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Home / Technical Articles / Pilot schemes for transmission line protection

When do we use pilot schemes?

Pilot schemes simultaneously measure and monitor system parameters at all terminals of a transmission line, local and remote, and then respond according to their predetermined functions. These schemes require the use of a communications channel that may be provided through pilot wires, microwave, fiber, or power line carrier.

Pilot schemes for transmission line protection
Pilot schemes for transmission line protection (photo credit: Siemens)

If the measured parameters exceed threshold values, appropriate actions are initiated.

Pilot schemes can generally be broken into two primary categories. Those categories are directional comparison and phase comparison. Directional schemes use directional distance relays for phase fault detection and either directional distance relays or directional overcurrent relays for ground fault detection.

The decision to trip is based on relay setting thresholds being exceeded and the faults being located in the predetermined direction for trip.

Phase comparison schemes are an extension of the differential protection principal. Currents from all line terminals are converted into a composite signal, transmitted to the remote terminals, and compared to the local terminal composite signal. The result of the comparison will result in a trip if the relay setting threshold is exceeded.

Phase comparison schemes are inherently directional and secure, not tripping for faults outside the protected zone of protection.


  1. Directional Comparison
    1. Blocking Schemes
    2. Unblocking Schemes
    3. Overreaching Transfer Trip Schemes
    4. Underreaching Transfer Trip Schemes
      1. Direct Underreach
      2. Permissive Underreach
  2. Phase Comparison
    1. Pilot Wire
    2. Single-Phase Comparison
    3. Dual-Phase Comparison
    4. Segregated Phase Comparison

1. Directional Comparison

Directional comparison schemes are divided into four categories:

1.1 Blocking Schemes

Directional comparison blocking uses distance relays as directional indicators and block initiation for phase faults. Either distance or directional overcurrent relays may be used for ground fault indicators and block initiation.

Each terminal has trip and start relays. The trip relay reaches toward the remote terminal and a little beyond. The start relay reaches backwards, away from the protected section. The trip relay attempts tripping when it operates unless it is stopped by receipt of a blocking signal (carrier, audio tone, or microwave) from the remote end. The start relays at each end initiate the blocking signal.

Thus, if only the trip relays see the fault, it is within the protected section and both ends trip. If the fault is just outside one end, the start relays at that end operate and send a block signal to the remote end, which would otherwise trip. The ground relays operate similarly.

A tripping delay is necessary to allow for the receipt of the blocking signal. A typical delay time of 6 to 16 msec is used to coordinate for the channel delay in communications.

The communication channel is not required for tripping the breakers since the breakers will trip in the absence of the blocking signal. Failure of the channel could result in overtripping of the breakers for adjacent line faults within the reach setting of the distance relays.

Blocking directional comparison is commonly used with on/off type carrier facilities. Since it is not necessary to drive a signal through a fault to operate this scheme, it is the most popular carrier relaying system. See Figure 1.

Blocking Directional Comparison
Figure 1 – Blocking Directional Comparison

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1.2 Unblocking Schemes

Directional comparison unblocking is similar to the blocking scheme except that the start relays are deleted and the blocking, “guard” signal is sent continuously. See Figure 2.

The communication signal for an unblocking scheme uses a frequency shift keying (FSK) channel.

For an internal fault, the frequency is shifted to the unblock, “trip” frequency. The receivers receive the trip frequency and close the output contact, which in series with the 21P relay output contact will trip the breaker.

For an external fault, within the reach of one of the 21P relays, the distant 21P relay will see the fault while the local 21P relay will not see the fault since it is behind the relay.

The distant 21P relay will shift its transmitter frequency to trip.

The local 21P relay will not send the trip frequency or close the 21P output contacts. The line thus stays in service. Should the receivers fail to receive a guard signal and a trip signal, the receivers will allow typically 150 msec of receiver contact closure to permit the 21P relay contact to trip the line.

After this time limit, the communication channel will lock out.

Directional Comparison Unblocking
Figure 2 – Directional Comparison Unblocking

This scheme is more secure since overtripping is avoided at all times with the exception of the 150-msec interval during the loss of signal. Reliability is improved since the communication channel operates continuously and can be monitored, providing an alarm in the case of failure.

The scheme is applicable for two-terminal and multi-terminal lines. Separate channels are required between each pair of line terminals.

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1.3 Overreaching Transfer Trip Schemes

Permissive overreach is also a simple scheme, requiring only one overreaching fault detector at each terminal. This fault detector sends both a trip signal and attempts local tripping through a contact on the receiver.

If both relays see a fault, both ends trip simultaneously.

The scheme appears similar to the directional comparison unblocking scheme of Figure 2. A trip signal is required for this scheme to trip. Power line carrier channels therefore are not recommended for these schemes since a fault could short out the carrier signal.

These channels are normally used with audio tones with frequency shift keying over microwave, leased line, or fiber-optic communications.

The overreaching transfer trip scheme provides highly secure transmission line protection since a trip signal is required from both ends of the line for tripping to occur. The dependability of the scheme may be less than the blocking schemes since the trip signal has to be received before the tripping is initiated.

The scheme is often used when an existing non-piloted scheme has communications added for piloting.

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1.4 Underreaching Transfer Trip Schemes

Underreaching transfer trip schemes include two variations: direct underreach and permissive underreach.

