Abstract
The increasing penetration of grid-connected PV rooftops at the distribution level still entails significant technical challenges affecting seriously the power quality indices such as harmonics and voltage fluctuations. Owing to the intermittent nature of renewable resources, the changing incident energy from renewables can generate considerable amounts of harmonics. Moreover, power electronic devices and nonlinear loads that are used frequently in the industry may exaggerate the harmonic distortions as well. Accordingly, utilizing suitable filtration techniques for harmonic reduction is crucial. In order to evaluate the impacts of grid-connected PVs in modern grids, a case study on power quality and voltage profile is conducted with a large grid-connected PV microgrid of 9570 kW, feeding a large hospital project in Saudi Arabia as an initial phase of implementing this project in the future. For eliminating the possible increase in harmonic distortion, a single-tuned filter is used to cope with the permissible limits according to the known IEEE standards. This filtering technique is chosen due to its advantages including the simplicity, suitability for significant integer harmonic orders, and low cost. For evaluation tests, a detailed simulation is developed by the ETAP program for the overall selected project as well as the aimed PV subsystem. Several simulation tests are conducted to investigate the harmonic distortion problem. The results show a significant reduction in the individual harmonic distortion (IHD) and the total harmonic distortion (THD) below 8% according to considered IEEE standards for LV networks. Both 6 and 12 pule inverters are considered. This is considered an important step in the realization of such large PV projects in the field.
1. Introduction
In Saudi Arabia, the majority of the electrical power is mainly relying on conventional resources till present. Hence, the environmental pollution and energy sustainability are major concerns recently; Saudi Arabia is planning for maximizing the utilization of renewable energy resources for power generation. Owing to the rapid increase of load demands with annual growth more than 6%, solar energy resources attract an increasing attention nowadays. Owing to the remarkable utilization of renewable energy in modern grids, multiple consequences arise with large-scale PVs into the distribution level regarding the power quality (PQ) indices, in particular, including voltage regulation, harmonic distortion, voltage unbalance, and flicker [1–3]. This is mainly due to the huge use of power electronic devices. Hence, different aspects have been remarked in distribution grids with the existence of PV systems including voltage problems such as voltage sag, voltage swell, voltage unbalance, and also frequency multipliers known as harmonics. This remarkably impacts the load factor and reduces the system efficiency as well as increases losses [4, 5].
Harmonics are usually used to describe the distortion of the electrical voltage or current waveforms. In distribution systems, they are typically due to some factors such as nonlinear loads, battery chargers, and motor-control devices. This harmonic content may be exaggerated by nonlinear devices in power systems such as static power converters, arc-discharge devices, saturated magnetic devices, and, to a lesser degree, rotating machines. Also, static power converters are usually engaged with nonlinear loads [6–8]. These harmonic contents may cause undesirable effects on transformers, causing an increase in copper and iron losses, and the phenomenon of crawling in motors. They can also generate negative sequence current components that produce reverse torque (torque breaking). On cables, they can produce different impacts such as an increase in the resistance of the conductor to the increase in temperature, and the resistance of the conductor increases as a result of the tendency of high-frequency current to be near the outer surface of the conductor, which is known as the skin effect. Moreover, they can impact the power factor remarkably [9].
Actually, the PV inverters have significant role for producing harmonics in the distribution level, where its harmonic-distortion contribution is owing to both intrinsic and extrinsic effects. Intrinsic harmonic distortions are a consequence to the converter and inverter deficiencies including the component and control loop nonlinearities, limited pulse-width modulation (PWM) resolution, and measurement inaccuracies. Other extrinsic harmonic distortion is mainly owing to the weakness of the grid as well as the accompanied distortions with the applied loads [9–12]. Although the earlier technologies of typical PV inverters had current THDs between 10% and 20%, recent grid codes set the Total Demand Distortion (TDD) of all distributed generators to be 5% [13–17]. Accordingly, a harmonic distortion of 5% or less is nowadays specified by the recent models of PV inverters with nominal operating conditions. However, the grid voltage distortions become normally uncontrolled as they are induced by nonlinear loads connected to the grid. Hence, a proper control method and/or modulation technique is required in conjunction to some suitable filtering actions with grid-connected inverters to mitigate such induced harmonics [18]. Also, some modern utilized inverters are equipped with embedded active filtering capability to compensate its current distortion. However, this optimized performance is guaranteed with near its rated output levels, where their resulting current THDs crossed the 5% limit remarkably when their power output dropped due to the intermittent nature of the solar irradiance [19]. Moreover, such harmonic problems can get even more severe with utilizing significant capacities of inverter-based Distributed Generators (DGs).
