• Transmission Line Impedance Transformer
• Transmission Line Transformer Calculator
• Jerry Sevick Transmission Line Transformers
• Transmission Line Conversion Transformers

1. For example, resonant EFHW antennas with 49:1 transformers would have to be wound as conventional transformers with losses of 15%-25% vs. 2%-5% for a lower ratio transformer (like 9:1) in a different antenna design using a transmission line transformer.
2. The long awaited revision of the classic book Transmission Line Transformers, by Jerry Sevick, is now in its 5th edition and has been updated and reorganised by Raymond Mack to provide communication engineers with a clear technical presentation of both the theory and practical applications of the transmission of radio communication.

Protective Relaying Schemes

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A substation can employ many relaying systems to protect the equipment associated with the station. The most important of these are: transmission and distribution lines emanating from the station, step-up and step-down transformers, station buses, breakers, shunt and series reactors and shunt and series capacitors.

Substations serving bulk transmission system circuits must provide a high order of reliability and security in order to provide continuity of service to the system. More and more emphasis is being placed on very sophisticated relaying systems which must function reliably and at high speeds to clear line and station faults while minimizing false tripping.

Most EHV and UHV systems now use two sets of protective relays for lines, buses, and transformers.

Many utilities still use one set of electromechanical relays for transmission-line protection, with a completely separate, redundant set of solid-state relays to provide a second protective relaying package or two completely separate redundant sets of solid-state relays.

For more difficult relaying applications, such as EHV lines using series capacitors in the line, some companies always use two sets of solid-state relays to provide the protection systems.

Transmission-line relay terminals are located at the substation and employ many different types of relaying schemes that include the following:

2. Direct Underreaching Fault Relays

These relays (Figure 2) at each terminal of the protected line sense fault power flow into the line. Their zones of operation must overlap but not overreach any remote terminals.

The operation of the relays at any terminal initiates both the opening of the local breaker and the transmission of a continuous remote tripping signal to effect instantaneous operation of all remote breakers.

For example, in Figure 2 below, for a line fault near bus A, the fault relays at A open (trip) breaker A directly and send a transfer trip signal to B. The reception of this trip signal at B trips breaker B.

3. Permissive Underreaching Relays

The operation and equipment for this system are the same as those of the direct underreaching system, with the addition of fault-detector units at each terminal.

The fault detectors must overreach all remote terminals. They are used to provide added security by supervising remote tripping. Thus, the fault relays operate as shown in Figure 2 and the fault detectors as shown in Figure 3.

As an example, for a fault near A in Figure 2, the fault relays at A trip breaker A directly and send a transfer trip signal to B. The reception of the trip signal plus the operation of the fault detector relays at B (Figure 3) trip breaker B.

4. Permissive Overreaching Relays

Fault relays at each terminal of the protected line sense fault power flow into the line, with their zones of operation overreaching all remote terminals.

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Similarly, the relays at B operate and transmit a trip signal to A. Breaker A is tripped by the operation of the fault relay A plus the remote trip signal from B.

Transmission Line Impedance Transformer

Likewise, breaker B is tripped by the operation of fault relay B plus the remote trip signal from A.

5. Directional-Comparison Relays

The channel signal in these systems (Figure 4) is used to block tripping in contrast to its use to initiate tripping in the preceding three systems. Fault relays at each terminal of the protected line section sense fault power flow into the line.

Transmission Line Transformer Calculator

Jerry Sevick Transmission Line Transformers

For example, in Figure 3 the blocking zone at B must extend further behind breaker B (to the right) than the operating zone of the fault relays at A.

Correspondingly, the blocking zone at A must extend further out into the system (to the left) than the operating zone of the fault relays at B.

For an internal fault on line AB, no channel signal is transmitted (or if transmitted, it is cut off by the fault relays) from any terminal. In this absence of any channel signal, fault relays at A instantly trip breaker A, and fault relays at B instantly trip breaker B.

For the external fault to the right of B as shown in Figure 3, the blocking zone relays at B transmit a blocking channel signal to prevent the fault relays at A from tripping breaker A.

Transmission Line Conversion Transformers

Breaker B is not tripped because the B operating zone does not see this fault.

6. Phase-Comparison Relays

The three line currents at each end of the protected line are converted into a proportional single-phase voltage. The phase angles of the voltages are compared by permitting the positive half-cycle of the voltage to transmit a half-wave signal block over the pilot channel.

For external faults, these blocks are out of phase so that alternately the local and then the remote signal provide essentially a continuous signal to block or prevent tripping. For internal faults, the local and remote signals are essentially in phase so that approximately a half-cycle of no channel signal exists.

This is used to permit the fault relays at each terminal to trip their respective breakers.

References //

• Standard handbook for el. engineers – Substations by W. Bruce Dietzman and Philip C. Bolin
• Wire-Pilot Relays by GE GRID