NEW LIFE FOR OLD THYRISTOR POWER RECTIFIERS USING CONTEMPORARY DIGITAL CONTROL

Copyright © 1999 IEEE

Copyright Material IEEE
Paper No. PCIC 99-15

Abstract - The paper presents information on a control and monitoring system retrofit of two unreliable 1978 14 MW rectifiers used in a Sodium Chlorate Plant. Reasons for the upgrade and expectations of the new system are discussed. Alternate solutions considered and the rationale supporting the choices made are reviewed. The paper summarizes the project from the initial investigation through the design process and selection of equipment. Justification of the expenditure, issues of mating old and new equipment, problems overcome during the implementation and the lessons learned are described. In closing, the suitability of this approach for other rectifier installations is addressed.

Index Terms - Industrial power rectifier, Upgrade

I. INTRODUCTION

CXY Chemicals operates and maintains in their North American Plants approximately 20 power rectifier systems with ratings of 8 - 34 MVA. Seven manufacturers are represented and ages range up to 42 years. The dependability of 1970s first generation thyristor power rectifiers has been particularly troublesome relative to the diode machines of the same period and present day thyristor rectifiers. The worst of these older thyristor units require costly ongoing repairs. Production losses have been substantial with repeated failures and extended downtime, particularly during periods of full load production. To maintenance personnel fighting repeated battles to keep the equipment running, the effect on morale has been negative. Despite all this, convincing management to replace the equipment based on poor reliability alone is seldom successful due to the high capital cost and lengthy production outages.

Increasing the plant's product output was the key element in obtaining funding for this project.

This case study of the owner's Beauharnois plant near Montreal, Canada summarizes how increased output and reliable production were achieved at a reasonable cost without replacing the rectifiers.

Photo 1 - Beauharnois Rectifiers

II. EXISTING EQUIPMENT

The Plant operates two 52 kA, 275 Vdc output, 13.8 kV input, industrial-type, outdoor water-to-air cooled rectifiers, each supplied with a close coupled forced-air cooled oil insulated transformer. The circuit design is ANSI 45/46 with interphase transformer. Two rectifier cabinets are attached to each transformer, each containing a single-way, three-phase thyristor bank. As the two three-phase transformer windings supplying the rectifier cabinets are phase shifted, the combined output of the two cabinets is 6-pulse. The single way rectifiers consist of 2 legs per phase with each leg containing 12 thyristors for a total of 144 per rectifier system.

Controls were original first generation electronic design utilizing discrete analog components with gate drive and isolator cards mounted in the power cabinet. Trigger Amplifier, Gate Drive, DC CT electronics, auxiliary relays, fan controls and the majority of the control system were placed in the control cabinet at the end of the rectifier enclosure. Operator interface cards, meters and an alarm panel were all located in the remote operator control room located 300 ft from the rectifier.

Each 52 kA rectifier system powers a separate production line, is controlled by its own HV circuit breaker, and is equipped with its own alarm panel and control station.

Photo 2 - Original Power Section

III. REASONS FOR UPGRADE

A. Control System Electronics

Aging analog control electronics were particularly troublesome. After some 20 years of operation, repeated failures of random components were typical of the final portion of the bathtub-shaped reliability curve often used to describe electronic components. The manufacturer was no longer in business and replacement parts were not readily available. Detailed drawings of existing components had degraded over time; reverse engineering and outsourcing of replacement cards were costly and time consuming. Many repairs required specialized knowledge, not available locally.

Automatic load control was inoperable. The operators simply adjusted potentiometers in response to process changes. Demand control consisted of an audible warning that signaled the operator to reduce load via the potentiometer.

Similarly, the integrity of the fuse/thyristor failure monitoring system was questionable at best, making identification of failed components a laborious process. When faced with failures or impending problems, the electricians had to shut down the rectifier and manually check the condition of cards, and up to 144 fuses and thyristors. Furthermore, without a first out thyristor alarm, the operator would not know that load needed to be reduced to protect the remaining thyristors. This greatly increased the probability of cascade failures and rectifier outages.

