21, rue d’Artois, F-75008 PARIS B1-101 CIGRE 2006 http : //www.cigre.org

400 KV VIENNA THE VIENNA 400 KV NORTH INPUT

Johannes VAVRA – WIENSTROM GmbH

Manfred WANDA – Prysmian OEKW GmbH Austria

SUMMARY Between 1978 and 1986 Wienstrom has built three 400 kV FF-cable links (south input) in Vienna (transmission station Pfarrgasse – substation Sued; substation Sued – substation Kendlerstrasse, substation Kendlerstrasse – power station Simmering), consisting of two three-phase circuits each with 1.200 mm² Co FF cables. Until today it is the only EHV connection for the city of Vienna. Depending on the actual increasing consumption of electricity, 2 % per year as an average, Wienstrom started in the early 90th planning a new 5,2 km long 400 kV cable input, combined with a new 9,1 km long 400 kV overhead line, in the northern area of the city of Vienna. In the pre-bid stage the following different solutions were foreseen (e.g. FF - PPL – XLPE). Main technical issues: Two transmission capacities were required by Wienstrom (600 MVA without cooling system and 1.040 MVA by using a lateral cooling system with water pipes). Program of the project: The civil works started in March 2004 and the commissioning was in November 2005. The present paper includes a description of the EHV Cable system, etc. There are two three-phase circuits, etc. The XLPE Cable consists of a 1200 mm2 copper conductor, XLPE insulation, copper wires screen in combination with welded aluminum sheath firmly bonded to the outer polyethylene sheath The metallic sheaths are connected in a Cross-Bonding system, etc. There are six laying sections for each system with cable lengths from 850 to 890m. The cables are laid in open trefoil with a depth of 2,7 m. All joints are installed in accessible joint bays.

In this paper there also is a description of the solution adopted to minimize the Electromagnetic Field (EMF) etc. The paper illustrates the water cooling system installed to increase the power rating in emergency conditions, etc. The temperature behavior of the cable system will be continuous monitored through fiber optics and a dynamic system (RTTR), etc.

1 General The WIENSTROM GmbH as local utility is responsible for production, transmission and

distribution of electrical energy within a supply area of about 2.000 km2, consisting of the

City of Vienna and the surrounding regions of Lower Austria. The annual demand on

electrical energy amounts to more than 11.000 GWh today, the maximum load has reached

nearly 2.000 MW. More than 50 % of the electrical energy is produced in own thermal power

plants with an installed power of about 1.500 MW shown in fig [1].

Fig [1] Energy production and demand of the city of vienna

Rather early the utility decided to build up a 400 kV transmission system because of the

following main reasons: The system has to transport electrical energy from the power plant in

Simmering to other parts of the city, it is a so called backbone for the 110 kV networks which

are divided into 7 groups in the meantime and finally it is the important input for additional

electrical energy in the south of Vienna shown in fig [2].

F

After finis

in 1984

substation

input in t

Bisamberg

ig [2] Electrical networks of the city of vienna

hing the three links in 1978 (power plant Simmering – substation Kendlerstrasse),

(substation Kendlerstrasse – substation Sued) and in 1986 (substation Sued –

Sued-Ost of Austrian Power Grid) the utility rather soon started to plan a second

he north of the city to connect the important substation Nord with the substation

of Austrian Power Grid shown in fig. [3].

The detailed planning started in the early nineties, but searching for the best technical solution

under consideration of the complex approval procedure needed a lot time and effort. Finally

the utility got an approval for a technical project quite similar to the one between substation

Sued and substation Sued-Ost from 1986, namely a combined link consisting of overhead

line, transmission station and cable system shown in fig [4, 5, 6].

This so called Vienna 400 kV north input now was built in the years since 2003 and could be

set into operation on December 4th, 2005. The project therefore is the issue of this report

whereas the main focus will be the description of the cable systems.

