Overvoltage Mitigation Techniques for PV Grid
In this case study, the authors have reviewed the conventional techniques used to mitigate the overvoltage problem and observe that they have several disadvantages. So, it is suggested a novel technique that relies on a minor modification made to the secondary windings of the distribution transformer: an extra tapping is pulled out from each of the secondary windings into a separate PV junction box which is the modified Point of Common Coupling (PCC)...
Main author - Vithal Narasinha Kamat
Co-authors – Nivea Chauhan, Harsh Bhatt,
Payal Bhadresa, Akash Sent
In order to achieve the goal of mainstreaming Photovoltaic (PV) systems in the rural environment, one of the main technical issues to be resolved is grid interconnection. Of the problems encountered in such an environment, ‘overvoltage’ is the top priority. Overvoltage incidents are more likely to occur on rural grid, in which, generally, the line impedance is higher and the load is relatively low. Many of the constraints, including overvoltage can be eliminated when infrastructure and other facilities are upgraded by designing the grid configurations and distribution capacities to meet future capacity growth. In addition, the following characteristics are identified as key recommendations for the future grid systems free of constraints on PV grid interconnections.
• Integrated system management using ICT (Information and Communication Technology)
• Extension of distribution capacities
• Development and widespread use of storage technologies or integration of either grid
load control or building load control with PV generation output.
• Provision of power quality that fits the corresponding application
Impact of PV Grid Interconnections
Over the past century, the standard electric power distribution model has been to generate power at large-scale power plants and distribute power to customers via power transmission lines. Power distribution infrastructure has also been designed with this model in mind. In recent years, however, we have been witnessing the appearance of many small-scale power plants on power networks as distributed power sources — such as photovoltaic power, wind power, and various types of co-generation power — gain traction. One side effect of this multiplication of power sources has been to make network electricity flow patterns much more complex, which in turn requires more sophisticated power regulation technologies than have been employed in the past.
Another concern with PV and other renewable energy forms is that they are intermittent power sources with substantial output fluctuations. As more of these power sources are interconnected with power grids, various risks come into view, such as lower electric power quality and stability.
AC power quality is a general term for indices that describe the impact on customer-device operation due to deviations from prescribed tolerances in the sinusoidal voltage’s amplitude, frequency, phase, and waveform. Various schemes have been proposed of parameters to evaluate the quality of electric power. Europe created the power quality standard EN 50160 in 1994 (revised in 1999), and the United States set out the IEEE Standard 1159 on electric power quality in 1995. The International Electrotechnical Commission (IEC) worked on establishing measurement methods for AC power quality parameters in conjunction with the global trend to deregulate the power industry. It set forth these methods in the IEC 61000-4-303 standard in 2003.
In this paper, we focus on voltage amplitude (overvoltage and undervoltage). For a voltage amplitude problem, it is required to arrive at a solution that can be implemented with minimal disturbance to the existing infrastructure. Overvoltage can be a nuisance that causes frequent inverter tripping. Hence, it is necessary to identify the root cause of the over-voltages and find not just potential but practical solutions to this problem.
The problem of overvoltage was being faced by the Dhundi Saur Urja Udpadak Sahakari Mandali (DSUUSM), a solar pump co-operative at Dhundi village in Kheda district of Gujarat. This co-operative was formed to sell surplus power produced in their farms to the electricity grid. We studied the problem of overvoltage and have arrived at a novel and elegant solution that addresses their problem.
Basics on Overvoltage/Undervoltage
Just as running water flows from a higher pressure point to a lower pressure point, electricity current flows from a higher voltage point to a lower voltage point. The water pressure and flow weakens as water is consumed along the way. In a similar fashion, the voltage of electricity decreases as it is consumed. Thus, the line voltage decreases relative to the distance from the voltage source, as well as the types of loads encountered (see Figure 1).
On the other hand, when the power generated by PV is more than the energy consumed at the point of use, the surplus electricity will flow back to the grid. In this case, the electric current flow reverses direction and the voltage rises as it goes to the end. This is not a significant issue in an urban grid, which can be characterized as a strong network with low grid impedance, and limited PV capacity. However, as PV penetration increases or when a large number of PV systems are installed on a rural grid with higher impedance, the voltage could exceed the upper limit. This issue is called overvoltage (see Figure 1).
