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Power Line Carrier System's Primary Purpose is Transfer Tripping
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Power Line Carrier System

Introduction to Power Line Carrier System


Coupling Capacitor Potential Device (CCPD)

Coupling Capacitor Potential Devices (CCPDs) or Capacitor Voltage Transformers (CVTs), aside from merely monitoring voltage levels, are used in teleprotection and communication system of the grid. This is accomplished by connecting a coaxial cable to the CCPD's base as shown in Figure-1. The medium of transmission is the transmission line conductors. Co-ax cable, as they are called, are shielded cable to guard the conductors against external interferences. Hybrids and filters are required when there are two transmitters in the PLC installation as explained below.

The primary purpose of Power Line Carrier communication System is transfer tripping and the relay used is the Distance Relay (Device 21). Distance relays at both ends of a transmission line are set at CT and CVT installation points to protect a portion of a transmission line on the side where they are installed. The length of a line protected by the distance relay are called their "zones" of protection. A transfer trip command is sent to the opposite side of a transmission line's circuit breakers when a fault occurs in the first zone of that distance relay, but lies in the second zone of the distance relay on the opposite side (please see our banner for this page). Instantaneous tripping of circuit breakers on both sides of the T/L is necessary to prevent possible damage to the electrical equipment at the other end since Zone-2 has a time delay of 0.2 - 0.5secs; it is a backup protection in case transfer tripping fails. PLC teleprotection may also include transformer, shunt reactor and breaker failure protection. Telephone communications and SCADA (Supervisory Control and Data Acquisition) telemetering kW, kVA, kVAR, Voltage, Power Factor and sending remote control data signals to a power plant or switchyard facility are PLCs' secondary purpose.

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The Line Trap

A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high voltage AC transmission power lines

Transmission line tower showing suspended type Line Traps -Wikipedia.org

Line traps are either encapsulated design or open style design and are designed to ANSI Standard C93.3 or IEC 353. The encapsulated design uses aluminum wire or cable, whereas the open deign utilizes flat aluminum bar. The continuous current of line traps range from 100A to 4000A with inductances of 0.2 to 2mH at 100kHz. Line traps are provided with a parallel capacitor to tune the line trap to the resonant frequency of the carrier signal frequency band and these are usually pre-ordered for frequency band to be used. The resonant frequency formula is shown below. Please take note the exponent "1/2" is the logarithmic equivalent of a square root symbol:

ω0 =1/(LC)1/2; The resonant frequency ƒ0 = ω0/2π = 1/2π(LC)1/2
Where L is the line trap's inductance in Henries; C the capacitance in farads and ?0 is the angular frequency in radians/sec and ƒ0 is the resonant frequency.

The parallel LC network provides high impedance to the frequency it is tuned in. At ƒ0 = carrier channel frequency, the carrier signal will flow towards the circuit as shown in Figure -1 and not to the station's busses where it will be dissipated and lost. The line trap provides a low impedance to 50 or 60hz power frequency current. The resonant frequency of the line trap LC network allows the carrier signal to travel via the path of the Capacitor Voltage Device (CVT) to the radio transceiver. A Line Tuning Unit (LTU) cancels the CCPD's capacitive reactance to the carrier frequency to provide a low impedance path for the carrier signal to and from the radio transceiver. The drain coil isolates the carrier frequency from ground.

230kV Double Circuit Bundle of Two 795MCM ACSR Transmission Line

230kV double circuit, bundle of two transmission Line using 795 ACSR cables

The number of line traps used depend on the length of the transmission line and its importance to maintain grid stability. The types coupling mode are briefly discussed below:

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Mode - 1 or Line to ground Coupling

Mode - 1 coupling uses only a single phase of the line which is the Line-B or the center phase. It usually utilizes only one line trap and 1 CCPD. PLC  operation in Mode-1 coupling is totally disabled in case of a solid center phase to ground fault since carrier signal will be totally dissipated to ground.

Note: Some designers in order to save on cost may opt for three line traps and three CCPDs for a important short length T/L link. This is beyond our scope and we leave it to one specializing on the subject to conduct a research on this PLC scheme. A good start is downloading some the PLC reference materials we have included in our "Suggested Readings" at the bottom of this page.

Mode - 2 or Phase to Phase Coupling

Mode - 2 coupling uses two phases of the line and are usually the Line-A and B phases. This system is used for medium length T/L circuit.

Mode - 3 or Three Phase Coupling

Mode - 3 coupling uses all the three phases of the T/L. This coupling mode is utilized for a very long transmission line circuit of more than 100kms length or for a very important T/L network and may even employ redundant PLC networks.