As the harmonic current injection by both loads and DGs increases, the levels of associated THD levels increase in the grid bus voltages. Hence, utilizing dedicated tool for mitigating these harmonics contents is required. Among these mitigating devices, both passive and active harmonic filtering techniques are common. In spite of the advantages of active filtering devices, single-tuned passive harmonic filters are still widely incorporated in whole-gird harmonic minimization studies. This is mainly due to their easier implementation, lower investment cost, and less maintenance [20–26]. On the other hand, the fast switching of high currents in active filters may cause electromagnetic interferences in power distribution systems. In addition, the technology of these types of filters still requires further research and development efforts [27].
The total harmonic distortion can be defined as a measure of the percentage of distortion caused by the presence of all harmonics as compared to the original wave (either in voltage or current). This compares the harmonic content of a waveform with the RMS value of the waveform as described in [28, 29],
Recent grid codes stipulate the Total Demand Distortion (TDD) at the PCC point of all distributed generators should not exceed 5% [15]. However, the THD limit can be raised to 8% for LV networks as illustrated in Table 1 according to the known IEEE standards [30].
As illustrated in Figure 1, Saudi Arabia is among the highest areas in the Middle East in terms of solar radiation, with an average horizontal solar radiation of 2,200 kilowatts per square meter. According to different measurements, the available brightness of the sun in Makkah region is typically ranged from 7.4 to 9.4 hours with an average time of 8.89 hours per day [31–33]. Hence, Saudi Arabia is distinctive obviously for large grid-integrated rooftop PV projects at the LV distribution level.
This paper is aimed at analyzing grid-connected PV subsystems in modern grids by presenting a real-world case study to understand and visualize the impacts on the distribution smart networks regarding the power quality and voltage profile. This will be conducted by exploring the design phase of a practical case study for utilizing a 9570 kW grid-tied PV subsystem feeding a large hospital project in Saudi Arabia. For this target, a detailed modeling of the electrical network of the considered project is first constructed in ETAP. Then, the aimed PV microgrids are designed and modeled as well. The quality of the generated power to feed the overall load is analyzed regarding the harmonic distortion problem. Finally, eliminating the resulting harmonic contents is realized by designing the proper single-tuned filtering as a practical, reliable, and economical methodology. Future steps regarding the feasibility study of this project, power management, and implementation phases are considered in the future.
2. Description of the Simulated Project
The first step in this project aims to design a solar PV system for generating the electricity need of Al-Noor Specialized Hospital in Saudi Arabia. This hospital is characterized with a large energy demand with energy consumption bill more than $1.5 million per month. The hospital is in the heart of Makkah city with the coordinates of the coordinates of [Lat/Lon] 21.387 and 39.859 as illustrated in Figure 2. The mean annual solar energy in this region is more than 2.0 MWh/m2 showing great prospects with solar energy [31]. This promising solar radiation facilitates utilizing feasible and cost-effective large PV rooftop power plants. Initially, a detailed modeling for the selected hospital was constructed in ETAP. As shown in the aforementioned figure, the considered hospital occupied a large and vast area with different buildings in addition to three large parking areas. This represents a typical project for implementing large-scale grid-connected rooftop PV subsystems. The hospital is fed from four different distribution substations, where each substation is equipped with a 67 MVA and 110/13.8 kV main transformer. This multiple feeding guarantees the continuity of the electrical power to the overall project. The electrical network of the overall hospital in the hospital consists of four distribution panels feeding loads of all buildings and air conditioning chillers as described in Figure 3.
Figure 3 illustrates the one-line diagram of the main panel feeding all hospital loads at the level of 13.8 kV. It consists of five distribution subpanels in addition to three backup diesel generators with a capacity of 1048 kVA for each one. Each subpanel has its loads via its distribution transformers as described below in Table 2. Figure 4(a) described the dedicated panel for air conditioning subsystem for the main building having 9 coolers with a capacity of 912 kVA for each one via two step-down transformers with a capacity of 5500 kVA. Four emergency diesel generators with a capacity of 750 kVA for each unit are adopted for the cooling subsystem as well. The AKU center is fed via a dedicated panel via a step-down transformer of and 2.5 MVA, and an emergency diesel of 750 kVA as described in Figure 4(b).
