Long-Distance Power Transmission for Renewable Energy Systems

Paul Cunningham

Sites with good renewable-energy power production are frequently far removed from the point of use. This is particularly true of wind and hydro systems, where the location of the energy source cannot be easily altered. Even in many solar situations, the site where the energy is used may be shaded and this may require going to more open or higher ground to find sufficient insolation.

Several techniques have been developed to solve the problem of power transmission over distance. They involve generating power at a voltage higher than battery voltage and stepping that down at the usage site. This enables lower current values to be used, which reduces transmission losses. 
(This is a modified form of the technique that utilities use.)

However, the first possibility to investigate is generating power at the battery voltage. Often a higher battery/inverter voltage (48 or 120 V) can be used to advantage. The larger transmission wire associated with direct transmission may cost less than the power conversion equipment otherwise needed. This also means a simpler system.

Most wind and hydro systems (unlike PV) generate alternating current first. This is then rectified (by the alternator) to direct current for storage in batteries. Transmission of alternating current over a longer distance is easier than transmission of direct current, because the power can either be stepped up or generated at a higher voltage. This enables lower current values and again reduces transmission losses.

Utilities require power to be generated at 50 or 60 Hz, as used by standard transformers. However, renewable energy systems do not need to match this; most transformers will readily accept higher frequencies with even an improvement in performance. Transformers can also be custom built for the task at hand. Solid-state transformers can also be used if the DC input needs to be converted to battery-voltage output.

For PV systems that produce DC output, the solid-state converter is the only option. These use high-frequency transistor topologies to convert a high input voltage into a lower battery voltage. They operate at high efficiency and are typically light and compact. Most are adjustable, which allows them to be tuned for optimum output. Some converters automatically optimize themselves and are know as maximum-power-point trackers or MPPTs. This is a very useful feature, because it makes the device more user friendly and the optimum generation voltage can vary widely with power output.

In a wind or hydro system, the power is often generated using a permanent magnet alternator. These usually operate at high efficiency and are brushless. They can often be supplied to produce the higher voltages associated with long-distance transmission.

Another option to consider is the use of induction generation. A standard three-phase induction motor with capacitors for excitation makes possible a simple, inexpensive generator. These are very reliable, brushless and can operate with high efficiency. When they are used with a tuneable converter, a very effective system is possible. Optimization of the power output would otherwise require the use of transformers with multiple output taps or changing capacitor values.

Which of these possible solutions should one use in practice? Three recent hydro installations have adopted different solutions, related to the needs of the application. At a site in New York state, hydro generators using about 20 litres/s from a 5 m head generate power at a nominal 240 V and 200 W. The power is transmitted to one site is 800 m away and a second 2 km away. At both sites the power is stepped down using standard transformers and rectified for 12 V batteries.

At a site in Arizona, a 3 m head uses about 15 litres/s and produces 130 W from the only surface water on a 200 square mile ranch. Two houses are supplied on a priority system. The first is only 50 m away and uses a solid-state converter; once the batteries are fully charged, the converter starts to reject the power. The generator voltage then rises from its nominal 40 V and the converter at the second house, about 200 m away, starts working and delivering power to the batteries there.

At an installation in New Brunswick, Canada, a flow of about 2 litres/s comes down a 50 m head through an 800 m poly pipe. This powers a hydro machine with an induction generator consisting of a standard three-phase induction motor with capacitor excitation. The power is send 500 m to a house where the 500 W is used directly without storage. Output is controlled manually, using a dimmer switch to shunt power to a dummy load to maintain the correct voltage! In most direct AC systems an electronic governor would do the job automatically.

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Paul Cunningham, Energy Systems & Design,
P.O. Box 4557, Sussex, NB Canada E4E 5L7
Tel: +1 506 433 3151 Fax: +1 506 433 6151