Original articleMitigation strategies to minimize potential technical challenges of renewable energy integration
Introduction
In general, power flows from the upstream network (the transmission network) to the downstream network (the distribution or low voltage network). Integration of renewable energy (RE) causes reverse power flows, i.e., feeding back into the grid as they are generally connected near the load centre, if the power generation from these systems is greater than the load in the local network. Therefore, RE integration introduces bi-directional power flows across distribution transformer (DT) and hence distribution network (DN) experiences with several potential problems that includes voltage variation, over loading of DT, poor power factor and harmonics injection in the DN which detracts the overall power quality (PQ) of the network [1], [2].
The intermittent nature of power output from renewable energy sources, in particular wind and solar, introduces potential technical challenges that affect quality of power observed including voltage fluctuation, frequency fluctuation, power system transients and harmonics, system blackouts, reactive power, low power factor, switching of electrical equipments, synchronisation problem, storage system, load management and forecasting and scheduling. Large scale RE integration in the DN causes voltage rise within the network and this voltage rise is significant in case of single phase PV system connections. The receiving end (customer premises) voltage may drop if RE is unable to support customer load demand, especially during peak demand periods [1], [2], [3], [4]. Moreover, the intermittent nature of RE causes uneven generation and hence might exceed the capacity of the connected transformer. Integration of RE also causes phase unbalanced conditions in the network due to uneven connection of RE source into different phases. The operation of the distribution network involves reactive power due to customer loads, line impedances and RE sources, in particular induction generators used in wind turbines which are unreliable for the smooth operation of the network [5], [6]. Inverters connected with RE sources, non-linear customer loads and power electronics devices introduce harmonics in the distribution network that causes overheating of transformers, tripping of circuit breakers, and reduces the life of connected equipment [6].
Significant research and development works are undertaken by various agencies throughout the world to investigate and mitigate the observed potential technical challenges to ensure reliable and uninterrupted power supply to the consumers. Albarracin and Amaris [6] investigated a voltage fluctuation model used for the evaluation of flicker assessment under sunny and cloudy conditions with photovoltaic energy sources. Results showed that irregular solar irradiation caused by cloud movement produced voltage and power fluctuations. The Gardner MA PV project [8] explores four areas: the effect on the system in steady state and during slow and cloud transients; responses of concentrated PV under fast transients; harmonic effects on the PV system; and the overall performance of distribution system, in which the total impact of high penetration of PV was evaluated. Results showed that 37% penetration of PV at Gardner was achieved without any significant problems. Asano et. al. [9], analysed the impact of high penetration of PV on grid frequency regulation which responds to short-term irradiance transients due to clouds. It was shown that break-even cost of PV is unacceptably high unless PV penetration reaches 10% or higher. Therefore, PV integration needs to be increased and impacts to be identified and mitigated. A comprehensive study was carried out by Fekete et al. [10] that analysed the harmonic impacts in both winter and summer seasons with 10 kW PV penetration on the distribution network. Recent studies by Ergon Energy, and Chant et al. [11] have explored the issues involved with small-scale PV penetration in urban networks. It was found that increased penetration exhibited increased voltage rise on LV networks, increased harmonic distortion and, as a result, load rejection occurs. The impacts of wind power on the power system, in particular voltage stability, power system stability and PQ characteristics were investigated by Pedro Rosas [12] using dynamic simulation. Results shows that wind turbine technologies with power converters can actively control the reactive power consumption which increased the voltage stability of the power system.
Appropriate design of electrical circuits with control systems can mitigate voltage fluctuations and harmonic distortion, provide reactive power compensation and power factor improvements and thus ensure PQ improvements of the power system. Customised power devices such as SVCs, STATCOMs, DVRs, thyristor controlled series compensators (TCSCs), static synchronous series compensators (SSSCs) and a combination of series and shunt active power filters are the latest developments of interfacing devices between grids and consumer appliances that overcome voltage and current disturbances and improve the PQ by compensating the reactive and harmonic power [13]. A STATCOM is the best performing device for reducing voltage fluctuations and harmonics as well as improving the PQ of the power network compared to other flexible AC transmission system (FACTS) devices. STATCOMs are faster, smaller and have better performance at low-voltage conditions, though the cost of STATCOMs is comparatively higher [14], [15].
