2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Overvoltage Protection in Wind Power Farms

Bernhard Richter PTHA Surge Arresters ABB Switzerland Ltd. Wettingen, Switzerland

bernhard.richter@ch.abb.com

Abstract-Lightning and overvoltage protection of wind power farms and wind power generators is addressed considering the different equipment and stresses. Standards are discussed and typical cases show that further investigations are necessary and new surge protective devices have to be developed and tested.

Keywords-wind power farm, wind power generator, lightning, overvoltages, MO surge arrester

I. INTRODUCTION

The increasing use of renewable energies makes it necessary to install wind power parks onshore and offshore. Due to the height of the wind power towers and the exposed places of installation they are prone to direct lightning, and consequently to overvoltages generated by the lightning strikes. Further on, the individual towers are connected with a.c. or d.c. links to substations and then to the grid.

The main electrical equipment that can be found in a wind power farm needing protection is:

Generator Frequency converter Driver control Wind turbine control L V side of transformer including LV switchgear HV side of transformer including HV switchgear Auxiliary circuits (e.g. warning lights)

Direct I ightning to the blades or tower has to be considered as well as possible overvoltages in the cables connecting the towers with the substation.

Based on the equipment that a typical wind power farm is composed of different stresses occur and appropriate measures for overvoltage protection are required. International standards have been developed recently to address the protection measures in case of specific applications, e.g. for wind power farms. However, they all address particular aspects only. But a general overview of the complete system is necessary to understand the different stresses, protection measures, fimction and application of surge protective devices.

In the following the existing standards are discussed, direct lightning effects are considered, as well as the protection of generator and converter in the nacelle. The necessity of

Erko Lepa PGD Generators

ABB Oy Helsinki, Finland

erko.lepa@fi.abb.com

overvoltage protection of the cable system connecting the wind power towers to a common substation is shown as well.

II. REVIEW OF STANDARDS

Often it is required by the users that wind power generators have to be equipped with overvoltage protection devices according to lEe TR 61400-24 "Wind turbine generator systems, Part 24 : Lightning protection". The only part in this TR addressing the generators is saying "Surge protection of electrical components - Electrical power equipment such as

motors, generators, transformers, and switch gears is designed to withstand high-voltage surges. Insulation of electrical power equipment normally withstands transient voltages in the kilovolt range. In the light of this, it is recommended that a surge arrestor or SPD rated above the operating line voltage and possible temporary overvoltages (TOV) be used. Othenvise, the surge arrestor or SPD may conduct current during normal line variations and have greatly reduced service life. Further guidelines for sizing SPDs and surge arrestors are found in 1EC 61643-22, 1EC 60099-5 and 1EC 61643-12." This is a very general statement and not helpful at all. lEe 61643-22 is about telecommunication, signaling and circuit board protection. lEe 60099-5 is addressing HV three phase systems only, and lEe 61643-12 gives selection and application principles for LV SPDs as they are installed in civil houses and industries, mainly DIN-rail mounted.

The recently developed CLClTS 50539-22 Low-voltage surge protective devices - Surge protective devices for specific application including dc. - Part 22: Selection and application principles - Wind turbine applications is referencing the EN 61643-12, which is similar to lEe 61643-12. The specific stresses including variable frequencies, superimposed oscillations and typically very severe ambient stresses are not addressed in any of these documents.

First it seems to be necessary to clarify the different wordings and definitions when talking about devices for over voltage protection:

Surge Protective Device (SPD) is a device that is intended to limit transient overvoltages and divert currents (def. lEe 61643 series). SPDs can have

978-1-4673-1897-6/12/$31.00 ©20121EEE

different technologies, e.g. spark gaps, metal oxide varistors (MOV), combinations of both, and contain disconnectors and auxiliary equipment. The wording surge protective device or short SPD is mainly used in the low voltage community for DIN rail mounted devices, and says nothing about the technology used and the principle of function.