Figure 1. Con ceptual diagram of Over-voltage
Voltage must be kept in a certain range as designated by laws, standards, or guidelines, which vary from region to region for purposes of appliances and machinery to operate properly. In order to control the voltage within the range, utility companies apply various technology counter measures.
One way is to control the line voltage to some extent by reducing the sending voltage from the distribution transformer; however, this may cause undervoltage of neighbouring lines connected to the same transformer with little backward flow, since it is difficult to independently control sending voltage from the same bank (see Figure 2).
Figure 2. Und ervoltage problem
Both overvoltage and undervoltage would have a negative impact on stable operation of the supply-side devices including generators and transformers. Additionally, there would also be an impact on the demand-side equipment. Overvoltage might damage or shorten the lifetime, while undervoltage could constrict the normal performance of electric equipment.
In Japan, Power Conditioning Systems (PCS) for PV systems are designed to control the voltage rise so as not to exceed the limit. Overvoltage can be completely prevented with this technology. However, a disadvantage is that the PV power output is dumped to control the voltage, leading to lower efficiency of the PV system. This can also lead to unfairness among users since the PV output at the end of the line tends to be restricted with higher priority. When investments are based on the PV production such as a feed-in tariff, the grid operation will affect the investment. Since the Japanese solution is both inefficient and unfair, it is not suitable for a democratic country such as India. Thus, overvoltage and undervoltage can be one of the biggest barriers to mass distribution of PV systems.
Instantaneous Voltage Change and Islanding
When faults such as lightning occurs on the grid network, the voltage around the fault point drops until the protective relay detects the fault and isolates the fault from the main grid by means of breakers. This is the typical case for instantaneous voltage change. The duration of the voltage drop is dependent on the operational time of protective relays and breakers.
Unlike other distributed generation sources, PV systems have little impact on instantaneous voltage change since fluctuations in the power output are relatively slow and the grid interconnection processes are appropriately controlled by power conditioners. One possibility for instantaneous voltage change occurrence by a PV system is simultaneous disconnection of PV systems by an unintended islanding function in the inverter that is too sensitive and the PV dropping off line.
The term islanding has historically been used to describe the undesirable event of a grid-connected PV generator failing to disconnect during a grid outage. However, as grid-connected PV systems have emerged to provide the dual purpose of acting as stand-alone generators during a grid outage, the term has been refined to intentional and unintentional islanding.
Unintended islanding is an electrical phenomenon in which PV systems within a certain network continue to supply power to the load even after the network is disconnected from the main grid for some reason (e.g., electrical problem). When a network is disconnected from the main grid, the PV systems in the network are designed to detect the abnormal power quality in voltage, frequency and grid impedance and to disconnect from the network immediately. However, if the power generated from the PV systems and that consumed in the load are by chance identical, the PV systems might not be able to detect the unintended islanding and will continue to supply power. It should be noted that there is little impact from unintended islanding since the possibility of unintended islanding operation is quite low.
Islanding operation can only be possible when the following three conditions happen simultaneously.
1. The power supply from the main grid stops for some reasons,
2. The power generated from the PV systems accidentally matches load
3. Islanding protection functions in the PCS failed to detect the islanding conditions
According to the IEA PVPS Task 5 report, the possibility of unintended islanding operation that continues for more than five seconds in a distribution line is 8.3×10-10 to 8.3×10-11 / year.
Currently, advanced unintended islanding detection schemes are available that minimize the risk of simultaneous disconnection of PV systems. Europe and the US are also considering change of timings for the PV system to drop off to have a slight (a fraction of the power frequency cycle) delay.
Figure 3. Unintended Islanding Operation
Voltage Imbalance, Harmonics, DC Offset, Frequency Fluctuation etc.
The other conditions for abnormal voltages could be – (a) Voltage Imbalance - when each phase voltage is different in a three phase system, (b) Harmonics - when voltages having an integer multiple of the fundamental frequency generated by the power electronics technologies distort the grid voltage waveform. DC offset, frequency fluctuations, etc., can also result in abnormal voltages, but these abnormalities are beyond the scope of the current paper.
