What is SMES?
In a Superconducting Magnetic Energy Storage (SMES) system, energy is stored within a magnet that is capable of releasing megawatts of power within a fraction of a cycle to replace a sudden loss in line power.
How does it work?
In standby mode the current continually circulates through the normally closed switch of the voltage regulator and power supply, and back to the magnet. The power supply provides a small trickle charge to replace the power lost in the non-superconducting part of the circuit. When a voltage disturbance is sensed, the controller directs real and reactive power from the inverter to the load, while automatically opening the solid-state isolation switch within two milliseconds. When the voltage across the capacitor bank reaches a pre-set level, the switch closes. This sequence repeats until normal voltage from the utility feeder is restored. This systematic transfer of energy from the magnet to the load keeps the load interruption free for optimum performance of critical processes.
How fast can it recharge?
The SMES recharges within minutes and can repeat the charge/discharge sequence thousands of times without any degradation of the magnet. Recharge time can be accelerated to meet specific requirements, depending on system capacity.
What are SMES units made from?
SMES units use liquid helium to keep the coil of niobium-titanium at 4.2K, the temperature required for its material to become superconducting. While in the superconducting state, the conductor material has practically no electrical resistance, which enables the coil to carry large currents with very little loss for long periods of time.
Why use an SMES unit?
Quality power is essential to keeping businesses running. Voltage sags can disrupt the operation of variable speed drives and induction motors with disastrous consequences. In fact, it has been estimated that the total cost to US businesses of this lost productivity is a staggering $15-30 billion per year.
Cost of downtime in general manufacturing
Voltage instability problems plague many companies with continuous processing using induction motors and variable speed drives. Consequences vary with each industry, but loss of production is always detrimental to profitability, and sometimes safety. Some examples:
• A study of a dairy producing dry milk showed that voltage sags of less than merely 90% of nominal voltage cause relays controlling the flame control valves to drop out and the milk supply to the dryers to be disconnected, shutting the plant down for upwards of five hours at an economic cost of $87,000 per event.
• The momentary voltage sag caused by the activation of arc furnaces at steel mills can bring down the rest of the plant, causing hours of lost production.
• Induction motor stalling at paper mills in northern Wisconsin has been shown to precipitate the voltage collapse of a utility grid serving almost a third of the state.
• Various heat treatments for metal fabrications are precisely controlled. A voltage disruption during the treatment process can result in lost time and, most importantly, ruined product.
• Modern electronic devices are sensitive to voltage dips as small as 15%. Large computer banks can benefit from the quality power assurance offered by a SMES.
Industry Specific Problems
Voltage instability problems plague many industries in particular. These include paper production, motor vehicle manufacturing, petrochemical, chemical and pharmaceutical industries.
In modern facilities, a continuous paper web runs through several independently driven, speed-synchronized units. Any interruption in a drive snaps the web, and that results in downtime for cleaning and re-threading.
• A voltage sag of only 250 msec can shut a paper machine down.
• Restarting the machine may take hours, if the machine is not damaged.
• Production losses and damages at one particular paper mill have been documented to average $50,000 per sag.
• Documentation from a paper mill in Stanger, South Africa showed that in eleven months there were 72 voltage dips, and half of which could have taken the plant down if an SMES had not been in place.
Motor Vehicle Assembly
Problems typically begin with controllers falling out of synch because of voltage sags and have effects all the way down the assembly line and even at other plants. Some examples:
• The EPA requires the total emissions from a company not exceed a certain level per vehicle. This requirement is met by producing many low cost, low emission vehicles so higher profit margin vehicles, such as SUVs, can exceed the EPA standards without sacrificing product quality. Thus, a voltage sag caused break in production on a light truck line in Michigan can stop production in a plant in Alabama.
• Opportunity costs in the automotive industry are particularly high for production delays because of the high value of the final product. If a car comes off the assembly line every 50 seconds, at $15,000 a car, a one hour line stoppage costs over one million dollars in lost product.
• The high level of automation in a modern assembly plant makes plants particularly susceptible to power quality issues. Step motors and programmable logic controllers both are very susceptible to voltage disturbances.
The problems in the petrochemical industry range from straightforward loss of production or costly cleanup to highly dangerous explosive situations and/or environmentally unacceptable chemical discharges.
• During the operation of a refinery unit with high throughput and tight quality standards, a power disturbance can mean thousand of barrels of off-specification product.
• Voltage sensitive process controllers can trip off with sags as small as 15%.
• Perhaps even more important than the costs of lost product and damaged material is the safety of human lives: how do you quantify the damages from a refinery fire started from a damaged controller
Chemical & Pharmaceutical Companies
As in the petrochemical industry, the problems in the chemical industry range from straightforward loss of production or costly cleanup to highly dangerous explosive situations and/or environmentally unacceptable chemical discharges.
• At one ammonium nitrate plant, a 1500 hp motor drives a blower which rotates at 3600 rpm. During a voltage sag, which typically last just 300 ms, the control circuit voltage dips and drops out the controls, bringing the blower (and the manufacturing process) to a halt.
• Voltage sags at a cyanide manufacturing plant in Winnemucca, NV shut down a process that took more than 24 hours to restart. Voltage sags were the direct cause of an average of four days of lost production a month until an SMES was installed.
• One drug manufacturer stated that if the final process goes down in a line voltage sag, "instead of getting a batch of thousands of expensive pills, we get one ´mongo´ lump."