Need to pick some brains guys/girls

Darkwood · Apr 6, 2016

I work on a site that has just had a 90Kw array system installed, grid linked with a customers KVA allowance of 60KVA (400v 3ph),

Now the site at present has a gen' set providing power for half the machinery, we are talking heavy stone cutting equipment with typical motor sizes or 30 - 45 Kw, the array has been sized for the yard to allow a gen' set to be switched off and thus maintain the sites power needs.

I have no issue with the sizing and the system fitted but what does concern me is these motors are VFD controlled, now these motors do have front end line reactors but this is where my technical know how limits me as I have zero experience of PV. There is going to be some harmonic distortion created and with an average of 60Kw generated by the PV with the additional 60Kva grid supply then I just wondered how the high non linear loading would effect the PV system (specifically the inverters) as we have a mixed supply stiff and soft approx 50/50.

Any thoughts of any issues, the way I understand it is the PV array is generated at a slightly higher voltage so as to feed surplus to the grid but this in effect means it would be the primary feed source for the loads with the grid taking up any slack.

Will heavy non linear loading with surpressed harmonics cause any issues with the PV inverters?, this is really the question I'm asking.

SibertSolar · Apr 7, 2016

Not aware of any issues with VFDs, and if the harmonics are suppressed then I wouldn't imagine any issues. The inverters are tested/certified for harmonics too, and I'd hazard that so long as the sensed grid connection doesn't fall outside the G59/3 parameters for voltage/frequency/ROCOF etc, then the VFDs should draw from the AC output of the inverter bank just fine, regardless of whether generation exceeds consumption or not. I'll stand to be corrected though of course.

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Darkwood · Apr 7, 2016

I only ask because if you use a generator for said same situe' then there is a derating factor that may need to be applied and careful balancing if numerous gen's of the non linear and linear loadings. Getting it wrong can cause all manner of problems from the VSD's tripping as the Gen's try to compensate for the instability (it gets complex as to why) to the harmonics and transients been amplified, as this is my first interaction on this scale with PV and with it been classed also like a Gen' supply as a 'soft supply' then harmonics etc cannot be soaked up like the Grids 'stiff' supply allows.

SibertSolar · Apr 7, 2016

Ok, understood. Still don't see an issue though tbh. The backup genset won't be active whilst the PV is on though, right? I'm assuming the PV is connected on the grid-side of the genny changeover switch and/or there's some form of inhibit interlock?

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Darkwood · Apr 7, 2016

A bit more info-

Although I'm ok with the intergrating of the PV/grid, here's a jist of the arrangement - half the site as it stands at the moment runs off PV/Grid but the other half runs off an independent gen' set (isolated circuits 'off grid'), the idea is to run the full site off the PV/grid set up until the PV output drops below a certain threshold of site demand then the gen-set takes over isolating half the site again. This side isn't an issue just ensuring the PV inverters won't read the wave distortion created by the numerous VSD's and try to regulate the voltage output to compensate, I'm personally hoping the fact we still have grid connection it will hopefully soak the distortion up as the grid is a stiff supply, but my original concern was that with the PV been slightly higher voltage then it may act as a prefered destination to react with the PV inverters, like I said before, it gets very complex and still isn't a fully understood even by the experts on harmonics but just putting it out there to see if anyone had come across issues with large arrays on sites with numerous VSD's, which would either create trip errors in the VSD's or errors in the inverters.

SibertSolar · Apr 7, 2016

There was a product made by Advanced Energy (formally Refusol, soon to be rekindled by the parent company Prettl perhaps) that was designed to do pretty much what you describe - with a grid-tied inverter for the PV, and a "hybrid" controller to power/demand-manage a diesel generator.

I'd imagine that the control required would need to encompass decent closed-loop feedback and some form of neutral and/or earth hard switching when running the generator and switching the temporarily off-grid portion of the site? The PV inverters tend to rapidly fluctuate their output as quickly as the irradiance fluctuates....could be tricky, even if you build in some hysteresis?

Doesn't sound like VFD loads are your biggest challenge to me....seems the control method and effectiveness might be a proverbial pita....

