Webinar: Diagnosing inverter overload – by Johannes Boonstra


Today, I will tell you something about overload. Overload of inverters, of Multis and Quattros. What is an overload of an inverter? Well, basically, an overload of an inverter is when the current in an inverter is too high and because the current in the inverter is too high, it could damage the inverter, or it could damage the Multi or the Quatro, so basically that should be prevented. There are two overload situations: the first overload is that the current is just too high but the inverter is still able to remain at a certain voltage, so if you have a 230 volts inverter you still have 230 volts on the output but the current is just higher than what the unit expects for. It can also happen that the load is so high that the inverter is also not able to maintain the voltage on to its level. That means that if you have a 230 volt inverter, the inverter is not able to reach those 230 volts. This is also an overload, and this is an even more critical one, so there is a distinction between those two overloads and how the inverter will react on each.

If you look at the picture you can see a row of black boxes, and the small black boxes are in fact FETs and these FETs are basically what is determining the maximum power of the inverter. The more FETs you have, or the more powerful FETs you have, the more energy the inverter can do. These FETs are being protected on current, so if the current is going too high, then it will tell the software and the firmware to stop. You will find these FETs basically in all the models that we have. The smaller model inverters have fewer or smaller FETs, and the larger inverters have more of these FETs. If an overload occurs, there are different situations: the unit will stop, or the unit will warn you, or it can also lock itself, but later on I can tell something more about it. If you have a system where you have more than one inverter, so you have a parallel or a three-phase system, if one of the units detects an overload, it will stop the whole system; so if you have a three-phase system and one of the phases has an overload detected, then the complete system will stop.

You will see that also on the LEDs of the master: if you have three phases, the master will blink and will tell you exactly what is going on in that system: if you have an overload or you have a low battery or you have a temperature alarm, or something like that. So depending on where the fault is, the LEDs will flash. If you have the units connected to the VRM site, you will not always be able to look at the LEDs, but you can look on the VRM site and see in a notification where the overload is, so they will tell you then exactly if it's on L1, L2, or L3, so then you have an idea on which phase of the system you have too much load connected or maybe the startup of that load is too high. If an overload is detected by an inverter, it is not automatically switched off immediately. It does that in a certain sequence: if an overload is detected, the first thing you will see is a pre alarm, an alarm that will flash the LED and the LED will tell you there is something going on.

It could be a temperature problem or maybe a low battery, but you will see a flashing LED and that flashing LED will tell you what is the probable cause of the switching off of the inverter later on. If the overload in this case continues, then that LED will lit up constantly and that will also mean the unit will be switched off, but you will always have these three phases where it goes through; so first a blinking LED, then the alarm, and then combined with switching off of the inverter. So basically, if something happens, you will see first on the LED what is the cause and in which phase the overload is located. Overload protection is controlled by the firmware and this firmware is also deciding if the unit should we be restarted or not. Basically every unit has a pre alarm and then a complete alarm, so it will switch itself off and it will try again to restart and it will try again exactly after 30 seconds.

These 30 seconds is a fixed value. That means that if the unit is starting on another time frame than these 30 seconds, most likely something else is causing the switching off. This can be an assistant, it can be something you programmed or it can also be something externally, that you cut the DC power or something. If the unit starts up in exactly 30 seconds then you know (it can be starting up and also switching off) it is caused by the firmware. If it is something else, well then you have to look for the cause of the overload or switching off outside the unit. It will be an external cause. Also, if you switch off the unit, you can also do that through the remote panel. So you can do that on the unit itself but you can also have a remote panel connected to the inverter and switch the unit off and on with that. If the unit switches off and restarts again, and switches off, and does that three times, if it started up three times again and within 30 seconds the unit stopped, so that overload of your existing load is too high, then the unit goes into the lock state, that means the unit will not try to restart itself, it remains off and it will show on the LED in what state it is.

