Copper loss refers to the energy which is dissipated by the resistance of the wire used to wind the coil in the generator (same applies for transformer and motor) as well as the transfer wire from the wind turbine. This dissipated energy creates heat in the wire. The term is applied regardless of whether the windings are made of copper or another conductor, such as aluminum. Hence the term winding loss is often preferred. Copper losses occur in all electrical devices because of the flow of electrical currents through conductors, all common conductors have electrical resistance. Some super-conductors have a reduced resistance or no resistance, but not commonly found in wind turbine applications at this time. Copper losses increase as the value of electrical current passing through the conductors increases. An increase in the temperature of the wire or conductor causes the resistance to of the wire to increase causing the copper losses to also increase.
Copper losses result from Joule heating stating that the energy lost increases as the square of the current through the wire and is in proportion of the electrical resistance of the conductors.
Copper Loss = I2R
where I (amperes) is the current flowing in the conductor and R (ohms) the resistance of the conductor. The copper loss being represented in watts.
Copper losses contribute to the reduction of efficiency in generators (motors, transformers, transfer lines, etc.). It maybe necessary to reduce copper loss in order to keep efficiency at optimum levels. Copper losses can be minimized by using conductors of large diameters in order to reduce the resistance per unit length of the conducting windings of the electrical device. The use of high voltages in electric power transmission systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents while receiving the same amount of power. Remember watts are power, watts equals volts times the amps (current).
A little about Joule heating (for further information)
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Joule in 1841. Joule immersed a wire in a fixed mass of water and measured the temperature rise due to a known current flowing through the wire for a period of time. By varying the current and the length of the wire he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the wire.
Q -proportional- I2 x R
This relationship is known as Joule's First Law. The SI unit of energy was subsequently named the joule and given the symbol J. The commonly known unit of power, the watt, is equivalent to one joule per second.
It is now known that Joule heating is caused by particles that form the electrical current (usually electrons) and the atomic ions that make up the body of the conductor colliding with each other. Charged particles in the electric current give up some of their kinetic energy each time they collide with an ion, increasing in the kinetic or vibrational (moving of mass) energy of the ions as heat, rising the temperature of the wire, which again increase the resistance.