High-Voltage Equipment

High-voltage equipment determines and controls the strength of the electric field generated between the discharge and collection electrodes. This is accomplished by using power supply sets consisting of three components: a step-up transformer, a high-voltage rectifier, and control metering and protection circuitry (automatic circuitry). The power system maintains voltage at the highest level without causing excess spark over between the discharge electrode and collection plate. These power sets are also commonly called Transformer-rectifier (T-R) sets.

In a T-R set, the transformer steps up the voltage from 400 volts to approximately 50,000 volts. This high voltage ionizes gas molecules that charge particles in the flue gas. The rectifier converts alternating current to direct current. Direct (or unidirectional current) is required for electrical precipitation. Most modern precipitators use solid-state silicon rectifiers and oil-filled, high-voltage transformers. The control circuitry in a modern precipitator is usually a Silicon-controlled Rectifier (SCR) automatic voltage controller with a linear reactor in the primary side of the transformer. Meters, also included in the control circuitry, monitor the variations in the electrical power input. A simplified drawing of the circuitry from the primary control cabinet to the precipitator field is shown in Figure 1.

The most commonly used meters are the following:

Primary voltmeter. This meter measures the input voltage, in a.c. volts, coming into the transformer. The input voltage ranges from 220 to 480 volts; however, most modern precipitators use 400 to 480 volts. The meter is located across the primary winding of the transformer. Primary ammeter. This meter measures the current drawn across the transformer in amperes. The primary ammeter is located across the primary winding (wires wound in the coil) of the transformer. The primary voltage and current readings give the power input to a particular section of the ESP.

Secondary voltmeter. This meter measures, in d.c. volts, the operating voltage delivered to the discharge electrodes. The meter is located between the output side of the rectifier and the discharge electrodes.

Secondary ammeter. This meter measures the current supplied to the discharge electrodes in milliamperes. The secondary ammeter is located between the rectifier output and the automatic control module. The combination of the secondary voltage and current readings gives the power input to the discharge electrodes.

Sparkmeter. This meter measures the number of sparks per minute in the precipitator section. Sparks are surges of localized electric current between the discharge electrodes and the collection plate.

The terms primary and secondary refer to the side of the transformer being monitored by the meter. Figure 2 shows the typical meters used on each ESP field and are located in the control cabinet.

The transformer-rectifier set ios connected to the discharge electrodes by a bus line. A bus line is electric cable that carries high voltage from the transformer-rectifier to the discharge electrodes (Figure 3). The bus line is encased in a pipe, or bus duct, to protect the high-voltage line from the environment and to prevent the line from becoming a potential hazard to humans. The high-voltage bus lines are separated, or isolated, from the ESP frame and shells by insulators. The insulators are made of no conducting plastic or ceramic material.

                                         Figure 1 -Schematic diagram of circuitry associated with precipitators

                     Figure - 2 . Typical gauges (meters) installed on control cabinet for each precipitator field

                                               Figure 3 -High-voltage system