Distributed generation (DG) refers to a variety of technologies that generate electricity near the end users. Some of the DG technologies are solar panels, and combined heat and power (CHP). They may serve a single structure (such as a home or an industry), or a major industrial facility, a large hospital, or a college campus, as part of a microgrid (a smaller grid that is also tied into the larger electricity delivery system). These systems may or may not be connected to the electric grid. When connected to the electric utility’s low voltage distribution lines, DG systems can support the delivery of clean, reliable power to additional customers and help reduce transmission and distribution losses.
Distributed Generation (DG) generally refers to small-scale (typically ranging from 1 kW – 50 MW) electric power generators that produce electricity at a site close to customers or that are tied to an electric distribution system. As of May 2015, India had a cumulative distributed Solar PV capacity of about 400 MW and is expected to witness significant growth owing to increasing economic viability and a facilitating regulatory framework in many states. It is one of the most promising DG source and their penetration level to the grid is also on the rise.
The integration of solar PVDG in power systems can alleviate overloading in transmission or distribution lines, provide peak shaving, and support the general grid requirement. However, improper coordination, location, and installation may affect the quality of power systems. When integrating DG, the inverter forms the heart of a grid-tied solar PV system and is responsible for the quality of power generated/injected into the grid. While it handles the important operating parameters such as voltage and frequency range, it also affects the quality of the solar power being injected into the grid.
Primarily through three major PQ issues:
Long duration voltage variations
Harmonic issues due to DG
Harmonics are electric voltages and currents that appear in the grid as a result of non-linear electric loads. In the solar system,
Harmonics are caused in the conversion of DC to AC power by the inverter.
Another factor that influences harmonic distortion in a power system is the number of PVDG units connected to the power system. The interaction between grid components and a group of PVDG units can amplify harmonic distortion.
The increasing use of harmonic-producing equipment on the customer side such as adjustable speed drives also creates issues like greater propagation of harmonics in the system, shortened lifetime of the electronic equipment, and motor and wiring overheating. In addition, harmonics can flow back to the supply line and affect other customers at the PCC.
Flicker issues due to DG
A DG installation may increase the flicker level during start/stop or if it has continuous variations in input power because of a fluctuating energy source. In the case of a solar energy generator, the output fluctuates significantly as the sun intensity changes. Moreover, Squirrel cage induction generators have a high possibility to make flicker level worse because of an inability to actively control terminal voltage.
It is typically caused by the use of large fluctuating loads, i.e. loads that have rapidly fluctuating active and reactive power demand. Flicker effect occurs when one generating source reactive power output increases or decreases faster than the remaining generators can compensate. Flicker does not harm equipment, but in weak grids with a higher possibility of voltage fluctuations, the perceived flicker can be very disturbing to customers.
DC Injection issues due to DG
Grid connected inverters are used to convert the DC power, thus obtained into AC power for further utilization. Thus, inverters connecting a PV system and the public grid are purposefully designed for energy transfers. However, due to approximate short circuit characteristics of AC network, a little DC voltage component can accidentally be produced by grid connected inverters which can create large DC current injections. If output transformers are not used, these inverters must prevent excessive DC current injection, which may cause detrimental effects on the network components, in particular the network transformers which can saturate, resulting in irritant tripping. This may also increase the losses and reduce the lifetime of the transformers, if not tripped. Moreover, the existence of the DC current component can induce metering errors and malfunction of protection relays and can create an adverse effect on the overall functioning of the solar power plant.
Other effects within transformers include excessive losses (i.e. overheating), generation of harmonics, acoustic noise emission, and residual magnetism. In addition, there is evidence for the seriousness of corrosion risks associated with DC currents in the grid.
Long Duration Voltage Variations
Overvoltage and undervoltage are generally not the result of system faults but are caused by load variations on the system and system switching operations. DG technologies, mainly the renewable systems like solar can cause long duration voltage variations. Small-distributed generation (less than 1 MW) is not powerful enough to regulate the voltage and is dominated by the daily voltage changes in the utility system. Small DG is almost universally required to interconnect with a fixed power factor or fixed reactive power control. Large voltage changes in distribution network are possible if there is a significant penetration of dispersed, smaller DG’s generating power at a constant power factor. Suddenly connecting or disconnecting such generation can result in a relatively large voltage change that will persist until recognized by the voltage-regulating system.
Emerich Energy have solutions for all types of Power Quality Challenges, The potential PQ Analysis and mitigation of Harmonics and other Variations are the solutions to increase hoisting of solar PV capacity in distribution grid.