Some of the limitations which should be taken in to account for correct, reliable and quantitative operation of the distribution system are given below:
Thermal Limitation on Power System:
Temperature determines the limiting load current for a device or a system.When the current is flowing in the lines or conductors in the power system generates I2R losses. These losses dissipate in the form of the heat radiated. This heat generated should be removed continuously else the temperature of the windings or conductors increases. This increase in temperature will destroy the insulation of the conductor or windings. At times, due to high short circuit currents conductors may melt because of the heat generated.
Therefore in a distribution system the loading on the system is limited by the thermal limitations. These loading vary appreciably for different parts of loading and climatic conditions. Thus a range of ratings will be available to coordinate with loadings under various conditions.
Economic Limitations on Power Systems:
Some cost is involved in the heat dissipation in the system components, i.e, in terms of the losses ( I2R) generated in the conductors or windings in the power system. The economic level of the loading in distribution system is reached when the cost of the losses generated in the system is equal to the cost of reducing the losses (an optimum value between the two extremes) is arrived at.
In a distribution system the normal component loading as usually well below the economic limit of the system and exceeds of this particular limits during short time in emergency (peak demand).
Voltage Drop in the Power System:
The thermal and economic limits are directly related to the magnitude of the load i.e, “quality of the supply”. An indirect limit is also set by the permissible voltage drop. Voltage limits as seen by the consumers are set which becomes the one of the determining factors in designing the components (voltage limits of operation for all electrical and electronic components we use decide the allowable voltage drop in the lime). Therefore voltage drop in the system should also play an important role in loading.
Fault Current capability in Power System:
The fault levels in the power distribution system goes on increasing with increase in the distribution power system. Depending on the system arrangement, the switchgear must be capable of either making and braking the available fault levels. With regard to the feeders, duration of the fault current flow is equally important and is set by the switchgear capabilities and relay grading requirements.Auto relcosures which may be justified on reliability grounds can unduly stress the line conductor from a fault capability view point
Over-voltages in Power System:
Apart from the voltage drops, components employed in the power system must be capable of withstanding abnormal voltage surges generated from within the system or from outside sources
Voltage flicker and dips in Power System:
The magnitude and frequency of fluctuations which can result in an annoying disturbances as well documented. Where equipment is operated under unacceptable levels, special supply arrangements are necessary. For example, separate LV distribution and sometimes transformers may be installed for the supply of welding loads. Supplies for arc furnace loads are usually from separate power transformers and feeders at a major supply point. Other obvious causes of voltage disturbances or flickers are associated with system faults and heavy current testings
Harmonic Interference in power system:
Electronic component manufacturers are quite vehement in increasing their demands for reasonably pure sine wave supply to ensure optimum operation of the computer installations.Because in increase in non linear loads (electronic loads) in the power system significant amount of harmonics is present in the power system. The distortion in the sine wave limits the use of power system components.
Frequency limit in power system:
For operation of the electrical components in power system certain frequency limits exist. + 1.5 and -1.5 are the permissible limits in the frequency variations in the power system.