The communications for this type of relaying are generally the same as for the overreaching systems, using audio tones with frequency shift keying over microwave, leased line, or fiberoptic communications channels.

1.4.1 Direct Underreach

This form of protection requires only a single distance fault detector at each end. It has to be set short of the remote end and will simultaneously trip the local breaker and send a trip signal to the remote end, which then trips directly upon receipt of the signal.

Note that local confirmation is not required upon receipt of a trip signal.

Though this scheme is the least complex, it is seldom used because of the high risk of false outputs from the communication channel, which would result in false trips. This risk can be minimized by using a dual-channel transfer trip, which requires the receipt of two signals from the remote end to effect a trip.

See Figure 3.

Direct Underreach Scheme
Figure 3 – Direct Underreach Scheme

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1.4.2 Permissive Underreach

This scheme is identical to the direct underreach scheme with the addition of an overreaching fault detector. The transfer trip signal requires local confirmation by this fault detector before tripping can occur. This increases the security of the scheme and the consequent range of application.

It is commonly selected when an existing step distance relay line is to have the pilot added. See Figure 4.

Carrier is not normally used since a fault could short out the communication signal and prevent the signal from reaching the remote terminal.

Permissive Underreach Scheme
Figure 4 – Permissive Underreach Scheme

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2. Phase Comparison

Phase comparison relay systems monitor the current direction at each line terminal of the protected line and transmit this information to the other terminal via a communication channel.

Each line terminal compares local and remote current direction and trips if the current is into the line from both terminals. The communication channel is normally an on/off type of communications, transmitting only when the overcurrent detector’s thresholds have been exceeded.

This system is immune to tripping on overloads or system swings since it operates on current direction only. It needs no potential source unless it has to be supervised by distance relays because of low fault currents.

Current or distance fault detectors are used to supervise tripping. These detectors have to be set above line charging current, which can appear to the relays as an internal fault at low loads. Internal timers have to be set to compensate for the transit time of the communication channel.

One of the most popular applications of this system is on lines with series capacitors because it is less likely that such a current-operated scheme will operate incorrectly for faults near the capacitors.

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2.1 Pilot Wire

This scheme is a form of phase comparison since it compares current direction at each terminal. The difference between this scheme and others is that a pair of telephone wires is used as the communication channel.

A special filter in the relay converts the three-phase currents to a single-phase voltage and applies this voltage to the wires. When current flows through the protected section, the voltages at each end oppose each other and no current flows in the operate coils.

When current enters the line from each end, the voltage on the pilot wire reverses to allow current to circulate through the operate coils and consequently trip both ends.

Special monitor relays sound an alarm if the pilot wire pair becomes open or shorted. The wire line has to have adequate protection against induced voltages and a rise in station ground potential but may not use carbon block protectors because the line has to remain in service while the protection is operating.

Neutralizing transformers and gas tubes with mutual drainage reactors, all with adequate voltage ratings, comprise the preferred pilot wire protection package.

This relaying has the advantage of simplicity and does not require a potential source. It does not provide backup protection.

Its application is limited to short lines a mile or so in length because of pilot wire cost and increased exposure. The system’s dependability is based on the integrity of the pilot wires themselves.

Many pilot wire systems have been replaced with other pilot schemes because of the failure of the pilot wire system to function reliably and securely.

Recently, pilot wire systems have been replaced with fiber-optic systems providing the communications systems, using a module to convert the output voltage to a light signal. These modified systems have provided a more dependable and secure protection system.

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2.2 Single-Phase Comparison

This scheme applies a sequencing network to the current inputs to the relay to produce a single-phase voltage output. This output is proportional to the positive, negative, and zero phase sequence components of the input currents.

This signal is squared so that the positive portion of the signal provides the positive portion of the square wave.

The negative portion of the signal provides the zero portion of the square wave. Two fault detectors are normally used to provide security, with the more sensitive detector used as the carrier start to transmit the signal to the remote end. The less sensitive detector is used to arm the comparison module for a trip upon the correct comparison of the local and remote signals.

See Figure 5.

Normally current-operated units are used as the fault detectors. In a case where the fault current is less than the load current, impedance-operated units may be used for fault detection.

The use of impedance fault detectors will increase the cost of the system because of the necessity of having line potentials for the operation of the relay.

Single-Phase Comparison Scheme
Figure 5 – Single-Phase Comparison Scheme

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2.3 Dual-Phase Comparison

This scheme is similar to the single-phase comparison scheme except that square wave signals are developed for the positive and the negative portions of the single-phase voltage output of the sequencing network. Each signal requires a separate channel for the transmission of information to the remote site.

This scheme can provide a slightly higher speed of detection since faults are detected on both the positive and the negative portions of the single-phase voltage output of the sequencing network.

This scheme is normally used with a frequency-shift channel, which is continuously transmitted. On a power line carrier it is configured as an unblocking scheme.

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2.4 Segregated Phase Comparison

This scheme is similar to the single-phase comparison scheme except that a square wave is developed for each phase of the transmission line. A communication channel is required for each phase to provide communications to the remote terminal.

Comparisons are made on each of the three phases. The operation of the scheme is basically as described above in the previous phase comparison schemes.

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  1. Design Guide for Rural Substations by United States Department of Agriculture

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming.

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