B. Repeated Failures of Power Thyristors

Over the years, the make-up of fuses and thyristors had become a mix of manufactures with unmatched characteristics. As well, both the integrity of the fuses due to aging and of the connections due to varying assembly methods had degraded. As a result, thermal and current sharing problems arose among the 24 thyristors per phase. It is noted that equal current sharing is critical for continued operation of the thyristors. If one thyristor conducts much more than its rated current, it will eventually fail by overheating. As thyristors fail, the remainder are forced to share the load and additional thyristors are stressed, creating a cascade effect. Massive paralleling was a technological necessity at the time of manufacture and demanded careful matching of components, and made uniform load sharing inherently difficult. One solution utilized was the purposely designed bus and balancing reactor design.

On occasion, replacement of up to 24 thyristors per leg and an equal number of fuses was required, a costly repair, not to mention the downtime required to complete the repairs.

As the accuracy and precision of the gating (firing) signals strongly affect current sharing and the rectifier's total output capability, the unreliability of the aging control components fostered power section component failures.

Investigation of the rectifier repair history established a close correlation between each case of multiple thyristor failure and an electronic control system problem.

C. Maintenance Expertise

In the past, Plant personnel have lacked technical insight into equipment operation and preventive maintenance. With the recent emphasis on improving reliability and plant production levels, maintenance electricians have become quite adept at repairing the electronics with the limited resources available. Nevertheless, the frustrations of recurring failures with no solution in sight were counterproductive to the morale of those trying to keep the equipment operating.

IV. EXPECTATIONS OF UPGRADE

The needs for the rectifier upgrade were established as follows:

1) 105% of full-load rating of the rectifier must be achieved with reliability. On occasion, the Plant had operated at 105% but at the expense of rectifier failures.

2) Good operating reliability must be achieved. This was defined as reducing or eliminating unscheduled outages to the point where production levels were unaffected.

3) The upgrade must provide the knowledge, documentation, and continuing parts supply to undertake any needed troubleshooting in-house.

4) Outside technical support must be readily available.

5) A new system must fit into the existing control cabinet; the firing and fuse monitoring equipment must mate up to the columns in the power cabinets. The operator interface must be compatible with the old control panel.

6) The plant downtime to install the new equipment should be limited to 10 days per rectifier.

7) One rectifier per production line must be kept operating during the upgrade of the other.

The following want list evolved:

1) A digital control system was desirable for its stable operation and ability to deliver precise thyristor firing timing, a serious shortcoming with the existing controls.

2) Implementation of the control system in software with user-friendly programmable parameters. The software should include online diagnostics to allow planning for scheduled repair outages.

3) The control system would automatically monitor the condition of the thyristor fuses and be set to alarm, reduce load and trip the system based on the number of thyristor circuit failures.

4) Troublesome auxiliary relays, meters and interface devices would be replaced with a digital graphical interface displaying metering, alarm and control information.

V. CONSIDERATIONS

Early in the project cycle, the restrictions on available capital meant a creative approach would be required to meet the expectations of the Plant.

The need to examine the feasibility of a partial retrofit became evident in the early stages of the project. Several essential questions required answers to ensure a successful result.

Photo 3 - Control Cabinet Prior to Removal of Old Controls

A. What Is Reusable?

The major unknown at the beginning of the project was the continued viability of the rectifier transformers. The condition of these costly units was a major criterion for a successful retrofit. Current and past dissolved gas analysis reports and an internal inspection helped to confirm the integrity of the transformers.

The power section of the rectifier was in surprisingly good condition despite the repeated thyristor failures. The thyristors and fuses, although unmatched, were of a style in current usage and commonly available.

Auxiliary components of the rectifier, including the cabinets, heat exchangers, plumbing and fans, were also in satisfactory condition.

B. What Should Be Replaced?

The operating history of the plant clearly indicated the necessity of replacing the control electronics with something maintainable.

