Electrical-Engineering-portal.com-Power Line Carrier Communication PLCC

7/29/2019 Electrical-Engineering-portal.com-Power Line Carrier Communication PLCC

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Power Line Carrier Communication - PLCC (photo: Zanith Transf ormers & Swithgears Pvt. Ltd)

http://electrical-engineering-portal.com/power- line-carrier-communication-plcc March 14, 2013

Power Line Carrier Communication (PLCC)

Asif Eqbal

Content

IntroductionMajor goal/Application of PLCC

Main Components of PLCC:

1. Coupling Capacitor

2. Line trap Unit

3. Transmitters and Receivers

4. Hybrids and Filters

5. Line Tuners

6. Master Oscillator and Amplifiers

7. Prot ection and earthing of coupling equipment

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PLCC Panel Block Diagram

Advantages of Digital PLCC over Analog ones

Typical signal to noise ratio calculation by considering a line of 295 kilometers

SNR (Signal-to-Noise Ratio) Calculation

Introduction

Use of PLCC in modern electrical power system is mainly fo r telemetry and telecontrol . Telemeans remot e. Telemetry refers to science of measurement f rom remote locat ion.

Dif ferent types of data transmission system can be used depending upon the networkrequirement and condit ions.

Main data transmission system for telemetry and telecontrol are:

1. Use of telephone lines

2. Use of separate cables

3. Power Line carrier communication

4. Radio wave micro wave channel

For

large power system power line carrier communication is used for data transmission as well asprotection of t ransmission lines. Carrier current has a f requency range of 30 to 200 kHz in USAand 80 to 500 kHz in UK .

Each end of transmission line is provided with identical PLCC equipment consisting of equipment:



3. Line Tuners

4. Line Traps

5. Power amplifier

6. Coupling capacito rs

Distance prot ection relay in relay panel at one end of the transmission line gets the input from

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PLCC scheme

CT and CVT in line. The output of relay goes to modem of PLCC.

The output of PLCC goes to couplingcapacitor and then to t ransmission lineand travels to another end where it isreceived through coupling capacitor

and inputted to relay and cont rol panelat that end.

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Major goal/Application of PLCC

PLCC in modern electrical power system substation is mainly used for

following purpose:

1. Carrier protection relaying of transmission line so that:

Inter trip command can be issuedby relay due to tripping of circuitbreaker at any one end.

To t rip the line circuit breaker nearest to t he fault , this is done by:a) Distance protection relay (V/I characteristics)b) Diff erential comparison methodc) Phase comparison method

Station to station communication between operating personnel

Carrier telemetring, electrical quantities that are telemetered are kW, kVA, kVAR, Voltageand Power factor etc.Following methods are used for telemetring as well as telecont rol:

Simplex

Duplex

Multiplex

Time division Multiplex

Many factors will affect the reliability of a power line carrier (PLC) channel.

The goal is to get a signal level to the remote terminal that is above the sensit ivity of thereceiver, and with a signal-to-noise ratio (SNR) well above t he minimum, so that the receiver can make a correct decision based on t he information transmitted.

If bot h of these requirements are met then the PLC channel will be reliable.

The factors affecting reliability are:

1. The amount of power out o f t he transmitter.

2. The type and number of hybrids required to parallel t ransmitters and receivers.

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PLCC component - Coupling Capacitor

3. The type of line tuner applied.

4. The size of t he coupling capacitor in terms of capacitance.

5. The type and size, in terms of inductance, of the line trap used.

6. The power line voltage and the physical conf iguration of the power line.

7. The phase(s) to which the PLC signal is coupled.

8. The length of t he circuit and transposit ions in the circuit .

9. The decoupling equipment at the receiving terminal (usually the same as the t ransmitt ingend).

10. The type of modulation used to t ransmit the info rmation, and the type of demodulat ioncircuits in the receiver.

11. The received signal-t o-no ise ratio (SNR).

The above list may not be all inclusive, but these are the major factors involved in the success

or failure of a PLC channel.

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Main Components of PLCC

1. Coupling Capacitor

Coupling capacitor connects the carrier equipment to thetransmission line. Thecoupling capacitor’scapacitance is of such a value that itoffers low impedanceto carrier f requency

(1/ωC) but highimpedance to power frequency (50 Hz).

For example 2000pF capacitor offers 1.5MΩ to 50Hz but 150Ω to 500kHz.

Thus coupling capacitor allows carrier f requency signal to enter the carrier equipment.

To decrease the impedance further and make the circuit purely resistive so that there is noreact ive power in the circuit , low impedance is connected in series with coupling capacitor to

form resonance at carrier frequency.

