Study of the Impact of Grid Disconnections on the Production of a Photovoltaic Solar Power Plant: Case of Diamniadio Power Plant

Amadou Ndiaye, Mohamed Cherif Aidara, Amy Mbaye, Mamadou Lamine Ndiaye

Laboratoire Eau-Energie-Environnement-Procédés Industriels (L3EPI), Ecole Supérieure Polytechnique, Université Cheikh Anta Diop (UCAD), Dakar, Senegal.

DOI: 10.4236/jpee.2023.116002                

Abstract

Today, renewable energy projects connected to the interconnected network, with powers of the order of tens of megawatts, are more and more numerous in sub-Saharan Africa. And financing these investments requires a reliable amortization schedule. In the context of photovoltaic systems connected to the interconnected electricity grid, the quintessence of damping is the amount of energy injected into the grid. Thus it is fundamental to know the parameters of this network and their variation. This paper presents an evaluation of the impact of power grid disturbances on the performance of a solar PV plant under real conditions. The CICAD photovoltaic solar plant, connected to the Senelec distribution network, with an installed capacity of 2 MWp is the study setting. An energy audit of the plant is carried out. Then the percentage of each loss is determined: voltage drops, module degradation, inverter efficiency. The duration of each disconnection is measured and recorded daily. The corresponding quantity of lost energy is thus calculated from meteorological data (irradiation, temperature, wind speed, illumination) recorded by the measurement unit in one-minute steps. The observation period is three months. The total duration of disconnections related to the instability of the electrical network during the study period is 46.7 hours. The amount of energy lost is estimated at 22.6 MWh. This represents 2.4% of the actual calculated production.

Keywords

Photovoltaic Power Plant, Disconnections, Network, Evaluation, Lost En-ergy

1. Introduction

Photovoltaic (PV) solar energy, having the sun as its primary source, is very appropriate for the production of electrical energy in some countries such as Senegal with a high solar potential (varying between 1850 and 2250 kWh/m²/year [1] ). It also has the advantage of simplicity of installation and commissioning, compared to other renewable energy plants. Senegal inaugurated in 2014, with the CICAD plant (2 MWp) its first PV plant connected to the 30 kV distribution grid.

Since 2016, an acceleration in the commissioning of PV plants has been noted. With the connection and injection of: Bokhol (20 MWp), Malicounda (22 MWp), Kahone (20 MWp), Ten Merina (29.5 MWp), Santhiou Mekhe (29.5 MWp), Sakal (20 MWp) and Diass (20 MWp in test phase), the PV power connected to the Senelec interconnected grid has increased from 2 MWp to 164 MWp between 2016 and 2019 [2] . However, various studies show the impacts of PV power plants on the grid such as: local voltage rise at the connection point [3] , voltage bumps [4] , rapid power variations [5] … But this connection imposes grid stability conditions that are not always guaranteed. This leads to disconnections of the inverters followed by energy loss during the period of grid instability. Depending on the output of the PV plant, the decoupling function can be internal or external to the inverter. The decoupling function is integrated in the inverter for small power sources, equipped with an inverter of less than 5 kVA. It is accepted that this decoupling protection function is provided by an automatic disconnector. Today, only the German standard DIN VDE 0126 is recognized. Two independent devices are connected in series, each with a disconnecting device for maximum safety. This device constantly monitors the quality of the grid by measuring voltage, frequency and impedance [6] . The decoupling function is external to the inverter for installations with a power rating of more than 5 kVA. The decoupling protection function is then provided by measuring relays that are independent of the inverter. Three types of protection are currently recognized for photovoltaic generators (GPV) connected to the public low voltage distribution grid [7] . The general objective of this work is to evaluate the performance of solar PV plants based on the impact of grid stability. This is a novelty in the field because most publications deal with the impact of intermittency on the interconnected grid.

Thus, based on the meteorological data and the energy produced by the plant, recorded over a period of three months, we will evaluate the losses due to the grid.

In this document, we first present the CICAD solar power plant as well as the methodological approach adopted. Then, the evaluation of the amount of energy lost as well as the corresponding duration will be presented in the results and discussion section.

