Sizing Contactor to control Photovoltaic DC

[Brians6]

I know this is a U.K. site but I'm hoping someone will be able to answer a qn. Recently I saw a that a company was producing a remote disconnect safety system for PV panels mounted on a roof.This is to disconnect the arrays automatically if the mains is disconnected or manually after a storm or fire. Because the coils are only ELV, only safety ELV will be on the roof once they are disconnected. The units are worth a fortune but they are just a 24 volt contactor in a box with connectors so that you can just fit it inline at the panels. I am reasonably familiar with contactors although it is some years since I did a lot of hands on work. I will be using some Sprecher CA7s for a contactor based genset changeover system in the near future so I've been looking up their specs & I have a range of their parts etc. My biggest array has the specs on the switchboard listed as 412V at 11.2 amps. I know this is DC & for safety it would be necessary to break both the positive & negative output leads. The trouble is I haven't ever worked with DC contactors & I can't seem to find DC specs for contactors. Of course, I may be looking in all the wrong places. Does anyone know what the DC load break specs for Sprecher CA7s are or can you suggest breakers designed for DC. Regards, Brian.

 

[Worcester]

Andy @ Sibert is your expert in this area, to summarise the challenges

AC Crosses 0 Volts 100 times a second (in the UK at 50Hz ) So 100 times / second you have zero volts and hence zero current flowing
DC has a permanent current flowing (except at night with PV ) so when you try to break you effectively have an 'arc' welder...

Manual DC Isolators therfore have a very fast opening rate (usually spring assisted) and also a larger air gap when open, hence you don't get the arce welder effect inside the isolator.

If you used a an AC ioslator to switch DC, the contacts are likely to just get welded together when you try to open them

As for specifcations,


This from the IMO Marketing literature:

The IMO SI range has a switching speed that is independent of the operator. The mechanism is such that there is no direct linkage between the operator handle and the switch contacts. As the handle is moved it interacts with a spring mechanism which, upon reaching a set point, causes all the contacts to “SNAP” over thereby ensuring a very fast break/make action which means that the arcs produced by the constant DC load are normally extinguished within 5ms. Many alternative solutions, particularly those based upon AC isolator design, have a switching speed that is directly linked to operator speed. Slow operation of the handle results in slow separation of the contacts and can produce arcing times of 100ms or more.
The SI range also features a “knife edge” mechanism so that when the unit is operated there is a double break on each pole with arcing effect occuring on the corners of the switch only. The main contact is made on an area where no arcing has occurred. The rotary nature of the contact mechanism also means that when the SI is operated a self-cleaning action occurs on the arcing points, thereby producing good contact integrity over the life of the product. A secondary advantage of this type of operation in photovoltaic applications is that, in the event of a supply to earth failure, the short circuit current pulls the contacts together thereby giving an extremely high short circuit current capability in the order of up to 1700A (product dependant).
Completing the package for all your PV instalation requirements is the IMO range of Solar Connectors. Rated up to 1000V 30A for 2.5-6mm2 cables, the IMO connectors feature a secure easy clip and release system providing the installer with a time saving and efficient connector solution. As with all IMO PV products the connectors have been tested to the highest standards and come rated with IP67 protection, UL-94-5VA fire resistance and provide class 2 protection.

Try Here: IEEE Xplore - SearchResult

Most DC switchgear specifications appear to relate to Mass Transit Systems! - hence for Solar PV, you often get modified / derated AC isolators.

If your goal is to isolate in case of fire, you don't want to start another!!

 

[Brians6]

DC Contactors & PV

Hi Andy, thanks for the reply. First one little qn. How do you get this system to start a new paragraph. It just deletes my returns. Now to the contactors. I was well aware of the problems with DC regarding arcing & load breaking capacity so that was why I was asking questions. Because of the extra danger of DC voltages, Australian installations are now required to have all the PV panel DC cabling in the ceiling etc to be in HD orange conduit of the type normally used here for underground use. It also has to be well marked at regular intervals. Every panel has to have isolation switches at the panel on the roof too but that doesn't allow for storm or fire damage making a metal roof live. That was why I was wondering if something more could be done. Our first panel was wired under the old rules so the wiring wasn't in conduit so I had it rewired when the new panels went on. I also had the conduits mounted up high so they can be inspected over the full length & aren't going to be stood on. The contractor had never done that before but said it still met regs & specs so was willing to do that Do you have any manufacturers or models of suitable contactors? When I try to do web searches almost all the results that I can find are just DC coil contactors & not DC load ones. Thanks, Brian.

