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Imagine this scenario… your plant has just undergone a major rehabilitation of its electrical system. It’s had multiple rewinds over the years and digital control and excitation systems have been added. The system is experiencing an all-time-high demand, and so every resource has been brought on line to meet the demand. The weather is hot and there are thunderstorms in the area. Everything seems to be fine.
And then it’s not. A lightning strike has taken down a major transmission line nearby. You nervously watch the control console and alarms, when suddenly, one of your plant units shows extreme overload. You think about your new equipment. In the space of seconds, your unit experiences catastrophic insulation failure because your protection system wasn’t able to handle the high transient energy spike. That unit is now offline during a critical peak load period.
M&I Materials has a solution to ensure your equipment will handle these high energy transients, which in turn prevents outages and increases plant reliability and resiliency. At best, testing equipment under the high energy conditions encountered during a major fault to be sure it will perform as anticipated is difficult. However, the High Energy Test Laboratory (HETL) provides original equipment manufacturers and owners a solution to this dilemma. M&I Materials has designed and commissioned this purpose-built test lab to provide certification that equipment is up to meeting high energy demands.
Improving reliability during times of high system stress
There are many scenarios that create high energy transients, and there are a variety of techniques to dissipate or suppress this energy. The situation becomes problematic, though, when considering high energy transients, as many surge suppression devices overheat and fail under such circumstances. These damaging transients can occur if a current surge exists for a long period of time, when a large amount of energy is stored in a system. To prevent damage or failure, this energy must be quickly dissipated.
In hydropower plants, it is quite common for excitation systems to be subjected to high energy transients. The exciter in the synchronous generator provides and controls DC supply to the electromagnetic field winding on the generator’s rotor. If there is a need to shut down the generator for maintenance purposes, or if a fault occurs, the high energy stored in the system must be discharged to prevent damage to the generator’s insulation and surrounding components. Failure to discharge can result in unplanned, costly periods of downtime. In extreme cases, the implications can be far more serious.
An electrical “safety valve”
A varistor is a resistor that has non-linear voltage and current characteristics. In comparison to a standard linear resistor, these characteristics allow the conductivity of the varistor to be very low at low voltages and extremely high at high voltages. Furthermore, non-linear varistors provide a much faster discharge time. These characteristics are beneficial and effective in dissipating high voltage transients in surge suppressor applications, thus preventing damage to, or failure of, electrical components or insulation systems.
Metrosil is a well-known name worldwide — so much so that it is often used as a generic term for a protection varistor. M&I Materials manufactures Metrosil varistors — in production for over 80 years — from a semi-conducting material based on Silicon Carbide (SiC). The properties of this material allow high energy to be shared between varistor discs, the number of which will vary according to “worst case scenario” transients. The ability to configure discs and de-excitation units in this way enables Metrosil to provide a customized electrical “safety valve” that will protect hydropower plant generator equipment and insulation from the effects of over voltages. This meets the demands of IEEE 421.6, which stipulates that a Silicon Carbide varistor in an exciter discharge system should clamp the magnitude of a high energy transient, absorb the energy stored in the field winding and provide a rapid discharge time to limit the extent of any damage.
Some of the largest hydropower stations in the world entrust Metrosil with protection of their plant equipment. Grand Coulee, La Grande, Mica, Revelstoke, Hoover, Itaipu, Xiangjiaba, Xiluodu and Three Gorges all use Metrosil varistors in their excitation systems. No matter what the plant size, a varistor can be an integral, affordable part of a protection system.
Testing and certifying dependability
In 2016, M&I Materials introduced the 8000 Series de-excitation unit. As part of this effort, they also designed and commissioned the HETL. HETL, with its ability to test and certify an excitation system, benefits owners and manufacturers. Metrosil’s Product Director James O’Brien says, “We refer to HETL as our very own on-site power station. With HETL we are able to run very high levels of energy through our de-excitation units, levels far beyond those likely to be encountered in their operating environment. This provides generator OEMs and suppliers with confidence, as they know their reputation is safe with us — their generators will withstand the ratings specified at purchase as proof of performance. Consequently, the confidence this gives to power utilities is invaluable, as Metrosil will protect their generator assets when problems arise with minimal disruption to power supply.”
Each Metrosil 8000 Series de-excitation unit is tested by HETL and certified as to its performance. A high energy test certificate is now included with the 8000 series. The certificate verifies that the varistors in the unit have already faced and discharged the highest surge they are likely to ever encounter. HETL makes high energy testing for excitation systems possible.