The operator interface consisted of a partially functional alarm panel, broken trend recorders, and worn manual potentiometer that all needed replacement.

Photo 4 - Control Cabinet Wall with Old Controls Removed (Partial)

C. General Improvements

The local attitude and level of interest in implementing an engineered solution were excellent. Knowledge of fundamental aspects of maintaining a power rectifier system was lacking. Education was required in the following areas:

1) Importance of and frequency of monitoring the dissolved gases in the rectifier transformer oil and monitoring temperatures of the rectifier power cabinet components.

2) Correct methods for installing replacement thyristors and fuses.

3) Importance of matching fuses and thyristors mounted in the same leg.

D. Gaining Corporate Approval

Approval for the expenditure hinged on the payback of a production improvement. The compelling question was whether the system was capable of additional output. Evaluation of the transformers and cooling system history indicated that overheating of the system was not a serious problem. New, precise control of thyristor firing and careful matching, installation, and thermal monitoring of components would provide better balance and improved operating temperatures. A first-out or high temperature alarm and automatic cutback in case of a thyristor failure would likely allow 105% output without impacting long-term reliability.

VI. SELECTION OF NEW EQUIPMENT

With the viability of the rectifier power section, auxiliaries and particularly the transformers reasonably assured, the next step was deciding what type of system to select and how to implement it.

A. Upgrade Options

1) Replace complete rectifier/transformer systems.

2) Retain transformers and replace rectifiers.

3) Replace the control, firing and fuse monitoring electronic systems.

4) Replace power section fuses, thyristors in addition to the electronics.

Options 1 and 2 were too expensive. Option 3 was necessary; option 4 was desirable but optional. Implementing a power section upgrade at a later date would be possible by careful selection of an equipment vendor at this time.

Next was the selection of the most appropriate system from the marketplace. After going out for quotations, choices included custom-designed systems as proposed from several engineering companies or adapting a system from an established rectifier manufacturer.

The rationale for the selection included:

1) Experience with similar system at other company-owned plants;

2) Proven reliability;

3) Vendor support;

4) Ease of reconfiguring, maintaining and troubleshooting;

5) Availability of local support;

6) Compatibility with a possible future upgrade of the power section.

The owner had purchased several new rectifiers from the same manufacturer in the past several years at competitive prices, with positive results. Accordingly, it was decided to order a semi-custom equipment package from the same manufacturer. In this instance the term "semi-custom" refers to the vendor's standard equipment packaged in a custom arrangement designed to fit into the existing rectifier design.

The owner elected to purchase new controls, gate drive and fuse monitoring systems. A remote digital operator interface completed the system.

Photo 5 - New Main Control Panel

B. New System

The new equipment consists of controls, gate drive panels, fuse monitoring circuits and a remote digital operator interface.

The control panel mounts on a wall within the control cabinet. The heart of the system is an industrial 486 PC running a RMX operating system. RMX is a real time operating system that is designed to improve reliability for industrial automation applications. Control system program changes are accessible by a laptop computer that modifies "sequencer" software, a soft PLC type ladder logic. A keypad is available for normal control functions in local mode.

Each rectifier control is provided with a current regulator of the proportional and integral (PI) type designed to control two 6-pulse rectifiers. Each 3-pulse side of a rectifier uses the controls for a 6-pulse system due to uncertainties in the original design to acquire optimum control and balance. The sides are designated "A" and "B".

The current regulator operates with a fast inner feedback loop and a slower outer loop. The regulator has a direct input to suppress firing signals. By going directly to the regulator, a very fast method of stopping firing under fault or emergency conditions results. In fact, the manufacturer's tests indicate this action will stop firing faster than the primary 15 kV breaker can open if a tripping signal occurs.

The annunciator is equipped with 160 programmable and 128 predefined points for faults and alarms. Diagnostics include predefined and programmable annunciator messages displayed on an LCD display. I/O consists of digital and analog inputs and outputs. Up to 20 "meters" and 16 "status lights" can be programmed and displayed in graphical format on the display. Firing uses a 120 electrical degree pulse-train to eliminate thyristor turn on failure. For electrical immunity, fiber optics are used to transmit the firing signals.