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Hybrid and Filters

Hybrids and filters comes in with a cost, they are used in PLC system when two or more transmitters are wired to a single coaxial cable to prevent intermodulation distortion (IMD). IMD is the amplitude modulation of two or more different frequency signals in a system that has no linear quantities and as such produces additional frequencies which is either the sum or difference of the original frequencies. Hybrid circuits cause considerable losses in the carrier path and thus, must be employed appropriately. High pass, low pass and band pass filters may alternately be used to isolate carrier signals from each other.

Get a good PLC communication expert as hybrids and filters may take a good chunk of your budget.

Figure 1 -Power Line Carrier System

The Line Trap and CCPD with the carrier signal path shown
CVT, Line Trap of Power Line Carrier System

When a transmission line fault is detected by the distance relay in Zone-1 of the protected line, a transfer tripping signal is sent to the other substation to trip the breakers simultaneously. The other side distance relay also detects the fault but it lies on its Zone-2 which will also trip the breakers after a certain time delay. Zone-2 detection is a backup in case transfer tipping fails.

The 4kHz Bandwidth

Carrier frequency carries the multiplexed audio channel of 300Hz - 3600Hz for communication over a single RF channel of 30 - 200 kHz or 80 - 500 kHz or depending upon your country's standard. Please take note that this is the frequency allocation for the whole grid and is conserved by using a certain transmit/receive signal that will be repeated - only - if they will not cause any interference to nearby PLC installation, other external radio communication network or aircrafts passing the vicinity. The selection of appropriate Tx/Rx for PLC links is called frequency planning. Typical PLC transmit/receive frequency is shown below:

Station A (422) - Tx: 422 - 426kHz; Rx: 426 - 430kHz

Station B (426) - Tx: 426 - 430kHz; Rx: 422 - 426kHz

Please take note that this is a 4kHz bandwidth: 426kHz - 422kHz = 4khZ, or simply a 4khz transmit and a 4kHz receive. The 4kHz bandwidth carries the 300Hz - 3600Hz multiplexed frequency band which carries the voice frequency channel of 300Hz to 2000hz with the remaining 2000hz to 3480hz of the channel bandwidth used for teleprotection, telemetering and remote control functions using Frequency-Shift-Keyed (FSK ) modulation. The use of the remaining 3600Hz-3480Hz = 120Hz is explained below.

PLC system uses the Frequency-Shift-Keyed (FSK) equipment since the equipment is less complex and provides better noise rejection characteristics compared to other types of modulation like the On-Off keying of the carrier frequency. The only disadvantage of FSK is its poor Automatic Gain Control (AGC), because the "0" and "1" logic (space and mark) is isolated only by a few Hz. However, a pilot tone which lies after the 3480 channel, i.e., from 3480Hz to 3600Hz, used for supervision of the PLC network, also regulates the AGC system and make it perform better to some degree.

FSK system uses two frequencies, the "Guard" and the "Trip" frequency. The guard frequency is continuously transmitted at low power during normal operation of the transmission line. When a fault is detected by the distance relay, the guard frequency shifts to "trip" frequency and all data and voice communications are stopped in the order of a few milliseconds for the system to use the whole available transmitter power; since a particular fault condition may greatly degrade or attenuate the signal-to-noise ratio of the carrier signal. This FSK operation is so fast that only a "click" is heard while one is using the network's telephone during a keyed transfer tripping operation.

Other bandwidths used in PLC communication include the 2kHz and the 8kHz, however please take note that the basic operation of the PLC system remains the same.

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Reliability of Power Line Carrier System

The reliability of a PLC teleprotection network is the signal-to-noise ratio (SNR) of the received signals on the other side of the link. This is influenced by the PLC coupling mode used, the transmitter power, hybrids and filters, line tuners and the length of the transmission line itself. Briefly stated, get that carrier signal to the other end in “good health” with the SNR ratio way above the minimum. A fault on the phase where a PLC system is connected attenuates the signal of a Mode-2 coupling to about 6db; for this reason, make sure that the transmitter has enough power to overcome this loss and comes in with a signal to the other end, strong enough to effect a successful tripping.

Note: There is no way for the signal to pass through a solid line to ground fault (bolted) in a Mode - 1 coupling using 1 CCPD and 1 line trap, however, solid ground faults are rare. Voice communication, telemetering and control channels might not be able to pass through a fault resistance, but the transfer trip signal using the whole transmitter power may still be able to pass some to effect a trip. In case this also fails, the distance relay zone-2 will still trip its corresponding breakers after a time delay of about 0.2 to 0.5secs. Also, since Mode-1 coupling are employed only on short or very short transmission lines, the overcurrent relays (Device 50/51) can still provide a good backup. The limitation of overcurrent relay is that the fault current may only be as high or slightly higher as the full load current during a medium to high resistance faults like "arching faults".

