Published On: May 25, 2015
In India, where the electricity demand is escalating and with its year-on-year growth rate (i.e. demand-supply gap of ~5.1% as of 2014), there is a need to add capacity in the system generation. Almost, all integrated power systems in India and world have been relying on centralized electricity generation such as large-scale hydro, coal, natural gas and nuclear power plants. Electricity is then distributed along long-distance with high voltage lines from centralized sources to the customers’ premise. However, the existing scenario is changing with the demand for clean, reliable and affordable electricity generation. These requirements are posing challenges to traditional methods of delivering electricity and hence the evolution of Distributed Generation has seen a huge potential.
The term Distributed Generation (DG) refers to electric power generation technology that is integrated within distribution systems, close to the point of use. Distributed generators are usually connected to medium or low voltage grid (generally until 33kV). They are not centrally planned and are typically smaller than 30 MW capacity. These systems have small and micro-generators connected directly to end users like factories, offices and households. Also, excess electricity generated by the directly connected consumers is fed into the active distribution network to meet demand elsewhere.
Considering the above scenario, tomorrow’s grid will be dominated by distributed generation – prime mover being sustainable energy for addressing climate change. The reasons for this change are numerous and debatable, but include such factors as environmental concerns, increase in cost to traditional energy sources and at the same time reductions in cost to new technologies, renewable energy generation incentive, etc. While end consumers have been generating their own energy for a long time, however with the increasing quantity and changes in methodologies, these generations pose many new challenges. One of the major challenges in Distributed Generation today is power quality issue due to various sensitive loads.
With DG systems, end consumers get benefits of backup generation as well as improved reliability of the power supply at lower cost. At the same time, it also increases the generation capacity of a power system without having an adverse impact on environment. All said and done, while there are benefits, this blog attempts to focus on the associated PQ challenges/issues and their ill effects with DG systems.
Power quality is an important aspect of power systems, which has a direct impact on efficiency, security & reliability and is therefore important to address when looking at distributed generation of electricity. There are important PQ issues that need to be addressed as part of the overall interconnection evaluation for distributed generation. Depending on the aspects chosen, distributed generation can either contribute to improve or deteriorate power quality where a common belief remains among consumers is that DG will improve power quality, and this potential for better quality is cited as one of the value attributes of installing distributed generators.
Distributed generation induces PQ challenges such as voltage variations, harmonics, flicker, voltage sags/dips etc.
PQ Issue#1: Long duration voltage variations:
Overvoltage and under-voltage 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 wind and 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.
Solutions to mitigate Long Duration Voltage Variations:
PQ Issue#2: Harmonics:
Harmonics are sinusoidal voltages or currents having a frequency that are multiples of the grid frequency, the frequency at which the supply system is designed to operate. Harmonics produce waveform distortion together with the fundamental voltage or current. Power electronic equipment is a major contributor of harmonics in the power system. The power electronic based variable-speed wind turbines connected to the distribution grid introduce harmonics into the distribution system. A number of distributed generation technologies rely on some form of power electronic device in conjunction with the distributed network interface, such as for example the DC-to-AC converter for photovoltaic systems. All of these devices inject currents that are not perfect sinusoidal.
Solutions to mitigate Harmonics
PQ Issue#3: Flicker:
Flicker is commonly seen due to rapid changes in the load or due to switching operations in the system. The term flicker is derived from the impact of the voltage fluctuation that causes light to shift brightness perceived by the human eye. Flicker is introduced into distribution system from the connection and disconnection of turbines, changing of generators on two-generator turbines, and by torque fluctuations in fixed-speed turbines as a result of turbulence, tower shadow, wind shear, and pitch changes. Flicker does not harm equipment, but in weak grids with higher possibility of voltage fluctuations, the perceived flicker can be very disturbing to customers.
Solutions to mitigate Flicker:
PQ Issue#4: Voltage sags/dips:
A voltage sag or voltage dip is a short term reduction in RMS voltage which can be caused by a short circuit, overload or starting of electric motors. Voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute. Wind turbines can cause voltage sags within the network. Depending on the generation technology, loading conditions and the interconnection location, DG might have the ability help reduce voltage drops at high-load situation. However, at low-load situation a massive excess of DG production, upper voltage limits could be violated. Solutions to mitigate Voltage Sags/dips
The nature of power grids is inevitably changing and is evidenced by the steady increase of distributed generation installations on grids worldwide. The benefits that DG provides to customers and utility and the increasing need for renewable energy sources have proven to outweigh the difficulties encountered in their implementation. In many cases, DG offers a means to improve grid operation through voltage support, decreased transmission losses, and increased reliability.
DG is good to improve overall PQ environment but has associated PQ issues for the network. It is advisable to address them right from green field stage and enjoy the fruit of sustainability encompassing good PQ environment too. If grids – or portion of grids – are evaluated on a more complete scale, the use of Distributed Generation may be more ideally optimized instead of developing in a “patchwork” fashion. It is likely that Distributed Generation will provide much higher portions of total energy production on the future grids and the challenge for engineers then, is to mitigate the adverse effects so that the benefits may be fully realized.
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