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TVA

Operations Continuing Training

Tennessee Valley Authority

TVA

WCF Unit 7 Precipitator Controls

2 Widow’s Creek Units’ 7 Precipitator Controls

Table of Contents

TABLE OF CONTENTS ......................................................................................................................................................................................

LIST OF FIGURES ..............................................................................................................................................................................................3

STUDENT OBJECTIVES ........................................................................................................................................................................... N/A

Precipitator System Overview ...............................................................................................................................................................5 SQ-300i Introduction ...................................................................................................................................................................................9 Alarms ............................................................................................................................................................................................................... 12 Rapper Controls .......................................................................................................................................................................................... 15 Explanation of Displays .......................................................................................................................................................................... 17

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Student Objectives

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System Overview Flue Gas Velocity and Flow Distribution: Flue gas ductwork is sized so that the gas velocity is high enough -typically about 50 feet per second - to keep fly ash entrained in the gas stream. Since the objective of a precipitator is to remove the ash, the first action must be to slow the gas down to about 5 feet per second. The reduction in velocity is accomplished by increasing the cross sectional area of the gas stream in the precipitator. A perforated plate at the transition from duct to precipitator evenly spreads the slowed gas over the face of the inlet field. The slow velocity allows ash particles to migrate across the gas stream to be collected on the plates, and keeps down turbulence which would scrub ash off of the plates. If the perforated plate is damaged or becomes clogged with ash, the gas flow will not be evenly distributed and poorer collection efficiencies will result.

Ash Collection: The precipitator is designed to collect the quantity and type of ash expected to be generated in the furnace. Energized electrodes and grounded plates are hung in an array in the gas path. Transformer/Rectifier sets raise and convert incoming 480 VAC to apply a DC voltage of 40 to 55 KV to the electrodes. This voltage potential causes a charged electrical field (corona) to develop around each electrode. As the flue gas carries the ash particle into the corona, the particle picks up an electrical charge. The strength of that electrical charge is proportional to the electro-chemical characteristics of the ash particle and the amount of time that the particle is exposed to the corona. As a general rule, lower sulfur coals produce ash which is more difficult to charge and larger precipitators or the addition of flue gas conditioning substances are required to ionize the ash.

After ionization, the charged ash particle is attracted to the plate, because the plate is at ground potential relative to the electrode. Movement of the ash particle from the electrode to the ground potential plate is current, and is measured in amperes, or miliamperes. The primary and secondary voltages, and primary and secondary currents are displayed by meters on the face of the control set cabinets.

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Since the movement of the ash particles is measured as current in the secondary, the current value is the critical indicator in assessing the efficiency of the precipitator field. The control circuitry for the TR sets contain logic which keys on the primary current of the transformer. The logic will adjust the secondary voltage - based on primary current - to achieve the highest voltage possible while minimizing arcing or overcurrent conditions. To asses the performance of a particular precipitator field, one must interpret the secondary voltage and current readings. Assuming that the meter is functioning properly, if no current is indicated, then no ash is moving from the charged electrode to the ground plate, and the field is not collecting ash from the flue gas. If a very high current is indicated (more than can be accounted for by the movement of ash) or the meter is erratic, then there may be a short circuit in the field. Indications for and causes of shorts, opens, and other failures are addressed in the attached fault tree. Normal secondary voltage is 40 - 55 KV, and normal secondary current is 100 - 300 miliamperes. Erratic voltage or current readings are evidence of arcing in the field, indicate abnormal conditions and should be minimal.

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Rapping: The objective in the rapping cycle is to move ash into the hopper while keeping it from being re entrained in the gas stream. The majority of the ash is loosely adhered to the plates by a weak charge, and disturbing it can allow it to be released to go out the stack. Lesser amounts of ash adhere to the electrodes and interfere with corona formation, and need to be moved first to the plates. Electrodes are rapped by drop rods. Rods are raised and dropped onto the rack supporting the electrodes at the roof of the precipitator. The intensity and frequency of the electrode rapping is adjustable. The shock is designed to shake ash loose from the electrode to improve corona development and allow the displaced ash to move to the plates. Plates are rapped by means of motor-driven individual tumbling hammers. The hammers are attached to a rotating shaft which traverses the section, and hammers are offset so that only one plate in each section is rapped at a time. As the shaft rotates, hammers fall sequentially on anvils attached to the bases of consecutive plates across the section. Every shock transmitted to the plates causes the ash to fall a portion of the way down the plate to the hopper, eventually falling off of the bottom of the plate into the hopper. The plates are designed so that gas velocity at the surface of the plate is minimal, preventing re entrainment of the ash. The intensity of the rapping is determined by the size of the hammers, but the frequency of rapping is variable. The motor driven shaft can be set to run continuously, or at timed intervals. The timing of the rapping cycle is coordinated with the transformer/rectifier (TR) sets. The TR is de-energized for the duration of a rapping cycle within a field, then re-energized. De-energizing the TR will cause the ash to be less tightly held at the plate and allow the ash to fall into the hopper more easily. Only one field per gas path (A or B) per unit is rapped at a time. The rapping sequence is critical to adequate ash removal. Each gas path is six fields deep. Each field collects about 80% of the ash that enters it; the first field collects about 80% of the total ash leaving the mechanical collectors, the second field collects about 80% of the ash not collected by the first field, and so on (The combined efficiency of the six field precipitator is about 99.7%). Since the earlier fields collect proportionately more ash, they must be rapped more often to prevent ash from accumulating on the plates and electrodes. The programmed rapping sequence is appears in the appendix, but shows that the first field is rapped six times per cycle, while the fifth field is rapped only once. If a field is removed from service for any significant length of time, the rapping sequence should be adjusted to account for the missing field.