The HETL is rated up to 2,000 volts, 8,000 amps and 2 MegaJoules, and its role in the testing process is the key to Metrosil reliability. HETL’s impact is becoming clearer as greater demands are placed on the energy industry to ensure the reliable delivery of electricity with the needs of a growing population and increased variable generation. With HETL, Metrosil guarantees that each de-excitation unit will quickly and safely clamp overvoltages — the proof lies within the 8000 Series test certificate. This ability provides peace of mind to plant owners, operators, protection engineers and investors alike and will enable OEMs to follow IEEE 421.6.
“HETL was developed in direct response to market needs,” says Metrosil’s Product Development Manager Ian Rowley. “We know that our units are called upon to protect many of the world’s largest hydropower plants — so we needed a way to prove they would perform under extreme surge conditions. Even with 80 years of manufacturing excellence and a proven track record, we were determined to go the extra mile to prove our capability. HETL is the result of that process and the response from our customers has been extremely positive.”
Professor Jicheng Li, a senior excitation expert in China and the chief technical consultant of excitation systems at the Three Gorges hydropower plant, recalls when he specified the Metrosil 6000 Series of de-excitation units into Three Gorges in 2000.
“Back then we did not fully understand the product, but we trusted the brand — Metrosil is highly reputable in the power industry,” he said. “As we increased operations our understanding deepened and Metrosil varistors proved to be a successful de-excitation solution. In fact, the 32 700-MW hydropower units that were commissioned in 2008 have remained in perfect condition for over a decade now. This has heightened Metrosil’s reputation in China — so much so, that large hydropower plants choose Metrosil SiC products for de-excitation as standard. This means that Metrosil now protects hundreds of assets across the province. In China, the hydropower industry appreciates Metrosil’s rich heritage and recognizes its spirit of continuous innovation. A noteworthy example is the 8000 Series of de-excitation units, which were designed alongside end-users to meet installation needs. High energy testing is also valued as it ensures reliability — a term that is certainly synonymous with Metrosil.”
Now re-imagine that same hot and humid “peak demand” day. You see the system spike in energy, realize a major line has tripped and watch as your machines successfully respond to the transient energy. You breathe a sigh of relief and remember that day, not so long ago, when the outcome was so very different.
But wait, there’s more!
Knowing that owners and manufacturers are extremely cautious, M&I Materials is already thinking about the next step. Looking ahead to future testing demands, the company has developed the Metrosil Event Monitor, which is due to be released later this year as an optional piece of hardware for excitation protection units. This “built-in” energy performance monitoring tool will allow users to compare the energy absorption of a varistor unit with its original HETL profile at any point in time.
“Innovation is one of our core values and so we will continue to invest in opportunities to meet the needs of OEMs and end-users,” says James. “HETL is truly one-of-a-kind. We can manufacture, test and ship our units worldwide with proven reliability as our key differentiator. Our generator de-excitation units, together with our Event Monitor, will increase reliability for hydropower, which globally is becoming more critical in the provision of safe and dependable energy.”
170 people with a global reach
M&I Materials focuses on specialized materials for industry and science. The company has offices in the UK, U.S., India, China, South Africa and UAE. Their customers are located in 60+ countries around the world. The company has been independently owned for 25 years. The head office in Manchester, UK, is a 13,000 m2 facility that features manufacturing facilities. The company products meet international standards, including ISO 9001 and ISO 14001 certification for commitment to environmental issues.
Click here to visit Metrosil’s LinkedIn page.
Early roots of the company date back to 1901 with the establishment of the Mica Insulator Company Limited, which was formed to manufacture micanite based insulation for Europe. In 1906, the company became the Micanite and Insulators Co. Limited, often referred to as simply M&I. In 1958, M&I joined Associated Electrical Industries Limited (AEI), a financial holding company that had amalgamated Metropolitan-Vickers, a name synonymous with excellence and research in manufacture, with other leading electrical manufacturing companies. AEI was acquired by the General Electric Company (GEC) in 1967, and in 1988 GEC merged with Compagnie Générale d’Electricité (CGE) to form GEC ALSTHOM. M&I changed its name to GEC ALSTHOM (M&I) Ltd in 1990, becoming M&I Materials Ltd in 1993 after a management buyout, when the company acquired the portfolio of brands it manages to this day — Apiezon®, MIDEL®, Wolfmet® and Metrosil®.
For more information, contact: Ian Rowley, Product Development Manager, [email protected] or Gemma Shackleton, Marketing Manager, [email protected]
