What is DPS and how is it used in photovoltaic systems?

The SPD (surge protection device) is an essential electrical protective equipment in electrical installations.

It is connected between the phases and protective grounding and has the function of protecting the installation in the event of surges caused by atmospheric discharges.

The device works by varying the impedance between two points, that is, the phase-neutral or phase-ground impedance. In normal situations, the DPS behaves like an open circuit, being practically invisible in the installation.

When a voltage surge is witnessed in the installation, the DPS becomes a closed circuit, diverting the current from the overvoltage conductor to ground, reducing the voltage perceived by other equipment connected to the circuit.

Voltage surges can be caused by atmospheric discharges, direct discharges, indirect discharges or network maneuvers. Electrical discharges and phenomena that cause overvoltage in a circuit have characteristic voltage and discharge current curves.

Atmospheric discharges have a voltage of the order of hundreds of thousands of volts and a current of the order of 30 kA. The curve that best represents the current wave caused by this type of discharge is called 10/350, where the number 10 refers to the 10 microseconds of rise time of the curve and 350 denotes the 350 microseconds that the current takes to reach 50% of the initial peak intensity. The appropriate DPS to block the effects of voltage rise caused by this current are classified as Type 1.

Direct discharges are those caused by the capture of lightning by a lightning rod system. As the current that the lightning rod system conducts is high, an equally high voltage appears, which can be shared with the building's electrical system through some common connection point or by magnetic induction. This type of discharge also has a 10/350 current waveform.

Voltage and current surges originating from the electrical network or induced by more distant electrical discharges have a wave behavior of 8 microseconds rising and 20 microseconds falling, receiving the name 8/20. The appropriate DPS to contain the voltage pulses caused by this characteristic curve is Type 2.

Type 3 DPS is suitable for 1.2/50 surge curves and is usually allocated to equipment with greater sensitivity to overvoltage or terminal equipment where the user is at risk of electric shock.

The overvoltages that this device protects can be caused by connecting and disconnecting loads, acting on a capacitor bank, short circuits and unwanted voltage variations coming from the external electrical network.

Description

It is not possible to use DPS for alternating current circuits in direct current circuits and vice versa. The internal insulation of components for devices suitable for alternating current is not sufficient to guarantee good insulation when used with direct current. This lack of insulation can lead to electrical arcs within the device, which could cause the component to burn or even cause a fire.

Due to its internal construction, the DPS may fail at the end of its useful life, causing a short circuit between the protected phases and ground. To avoid this short circuit, some devices contain an internal fuse or mini-circuit breaker, which will act if this failure occurs. The information about the electrical operating and protection quantities found in a DPS is:

• U_w: Maximum continuous operating voltage. It characterizes the maximum operating voltage that the DPS can withstand without its overvoltage protection being activated;
• U_P : Protection voltage level. It is the maximum voltage between the DPS terminals at the instant it is active and conducting discharge current equal to I_n or I_imp. In other words, it is the maximum voltage that the circuits downstream (that is, after) the DPS receive;
• I_imp: Nominal protection current for type 1 SPD. It is the maximum protection current that a type 1 SPD can divert to protective earth. The values of I_imp typically occur in direct electrical discharges;
• I_n: Nominal protection current for Type 2 and 3 SPDs. It is the maximum value of current that the Type 2 SPD diverts to the protective earth. The DPS must withstand this current for at least 19 activations;
• I_cc or I_SC: This is the maximum short-circuit current that a SPD with an internal fuse or mini-circuit breaker can withstand. In the event of a failure in the DPS, it must be capable of conducting the short-circuit power until it is interrupted by the DPS itself, or by other protection existing in the circuit;
• I_MAX: This is the maximum current that a SPD can divert to protective earth. The ability to divert current when it reaches I_MAX only occurs once. The DPS is damaged and can no longer be rearmed.

Mandatory use

The Brazilian standard ABNT NBR 5410 – Low voltage electrical installations – governs electrical systems that have alternating voltage up to 1000 V or direct voltage up to 1500 V.

All electrical installations that fall within the scope of the standard must follow the requirements set out therein. The standard provides that protection must be sought for humans, animals and property that could be damaged in overvoltage events.

Installations with metallic masses and energy conductors exposed to the weather are classified according to the NBR 5410 standard as risk of atmospheric discharge AQ3, and overhead power lines AQ2, according to table 15 below.

Section 5.4.2 of the standard provides for the mandatory use of a transient overvoltage protection device for circuits classified as AQ2 and AQ3 and that they must be protected by a SPD or equivalent device.