In order to enhance the terminal voltage quality, SVCs were used for reactive power compensation of wind power induction generators [16]. From simulation results, it was observed that the STATCOM can considerably improve the voltage profile at the PCC by regulating the reactive power of the grid during faults and maintaining an appropriate level of voltage sag on the grid and prevents the turbine from being disconnected from the grid during certain levels of voltage sag on the grid side [17]. A STATCOM based control mechanism is used to reduce the power quality problems as well as harmonics on integrating wind energy into the grid [15], [18].
On the other hand, energy storage plays an important role in facilitating large-scale RE integration by supporting peak load demand and peak shaving, improving voltage stability and power quality. It also maintains constant grid power and reduces GHG emissions by maximising RE utilisation. Batteries are one of the most cost-effective energy storage technologies for power applications such as regulation, protection, spinning reserve and power factor correction [19]. Lead-acid and lithium-ion batteries are the renowned and effective storage technology today. Hybrid battery super capacitor energy storage systems are expected to be able to play a major role in power smoothing, power quality improvement and low voltage ride through in a wind energy conversion system [20].
From the literature, it is observed that most of the available research was carried out primarily in USA and Europe [5], [6], [7], [9], [10]. However, the distribution network characteristic of Australia is different compared to other developed countries in many forms and research conducted by other countries could not simply be adopted without further research in Australian context. For example, USA is a densely populated country with a population density of 32.86/km2 and provide electricity to 323.1 million inhabitants as of 2016 [21]. On the other hand, Australia is very lightly populated country with a population density of 3.09/km2 and provide electricity to 24.21 million inhabitants as of 2016 [22]. The total geographical area of USA and Australia are 9.525 million km2 and 7.692 million km2 respectively. USA’s population is 13 times more than the Australian population though geographic area of USA is only 1.24 times larger than the Australia [23]. Therefore, USA’s transmission and distribution network may comparatively larger but not disperse like Australia. Therefore, electricity costs are more than double in Australia compare to USA. Moreover, 1-phase and 3-phase nominal voltage at the rear end of the distribution network are 240 V and 415 V with line frequency of 50 Hz in Australia while 120 V and 208 V with line frequency of 60 Hz in USA respectively. On the other hand, higher end distribution voltages in Australia are 66 kV, 22 kV and 11 kV while in USA 69 kV, 13 kV and 4 kV.
Recently authors [24] conducted an experimental and simulation study to investigate the technical adverse impacts with PV penetration into the Australian low voltage distribution network and identify possible mitigation measures using FACTS devices. From both the experimental and simulation analyses it can be evident that the major impacts observed on integrating PV into the LV networks are: voltage fluctuations and harmonics injection that reduces the overall PQ of the power systems network. Study also proves that the use of FACTS devices reduces the level of potential adverse impacts in the LV network.
Therefore, this study aiming to develop a model that investigates the potential technical challenges with the higher renewable energy integration into the distribution network using power system simulator PSS SINCAL [25]. In addition to the Berserker Street Feeder (Frenchville Substation, Rockhampton, Australia), IEEE 13 Bus network was also selected as suitable network for investigation of large-scale deployment of RE into the grid. By increasing PV/wind integration into the network, the points at which distributed generation starts to have an impact on utility grid operations has investigated with regards to network performance standards. Worst-case scenarios were identified and measures were taken to reduce the level of adverse potential impacts.
Section snippets
Power quality problems
With increased penetration of RE to the grid, the key potential technical challenges that affect quality of power observed include: voltage fluctuation, active and reactive power, power factor and voltage and current harmonic.
Case study: Berserker Street Feeder
To investigate the potential technical impacts of RE integration to support the loading of a Feeder, this study initially developed a model for Berserker St Feeder, Frenchville, Rockhampton, Australia. Later, similar analysis has been carried out using the IEEE 13 bus network.
Model evaluation
IEEE 13 bus test feeder is selected in this study to explore the potential technical challenges in particular, voltage regulation and harmonics injection with different levels of RE integration. This test feeder includes unbalanced load distribution in all nodes as shown in Table 3 and load profile in Fig. 25 and further information can be found [35]. This study also identifies suitable mitigation measures to reduce the adverse impacts using customised FACTS devices comparing shunt capacitor
Conclusion
Integration of renewable energy sources into the grid introduces potential technical difficulties due to its intermittent nature, bidirectional power flows, and nature of power generation that need to be investigated and mitigated as part of developing a sustainable power system for the future. From the modelling analyses, it has been clearly evident that integration of RE cause’s uncertainties in the Rockhampton power network and introduces adverse impacts on voltage regulation, power
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