Automatic voltage control

An automatic voltage control is an electronic device used to regulate the application of direct current (DC) power into a field of an electrostatic precipitator.
(PIC OF MVC4 FACE PANEL AND PIC OF INTERFACE BOARD)
Theory
• Optimize power application - The primary purpose of a voltage controller is to deliver as much useful electrical power to the corresponding electrostatic precipitator field(s) as possible. This is not an easy job; electrical characteristics in the field(s) are constantly changing, which is why a voltage controller is required.
• Spark reaction - When the voltage applied to the electrostatic precipitator field is too high for the conditions at the time, a spark over (or corona discharge) will occur. Detrimentally high amounts of current can occur during a spark over if not properly controlled, which could damage the fields. A voltage controller will monitor the primary and secondary voltage and current of the circuit, and detect a spark over condition. Once detected, the power applied to the field will be immediately cut off or reduced, which will stop the spark. After a short amount of time the power will be ramped back up, and the process will start over.
• Protect system components by adhering to component limitations - The Transformer Rectifier set (TR set) can be damaged by excessive amounts of current or voltage flowing through it. Each TR set has voltage and current limits established by the manufacturer, which are labeled on an attached nameplate (PIC OF A NAMEPLATE). These nameplate limit values (typically primary and secondary current, and voltage) are programmed into the voltage controller. Through metering circuits, the voltage controller will monitor these values, and ensure these limits are not exceeded.
• Tripping - When a condition occurs that the voltage controller cannot control, often times the voltage controller will trip. A trip means the voltage controller (by way of the contactor) will shut off the individual precipitator power circuit. A short inside the electrostatic precipitator field caused by a fallen discharge electrode (wire), or a shorted out Silicone Controlled Rectifier are examples of conditions that a voltage controller cannot control. (PIC OF CLOSE-UP OF TRIP LIGHT ON MVC4 FACE PANEL)
Operation
To maximize electrostatic precipitator efficiency a voltage controller usually attempts to increase the electrical power delivered to the field. However in some conditions a voltage controller must just maintain power at a constant level. Increased electrical power into the electrostatic precipitator directly correlates with better precipitator performance, but there is a limit. If too much voltages is applied for a given condition (as mentioned in the spark reaction section), a spark over will occur. During a spark over precipitator performance in that field will drop to zero, rendering that field temporarily ineffective.
To overcome the crippling effect that spark over has to increasing the electrical power in the precipitator field, spark response algorithms have been developed that will interrupt power upon detection of a spark, then ramp power back up to a high level. These response algorithms can greatly influence overall precipitator performance.
(PIC OF SPARK RESPONSE WAVEFORMS)
Response algorithms will vary by manufacturer. Some voltage controllers may also have additional algorithms such as back corona detection and suppression, intermittent energization, reduced power rapping, and performance optimization.
History
• The first Automatic Voltage Controls used simple discrete component analog circuitry to detect sparking and regulate power delivery to the primary of the TR set. The regulating device was a saturable (core) reactor. The saturable core reactor can, by changing inductance over a wide range, regulate power flow, but it is unable to stop power flow. This type of high voltage power system generally used dual bushing TR sets running in double half wave mode to provide a power off period (quench) every other half cycle of the power line.
• The next evolution in Automatic Voltage Controls was replacing the saturable core reactor with a pair of inverse parallel Silicon Controlled Rectifiers (SCR). The SCR completely stops power flow after each half cycle of the power line, enabling the SCR based Automatic Voltage Control to fully quench detected sparks. A non-saturating Current Limiting Reactor (CLR) is placed in series with the TR primary to limit surge current during spark over events and to smooth primary current waveforms. The actual controller sophistication was unchanged. The SCR / CLR power regulating elements remain in use today. From this point on controller design progressed.
• Automatic Voltage Control circuitry migrated from discrete components to analog IC implementations. This change improved reliability and stability of the analog circuitry over a wider temperature range and with time.
• Next the simple control algorithms of the analog Automatic Voltage Controls were implemented in relatively simple microprocessors in the 1980s. The microprocessors had sufficient speed to provide reasonably accurate metering of currents and voltages and to quench sparks in the half cycle following spark detection.
• In the 1990s, faster microprocessors supported more sophisticated control algorithms, diagnostic features and remote communications to host data gathering PC's.
• Also in the 1990s several TR manufacturers introduced high frequency switching power supplies with integrated controllers. Three phase AC is converted off the line into a DC power bus. An IGBT based inverter energized the primary of TR at a frequency in the range of 10 kHz. to 20 kHz. The high frequency operation greatly reduces the size of the actual TR windings and core compared to a 60 Hz. conventional TR. Another benefit is a factor of 10 reduction of ripple in the DC voltage supplied to the field which usually correlates to better collection efficiency.
• In the 2000 timeframe, highly integrated microcontrollers and economical complex analog integrated circuits made possible the design of Automatic Voltage Controls requiring absolutely no calibration.
Signals Monitored
• Primary current circuit - measures the AC current flowing through the primary side of the precipitator power circuit. Current Transformers (CTs) are typically used as the measuring device. (PIC OF A CT)
• Primary voltage circuit - measures the AC voltage on the primary side of the precipitator power circuit. This is done via a direct connection to the main power wires typically just before the transformer rectifier set. Primary voltages are between 400 and 600VAC (in North America).
• Secondary current circuit - measures the DC current on the secondary side of the precipitator power circuit. Since currents are very low on the secondary side, the voltage controller has a direct measurement circuit. Currents on the secondary side are usually in the 1 to 2 Ampere range.
• Secondary voltage circuit - measures the voltage on the secondary side of the precipitator power circuit. This is done via a voltage divider since voltages on the secondary side are too high to measure directly. Secondary voltages can be in the area between negative 45,000 and 100,000 volts DC. The voltage divider will convert the high voltage to a proportionate microampere current for direct measurement by the voltage controller.
Components in precip circuit (PIC OF PICTORIAL SCHEMATIC)
• Circuit Breaker
• 480V to 120V transformer
• Contactor
• Current Transformers (CTs)
• Silicone Controlled Rectifier (SCR)
• Current Limiting Reactor (CLR)
• Transformer Rectifier (TR) Set
• Voltage Divider
• Bus Lines
• Wires / Plates
Maintenance
Most voltage controllers require calibration over time. Recent advancement in voltage controller design has produced some voltage controllers that never need calibration. This is significant because it eliminates the need for electricians to work in live high voltage cabinets to perform yearly calibrations, which is an Arc Flash hazard. Due to recent laws in Arc flash safety, installing voltage controls that don’t require calibration is becoming a precedent. Otherwise keeping the cabinets and components clean, connections tight, and components properly ventilated is all that’s needed for maintenance.

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