PV DC switching back from Australia to see the importance of choosing appropriate PV DC switch

Polaris solar PV net news: an increasing number of Australia due to OEM sales of photovoltaic solar company DC switching unqualified leads to close down. Almost all Australia dealers chose to import cheap OEM DC switch to sell, switch OEM easier, only the name and packaging, the original combined with easy second factory belonging to the small workshops, no brand recognition, small scale and are willing to cooperate.

And PV quality problems led to a number of DC switch Australia solar companies go out of business

Dealers by posting on cheap DC switch in Australia to sell local brands to increase added value. Dealer OEM products need to assume all warranty services later, assumed all responsibility for product problems. In this way, once a product has quality problem, will allow dealers to take higher risks, affect their own brand, which is also the main cause of the collapse of these companies. These DC switch-the main problem is this:

1. contact high impedance caused by overheating or even fire, 2. switch doesn’t turn off, switch the handle in a ‘OFF’ State, 3. not completely cut off cause a spark; 4. allowable operating current is too small, prone to overheating, make the switch interrupter is damaged or even shape deformation. Selling a company in Queensland, according to test safety of DC switch, on a rooftop solar system in users at least 70 fires were sparked, another electrical fire risk to tens of thousands of worried homeowners. Advancetech headquartered on the Sunshine Coast is a long established company, whose motto is “tried, tested and trusted.”

On May 12, 2014, the Queensland Attorney-General JarrodBleijie ordered the immediate recall by the import and sales of 27,600 Advancetech solar DC-switch, the PV DC switching at the time of importation was renamed “Avanco”. On May 16, 2014 Advancetech entered bankruptcy, all secondary distributors and installers had to bear the cost of replacement products and risks. This shows that it’s not about what you bought but who do you buy things and its potential risks.

Australia recalled brands relate to: GWRPTYLTDTradingasUniquipIndustries the company’s DC switching, recalled due to overheating and cause a fire.

NHPElectricalEngineeringProductPtyLtd the company’s DC switching, recalled because if the switch handle to ‘OFF’ status, but contact is always ‘ON’, switching cannot be turned off.

DKSHAustraliaPtyLtd PVPower brand-DC switch recall is again overheating and cause a fire. Eltechindustries’s DCISO-MAX brand of DC switching, largely due to the design flaw does not completely cut off even fire arc and overheating. Currently, there are a lot of so-called DC circuit breaker is not really a DC circuit breaker, but improved by AC circuit breaker. PV systems are generally disconnected from voltage and current is higher in case of an earth fault, high short-circuit currents will pull together contacts, resulting in a very high short circuit current, the maximum can be up to thousands of amps (depending on product).

Photo: AVANCO brand PV DC switch recall

Figure five: DC and AC

Especially common in PV systems parallel input or multi-channel multiplex panels panels independent inputs, so you need to cut off the DC parallel input or multi-channel multiplex panels panels-DC independent inputs, where the arc of DC switch capacity requirements will be higher, the improvement of DC circuit breaker on the PV system will be a lot of risk.

Each cycle of the alternating current have a natural zero point, zero-crossing easier arc; and DC is not zero, arc difficult to extinguish. AC arc easy arc of DC system more difficult. Because the system is different, the principle of the arc are different, therefore DC and AC breaker breaker there is a big difference in structure and performance. Relative to the alternating current circuit-breakers, the arc-extinguishing devices of DC circuit breaker additional to increase arc.

Any AC circuit breaker most materials applicable to the exchange of circuit design and manufacture, which means that no matter the load voltage is 230 v (AC) or 400 volts (AC), it is fifty-sixtieths Hz sine wave. In pass broken Exchange electric Shi, must to note voltage of features is it must will through 0 v, so, despite different of load situation has different, but current will gradually itself disappeared-here refers to of is even circuit breaker device in power peak of when switch, and touch head between has electric arc into, power voltage reduced to 0 v of process also means with load of voltage also will tend to Yu zero, arc was out off. However, the total DC voltage to remain constant, through contact between the power is always the same, unless the load to zero. If the load is 500 volts (DC), 25 amps, so it is now, after a second, a minute later, after one hour is 500 volts, 25 amps-remain constant. And exchanges are not the same, all DC load throughout the shutdown process in the DC load is constant between contacts of switch; does not pass through the 0 v of direct current level unless there is a system power failure (or other errors).

