Energy Transmission System Design Considerations and Constraints

Electric energy transmission from power plants to substation centers are growing with increasing power demand today. As transmission systems expand over the decades, excess capacity available on transmission lines seems to be consumed with system growth, or with transmission users developing more economical plans to meet the system demand. The expansion leads to more consumption which promotes more towards the expansion. Understanding the considerations and constraints involved with designing transmission systems will give engineers insight into how this affects operations and reliability.

Transmission Constraints

Expansion growth leads to users consuming more and more energy based on their demand. Energy transmission congestion results when energy transmission can no longer accommodate the increased power flow. Reasons for transmission congestion can vary, but the common demand issues for power flow on a specific route is not possible without risking its reliability. Lets identify common constraints and the consequences associated with them.

Thermal Constraints

Transmission lines have their own thermal limit that can result in sagging lines if it is exceeded. This can result in a line fault, where electric arcing is experienced to the nearby vegetation, structures, and of course the ground. When this happens protective transmission components remove the faulty line in order to preserve terminal equipment from serious damage.  

        When the line is remove for repair other transmission lines experience increased loads to compensate for the loss. Overloading can result, which can lead to thermal limits exceeding its operational constraints. If this situation is not properly contained quickly the other lines compensating for the loss can experience the exact same scenario.

        Understanding that this temporary fix is only for emergency situation and that energy transmission lines can still exceed its thermal limit. For this reason energy transmission lines often have an emergency rating. This rating gives a specific amount of time that allows higher load transfers in order for to minimize the chance of hitting the thermal limit.

Voltage Constraints

Generally the energy transmission line reactance at the receiving ends is much less than the voltage applied at the starting end. Larger voltage deviations higher or lower than the nominal voltage value may cause equipment damage for the consumer or the provider. Which gives reason for having an operating voltage constraint to maintain operation that meet requirements. This constraint is much more important in areas where energy transmission lines are scatter and lengthy.

Operating Constraints

Loads constantly change, this can either be small or large changes. Relatively small changes in load generally occur when mechanical power on the generation side adjust to electrical demand. As long as the variation is small the connection between systems can remain in synch. The system would remain stable as long as the loads do not gain in magnitude and oscillate at low frequencies. These oscillations can lead to problematic voltages and frequency issues that can lead to instability and possibly outages.

        Large oscillations occur due to servicing, faults, or disruption in energy transmission lines. Larger frequency ranges can cause uncontrollable situations that could result in non-steady-state instability. Preventative measures are necessary to minimize the potential instability.

        Voltage instability happens when systems are exposed to larger reactive power flows. This is a result of the voltage difference from the starting to the receiving end of the line. This resulting in voltage drops at the receiving end. Lower voltages increase current and can contribute to losses. Voltage collapse is the end consequence. Potential causing equipment damage and possibly outages.