Wisconsin Public Service Corporation has ordered a system comprising six Distributed Superconducting Magnetic Energy Storage (D-SMES) units. This is a new step in enhancing grid stability.

Demand for access to low-cost electric power is straining the physical limits of today’s transmission grids. The result? Voltage drops and possible blackouts. The traditional solution? More transmission lines or banks of capacitors. American Superconductor’s alternative solution is to deploy distributed superconducting magnetic energy storage (D-SMES) units at key points in the transmission network. When voltage sags occur, D-SMES instantaneously injects bursts of real and reactive power, resulting in stabilization of the grid.

Earlier this year, American Superconductor won a deal with Wisconsin Public Service Corporation (WPS) to install an energy storage system. The deal is the first application of D-SMES technology to improve a portion of a utility’s transmission grid.

The WPS Northern Loop is a 115 kV transmission system that stretches northward from Wasau, Wisconsin – a portion of the system that is somewhat isolated from the rest of the WPS grid – with a peak load of approximately 200 MWe. Expanding the load on this 200 mile long loop gradually caused the network to experience significant voltage instability problems, including momentary voltage depressions. Contingency analyses have indicated that, for certain line openings under known loading conditions, WPS faced the possibility that the grid would not be able to recover its nominal voltage, making persistent undervoltage or voltage collapse a distinct possibility.

The stability problem arose because of rapidly increasing load conditions in the region. In addition, being largely rural in character, the area that lies north of Wassau experiences significant summer peak loads as a result of tourism and vacation home construction. Moreover, a feature of the rural economy is its high concentration of paper mills, with large inductive motor loads. The heavy inrush of current drawn by these motors following system faults compounded the burden on the grid and, given the real and reactive power losses that occurred over the lengthy transmission lines, made voltage recovery more problematic.

WPS pursued the construction of a 250 mile 345 kV transmission line from Duluth, Minnesota to Wausau to provide a long-term solution for reliability concerns. This line will fortify the link between WPS and Minnesota Power, and strengthen the transmission grid on a regional basis. This line is due to enter operation in 2002.

However, WPS recognized the need to take immediate action to resolve the instability problem. A number of options were considered, including conversion of the 46 kV line to 115 kV, construction of additional 115 kV lines, traditional shunt compensation, series capacitance and the addition of Static VAR Compensators (SVC) to support the grid. Cost estimates for these more traditional alternatives ranged from $6-15 million.

WPS chose to solve this problem using six D-SMES units to improve voltage stability in the utility’s Northern Transmission Loop. By placing six units at five key substations on the Northern Loop, the system injects real and reactive power into the grid.

WPS evaluation

Network analysis that was carried out on the WPS system indicated that, under certain contingencies, voltage drops to 0.5 pu or lower were possible. The performance criteria set for all options required that system voltage be restored to a level of 0.90 pu within 0.5 seconds, and 0.95 pu within five seconds of a triggering event. American Superconductor configured D-SMES power electronics to provide large power bursts to enable rapid voltage recovery.

WPS selected D-SMES to solve the problem for the following reasons:

  • Performance. The stored power capacity of D-SMES made possible a more rapid response to voltage sags than other solutions.
  • Flexibility. The modular design of the D-SMES system made it easier to site, reconfigure or move to other locations in response to changing load conditions.
  • Local power quality benefits. In addition to grid stability benefits, the injection of real and reactive power at multiple points provided power quality benefits to customers served by feeders connected to the D-SMES units.
  • Short lead time. The small size and modularity of D-SMES allowed installation and service in less than 12 months, without delays resulting from siting, permitting or construction issues.
  • Lower cost option. The total cost for six SMES units was $4 million, compared with up to $15 million for other options.

    How does SMES work?

    A superconducting magnetic energy storage (SMES) system is a device for storing and instantaneously discharging large quantities of power. These systems have been in use for several years to improve industrial power quality, protecting customers vulnerable to voltage dips. A Distributed SMES (D-SMES) enables utilities to improve system reliability performance by correcting voltage stability and low voltage problems.

    The SMES recharges within minutes, and can repeat the charge/discharge sequence thousands of times without any degradation of the magnet.

    The SMES system uses an electric energy storage electromagnet wound with low-temperature superconductor (LTS) wire. The superconducting properties of the electromagnet, which is chilled with liquid helium, allow it to carry large currents without significant electrical resistance, and to be quickly charged and discharged.

    Power electronics contained in the SMES system, upon detecting a drop in voltage, can trigger a discharge of electricity and restore 90 per cent of the original voltage level in less than a second.

    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 on the transmission network, the controller directs real and reactive power from the inverter to the utility system. This rapid transfer of energy from the D-SMES to the utility system leads to a highly reliable performance on the transmission system.

    The D-SMES system

    D-SMES systems consist of multiple SMES units installed at substations throughout power transmission networks to provide enhanced transmission network reliability and capacity.

    D-SMES systems are connected to utility’s grids at substations, and protect the grids from the destabilizing effects of short-term events, such as faults that are caused by lightning strikes, downed poles, sudden changes in customer operations, and switching operations. In many cases, D-SMES is a cost-effective way to reinforce a utility grid without the expensive need to construct new lines. In addition, rather than protecting any specific customer, D-SMES protects the stability of entire regions of the grid, thus protecting many customers simultaneously.

    American Superconductor has focused on SMES systems as its product platform to address the need for solutions for power quality and reliability problems. Protection against power quality problems such as voltage sags can provide significant economic value to large industrial users of power. Various industry sources have estimated that the cost of power quality problems to industry in the USA alone to exceed $26 billion per year. Protection against power reliability problems, such as voltage instability and low voltage can also provide stablization of transmission networks and an increase in effective transmission capacity.

    The problem facing industry is becoming more severe as manufacturing operations increasingly shift to microprocessor controlled machinery, which is highly sensitive to voltage sags. More than 80 per cent of the power disturbances that cause plant shutdowns are voltage sags and interruptions lasting two seconds or less.

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