Effects of direct current on molybdenum boosting electrodes

René Meuleman discusses the latest developments in SCR controlled glass melting furnace boost systems.

Electrical furnace heating systems based primarily on stepped or regulated transformers have been used for decades. With molybdenum electrodes, placed directly in the glass melt, the glass is heated by running high currents between them. These electrical boosting systems are normally used to increase the throughput of glass furnaces, applying heat to the lower regions of the melt when melting dark glasses and enforcing specific glass flow currents in the melt.

As an alternative to stepless variable voltage transformers, silicon controlled rectifiers (SCRs/thyristors), in fact high current solid-state switches, are also used. Since modern digital signal and microprocessor technology became available to control SCRs with extreme precision, it is also possible to have these SCRs on the secondary side of the transformer, saving investment in a second step down transformer.

Today, different SCR firing modes are available and modern dynamic load tap changing strategies are applicable to improve power factor and harmonic distortion. With SCRs directly connected to molybdenum electrodes, small direct currents can appear, leading to an unwanted decrease in electrode lifetime. To overcome this major drawback of having SCR control directly controlling the molybdenum electrodes, DC elimination strategies were successfully introduced. Today, SCR controlled glass furnace boosting systems are fully accepted by the industry, providing high precision, high performing and maintenance free solid-state technology.

Electro chemical effects on electrodes and DC enforced cathodic electrode protection remain largely unexplored. This paper focuses on the advantage of modern SCR technology, DC elimination methods and catholic protection strategies to potentially elongate molybdenum electrode lifetime.

Objectives

Lately, there seems to be a trend to increase boosting power to be able to increase furnace pull rates by remaining with the same furnace footprint. By increasing pull rates by applying more boosting power, the boosting system’s reliability becomes vital. Next to that, furnace boosting systems are an important tool to increase furnace glass temperature control performance and improved melting/fining behaviour.

Boosting has become an important controlled value for advanced process control systems as well. Modern high power electronics and transformer designs open up new possibilities for innovative and power saving furnace boosting layouts and avoid common disadvantages at the same time. However, one of the frequently noted disadvantages of this technology is possible DC (direct current) side effect of silicon rectifier controllers and its consequence on the electrode’s life expectancy. Electrode lifetime is extremely important for glass manufacturers as they want to avoid the extremely high costs of electrodes, the risks of full forwarding electrode operations and major boosting downtimes.

Since there is considerable misunderstanding about DC components and possibilities of DC elimination, this paper attempts to clarify some of these issues and separates sense from nonsense. It will focus mainly on so-called SCR (silicon controlled rectifier) systems.

Fundamentals

There are many ways of controlling the electrical power of a furnace boosting system. Before high power variable transformers became available, multiple tapped transformers, the power could be controlled only in multiple predefined steps and in many of these applications, the transformer’s primary voltage had to be switched off before the secondary taps could be switched. This led to unwanted high voltage switchgear wear and eventually, to unwanted boosting interruption due to necessary switchgear maintenance. Negative effects of incoming power fluctuations are difficult to control with such systems and therefore those incoming line fluctuations could have a direct and relatively quick and uncontrollable effect on glass temperatures. Today, commonly variable high power transformers are installed. The main advantage of these transformers is that they are capable of applying continuous controlled power to the boosting electrodes. These transformers are capable of controlling incoming power line fluctuations and are easily adapted to furnace (advanced) control strategies. The main disadvantage is that the primary slide wires are subject to wear, especially in those applications where they are used to control constantly incoming line fluctuations.

Repairs of mechanical parts in such transformers are expensive and time-consuming. Because they are normally oil-filled, they also have to be placed at a distance from the furnace and will consequently need to have long electrode wiring or busbars with related, unwanted power losses.Simultaneously, high power silicon-controlled rectifier systems became available and those systems also found their way into furnace boosting control systems. Their main advantages are that they are based on solid-state technology and will show no signs of wear, that they are continuously controllable and that they are capable of precise power, voltage or current control. Although the high power part of these controllers basically are still the same, the digital microelectronics have taken over the part that controls these SCRs, resulting in new ways of control and increased precision.

Direct current on electrodes

Early in the introduction of SCR-controlled systems, the so-called DC component discussion became an issue for the industry. However, to understand these issues, we have to understand the course of DC components, their possible effects on the process and ow that reflects on todays’s modern SCR-controlled systems. There are even DC currents that have a positive effect on the process and this effect deserves further study.The corrosion of molybdenum is influenced by many causes, which cannot always be separated. Generally and unfortunately, any DC components on molybdenum boosting electrodes are considered to be harmful for the glass melting process and/or electrode wear/lifetime.

Some glass manufacturers do not even want SCRs directly connected to their electrodes, preferring to have a transformer between the SCR control and electrodes. Some boosting system suppliers claim to be capable of measuring these DC components down to several millivolts but fail to specify what they measure. Some claim to measure and correct the DC component by shifting positive and negative phase angles in a way that they are counter-productive.

Some introduce a small DC power supply that inserts a specific counter-productive DC current. AS will be seen, all seem to be valid solutions but are difficult to prove their value. As far as I was able to find, there are three different DC components that should be considered:

• SCR control coursed DC components
• Electro-chemical DC components between the molybdenum electrodes and the glass
• Cathodic protection applied DC current.