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Ash Removal: After being collected in the hoppers, the ash must be moved to the ash pond. Vacuum is developed in the hydrovactor by forcing water from the ash sluice pumps through a venturi. The vacuum is applied through a system of headers and transport pipes to the fly ash valve at the base of each hopper. Segregating valves allow vacuum to be individually applied to the ash transport piping of each row, and fly ash valves at the bottom of each hopper are used to empty one hopper at a time. Through an automatic sequencing control panel, the segregating valve on one hopper row will be opened, and then the first fly ash valve will be opened to system vacuum and ash is drawn from the hopper into the transport lines. Ash transport air is admitted through a regulating check valve in the transport pipes at the end of each hopper row. The check valve is sized to admit the appropriate amount of transport air to facilitate movement of the ash. The sequencer in the control panel keys on system vacuum. It is extremely important that design vacuum parameters be maintained. Adjusting vacuum trip settings to compensate for low vacuum caused by leaks will result in inadequate ash removal, hopper plugging, and improper sequencing. As vacuum rises, a flyash valve is opened and ash is drawn from the hopper. When the hopper empties, vacuum will drop and the sequencer will close the fly ash valve, wait for vacuum to return to the specified value, and then open the next fly ash valve in the sequence. Ash removed from the hopper travels through transport pipes to the hydrovactor where it is mixed with the water used to develop vacuum in the venturi. Upon leaving the jet, air is removed from the mixture in a vented separator tank, and the water/ash slurry is allowed to gravity flow to the ash sluice trench.

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SQ-300i Operator Training

When the control software initiates, the first Operate screen will appear. There are seven Operate screens; each have the same basic format but provide a different set of data. Each of the operate screens contain an operating/functional area which displays real time data in the form of meters, digital displays and /or trends. To move from one Operate screen to the next, simply touch or click anywhere in this area of the screen. Once the control displays the seventh Operate screen, another tap will return you to the first Operate screen. Each Operate screen also contains a status area which displays the name of the currently selected SQ-300i, as well as its status.

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The next row contains buttons used for navigation to other screens and controls.

• The Functions/Operate button toggles the display between Functions Mode and Operate Mode.(While the display is in Operate Mode, the button will read “Functions” which will return you to the SQ-300i Settings screen.

• The Table View button allows you to view all of the controls in a list that includes key operating parameters for each.

• The Back and Next buttons will send the display to the previous or next control. • The Jump To button allows you to select a specific control.

Control Name

Status: Run, Halted, Main Power Off, Spark, Arc, Limits,

Previous Screen

Next Screen

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Press or click the Run button to start SQ-300i operation if the system is halted. If the system is already operating, this button will restart ramping.

• Press or click the Halt button to stop SQ-300i operation by halting the firing of the SCR’s. If the system is already halted due to an alarm, press or click this button to clear the alarm and then press or click Run to restart.

• Press or click the Trending, Scope, VI Curves or Energy Management button to access features for the control listed at the top of the screen.

• Press or click the Set SQ Parameters button to access programming for the control listed at the top of the screen.

• Press or click the Systems Settings button to access the second Functions screen.

• Press or click the Operate button to return to the Operate screens. • Press or click the Table View button to view all of the controls in a list that

includes key operating parameters for each. • Press or click the Back, Jump, or Next button to navigate to other control

screens. These settings are password protected and will only be accessed by engineering and qualified electricians.

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The SQ-300i provides six alarms to alert you to abnormal operating conditions. The trip points for these alarms can be individually set in the same manner as the Operating Limits. Determine the Alarm Limits Each of the overcurrent alarm limits should typically be set at 25% above each of the operating limits. To determine the percentages, do the following: Multiply the limit setting entered in the Operating Limit screen, by 1.25. Example: If the primary current limit is set at 200 AAC, multiply 200 x 1.25. The resulting primary overcurrent alarm limit is 250. Disabling alarms can be very dangerous. The decision to disable an alarm should only be made by a qualified electrical technician experienced with precipitator operations and the SQ-300i.

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Alarms Primary Overcurrent Alarm This alarm will occur if the measured primary current has exceeded this setting for 2 seconds. A setting of 0 disables the alarm for troubleshooting. The default/safe value is 200. Primary Under Voltage Alarm This alarm will occur if the measured primary voltage is below this setting for 30 seconds and primary current is at least 20% of operating limit. The maximum setting allowed is 700 volts AC. The default/safe setting is 0. Secondary Overcurrent Alarm The secondary overcurrent alarm occurs if the measured secondary current has exceeded this setting for 2 seconds. The default/safe value is 1990.

