Miniaturized Isolation Amplifier for Current and Voltage Measurement in Small Wind Power Systems

Introduction

As one of the most promising alternative energy sources, wind power generation has not been affected by the financial crisis and economic recession and is expected to continue to grow at an annual rate of 22.4% over the next five years [1]. Large wind farms have also begun to expand into offshore deepwater areas, such as the 25MW Arklow offshore wind farm in the Irish Sea and Cape Wind, the first offshore wind farm in the United States, which can provide 420MW of power generation capacity.

In addition to focusing on large multi-MW wind turbines, which are expected to continue to grow in size and number of installations, most power inverter manufacturers face market opportunities in the small wind turbine market <100kW. Although the market for wind power inverters is relatively small compared to solar power generation, it is growing significantly. Many new developments are affecting this field, including Building Integrated Wind Energy (BIWE), such as the 1kW AVX 1000 and the 60kW WindCube?, and further developments in vertical axis wind turbines, such as the UGE 10kW VAWT [2].

Focus on the small wind turbine market

The US small wind turbine market grew by 78% in 2008, adding 17.3 MW of generating capacity, while the global growth was approximately 53% and 38.7 MW in the same period. US manufacturers accounted for 49% of global small wind turbine sales in 2008, continuing their long-standing dominance [3].

For the US commercial market for small wind turbines (21-100 kW), growth mainly came from increased production due to greater private investment. In the residential market, which has the largest market share, the market growth was driven by cost reductions brought about by investment and mass production. Rising household electricity prices and greater public awareness of this technology are also one of the reasons for the market growth. In

Europe, there are currently more than 40 manufacturers, mainly located in Germany and Spain, actively competing in the small wind power generation market. These manufacturers produce small wind turbines with power capacities ranging from less than 1 kW to hundreds of kW.

Small Wind Power Generation System The

small wind power generation system in Figure 1 usually includes a wind turbine, a generator, an AC-DC rectifier, a charge controller and a rechargeable battery module, an inverter, wiring, and a tower to support the entire system, among which the charge controller and battery module are usually required to meet the requirements of remote offline wind energy systems.

Figure 1: Simplified block diagram of a small wind power generation system.

Efficiency is key

Due to the variable nature of wind speed, many wind power generation schemes use variable speed control technology to maximize the energy from the wind and minimize the turbine drive-train loads. Many different variable speed control methods have been proposed and discussed in the industry, with the common goal of optimizing efficiency [5, 6].

In addition to the wind speed control part, the inverter also plays a very critical role in the power conversion process of the wind power generation system. Taking the wind turbine variable speed power generation equipment as an example, the inverter is the basic equipment that connects to the power grid and provides power that meets the power requirements. The inverter can be a commercial single-phase or industrial three-phase discrete component or module, and is controlled by a digital signal processor (DSP) to provide high-efficiency energy conversion. Efficient inverter systems require precise timing control and safety isolation of power equipment to prevent harmful high-voltage switching transients from damaging the controller and endangering operators.

Taking a 30kW power conversion system as an example [6], a simple AC-DC-AC converter and modular control method are used in the wind energy system connected to the power grid. Figure 2 is a similar block diagram of this power converter. Since the voltage and frequency of the generator output vary with wind speed, a DC-DC boost chopper circuit is used to maintain a stable DC link voltage. The input DC current is regulated to follow the preset optimized current reference so as to operate at the maximum power output point of the turbine system. When connected to the power grid, the PWM signal controls the IGBT through the ACPL-332J gate driver [7], which provides current to the power line by regulating the inverter DC voltage.

Figure 2: Block diagram of a small wind turbine system using an AC-DC-AC converter.

In order to achieve optimal system efficiency, the converter shown in Figure 2 and other references [6, 8] must feed back important current and voltage information to the digital signal processor (DSP) for calculation and effective control. This information may include the DC link current, generator phase current, inverter output phase current, and DC link voltage. Such requirements have brought about business opportunities for current and voltage sensors, which not only need to have sufficient accuracy and response speed, but also need to provide high switching noise rejection ratio and safety insulation capabilities at a high cost-effectiveness.

Figure 3: ACPL-C78X series gain frequency response diagram.

Although there is a disadvantage of shunt resistor power loss when measuring current and voltage using isolation amplifiers

, shunt resistor current sensing is a commonly used method for current measurement due to its advantages such as high linearity, low cost and design flexibility. With the development of better heat dissipation performance and lower resistance shunt technology, power loss can be minimized by reducing the size of the shunt signal.

Specifically designed to meet the stringent requirements of power conversion systems, the ACPL-C78A/C780/C784 miniaturized isolation amplifiers can accept ±200mV signals and are very suitable for shunt-based current sensing applications. By selecting the appropriate shunt resistor size, a wide range of current monitoring applications from less than 1A to more than 100A can be performed. The

ACPL-C78X series uses advanced sigma-delta analog-to-digital conversion technology and fully differential methods to achieve 1% gain error (ACPL-C78A), ultra-low nonlinearity of 0.004%, and a wide bandwidth from DC to 100kHz. Please refer to Figure 3. Available in a stretched SSO-8 package with 8mm creepage and clearance, the ACPL-C78X provides robust electrical isolation and is certified to 1140V working voltage per IEC/EN/DIN EN 60747-5-2, dual protection to 5kVrms/1min per UL 1577, and common mode rejection of 15kV/μs[9].

Conclusion

While different market conditions have given the wind industry a different look in each region, from the successful implementation of tariff protection systems in Europe to the combination of regulations, subsidies, and tax incentives adopted in North America and Asia, the wind energy industry is expected to grow rapidly. Although many eye-catching multi-MW wind power equipment has begun to enter deep water offshore areas, the majority of the power inverter business opportunities are still in the small wind turbine market.

The ACPL-C78X miniaturized isolation amplifier provides the precise current and voltage measurements required for optimized efficiency in typical small wind power designs. High switching noise rejection and high isolation voltage result in smooth inverter operation and safety for controllers and operators.