A NBR 5410 it also determines which type of SPD allocation in the alternating current part of the system should be made based on the grounding and neutral characteristics of the electrical installation.

An example of connecting a DPS to an electrical panel is shown below.

NBR 16690 replaces the requirements of standard NBR 5410 in the direct current part of the photovoltaic system, and recommends that the guidelines of standard NBR 5419 – Protection of structures against atmospheric discharges be followed.

NBR 5419 says that all conductors of electrical power and signal systems must be connected directly or indirectly connected to the equipotential bond, and live conductors must only be connected through SPD. Therefore, the direct current part must have a SPD that protects both poles of the system.

Coordination and selectivity

Complete electrical protection provides for the coordination and selectivity of DPS action. The strategy adopted to dissipate the energy contained in an electrical surge before it reaches the equipment to be protected is to combine DPS types 1, 2 and 3 in order to allow each of them to act in the correct order.

This combination typically occurs with the Type 1 SPD being connected to the building's power input. Type 2 DPS is allocated to each secondary distribution board and Type 3 is allocated to each sensitive equipment that requires protection. Type 1 DPS can be replaced by Type 1+2 DPS.

The previous figure shows that if a voltage surge occurs, the Type 1 DPS must act first, reducing the voltage that the Type 2 DPS will receive. Likewise, the action of the Type 2 DPS occurs before the action of the Type 3 DPS, reducing the tension to which the Type 3 DPS will be subject.

The last step of voltage reduction occurs in the Type 3 DPS, thus protecting the most sensitive equipment. In each of these steps, the energy involved in the voltage surge was dissipated to protective ground. P

In order for there to be perfect coordination between the DPS devices, the response time for activating each device must be respected, in such a way that the DPS furthest from the load must act first than the DPS closest to the load. This information is released by DPS manufacturers.

Table 1 – Technical information on DPS suitable for PV systems. Note the device's operating time. Source: DPS datasheet for DGM PV2 solar systems from manufacturer Dehn

The NBR 5410 standard does not require the use of Type 1 DPS for installations without SPDA.

Lightning protection system and DPS considerations

The standard on SPDA (atmospheric discharge protection systems) NBR 5419 defines the correct type of SPD for the installation based on the distance 's' from the lightning down conductor and the other installation circuits. Based on distance, the standard recommends using Type 1, Type 1+2 or Type 2 DPS.

DPS sizing for photovoltaic system

The use of DPS in photovoltaic systems aims to prevent direct or indirect atmospheric discharge from causing harmful effects on the installation. For complete protection of the equipment, there must be at least one SPD between the photovoltaic array and the inverter and at least one SPD between the inverter and the electrical grid.

This topology protects the inverter from both discharges in the photovoltaic array and overvoltages arising from the network external to the installation. The DPS located between the array and the inverter must be suitable for direct voltage operation. To reduce the effect of current induced by atmospheric discharge, the loop formed by the modules' cabling must be kept to a minimum.

The DPS that protects the inverter on both the AC and DC sides cannot be more than 10 meters from the inverter, as in the event of an atmospheric discharge, induced current may appear in the circuit between the protection and the equipment. It is also worth remembering that the standard prohibits mixing AC and DC circuits in the same panel.

We must have a board only for AC and another only for CC. As both the positive and negative poles of the photovoltaic array can carry current induced by lightning, both need to be connected to a SPD.

There are already DPS devices on the market suitable for photovoltaic systems, compatible with their typical operating voltages and with electrical terminals for connecting the two poles on the same device.

DPS application example

A system containing two strings of 10 330 Wp panels is connected to a 6 kW inverter. The inverter is single-phase, with an input voltage of 220 V. Analyzing the panel datasheet we see that the association of 10 modules in series produces a maximum voltage of 469.8 V.

Therefore, we should look for a DPS with Uc greater than this value. The type of DPS selected depends on the existence of an SPDA and the separation distance. In this example we will consider that the system is installed in a building without SPDA. For this type of building, the recommended DPS for the photovoltaic system is Type 2.

Table 2 – P6C-36 family data sheet. Source: BYD

It is necessary to provide overvoltage protection on both strings, therefore the string box will have 2 DPS devices. To protect the inverter, a DPS suitable for AC circuits and with Uc greater than the inverter output voltage, which is 220V, as specified in the data sheet below, will be selected. For inverter protection on the AC side, the DPS must be Type 2 or Type 1+2. This will protect the inverter and the installation from overvoltages.

Table 3 – Sunny Boy inverter family data sheet, showing the maximum voltages supported by the inverter. Source: SMA