quality analyzer

measures line voltages and currents at the wind farm electrical

substation.

Both power quality analyzers are linked via wifi to a

UMTS/GPRS modem, allowing the remote accessing to the

power quality analyzer configuration and their recorded data.

Figures 3 and 4 show the three-phase voltage waveforms,

the amplitude and the angle of the voltage space vector,

[9], of two measured voltage dips. The voltage space vector

is a complex magnitude obtained by combining the three-

phase instantaneous voltages va, vb and vc, representing

therefore the voltage dip uniquely through an instantaneous

complex vector, V(t). Once this complex voltage is given,the three instantaneous phase voltages can be obtained by the

projection of V(t) along the three axis: ej0, ej2

3 and ej2

3 .

These two voltage dips together with others measured for

months in a Spanish wind farm have been characterized

according to the methodology explained in section II, [18].The characteristics of voltage dips at these equipment ter-

minals vary according to the transformer winding connections

where they are located. So, in general, the difference in voltage

dips performance magnitude and phase angle is a result

of the propagation of voltage dips from the fault location

to the terminals of wind farms and wind turbines, taking

into account the transformers and the lines located between

these two electrical points, [19]. In [2] the importance of the

winding transformer connections have been deeply studied,

summarizing that the Yd transformer winding connection has

the strongest influence, reducing the number of severe voltage

sags and increasing the number of medium voltage sags. On

the other hand, the Yd1, Yd11, or Yd5 transformers maintainthe voltage dip magnitude, but the associated phase shifts are

completely different from each other.

Therefore, transformer winding connections in the substa-

tion and wind turbines must be taken into account, being usual

a Dy11 transformer in the wind turbine 690/20000 V

and Yd11 transformer in the wind farm electrical substation

20/66 kV, as it is shown in the figure 2.

Figure 5 shows the classification of measured voltage dip

according to the characterization established in [18]. It can

be seen that voltage dips are undoubtedly classified. More-

over, real data measured results are not over the theoretical

curves on the contrary of simulation based results but

in any way very near to them. This is due to factors suchas fault impedance neglected in the theoretical curves and

simulations and load effects.

On the other hand, this characterization and classification

method presents some disadvantages such as, obviously it is

not adequate to multistage voltage dips, in which the fault

and therefore, the voltage dip itself evolves to a different

type. Moreover, during the evolution in time of the fault, the

voltage dip can not be correctly characterized, since it is based

on the theoretical relationship between the minimum phase-to-

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.5

1

0.5

0

0.5

1

1.5

Time (s)

Vo

ltage(pu)

(a) Voltage waveforms

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

Time (s)

Voltage(pu)

(b) Space vector voltage (amplitude)

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5200

150

100

50

0

50

100

150

200Voltage space vector angle

Time [s]

Angle[Deg]

(c) Space vector voltage (angle)

Fig. 3. Example of voltage dip measured


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0 0.5 1 1.5

1.5

1

0.5

0

0.5

1

1.5

Time (s)

Voltage[(pu)

(a) Voltage waveforms

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

1.06

Time (s)

Voltage(pu)

(b) Space vector voltage (amplitude)

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

200

150

100

50

0

50

100

150

200

Time (s)

Angle(deg)

(c) Space vector voltage (angle)

Fig. 4. Example of voltage dip

neutral voltage and the minimum phase-to-phase voltage. In

the application of this method to measured voltage dips, as

the presented here, other considerations must be taken into

account, as the selected point reflects the voltage dip type.

So, minimum density of measured data around zones must be

established, and only these zones accessed for consideration in

the classification of dips. Finally, as it was commented above,

it is usually assumed that the voltage profile during voltage dip

is rectangular, failing in the characterization of non-rectangular

dip, overestimating it.

IV. CONCLUSIONS

Voltage dips are a major concern in wind energy indus-

try, due to the imposition by some national grid codes of

requiring uninterrupted generation throughout power system

disturbances, such as the voltage dips.

Two power quality analyzers have been installed in a wind

farm, being the first one located inside a multi-megawatt wind

turbine, and the second one in the wind farm substation.

Voltage dip records from instantaneous voltages measured

using the power quality trigger abilities have been char-acterized taking into account their waveforms along the dip.

Basically, the presented method previously applied by its

authors to simulated voltage dips obtains a value for each

voltage dip according to the relationship between the minimum

phase-to-neutral and the minimum phase-to-phase voltages,

being classified according to the distance of that point to six

theoretical curves.

Disadvantages of the used method have also been high-

lighted, such as being this point representative of the voltage

dip without taking into account measurement errors, for

example or the errors in the characterization of multistage

voltage dips.

ACKNOWLEDGMENT

The financial support provided by the Ministerio de

Educacion y Ciencia ENE2006-15422-C02-01/ALT and

ENE2006-15422-C02-02/ALT is gratefully acknowledged.

The authors also would like to thank to Mr. Juan Manuel

Abellan from Dea y Energas Renovables, the Moralejo

wind farm people located in Alpera, Albacete (Spain) and

Gamesa technicians for their support in the measurements.

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

PN voltage (pu)

PP

voltage(pu)

A,E

B

C

D

F

G

Fig. 5. Relation between phase-to-phase and phase-to-neutral measured dip magnitudes

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voltage dips characterization at wind projects

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