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PLCC component - Transmitters and receivers

2. Line trap Unit

The carrier energy on the t ransmission line must be directed toward the remot e line terminaland not toward the station bus and it must be isolated f rom bus impedance variations. Thistask is performed by the line trap.

The line t rap is usually a form of a parallel resonant circuit which is tuned to the carrier energy

frequency.

A parallel resonant circuit has high impedance at its tuned frequency, and it then causes mostof the carrier energy to f low toward the remote line terminal. The coil of the line trap provides alow impedance path for the f low of the power frequency energy.

Since the power f low is rather large at t imes, the coil used in a line trap must be large in termsof physical size.

Hence a line trap unit /Wave trap is inserted between busbar and connect ion of couplingcapacitor to the line. It is a parallel tuned circuit comprising of inductance (L) and capacitance

(C). It has low impedance (less than 0.1?) for power frequency (50 Hz) and high impedance tocarrier f requency.

This unit prevents the high frequency carrier signal from entering the neighboring line.

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The carrier t ransmitt ers and receivers are usually mounted in a rack or cabinet in the cont rolhouse, and the line tuner is out in the switchyard.

This then means there is a large distance between the equipment and the tuner, and theconnect ion between the two is made using a coaxial cable.

The coaxial cable providesshielding so t hat noisecannot get into the cable

and cause interference. Thecoaxial cable is connected tothe line tuner which must bemounted at the base of thecoupling capacitor.

If there is more than onetransmitter involved per terminal the signal must gothrough isolat ion circuits,typically hybrids, beforeconnect ion to the line tuner.

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The purpose of the hybrid circuits is to enable the connection of two or more transmitterstogether on one coaxial cable without causing intermodulation distortion due to the signal fromone t ransmitter affecting the output stages of the other transmitter. Hybrids may also berequired between t ransmitters and receivers, depending on t he application.

The hybrid circuits can, of course, cause large losses in the carrier path and must be usedappropriately. High/low-pass and band-pass networks may also be used, in some applications,to isolate carrier equipment f rom each ot her.

5. Line Tuners

The purpose of the line tuner in conjunction with the coupling capacitor is to provide lowimpedance path for the carrier energy to the transmission line and a high impedance path to

the power f requency energy.

The line tuner/coupling capacitor combination provides a low impedance path to the power lineby forming a series resonant circuit tuned to the carrier f requency.

On the other hand, the capacitance of the coupling capacitor is high impedance to the power frequency energy. Even though t he coupling capacitor has high impedance at power frequencies, there must be a path to ground in order that the capacitor may do its job. Thisfunction is provided by the drain coil, which is in the base of the coupling capacitor. The draincoil is designed to be low impedance at the power f requency and because of its inductance itwill have high impedance to the carrier frequency.

Thus the combination of the line tuner, coupling capacitor, and the drain coil provide thenecessary tools for coupling the carrier energy to the transmission line and blocking the power frequency energy. One last function of the line tuner is to provide matching of impedancebetween the carrier coaxial cable, usually 50 to 75 ohms, and the power line which will have animpedance of 150 to 500 ohms.

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6. Master Oscillator and Amplifiers

High frequency carrier signal is generated in oscillator.

Oscillator can be crystal oscillator with which operation for a part icular bandwidth can beachieved. The output voltage of a oscillator is held constant by voltage stabilizer.

The output o f oscillator is fed to amplifier so that loses in transmission can be compensated.Losses occurring in carrier current is termed as attenuation of carrier signal.

They are mainly: Losses in coupling equipment which are constant losses for a given carrier f requency bandwidth.




PLCC component - Master oscillator and amplif iers

Protection and earthing of couplingequipment

Line losses vary withlength line, size of line,weather conditionetc…These losses for underground line ismore than overheadline.

Frequency spacing is aprocess using diff erentcarrier f requency intwo adjacenttransmission lines.Wave trap/Line traphelp in accomplishing this.

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7. Protection and earthing of coupling equipment

Over voltage can be caused due to lightning, switching and sudden loss of load etc.

They produce stress on coupling equipment and line trap units. Non linier resistor in serieswith protective gap is connected across the line trap unit and inductor of coupling unit .

The gap is adjusted to spark at a set value of over voltage.

Coupling unit and PLCC equipment are earthedthrough a separate and dedicated system, sothat ground potential rise of stat ion earthingsystem does not affect the reference voltagelevel/Power supply common ground of the PLCCequipment.

In this regard that is earthing of PLCC and ot her

communication/Instrumentation/Electronicequipment please refer to NEC Article 645 for data centers (IT equipment.)

