Protection Relays

Protection relays monitor the current and the voltage, measured by the current and voltage transformers. Knowing these values, the relay can then calculate the impedance between the OHL and the track. Impedance is similar to resistance, except that it only applies to AC electricity.

Impedance, measured in Ohms, is known by the letter z, and is calculated in the following way:

Impedance = voltage / current

On the OHL network, the value of impedance is relatively high. The voltage is approximately 25 000V and the current is approximately 600 A. Therefore the impedance is approximately 42 Ohms.

If the value of impedance were to drop by an amount, outside of a set tolerance, the protection relay would initiate the contact breakers into opening, and therefore cut off the supply.

Reasons for the impedance dropping can vary. These can be:

• Magnetic tape, the kind from cassettes, touching both the OHL and the track
• De-wirement
• To many trains in the section, resulting in too much current being drawn

Neutral Sections

These are in place to separate the voltage between two feeder stations. The simple following diagram emphasises this point.

The neutral sections are, as the term applies, void of electricity. If the pantograph of an electric train were to stop under a neutral section, the train would not be able to start again. This may seem like bad design, but this is the only way of keeping the different electricity supplies apart.

The reason for keeping apart the electricity supplies is in case two different supplies are in different phases. Each supply is single phase, but can be one of many phases. If two different phased electricity supplies where to clash, big problems would be presented.

Detailed neutral section.

At the neutral section between the feeder stations, a Track Section Location (TSL) or a Track Section Cabin (TSC) can be found.

Current and voltage transformers

These are in place to measure and transform the current and the voltage to the overhead line. These are directly linked to the protection relays.

Circuit breakers

These ensure the flow of electricity to the OHL. Circuit breakers are basically large switches and can cut off the flow of electricity to the OHL. Usually, all being well, the breakers are in the position to make the circuit, that is, electricity will flow. Should the protection relay detect a problem with the OHL, specifically, a drop in impedance, it will operate the relevant breakers to cut off the power.

These circuit breakers can also be operated by the Electrical Control Operator (ECO) in the case of possessions and emergency work.

These breakers can now be found indoors, but the initial trend was to place them on structures.

Structure Mounted Outdoor Switchgear (SMOS)

SMOS is the equipment found at feeder stations, which is mounted onto masts or other structures. For a while, this kind of equipment was considered to be the best, but engineers are starting to realise that indoor switchgear is much more effective.

SMOS.

An overall simplified diagram of The OHL structure can be seen below:

General OHL layout.

Track Section Locations / Cabins (TSL / TSC)

A site visit to Leeds, Armley was also undertaken, to look at a TSL. This falls under distribution, and the job of a TSL is to provide an electrical bypass. The Leeds area is shown below.

OHL of Leeds area.

A simplified version of the TSL is seen below.

TSL.

A TSL and a TSC do exactly the same thing. The difference between them is that the TSC is housed indoors whereas a TSL is not. If the power supply was to fail at a feeder station, the electricity from the next feeder station along could be bypassed the neutral section via the TSL and used to supply the next section.

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