Major Interconnection Issues

Generally, DG-based power systems can operate in independent or stand-alone and grid-connected modes. In fact, in the first mode, the capacity of a DG unit is selected only based on the load requirements. But, in the latter case, this constraint is not determinant. Although the grid-connected operation mode usually is preferred due to the bilateral energy exchanges, the islanding condition is a main concern that should be considered.

Islanding means that one or several power plants (i.e., DGs), isolated from the national power grid, supply a part of the electrical network independently and following some faults in the main network. Operating in islanding condition is undesirable, as this mode may cause unwanted problems such as creating hazards for maintenance and repair staff, and equipment damage due to the instability in voltage and frequency.

Certain issues that arise due to the interconnection are discussed below.

Technical Concerns:

•    Stability: Interconnection of the DG to the grid affects the rotor angle, voltage, and frequency stability of the grid. Based on the type and size of the generators, DG either improves or worsens the stability of the system.

•    Power quality: The power quality of the grid has recently become a problem with the increased use of power electronic devices. Most of the distributed generators are interfaced to the grid through power electronic circuits. Use of these power electronic interfaces increases the already existing power quality problem.

•    Voltage fluctuations: Power injected by certain DG technologies, such as wind turbines and PV power plants, is fluctuating. This results in the local voltage fluctuation.

•    There is a limit on the number of DG units that can be connected to the system. This limit depends on the size and type of the system. The supplied reactive power must be equal to the reactive power demand to maintain the voltage level of the system in the allowed region. Connecting more DGs may increase the reactive power supplied, thereby increasing the voltage level of the system considerably.

•    DGs increase the short circuit current during a fault, which creates more challenging protection requirements. Therefore, to use DGs, improved protection devices need to be used, which adds to the cost of the system.

•    Increasing the penetration level of DGs is difficult and it also takes a long time to locate any fault in the system. Moreover, the direction of current flow becomes unpredictable.

•    Existing radial distribution system and control are designed to handle the power flow in just one direction. When DGs are connected, power flows in both directions. Therefore, existing systems need to be upgraded.

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