Before discussing DC current effects on electrodes, it is necessary to consider that almost all publications proved that overall electrode wear is greatly affected by current density on the electrodes and the chemical composition of the glass. ‘The amount of corroded molybdenum is approximately equivalent within broad limits of the alternating current loading. Extending the surface of the electrode is a simple method to reduce molybdenum corrosion as a lower density of heating current is obtained. Therefore, the first consideration should be the amount of A/cm2 that is applied to the electrodes before other issues are taken into account.

SCR control coursed DC components

To understand the course of possible DC currents, the basics of SCR control need to be explained. Fundamentally, SCR controllers build on two anti-parallel thyristors. A thyristor can be seen as a solid-state, one way switch that can be switched on via a so-called ‘gate’ at any time during the alternating sinus wave, as long as the anode is positive and the cathode is negative.

The thyristor will stop conducting as soon as it runs through the zero voltage of the sinus wave. In the anti-parallel configuration of these two thyristors, one will take care of the positive part of the sinus wave and the other will look after the negative part. Thus, a pair of thyristors of this type is capable of controlling the alternating current.

As long as both thyristors are physically the same and they are fired at exactly the same phase angle, both positive and negative conducted sinus wave parts will be the same and no additional DC components will be introduced. Bear in mind that to be able to control these phase angles, it is important for the controller to predict the exact zero crossing of the applied AC (alternating current).

In the early days of SCR control, based on analogue firing technology, precise firing was questionable and high power SCR manufacture was difficult. Today’s SCR production quality and Eurotherm applied digital signal processor controlled firing method normally will not introduce DC components and basically therefore, no correction is required. In conclusion, SCR control coursed DC currents can only be introduced by SCR if they are directly connected to the boosting electrodes, operated in phase angle mode and/or if DC currents appear, they will always run between the boosting electrodes.

Electrochemical DC Component influences

Molybdenum is a metal of high strength and melting point (2610oc) and therefore, it meets the most important requirements to be used as a boosting electrode. There are a few exceptions like platinum, tin-oxide and graphite, the most commonly used materials for boosting electrodes.

Molybdenum and glass, especially under high temperature conditions, will chemically interact. Since molybdenum is one of the less noble, it will be oxidised by nearly all other polyvalent cations, causing specific amounts of DC currents running between electrodes and the glass melt, thus in between electrodes and ground electrode.

Cathodic protection applied DC current

According to the study of B Fleischmann and K W Mergler, electrochemical protection of molybdenum electrodes is possible. With the superposition of a negative direct current on the heating alternating current, the formation of a MoSix layer is observed that decreases the corrosion. However, the effect seems to be based on the DC current applied to the electrodes in respect of the melting tank refractory or a counter electrode.

However, some positive effects were also noticed when such a current was applied between the heating electrodes themselves. In-situ tests are needed to define the best results. Nevertheless, industrial tests on an all-electric melted proved that molybdenum loss of electrodes was nearly halved.

To be able to apply such a cathodic protection DC current, a middle tap on the boosting transformer is needed and the SCR power control part should be on the primary side of the transformer.

SCR-based boost control system layout

Since a huge amount of power is normally applied to the boosting electrodes, reaching from several kVAs up to several MVAs, generally the incoming lines are directly used to feed these systems. Incoming line tensions extend from 8kV up to 25kV.However, the boosting electrodes normally run at several hundred volts, depending on electrode line-up, glass composition and power ratings. Normal boosting tension will be around 200 – 300Vac. To convert the incoming line high voltage into the suitable electrode voltage, transformers are used in different system configurations.

Single transformer and single SCR control

A single transformer changes incoming line tension directly into the suitable electrode voltage. The SCR controls the amount of power by phase angle firing. Due to the single SCR, normally such applications will run in phase angle firing mode, resulting in poor energy efficiency and high harmonic line distortion. In such a configuration, DC components might be introduced and the application of a DC elimination system should be considered.

Step down transformer, on-load LTC and boosting transformer

A standard transformer changes the incoming line tension down to a predefined and standardised voltage. On-load LTC guarantees best energy efficiency and minimum harmonic distortion. A second (water-cooled) transformer near the electrodes provides minimum cabling/busbar losses. No possible SCR coursed DC components will be transferred by the boosting transformer and there is no need for a DC elimination system or fear for negative DC side effects.If desired, the system is adaptable to cathodic protection DC feed in on a middle tap at the secondary side of the boosting transformer.

Conclusions

Solid-state silicon controlled rectifier technology has come a long way since the introduction of glass furnace boosting systems. Today, ultra-fast microprocessor control and digital signal processing are part of modern free programmable SCR controllers. Most of the asynchronous firing issues are solved and the latest on-load become available. These methods increase efficiency, minimise harmonic distortion and stabilise total incoming line loads. Together with innovative and efficient transformer designs, the future furnace boosting system will look different.

These designs are not only capable of eliminating DC components but also open up for cathodic protection techniques.Many boosting and DC elimination studies have
been conducted but there is still little understanding of the subject in the industry. Increased use of boosting, the need for better performing furnaces, minimised maintenance efforts and an ongoing desire for best glass quality at acceptable costs will force us all to understand boosting better and leave conservative attitudes behind. However, the glass furnace is still one of the most expensive assets in a glass plant and nobody will put that at risk. By applying the SCR LTC duo transformer concept on boosting systems, the best of both worlds is available…no risk of unwanted DC components and a system that is still open for cathodic protecting DC infeed.

More tests on existing boosting systems should be undertaken to be able to separate the sense from the nonsense and move forward to innovative boosting designs.

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Article published in Glass Worldwide, issue 31, 2010.