Automatic control systems

5.6.1 Early current control

With the rapid increase in industrialisation following World War II and the tightening of particulate emission regulations, it became important to be able automatically to optimise precipitator performance by operating the precipita- tionfield close to the breakdown voltage at all times. The first attempts utilised the rapid increase in primary current resulting from flashover and a simple ‘ramp–detect–back off’ approach was developed. Although an improvement over manual adjustment of the voltage, to minimise wear on the moving con- tacts of the auto-transformer then in common usage for primary voltage con- trol, the ramp rate had to be fairly slow, with a 30 s response time being typical. The overall system therefore lacked inertia to control the precipitator in order to achieve optimum performance.

Figure 5.20 indicates how the mean operating voltage on the precipitator changes with flashover rate. This shows that for a specific application, the opti- mum precipitator voltage is achieved with a flashover rate of around 120 per minute. Once the optimum sparking rate has been reached any further increase in primary voltage results in a decrease in mean operating voltage and hence efficiency at the expense of increasing power usage because of ‘active’ time lost through arcing. Although for the particular application, Figure 5.20 indicates an optimum sparking rate around 120 per minute, for different operating condi- tions the optimum rate may differ being basically site/application dependent.

Figure 5.20 Effect of flashover rate on mean voltage on precipitator

With the change from auto-transformer to saturable reactor for primary con- trol, the ‘ramp–detect–back off’ approach enabled the system to have an improved response time of some 50 ms. This meant that the precipitator voltage could be maintained closer to the breakdown condition with changes in operat- ing conditions within the precipitator field and hence achieve improved overall performance levels.

5.6.2 Voltage control method

Since the precipitator performance is proportional to the operating voltage squared (see Equation (2.14) ), it became important to develop a means of moni- toring the operating voltage on the discharge electrode system. This was achieved by series connecting 1 M resistors to produce a string having a total resistance of around 100 M. This string was connected directly to the HT line and the lower end connected to earth through a 30,000  resistor in parallel with a micro-ammeter; this approach readily enabled the average electrode voltage to be evaluated/measured. Because of the large time constant of the circuit, only average values can be measured using a resistor chain; for instantaneous and peak voltage measurements the voltage divider must be of a capacitance type. [Note. To protect personnel and instrumentation against potential high voltages, the voltage micro-ammeter and secondary current milli-ammeter must be connected across substantial shunt resistors connected to earth.]

Figure 5.21 indicates the relationship between input voltage, precipitator voltage and emission for a power generating plant precipitation field, which supports that the minimum emission, i.e. optimum collection efficiency, is coincident with the maximum precipitator operating voltage. This led to a

Figure 5.21 Principle of hill climbing form of controller

system of ‘hill climbing’ being developed as an alternative to the ‘ramp/fall back’ approach; here, instead of rapidly ramping the primary voltage up to flashover, the primary voltage was steadily raised in steps until flashover was achieved, at which point, the control system lowers the primary voltage by one or more small, 0.5 kV equivalent controllable steps, depending on the basic operating conditions to eliminate the flashover. After a certain time period, the raise system is again initiated which slowly lifts the voltage up to flashover and the whole control system cycles about the maximum flashover voltage in a controlled fashion.

There have been numerous discussions within the precipitation industry as to which system results in the best precipitator performance; however, under ideal operating conditions both can produce similar efficiencies and there is little to choose between them. The ‘hill climbing’ approach, however, tends automatically to select the optimum operating voltages, whereas, the ‘ramp/fall back’ approach may need manual adjustment to optimise the sparking rate.

Although either approach can result in better overall performance levels than manual forms of control, it is important that the AVC operation produces a certain degree and intensity of flashover. This is necessary to maintain the dis- charge electrode emitting points clear of deposit, otherwise the true breakdown voltage tends slowly to ‘fall back’, which results in a lower than optimum effi- ciency. Site investigations have indicated that unless the control point corre- sponds to a similar voltage value as that obtained from the V/I ‘clean plant’ characteristic, then the AVC can operate, apparently satisfactorily, at a voltage some 3–4 kV lower than optimum, which results in a significant fall in corona current. (A recent test indicated that by increasing the flashover rate and inten- sity resulted in an additional 200 mA being supplied to an inlet field, with corresponding increases in the downstream fields as a result of reduced space charge effects.)

To minimise reflected transients arising during flashover, which can impact on the circuit components, particularly the rectifiers, the output line connection is typically fitted with heavy duty line control resistors or high voltage inductors of some type. Following a flashover there is always a significant ‘inrush’ current to recharge the capacitive element of the precipitator, since during the break- down the time constant of the precipitator is almost zero being a capacitor discharge. For installations employing high voltage cable rather than direct bus connection, these line control resistors or secondary inductors are essential. Depending on the length and total capacitance of the cable and other circuit components, the induced voltages resulting from ‘ringing’ can be at least twice the nominal operating voltage of the precipitator and have been known to result in cable failure.

5.6.3 Computer control methods

Most operational plants presently employ computer based automatic voltage control systems. These, dependent on the programme and using electronic

switching, have response times in the order of milliseconds which overcomes the in-built mechanical limitations of the earlier approaches.

With the rapid development of computers during the late 1970s and early 1980s, leading to the production of A/D converters, analogue readings from primary and secondary ammeters and voltmeters can be produced as digital signals and directly used in specially adapted computer programmes. These pro- grammes tend to be specific to any one manufacturer, and since each manu- facturer will have a preferred method of control philosophy, it is only possible to consider general principles of operation. These approaches will be reviewed in Chapter 6, but are mentioned here to complete the section on component and energisation system developments.