Primary input control systems

5.5.1 Manual methods

The earliest system for controlling the input voltage was simple manual tap changing on the transformer primary feeding the high voltage unit. The control achieved by this system was rather coarse and time consuming, because the precipitator had to be electrically deenergised during the connection change;

Table 5.2 Acceptable test results for long term operation

Test method BSS 142 1972 ASTM (method)

Neutralisation number Max 0.4 mg KOH g−1 Max 0.4 mg KOH g−1 (D664)

Dielectric breakdown voltage

30 kV (2.5 mm gap) 22 kV min. (D877)

Interfacial tension n.a. 18 dynes DM−1 min. (D971)

Power factor n.a. 1.0% Doble limit (D924)

Colour n.a. 4 max. (D1500)

Water content 50 p.p.m. max. 55 p.p.m. max. (D1533)

Viscosity 40 cSt at 20°C n.a.

Sludge 0.10% max. n.a.

DDF 0.005 at 50 Hz n.a.

Flash point,°C 140 min. n.a.

consequently the unit was rarely optimised and the operators left the unit on a lowish output level to minimise their involvement.

The manual change over was subsequently replaced by a switch system, but again, since it was manually controlled, the TRs tended to be left in a ‘safe’ trouble free system by the operators hence optimum performance was rarely attained in practice.

5.5.2 Motorised methods

The development of the motor controlled auto-transformer or ‘Variac’ resulted in the operators being able to control the input supply voltage remotely by simply energising the motor control. The auto-transformer or Variac comprises a primary winding having a motor driven movable contact running along the winding surface, so that a continuously variable input voltage can be tapped off to feed the fixed secondary winding.

At the initial stage of application of this approach, precipitator instrumenta- tion was rather basic, usually consisting of a voltmeter across the HT trans- former, a primary ammeter and a secondary milliammeter connected across a shunt resistor in the earth leg of the rectifier circuit. As regards control, the primary voltage was manually increased until flashover was detected, then the voltage ‘backed off’ by some set amount to minimise circuit tripping. Unlike other electrical equipment that rarely experiences a flashover/breakdown situ- ation, that used for precipitation duty is always seeking the maximum voltage applicable to the electrode system, that is, just at the point of electrical breakdown, hence the equipment must be designed to cater for this condition.

A precipitator is electrically represented as a ‘leaky’ capacitor, that is, the electrode system forms the capacitor and the corona current the resistive com- ponent. The self-capacitance of a reasonably sized precipitation field can be some 40 pF and hence at an operating voltage of say 60 kV, the total charge stored within the system (q= CV2_{), is some 144 J, which can be dissipated during}

aflashover. In the early equipment, in order to limit the primary current rise, a resistance of around 0.5  was typically included in the primary, but could be adjusted up to some 2  dependent on the operating conditions experienced by the precipitator.

A typical circuit using a motorised Variac with line current limiting resistors is illustrated in Figure 5.16.

To eliminate spark erosion and wear on the moving contact of the Variacs, moving coil regulators were used as an alternative to control the voltage input. In one arrangement, the primary transformer feeding the HT transformer com- prises two windings, the secondary of which can either move horizontally along the axis of the primary or rotate within the primary such that the magnetising flux can be changed depending on the position of the secondary. Otherwise the operation of the system is similar to the circuitry illustrated in Figure 5.16.

On later equipment designs, in order to minimise current surges during flashover and arcing and simultaneously reducing power losses, the total circuit

impedance is arranged to be around 40 per cent, with 5 to 10 per cent being in the primary windings of the transformer, the remainder being in added as an inductor, typically having an inductance between 5 and 20 mH. The 40 per cent total impedance limits the primary current rise to around 250 per cent of the design line current of the transformer. Although higher values of total imped- ance would reduce the current rise, the time lost in re-establishing the optimum voltage on the precipitation field can significantly increase, with the resultant fall in precipitator performance during the recovery time.

5.5.3 Saturable reactors

An intermediate development was the current limiting saturable reactor, which was used prior to the significant changes resulting from the development of silicon based devices in the 1960s and 1970s leading to the current usage of thyristors or silicon controlled rectifiers (SCRs) as primary control devices. The principal advantage of the saturable reactor or magnetic amplifier over the earlier forms of control is the ability to control high levels of power by the application of small control signals.

The saturable reactor consists of an inductor, with a split core, having a secondary d.c. winding, the current through which determines the total flux developed and hence the inductor’s overall impedance. Its operation, therefore, depends on the variation of the inductance of the iron cored coils due to the level/change of magnetic flux within the core. A circuit diagram using this form of controller is illustrated in Figure 5.17.