F F

ig. [4] overview of the 400 kV north input of Vienna ig. [5] 400 kV GIS in SS Nord

1.1 Descri In the year

in Vienna.

suppliers (

each with

formation

transmissi

Up to the e

increasing

early 90th

decided to

Bisamber

link insid

Similarly

to operate

Fig. [6] OHL connection to SS Bisamberg

ption of the network

1978 Wienstrom has started with the building of the three 400 kV cable-links (south input)

For each system there were two main-contractors in Vienna with two different cable

AEG, BICC, SIEMENS, PIRELLI). Each system was build as a double three-phases circuit

1.200sqmm copper conductor, fluid-filled cable. The laying arrangement was in flat

with additional cooling pipes in a thermal stabilised backfill material to increase the

on power in later years.

nd of 2005 it is the only EHV connection for the city of Vienna. Depending on the actual

consumption of electricity, 2 % per year as an average, Wienstrom started in the

planning a new EHV connection for the city of Vienna in the northern area. It was

connect the Wienstrom substation “UW Nord” with the Verbund substation “UW

g” throw a ~ 13,2km long EHV system consisting of a 5,2 km long 400 kV cable

e Vienna, and with an 9,1 km long 400 kV overhead line outside of Vienna.

to the existing FF 400kV cable connections in Vienna, the new system is designed

at two different power ratings: at the beginning each circuit will carry 620 MVA

and in case of future power demand, the power rating will increase to 1030 MVA with the

activation of the forced water cooling system.

1.2 Route of the cable

The cable portion of the 380kV link is consisting of two circuits each long 5,2km connecting

the UW Nord Substation Switchgears at one end and the overhead line in the transmission

station Dr. Nekowitsch Strasse at the other side.

The selected route crosses a very populated area along streets large enough to lay the two

parallel circuits with a minimum inter-axial distance of approx. 5m.

The two systems were arranged at the sides of the streets, under the pavements, in order to

minimize the effect on traffic during the installation works.

The route crosses the river Marchfeldkanal and the railway Nordbahn. Since they are to too

close to be crossed with conventional installation techniques, the choice was to build an

underground cable pathway, consisting of two circular tunnel 20m below the ground.

1.3 Constraints

The cable system was required to respect a series of constraints imposed by the owners of

other underground infrastructures and by National and/or local laws.

The requirements that mostly affected the technical solutions are the limitation of the Electro

Magnetic Field (EMF) and the limitation of the interference on the telecommunication system

in case of a phase-to-Earth fault external to the cable system.

The EMF is to be equal to or lower than 15 microT at the soil surface at any point along the

cable route. Such value, imposed by the Viennese local authorities, is considered as a safety

value to avoid any interference with portable electro-medical devices such as pace-makers,

although the relationship between the effects of the EMF on human health and/or on electro-

medical devices has not been proven on scientific basis.

In case of a phase-to-Earth fault external from the cable system the fault current shall flow in

the cable metallic sheaths, rather than the soil, in order to minimize the interference with

telecommunication cables laid parallel to the 400kV connection.

It is common practice to achieve this result imposing a maximum value of the ratio between

the fault current in the Earth and the total phase-to-Earth fault current. This value, known in

literature as Cable Reduction Factor (k), shall be limited to 0.05, meaning that the maximum

fault current in the soil shall be no more than 5% of the total fault current.

2 Thermal design

2.1 Trench arrangement

The existing FF cables in Vienna are laid in flat formation at a depth of 1,7m from the soil

surface.

Due to the new requirement regarding the magnetic field, such arrangement was not possible

unless an additional compensation system against EMF were installed.

A new arrangement with the cables in open trefoil formation, spaced 270mm, at a depth of 2,7

m shown in Fig. [7] was therefore adopted for the following reasons:

• the trefoil geometry and the increased depth limit the EMF at soil surface to 15

microT,

• the cables need to be spaced so that the forced cooling system is more effective.

The axial spacing between the cables of 270mm is the result of an accurate study which aim

was to find the best compromise between the limitation of the EMF (the closer, the better) and

the achievement of the power ratings (the further, the better).

Fig. [7] laying arrangement

2.2 Cable cooling

2.2.1 Natural cooling

The current ratings calculation for natural cooling was based on the IEC standards [1]

assuming the following environmental conditions:

• Load factor = 1

• Max soil temperature = 20°C

• Laying depth (to bottom of cables) = 2.7m

In such conditions the power ratings of 620MVA can be achieved without exceeding the

maximum admissible conductor temperature of 90°C.