The control system is shielded and equipped with an LCD display and minimizes the effects of the surrounding magnetic field and EMI/RFI.

The gate drive, fuse monitor and protection circuits for each leg (total six per rectifier) are mounted on panels attached to the base of the columns in the power cabinet.

A fiber optic serial interface connects the control system to the remote digital input panel, where a keypad is mounted for operator input signals. The Remote Digital Input Panel is mounted behind the existing operator control console.

A CRT mounted in a shielded box recessed into the control console displays control, metering and alarm information to the operator.

1) Main Control Panel (photo 5): The supplier built their standard PC-based control system on a custom two-piece flat panel approximately 7.5-ft high by 4.5-ft wide and 16.5-in depth, designed to fit on a vacant wall of the walk-in control cabinet. CPU, firing interface, I/O, metering, protection and the fiber optic gating signals and control interface attach to this panel.

Photo 6 - New Gate Drive/Fuse Monitor Panels

2) Gate Drive and Protection Assembly (photo 6): Gate drive, fuse monitor and protection panels for each leg attach at floor level behind a clear acrylic cover. Pilot fuses microswitches and gate leads fasten on vertical glass fiber columns replacing the old columns which supported the gate protection cards (photo 2).

3) Operator Interface: The operator interface consists of a main breaker close/trip switch, E-stop, keypad and computer CRT monitor. The single CRT displays in graphical format all metering, alarm and control information. It replaces numerous chart recorders, panel meters, and a panel alarm. A VGA display extender amplifies VGA signal from the LCD display in the rectifier control cabinet and displays an identical image on the remote CRT through a multi-conductor co-axial cable. The keypad connects to the remote digital input panel I/O card and fiber optic modem transmits the signals to the main control system located in the rectifier.

Photo 7 - Remote Digital Input Panel

VII. INSTALLATION

The list of direct participants included the project manager, consultant, installation contractor, plant project engineer, plant maintenance electricians, process and production staff. To upgrade one rectifier while keeping the second in service required a concerted effort by all members of the team. Control systems of the two believed to be independent rectifiers turned out to be interconnected, requiring a methodical procedure to decommission and install the new equipment.

The original schedule called for the installation during a period of low production demand. A relatively leisurely schedule was called for with an installation contract based on a 3-week cycle for each rectifier; two 5-day weeks to install the equipment; 1 week to start up and commission each system. Typically, when the installation period approached, production demands changed and the careful scheduling became redundant.

Several meetings later, it was decided to use the original schedule for the first rectifier and speed up the work on the second unit once all the difficulties were discovered and resolved.

Commissioning consisted of wiring checkout, preliminary system checks and correct phasing tests. The phasing checks must extend from the phase reference supply through the control to the gate on each thyristor. Measuring each gating signal for the correct 1-ampere dc pulse of 120 electrical degrees assures the correct signal to each thyristor. Synchronization was confirmed using a dummy load. Finally, the rectifiers were operated with the process load; first in "Test Mode" to determine the feedback scaling, polarity and load time constant, then switching to automatic and fine tuning step response and gain.

The first rectifier was equipped and commissioned within the scheduled time. The second unit took fifteen days or half the time of the first.

Photo 8 - Before/After Operator Console

VIII. REFLECTIONS AND COMMENTS

One of the more perplexing problems that surfaced during commissioning affected the fiber optic firing circuits. Disconnecting the fiber optic connections one sunny day to complete a test, the power supplies to the gating circuit were inadvertently left on. Suddenly one of the team smelled the distinctive odor of smoking electrical parts. Sunlight shining through an open rectifier door had turned on the gating circuits via the open optical connection. The gating circuits designed for a 1/3-duty cycle had failed when turned fully on for several minutes. Fortunately, damage was limited to several power resistors.

