Modern mains frequency energisation and control
6.4 Automatic voltage control and instrumentation
Most investigators in this field have emphasised the importance of optimising electrical energisation in order to obtain the maximum collection efficiency. To accomplish this, two important aspects have to be taken into account.
(a) The size of the TR set and the energised bus section must be reasonably matched.
(b) The unit must have an efficient automatic voltage control (AVC) unit to optimise electrical operation under all operating conditions.
As already indicated, the key parameter governing the total corona power delivered by the TR set to a particular bus section is the firing angle of the thyristors. This angle is determined by the control unit for every half cycle of the supply frequency and must have the correct value according to the existing operating conditions within the precipitator.
The performance of any automatic control system is closely related to the type of instrumentation used to establish the precipitator conditions at any given time. The following section briefly reviews the signals typically used by the AVC units for control purposes.
The type of signal used for control purposes is typically dictated by the design approach used by different manufacturers and consequently are not always the same. The Europeans have a long tradition of using the actual precipitator current and voltage, the so-called ‘secondary values’, whereas the Americans have preferentially used the ‘primary values’, but in recent years the tendency has been to also incorporate the secondary values in their AVC units.
The use of r.m.s measuring devices along with high resolution analogue to digital converters enables modern microprocessor approaches to accurately sample the average operational voltage and current values of the precipitation fields. The actual parameters, which are used for control purposes to optimise performance in these later microprocessor based AVC units, are stored in a non- volatile memory using a key pad approach, and are not subject to change as could occur with earlier forms of controller.
Using the elements described above, coupled with a satisfactory control algo- rithm, it is possible to improve the controller response such that arcs within the precipitator are extinguished on a timely basis, thereby promoting improved
precipitator performance. When handling difficult high resistivity dusts, etc., improved control techniques, e.g. intermittent energisation, can be used to mitigate any fall in performance as will be examined in Chapter 7.
Furthermore, the installation of opacity (or extinction) meters in chimneys is becoming more common, and in some countries is now compulsory, especially in connection with new and retrofitted plant. The signals delivered by these meters are used for continuous monitoring of the stack dust emission (CEM), but sometimes are also used by the AVC units for optimising power consump- tion. The purpose of the opacity meter in conjunction with the control units is to:
(a) optimise the operation of the precipitator to achieve maximum efficiency, and to
(b) achieve energy savings for a set emission/opacity under easy operating conditions.
The signals typically used by the AVC to control precipitator operation are depicted in Figure 6.6.
6.4.1 Secondary metering approach
In Europe, the TR equipment is normally fitted with secondary metering for measuring the precipitator voltage and current. The mean voltage is measured by a voltage divider, usually of the resistance type, and the current by means of a current shunt resistor connected in the ground or earth return connection of the rectifier. For the measurement of the peak and trough voltages some form of capacitive divider is used.
In modern AVC control units the following quantities are normally measured and displayed:
Figure 6.6 Signals used in the instrumentation and automatic control of a high voltage power supply
mean precipitator voltage,
peak precipitator voltage (peak of ripple voltage),
trough precipitator voltage (minimum of ripple voltage), and precipitator mean current.
The mean values were initially used for control tasks but in the last decade, the importance of measuring the minimum or trough voltage value has become more significant [1]. This value is vitally important in evaluating the operation of a precipitator collecting high resistivity dust. Other important tasks using this feature are the automatic detection of back-corona and the control of the degree of intermittence, when operating under intermittent energisation [1, 2].
The peak voltage value, as previously indicated, is important in determining the optimum sparking level, i.e. maximum operating voltage in the precipitator and for optimising the electrical operation, e.g. during voltage recovery after a spark, and for computing the corona power.
In the past, the values of minimum and peak voltage were measured with an oscilloscope using a capacitance type voltage divider connected to the discharge frame unless one was actually built into the TR set. This measurement was, however, cumbersome for less experienced plant personnel, but today it is a standard feature in many modern AVC units using solid state integrated circuitry (ICs).
6.4.2 Primary metering system
In addition to the secondary values, primary input measurements are also used and displayed. These signals are used by some designs for automatic voltage control and/or monitoring tasks. It is now recognised, however, that the use of the secondary values is far superior for the automatic control and in the opti- misation of the precipitator operation, e.g. spark detection, voltage recovery after spark and back-corona detection.