Since the cooling pipes are filled with gas at the first stage, a more accurate calculation based

on finite elements analysis was done to verify the negligible effect of the empty pipes on the

cables.

2.2.2 Forced cooling

The current ratings calculation for forced water cooling was based on the calculation method

proposed in Electra [3].

For each circuit the cooling system is consisting of two separate loops of pipes, each covering

half of the cable route.

Figure [8] shows a schematic view of the cooling system for each cable circuit: two separate

loops of four pipes. The inlet cold water flows into the two pipes at the bottom of the trench.

The water direction is then inverted at the central joint bay where the two pipes at the bottom

are connected to the two pipes at the top.

5200 m

2600 m 2600 m

Cooling system A-1

Cooling system B-2 Cooling system A-2

Cooling system B-1

Figure [8]: basic layout of forced cooling system (one cable circuit)

In order to ensure the required power rating of 1040MVA it is necessary to choose the right

combination of inlet water temperature and water flux.

The curve showing the combination of these two parameters is shown in Fig. [9]

Figure [9]. Curve showing the possible combination of inlet water temperature (on the y axis)

and water flux (on the x axis) to achieve the required current ratings of 1040 MVA.

2.3 Installation in tunnel

The railway and the river « Marchfeldkanal » are crossed through a cable pathway in tunnel

20 m below the ground and approx 125 m long shown in fig. [10].

The tunnel are two, one for each circuit, the shape is circular with a diameter of 2,5 m.

The cables reach the tunnels at the mention depth by two vertical slides at the two ends of the

tunnel.

For safety reason against fire propagation, the cables where covered with thermal stabilized

backfilling at any point inside the tunnel.

Due to the depth of such installation the magnetic field does not need to be limited and

therefore the cables could be installed in flat formation.

The cooling pipes were also covered and placed in horizontal position with the cables.

Figure [10] : arrangement in the tunnel during the cable laying of one system

2.4 Cooling of the joints

related to the current rating capacity of the system, and particularly

critical aspect, as the heat due to the losses within the

e.

hnology difficulties arise when forced cooling is applied to the system.

h heat due to

s therefore to install the joints inside an air filled chamber so to minimize the

ive in terms of heat removal in free air. Nonetheless their

lar

• the distance between the joints was reduced as much as possible compatibly to the

• y

o concentric cable loops was added.

o keep the joints as close as possible and to ease their heat dissipation, the accessory’s outer

s

Joints pits dimensions are

to the effects of the forced cooling.

The thermal design of the joints is a

conductor must dissipate radially through a larger insulation with a higher thermal resistanc

In normal circumstances the joints are however designed to be used in conjunction with the

cable.

The tec

In a direct buried installation, if the cooling pipes are not able to dissipate enoug

the joint dimensions, the distance between joints should be increased to create a more

favorable heat exchange with the surrounding material.

This inevitably leads to an increase in the joint pit dimensions and an unacceptable increase of

the magnetic.

The choice wa

mutual heating between the joints.

The cooling pipes will not be effect

arrangement was specifically designed to ensure natural ventilation due to their different

temperatures. Such thermal behavior was verified by means of finite elements analysis.

The reduction of the magnetic filed below the required limit was achieved with a particu

arrangement summarized as follows:

spacing necessary for the jointing activities,

the phase transposition is placed inside the ba

• a passive compensation system consisting of tw

T

protection was kept as minimal as possible: heat shrinkable tubes over the copper casing.

Such solution is used typically in a dry environment. For such a reason the joint chamber i

designed to be completely watertight. Water pumps, sealing gaskets, etc. were used for this

purpose.

Such arrangement is shown in fig. [11, 12].

Figure [11] : joint bay arrangement

Figure [12] : finished installed joint bay arrangement with phase transposition and cooling

pipes

3 Design of the cable

The cable is consisting of a single core 1200 mm² XLPE insulated cable designed for

Um=420 kV, maximum continuous operating voltage, and Up=1425 kV, lightning impulse

withstand voltage.

Insulation consists of extruded cross-linked polyethylene (XLPE) suitable for operation at

conductor temperature of 90 °C, extruded simultaneously with the semi conductive conductor

and core screen (triple head extrusion).

