Line Traps

2

Basic Function of Line Traps

Power Line Carrier (PLC) is a common method of Power SystemCommunication, such as telepro-tection, voice and data communi-cation, etc. It has developed thereputation of being one of themost economical and reliable formsof communication and versatile inits application. Fig.2 shows a basicPLC system consisting of three distinct components:

• signal carrying medium (HVtransmission line)

• communication apparatus(transmitters, receivers and associated components)

• coupling/blocking equipmentsuch as coupling capacitors, linetuners (coupling devices) andLine Traps.

Line Traps are connected in serieswith HV transmission lines. Themain function of the Line Trap is topresent a high impedance at thecarrier frequency band while introducing negligible impedanceat the power frequency. The highimpedance limits the attenuationof the carrier signal within thepower system by preventing thecarrier signal from being:

• dissipated in the substation

• grounded in the event of a faultoutside the carrier transmissionpath

• dissipated in a tap line or abranch of the main transmissionpath.

Line Traps

Introduction

With over 40 years of successfulfield experience Trench is therecognized world leader in thedesign and manufacture of air-core dry-type inductors for electricutility applications. The unique custom design approach, alongwith fully integrated engineeringand manufacturing facilities inboth North America and Europehave enabled Trench to becomethe technical leader for highvoltage inductors world wide.

Line Traps for Power Line Carrier(PLC) communication systemsrepresent a significant applicationsegment for high voltage inductors.

Fig. 1Post mounted

Line Traps

3

Design and Construction

Line Traps are designed to meetANSI standard C93.3, IEC stan-dard 353 or other internationalstandards.

The major components of a LineTrap are the main coil, tuning device and protective device (see Fig. 3).

Fig. 4Suspension mounted

Line Traps

Fig. 2Principle scheme of a

PLC system

Tuning Device

Terminal

Main Coil

Pedestal(Optional)

LightningArrester

Bird Barrier (Optional)

Since Line Traps are series connec-ted with the HV transmission line,they must be designed to with-stand the high mechanical forcesgenerated by the short circuit (s/c)current associated with the HVtransmission system.

Fig. 3Line Trap

components

1 Coupling Capacitor 2 Line Tuner 3 Transmitter/Receiver

Line Trap Line Trap

Station A Station B

4

Main Coil

The main coil of a Line Trap is anair-core dry-type power inductor.

Trench offers Line Traps with eitherof two well established windingtechnologies:

• Encapsulated design (E-design)

• Open style design (O-design).

Both technologies fully complywith power system and PLC requirements and are thereforeapplied over the full range of com-monly specified main coil ratings. However, in the upper limits of inductance and current ratings theencapsulated design is the preferred concept (see Fig. 5).

The winding of the encapsulateddesign utilizes aluminum wire orcable, whereas the open style winding employs aluminum flat-bar conductor. All power currentcarrying components utilize welded connections.

High mechanical strength of thewinding is either achieved by resinimpregnated, fiberglass reinforcedencapsulation (E-design) or by fiberglass spacers which are resinbonded between turns (O-design).

The winding is terminated at bothends on a system of aluminumbars, denoted as the spiders whichare tensioned together by fiber-glass ties (E-design) or fiberglassrods (O-design). These spiders areadditionally used for

• the electrical connection to theLine Trap by terminal pads orstuds

• providing the hardware for lifting (lugs), mounting (pede-stals) and corona protection (bells,rings)

• connecting the tuning and protective device across the main coil.

Trench can provide the completerange of standard ratings(inductance, continuous and s/ccurrent ratings, system voltage) inaccordance with IEC 353 or ANSIC93.3 standards (see Fig.5). Customized units are also availableto meet specific customer require-ments, such as specific inductance,current (s/c or continuous), lowloss requirements, etc.

a

Rated Currents Rated InductanceIEC 353 IEC 353

Continuous Short-timeA Series 1 mH at 100 kHz

kA/1sec

100 2,5 0,2 0,25 0,315 0,4 0,5 1,0 2,0200 5 0,2 0,25 0,315 0,4 0,5 1,0 2,0400 10 0,2 0,25 0,315 0,4 0,5 1,0 2,0630 16 0,2 0,25 0,315 0,4 0,5 1,0 2,0800 20 0,2 0,25 0,315 0,4 0,5 1,0 2,0

1000 25 0,2 0,25 0,315 0,4 0,5 1,0 2,01250 31,5 0,2 0,25 0,315 0,4 0,5 1,0 2,01600 40 0,2 0,25 0,315 0,4 0,5 1,0 2,02000 40 0,2 0,25 0,315 0,4 0,5 1,0 2,02500 40 0,2 0,25 0,315 0,4 0,5 1,0 2,03150 40 0,2 0,25 0,315 0,4 0,5 1,0 2,04000 63 0,2 0,25 0,315 0,4 0,5 1,0 2,0