Peak Power Supply Effect of a PV System
Generally, electricity demand increases during the day time and decreases during the night time, although it is heavily dependent on regional conditions. Since PV systems generate electricity in the daytime they can contribute to supplying the peak load. Especially, for countries with a relatively hotter climate, PV is expected to offset the increase in cooling demand during the summer (Figure 4). Large office buildings in urban areas have to cool year-round due to heat loads from both people and electronic equipment. On the other hand, it is not easy to quantitatively assess the effect of peak power supply by PV systems. For example, PV systems cannot supply electricity in the evening when the demand remains relatively high in many countries; therefore, the effect is limited (Figure 5). It is also pointed out that solar energy is an intermittent energy source that requires a back-up generation plant to some extent in order to ensure supply security.
Figure 4. Peak Power Supply - 1
Figure 5. Peak Power Supply – 2
The peak power supply effect of a PV system can be significantly enhanced through coupling with a small-scale energy storage system such as batteries (peak-shifting). If the system stores power during times of high PV output and discharges the power when it is needed, the power supplied from the grid during peak hours would be reduced. Also, in large buildings with cooling loads, the energy controller of the cooling equipment can be interfaced with the operation of the PV to effectively use the thermal mass storage of the building to support the intermittency.
The imbalance between power supply and demand leads to fluctuation of grid frequency or voltage, which could cause equipment damage on the demand side. However, electricity demand (load) changes every minute. In order to efficiently respond to the changes, utility companies generally classify and operate power plants independently. Example classifications are: base load power for constant output, middle load power for changing load and peak load power for peak demand.
Peak-power generators do not usually operate during off-peak hours. Therefore, the capacity factor for power plants is relatively low and the cost is high. To reduce the need, and therefore the cost, for peak-power generators, utilities strive to reduce the peak demand through demand-side management programs. Utilities also price electricity higher during peak periods with time-of-use and demand-rate tariffs. Consequently, utilities benefit from reduced peak demand via supply of PV power, and the PV owner benefits as well. Moreover, if the PV owner is on a demand rate or time-of-use rate, the PV electricity is displacing higher-priced electricity, and the benefit of energy cost savings is greater.
It describes various counter measures that can be taken if deterioration in power quality is observed (see Table 1).
Some of the equipment used to counter overvoltages are:
i. Line Voltage drop Compensator (LDC)
ii. Phase modifying equipment, such as Static Capacitor and Shunt Reactor
iii. Step Voltage Regulator (SVR) or Transformer with on-load tap changer.
iv. Thyristor Voltage Regulator (TVR)
v. Static VAr Compensator (SVC)
vi. STATic synchronous COMpensator (STATCOM)
vii. Dynamic Voltage Restorer (DVR) using Storage Devices
viii. Power Conditioning System (PCS) with function to suppress rise in grid voltage
ix. Passive Filter or LC Filter
x. Active Filter
xi. Sodium-sulphur (NaS) Battery
xii. Unintended islanding detection system (Passive system)
xiii. Unintended islanding detection system (Active system)
xiv. Transfer trip equipment – signals from transformer directly to device via communication lines
xv. OLTC fitted transformers
xvi. Batteries or Super-Capacitors Energy Storage
Unfortunately, many of the counter measures listed above have their own disadvantages. Thyristor Voltage Regulator (TVR) has disadvantage of long term reliability when installed on a pole. Cost is a bottleneck in Static VAr Compensator (SVC) technology and Static Synchronous Compensator (STATCOM). Line voltage Drop Compensator (LDC) has a problem of overvoltage suppression. Step Voltage Regulator (SVR) has disadvantage of low response time.
Tap changer has limitations in switching time. The main problem associated with On Load Tap Changer (OLTC) fitted transformers is the communication infrastructure cost associated with this kind of procedure. Test results show that in order to implement a cost effective approach we need to minimize tap operations, voltage profile issues monitoring should be only located at the end of the feeder and the ideal cycle length should be 30 minutes, thereby, causing less impact in the OLTC usage.
