When talking about a photovoltaic system, we often think simply of PV modules, but a typical power plant of this type is much more complex. It also consists of converters (Inverters), measuring switchboards on the AC side connected to the power grid, special solar cables, connecting elements, support structures, as well as switchboards with protection on the DC side - such as surge arresters and overcurrent protection. It is to them and the issue of their selection that we will devote this article.
DC side of the PV system
Typically, a prosumer photovoltaic power plant has a very simple scheme. PV modules are placed on the roof of the building, inside there are switchboards and converters, as well as a measuring switchboard with a meter for the energy produced. All these elements are connected with two- or four-core cables.
In DC switchboards, devices are usually installed to protect the installation against overload or short circuit, surge arresters and a DC circuit breaker. The main purposes of these protections are:
- protection of photovoltaic modules against short circuits, overloads and reverse short circuit currents,
- protection against switching overvoltages,
- protection against surges resulting from lightning discharges.
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Dangerous overvoltages in sensitive photovoltaic systems result from lightning discharges or switching overvoltages induced in the loops of series-connected PV modules. Therefore, overvoltage protection is one of the most important types of protection for the DC part of the photovoltaic installation.
What is a surge arrester?
It is designed to repeatedly discharge discharge currents resulting from overvoltage in a photovoltaic installation. It can therefore operate for many years if it is not damaged or if there is no surge or discharge current exceeding the capabilities of the given arrester. After reacting to the overvoltage, the device should return to its original state.
Due to their construction, surge arresters are divided into:
- Cut-off limiter - using primarily gas and air spark gaps.
- Limiting arrester - it mainly uses varistors or special diodes.
- Combined arrester - combining the two above types, i.e. a combination of a varistor and a gas spark gap.
ETI surge arresters
Type 1 or Type 2 surge arresters ?
Currently, there are three types of photovoltaic modules on the market: polycrystalline, monocrystalline and thin-film. They differ in the level of surge resistance, which the manufacturer should specify in their specifications. The selection of surge arresters will largely depend on this parameter. They are usually adapted to nominal voltages ranging from 500V to 1500V DC. With proper installation and grounding, they can provide surge protection at a level of 3 - 5 kV.
Depending on the size of the photovoltaic power plant, two types of surge arresters are used: T1 or T2. The first is used to protect the installation against surges caused by direct lightning strikes to the photovoltaic system or in close proximity. The second protects the system against switching surges or surges caused by indirect lightning strikes.
Since the energy from a direct discharge is usually very large, the T1 surge arrester has a spark gap or a varistor with greater strength. This makes Type 1 necessarily more expensive. Type 2 is recommended for use in facilities without an external lightning protection system.
Sometimes, in a PV electrical installation with a lightning protection system, a T2 surge arrester can be used. This is because the voltage on the DC side is usually much higher than the voltage on the AC side, where Type 1 surge arresters are used. In the event of a direct discharge, the surge arrester on the AC side will therefore react earlier.
General diagram of the PV module system and surge protection
Due to the risk of damage to surge arresters, remember to periodically check their condition – the inspection window. In the event of a surge arrester failure, the red color will appear in its inspection window. The user will then be informed about the need to replace the replaceable element of the arrester.
Where to install a surge arrester?
The surge arrester should be placed near the protected object. which it protects. In the case of a photovoltaic power plant, these will be PV modules and an inverter. If the length of the cable connecting the modules to the inverter does not exceed 10 meters, it is enough to install one arrester in each string, locating it as close as possible to the device with the lowest resistance to surges. Usually, this will be a DC/AC inverter. However, it is worth checking the technical data of the photovoltaic modules to be sure.
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If the length of the cable between the above-mentioned elements exceeds 10 meters, one T1+2 or T2 surge arrester (depending on whether there is an external lightning protection system or not) should be installed at the modules, and a second one - usually of the same type - should be installed at the inverter.
Why use surge arresters?
Example of an isolated lightning protection system
Due to its construction and location, a photovoltaic installation is exposed to direct and nearby lightning strikes. In Poland, there are an average of 20-30 storm days. This means that every photovoltaic power plant is exposed to serious damage as a result of lightning strikes. These can destroy primarily expensive components, such as the modules themselves or DC/AC inverters. As a result, within a second, the owner of the installation may be exposed to power outages and expensive repairs to the PV system. This fact is not changed by the power plant's insurance. Even if the installation is covered by it, in the event of a failure, the insurer will first check whether all formal conditions have been met - including appropriate safeguards. Their absence will probably result in a refusal to pay compensation. In order to avoid such a risk, it is essential to ensure that surge arresters are installed in every PV photovoltaic power plant.
Surge protection from ETI Polam
If you are looking for reliable surge arresters, check out the ETI Polam brand. It offers all the necessary devices to protect photovoltaic systems against overcurrents and overvoltages, including ready-made switchboards. Here you will find switchboards equipped for individual configuration of PV modules - adapted to the number of modules in the string, the number of strings connected in parallel, the type and power of the converter and the number of converter inputs on the DC side. Secure your photovoltaic power plant with an experienced manufacturer!