Continuous Short-timeA Series 2 mH at 100 kHz

kA/1sec

100 5 0,2 0,25 0,315 0,4 0,5 1,0 2,0200 10 0,2 0,25 0,315 0,4 0,5 1,0 2,0400 16 0,2 0,25 0,315 0,4 0,5 1,0 2,0630 20 0,2 0,25 0,315 0,4 0,5 1,0 2,0800 25 0,2 0,25 0,315 0,4 0,5 1,0 2,0

1000 31,5 0,2 0,25 0,315 0,4 0,5 1,0 2,01250 40 0,2 0,25 0,315 0,4 0,5 1,0 2,01600 50 0,2 0,25 0,315 0,4 0,5 1,0 2,02000 50 0,2 0,25 0,315 0,4 0,5 1,0 2,02500 50 0,2 0,25 0,315 0,4 0,5 1,0 2,03150 50 0,2 0,25 0,315 0,4 0,5 1,0 2,04000 80 0,2 0,25 0,315 0,4 0,5 1,0 2,0

Rated Currents Rated InductanceANSI C93.3 ANSI C93.3

Continuous Short-timeA kA/2sec

mH at 100 kHz

400 15 0,265 0,53 1,06 1,59 2,12 2,65800 20 0,265 0,53 1,06 1,59 2,12 2,65

1200 36 0,265 0,53 1,06 1,59 2,12 2,651600 44 0,265 0,53 1,06 1,59 2,12 2,652000 63 0,265 0,53 1,06 1,59 2,12 2,653000 63 0,265 0,53 1,06 1,59 2,12 2,654000 80 0,265 0,53 1,06 1,59 2,12 2,65

5

Note: All ratings available with E-design

Shaded ratings not available with O-design

Fig. 5 Values of continuous

current, short time currentand inductance

6

Tuning Device

The tuning device, connectedacross the main coil, forms ablocking circuit which provideshigh impedance over a specifiedPLC-frequency range. Depending on the type of tuning(see below) the tuning device consists of capacitors, inductorsand resistors, all having relativelylow power ratings, compared tothe main coil. For environmentalprotection these components are mounted in one or more fiberglasshousings. The tuning device is installed inside the main coil. To meet changing PLC frequencyrequirements the tuning device iseasily accessible for replacementor field adjustment (if applicable).

The bandwidth of a Line Trap isthat frequency range over whichthe Line Trap provides a certainspecified minimum blocking impe-dance or resistance. Minimumblocking resistance should be specified if the potential exists forthe reactive component of theLine Trap impedance to resonatewith the substation impedance.The achievable bandwidth can beexpanded by increasing the maincoil inductance. Different types of tuning may beexpanded by increasing the maincoil inductance.

Different types of tuning may besupplied.

• Single Frequency Tuning

If narrow blocking bands are required single frequency tuning isthe simplest and most economicaltype of tuning available. Fig.6shows a typical schematic andblocking characteristic. Within thisnarrow band, however, high blocking impedance can be provi-ded, resulting in excellent PLC signal isolation.

• Double Frequency Tuning

The double frequency tuning arrangement blocks two relativelynarrow bands of frequencies.Otherwise, the blocking characte-ristic is similar to single frequencytuning.

For proper operation and isolationof the tuned bands a minimumfrequency separation must bemaintained between the peak tuning frequencies. This is 25 kHzor 25 % of the upper tuning frequency peak, whichever is greater. Fig.7 shows a typicaldouble frequency schematic andblocking characteristic.

Fig.6Single frequency tuning

Fig.7Double frequency tuning

Fig.8

Wideband tuning

b

Z Blocking impedanceR Resistive componentf Carrier frequencyfm Resonant frequency

(Geometric Mean Frequency GMF)

f’1 f’2 Frequency limits of blocking impedance

f1 f2 Frequency limits of resistive blocking impedance

7

Fig.10Selftuned Line Trap

blocking characteristics

• Wideband Tuning

Wideband tuning is the most common type of tuning as it efficiently utilizes the main coil inductance. Wideband tuned LineTraps are suitable for multi-chan-nel applications, since relativelyconstant impedance is obtainedover a broad frequency range. Thistype of tuning provides high band-width flexibility for future changesor expansion of PLC frequencies.PLC channels can be placedanywhere within the blockedbandwidth. Fig.8 shows a typical widebandfrequency Line Trap schematic andblocking characteristic.