Surge Arrester (SA) is a device intended to limit transient overvoltages to a specific limit (e.g. def EN 50526-1). This is a general terminus used for surge protective devices in medium and high voltage systems and in the specific case of traction systems. The technology is based on metal oxide varistors and can contain spark gaps. Older designs of surge arresters have as active part a series connection of SiC resistors and spark gaps (see also the definition in lEC 60099-1).

Metal-oxide surge arresters without gaps (MO arrester) is an arrester having nonlinear metal-oxide resistors connected in series and/or in parallel without any integrated series or parallel spark gaps (def IEC 60099-4).The wording MO arrester, or shorter arrester, is used generally in the medium and high voltage community.

The in the market existing SPDs, SA or MO arresters have been developed and tested according to the existing international standards of lEC, CENELEC and some national standards as for instance lEEE/ ANSI. These standards consider a.c. systems with power frequency of 50/60 Hz only. This means that all devices installed in other systems had to be tested with reference (but not according) to the existing standards. This does not reflect the real stresses in the system. In recent years some new standards and guidelines for selection and application have been developed on European level. With the new standards comparability between different products can be achieved and a good basis for development and testing of new products for specific applications is given. However, the new standards are based on the well-established test methods for a.c. applications. Voltage stresses like d.c. or d.c. with superimposed impulses and a.c. voltages with higher frequencies or high frequencies in general are not covered. For HYDC systems a new standard is under discussion and development in lEC TC 37. It is important to know first where the arresters and different surge protective devices will be installed and what stresses are to be expected from the system before requirements and test procedures can be described. This is the reason why for instance in Cigre working group A3.25 of study committee SCA3 High Voltage Equipment research is done on the energy handling withstand capability of surge arresters. The results will influence the actual work on the new Rev. 3.0 of lEC 60099-4.

III. LIGHTNING PROTECTION

Fig. 1 shows the system configuration of an off-shore wind power park. The different wind in fed (generator and transformer) are connected over a medium voltage cable network with system voltages Us = 33 kV. The transmission to the mainland is done through a 155 kV cable connection.

The used medium voltage cable network has an arc­suppression (Petersen) coil earthing to compensate in case of a single earth fault. In addition it has a charging coil, which in connected to the medium voltage bus bar and compensates the capacitive charging current. In Fig. 1 is only one wind power tower shown as example, altogether there are 21 generators (7 per track) connected to the medium voltage bus bar over the three cable tracks.

net¥.ork 155kV trCllSfcrrrer transnission SOMVA

33kV

cctlleretvvurk

dlarging

coil

i .. ndJXlv.€r � generation

Figure I. System configuration of an off-shore wind power fann (simplified)

The very high towers of wind power parks are naturally prone to severe lightning. Special measures have to be taken to avoid damages due to direct and nearby lightning. Current surges and overvoltage as result of lightning may endanger the installations and equipment and have to be considered. Critical and therefore of interest for the performance of surge protective devices (SPDs) are the system preconditions like different current and voltage wave shapes and frequencies.

Blades

� Tower

Warning Lights /�Meteo

� Nacelle / Generator

In Fig. 2 a typical example of a wind turbine tower is shown. It needs to be differentiated between different stresses. The rotator blades and towers are prone to direct lightning. Therefore, the path of the lightning current from the point of

stroke down to the earthing point has to be known and can be critical. Lightning currents can be critical for bearings or gearbox elements. High current densities may burn the surface of bearing balls, as shown in Fig. 3. In order to protect the bearings, the lightning current has to be diverted via a low impedance path, while the bearing structure must have high impedance; Fig. 4 is showing the principle [1].

: Desirable \current path

.�

Gearbox

II Sliding

contacts

lEG 186!i102

Figure 4. Current path for lightning current (possible solution)

The tower itself should be protected and the SPDs in the tower should be selected according to the lightning protection zone (LPZ) principle and to the required protection level of the equipment to be protected.