Choose second, DC switching of several criteria

How to choose suitable for PV system DC switch it? the following criteria may refer to:

1. try to choose a big brand, in particular through the internationally recognised brands.

PV DC circuit breaker main European certification IEC60947-3 (European common criteria, most Asia-Pacific countries), UL508 (United States standard), UL508i (for PV system DC United States standard), GB14048.3 (domestic standard), CAN/CSA-C22.2 (Canada standard), VDE0660.

All the international brands such as certification, such as the United Kingdom of the IMO and Netherlands SANTON, most of the brands were domestic IEC60947-3 only through a universal standard.

2. Select the DC circuit breaker arc function.

ARC extinguishing effect is assessing the DC switch is one of the most important indicators. Real have specialized arc-extinguishing devices of DC circuit breaker and load turned off. General real of DC circuit breaker device of structure design compared special, handles and touch head no directly of connection, so pass broken of when not directly rotating touch head and disconnect, but has special of spring for connection, dang handles rotating or mobile to a specific points Shi triggered all of touch head “suddenly disconnect”, thus produced a very quickly of pass broken action, makes arc time continued compared short.

General international brands of photovoltaic DC switch been killed within a few milliseconds of arc, such as IMO SI systems claims to be in 5 milliseconds of arc. General AC circuit breakers improved over the arc of DC circuit breaker for 100 milliseconds or more.

Figure six switching with independent external DC switch panels

Breaking a string of seven panels, external DC switch

3. resistant to high voltages and currents.

PV system voltages up to 1000V (United States 600V), you need to disconnect electrical current based on its brand power, photovoltaic systems and connection is parallel or multiple independent connections (multi MPPT) is different. DC switching voltage and current by the need to disconnect the PV array voltage and current decisions. PV DC circuit breakers in the selection according to the following experience:

Voltage =NSxVOCx1.15 (Formula 1.1) current =NPxISCx1.25 (Equation 1.2)

NS – the number of panels in series

NP – number of battery packs parallel VOC – the battery open circuit voltage and short-circuit current ISC – modules of 1.15 and 1.25 is the experience factor as big brand DC 1000V DC voltage switch can be disconnected, even disconnected 1500V DC input. Major brands of DC switch power series, such as ABB’s PV DC switch-there are hundreds of series products, DC switch for IMO to focus on distributed PV systems, can provide a 50A,1500V of DC switch. While some small factories generally provide only 16A,25A DC, its technology and process it is difficult to produce high power PV DC switch.

4. product model range.

Generic brands of DC switch has a variety of models to meet demand for different occasions, have an external, built-in, by series and parallel terminals meet multiple MPPT chosen, locking and non-locking, and meet a variety of installation methods such as pedestal mount (installed on the combiner boxes and cabinets), and Panel Mount single hole and so on.

5. high flame retardancy and protection.

Casing, a base material for General DC switch or the handles are plastic, with its fire-retardant properties, and can generally meet the UL94 standards. Quality DC switching shell or body can meet UL94V-0 standards, and handles General meet UL94V-2 standards.

Second for inside the inverter DC switch, outer handle can switch, which generally require the protection degree of the switch is at least satisfy the test requirements for machine protection class. Currently is widely used in the industries of string inverters (generally less than 30kW of power level) generally meet the whole machine protection class of IP65, which requires built-in DC switch panel and machine installation of the sealing requirements are relatively high. For external switch if installed outdoors, are required to meet at least the IP65 protection rating.

Three selection steps, PV system DC switching

PV DC selection General switch by preliminary estimates of key parameters and ensure that an adequate allowance. Power output of the photovoltaic panels photovoltaic system itself affected by the weather, such as ambient temperature, the inverter power point tracking effect and, secondly, PV inverter itself have input power limit and protection, maximum permissible input voltage and current limit and protection of last DC switch rated shutoff capability and ambient temperature are also related.