So it can be multiple causes that unit switches off: it can be overloads, can also be overload and low battery, it could also be overload and temperature, for example, and these LEDs will constantly lit up on the unit, that means if you come to a system and you see these LEDs you know for what reason that unit was switched off. The LEDs will never get better, they can only get worse, so you get always the maximum reason for the unit to be switched off, so if something happens during the switch off you will remain seeing that on the LEDs. So this is what I said. You can restart the unit when it is locked by using the front switch on the inverter. You can also use the remote control, but you can also, when you are not on the site, use the VRM portal to do that. I will show you later on how to do that exactly. If the unit starts up and within 30 seconds nothing happens, so it continues to run but an overload occurs after a minute or two minutes or more, then the timer is cleared, that means that the three times trying and then locking itself is always within 30 seconds. If you come through these 30 seconds then basically the timer is reset and you have again three attempts that you could possibly try to start up the inverter.

If the inverter locked itself but is on a remote location and you can connect to the unit with the remote console, then you can go to Device List, select the Multi or the Quattro that you want to reset, therefore you go to Set Up, and if you are in Set Up you will see three options: Redetect system, System reset, and Alarms. And if you go for a System reset, the unit, if it is locked, will be switched off and on again, so then again you have your three attempts. If it continues to do so, then something is really wrong and then you have to find another solution, but at least with the VRM you can remotely reset the unit, if possible. If you have three phases of course you are resetting the master and then the units should restart again. Well, basically what can an inverter handle? The inverter can handle a maximum amount of current, and that maximum amount of current is determined in the design of the unit.

If, for instance, you have a unit like this example which is an 8000VA, so 48V/8000VA, this is the current that it can do for about 30 minutes. So if you divide the 8000VA by 230 volts you will find that it is 35 Amps, so the 35 Amps is the maximum current the working inverter can handle for 30 minutes. Those 30 minutes are for normal ambient temperature. If it is too warm, warmer than the specifications on the data sheet, you have less power, if it is colder, you can even have more power than you can use for that inverter. In this, the measurement is in RMS, which means that the power factor is irrelevant because all the current is counted and measured, that means if you have a reactive or resistive power, it is the same measurement. The RMS value (true current) is used for determining how much the inverter can handle, also the RMS power is the power that the inverter actually does supply. How can you determine that? Well, you can determine the current by using an RMS meter, so if you are using a power clamp like the one on the picture, make sure you have an RMS meter because then you can see actually the same value that the inverter is measuring. Going back to what the inverter can handle, basically you have three different overload situations to take into account: the first one is the maximum power, and the maximum power when the voltage is not at a level where it is supposed to be.

So if you have a 230 volts inverter, but the inverter is not able to reach those 230 volts, it will try to supply that low grade energy. This is what you will see for instance when you start up an air conditioning of a large motor with a high startup power, if it sees that the maximum current is there but the voltage is not at its level, it will try it for 30 cycles, which is a little more than half a second. So it will start up, no matter what the voltage is, and try with the maximum current that is available to start up their load. If the voltage reaches the set point then it is okay, if it does not reach the set point within these 30 cycles, then the unit immediately switches off. The other maximum power is when the voltage is reached, so if you have a 230 volt inverter, you have 230 volt on the output and the current is maximum.

The maximum power can then be about two minutes. This is the power that you can also see in the data sheet: if you have a 5k unit, a 5000 watt unit, you will see sometimes the maximum power on the data sheet says 10 kilowatts, well that is for these two minutes while it is able to handle that power. The third one is a 30% overload. This is basically what I said in the example already: you divide the power by the voltage and then you have the 30 minutes load. This depends heavily on the ambient temperature. If the ambient temperature is higher than the normal that you expect, then you have less power, and if the temperature is lower then you even have some more time to use this overload power. So, basically, what it comes down to is if the voltage is to a set point or not, and what is the ambient temperature.

If you are designing a system, always go for the safe margin, so use the 30% if needed, but never count on options 1 and 2 because these are only extreme situations. You should try to avoid using these powers to specify a system. If you take that into real life, what can you connect to an inverter, you have to take into account that some single-phase engines, single-phase electrical motors, can draw about six times the nominal power. That means that if you have 1000 watt inverter, you can only have a small single-phase motor connected to it because a single-phase motor, in order to have it started, takes six times its nominal power, so it means that you only have 10% of that inverter more or less to start up that engine. The other way around, if you have a 1000 watt electrical motor, you need about 6000 watts startup power to get this motor running. On three-phase it is slightly better, a three-phase motor draws about three times its nominal power at startup, that means also in inverters, so inverters handle three-phase electrical motors better than single-phase electrical motors.

