Thyristor Bridges

15.2.2 Thyristor Bridges

Thyristors used in rectifier and inverter bridges are usually of two types. The first type is a three- terminal semiconductor that can only be turned ‘on’ by a control or ‘firing’ signal applied to its

Table 15.1. Operating modes of a three-phase diode bridge Mode

Rectifier angles

Approximate

Power factor

Power factor

angle φ

cos φ 1 0 0 0 0 1.0

HARMONIC VOLTAGES AND CURRENTS

‘gate’. It cannot be turned ‘off’ by the control signal. It can only be turned ‘off’ by forcing the anode current to zero, which is achieved by a special circuit that is connected across the anode and cathode, see References 6 and 9. This was the first type to be developed. In recent years a second type has been developed that can be turned ‘off’ by applying a reversed polarity control signal to the gate. This device is usually called a ‘gate turn off’ thyristor or GTO. Both devices are either in their fully ‘on’ state or their fully ‘off’ state when operating in normal bridge circuits. There is not an intermediate state such as found with transistors.

Thyristor bridges are used where the DC output voltage needs to be varied. For example for control purposes such as varying the speed of motors or for protective purposes such as limiting the maximum DC output current that can flow when an external short circuit occurs.

The basic circuit of a thyristor bridge is almost the same as that for a diode bridge. The essential differences are the replacement of the diode elements by thyristor elements, the inclusion of a controlled firing system for the thyristor gates, and in some cases the application of forced commutation circuits, see Figure 15.1.

15.2.2.1 Commutation

The commutation processes for Mode 1 operation of delay and current transfer are essentially the same as the diode bridge, except that the delay angle α is now controlled instead of occurring naturally ◦ ◦ and can be extended to 90 from 60 . The current transfer occurs in the same manner and gives rise to the same angle u.

Control of the triggering pulses to the thyristors needs to be carefully managed when the commutation is in Modes 2 and 3, otherwise the operation of the bridge may become unstable, see Chapter 7 of Reference 1.

The normal control range of the delay angle α is from zero to 90 ◦ , over which the average DC output voltage decreases from its maximum value to zero. In a good design of the bridge, with

an appropriate reactance in the supply transformer and enough inductance in the DC load circuit, the practical operating region is ensured to be within the Mode 1 operating range. If the load is a motor then it will produce an emf that has a magnitude roughly in proportion to the shaft speed. During transient disturbances there may be a wide mismatch between the output voltage of the bridge and the emf within the motor. The mismatch will cause a large current to flow, e.g. if the motor suddenly stalls, which may drive the bridge into a Mode 2 or 3 operation unless the protective control circuits rapidly take corrective action to prevent such operation.

15.2.2.2 Harmonic components

The shape of the waveform for the AC current in the supply lines to the bridge will be the same as that for the diode bridge. Hence the harmonic analysis will yield the same results for practical operating conditions. Table 15.2 shows the harmonic components for the range of u between zero

and 60 ◦ . The fundamental component is taken as reference.

15.2.2.3 Distortion upstream of the bridge

The installation of a rectifier bridge that has a relatively high power rating with respect to its supply will cause significant distortion to the supply line currents and line voltages.

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Table 15.2. Variation of harmonic coefficients with the commutation angle u

Harmonic Magnitude of the coefficient b n at different values of u in degrees number

The near-rectangular line currents will produce volt-drops in the series resistance-reactance cables, overhead lines and transformers. These volt-drops will be non-sinusoidal and will distort the waveform at their intermediate points of connection. At such points there may be a switchboard or distribution board and the loads connected to them will experience the distorted voltage waveform.

The line voltage waveform at the primary terminals of the transformer that feeds the bridge will be distorted by the short commutation pulses. These are often called ‘notches’. At the thyristors or diodes the notches have a near-zero base due to the temporary short circuit during the commutation. Immediately upstream of these elements is the impedance of the transformer, and beyond that the impedance to the main source of supply. A potential divider circuit exists between the bridge elements and the source of supply. Consequently the higher the transformer impedance the lower will be the impact of the commutation notches. Suppose the bridge is fed from a motor control centre that has its own feeder transformer. Since the feeder transformer and its upstream circuit has a finite impedance, there will be a certain amount of distortion to the voltages at the busbars of the motor control centre. The notching distortion injects high frequency currents into all the loads and instrumentation connected to the busbars. In many situations the loads are not sensitive to this form of distortion, but a few in a particular situation may be adversely affected, especially power factor correction capacitors and capacitors in fluorescent light fittings (if fitted). Retrofitting filters to an existing set of loads on a switchboard or motor control centre may be a difficult task to complete satisfactorily. Some instrumentation within or supplied from the switchgear may be requiring timings pulses or triggering signals that are derived from the busbar voltages. These signals may be disrupted by the presence of notching distortion.

The presence of high frequency harmonics in the power supply lines leaving the switchgear can cause mutual coupling to electronic and telecommunication cables if they are routed in close proximity to the power cables. This can occur especially if the cable racks run parallel to each other over an appreciable distance. As a ‘rule-of-thumb’ guide, derived from Table 13.1, the spacing (d) between power and electronic cables should be at least,

d≥ 300 + 1.75I n millimetres

Where I n is the current rating of the power cable.

HARMONIC VOLTAGES AND CURRENTS

The spacing need not be greater than about 1000 mm unless the parallel route length is very long.