Design & Construction Considerations For Wtsu Transformers

Design and Construction Considerations for WTSU Transformers

Design and Construction Considerations for WTSU Transformers

Introduction The conversion of wind energy to electrical power is one of the world’s fastest growing industries. In the US alone, wind power capacity has grown by a sizable average of 29% a year for last five years; wind power now contributes a little over 1% of the total US energy requirement. Giant sprawling ‘wind farms’ will soon step off the trade magazine spreads and become a common sight. Like with any other electrical transmission and distribution system, power transformers are at the very heart of power generation using wind energy. The initial design of a transformer can have profound implications on the future profitability of wind farms and thus, design and construction of transformers specifically for use in wind farms - including Wind Turbine Step-Up (WTSU) Transformers - assumes critical significance. Even though wind generation technology has shown marked improvement over the last few years, it continues to present some important system design, control and operation challenges. If these issues are not properly addressed at the right time, they may lead to significant system disturbances especially when interconnected with the existing power grid.

Wind Turbine 101 A wind turbine is a prominent symbol of the huge renewable energy generation market. Wind turns the turbine blades, which spin a generator shaft and creates electricity. A local transformer is then used to “step up” the electrical voltage, so that the electricity can then be delivered through transmission and distribution lines to domestic users. Wind turbines generally produce electricity when winds are at speeds of 8 mph or more. They shut down when wind speeds exceed 55 - 60 mph, for safety reasons. Modern wind turbines usually make use of a rotor with three large blades, ranging between 40 to 80 meters in diameter, to capture wind and extract energy from the largest possible volume of air. The blades are set at different angles to cope with varying wind speeds, and the generator and the blades can be turned to face the changing direction of the wind. The wind turbines are mounted on 40 to 100-metertall towers, so as to capture stronger wind flows.

Wind Turbines: Basic Components Electricity-producing wind turbines have four basic components: • •

•

A rotor consisting of three blades A generator that produces electricity in the form of alternating current A control and protection system that optimizes performance and keeps the machinery operating within safe limits A tower that raises the rotor off the ground.

Wind turbines come in different sizes and can be • used in both small and large-scale applications. Single small turbines, up to 300 kilowatts, can be used in a variety of applications, including battery charging, providing power to remote cottages or communities, and powering farms and industrial facilities. Utility-scale turbines may be 500 kilowatts and larger. These are often grouped together in wind farms or wind power plants to feed the electrical grid. By grouping wind turbines into wind farms, it is possible to generate electricity more economically and to produce enough energy to power thousands of homes. This also makes it cost-effective to maintain and operate turbines.

2

Design and Construction Considerations for WTSU Transformers

What are Wind Turbine Step-Up Transformers? A Wind Turbine Step-Up (WTSU) Transformer plays a critical role in converting the generator output to transmission levels and passing it across the interconnected power grid to end users. Wind turbine output voltages typically range from 480 volts to 690 volts. This turbine output is then delivered to the WTSU transformer and transformed to a collector voltage of 13,800 to 46,000 volts. The role of the WTSU transformer is critical and, as such, its design needs to be robust. WTSU transformers in today’s wind generation schemes have to cope with a combination of: • • • •

Wide variations in loading Harmonic and non-sinusoidal loads from associated control electronics and generators Sizing without protection for over-voltage, under-voltage or over-loading Requirement to ‘ride through’ transient events and faults

All of this sets the WTSU apart from its more conventional, off-the-shelf counterparts. It is neither a conventional distribution transformer nor is it a conventional generator step-up transformer.

The Need for Special Design and Construction Considerations Wind resources are often located in remote areas, far from existing utility facilities, and receive widely varying intensities of energy. These factors make wind a highly fluctuating energy resource that can experience major power swings of up to 25%. About 10% of the time, wind may produce an hourly output from 5-20% of capacity. Such variability may affect power systems negatively. Conventional distribution transformers and power generator step-up transformers usually experience more constant loading at higher levels. The thermal stress on insulation is thus naturally higher. WTSU transforms don’t suffer from these problems, but the lighter, more variable loading leads to other problems, such as: •

Core Losses Core losses can become a significant economic factor for lightly loaded or idle transformers. Operational scenarios with an average loading of 30-35% make using conventional price evaluation formulae inapplicable.

•

Thermal Cycling Varying loads can put repeated thermal stress on the winding, clamping structure, seals and gaskets. Thermal cycling also causes nitrogen gas to be absorbed into the hot oil, only to be released as the oil cools, forming bubbles which can migrate to the insulation and windings, creating hot spots and partial discharges and damage insulation.

Off-the-shelf distribution transformer and power generator step-up transformer designs cannot cope with these issues effectively, and will display higher incidence of insulation and dielectric failure.

3

Design and Construction Considerations for WTSU Transformers

The Solution: Custom-Built WTSU Transformers Custom-built WTSU transformers can be built from the ground up with these considerations in mind. The use of cruciform cores, more robust windings, clamping structures, seals and gaskets, and protective measures that prevent hot spots and partial discharges can all contribute to lengthening transformer life and improving reliability. Much like rectifier transformers, WTSU transformers must be designed for harmonics, additional loading, and have electrostatic shields to prevent transfer of harmonic frequencies between the primary and secondary windings. Off-the-shelf designs cannot really include all of these features and design considerations.

Pacific Crest Transformers Pacific Crest Transformers (PCT) has a long history in the designing of custom-built, energy efficient transformers for the renewable energy sector. Way back in the 1980s, PCT committed to designing and manufacturing superior quality, custom-built and specialty transformers in the most cost-effective and responsive manner possible. With over 90 years of continuous experience in building Padmount, Station and Secondary Unit Sub Transformers, PCT specializes in environmentally friendly and efficient liquid-filled distribution transformers. Pacific Crest’s transformers are designed to increase efficiencies in the renewable energy market. PCT grounding transformers are especially designed to withstand harsh wind farm grounding duty and PCT’s WTSU transformers, have robust round coil designs assembled on miter-cut cruciform cores for enhanced strength, maximum cooling, longer life and lower total cost of ownership.

4