For Mode-2 coupling, a fault resistance in two phases where the PLC network is connected can attenuate a signal to a point where it might be too weak to effect action at the receiving end. This is where a security measure - the “150mS unblock window” - that is added to the receiver logic, comes into action to compensate for a loss channel or weak signal, and trips the system. The 150mS unblock window provides the added reliability of a blocking system while insuring a permissive system. A normal signal loss is about 6db when only one phase is faulted, and the signal in the unaffected PLC line is still healthy enough to carry out a transfer trip (we welcome comments here).

Figure 2 - PLC System Sectional Diagram

Power Line Carrier CVT and Line Trap Section Drawing

The Distance Relay

As mentioned, a PLC system's main function is teleprotection. and the relay used is the distance relay (Device 21). Electromechanical distance relays have two balancing coils with one coil energized by the voltage and the other by the current. The voltage/current ratio produces a balanced torque, i.e., the torque produced by the voltage and the torque produced by current balances each other with the current element delivering a positive “pickup” torque while the voltage element delivering a negative torque for reset.

Impedance may be a poor term to describe an impedance type distance relay and it might be confusing since impedance as we all know is the vector sum of resistance and inductive reactance in transmission line circuit. Impedance when used in distance relay application, however may involve only the resistance or the reactance of the circuit. Impedance relay fault detection is similar to overcurrent relay, but this time it is a voltage restrained overcurrent relay. During a fault, voltage “V” falls down while current “I” rises in proportion to a fault resistance. The value V/I during a fault represent a resistance (R = V/I) in proportion to its distance from the relaying point. Knowing the resistance of ACSR cables used in Ω/kilometer, the location of the fault can be easily determined.

Distance Relay Circle Diagram

Circle diagram or R-X diagram of an impedance relay is used to plot its operating characteristic with the voltage and the current; and its displacement angle Ø converted to two variables X and Y. The circle R-X diagram that crosses the y and x axis (to y and -x) is the relay's back impedance coverage that might present some coordination problems with the adjoining line on the same bus. For this reason, modern digital impedance relays now have lens type R-X diagram that provides better selectivity.

We have so far discussed only the impedance type distance relay. There are two other types, and they are the reactance and mho type distance relays. These are discussed in length in General Electric's Distance Relays Fundamentals shown in the suggested readings column at the bottom of this page.

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Some Forms of Protective Relaying Schemes

Blocking

A blocking mode of protection scheme means that an external fault is detected by a distance relay that is outside of its protected zone, since Zone-2 extends up to 20% of the next interconnected T/L link. The circle diagram portion of a distance relay that falls on the –x axis may also see this fault when well within its reach (ex.: busbar faults). A PLC channel is not required to prevent the breakers in the unaffected T/L section from tripping since protection schemes have pilot channels transmitting programmed information on when to trip or not to trip the unaffected line. This scheme uses the “on-off” carrier type, i.e., the system is off and turns on to send a blocking signal only when it detects a fault .

Permissive Overreach

When a Zone-1 of a distance relay detects an internal fault that is in the Zone-2 of the distance relay at the opposite link, the PLCs guard frequency shifts to trip frequency to send a trip command. Please see the banner above for reference.

Directional Comparison

Similar to differential relaying, the directional comparison monitors the direction of the current. When a fault occurs within a protected line, the current at both ends of the line will flow towards the faulted internal line section.

The Fiber Optics

Overhead ground wire, aside from merely protecting transmission lines from lightning strikes is now used as a communication link between two interconnected switching facilities with the use of OPGW cable. Fiber optics cable is a high speed data transmission system and can provide all the channels used in PLC. OPGW, or Optical Ground Wire, is defined by IEEE as “a cable that contains a tubular structure with one or more optical fibers in it, surrounded by layers of steel and aluminum wire.”  OPGWs are installed at the top of high voltage transmission line towers (pylons) in a similar fashion that OHGWs are fitted and mounted. A fiber optics is well insulated and protected against physical damage by the OPGW steel and aluminum wire. The use of fiber optics frees the PLC system of voice, telemetering and telecontrol channels so the PLC can be dedicated for teleprotection only. Fiber optics can be used in lieu of microwave communication; its only disadvantage is, it must be well protected when installed underground or under water. Using OPGW has disadvantages, and that is the difficulty and expense of splicing it in case it sustains damage or needs modification. It has a high cost per lineal meter, too, so it must be ordered with almost zero wastage upon installation. Fiber optics are easy to install on T/Ls under construction, but can also be used in existing pole type T/Ls by installing special mounting brackets for support. Replacing the OHGW with OPGW in existing systems must be done on an energized transmission line using a technique called Live Line Working, since shutting it down will incur a great financial loss to the utility company.

References:

- Power Line Carrier Frequency Planning Design Guide Guideline by Escom

- Power Line Carrier Channel and Application Considerations for Transmission Line Relaying

- Transmission Line Protection Principles

- Line Protection with Distance Relay

- Use of the R-X Diagram in Relay Work



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