There are two basic designs of wind turbines (direct drive turbine and double fed turbine) that may require different surge protection measures. Further on the different equipment mentioned above will see different stresses from lightning and overvoltages and need different types of surge protective devices. As long as different voltage levels and voltage wave shapes exist in a wind power installation different standards and concepts have to be considered. The different voltages at the different types of equipment are in principle:

- AC voltage with different frequencies - DC voltage - AC/DC voltage with superimposed impulses

- low voltage (::; 1 kV)

- medium voltage (> 1 kV)

IV. OVERVOLTAGE PROTECTION

Overvoltage protection in the nacelle is needed between the generator and the converter, see Fig. 5. While the voltage at the transformer side of the converter is sinusoidal with power frequency of 50 Hz or 60 Hz, the voltage at the generator side is varying in frequency, height and may have, due to the pulse width modulation (PWM), high frequency contents.

Figure 5. Electrical train drive. full converter concept.

1.1

1.0 .....

'peoo �.9.r' ••• • 1.9

• 0 • V I.'

1.1 • 0 • 'J

1.6 • 0 .. 'J Up/UIi: • 0 .. V

1,5

I,' • 0 .. u 1,3 • 0 • V

1,1 . :-. ffiAJ#\ 51� v

1,1

� .tRN .....

J\I W I,D

1 10 10

IlmJ

• t,= 0,05 �s o �=O,1 �s .. � = 0,2 �s 17 t,= 1,0 �s

FIgure 6. Voltage at the generator termmals as functIOn of the cable length

Highest practical switching frequencies are used in the drive design. Sine filters are generally not used, which brings additional problems for the selection of surge protective devices. The PWM pulse rise times are so short that their propagation along the cable between generator and converter can change the pulse shape and create overvoltage spikes.

Depending on the cable length the occurring overvoltages may vary, see Fig. 6.

For choosing the right voltage levels for the surge protective devices the performance of the surge protective devices under the mentioned stresses has to be known. Depending on the technology of the intended surge protective devices different voltage and current ratings may be necessary. Investigations to design optimized overvoltage protection in the described cases are under way. Especially the long term performance of the materials used has to be considered. Combined solutions with disconnecting devices may be necessary. The very severe ambient conditions like increased ambient temperature of 110 °C and vibration due to wind and the rotor movement have to be considered as well when designing an optimized solution.

The performance of the used components (MO surge arrester and fuse including status indicator) under standard ambient conditions are well known. To ensure reliable service in the nacelle of a wind power generator performance tests have been done in ambient temperatures up to 120°C. It could be shown that, considering the expected de-rating in this temperature range, can be compensated by selecting appropriate components. All tests have been successful.

V. PROTECTION OF THE CABLE NETWORK

Large wind power farms, as for instance off-shore installations, have long cable connections in-between the individual wind turbines. In this case mostly switching overvoltages in the medium voltage cable system have to be considered, besides the protection against lightning overvoltages. System studies have to be performed from case to case.

To determine the maximum overvoltages in the medium voltage system only switching-on of cable sections and earth faults between phase and earth are considered. Several calculations have been performed [2]. Because of the adapted earthing principle of the neutral it is possible, that after disconnection of cables the switched-off cables keep a capacitive charge. Reclosing with opposite phase to earth voltage leads to the highest overvoltages, which is especially critical during a single phase earth fault.

High overvoltages during reclosing are to be expected when during a unipolar fault a faultless cable is disconnected and then reconnected. The consequence is that the pre-charge of the cable has the value of Uo = l.73 p.u. (phase to phase voltage) and this process leads to high overvoltages. Switching-on with pre-charge (connection of an entire cable track when two are already connected) the highest overvoltage occur. For simplification only this case is simulated. These calculations have been made as well with as without surge arresters in the network.

Fig. 7 shows the electric diagram. The connection of the cable track is made through the circuit breaker S2, the single phase fault exists already in the connected power supply unit.

