Transformers Chapter 11

Figure 11.8 – Current transformers used for differential protection in a electrical supply circuit

This system protects against line-to-line and line-to-earth short circuits on the feeder lines between the generator and the generator circuit breaker (GCB). Doughnut current transformers are placed around the feeder lines and secondary windings of each pair in series opposition to ensure that the full output from the generator passes to the load. Under no fault conditions, the currents at each end of the feeder lines are equal, so the induced EMF is in balance and no current flows to the differential protection relay. If a difference in current of 30-40 amps exists, a signal flows to the protection relay, which instantaneously trips the generator control relay (GCR) and the GCB, thus automatically disconnecting the generator from the system.

TRANSFORMER RECTIFIER UNITS A transformer rectifier unit (TRU) is used to convert AC into relatively smooth DC An example of a simple TRU circuit is that which is used in a car battery charger, as shown in figure 11.9.

Figure 11.9 – Transformer rectifier unit components

This device takes the mains 240 VAC and converts it to approximately 14 VDC to charge the battery. This is achieved by a transformer, which first steps down the AC voltage to a reasonable level and then converts it via a bridge rectifier assembly into DC. Electrics (Rev Q210) 11-5 Chapter 11 Transformers

Most large aircraft AC generator systems have dedicated TRUs, which operate on the same principle, although they are slightly more sophisticated. A typical unit is illustrated below.

Figure 11.10 – Transformer rectifier unit

The TRU that is fitted to an aircraft is typically supplied with 115 V 400 Hz three-phase AC, which is stepped-down through a three-phase star-star wound transformer and changed to 28 VDC by a six-rectifier bridge assembly. The output from the TRU is then fed to the aircraft’s DC busbars.

Each TRU has the following basic protections:

Overheat When operating, most TRUs are cooled by air from a thermostatically controlled cooling fan. If the TRU overheats (150°-200°) due to fan or other failure, a warning light illuminates on the flight deck. The TRU should then be switched off, either manually or automatically.

Reverse Current. When the TRUs are operating in parallel with some other power source, the failure of a rectifier in a TRU can cause a reverse current to flow into it and may even cause a fire. Reverse current protection in the failed TRU is designed to sense the fault current when it reaches approximately 1 amp, and disconnect the TRU automatically from the DC bus bars.

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Figure 15.10 – Current flow vs. voltage polarity or bias

If the negative terminal is connected to the N-type material, the diode is forward biased and current flows (i.e. it is in a conducting state), as shown above. If the diode is reverse biased, the positive terminal attracts electrons in the N-type material away from the junction. The negative terminal similarly attracts the holes in the P-type material, increasing the thickness of the depletion layer, as shown in figure 15.11.

Figure 15.11 – Depletion layer with reverse bias

If the diode is forward biased, electrons are attracted from the N-type material across the depletion layer to the positive terminal and the holes are attracted to the negative terminal, as shown in figure 15.12.

Figure 15.12 – Depletion layer with forward bias

A forward biased diode acts as a closed switch and a reverse biased diode as an open switch.

Electrics (Rev Q210) 15-5 Chapter 15 Semiconductor Devices

USE OF DIODES Diodes in their basic forms are used for rectification (or conversion) of AC into DC, for example, in a battery charger circuit, as shown in figure 15.13.

Figure 15.13 – Typical diode rectifier bridge

The diodes offer an easy path for currents to flow in one direction and offer a high resistance path in the opposite direction. During the positive cycle (1), current flows through diodes 1 and 3, whilst diodes 2 and 4 are switched off. The reverse occurs during the negative cycle, producing a DC output. The following special types of diode exist:

ZENER DIODE This is a special type of diode, which consists of a reverse-biased silicon P-N junction and is represented by the symbol shown in figure 15.14.

Figure 15.14 – Zener diode symbol

This type of diode is designed to operate normally when it is forward-biased, but unlike a conventional diode, it will also operate when high reverse currents are applied. When the reverse- bias voltage reaches a set value, the zener diode will break down, and thermal avalanche occurs. When this happens, one electron gains sufficient energy to knock others out of the valence band, causing a rapid increase in current flow through the diode, as shown in figure 15.15. This typically occurs from 4 to 75 V, depending on the design.

Figure 15.15 – Typical zener diode bias properties

15-6 Electrics (Rev Q210)