(a) Dedicated panel for feeding nine chillers
(b) Dedicated panel for feeding the AKU center
2.1. Load Profile Description
Profiling the overall loading of the project is essential for characterizing the aimed PV system as well as visualizing the future energy management system. The selected hospital is characterized with a very high electricity consumption, where its monthly electricity consumption in kWh is illustrated in Figure 5 showing the average monthly loads for three years from 2018 to 2020. The maximum energy consumption of the hospital is during August, whereas the minimum consumption is in January. The remarked large variation of the related energy consumption is mainly owing to the air conditioning performance according to the weather circumstances throughout the year.
3. Designing the PV-Based Rooftop System
The designed PV system for the considered hospital is grid-tied one to benefit with the solar energy to supply the electricity need of the hospital during the day. Then, possible excess energy can be supplied to the national grid during the daytime and reused at the night. In this project, 7 PV-based rooftop microgrids were designed with a total capacity of 9570 kVA. These proposed PV subsystems were designed using the governmental PV designing tool, Shamsi, as a practical and versatile designing tool for such systems provided by the Saudi Electricity Regulatory Authority [34, 35]. Shamsi aims to guide the users in evaluating the expected benefit from solar panels installation by calculating the expected produced energy, providing financial feasibility, and several other services. Based on the available roofs of all residential and administrative buildings, the main building and medical centers as well as empty spaces and parking lots, the aimed rooftop PVs are utilized with an almost area of 63,800 square meters. Seven different PV microgrids were assigned as described in Figure 6.
(a) 7 microgrids power with building details
(b) Schematic of the utilized 7 microgrids
Shamsi is an online governmental portal that aims to guide professionally providing technical, financial, and feasibility analysis of grid-tied PV rooftop systems. It provides three different functions including the “Solar Calculator” to estimate solar energy production in a selected location. “Financial Calculator” helps to estimate installation cost and electricity bills saving, and “Contractors & Quotations” helps to connect consumers with certified contractors and facilitate quotations. The “Solar Calculator” facilitates a professional tool to get a precise estimation of the expected generated energy related to a specific area or a specific system size. It will provide the users with a detailed output about the energy expected to be produced monthly after installing the solar panel according to the selected technology [34]. Moreover, it can be directly linked to updated maps of the considered area. Shamsi facilitates designing rooftop PVs by maximizing the possible space of the selected area based on the updated satellite maps with practical modules data using polycrystalline, monocrystalline, or thin films.
Microgrid-1 is installed via two PV subsystems on the main building with a total capacity of 1900 kVA. Each subsystem is designed with a polycrystalline silicon panel of 229.3 W panels having 130 parallel strings with 34 series ones in each one. They generate together 1900 kVA with 400 V level. For illustrating the designed steps, details for microgrid-2 are described consisting of different nine PV subsystems on a group of buildings including 30 residential villas and building no. 6, building no. 7, and the warehouse no. 2 with a total capacity of 967.33 kVA at 400 V. Table 3 illustrates the description of the assigned nine PV subsystems according to the available area for the related buildings.
The available area for the first PV subsystem is 614.33 m2 facilitting installing 408 solar panels with overall 93.55 kW. It is equipped with a 229 watts PV panel including 34 series panels and 12 parallel ones. Building no. 7 is characterized with an area of 1014 m2 with 680 solar panels 34 series and 20 parallel ones, respectively. They can generate together 155.9 kW. Similarly, other PV subsystems are designed to provide an overall energy of 967.33 kVA.
Similarly, microgrid-3 consists of five PV subsystems on a group of buildings including building no. 14, building no. 15, building no. 16, and also the pharmacy and park area-6 with a total generated power of 875.7 kVA. Microgrid-4 consists of eight PV subsystems on a group of buildings including building no. 12, building no. 13, store no. 1, maintenance building, park-5, outpatient clinics, administration building, and pumping building with a total capacity of 1209.28 kVA. Microgrid-5 consists of nine PV subsystems on a group of buildings including building no. 2, building no. 3, building no. 4, building no. 5, four free spaces, and an education and training building with a total capacity of 2722.4 kVA. Microgrid-6 consists of five PV subsystems on a group of buildings including building no. 8, building no. 9, building no. 10, building no. 11, and building no. 1 with a total capacity of 733.8 kVA. Finally, microgrid-7 consists of eleven PV subsystems on a group of buildings, four parks, a market building, reception building, the mosque, AKU building, OPTH building, RHC building, and the DNC building with a total capacity of 1161.89 kVA.
