TRANSMISSION AND DISTRIBUTION SYSTEMS

RESISTANCE LOSSES

Power lines and cables can be designed in many ways and be made of many different components. However there is one ingredient that all the electrical wires have in common, it is conductor. Conductors are materials that have high conductivity thus they are permitted to transmit power with low losses. Equation that tells about resistance losses viz power lost due to wire heating looks like:

Where: P – power lost due to resistance losses

I – current flowing through the wire

R – resistance of the wire

As can be seen from the foregoing resistance losses are caused by currents that flow in the wires. The resistance depends on the conductors i.e. on their conductivity, gauge and its length. Higher conductivity makes lower resistance since conductivity is a reciprocal of resistivity. The gauge affects inversely proportional and length proportional in the resistance. Thus it is good to use an appropriate conductor to the wire in terms of financial and effective conditions. Another element in the formula stands for current flowing in the wire. The amount of current that affects the losses is in a square thus e.g. ten times lower amount of current flowing through the wire induces a hundred times lower losses. The current can be decreased by increasing the level of voltage which can be done by transformers. It can be concluded that transmitting big amounts of power in long distance with low resistance losses demands very high voltage power lines. However it also suffers from limitations. Higher voltage lines require taller pylons, longer insulators and bigger space between wires. Also very high voltage causes corona discharge which induces extra losses.

Another problem which affects the resistance losses is an amount of reactive current. Some units e.g. electrical machines consume real power which is consequently changed into the work. However they also need reactive power to work properly. Reactive power is not consumed by the units, it fluctuates between generators and loads without doing any work. Reactive power is used to create and maintain electromagnetic fields. However the flow of reactive power effects in growth of reactive current which produces extra transmitting losses. This is why it is important to curtail reactive power in the wires. There are two ways of compensating reactive power.

  • Capacitors – Capacitors work as the reactive power generators. When they are installed near loads, they feed the appliances with reactive power, thus there is no need to move reactive power from far away generators. Capacitors are easy to install and very efficient, they need 3-5 watt hours to generate 1 kilovar hour.

  • Synchronous compensators - Synchronous compensators are synchronous electrical motors which run at no-load. As with capacitors, synchronous compensators can generate reactive power. However they are much more flexible, they can be adjusted by excitation current. They main disadvantages are that they use more power (they need 10-20 watt hours to generate 1 kilovar hour) and they are comparatively larger than capacitors.

LOSSES IN TRANSFORMERS

Transformers are electrical devices used for transferring the electrical energy from one circuit to another with constant frequency, often with converting voltage from one level to another. Transformers are considered as the electrical machines and they are characterized with the highest efficiency. The efficiency depends on many elements such as design, size and operating load and it varies usually between 95% and 99%.

There are two kind of losses in transformers. First one are no-load losses also known as core losses and as the name suggest, they are regardless of the load (Graph 2). They are caused by hysteresis losses in the ferromagnetic core, eddy currents in the core, resistance losses due to no-load current and dielectric losses. The other kind of losses are load losses which are caused by resistance losses in the windings.

  • GRAPH 2: TRANSFORMERS LOSSES VS PERCENT OF LOAD BASED ON [8]

    • Life span of transformers depends on their load, also the losses get higher while the transformer gets older. This is why it is important to select an appropriate transformer to match the load demands. Along with the development, transformers are more and more efficient, it is said that if all the 30 years old transformers were swapped to the new ones all the transformer losses would be decreased by 90%.

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