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Weidmüller terminal blocks for 1000 VDC applications

13 September 2010

Increased safety and reliability when operating photovoltaic systems. – Partial discharge test for DC voltages. – To date a unique test of terminal block quality.

Weidmüller subjects its terminal blocks destined for installation in photovoltaic systems to a partial discharge test with DC voltage. This is to date a unique test of quality of terminal blocks. Put succinctly, Weidmüller is the first and presently the only manufacturer that tests the long-term stability of the insulation of its terminal blocks under "DC conditions". That enables Weidmüller to guarantee its terminal blocks are able to withstand a voltage of 1000 VDC under all climatic conditions. And it is only terminal blocks that have passed this test of quality that ensure safe and reliable operation of PV systems at voltages of 1000 VDC.

Insulated lines react completely differently to partial discharge tests with AC voltage than to partial discharge tests with DC voltage. It is not possible to draw conclusions as to whether a terminal block is suitable or not for 1000 VDC applications simply by considering the thickness of the insulation, its dimensions or other physical attributes in isolation. When dealing with DC applications these design features of a terminal block must be considered in combination, because it is in combination that they qualify a terminal block to provide guaranteed reliability. For that reason Weidmüller approves each type of terminal block for different configurations of terminal rails.

A list of approved terminal blocks is available for downloading from the Weidmüller website. To achieve greater system voltages individual panels are connected in series to so-called 'strings' during the installation of photovoltaic modules. For these applications Weidmüller subjects its terminal blocks for 1000 VDC to a partial discharge test with DC voltage. This non-destructive method of testing the quality of insulated lines facilitates recognition of microscopic discharges that can 'eat' their way through an insulated line over days, weeks or even years and finally lead to the breakdown of the insulation and subsequent damage. Classic dielectric tests are unable to determine the very low energy discharges. Only terminal blocks that have passed the partial discharge test – and as a result have proven their long-term stability – are suitable for trouble-free use in photovoltaic systems as well as all other DC applications in the control cabinet.

In the past manufacturers have concentrated on specifying clearance and creepage distances to determine the rating data of terminal blocks. As a result of increased internal quality standards Weidmüller had already introduced AC partial discharge tests for its terminal blocks. Now Weidmüller is going one step further and is the first and presently the only manufacturer to carry out a partial discharge test with DC voltage. The partial discharge test is recognized as a reliable procedure to verify the long-term quality and safety of an insulation system. This test is performed when particular insulation lines have to fulfil especially stringent quality requirements. The unit of measurement for a partial discharge is the coulomb [C] (1 C equals 1 As).

The usual measurement values for partial discharges are between 1 and 10000 pC (pico-coulomb). Weidmüller tests each individual type of terminal for suitability in 1000 VDC applications, such as those used in photovoltaic systems and others. The consequences can prove fatal if the wrong terminal block is installed in these systems. The following scenario serves as an example: When a non-approved terminal block is utilised material fatigue can result in heat and finally arcing. The insulation is no longer doing its job. The result is a short-circuit. And because the photovoltaic module carries on operating continuously the short-circuit remains, which can lead to the entire module going up in flames. The reason being, no fuse is generally installed before the string terminals. Two factors are decisive for the safe operation of photovoltaic systems: the right terminals and their layout, meaning arrangement on the terminal rail.

The terminal rail is utilized to group the various PV strings per potential; in other words, a group of terminal blocks collects 'positive' and the other set of terminal blocks 'negative'. For instance, each row of terminal blocks consists of a 35 mm2 and 2x five 6 mm2 terminal blocks linked with cross-connectors. The 6 mm2 terminals connect the 'strings', the 35 mm2 terminal provides the connection to the inverters. An 8 mm wide end bracket separates the 'positive' and 'negative' groups of terminal blocks. As a consequence there is no risk of a partial discharge, because the width of the end bracket ensures a lower electric field. It is insufficient to use an end plate or a partition plate instead of an end bracket.

A partial discharge is possible. The clearance from the top-hat rail to the live components of a terminal block is not a critical factor in this application. The critical point is the side-by-side mounting of the terminals; this is where partial discharges can occur. It is for this reason that Weidmüller terminals approved for 1000 VDC should be used. If, on the other hand, the strings are collected in groups of terminal blocks then terminal blocks from Weidmüller with less than 1000 VDC ratings can be utilized for this arrangement. Terminal blocks are utilized in a variety of sections of PV systems – these are "DC and AC system sections". Installed in series and parallel the PV panels are connected together to 'strings' – the DC system section. Inverters have a DC and AC side – because the mains grid requires AC.

Different fusing solutions and measuring devices require AC or DC. That is why the application location and local conditions are decisive when choosing terminal blocks. The basic consideration is: is 1000 VDC required to the adjacent terminal or not – or exclusively to the mounting rail. Conclusion: Terminal blocks suitable for 1000 VAC in accordance with IEC 60947-7-1 are not automatically suitable for 1000 VDC. The IEC standard takes only clearance and creepage distances into consideration. That is why Weidmüller has increased its quality demands and tests the partial discharge characteristics of its terminal blocks with AC and, in addition, with DC voltage. Through this approach Weidmüller secures the long-term operability of PV systems operating under maximum load conditions.

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