The importance and method of choosing the right photovoltaic DC switch
Sunday, 17 October 2021
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The importance and method of choosing the right photovoltaic DC switch

 

1. The quality of photovoltaic DC switches has caused many Australian solar companies to close their doors

More and more Australian solar companies have closed their doors due to unqualified OEM PV DC switches. Almost all Australian distributors choose to sell imported cheap DC switches by OEM.

Firstly, it is easier to OEM the switches. Only the brand name and packaging are replaced, and the original factory is easy to cooperate.

Secondly, these original factories are often small workshops and nothing. Brand awareness, small scale, and willing to cooperate. Distributors can increase the added value of cheap DC switches by labeling local Australian brands for sales. Distributors need to assume all subsequent quality assurance services for the OEM products and assume all responsibilities for product problems.

In this way, once the product has quality problems, dealers will take a higher risk and affect their own brand influence. This is also the main reason for the bankruptcy of these companies.

The main problems with these DC switches are:

1. The high resistance of the contact causes overheating and even fire;

2. The switch cannot be turned off normally, and the switch handle remains in the ‘OFF’ state;

3. Not completely cut off, causing sparks;

4. Because the allowable operating current is too small, it is easy to cause overheating, damage to the switch interrupter or even shape deformation.

 

A Queensland company sold DC switches that had been tested for potential safety hazards and caused at least 70 fires on the solar systems on the roofs of users. In addition, there are tens of thousands of homeowners who are at risk of electrical fires worrying.

Advancetech, headquartered in the Sunshine Coast, is a long-established company whose motto is "try, test, believable". On May 12, 2014, Queensland Attorney General Jarrod Bleijie ordered the immediate recall of 27,600 solar DC switches imported and sold by Advancetech. The photovoltaic DC switches were renamed "Avanco" when imported. On May 16, 2014, Advancetech went into bankruptcy liquidation, and all installers and secondary distributors had to bear the costs and risks of replacing the defective products.

This shows that the key is not what you buy but who you buy from and its potential risks. Related information can be found at

http://www.recalls.gov.au/content/index.phtml/itemId/1059088.


Picture 1: AVANCO brand photovoltaic DC switch recall notice

 

In addition, the brands recalled in Australia also involve:

The DC switch of GWR PTY LTD Trading as Uniquip Industries was recalled because of overheating and fire: http://www.recalls.gov.au/content/index.phtml/itemId/1060436

NHP Electrical Engineering Product Pty Ltd's DC switch, the reason for the recall is that when the handle is switched to the'OFF' state, but the contact is always in the'ON' state, and the switch cannot be turned off: http://www.recalls.gov.au/ content/index.phtml/itemId/1055934

 

At present, there are many so-called DC circuit breakers on the market that are not real DC circuit breakers, but are improved from AC circuit breakers. Photovoltaic systems generally have a relatively high disconnect voltage and current. In case of a ground fault, the high short-circuit current will pull the contacts together, resulting in a very high short-circuit current, which can be as high as kiloamps (depending on different products). Especially in photovoltaic systems, it is common to have multiple parallel input of solar panels or independent input of multiple solar panels. In this way, it is necessary to cut off the parallel DC input of multiple solar panels or the independent DC input of multiple solar panels at the same time. The arc extinguishing ability of DC switches in these situations The requirements will be higher, and the use of these improved DC circuit breakers in photovoltaic systems will have great risks.

 

1. the correct selection of several standards for DC switches

How to choose the right DC switch for the photovoltaic system? The following standards can be used as reference:

1. Try to choose big brands, especially those that have passed international certification.

Photovoltaic DC circuit breakers mainly have European certification IEC 60947-3 (European common standard, followed by most countries in Asia-Pacific), UL 508 (American general standard), UL508i (American standard for DC switches for photovoltaic systems), GB14048.3 (domestic general Standard), CAN/CSA-C22.2 (Canadian General Standard), VDE 0660. At present, major international brands have all the above certifications, such as IMO in the United Kingdom and SANTON in the Netherlands. Most domestic brands currently only pass the universal standard IEC 60947-3.

2. Select a DC circuit breaker with good arc extinguishing function.

The arc extinguishing effect is one of the most important indicators for evaluating DC switches. Real DC circuit breakers have special arc extinguishing devices, which can be switched off on load. Generally, the structural design of the real DC circuit breaker is quite special. The handle and the contact are not directly connected, so when the switch is turned on and off, the contact is not directly rotated to disconnect, but a special spring is used for connection. When the handle rotates or moves to At a specific point, all the contacts are triggered to "suddenly open", thus generating a very rapid on-off action, making the arc lasting relatively short. Generally, the arc of the photovoltaic DC switch of the international first-line brand is extinguished within a few milliseconds. For example, the SI system of IMO claims that the arc is extinguished within 5 milliseconds. However, the arc of the DC circuit breaker modified by the general AC circuit breaker lasts for more than 100 milliseconds.

3. Withstand high voltage and current.

The voltage of a general photovoltaic system may reach 1000V (600V in the United States), and the current that needs to be disconnected depends on the brand and power of the module, and whether the photovoltaic system is connected in parallel or multiple independent connections (multi-channel MPPT). The voltage and current of the DC switch are determined by the string voltage and parallel current of the photovoltaic array that needs to be disconnected. Refer to the following experience when selecting photovoltaic DC circuit breakers:

Voltage = NS x VOC x 1.15 (Equation 1.1)

Current = NP x ISC x 1.25 (Formula 1.2)

Where NS-the number of battery panels in series NP-the number of battery packs in parallel

VOC-battery panel open circuit voltage

ISC-short circuit current of battery panel

1.15 and 1.25 are empirical coefficients

Generally, the DC switches of major brands can disconnect the system DC voltage of 1000V, and even design to disconnect the DC input of 1500V. Big brands of DC switches often have high-power series. For example, ABB's photovoltaic DC switches have hundreds of ampere series products. IMO focuses on DC switches for distributed photovoltaic systems and can provide 50A, 1500V DC switches. However, some small manufacturers generally only provide 16A, 25A DC switches, and its technology and technology are difficult to produce high-power photovoltaic DC switches.

4. The product model is complete.

Generally, big brands of DC switches have a variety of models that can meet the needs of different occasions. There are external, built-in, terminals that can meet multiple MPPT inputs in series and parallel, with and without locks, and more satisfying. Various installations Ways such as base installation (installed in the combiner box and power distribution cabinet), single-hole and panel installation, etc.

5. The material is flame-retardant and has a high degree of protection.

Generally, the housing, body material, or handle of DC switches are all plastic, which has its own flame-retardant characteristics and can usually meet the UL94 standard. The casing or body of a good-quality DC switch can meet the UL 94V0 standard, and the handle generally meets the UL94 V-2 standard.

Secondly, for the built-in DC switch inside the inverter, if there is an external handle that can be switched, the protection level of the switch is generally required to at least meet the test requirements of the protection level of the whole machine. At present, the most widely used string inverters in the industry (generally less than 30kW power level) generally meet the IP65 protection level of the whole machine, which requires the built-in DC switch and the tightness of the panel when the machine is installed. For external DC switches, if they are installed outdoors, they are required to meet at least IP65 protection level.



Picture 2:An external DC switch for making and breaking multiple strings of independent battery panels


Picture3: An external DC switch that turns on and off a string of battery panels