The solar irradiance data and the average monthly solar irradiance data were obtained from the Shamsi portal. Then, the percentages of the monthly solar energy along the entire range of the year were described as seen in Figure 7.
It is worthy to note that designing grid-tied PVs requires detailed analysis by considering local parameters. On the other hand, solar panel manufacturers provide the performance of their PV panels under standard test conditions (STC) with a solar radiation of 1000 W/m2 and surface temperature of 25°C. These parameters may be deviated in the real field.
4. Simulation Tests and Filtering Design
As explained earlier, adopting grid-tied PV subsystems may affect the power quality remarkably. Hence, the variation of the THD at the coupling buses should be investigated. For this target, a detailed simulation for the overall project was constructed via ETAP [36]. This well-known simulation program helps to investigate the performance of electrical networks deeply. Moreover, it can help to adopt the aimed tuning filters including single-tuned ones with bypass, high pass (damped), high pass (undamped), 3rd order damped, or 3rd order C type [37]. Accordingly, it represents an ideal candidate for performing the aimed simulation tasks in this study.
4.1. Overall Project Model Development
As mentioned previously in Figures 3 and 4, the overall grid of the hospital was constructed in different panels fed from 4 large 110/13.8 kV transformers. Based on the demonstrated one-line diagrams in the aforementioned figures, the corresponding ETAP model was constructed with the complete data of each power elements, loads, and cables. Figure 8 illustrates the constructed overall conventional model of the selected hospital in ETAP without the designed PV-based microgrids demonstrating its load flow analysis according to the main grid. Similarly, the corresponding ETAP model for the air conditioning system and the AKU center were developed according to their corresponding one-line diagram in Figure 9.
(a) ETAP model for the AKU center
(b) ETAP model for chiller 6 to 9
(c) ETAP model for chiller 1 to 5
The next step is to characterize the resulting total harmonic distortion (THD) with the overall loading without the aimed PV system as described in Table 4. An acceptable THD profile was realized in such situations according to the common IEEE standards for the selected the main buses 3, 4, and 5, respectively, as illustrated in Figure 10. Figures 11–13 illustrates the corresponding phase voltage and its corresponding harmonic spectrum at buses 3, 4, and 5, respectively. These busbars represent the coupling points to the grid. As remarked from the illustrated results, the voltage levels as well as the related THD at the selected buses were kept in an acceptable range according to the common standards.
(a)
(b)
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4.2. Impacts of Added PV Rooftops
Similarly, the resulting THD of the constructed system with utilizing the adopted 9750 MW rooftop PV subsystems was investigated as well. First, economic inverters were considered in this test with 6 pulse inversion mode. As seen in Table 5, an exaggerated THD profile was realized for the selected buses beyond the limit of the common IEEE standards. This was illustrated in Figure 14 for the aforementioned buses as well. For example, Figure 15 illustrates the corresponding phase voltage and its corresponding harmonic spectrum at bus 4 in this condition.
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(b)
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The performance of the adopted inverters was improved with utilizing 12 pulse inverters with all constructed PV subsystems. Then, the resulted THD levels at the considered buses were estimated as illustrated in Table 6. Although the resulted THD levels at the selected buses were remarkably reduced as compared with the corresponding ones with 6 pulse inverters, it exceeded the permissible THD limit with 10.53% at bus 4. This certainly clarified the realized improvement regarding the resulting harmonic distortion with utilizing improved inverter units with advanced control paradigms.
Actually, PV inverters play a basic role in the imposed harmonic distortion in the distribution level. Although modern inverters are equipped with precise control mechanisms and sometimes some suitable filtering regimes, the resulting harmonic distortion may cross the allowable limits. Different reasons participate in these circumstances including the intermittent nature of the solar irradiance, the inverters’ sizes, PWM control nature, and the DC link voltage variation [18]. PWM harmonics are owing to the utilized switching frequency and its multiples due to the dead-time and nonlinear turn-on/turn-off delays [38]. Hence, utilizing single-tuned filtering represents a practical and economical solution.
4.3. THD Reduction Using Single-Tuned Filtering
Different filtering techniques are practically utilized in eliminating harmonics in power systems. Passive filters are characterized as a resonant circuit with a low impedance at its tuned frequency to be trapped. Passive filters can be connected in series, in shunt, or in combination of series/parallel. Although such types of filters have some drawbacks, they are preferrable as favorite solutions in industry to suppress the harmonic pollutions for the reasons that they are very common and economical. Single-tuning filters are widely used for their simplicity and reliability. It diverts the harmonic current path from the normal flow path through the filter, where the filter’s impedance, Zn, can be computed as a function of its parameters as
The frequency order of the filter is equal to the resonant frequency at which the impedance of the capacitor is equal to the impedance of the inductor as described in Figure 16. Then, the harmonic current will flow in the filter and dissipate in the power system.
