Q&A: Fire Safety (follow-up)

One month ago we shared a Q&A about Fire Safety. This follow-up will give more details about Sungrow’s built-in protection features to reduce fire hazards within the PV-system. Sungrow is an innovator of the latest and best practices for inverters for PV-systems world-wide. Continuously developing and improving their built-in protection features. In this follow-up Q&A we ask Sungrow about their best fire safety features.

 

Feature 1: Arc Fault Circuit Interrupter

We are pleased to inform you that Sungrow received the world’s first DC arc detection and interruption technical report issued by TÜV SUD. The report outlines Sungrow SG110CX has been tested according to the newly recognized international standard IEC 63027 ED1 (draft), 82/1636/CDV, underlining the global leading position on product reliability and safety.

What is an arc fault?

Arc fault is an unintentional electrical discharge phenomenon between one or multiple conductors, resulting in an instantaneous spark generated by the current flowing through an insulating medium (i.e air).

What causes arc faults?

The causes of arc faults may arise from product or planning failures, installation mistakes as well as external influence (i.e. lighting). They are mainly caused by loose or poor contacts, broken wiring, moisture and corrosion of wires or aging of insulating materials.

What is the arc fault effects?

An arc fault produces high temperatures which can result in fire hazards. Within an electrical system, arc faults may cause the surrounding insulating materials to decompose or reach the ignition point.

What is Sungrow’s solution to prevent / detect arc faults?

In order to improve the detection accuracy, Sungrow adopts three-level detection and two-level closed-loop algorithms for arc detection. Once the arc detection function has been activated, it will identify a fault arc and will interrupt the circuit in less than 200 ms (accuracy is up to 99.9 %).  These features increase personal safety, protects equipment and prevents damage of the solar system.

Can older models be retrofitted with the AFCI technology?

Older versions like V11 of the CX Series cannot be retrofitted with this feature. Only the CX inverters of version V112 are equipped with Arc Fault Circuit Interruption.

 

Feature 2: Insulation Resistance Monitoring Unit

The PV system’s array insulation resistance monitoring function serves to reduce the risk of electric shock or fire caused by DC ground faults. For non-isolated inverters, DC ground faults form a loop between the fault point and electrical grid, leading to a fault current.  In this respect, before the inverter is connected to the grid, the array insulation resistance value must be measured and verified. If the insulation resistance value is lower than the threshold value required by the relevant standards, an alarm is promptly issued, and the inverter will not be connected to the grid.

How does the feature work?

The entire process is divided into four stages: initial standby, standby, start-up and operation. In the „start-up“ stage, the insulation resistance is checked against the default threshold protection point. If the resistance from the positive and negative pole to the ground is exceeding the threshold value, the inverter is allowed to be connected to the grid as normal. If the initial measurement identifies that the insulation resistance is less than the threshold value, the inverter will enter in stand by for 10 minutes. If the second measurement gives the same result, the inverter will enter in stand by for another 10 minutes. If the third measurement also fails to meet the minimum threshold, the device shall report a fault and connection to the grid will be stopped.

What is the minimum threshold?

According to section 4.8.2.1 in IEC62109-2, the minimum value set for the insulation resistance threshold may not be lower than R=VMAX PV/30mA.

 

Feature 3: Residual Current Monitoring Unit

For non-isolated inverters, residual current can affect personal safety and may cause fires. Therefore, detection and protection are extremely important.

What does the Residual Current Monitoring Unit?

Residual current protection is divided into two protection methods: continuous residual current protection and sudden current change protection. During the normal operation, the inverter relays are switched on to connect the inverter to the grid. The Residual Current Monitoring Unit (RCMU) keeps monitoring the residual current by sampling the sum of the three-phase output currents. After wave filtering and amplification, the result is sent to the DSP control unit. If detected residual current exceeds the pre-set threshold value, the inverter will disconnect within the timeframe specified by the relevant standards and report a residual current fault.

What is the threshold value for the residual current?

The threshold value must be set according to section 4.8.3.5 in IEC62109-2, which is the same as required in UL1741 CRD Sections 87-100, table 89.1 and 89.2.

 

Feature 4: DC Switches and DC&AC Surge protection devices

Devices used to safely disconnect the DC circuits in emergency situations or protect the inverter against dangerous overvoltage (direct or indirect lightning, transient overvoltage during switching operation) which may cause fire.

 

PDF download of the Q&A Fire Safety – follow-up with Sungrow