Energy storage devices such as NaS batteries or super-capacitors are very expensive making their choice uneconomical. Moreover, the interruptible characteristic of renewable energy has a detrimental effect on battery life due to stress.
We studied the problem of overvoltage being faced by the farmers of the solar pump co-operative at Dhundi village, in the Anand District of Gujarat. This co-operative was formed by bringing together six vegetable farmers in Dhundi village (see Figure 6). The solar pumps were connected to the power grid of the local electric utility – Madhya Gujarat Vij Company Limited (MGVCL), and a 25 year power purchase agreement was signed allowing the farmers to sell back the surplus energy at a rate of `4.63 per kWh equivalent of solar energy.
Figure 6: Line diagram of Dhundi village
The current installed capacity of the Dhundi solar co-operative is 56.4 kW, which is estimated to generate nearly 85,000 kWh units of energy annually, if 300 sunny days and 5 units per kW are assumed. The six farmer members would use 40,000 units to irrigate seven acres of farmland and the balance 45,000 units would be injected into the grid to fetch over `2 lakh in revenue from the sale to MGVCL. The problem of overvoltage is causing the grid tie invertors to trip, thereby, causing a substantial loss in this revenue. Hence, in this paper we suggest an elegant solution to the overvoltage problem to minimize the loss of revenue to the farmers.
Observations during Field Visit
We visited a solar pump house of one of the six members of the co-operative on a cloudy day. The PV installation comprised of a group of six large swivel type PV panel mounts each of which was rated for 250 We at an output voltage of 144 Volts. There were six mounts; the connection of which were in series, hence, the total output from the PV panels to the solar pump house was 864 Volts and 7 Amps, representing a total of 6 kWe of solar generation from this single farmland. There are six such members with different capacities and the total installed capacity of all the members of the solar co-operative is 56.4 kWe.
The following observations were taken when the Grid-Tie inverters were connected to the solar panels instead of the pump. The output of the inverter was observed to vary from 2.30kWe to 2.41 kWe, which was less in comparison to its rated capacity (6.0kWe) due to cloud cover.
The voltage readings were taken before and after connection of the Grid Tie Inverter to the PV grid. In the meter room that housed the individual meters and the net meter, the voltage readings recorded are as shown in Table 2. It can be observed that there is, on an average, a 3.5 Volt increase in each of the three phases when the inverter is connected.
We also observed rains (a mild shower) and the PV output dropped. Table 3 gives the power output and the corresponding current in each of the three phases from the grid-tie inverter during the rains.
It may be noted that the meter room is a central one that contained all the six individual meters from the six farmer members of the co-operative. This means that the generation of all the PV installations gets accumulated in this meter room.
The central meter room also has a common main Net meter from which a separate cable emerged that ran as an overhead cable all the way to a junction box or PCC (Point of Common Coupling) located about one kilometer away from the meter room. In this junction box, the PV grid output gets fed (injected) into the utility (MGVCL) grid.
About 400 meters further down from the junction box (see Figure 6), we located the utility’s (MGVCL) distribution transformer which was positioned close to the pump house and overhead tank that fed water to the village. This single distribution transformer of 100 kVA capacity fed electric power to the whole of Dhundi village. The readings taken on the transformer meter are as shown in Table 4.
From the distribution transformer meter readings, we can observe that the transformer output voltage is about 5 to 12 volts higher than the voltages observed at the farm when the PV generation was off. This indicates that the impedance (essentially resistance) of the long rural lines (length greater than one kilometre) is high.
Though we were unable to take readings at the distribution station while the PV generation is ON, it is clear that the voltage at the transformer output would increase further. When all the PV sources are simultaneously injecting into the PV grid, the voltage at the grid tie inverter points would easily exceed 270 Volts which is the limiting value beyond which the Grid tie inverters are designed to trip.
In Dhundi, we observe all the conditions present that can promote overvoltages, namely:
i. Integration of generation (PV) in a rural grid
ii. Reverse power flow, and
iii. Rural area far away from the substation transformer.
