Power Quality Issues Caused by Distributed Generation

To mitigate the effects of Load-Shedding, many homeowners have decided to install PV Solar Energy systems. And, as I said before, they are more worried about how much it would cost than the quality of the products. If they can pay the lowest possible price for the entire installation, they are happy and do not care so much about what they get.

During the early stages of Load-Shedding, someone from our neighborhood asked me for advice regarding a PV system, but when I gave him a rough estimate of what it would cost, including the cost of a well-engineered design, I could see that the price was far more important than the quality of the system. Those advertising in the newspaper say that something the with same output would cost far less. Not long after he had his PV system installed, he told me that their washing machine had broken down and that they were compelled to replace it. I did respond since I knew what the cause of it was.

As in many other countries, PV systems with microgrids has become an unavoidable choice. This, in turn, bring along new challenges for utility companies. Naturally, if those employed by the utility companies are awake, knowledgeable, and experienced, they can proactively conduct power quality monitoring exercises. With the ever-increasing expansion of renewable energy systems, the most important power quality challenges that is brought about, is the harmonic distortion which affect the voltage and current quality at the point of common coupling (PCC), and negatively affects the loads.

We are all aware that there is a wide variety of inverter models are available on the market with new ones added constantly. Consequently, the harmonics delivered to the main power grid will thus vary according to inverter types and their control strategies. Utility companies therefore must have the knowledge, skills, and instrumentation to investigate the impact of these components that accompany the renewable (embedded / distributed) generation.

The technical consequences of distributed generation are dependent on the size of the system and its location in relation to the main power generation grid. These influences are reverse power flow, overvoltage along distribution feeders, voltage / current harmonics, phase unbalances, power losses, voltage control disturbances, low power factor, and Electromagnetic interference problem.

To overcome these effects, grid measurements, data analysis, and system modelling are needed for different parameters of grid and electricity generated by the renewable energy suppliers to find solutions and to make this resource more reliable.

Power quality parameters such as inrush current, power factor, total harmonic disturbance (THD), and frequency fluctuation should be recorded and studied regularly.

Inrush Current is a small predictable difference between distributed generating station’s voltage and utility grid voltage which can produce transient inrush currents. This can cause a temporary voltage sag at the neighboring buses, nuisance trips, thermal stress of the power components and several other problems.

With Under Voltage / Overvoltage, the grid connected distributed generating systems only inject active power into the utility grid, which may change the value of reactive power that flow in the system.

Output power fluctuation is one of the main factors that may cause severe operating problems for the utility network. This occurs because of variations in solar irradiance caused by the movement of clouds and may continue for minutes or hours. This may cause power swings, voltage flickers and fluctuations of over / under voltage.

Power factor often decreases to unacceptable levels due to high-powered distributed generation which sits on lightly loaded short distribution lines.

Current and voltage harmonic distortion, voltage and current distortions is generated by nonlinear loads such as power inverters that are used in solar systems where the currents flowing through the impedances of the grid affecting the voltage nodes. Harmonics produced under such circumstances can cause series and parallel resonances, overheating in transformers and capacitor banks, and false operation of protection devices that may reduce the reliability of power systems. A total harmonic distortion voltage (THDu) value of between 5% and 8% indicates significant harmonic distortion. Some equipment malfunctions may thus occur. The acceptable current and voltage harmonic level generated in the microgrid was specified by the IEEE standards 519 and 1547 and IEC-61000-3 standard, and this should not exceed a THDu of more than 8%.

Frequency fluctuation is an important factor in power quality. Any change between the produced and the consumed power may lead to frequency fluctuation. Small sized solar systems may cause frequency fluctuation to be insignificant compared with other renewable energy-based resources. However, this issue may become more severe by increasing the penetration levels of solar systems. Large enough frequency fluctuation may change the winding speed in motors and may damage generators.

Although there may be more solution available to moderate the impact caused by distributed generation, one suggestion is to switch off some or all the PV panels when flowing current is under the critical value, or rather divert the power to charging a battery bank. A second suggestion is the use of passive or active filters.

When new solar farms are proposed for connection to the municipal distribution network where this is perhaps far more critical, neither solution are compulsory, as far as I understand. I recently saw a proposal for a grid-tie solar farm without batteries nor active passive or active filters. But, as I said earlier on, utility companies need employees who are awake, knowledgeable, and experienced to know what to look for. Alternatively, utility companies can contract a company, us, to regularly conduct power quality monitoring exercises on their behalf.