The cable is provided with a longitudinally welded aluminium sheath, thick enough to

withstand the rated Phase-to-Earth short circuit current. The metallic sheath acts also as a

radial water barrier.

Before the metallic sheathing, a longitudinal water barrier is applied to limit the water

penetration along the power core in case of cable external damage.

The anti-corrosion protection consists of an extruded PE sheath. The cable outer sheath has an

overall thin layer of graphite to permit the after laying voltage test of the sheath.

The metallic sheath was designed to fulfill the requirement of a cable reduction factor of 0.05.

The calculation of this parameter has been verified in the worst working condition, i.e. with

the cable sheath at its maximum working temperature and neglecting the parallel of the other

cable sheaths.

With this respect it was necessary to add copper wires screen below the aluminium sheath.

A schematic drawing of the cable construction is shown in fig. [13].

Figure [13]: used 400 kV cable design for the Vienna north input

4 Installation

4.1 Installation in trench

According to the required transmission power of 1.040 MVA by using the lateral cooling

system and the maximum magnetic field emission (EMF) it was calculated from Prysmian to

install the cable system in an depth of 2,70m in an open trefoil with an phase distance of

270mm. The cooling pipes are installed in an distance of 50mm to the cables and in the centre

of the three phases are the pipe for the Fibre Optic cable to control the temperature of the

systems. The minimum distance between the two systems are 5,0m.

All this cables and pipes are direct buried in an block of ~ 1.100mm x ~ 600mm with thermal

stabilised material with an maximum value of 1,2 Km/W in dry condition. Before and during

the trench back-filling operations an authorised test institute has verified the thermal

properties of this material.

The distance between the cables and between the cables and the pipes was ensured by the use

of plastic spacers specifically designed for this project.

The open trefoil arrangement, the presence of pipes for forced cooling and FO cables at

different depths, implied a complex installation procedure: it was necessary to lay the

different sets of power cables and pipes layer by layer.

Due to the drum size (Flange 4,2m; Width 2,5m; weight 32 ton) the transportation of the cable

lengths (average 860m) was only allowed during the night on a very limited time span.

Therefore it was possible to lay only one cable length per day.

The cables were laid on cable rolls placed all along the open trench and in some sections

additional caterpillars were necessary; the pulling force was applied to the cable head only.

After the cable laying, the cable oversheath was tested at 10kV DC to be sure that the cable

was not damaged during the cable laying. Each cable was immediately protected after the

cable laying with special plastic-gaps shown in fig. [14], to minimize the risk if stones, tools,

etc. that may have fallen down from the entire height of the trench.

Figure [14] : special plastic gaps over the 400kV cables after laying

The last cable sections were laid in winter 2004: since the air temperature in the period were

far below the minimum allowed temperature for the cable laying an air conditioned scaffold

was erected on the temporary storage place over the drums. Also after the delivery to the

laying place it was build an additional scaffold and continues the heating up to starting with

the cable laying.

4.2 Installation of the terminations

At both ends of the circuit (UW Nord S/S and also at Dr. Nekowitsch Straße overhead line) it

was necessary to snake the cables to minimize the force of the conductor on the terminations

due to the metal thermal expansion under operation.

Inside the UW Nord S/S two sets of three GIS terminations were delivered and installed. All

dimensions and details were agreed together between the wnd user, the cable supplier, and the

GIS supplier.

For the installation of the outdoor terminations in KÜ Dr. Nekowitsch Straße, Wienstrom has

build a temporary scaffold for the protection of the materials and the safety of the personnel

during termination works.

4.3 Construction of the joint bay and installation of the joints

All the cables, pipes and accessories inside the joint bay were fixed with special steel works

and special cleats so to ensure the watertightness of the chamber. For safety reason after final

installation from Prysmian, Wienstrom has installed in each joint bay sensors against, water,

gas and opening the joint bay entrance.

5 Tests

5.1 Type and factory testing

Cable and accessories were tested to prove their suitability to withstand the electrical and

mechanical requirements of the IEC 62067 standard.

A type test was conducted on a loop consisting of two GIS terminations, one Outdoor

termination, two joints (normal and sectionalized) and 30m of cable.

All manufactured cable lengths and accessories passed the routine and sample tests required

by the IEC standards.