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Secondary Under Voltage Alarm The secondary under voltage alarm occurs if the measured secondary voltage at the KV input is below this setting for 30 seconds and secondary current is at least 20% of operating limit. The default/safe value is 0. Imbalance Alarm The imbalance alarm will occur when there are missing or abnormal half cycles in the secondary current waveform. This is normally caused by an open Main Line SCR. The default/safe value is On. Alarm Contact Type The data entered in this screen will determine whether the alarm relay output on the SCR Firing module will be Normally Open (NO), or Normally Closed (NC). Generally, the alarm relay output is tied to an interposing relay that can interrupt the power to the coil of the main contactor. If this relay uses a NC contact, then the alarm output should be set to NO so that when the relay is energized, it opens its NC contacts in the event of an alarm. If the relay uses a NO contact, then the alarm output should be set to NC. The default/safe value is Normally Open.

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Rapper Controls WinRap Display Rapper Status The Rapper Status page enables you to view various rapper facts.

• The first field shows when the rapper

was last fired. • The next field indicates how much

current was drawn when the rapper fired.

• The Status field gives the current condition of the rapper. ( OK, Failed).

• The Strike Count lists the number of times the rapper has been rapped. (This is a life-long count)

• The Fail List shows the time and date of rapper failure, and the type of failure.

• Use the Clear Fail List button to empty the Fail List for the selected rapper.

• Use the Reset Strike Count button to set the Strike Count to zero for the selected rapper.

This number is assigned by WinRap to further identify the rapper.

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Cycle Time The Cycle Time indicates the minimum amount of time it takes the rapper group to perform one complete run; time starts when the first rapper in the group is fired, and ends when the group begins again (by firing that same rapper). The Rest Time is the period that follows the last rapp (and its corresponding “time between” setting), at the end of the cycle.

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Explanation of Displays The first operate screen is the main display. This screen displays the key precipitator operating values, as well as any limit or alarm messages. During normal operation, the Operating area displays the real time precipitator performance through readings that are updated continuously. ( The only exception is Spark Rate, which is updated once per minute.) Primary voltage, primary current, average secondary voltage, and secondary current are displayed on virtual meters. Each meter shows the activity of one parameter. The current value of the parameter, which is constantly changing, is displayed underneath each meter, and the limit is displayed in the upper-right corner of each meter. The scale was determined when programmed.

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Definitions: Primary Voltage: True RMS primary voltage is displayed as Volts AC, and is updated continuously. Primary Current: True RMS primary current is displayed as Amps AC, and is updated continuously. KV Average: Average secondary voltage is displayed as KV DC, and is updated continuously. Secondary Current: Secondary current is displayed as mA DC, and is updated continuously.

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Beneath the meters, on this Operate screen are six other parameter readings:

Spark Rate: SCR Firing Angle:

Average Form Factor:

KV Max:

KV Min:

Average Line Frequency:

The actual spark rate, in sparks per minute, updated once every minute. The SCR firing angle is displayed in degrees, and is constantly updated. Is a number used for trouble shooting. KV Max (KV Peak) is displayed in KV DC. And is actively used in the operating parameters. KV Min is displayed in KV DC. And is also used actively in the operating parameters. Average line frequency is displayed in Hertz, and is updated every half second.

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Voltage Source: The line voltage source is used not only for power, but also for critical timing in firing the SCR’s. An accurate determination of time relative to the line voltage supply is made to synchronize the firing of the SCR’s to the supply. In order to obtain this synchronization, the SQ-300i produces a pulse each time the line voltage source crosses zero. Some equipment, such as adjustable speed drives, can distort the line voltage source and cause false zero crossings. This can create interference with proper zero crossing detection. These problems are often intermittent and difficult to detect. The system has filters to prevent this, but this screen provides an additional troubleshooting tool to detect these problems which may be affecting other plant equipment.

The second operate screen provides three more precipitator operating values, and displays limit or alarm messages. The first two parameters are displayed as meters. Input Power: Is displayed in KVA, and is updated continuously. Output Power: The real time rate of power usage is displayed in KW. SCR firing angle is displayed using a real time trend. This 3.5 minute trend updates continually, providing insight to any trends that might be developing. The current value is displayed above the trend. The Y-scale is entered when programmed. SCR Firing Angle: The SCR firing angle is displayed in degrees.

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The third Operate screen provides real time trends for Secondary Current and KV Average. Touch the screen anywhere in the top section to advance to the next Operate screen. The fourth Operate screen provides real time trends for KV Max, KV Average and KV Min.

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Table View The status area of each Operate screen includes a button that allows you to switch to the Table View. The table view allows you to view all of the controls in a list ( rather than one control at a time). This list will update the status and readings for each SQ-300i about once every two seconds. To access the Table View, press or click the Table View Button.

• If the SQ-300i control is halted, has an alarm, or is not communicating, the Control and Status columns will be red. ( Otherwise, they will be green.)

• To change the order of the controls in the list, go to Manage Controls. • To exit the Table View, press or click the Close button.

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