Persistent electricity shortages have been one of the key bottleneck for sustaining India’s growth rate. While the energy demand is increasing day-by-day the centralized power generation systems are facing twin constraints of fossil fuel shortage and the need to reduce carbon emissions. There is an increasing emphasis on Distributed Generation (DG) networks with an integration of renewable energy systems (primarily Solar energy) into the national grid, leading to efficiency in the power systems and reduction in emissions. The Indian Government has setup a massive and ambitious target of 20GW distributed generation by 2022. (Source: MNRE)
This recent policy thrust and regulatory directive for Solar Photovoltaic (PV) based grid connectivity and net metering facility encourages the rise of the solar energy penetration into the grid (grid-connected/stand-alone and off-grid both), however, it is not clear, if Power Quality (PQ) aspects has been given due consideration. Most of the available integration of renewable energy systems to the grid take place with the aid of power electronics based converters. The primary use of these power electronic converters is to integrate the Distributed Generation capacity to the grid in compliance with the defined power quality standards. However, it is clear that high-frequency switching of converters can inject more harmonics or cause voltage disturbance to the systems, creating major PQ issues if the integration is not implemented properly.
This blog focuses on key PQ issues emanating from Solar Photovoltaic integration with the grid and their potential mitigation solutions.
SOLAR PV DISTRIBUTED GENERATION (DG) – A PROMISING STEP TO MEET INCREASING ELECTRICITY DEMAND IN INDIA
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.
On one hand, the greater amount of photovoltaic distributed generation (PVDG) integrated with grid promotes the utilization of solar resources, while, on the other hand, it also brings new challenges in the planning, designing, operation, power quality and safety of such systems. Most conventional power systems are designed and operated such that generating stations are far from the load centers and use the existing transmission and distribution system as pathways. Especially, after a PVDG is connected to a distribution network, the topology and the direction of flow of power in the grid are changed. This creates a PQ impact on end-users, as it is influenced by photovoltaic power output characteristics.
Although, DG integration with the grid provides various system benefits in terms of improved grid reliability and power quality, deferring of new or upgraded grid investments, reduction in T&D losses, etc. however, since the distribution grid was not designed keeping in mind the potential high penetration of DG, there are valid technical concerns about power quality and the general impact of DG on the low tension (LT) grid. These apprehensions are further heightened due to a lack of clear documentation and understanding of these concerns and their potential solutions.
KEY PQ ISSUESDUE TO SOLAR PVDG
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:
- DC Injection
- 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 accidently 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 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 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.
POTENTIAL PQ MITIGATION SOLUTIONS AND THEIR BENEFITS
Some of the potential PQ mitigation solutions to increase hoisting of solar PV capacity in distribution grid are as mentioned below. The solutions can be adopted by Utility as well as Customers too.
- Static VAR Compensators (SVC) is an excellent device, which uses a combination of capacitors and reactors to regulate the voltage and it prevents from fluctuating it. A SVC is typically made up of the coupling transformer, thyristor valves, reactors, capacitance (often tuned for harmonic filtering). The main advantages of SVCs over mechanically switched compensation are their near-instantaneous response to change in the system voltage. For this reason, they are often operated at close to their zero-point in order to maximize the reactive power correction. They are in general cheaper, faster, and more reliable than dynamic compensation schemes such as synchronous compensators (condensers). Due to its high-speed switching and simple control, it is widely used by utilities and industries worldwide.
- Unified Power Quality Compensator (UPQC) mitigates voltage flicker/imbalance, reactive power, negative sequence current and harmonics. UPQC is one of the most effective devices for mitigating these issues. It consists of combined series and shunt active power filters (APFs) for simultaneous compensation of voltage, current disturbances, and reactive power. They are applicable to power distribution systems, being connected at the point of common coupling (PCC) of loads that generate harmonic currents. UPQC has the ultimate capability of improving the power quality at the installation point in the distribution system.
- Custom Power Devices like D-STATCOM, DVR, etc. are capable of mitigating multiple PQ issues associated with distribution utility and end-user appliances.
Distributed Static Compensator (D-Statcom) is a FACTS device, which is used for balancing source current, power factor correction, harmonic mitigation and has the capacity to maintain bus voltage sags at the required level by supplying or receiving of reactive power in the distribution system.
Dynamic Voltage Restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage variations and improves the voltage stability. It is a series of connected compensator designed to maintain a constant RMS voltage levels against the voltage disturbances. The main function of a DVR is to protect sensitive equipment likes Variable Frequency Drives (VFDs), Induction motors, etc. from voltage variations coming from the grid.
- From end-customer side solutions, reducing injection of solar PV power into the grid to overcome voltage and congestion issues through
Increased self-consumption of PV
Curtailment of power injected at point of common coupling (PCC) by limiting it to a fixed value
Storage during periods of peak solar generation
Load shifting through tariff incentives or demand response
The nature of power grids is inevitably changing and is evidenced by the steady increase of distributed generation installations on grids. The integration of DG into a system has a positive as well as a negative impact depending on the operating features and the DG characteristics. DG is good to improve overall PQ environment but has associated PQ issues for the network. The effect of DG on power quality depends on its interface with the utility system, the size of DG unit, the total capacity relative to the system, the size of generation relative to load at the interconnection point, and the feeder voltage regulation practice. DG will provide high portions of total energy production on future grids and it is imperative to mitigate the adverse PQ effects so that the benefits may be fully realized.
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(3) Report on Grid Integration of Distributed PV in India – A Prayas (Energy Group) Report, July 2014
(4) Analysis of Output DC Current Injection in Grid Connected Inverters–Sneha Sunny George, Robins Anto, Vol. 3, Issue 9, 2014
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