Limitations of field test equipment prevented the testing of the short length 7.6 m (25 ft) of fiber optic cables used to deliver gating pulses to the thyristors. On start-up several of the cables were found to be defective, likely from lack of care in handling when they were removed and re-pulled into the control box. As the cable size and connector type were an older standard, connectors were in short supply. Accordingly, repairs involved both the re-termination of existing and installation of replacement cables.

The VGA extender system used to transmit graphical images to the operators control console from the main control resulted in a low cost solution without the expense of a separate HMI system. Actual operator inputs were accomplished with a very robust keypad/digital I/O/fiber optic modem system. With the plant expecting to install a DCS system the following year, switching the graphics over to the DCS is under consideration.

The original control used a number of auxiliary relays and switches to interface control, alarms and monitoring signals. The new system eliminated most of these devices. Nevertheless, corrosion and wear in the few that remained caused most of the startup difficulties. In retrospect, replacing all these devices would have resulted in a smoother startup.

Benefits of the new control system include better balance, slightly cooler transformers and greatly reduced thyristor failures due to tighter control.

The plant has adopted a structured procedure to procure, install and monitor thyristors and fuses to help ensure current balance and minimize future thyristor failures. Matching sets of thyristors and fuses for each leg was critical for the planned increase in output.

Digital control and the use of fiber optics improved the accuracy and precision of firing. Matching of components and ensuring proper operation of the cooling systems all contributed to additional output through incremental improvements.

On the final day of commissioning, the rectifiers reached full load and the load was increased to 105% the following day. The rectifiers have run constantly since that time, except for one control card that failed due to infant mortality.

IX. SUITABILITY FOR OTHER APPLICATIONS

This application is suitable for similar thyristor rectifiers when seeking improved reliability with limited funds. To help assure a successful outcome, the following is applicable:

1. The condition of the rectifier transformers is critical. A rewind or major repair is costly and time consuming. Usually custom-built, with a complex configuration, these transformers are very difficult to source with suitable specifications on the "used" market.

2. A history of dissolved gas analysis preferably with an internal inspection is very helpful during the decision making process. Records from the original manufacturer can provide valuable design information in order to evaluate the transformers additional output capacity.

3. Condition of the enclosure, cooling systems, balancing reactors, maintenance history, operating environment and the original rectifier design constraints are all important considerations. In some cases, it may actually be less expensive to replace the rectifier and/or transformer.

4. In order to sustain the intended long-term operation of a retrofit, emphasize the need for plant stakeholders to take ownership and understand the importance of performing preventive maintenance, stocking sufficient spare parts and maintaining technical support contacts.

Photo 9 - Installation Complete

X. CONCLUSION

The feasibility study was completed summer 1997 and the upgrade completed fall 1998.

To date, plant operation has proven that reliable performance and increased output are achievable on a problematic early design thyristor power rectifier.

By installing up-to-date digital controls supported by the implementation of practical preventive maintenance methods, a successful retrofit has met the requirements of the project.

Lower capital requirement of the upgrade versus replacement made the difference between corporate approval and having to stay on the path of reactive repairs and extended downtime.

The continued reliable operation of the rectifiers will depend on closely monitoring temperatures and transformer dissolved gas levels. Summer 1999 ambient temperatures at the plant will approach 100°F (40°C) and will provide valuable information on the ability to run at extended output year round.

XI. REFERENCES

[1] Ray Bernadelli et al, "Control Modernization of SCR Rectifiers with Continuous Device Current Monitoring," IEEE PCIC Conference Record, PCIC 97-19, pp 183-190.

[2] R.L. Doughty et al, "Optimum Electrical System Design for a modern Chlor-Alkali Plant", IEEE PCIC Conference Record, PCIC 88-51, pp 139-150.

[3] Power Converter Handbook, Canadian General Electric Co. Ltd., 1976.

[4] IEEE 34.2-1968, USA Standard Practices and Requirements for Semiconductor Power Rectifiers.

[5] MI-7805, Megaverter Instruction Book, Oxymetal Industries Corporation.