That is because basically the three-phase motors already have a rotation in their fases so it is easier for an electrical motor to start up. The other thing for inverters that you have to take into account is inductive load and capacitive load. An inductive or capacitive load is difficult for an inverter to handle because the inverter is basically made to have a power factor of 1. If you have an inductive or capacitive load you have to take extra margin in your inverter. Your inverter, if you want to be on the safe side, needs about a 100% reserve to be able to handle these capacitive or inductive loads. The 100% margin is for a power factor of approximately 0.7-0.8, something in that range. In those situations the inverter is able to handle that load. So these three things: start-up power single-phase, start-up three-phase are important, as well as capacitive or inductive load, so basically if you are designing a system build in these margins so that the inverter is able to start up and handle these loads.

What is also important in inverter systems, but also in other systems of course, is the circuit breakers. Circuit breakers come in many forms. I will tell you a little bit about the differences in circuit breakers. Circuit breakers trip on two criteria: one is the magnetic criteria and the other one is the thermal one. The magnetic one is for fast response and the thermal one is for overload over time, so to say. If you look at this small graph, you can see three types of curves, the B, C and D curve, and you can see that basically B has a low magnetic threshold, that means for normal household situation that will handle start-up power. The normal C is for more in-rush current so that means it will allow you more magnetic threshold, so it will start up more easily high start-up powers. D is even higher, it can do 10 to 14 times the nominal power, that means start-up power is more forgiving than curve B.

So depending on which type of loads you have in your system you choose also which type of circuit breakers you are going to use in the system. The other thing that I think is also often used is the RCD breaker. The RCD breaker trips on magnetic threshold and on thermal threshold but also has a fault current detection, which means that if more energy is supplied to the system than comes back up because the electrical circuit now is broken, so that the amount of current that is going through the circuit equals what you get in return of that circuit. If you lose current, so you can have a fault current, then this is detected by the RCD. If you have an RCD used in combination with a Multi or a Quattro we also advise to have the RCD on the output of the Multi so it is also working in inverter mode, so that you have both the protection for pass through and also in inverter mode.

I think I am already at the end of my presentation. So I invite you to, if you have any questions, do ask. Thomas: Explain the 30% overload again please. Basically, Thomas, a 30% overload, that is the power that the inverter can handle on current, but because of that power the inverter will heat up and that heat up eventually will make the inverter stop. We had the inverters named after what they can handle in 30 minutes, that means if you have a 5 kilowatt unit, you cannot use it 24/7 at 5 kilowatt because the unit eventually will heat up, so if you have a 5 kilowatt unit you can use it for 30 minutes under normal ambient temperature. Matty: I would like to hear something more about motors and inverters. Basically what I explained is that with electrical motors you have two different options: you have single-phase and three-phase, and basically what is important with the motor is that you can go through the start-up phase. On the three-phase it is quite easy because three-phase system gives already a rotation magnetic field, which means also the start-up therefore will be lower.

It is easier to start up a three-phase motor. This means if you have a three-phase engine which is 1000 watts, your inverter system only needs to handle about 3 kilowatts to be able to start up that motor. If you have that same motor but in a single-phase configuration that means the inverter should be much more powerful because it needs to take more energy to the motor to get it running and then it takes about six times the nominal power, and six times nominal power means that, in the example before, for a 1 kilowatt motor you take 3 kilowatt at least because, that is already on the edge, but a 3 kilowatt can supply 6 kilowatts of maximum power, so theoretically that would be able to start up 1000 watts electrical motor. However, if you are going to count on the maximum power with that tight margin, it is perhaps better to select an inverter with more capacity, more margin. So a 5kw or maybe even an 8kw unit should be more reliable in that respect. -Johannes, I do not know if you can hear me.