Regarding the design process, the passive filters are tuned at frequencies with tolerant values of ±3% to a level of ±15% far from the harmonic frequencies anticipated for suppressions. This is because the ratings of inductors and capacitors may change due to manufacture tolerances [39–41]. Damped filtering produces the sequential resonance at the fundamental frequency to reduce power loss and attenuates frequencies above the specified harmonic order. This filter is preferable to higher frequencies than a single-tuned filter because of the increased resistance of the filter at a monotonic frequency. Because the damped filter’s resistance is close to the resistance value at high frequencies, the performance of this filter at high frequencies is better than the single-tuned one [42–44]. Then, single-tuned filtering is utilized in this work as the most widely used filter due to its low cost compared to other filters [45–48]. The utilized single-tuned filters at buses 3, 4, and 5 are computed as follows.
4.3.1. Bus-3 Filter Design
At bus 3, the highest harmonic content is for the 7th order with was 5.04% at a power factor of 87% with a current of 1701 A. With the proper selection of the corresponding filter at this bus, the power factor was set to 95% reducing the seventh harmonic order to 0.05%. Then, the total THD was reduced to 9.36%. Another single-tuned filter was also designed for the 5th harmonic order having a percentage of 6.75%. Then, a power factor of 95% was realized with 0.11% of the 5th order. This resulted in reducing the overall THD to 5.81%, which agreed with the known IEEE standard as illustrated in Table 7. The corresponding spectrum for the bus voltage was illustrated in Figure 17. Accordingly, the improvement of the single-tuning filtering to meet the allowable THD limit at this voltage level is clear.
4.3.2. Bus-4 Filter Design
Similarly, three single-tuned filters were utilized for the 5th, 7th, and 11th harmonic orders at bus 4 as described in Table 8 resulting in reducing the overall THD to 6.52% which copes with the common IEEE standard at this voltage level as illustrated with the related voltage spectrum in Figure 18.
4.3.3. Bus-5 Filter Design
For bus 5, the highest harmonic content was for the 5 orders with 5.32% and a power factor on this bus of 80% with a current equal to 998 A. Then, a single-tuning filter was utilized at bus 5 to eliminate the 5th order. As shown in Table 9, the added filter succeeded to reduce the THD of the bus to 7.73 at a power factor of 95%, which meets the related IEEE standard at this voltage level. The related voltage waveform and its spectrum were demonstrated in Figure 19.
5. Conclusion
In this paper, the designing phase of a 9750 MW grid-tied rooftop PV subsystem was described with a real hospital in Saudi Arabia. The overall PV systems were constructed in 7 different microgrids according to the available buildings and their available areas. Details of the loads as well as their related electrical network were precisely modeled in ETAP. As concluded from the results, utilizing large grid-tied PVs provided remarkable harmonic distortion excessing the permissible common limits in LV networks. However, utilizing recent inverter technologies with 12 pulses or more can partially eliminate the imposed distortion. A single-tuned passive filter was used to reduce the resulting THD that exceeded the allowable limits according to the IEEE-519 standards. Required filter parameters for buses 3, 4, and 5 were computed to reduce the total THD to meet the known IEEE standard, where the THD at the aforementioned buses was reduced from 10.87%, 17.37%, and 12.11% to 5.81%, 6.52%, and 7.73%, respectively. The results corroborated the efficacy of single-tuning filters with large rooftop PVs in the distribution level to reduce their impacts on the harmonic contents remarkably. It is technically advantageous with its cheap, reliable, and simple utilization as compared with other tuning methodologies. Hence, it represented a preferable choice for PV projects in the distribution level.
Appendix
Single-Tuning Filter Design
Designing the aimed single-tuning filters is basically performed according to simple active/reactive power relations as seen below as
is the three-phase capacitive reactive power, is the capacitive reactance at the fundamental frequency, is capacitance, and are the inductive reactance and inductance and is the quality factor among between [30–33].
Data Availability
All required data in this paper are found in paper or in the relevant references in the paper.
Conflicts of Interest
The authors declare that they have no conflicts of interest.