Environment when certain panels (arrays) output DC power inverter-following impact with increasing voltage, DC switching current capability drops (thermal effects). Generally use DC switch-off panels actual output voltages and currents, weather, environmental stability, inverter power track take into account.

The easiest method is to choose the DC switch (both external and internal) to meet the requirements of formula as above 1.1 and 1.2. Because solar panels cannot work on the Max voltage and short-circuit open-circuit current, so the General external DC switch-reference using the maximum power point voltage and current:

Voltage =NSxVMPx1.15 (Equation 1.3)

Current =NPxIMPx1.25 (Formula 1.4) for the DC switching built-in, you may also be subject to maximum input voltage and current of the inverter itself limits,

In order to save costs, according to DC switching switching panels actual output voltage and current, no more than the selected protection of voltage and current of the inverter used. In General, the DC switching switching V-1 curves need to envelope allows the inverter input DC voltage and current curves. The whole PV DC switching options according to the following steps:

1. determine the system voltage.

Choice of maximum permissible voltages needed to achieve PV system DC switch voltage requirements. Common 600V and 1000V. General access system voltage up to 600V single phase inverter, three-phase string or power inverter system voltages up to 1000V.

2. determine the number of separate battery strings.

If it is a DC switching built-in, integrated inside the inverter, and independently of the inverter MPPT number relationships. Common inverter MPPT is a single road, MPPT dual, there are a few 3-way on grid-connected inverter of MPPT. Power rating of the inverter in the design for single MPPT;3kW~30kW for 1kW~3kW dual MPPT, a small number of companies also have a three-way the inverter MPPT; large inverter above 100kW is usually entered through the combiner box after Confluence along the MPPT. Independently of the inverter’s MPPT built-in DC switch determines the number of selected independent opening segment of logarithms.

If it is an external DC switch may design and networking relationships, you can choose with breaking strings Panel of external DC switch or select only external DC switch for breaking a bunch of cells.

3. determine the battery voltage and current.

By battery voltage and current parameters, general insurance calculated according to the formula for 1.1 and 1.2, 1.3 and 1.4 can also be based on formulas. If you understand the structure of the PV inverter, inverter manufacturers in particular in choosing a built-in DC switch, in order to effectively save inverter machine costs, through research on inverter DC voltage/current curve to select, guarantees choice of DC switching curve in all kinds of weather conditions and temperatures enveloping DC inverter input switch and the current curve.

4. analysis of using environmental and installation methods.

According to use conditions to determine the working temperature, protection and fire rating, usually good quality DC switching is stable at-40 ° to 60 °. Common external DC IP65 degree of protection, built-in DC switch installed to make the whole machine by IP65. Fire rating as the shell or body material UL94V-0, handles UL94V-2. Installation panel mount, Pedestal Mount, single-hole installation.

5. identification of specific models.

If 1~4 meet, you need to select a specific model from the preferred brand. Specific types of specifications is segmented through the current, that exactly match the DC switch is hard to find, some headroom left to select the specifications of DC switch higher than the theoretical calculations. Common 16A,25A and 32A, some major manufacturers will have 40A,50A,600V or 1000V rated voltages.

As domestic PV policy clarity, PV power station added installation increase year by year, especially distributed PV systems started, individual household rooftop PV system installation will be more and more popular. PV DC switch security protection of these systems will be the most important part, PV DC switch is directly related to the stability and reliability of PV power systems stability and benefits, directly affect the safe and reliable operation of photovoltaic systems, so choose a qualified PV DC switch will be crucial.

Domestic market called the PV DC switch AC switch or its product improvement, is not actually in a PV system with safe arc, high power cut off switch. ARC-quenching ability of these AC switch, rated with load cut off power are far from enough, prone to overheating, leakage, and sparks when burnt to destroy the entire photovoltaic power plant.

Original title: PV DC switching back from Australia to see the importance of choosing appropriate PV DC switch

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