Photovoltaics (PV) is a technology that converts sunlight directly into electricity using unique materials known as photovoltaic cells. PV cells are made of semiconducting materials, such as silicon, that absorb photons of light and release electrons, which can be captured and used as an electric current. The amount of electricity generated by a PV cell depends on the intensity of the sunlight, the size of the cell, and the efficiency of the materials used.

PV technology has been rapidly advancing in recent years, and it is now a mature and cost-effective way to generate electricity from renewable energy sources. PV systems are used in a wide range of applications, including small-scale installations for powering devices such as calculators and outdoor lighting, as well as large-scale solar farms connected to the electric grid.

Calculating Design Voltage and Current for a PV Array

The design voltage and current for a PV array are essential parameters that must be calculated to ensure the system operates correctly and safely. The IET Code of Practice for Grid Connected Solar Photovoltaic Systems provides guidelines on determining the design voltage and current.

Step 1: Determine the Module Characteristics

Find the open circuit voltage (V_oc) and short circuit current (I_sc) of the PV module. For example:

Step 2: Calculate the Maximum System Voltage and Current

Calculate the maximum system voltage (V_max) and maximum system current (I_max) using the following formulas:

• V_max = V_oc x N
• I_max = I_sc x M

N is the number of modules in a string, and M is the number of strings in parallel. For example:

Step 3: Apply the IEC CoP Correction Factors

Apply the IEC CoP correction factors to the maximum system voltage and current to obtain the design voltage and current:

• Design Voltage = V_max x 1.15
• Design Current = I_max x 1.25

These correction factors account for temperature and other variations that may affect the actual performance of the PV array. For example:

So, for the first example, the design voltage and current are 517.5V and 33.75A, respectively, and for the second example, they are 414V and 40A, respectively.

To connect a PV system to the grid, several components are required:

1. PV Array is where solar energy is captured and converted into DC (Direct Current) electricity. The PV array consists of multiple solar panels linked together.
2. Inverter: DC electricity from the PV array is directed to an inverter. The inverter converts DC electricity into AC (Alternating Current) electricity, which is compatible with the grid and home appliances.
3. AC Distribution Board: AC electricity from the inverter is then routed to an AC distribution board. This component distributes the electrical power to different loads within the property and also connects to the meter.
4. Meter:  measures the amount of electricity generated by the PV system and supplied to the grid. In many setups, this can be a smart meter that can also track electricity drawn from the grid.
5. Grid: electricity flows from the meter to the national grid, where it is distributed to various consumers.

Each component is represented as a node in the diagram, with arrows indicating the direction of electricity flow from the PV array to the grid.

UK Compliance

In the UK, the connection and operation of photovoltaic (PV) systems and other types of electricity generation units to the grid are governed by specific compliance requirements, primarily G98 and G99, set forth by the Energy Networks Association (ENA). These requirements ensure that all generation units, regardless of size, operate safely, reliably, and efficiently within the grid infrastructure.

1. G98:  applies to smaller-scale generators with a capacity up to and including 16A per phase, which is roughly equivalent to installations under 3.68kW (single-phase) or 11.04kW (three-phase). G98 covers the technical requirements for the connection of such microgeneration equipment to the low voltage electricity distribution networks. Most domestic PV systems fall under this category.
2. G99: for larger installations that exceed the specifications covered by G98, G99 comes into play. It is aimed at installations that are directly connected to the distribution network or are above the threshold set by G98. G99 includes more comprehensive and stringent requirements due to the potential impact these larger installations could have on the grid. It covers aspects such as engineering recommendations, planning, and operational procedures to maintain grid stability and safety.

Both G98 and G99 include guidelines on aspects like synchronization, islanding protection, fault level contributions, and power quality. Compliance with these standards is crucial for gaining permission to connect to the grid and for the ongoing operation of PV systems, ensuring they do not adversely affect the grid's operation and other connected users.