5.2 Testing after installation

5.2.1 Voltage tests

At the completion of works the installed power system was successfully tested in accordance

with the requirements of the IEC62067.

The cable oversheath was tested applying the DC voltage of 10 kV for 1 minute.

The main insulation was tested applying the AC voltage of 260kV for 1 hour. Due to the high

voltage test level a mobile resonant generator was carried on site by the cable supplier.

5.2.2 Partial Discharge (PD) Measurement

The PD measurement will be performed at three different moments:

- At commissioning

- After the trial period

- After the warranty period

The method used for this project, successfully experienced on other EHV cable installations,

considers individual off-line measures (i.e. not contemporary to the AC HV insulation test).

The subject method provides the possibility of measuring PD activities in each cable

accessory (100% circuit tested using 100% of the sensors) at different times. The PD

measurement instrument will be connected to bonding leads inside the link boxes used to

sectionalise the earthing connections from of the cable sheaths at joint and termination

positions.

At present time there are no standards defining a threshold for the maximum PD activity on

site, mainly due to the external and background noise level that can be neither foreseen nor

limited below a given value. In order to solve this problem, a PD measurement system based

on fuzzy logic was used: such system is indeed able to recognize the waveform of specific

defects inside the cable or the accessories, despite the absolute value of the PD activity.

In order to ensure that the Partial discharges are activated, the voltage test is higher than the

system voltage, i.e. 1.1 x U0.

With this respect the voltage test will be provided by suitable mobile equipment that will be

connected to the circuit in order to apply the required voltage level during all the

measurements.

6 Real Time Thermal Rating (RTTR) Monitoring Systems

RTTR is a system which provides the capability of continuous monitoring of the thermal

performance of high voltage underground cables and accessories enabling the operator to take

advantage of the real time conditions rather than using traditional conservative assumptions

on cable ratings.

Prysmian’s RTTR system (OptoPower®) consists of two main parts.

The first element is a remote data acquisition system (a DTS system), which collects all the

information about the cables physical variables by means of suitable sensors (in this case,

fiber optic cables laid on top of the hottest phase).

The other part is essentially a computer which receives all the data from the sensors, and

computes it by means of suitable software, in order to calculate in real time the cable

performance, both in steady and in transient state.

The software is able to monitor the current conditions and operation of the system, predict the

overload capacity, or monitor possible changes in environment surrounding the cable, thus

predicting possible emergency conditions enabling optimization of the system operation or

preventive measures.

7 Conclusions

The design and the installation of two 400kV cable circuits with XLPE insulation capable of

meeting the requirements of power ratings (620MVA naturally cooled and 1040MVA forced

cooled) respecting all the imposed constraints (EMF, cable reduction factor, etc.) was

described in this paper.

This experience demonstrates how the XLPE insulating technology could be used as a

suitable alternative solution to conventional OF cables for high voltage connections coupled

with forced water cooling systems.

8 References

[1] IEC62067 - POWER CABLES WITH EXTRUDED INSULATION AND THEIR

ACCESSORIES

FOR RATED VOLTAGES ABOVE 150 kV (Um = 170 kV)

UP TO 500 kV (Um = 550 kV) –

TEST METHODS AND REQUIREMENTS

[2] IEC60287 – Electric cables – calculation of current ratings

[3] The calculation of continuous ratings for forced cooled cables – Electra No.66

[4] First 380kV bulk power transmission system with lateral pipe external cable cooling in

Austria. Cigrè paper 21-09 – 1980 Session – August 27 – September 4

[5] OEZE-Sonderheft „400kV-Verbindung UW Simmering – UW Kendlerstrasse“

(Oesterreichische Zeitschrift für Elektrizitaetswirtschaft, 32. Jg., 1979, Heft 9/10) [6] OEZE-Sonderheft „400kV-Verbindung UW Kendlerstrasse – UW Sued“ (Oesterreichische Zeitschrift für Elektrizitaetswirtschaft, 38. Jg., 1985, Heft 7/8) [7] OEZE-Sonderheft „400kV-Verbindung UW Sued – UW Sued-Ost“ (Oesterreichische Zeitschrift für Elektrizitaetswirtschaft, 40. Jg., 1987, Heft 8)