Interesting project though, I'm sure you'll get better input from some of the more experienced guys on here in due course. Good luck with it, let us know how it goes and if we (Sibert) can be of any assistance,

Andy

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SibertSolar · Apr 7, 2016

PS: a SolarLog device plus a Smart Relay Station would probably give you the measurement and control functionality required (plus some ancillary hardware)

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Gavin A · Apr 7, 2016

darkwood - my concern with that set up would be the potential for the presumably fairly large start up surge currents to cause sufficient voltage swings to result in the inverter tripping out. Those settings can be adjusted with the right codes, but only supposed to happen with DNO approval.

Some inverters actually have a lot of settings on them that can be used to aid with phase balancing, power factor correction etc. but pretty much nobody in the UK uses any of that because it needs DNO approval to alter it from the nominal PF of 1. SMA in particular can do all of this, I think ABB can do a lot of it.

I think I have an SMA document on this stuff kicking around somewhere if you pm me your email addy.

SibertSolar · Apr 7, 2016

Wouldn't using VFDs actually help minimise initial startup current surges though?

Also, what a VFD does is pretty much what goes on in a PV inverter anyway - DC to DC conversion + PWM

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Darkwood · Apr 7, 2016

Gavin A said:
darkwood - my concern with that set up would be the potential for the presumably fairly large start up surge currents to cause sufficient voltage swings to result in the inverter tripping out. Those settings can be adjusted with the right codes, but only supposed to happen with DNO approval.

Some inverters actually have a lot of settings on them that can be used to aid with phase balancing, power factor correction etc. but pretty much nobody in the UK uses any of that because it needs DNO approval to alter it from the nominal PF of 1. SMA in particular can do all of this, I think ABB can do a lot of it.

I think I have an SMA document on this stuff kicking around somewhere if you pm me your email addy.

I will be using a synchronised changeover so the swap should be pretty smooth, as for motor inrush, this is limited using vsd's as ramp up times are gradual.

Gavin A · Apr 11, 2016

ok so it probably won't be the inrush, but what happens when the sun drops behind a cloud? Is the generator still going to be running in parallel with the grid & PV? and is it able to ramp up to cope with that rapid demand increase? If not then you're going to be getting significant voltage variations as you'll go from pulling say 60kVA from the PV, 40 from the grid to 90kVA from the grid and 10kVA from the PV in a matter of seconds, which could well end up with the on site voltage dropping outside of the allowable limits and the inverter tripping out for undervoltage and making the situation worse.

Darkwood · Apr 11, 2016

Gavin A said:
ok so it probably won't be the inrush, but what happens when the sun drops behind a cloud? Is the generator still going to be running in parallel with the grid & PV? and is it able to ramp up to cope with that rapid demand increase? If not then you're going to be getting significant voltage variations as you'll go from pulling say 60kVA from the PV, 40 from the grid to 90kVA from the grid and 10kVA from the PV in a matter of seconds, which could well end up with the on site voltage dropping outside of the allowable limits and the inverter tripping out for undervoltage and making the situation worse.

To clarify, I have had zero input on design of the PV system and any calculations for the site, the cloud situe' you mention will not be a problem as I will be setting up a threshold for change-over while there is still adequate power from the PV when the gen' is brought in, the gen' will only pull in if the threshold is met for X amount of time, the threshold will be higher than the requirements of the site and the gen' once running will run for a min period of time regardless of PV meeting its power needs again. This will stop any voltage drop issues, it will also stop the gen' cycling on/off.

The control system for the change-over will be monitoring the full site for supply and demand as well as the PV system and light levels- I don't foresee any volts-drop issues like you mention.

When the Gen comes into play part of the site will be removed from the grid/pv in stages and loaded onto the generator after it has sychronised with the grid this will avoid shock loading on the gen' which would see a VD that may cause issues.

Gavin A · Apr 11, 2016

I'd picked up that you were coming at this after installation design etc.

In case it's useful, some inverters have relays that can be set at different levels of output, or possibly voltage, that could be used to trigger the contactors for the changeover to generators. Not sure if that helps or not, but thought I'd mention it as a possibility - some also come with modbus coms output.