Challenges and Opportunities for Power Grids with Distributed Generation (DG)
Distributed Generation: impact on Protections and Power Quality
Distributed generation (DG) refers to the use of small-scale power sources that are connected to the distribution network or directly to the end-users. DG can provide various benefits such as reducing transmission losses, enhancing reliability, increasing energy efficiency, and diversifying energy sources. However, DG also poses some challenges to the existing power system, especially in terms of protection and power quality.
Protection is the function of detecting and isolating faults in a power system to prevent damage to equipment and ensure safety of personnel. Power quality is the measure of how well the voltage, frequency, and waveform of the supplied electricity meet the needs of the loads. Both protection and power quality can be affected by the presence and operation of DG units in a distribution network.
Some of the main impacts of DG on protection and power quality are:
- Fault current contribution: DG units can contribute additional fault currents to the distribution network, which may change the direction, magnitude, and duration of the fault currents. This can affect the coordination and operation of the existing protection devices, such as overcurrent relays, fuses, reclosers, and circuit breakers. For example, DG units may cause false tripping, nuisance tripping, or delayed tripping of protection devices, or may prevent them from tripping at all. To avoid these problems, proper coordination and settings of protection devices are required, taking into account the possible fault current contribution from DG units.
- Islanding: Islanding is a condition where a portion of the distribution network remains energized by one or more DG units while being disconnected from the main grid. Islanding can pose serious risks to both the utility and the customers, such as equipment damage, safety hazards, power quality issues, and synchronization problems. Therefore, islanding should be detected and cleared as soon as possible by disconnecting the DG units from the network. However, islanding detection can be difficult in some cases, especially when the load and generation are well balanced in the islanded area. Various methods have been proposed for islanding detection, such as passive methods (based on voltage or frequency deviations), active methods (based on injecting disturbances or signals), and hybrid methods (combining passive and active methods).
- Voltage regulation: Voltage regulation is the process of maintaining the voltage level within acceptable limits in a power system. Voltage regulation can be affected by DG units in several ways. On one hand, DG units can help improve voltage regulation by injecting reactive power or by reducing the voltage drop along the distribution lines. On the other hand, DG units can cause voltage regulation problems by creating voltage fluctuations or overvoltage due to their variable output or switching actions. For example, DG units based on renewable energy sources such as wind or solar may cause voltage flicker or voltage sags due to their intermittent nature. To mitigate these effects, proper voltage regulation devices and strategies are needed, such as tap-changing transformers, voltage regulators, capacitors, or reactive power controllers.
- Harmonics: Harmonics are sinusoidal components of a periodic waveform that have frequencies that are multiples of the fundamental frequency. Harmonics can distort the shape of the waveform and cause various power quality problems, such as overheating of equipment, malfunctioning of devices, interference with communication systems, and increased losses. Harmonics can be generated by DG units that use power electronic converters or nonlinear loads. For example, DG units based on inverters may inject harmonics into the distribution network due to their switching actions or control schemes. To reduce harmonics, proper filtering devices and techniques are required, such as passive filters (based on capacitors or inductors), active filters (based on power electronic converters), or hybrid filters (combining passive and active filters).
In conclusion, DG has both positive and negative impacts on protection and power quality in a distribution network. To ensure safe and reliable operation of both the utility and the customers, proper planning, design, coordination, control, and monitoring of DG units are essential.
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References:
[1] Distributed Generation: impact on Protections and Power Quality https://ieeexplore.ieee.org/document/6502846
[2] Impact of distributed generation on distribution protection and power quality https://ieeexplore.ieee.org/abstract/document/5275777
[3] Impacts of Distributed Generations on Power System: Transmission Congestion Management https://www.igi-global.com/chapter/impacts-of-distributed-generations-on-power-system/245644
[4] Protection and Power Quality Impact of Distributed Generation https://era.library.ualberta.ca/items/b9490da2-7d73-4e3f-bdc7-4d5c00873713