Hence, the frequent tripping of grid tie inverters comes as no surprise.
Proposed Overvoltage Mitigation Technique for Dhundi
In counter measures section, we have described various overvoltage mitigation techniques that would help to mitigate overvoltage, however, each of them have disadvantages that make them less attractive. In this section, we propose a solution that would suit the Dhundi rural installation. The constraints for Dhundi arise since PV installation was already complete; the proposed technique should not disturb the installation in a major way. The cost of the proposed technique is also of significance and it is desirable to have a least cost solution. Due to the urgency, the time required to implement the solution at Dhundi should also be minimum.
Proposal : Transformer with a Secondary Tapping
Unlike the techniques given in counter measures section, here we propose a novel technique that, not only satisfies all the above constraints, but also requires only a minor modification to the distribution transformer.
The transformer used in Dhundi is over-rated at 100 kVA to account for future addition in PV generation capacity. This 3 phase distribution transformer is like any other used in the Indian distribution system. It is a simple step down with three numbers of 11 kV input (primary) windings in a delta configuration and three numbers of 240 V output per phase secondary windings in a star configuration.
Though India now is expected to conform to the international regulations that suggest 230 Volts instead of 240 Volts, the lower voltage is seldom observed in a grid surplus Indian state since the utility revenues would fall. The rural LT lines also extend over long distances, and hence with the high line resistances, it is inappropriate to suggest to the Indian utilities that they lower down the output voltage of the transformer secondary.
Hence, the minimum changes that we recommend in our proposal are as follows (see Figure 7):
Figure 7: Modifications proposed for Dhundi village
1. Get a Tapping out from each of the Transformer Secondary Windings: While keeping the number of turns of the primary and secondary the same, a minor change would take out a tapping from the secondary after 90% of the turns have been wound. Hence, when the nominal voltage at the secondary of a loaded transformer is 240 Volts, the voltage at this tap would be 216 Volts.
Since our transformer is lightly loaded, when the voltage across the secondary is 260 Volts, the voltage at the tap would be 234 Volts.
2. Shift the PV injection point: On the secondary side of the modified transformer we would now observe two junction boxes (see Figure 7). The 260 Volts (Nominal rated: 240 V) secondary output winding gets terminated in the regular junction box that is feeding the Dhundi village load. On the other hand, the 234 Volts (Nominal rated: 216 V) secondary output winding would be terminated in the PV junction box (see Figure 7). This means that for Dhundi village, the PV grid cables need to be extended further by another 400 meters to this new PV junction box from where the PV power would get injected into the utility grid system (also see Figure 6).
With the above changes, the Dhundi village would not be facing an undervoltage problem in the evening or night when the PV generation is OFF and the load is heavy. At the same time, during the day, when PV generation is maximum, the grid tie invertor would not face the problem of tripping since it feeds into a secondary tapping that carries a lower voltage.
We consider this proposal as the most elegant, since the changes are minimal, and so is the cost. This proposal is best suited for Dhundi, as the co-operative has already invested in laying a separate PV grid till the feed junction point or the Point of Common Coupling (PCC) that is located close to the transformer center. Under the proposed modifications, the new PV Junction box becomes the PCC and hence the PV grid needs to be extended further by 400 meters (see Figure 6 and Figure 7).
Worldwide, side by side with distributed renewable integration, we observe the power system to be evolving into a smarter and more efficient grid. However, till such a time that our Indian villages become equipped with such grids that are smart enough to mitigate the overvoltage problem with ease; we need elegant solutions that can be implemented with minimal changes to the existing infrastructure.
In this paper, we have reviewed the conventional techniques used to mitigate the overvoltage problem and observe that they have several disadvantages. Hence, we have suggested a novel technique that relies on a minor modification made to the secondary windings of the distribution transformer: an extra tapping is pulled out from each of the secondary windings into a separate PV junction box which is the modified Point of Common Coupling (PCC). The PV output is connected to this PCC / PV junction box. The load connections and the rest of the distribution system remain unaltered. Since it is minor modification that will help promote PV on a large scale, we hope to see this modification implemented in all the distribution transformers that are manufactured in India in the near future.
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