• Selftuned Line Traps

Selftuned Line Traps do not requirethe use of tuning devices. Theblocking characteristic as shown inFig.10 is achieved by simply utili-zing the self-capacitance of themain coil winding. The inductanceof a selftuned Line Trap is higherthan that of a tuned Line Trap.

Fig. 9Suspension mounted 380 kV,

1.0 mH, 2100 A Line Trap

8

Protective Device

The protective device is a surgearrester connected in parallel withthe main coil and the tuning device. It protects the main coiland the tuning device by reducingthe transient overvoltages to levelscorresponding to distribution voltage class insulation.

The insulation level of the maincoil and tuning device is coordinatedwith the surge arrester protectivecharacteristics.

Trench Line Traps are equippedwith advanced metal-oxide typesurge arresters having a dischargecurrent rating of 10kA. Surgearresters with higher dischargecurrent or high energy dissipationarrangements are also available onspecific request.

Mounting and Connection

Trench Line Traps can be mountedin several configurations. Suspen-sion mounted Line Traps areavailable with either single point ormulti-point suspension brackets.Line Traps can also be pedestalmounted directly onto couplingcapacitors (CCs), capacitive voltagetransformers (CVTs) or station postinsulators. Trench offers severaltypes of mounting pedestals:

• single insulator support pedestal

• multi-insulator support pedestal

• insulated pedestal

Other than the insulated pedestal,all pedestals are electricallyconnected to the lower terminalof the Line Trap, and as such canbe used as both the electrical andmechanical connection to the CCor CVT (see Fig.11a). Should it benecessary to utilize the upper LineTrap terminal as the connection tothe CC or CVT, a pedestal insulatedfrom the bottom end (bottom spider) of the Line Trap must beused in conjunction with a specialinsulated connection rod suppliedby Trench (see Fig.11b). All pedestals can be custom madeto suit customer requirements.

Terminals supplied on the Line Trapscan be either pad or stud type.Each type is manufactured tomeet the applicable IEC or NEMAstandards. In addition, terminals can be located on virtually any spider arm, ensu-ring total flexibility to meet individual re-quirements. Terminal details and termi-nal orientation are shown in Fig.12 and

Fig.13.

c

Fig. 11a Fig. 11b

Station

Station

Line

Line

CouplingCapacitor

CouplingCapacitor

Line Trap connection

9

Fig.12

Fig. 12Terminal orientation(by special request may be situatedat any spider arm location. Number of spider arms is obtainedfrom actual quotation drawing, typically 4, 6 or 8)Et: is used to define the top terminal location, at centre or atany of the spiders.Eb: is used to define the bottomterminal location; at centre or atany of the spider arms.

Fig. 13Standard terminal types plated orbare aluminum

Note:Unless otherwise specified, flatterminal pads will be vertically oriented to reduce eddy currentheating. (ie. terminals oriented sothat the coil’s axis is in the planeof the terminal.)

Fig.13

Top view

10

Calculation of tapping loss (At)and blocking attenuation (Ab)

Z1 = Characteristic impedance ofthe line

The impedance of substation Zs isassumed to be 0 Ohms.

• Center Frequency (fc)

fc is the mean frequency of the blocked bandwidth limit frequencies (f1, f2).

Definiton of BlockingTerms

The blocking requirement of aLine Trap is dependent on thecharacteristic impedance of thetransmission line where PowerLine Carrier is to be applied. TheLine Trap blocking requirementscan be specified in terms of:

• Blocking Impedance (Zb):

Zb is the complex impedance ofthe complete Line Trap within aspecified PLC frequency range.

• Blocking Resistance (Rb):

Rb is the value of the resistive

component of the blocking impedance, within a specifiedPLC frequency range.

• Tapping Loss (At)

At, also known as “InsertionLoss”, is a measure of the lossof power sustained by a carrierfrequency signal due to the finite blocking ability of the LineTrap. The tapping loss of anideal Line Trap should be verylow and approach zero.

• Blocking Attenuation (Ab)

Ab is a measure of the relativetransmitted carrier frequencysignal which enters the trappedcircuit section of network. Theblocking attenuation of an idealLine Trap should be infinitelyhigh.

Equ. I At (dB) = 20 log10 (1+ ___ )Z1

2Zb

Equ. II Ab (dB) = 20 log10 (1+ ___ )Zb

Z1

fc=������f1x f2

11

1,0 mH Line Trap

0,315 mH Line Trap0,2 mH Line Trap

0,5 mH Line Trap

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www.trenchgroup.comSubject to change without notice04.2000

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