Solar energy has become one of the most popular renewable energy and the number of PV systems in use has rapidly grown in recent years. Safety is always the main focus of attention.
A solar system is normally composed of a PV array, inverter, combiner box and monitoring platform. As a key component in the system, the inverter undertakes the task of both generation and protection. This article focuses on how to prevent solar fires, protect personal safety and protect the devices from the inverter side.
- How to prevent solar fires?
Targeted solutions will be taken for a different causes of solar fires. Here will analyze two main causes and solutions. AFCI for DC Arc fault and RCD for residual current.
Figure 1 fire accident
In the discussions about the fire hazard in the PV system, the DC side is always the focus. And the major fault that occurs on the DC side is the DC arc fault. Arc faults may be caused by several reasons such as faulty components, installation errors, or mechanical damage and aging, including Loose or separated joints, connections, or terminations; damaged or aged insulation; damp and broken cable and bad contact.
Electric arc fault often leads to variations in physical characteristics and electrical characteristics of the circuit, the characteristics of the arcing current shall be reflected by the characteristics of the DC bus current of the PV system. As Figure 2 shows, GoodWe adopts the solution based on the detection of arcing current. DC current data collected through CT are transmitted to ARM. The voltage signals are processed by using the FFT algorithm, which can identify the arc fault that occurs. If an arc fault or faults are identified, the DC pathway will be automatically cut off within 2 seconds.
Figure 2 principle of GoodWe’s AFCI
GoodWe inverters with the 2nd Generation AFCI function which trip time is less than 0.5s (much better than 2.5s industry standard) will escort the safety of PV plants.
PV systems produce residual currents like any other electrical equipment. Residual current in the PV system, which refers to the leakage current from PV to the ground, could be caused by the parasitic capacitance between the PV arrays and the ground and ground fault. As figure 3 shows, a loop will be formed between the AC grid, inverters, PV modules and ground. If the continuous leakage current exceeds a limit value, the carbon deposits and insulation can quickly ignite and cause a fire hazard.
Figure 3 cause of residual current
According to the requirement in IEC 62109-2:2011, the inverter shall provide residual current monitoring, i.e RCMU, which shall measure the total (both AC and DC Components) RMS current and activate an alarm. In addition, an external RCD (Residual Current Device) can also be used to detect these currents and disconnect the circuit from the source automatically according to the preset threshold value. To prevent the fire risk RCMU or RCDs should be equipped with a maximum of 300mA of rated residual operating current for inverters with rated output power less than 30kVA. Inverters with a rated out power greater than 30KVA shall disconnect if the detected continuous residual current exceeds a maximum of 10 mA per kVA.
- How to protect the human safety?
Just as with another electric system, the PV systems present the risk of electric shock. How to protect the human from the inverter side? This chapter focus on RCD and external emergency switch.
As figure 4 shows, leakage currents can flow through a human body to the ground and form a loop between humans, inverters and AC grid, which results in a risk of electrical shock. This risk also can be prevented by the protection method of RCDs.
Figure 4 risk of electrical shock from residual current
The RCD integrated into non-isolated grid-tied inverters is required to have a limit value for sudden changes of 30mA. That means the inverter will disconnect from the mains within a time when the current exceeds 30 mA.
Shutdown on AC side
The inverter as the key unit in a PV system is connected with the AC grid. GoodWe inverters provide remote shutdown solutions for system security protection by cutting from the grid on the AC side.
In some European countries, the inverters are required to be equipped with a remote shutdown function.
The remote shutdown function that comes with the inverter can be realized by a shutdown function circuit integrated into the inverter and a switch. It requires the combined action of on-off switch, long-distance communication and control mechanism via signals.
Figure 5 schematic of remote shutdown
The on-off switch can be deployed in a remote place，where the operator can turn on or off the switch at a safe distance to control the inverter. Communications cables such as RS485 are used to connect the inverter and switch to transmit signals of the remote shutdown. To realize communication between the multi-inverter and one switch, the inverter should be connected hand in hand as shown in figure 5. When the operator turns on the switch, signals of remote shutdown shall be sent to multiple inverters at the same time, and the inverters should disconnect from the grid and stop power output within 0.5s.
To satisfy Indian customer needs, GoodWe also provides an Emergency shutdown solution.
- How to protect the devices?
The installations of PV Plants that arise from their exposed nature and collection areas make them vulnerable to overvoltage surges from lightning. In a lightning-prone area, the lightning-induced array failures can be more detrimental to solar equipment. SPDs are particularly crucial to protect against damage from surges including direct lightning strokes and indirect lightning.
GoodWe inverters integrate SPD I or II on the DC side and SPD II on the AC side. SPD I can discharge the back-current from lightning spreading from the earth conductor to the network conductors. SPD II prevents the spread of overvoltages in the electrical installations and protects the loads.
SPD connected in parallel has a high impedance. Once the transient overvoltage appears in the system, the impedance of the device decreases so surge current is driven through the SPD, bypassing the sensitive equipment.
Figure 7 SPD System Architecture
PV Systems are generally a safe technology. Nevertheless, like any other electrical equipment PV systems constitute a risk of fire, electric shock which is dangerous to human.
To prevent the potential fire risk it is important to analyze the cause of the fire. In this article, we discussed two main causes and proposed the relevant solutions. In critical applications, AFCI should be equipped for a reduced risk of DC arc fault. RCMU and RCD can against the fire risk, which is caused by the residual current. RCD can also be a life-saving device that can prevent humans from getting a fatal electrical shock. To minimize the risk which the human contact with the live cables, the emergency switches of the remote shutdown is used to separate the power from the Ac grid. Also, we provide device protection like SPD against damage from surges.