Pilot relaying refers to the communication network implemented on the high voltage transmission line (T-line) to transmit “trip or don’t trip” signal to and fro between two or more substations. The intent here is to trip the selected few circuit breakers as fast as possible when a fault strikes the T-line, therefore, protecting it.
The actual detection of the faults on the T-line is performed by the protective relays like the Schweitzer’s SEL-421, SEL-321, SEL-311L or the General Electric’s GE-L90. These devices interface with the communication or pilot relays to enable the transmission of trip signal to the remote end substations. Ofcourse, the relays that engage the communication equipment, will also trip its own circuit breakers contingent on the logic programmed into them and the breakers trip coil wired to it.
The pilot relays can transmit the trip or don’t trip signal through various channels. The following channels are commonly employed.
- The signal generated by the pilot relays can be transmitted through the power line itself by means of a line tuner. The line tuner couples the pilot relay’s carrier signal to the power line’s 60Hz signal. To make sure the signal does not travel beyond its intended destination, wave-traps are used. This setup of a line tuner and a wave-trap is known as the Power Line Carrier (PLC.) Where the protected line is considered critical infrastructure, one more set of PLC is wired to the outer phase.
- If not through power line, carrier signals can be sent to the remote end stations using the all too common telephone lines. The utility can lease a telephone line to transmit a tone which can be switched to indicate a fault condition.
- If the T-lines are short, installing a fiber-optic cable along the length of the line (on the shield wire) makes sense. In this case, no elaborate set-up like the line tuners, or wave-traps, or carrier equipment is required. The fiber-optic cable is directly hooked to the protective relays on the either end of the line. You will need a multiplexer if you are forming a network of substations using the fiber optic cable. A JungleMUX SONET multiplexer can be used at every substation that relies on this pilot channel.
- If the line of sight between the two substations is clear then a microwave tower can be installed on each end. The towers can directly transmit the trip signal to each other.
Pros and cons of pilot channels
There are pros and cons of each above mentioned communication/pilot channels. The PLC and the fiber optic cable relays information faster than the telephone line and is less susceptible to noise. However, both these channels fail when either a fault occurs on the T-line, disabling the PLC, or the shield wire carrying the fiber optic cable snaps. It is therefore important to identify the pilot relaying scheme that is best suited for the given pilot channel.
Pilot Relaying Schemes
There are many pilot relaying schemes that can be implemented on the pilot channels. The following are typically employed by the utility.
- Directional Current Blocking Scheme (DCB)
- Permissive Overreaching Transfer Trip Scheme (POTT)
- Directional Current Un-Blocking Scheme (DCUB)
- Current Differential Scheme
- Direct Transfer Trip Scheme (DTT) (This is technically not a pilot scheme but requires a pilot channel)
It is important to note the following:
We know that a carrier signal cannot be transmitted through the power line that has a fault on it. In this scenario, the DCB scheme is the best choice. In the DCB scheme, a lack of carrier signal and a fault on the line initiates tripping of the circuit breaker on either end of the line. Where a trip signal from the remote end substation needs to be acquired to trip the circuit breakers, a telephone line or a microwave tower is used. This ensures a secured transmission and reception of the signal which is not affected by a fault on the T-line. POTT, and DTT schemes use this latter pilot channel.
In case no leased lines are available or a microwave tower is not reliable enough, then PLC equipment – liner tuner and wave trap, is installed on two or all three phases of the power line. This ensures transmission of trip signal even if one line goes down and shorts the signal. (Hopefully not all three go down)
- DCB or DCUB —-> PLC
- Current Differential —-> Fiber (Optical Ground Wire)
- POTT or DTT —-> Fiber, Telephone circuit, Microwave, or PLC (Multi-phase coupling required when using PLC)
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