During operation, the saturable reactor relies upon its inductive reactance in order to control the TR input power, but in doing so various difficulties can be experienced. Firstly, the saturable core reactance response is very slow in respect to changes in the d.c. energising current settings; and secondly, the creation and collapse of the magnetic field in the saturable core takes a relatively long time when compared with the standard 10 ms half line cycle. Several sparks may occur before impedance levels can be increased sufficiently to quell the undesir- able element of any sparking. Once established, an arc can persist for a longish Figure 5.16 Typical circuit diagram using a Variac and primary resistors

period of time, which can not only result in damage to the precipitator internal elements, but during this period the performance will be compromised.

The second problem with the saturable core reactor is that it is virtually impossible to shut the TR input current down entirely. The impedance of the saturable reactor is minimal for zero d.c. magnetising current, increasing with rising current flow until magnet saturation is achieved. This minimal impedance value can result in a serious ‘switch on’ problem, because without d.c. current flow, the maximum line voltage is directly applied to the HT transformer pro- ducing maximum output voltage which will lead to severe flashover within the precipitator. This situation, however, can be avoided by delaying the actual clos- ure of the contactor feeding the HT transformer until the d.c. circuit is fully energised and supplying current to the control winding of the saturable reactor.

Although the saturable reactor can produce a satisfactory control system for most applications, if a ‘soft arc’ develops then the controller will continue to feed current into the arc at around 150 per cent of the maximum line current even at maximum d.c. energising current. If this condition is encountered in practice the only way of returning the unit to normal operation was to isolate/ disconnect the primary feed to the HT transformer for a short period of time in order to dissipate the arc.

5.5.4 Silicon controlled rectifiers (SCRs)

The silicon control rectifier or thyristor is shown schematically in Figure 5.18. Its operation makes use of the breakdown of a reverse biased p-n junction con- tained within two outer layers referred to as the anode and cathode. The arrangement and characteristic curve is also illustrated in Figure 5.18.

The arrangement indicates that both the anode and cathode junctions are forward biased, but the control junction is reverse biased. Under this condition the current through the device is made up of electrons in the p-n control layer Figure 5.17 Typical circuit employing saturable reactor control

diffusing towards the anode and the ‘hole’ carriers in the n-type blocking layer migrating towards the cathode. Initially the current is fairly low and represents the saturation current of the reverse biased control junction; as the voltage across the device is increased, the current rises until the breakover point is reached. At this point the reverse field is strong enough at the control junction that avalanche breakdown occurs, allowing further current to pass such that the overall resistance of the device drops sharply. The potential difference across the device correspondingly falls and ‘kicks back’ to the normal low conduction characteristic. This breakdown condition is reversible and the original state may be obtained by reducing the applied voltage until the current falls below the holding current.

Figure 5.18 Thyristor arrangement (top) and characteristic curve (bottom)

The junction breakdown can be induced earlier by applying a positive potential to the control layer with respect to the cathode so that the reverse field is increased. The control layer is then called a ‘gate’ and an electron current flows into the control layer and hence to the anode. Breakdown can now occur at a much lower voltage and the current through the device is independent of the ‘gate’ potential and can only be reduced by reducing the anode-cathode voltage.

A typical thyristor ‘turn-on’ relationship between the gate current and the gate cathode voltage is illustrated in Figure 5.19. This, in addition to indicating maximum and minimun gate voltages and currents, also illustrates the effect of the ‘turn-on’ characteristics under differing temperature conditions.

For a transformer rectifier primary voltage controller, two thyristors are con- nected in an anti-parallel configuration, as indicated in Figure 5.14. One thyris- tor controls the positive half cycles and the other the negative half cycles. Both thyristors are balanced in that the gate ‘switch-on’ voltages are similar and are protected against dv/dt damage by a snubbing resistor/capacitor network; they are also typically fused against current overload. In operation, two conditions must simultaneously apply, the first is that the devices must be forward biased and the gate voltage must be adjusted to provide a ‘trigger’ signal. By varying the time delay between when these conditions are satisfied, the amount of time the thyristor is conducting varies; this variation is referred to as the thyristor conduction angle, which can range from zero, ‘thyristor off’, to full 180°, i.e. full firing. Typical waveforms exiting the thyristor assembly are also illustrated in Figure 5.14, from which, it can be seen that the r.m.s power in the primary circuit is controllable by adopting different firing angles.

Figure 5.19 Typical thyristor control characteristics

The majority of operational precipitators now use silicon controlled rectifiers or thyristors for control of the primary power and their usage will be covered in greater detail in Chapter 6.