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Advantages of Digital PLCC over Analogones

1. Immune to noise in processing and storagestages, as it is completely digital.

2. Digital: Require less no. of circuits (hardware), since Digital Processor is a single chip.

3. Processing is accurate and reliable.

4. Frequency conversion is done in a single step (Digital Conversion).

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5. Digital processing allows the applicat ion of a wide range of mathematics. ( Analog processing is limited by the availability of devices to perform desired functions, while)

6. Equalizat ion is perfect: High-resolution digital filtering gives very flat f ilter response asdesired.

7. The performance of digital circuits, opposed to analog, is relat ively independent of actualcomponent values in the implementing circuit. Therefore, digital systems more reliablyreproduce the desired responses in spite of temperature variat ions or component aging.

8. In addit ion, in digital circuits there is litt le need for component matching.

9. Simplif ied Product ion: Lower Parts count and improved testability.

Power Line Carrier (PLC) Signal propagat ion along high voltage lines depends ent irely on t heconstruction of t ransmission lines, mainly on t he conf iguration and characterist ics of allconductors and on the ground resistance optimum coupling allows to make the best use of agiven transmission line.

Transposit ion may introduce addit ional at tenuation which can generally not be predicted with

simple rules. Most transposit ion schemes result in high attenuation poles at certain frequenciessuch frequencies cannot be used for PLC communications.

Forbidden Frequency Ranges may be determined as explained in CIGRE Paper 35-02,Senn/Morf – Optimum PLC Arrangement on Transposed Single Circuit power Lines – (August,1984)

In critical cases, however, computer calculation may be necessary, for which the following data is required:

1. Height of each conductor above ground (at the towers)

2. Sag of conductors (between towers)

3. Horizontal distance (between conductors)

4. Number of conductors per phase (single or if bundle spacing)

5. Outer diameter of conductors, material of conductors

6. Number of strands at t he circumference (outer strands)

7. Diameter of strands

8. Same information (a) to (g) for ground wires

9. Tot al length of transmission line

10. Sketch of phasing arrangement showing type and number of t ransposit ions and distancebetween t ransposit ions (if double system, each scheme required separately)

11. Earth resistivity in Ohm meters, if not known, state whether around 300 or 1000 or 3000communication separately.

12. Coupling arrangement (phase to ground of phase to phase)

13. Available carrier f requency range

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Typical signal to noise ratio calculation by considering a line of 295kilometers

Frequency line section: 140/144 KHzLine Voltage: 400 KVLine configuration: 3 transposit ions at equal distanceLine length: 295 KmsConductor diameter: 31.77 mm

No. of bundles per conductor: Two

Overall loss = Line att enuation + Coupling loss

Line attenuation (aL) = a1 x L + 2aC + aadd

Where:

a1 = at tenuat ion constant of the lowest loss made in dB per Kmac = model conversion loss in dBaadd = additional loss caused by discontinuities e.g. coupling circuits, transposition etc. in dB

a1 is a constant which depends uponf = f requency in KHzd = conductor diameter in mmn = No. of bundles

Line configuration = No of transposition at equal intervals

Upon subst itut ing corresponding values with certain approximation we get a1:

a1 = 0.029 dB/Km

Line attenuation, aL = 0.029 x 295 + 2 x 0 + 10 = 8.55 + 10 = 18.55 dB

Coupling Loss = Loss in Coupling equipment + tapping loss + paralleling loss + by passlosses in case of bypasses + cable loss.

= 2 + 2.6 + 1 + 0 + 0.5= 6.1 dB

Overall loss = Line attenuation + Coupling loss = 18.55 + 6.1 = 24.65 dB

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SNR (Signal-to-Noise Ratio) Calculation

Signal level (speech) = +35 dB & 38 dB (Corresponding to 20 Watt (43 dBm PEP) and 40 wattsrespectively)

Noise level (Corona noise) in 2.2 KHz bandwidth = -13.5 dBCorrection considering Psophometric factor = -2.0 dBNoise level in speech band (300 – 2400 Hz) = -15.5 dB

Equipment noise above external = -60 dBm is very low corresponding to noise so notconsidered in calculationSignal level (speech) at receiver side on line side = +35 – (Line attenuation + Coupling loss)

= +35 – (18.55 + 6.1) = +35 – 24.65 = 10.35 dB




Signal to noise ratio = (Signal level (speech) at receiver side on line side – Noise level in speechband)

= +10.35 -(-15.5 dB) = 25.85 dB (considering PLC terminal power output as 20 watts)

= 28.85 dB (considering PLC terminal power output as 40 watts which is recommended for better SNR).

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Electrical-Engineering-portal.com-Power Line Carrier Communication PLCC