2. Methodology

Based on the daily reports from the Abdou Diouf International Conference Center (CICAD) power plant, the meteorological data recorded and additional physical measurements carried out in the PV field, all the losses, the number and duration of interruptions as well as the quantity of energy produced day by day are aggregated. The working method adopted, consists in first making an energy audit of the plant in order to identify all the existing losses. Then determine the percentage of each type of loss on production. Finally, the total duration of the disconnections and the corresponding amount of energy during the study period are evaluated.

2.1. Presentation of the CICAD Plant

The Centre International de Conference Abdou Diouf (CICAD) Solar PV power plant is the first PV power plant interconnected to the Senelec power grid. It is located in Diamniadio (30 Km from Dakar Figure 1) and was commissioned in November 2014. The plant with a total installed capacity of 2 MWp, is connected to the 30 kV distribution grid through a 2 kVA, 30/400kV transformer.

The CICAD plant is composed mainly of the following elements:

- A photovoltaic field.

- 116 boxes or junction boxes.

- Fuse grouping boards.

- Two inverters branded GAMESA of 1 MW each.

- A 2 kVA transformer.

- Cables.

- Parameter monitoring system (SCADA).

- Protection equipment.

- A meteorological station.

Table 1 summarizes the description of the plant. The technical characteristics of the modules used are given in Table 2.

Table 3 shows the recommendations for decoupling protection according to the German standard DIN VDE 0126.

Table 4 shows the technical specifications of the GAMESA E1 inverter, given by the manufacturer. The inverter acts as a DC-AC converter. This allows the energy produced by the PV field to be injected into the public electricity distribution grid. It also integrates coupling-decoupling functions as well as maximum and minimum protection of electrical parameters (voltage and frequency).

2.2. Different Causes of Performance Degradation

The performance of a PV plant decreases over time due to a degradation process of the PV system, especially the PV panels. Also, the inverters, transformer, connectors, and protection system can be affected by degradation [9] . Table 5 shows all the detected degradations as well as the causes and consequences.

2.3. Assessment of Plant Losses

The evaluation of the system losses of the power plant is necessary to control the amortization plan and the follow-up evaluation of the investment. It also allows to evaluate the quality of the Senelec distribution grid in the area.

Figure 1. Geolocation of the CICAD site.

Table 1. Description of the CICAD plant.

Table 2. Technical specifications of the module operated at the CICAD plant.

Table 3. Types of decoupling protection in BT.

Table 4. Technical specifications of the inverter [8] .

Table 5. Summary of the main sources of plant performance degradation.

The amount of energy lost is obtained by subtracting the theoretical energy of the plant from the actual energy obtained during an hour of time. The meteorological and electrical parameters are measured and known for the determination of the theoretical energy that should produce the plant. It will remain only to deduct the losses of energy due to the disconnections by instability of the electric network. Thus we distinguish:

- The efficiency of the inverter which is the ratio of the output power of the inverter on its input power.

${\eta }_{ond}=\frac{P_{ac}}{P_{dc}}$ (1)

η_ond: Inverter efficiency.

P_ac (kW): Alternating power inverter.

P_dc (kW): Inverter continuous power.

- Voltage drops which are losses due to the connection cables at the junction box and the electrical tables.

$\Delta U=\frac{\rho \ast L\ast I}{S\ast U}$ (2)

- The form factor is one of the most important values for evaluating the efficiency of a photovoltaic system.

$FF=\frac{P_{\max }}{V_{oc}\ast I_{sc}}$ (3)

$P_{\max }=V_{opt}\ast I_{opt}$ (4)

It is equal to 0.8 for plants with normal operation [10] .

2.4. Application

In order to determine the coefficients for the various losses accurately, we used the peak data for each month over the three month study period (January, February, and March). The data are measured and recorded in one-minute steps. Table 6 shows the meteorological data corresponding to the monthly peak irradiance.

- The loss coefficient at the inverter is represented by the letter K₁. The conversion efficiency is a measure of the losses incurred during the conversion from DC to AC. These losses are due to several factors: the presence of a transformer, magnetic losses and associated copper losses, and self-consumption of the inverter. Table 7 shows some values collected at the inverter level to calculate its real efficiency.

- Voltage drop coefficient K₂

The voltage drop coefficient K₂ expresses the connection cable losses. The voltage drop must be calculated for each cable of the PV array, each cable of the PV junction boxes, and for the cable of the inverter. The cumulative voltage drop of the cables between each string and the inverter is then calculated. Table 8 shows the current and voltage measurements at the various nodes of the system.

