By: James J. Benke, Engineering Manager at Eaton Corp.
Renewable energy sources will help meet the challenge of electrical energy consumption which is projected to double in the next 20 years. Wind power, a critical part of the solution, is one of the fastest growing sectors of the global power generation industry.
Wind farms require a complex collection of equipment that must work together efficiently to generate electricity and transmit it to the utility grid. Here’s a thumbnail sketch of the power flow on a wind farm: First, utility-scale turbines capable of generating thousands of volts, send electricity to inverters to produce a uniform frequency and constant voltage. This cleaner power then goes to a substation transformer that increases voltage to distribution levels, typically 15 to 38 kV. Distribution power flows from the substation transformer to a collector where the power is combined with that from other turbines. In some cases the electricity is used locally, though it is typically sent to the substation where voltage is increased further for efficient delivery to distant cities and other loads. Each turbine and most of the transformers in this setup can be isolated using circuit breakers.
Medium-voltage circuit breakers with enhanced capabilities such as protective and monitoring devices, make it easier to get generated power so it can be distributed and used. The breakers are engineered to improve uptime, reliability and sustainability, while enhancing safety.
Protecting electrical equipment
Significant wind is both remote and variable, which means wind projects must be sited at such locations, often far from major markets. This adds significant dependence on long-distance transmission.
Long transmission lines are an occupational necessity because the nation’s vast power-supply system must deliver power from large, centrally dispatched power stations to a mostly passive distribution network. As a result, connecting embedded plants, such as wind farms, can require local system modifications. These might include switchgear or transformers to adapt the wind plant to grid-voltage and current-flow changes. Designers of such systems must address possible effects on the local voltage, thermal ratings of the wires, and the local fault level.
Some circuit breakers are engineered to address system wind-farm requirements, primarily controlling high over-voltages. These can come from construction accidents that cut underground cables, rodents and snakes that gnaw wires in transformer housings, lightning strikes, and other grid-related faults. If the turbine is not isolated from sudden high voltage, its generator will suffer damage that likely puts the turbine out of production for long and costly repair periods. In all cases, they must sense high voltages and quickly isolate the turbine.
Circuit breakers are more than switches. They improve uptime and reliability by recognizing conditions that do not require a disconnection, events that often trip older equipment.
Previous designs often required that a technician visit the job site – frequently a remote location – to verify the turbine remains job worthy. The technician much also reset the equipment, which can be a hazardous task because of the potential for arcs.
Environmentally friendly circuit breakers improve sustainability by avoiding the use of sulfur hexafluoride, SF6. This is a colorless, non-flammable gas used in circuit breakers on applications above 38 kV, rather than relying on air as an insulator and arc preventer. The gas was found useful in the U.S. to insulate switchgear in higher-voltage equipment.
But if some gas should leak during operation, or if cleanup is required after equipment failure, toxic byproducts pose safety concerns for those nearby. Also, dismantling and disposing of circuit breakers that rely on SF6 can result in similar exposure.
It is not necessary to rely on the gas for insulation. Recent vacuum-interrupting-based equipment can reliably switch potentially damaging high voltage and high-amp currents. They require no cooling or ventilation equipment, and avoid the use of SF6.
What’s more, a few circuit breakers are typically more compact than those they replace, which allows placing voltage transformers in a top compartment of the switchgear. This further reduces equipment footprint. And because switch gear are often mounted in a separate building, a more compact footprint means a smaller (less-expensive) building. For recent circuit breakers, such as Eaton’s VCP medium-voltage line, this means:
-Transformers can mount above the breaker to save space
-Encapsulated poles protect high-voltage components from the environment. High-humidity locations or those near oceans are subject to salt spray that can corrode components and encourage arcing.
-Motorized levering allows remotely racking the breaker, thereby enhancing personnel safety. This lets a technician position a circuit breaker in front of its switchgear, step away from the breaker, and signal a remote location to initiate the breaker insertion. The technician need not touch the equipment when it is energized.
Accessories for circuit breakers
All circuits are different so today’s breakers engineered for wind-turbine applications can be customized with accessories. A few include:
-Self-aligning and coupling primary and secondary disconnecting devices
-Trip-free interlocks prevent moving a closed breaker into and out of the connect position
-Coding pins ensure that only breakers of the correct rating are inserted into the enclosure
-Distinct latch positions for disconnect, test, and connected.
-Spouts to accommodate the mounting of six sets of current transformers per phase. WPE
Step away from the circuit breaker
Electric arcs in circuit breakers come from thermal ionization that occurs when current flow is interrupted as conductors separate. Thermal ionization can generate temperatures to 35,000°F during an arc flash. Enhanced safety precautions and the need to protect personnel from the dangers of potential arc-flash occurrences suggest the need to increase the distance between the operator and the front of a switchgear lineup during racking operations.
Eaton engineers have devised a motorized remote racking device that inserts or removes draw-out circuit breakers used in the company’s VacClad-W switchgear. The unit lets an operator move a VCP-W breaker between three pre-determined positions within the circuit-breaker compartment. Standing outside the arc-flash boundary of the equipment, an operator can use remote controls to disconnect the breaker, test it, or connect it to its circuit.
A few other features include:
• Racking motors are powered by an external 120 Vac source
• Permanently installed racking motors eliminate the need to lift, manipulate, or align heavy, bulky equipment
• 25-ft umbilical cord connects the hand-held controller to the circuit-breaker compartment
• Circuit breaker safety interlocks remain intact per IEEE C37.20.2
• Logic built into the device senses interference issues, and will not accept a command to connect a circuit breaker that has experienced interference until after it has been moved to the disconnect position.
• Controls can be integrated into switchgear secondary control circuits, or SCADA systems using ModbusT interface, or discrete wiring.
Filed Under: O&M, Turbines