The primary values, however, can be used in various monitoring tasks, such as the determination of
•
r.m.s. value of the primary current,•
r.m.s. value of the primary voltage,•
active power delivered to the TR set, and•
apparent power delivered to the TR set.The primary voltage and current are usually measured by means of potential and current transformers connected into the primary. This approach enables the resultant outputs to be readily connected to the AVC unit, since the transformers provide electrical isolation and, dependant on the turns ratio, adequate signal levels.
The active power of the electrical equipment has two components: (a) the corona power delivered to the bus section, and
(b) the losses in the TR set (transformer iron and copper losses, silicon rectifier diode conduction losses, etc.).
The active power delivered by the a.c. supply to the equipment has two main components:
(a) the active power delivered to the TR set, and (b) the conduction losses within the thyristors.
Because the latter is negligible, compared with the corona power, the active power measured by means of the primary values is approximately equal to the active power delivered to the TR set. The equipment primary values are also important in monitoring tasks, such as
state of the phase control thyristors,
the value of the transformer current, i.e. over current, the value of the transformer voltage,
magnetic saturation of the transformer, and the value of the form factor, etc.
6.4.3 Opacity signal and full energy management 6.4.3.1 Energy saving system
In the past, certain regulatory bodies have raised objections to systems which target set point (as opposed to optimum) emission levels. This has been success- fully countered by the argument that to reduce the emission from 50 mg Nm−3 down to 25 mg Nm−3 consumes a great deal of additional power. Figure 6.7
Figure 6.7 Effect of precipitator power input on opacity level
indicates that the additional power required on a large precipitator installation to reduce the emission by 50 per cent can be significant, hence the overall effect on global emissions in winning the fuel, transporting and preparing it for combus- tion and then having a 30 per cent electricity conversion efficiency, is significantly greater than that resulting from the slightly higher particulate emission. In any event, various companies have developed critical point control (CPC) type algo- rithms for controlling the power usage, which overcomes the ‘set-point’ approach to some extent [3]. It will be appreciated that the efficiency of any given installa- tion, and hence performance, is controlled by the specific power usage. For plants where the gas flow is lower than the design volume or the fly ash precipitates more readily than anticipated from the firing of a higher sulphur or lower ash content fuel, then the basic design efficiency can be readily exceeded. This in turn not only results in a very low emission but a significantly increased power con- sumption because of the reduction in space charge effects. It will be seen from Figure 6.7, which relates stack opacity/emission to specific power input, that, under typical operating conditions, as the power is reduced the opacity initially remains fairly constant, until at a certain power level it begins to increase rapidly. By choosing a certain opacity level as a critical control point (CP), it is pos- sible to curtail only the excess power used by the precipitator. By continuously monitoring opacity levels this routine assures that input power levels are reduced only to the point of a defined opacity rise (as low as 0.1 per cent). A set point is not used, and the system accounts for boiler load swings and possible changes to fuel blends by searching for the optimum performance levels based upon these factors. Figures 6.8 and 6.9 illustrate a typical control operation on an existing power plant precipitator installation handling a relatively easy fly ash, indicating it is possible to control the emission satisfactorily even following a power reduction of some 50 per cent.
6.4.3.2 Energy management systems
The principle used is depicted in Figure 6.10, which shows a three-field precipita- tor with two bus sections per field, each section being energised by a separate TR set, which in turn is controlled by its own AVC unit.
The opacity (or extinction) meter is mounted in the stack and delivers a 4–20 mA signal to a converter, where the opacity signal can be filtered, converted to a digital signal, etc., before it is applied to the control units. This signal is a measure of the actual dust emission in the stack, based on a gravimetric measured calibration, and is compared with a set point in each control unit, which results in a certain corona power control action in order to accomplish a particular objective.
This is a simple and economic solution and must not be confused with the more expensive approach, where the control units are connected to a common communication bus, which is connected to a ‘master’ or plant computer, via a ‘gateway’ unit. This computerised central control system for precipitators is covered later in a following section.
Figure 6.8 Critical control point operation
Figure 6.9 Results of power optimisation using critical point algorithm