I can hear you, yeah. -Good. Is there any benefit between using different types of battery technology, do you get a better performance on a Lithium vs. LED on an overload condition? – Yes, the question is if the battery has influence on the overload. It does, because of the voltage. An inverter makes an AC voltage out of DC, that means the better quality DC you have, the easier it will be for the inverter, so certainly a Lithium battery, which has a very firm DC voltage, will make performance on an inverter better than a LED battery, which drops kind of easily in voltage when you have a big overload condition there. So, yes, the size of the battery matters, also the chemistry of the batteries – if you have LED or Lithium – but also the wiring, of course, also use efficient wiring to prevent having a lot of losses in your system. I have a question on my list: do you think it is possible to use a 3000 Multiplus with a load that consumes 2500 W on a peak for 0.5 sec every 3 sec all day long? and for the rest 2.5 sec..

. This is a difficult one, but basically, a 3000VA Multiplus can handle 2500 watt peak but it depends also of course on the temperature because you are basically at the limit of what the unit can do, but theoretically you can do that. And for the rest 2.5 sec 100 watt, that is an easy one, that it will certainly do. But again, if you are designing a system, please try to design that system with a little wide margin, so it has a margin built-in and is reliable, because over time the batteries get a little bit worse or you can have different ambient temperature situations so you could end up in a critical situation, but theoretically, Filippo, this should work. Matty, you want to know what the biggest Multi installation ever done by Victron is. Is that correct? Do I read that correctly? Yes.

Well, from the top of my head, but I can be wrong, it is 270 kilowatts, that is the biggest one we ever did, but we do not advise people to go that large because building these systems requires a lot of engineering, especially when you have a three-phase system you make this system with a lot of units, that means you have a lot of current distribution that you have to do, so therefore it is not supported to make the system that large. What we try to do is have the maximum of inverters at six units and basically that comes down to about 100-150 kilowatts maximum. We also get Matthijs. Matthijs sends also a comment. There is a link on Victron Energy live there you can see a manual on parallel and three-phase systems and there is also a description on what the maximum size is and which things to consider. Emmanuel: can you say a little bit more about inductive and capacitive loads with examples versus inverter power and capability? Basically, Emmanuel, if you have capacitive power that means the unit needs to supply RMS power, so on energy that is more than basically what you should expect normally and therefore the inverter will have a hard time, so if you have a lot of fluorescent lights or you have other things with inductive or capacitive for an inverter, you will have more power to supply than what meets the eye, so therefore you have to take that into account, that you need to have a bigger inverter than the lights that you connect.

Like I said, if you have a power factor of 0.7 or around that, then you take at least a 100% margin in your inverter because your inverter needs to be able to power that extra load. Matty asked: To run a motor three-phase of 10kVA I need a 20kVA inverter? Yeah, if you have a three-phase motor of 10kVA that means you have about… let's say 3kVA per phase you need. You need to have a 20kVA inverter system, indeed you have to have three times an 8 kilowatt to run that motor. It depends a little bit on what that motor is connected to. If you have something which is easily started up you can do that. There are some specific loads that also are connected to something which is immediately taking a lot of energy. In that perspective you can also do different things, for instance, a frequency controller.

A frequency controller makes the motor start up slower than a direct drive and therefore a frequency controller can help improve the start of capacity of an inverter. You have also phase-cutting devices which lower the voltage, which normally, for the grid, means that you have less power from the grid to start up a motor but, if you use these kind of appliances for inverters, they actually work the other way around, because they even give the inverter a stretched startup period, so only frequency drives are good to work, soft starts are actually not that good to be used with inverters. I get another question: the BMS low voltage will give a red bar on the VRM but also a low voltage set on the dynamic cutoff settings. Both will appear red on the VRM? I would like to distinguish them. Basically, the dynamic cutoff is something within the inverter itself and at the moment is a part of ESS so it is software driven alarm and actually it has not so much to do with overload of the inverter, it has more to do with what the battery is able to handle, and if you want the battery to be on discharge voltage, so to say.

So it has not so much to do with the inverter itself on overload. If you do not want to have dynamic cutoff, simply do not use ESS because ESS has that, or you can also change the settings of course so that you have a higher voltage level and then basically you do not see that alarm anymore. Thank you for listening and well, perhaps another time! Bye bye..