After energy is produced, it needs to be transferred to the electric energy grid, frequently referred to as the electric power grid, so that it can reach consumers. This is true of every form of energy, from renewables to fossil fuels.
In this article, we outline three energy conversion methods that take energy from production sources and transfer it to the electric energy grid. First, we discuss the traditional energy transfer method: synchronous generators. This energy transfer system has been employed for decades because it easily integrates with traditional energy production facilities, typically fossil fuel burning power plants.
We also explore two new forms of energy transfer that are less expensive and used to connect renewable energy sources to the grid: induction generators and inverters. Induction generators are generally used with wind turbines, and inverters are used with all solar installations and some wind turbines.
The Importance of Energy Transfer
It is vital that energy be transferred to the electric energy grid within certain voltage parameters so that it reaches consumers in a safe, usable form. Energy is consumed by induction motors, synchronous motors, heaters, furnaces, power supplies (such as AC/DC converters for laptops), lights, refrigerators, and more.
Motors and appliances require that energy always arrive in the form of 120 volts and 60 Hertz. Consumers expect that every outlet in every building always delivers energy within these parameters so that they can safely use the energy.
Energy production facilities need to be designed so that the performance of the electric energy grid meets expectations, that is, voltage and frequency are within the necessary constraints to provide high reliability.
Traditional Energy Transfer: Synchronous Generators
Traditional AC energy sources are synchronous generators with speed and excitation controls. Speed controls maintain power system frequency at 60 Hertz. Excitation controls maintain power system voltage near nominal voltage. Synchronous generators supply MW (output power) and MVAR (excitation energy) to the electric grid. The need for MW and MVAR is a fundamental consideration when operating electric systems.
Synchronous generator terminal voltage is controlled by an automatic voltage regulator within the range of 95% to 105% of rated generator voltage.
All synchronous generators operate at the same speed, 60 Hertz. Automatic speed controls reduce input power when frequency rises above 60 Hertz and increase input power when frequency drops below 60 Hertz. These generators have been used for decades and are reliable, but expensive.
Renewable Energy Transfer, Option 1: Induction Generators
An induction generator produces MW while extracting MVARs from the electric grid and rotates slightly faster (1240 RPM) than a synchronous generator (1200 RPM). Although induction generators are not equipped with voltage regulators, they may be equipped with automatic speed controls. Actual MVAR extraction and speed are a function of generator load.
At low production levels, induction generators rotate at speeds that are close to nominal (1200 RPM). At high production levels, induction generators rotate at speeds that are a few percent above nominal (1240 RPM). Induction generators are a less expensive option due to the fact that voltage regulators are not installed.
Renewable Energy Transfer, Option 2: Inverters
Inverters, while used for a similar purpose as generators, are a very different piece of equipment than either synchronous generators or induction generators. Inverter outputs are controlled by algorithms developed by inverter manufacturers. This means that frequency, voltage, and power control setpoint considerations are manufacturer specific.
This also means that inverter manufacturers need to provide inverter models if inverters are to be included in energy grid simulations. Accurate models for each manufacturer’s inverter are needed for normal operating, fault, and fault recovery calculations.
Inverters are integrated into solar energy production because solar panels produce DC energy. Inverters are used with wind energy induction generators when “fault ride through” is an essential consideration. Of course, adding inverters to improve the performance of wind energy induction generators increases the cost of the energy production facility.
Energy Transfer and the Next Generation Grid
Considering energy transfer options is vital to the next generation electric power grid because renewable energy sources are expanding. The traditional method of energy transfer is not the most efficient and cost-effective form of energy transfer for solar and wind energy conversion.
However, these two forms of energy transfer have different responses to faults on the grid and grid recovery after a fault. Next week, we will explore the response of induction generators and inverters during fault recovery conditions.