33kV S2 cable network

network 1S5kV transformer u

"l transmission 50MVA

-

j ", u"l charging

coil

wind power generation

Figure 7. Reclosing during an existing single phase fault

-

The maximum voltages according Table I result. In this case the maximum voltages is UKI = 12l.38 kV (= 4.50 p.u.) at the end of the connected cable track. The voltage at the bus bar is with Us = 73.67 kV (=2.73 p.u.) much smaller, because at this position there is no complete reflection. The given values have been determined without surge arrester in the network.

TABLE I. Maximwn voltage when reclosing a cable track

(single phase fault)

Parameter Without SA With SA

us, kV/p.u. 73.67/2.73 75.20/2.79

UK]' kV/p.u. 12l.38/4.50 88.96/3.30

UK2, kV/p.u. 89.90/3.34 98.33/3.65

Fig. 8 and Fig. 9 show the transient voltages at the transformer tenninals and at the ends of the connected cable section.

o 3 6 (file Einschalb.mg-Kabelslrang-VorladlSlg-ErdschllB.pl4; x-varl)

9 12 Ims] 15 v:XOCI03A v:XOOO3B v:XOOO3C

Figure 8. Voltages Us at the transformer tenninals (phases A. Band (Cl

-12�---+---+---+--�--�--�--�----r---r-� o 3 6 12 [ms] 15

(tie EinschaHlXIg-Kabelslrang-Vorladung-EltIschluB.pl4; x-varl) v:X0C61A v:X00518 v:X0051C

Figure 9. Voltages UK] at the cable ends (phases A, B and C)

The simulation under the given circumstances leads to the highest overvoltages. Therefore in a second step surge arresters phase to earth at each end of the three cable tracks are considered, in addition a surge arrester at the transformer neutral to earth is applied. Under these circumstances the maximum voltages are considerably reduced, a reduction from Umax = 12l.4 kV to Umax = 98.3 kV can be observed. The residual voltage for a current wave 8/20 f..lS is taken into consideration for the surge arrester phase to earth (Ue = 36 kV). The surge arrester between the transformer star point and the earth (Ue = 23 kV) is taken into account with a current wave shape of 30160 f..lS.

VI. SUMMERY

Alternative sources producing electrical energy, like wind power farms, are discussed since long and in some extend are successfully in service in different parts of the world. The occurring stresses due to lightning, and the voltage wave shapes in the wind power generators are in some cases different from the standard a.c. voltages. Because of the severe stresses due to direct lightning and rough ambient conditions, like high

temperatures in the nacelle and vibration, special precautions have to be taken to ensure reliable service of the equipment. This stresses have to be considered for the development, testing and application of surge protective devices. In recent years technical committees have started on European level to work on selection and application principles, requirements and test standards for surge protective devices intended to be used in wind power generators, and wind power parks in general. This results in new definitions, test procedures and sometimes in new products. Basic research was done to understand the occurring stresses from the system and their influence on the material and performance of the surge protective devices.

The actual standards, as for instance lEe TR 61400-24 "Wind turbine generator systems, Part 24: Lightning protection" gives not really guidance for the user. Additional standards have to be consulted to cover the different aspects of lightning and overvoltage protection.

The individual wind turbines are connected via a cable system with the grid. Overvoltages in the cable system are mainly due to switching operations. Earth fault conditions in a 33 kV cable system of an offshore wind power park have been considered and calculations of overvoltages due to switching and earth faults have been done. Maximum occurring voltages are represented and the positive effect of application of surge arresters to reduce the overvoltages is shown.

VII. REFERENCES

[I] IEC TR 6[400-24, frrst edition 2002-07:Wind turbine generator

systems - Part 24: Lightning protection.

[2] G. Balzer, T. Werner, B. Richter, "Overvoltages in MY Cable

Networks of Offshore Wind Power Parks", CIRED Workshop­

Lisbon 29-30 May 2012, Paper 138.