ΔU_a: Voltage drop of the cables connecting the junction box to the inverter.

ΔU_b: Voltage drop of the cables connecting the PV panels to the junction box.

$K_2=100\%-0.271\%=99.7\%$

- The tilt coefficient, this value is related to the angle of inclination, orientation and fixation of PV modules. It is measured using the solar disk in the Dakar region. The CICAD PV solar power plant has a PV module tilt of 6˚C and an orientation of 24˚C south. The tilt coefficient (K₃) is therefore 0.99.

- The form factor coefficient K₄ is the average of the calculated form factor values divided by 100.

Table 6. Hourly data for the monthly daily peak.

Table 7. Comparisons of the power ratios between the input and the output of the inverter.

Table 8. Voltage drop calculation.

$K_4=\frac{100-\left(\text{averageofFF}\right)}{100}$ (5)

$K_4=\left(100-0.75\right)/100.K_4=0.99$

- Calculation Power injected into the grid (P_inj)

The electrical power at the output of the inverter is the module power multiplied by the different loss coefficients of the PV system.

$P_{inj}=P_{field}\ast K_1\ast K_2\ast K_3\ast K_4$ (6)

The field power is the power of all the photovoltaic modules in the plant. It is obtained by multiplying the power of a module by the total number of solar panels [11] .

$P_{field}=P_{pv}\ast N_m$ (7)

P_field: PV field power; N_m: number of modules

The module power P_pv represents the measured module power. It is obtained from the measured values of the open circuit voltage, the short circuit current and the form factor [10] .

$P_{pv}=V_{co}\ast I_{cc}\ast FF$ (8)

3. Results and Discussion

The coefficients of the different losses are calculated, the duration of the disconnections noted as well as the meteorological parameters. We can calculate the amount of energy lost during the whole observation period and their percentage on the global production. Table 9 shows a daily report of the CICAD solar power plant and Table 10 shows the results obtained from 02 to 09 January 2017.

Table 9. Daily CICAD report of 02/18/2019 [12] .

Table 10. Summary from 02 to 09 January 2017.

The special case of Table 9 shows how the amount of energy produced, the duration of disconnections as well as the daily peak, are recorded in the daily report. This makes it easy to aggregate the total duration of disconnections and the amount of energy produced during the study period.

We note during the period from January 02 to 09, 2017, there were several disconnections due to the instability of the electrical grid, During the period from January 2 to 9, 2017, there were several disconnections due to the instability of the electricity network, with a peak of lost energy of 879 KWh recorded on January 7, 2017. The causes of grid instability are of several types among which we can mention: automatic load shedding due to lack of production, fault tests, grid incident, etc. The total duration of generation interruptions during this period is 2.55 hours and the corresponding lost energy estimated at 2.1 MWh.

Based on the complete summary of the three months studied, we present in Table 11 the amount of energy lost, the total duration of disconnections as well as the amount of energy produced during our study period.

Table 11. Quarterly summary of network unavailability.

It can be seen that the monthly producible during the first quarter of the year is around 3 Gwh. But the lost energy rates for the months of February and March are almost nil (0.3%) unlike the month of January which recorded a rate of 7%. This difference is explained by the high duration of network instability during the month of January, which corresponds to an inverter disconnection duration equivalent to 22 hours.

4. Conclusions

An evaluation of the amount of energy not produced as a result of grid disconnections is presented in this study. The different technical energy losses are determined and classified into four types: aging of the photovoltaic modules, tilt of the photovoltaic modules, voltage drops and inverter efficiency. The factor of each type of loss is taken into account in the evaluation of the amount of energy produced by the photovoltaic field. The first results obtained after the study of three months of operation of the disconnections of the CICAD power plant, due to the instability of the Senelec distribution grid are:

- The amount of energy lost is estimated at 22.509 kWh.

- The percentage of energy lost is 2.4% of the total calculated energy that the plant was to produce.

- The total duration of the disconnections is equal to 46.7 hours.

However, it is very difficult to quantify the unproduced energy with precision. Finally, it should be noted that it is very rare to find research that gives results on the energy lost due to the instability of the electrical grid.

In the future, studies on longer periods and on larger power plants would be very relevant as well as work on the causes of disconnections.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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