 

[jason121]

Could you wire them thru a AC contactor but use two contactors together giving six poles. Oversize the contacts 4x ?

 

[Worcester]

Hi Brian - you say isolator on the roof - do you have one for EVERY PANEL or one for each string? So if you put isolators on the roof - how are they supposed to access it?

A link to some photos if you haven't enough posts yet to put them in here (photobucket etc) would help.

Stanton do a remote breaker (firemans switch, costs a fortune, of course it's all their bits inside) PV Installation - PUBLIC | Sibert-Online abd it's designed to be installed at STRING level, not panel level.

If you put a double pole isolator across EVERY panel, you are only switching 30-60V and 10-5 amps, so at the panel level it's not a heavy load. Also the actuating voltage doesn't need to be high, so assuming you have a normally open 'switch' what you want is say a 12V double pole relay rated for 60V 10A, DC ...

I'm not sure if I'm correct on that..... as in a 'closed circuit' condition it would only be the panel voltage, when it goes / tries to go open circuit at the panel the Potential may be String Voltage-1 Panel.....

If the circuit has gone 'open ciruit' - i.e. main string isolator open, then you are switching Panel Volts at zero current, if through storm damage, you got a string shorting via you nice metal roof...

Need to get my brain in gear for this one!

IF - it only needs to switch one panel then searching Farnell (Element14 for you?) I just found an 'automotive' relay - 12v DC actuation, Contacts rated at 75V DC and 20A, cost UK£2.42 (plus delivery from the US ! )

IMO PRECISION CONTROLS - SRL-1CT3-SL-12VDC - AUTOMOTIVE RELAY, 12VDC, 20 A, S a loPDT-CO
SRL-1CT3-SL-12VDC - IMO PRECISION CONTROLS - AUTOMOTIVE RELAY, 12VDC, 20 A | Farnell UK
or this:
IMO PRECISION CONTROLS - SRMA-1CT-SL-24VDC - AUTOMOTIVE RELAY, SPST-NO, 24VDC, 20A (24VDC Actuated, Contacts rated at 60V DC / 20A) Uk£2.16 plus delivery
SRMA-1CT-SL-24VDC - IMO PRECISION CONTROLS - AUTOMOTIVE RELAY, SPST-NO, 24VDC | Farnell UK

Of course you'll need to put it in an IP65 box with a pair of chassis mount MC4's (in and out +-) and a waterproof socket (maybe in / out) to power the relay, in/out would allow you to daisy chain them)
52213-0211 - MOLEX - RECEPTACLE, 125V, BLACK, 2WAY | Farnell UK

 

[SibertSolar]

Brian - the Santon Firefighter's Safety Switch system disconnects the strings as Worcester has stated. They use a 24vdc motor to turn their DC contact block on/off on the roof. The system is designed to provide the fire service with a visual confirmation that there is no DC running inside the building, once the strings are switched to open-circuit on the roof.

The Solace solution from Area Energy has the provision to add panel-level DC switching to the base fire/smoke/alarm disconnection solution they offer. Not sure what type of contactors they use within their panel-level boxes but they will have a suitable DC21 rating.

ELV switching at panel-level is not a regulatory requirement here in the UK, yet, as far as I am aware. We have provided a lot of the Santon FSS systems to clients but I'm not aware of a single Solace installation as yet but you are best checking with Area Energy about that.

From a DC switching point of view, the rating you would typically be looking for in a switch/contactor is the DC21 values (switching of a resistive DC load, with moderate overloads). However, this only tells half the story as the actuation/design of the switch is also an important factor, as Worcester has highlighted with the IMO Precision Controls info.