 Welcome to class 10 in topics in power electronics and distributed generation. In the last class we are looking at the issues of fault coordination and we looked at the case where you had a existing feeder and then distributed generation source was added to the feeder and then we looked at what would what is the resulting system. So, we saw that in general in a distribution system the fault current levels can actually increase decrease or stay the same. So, if your fault current level changes during the addition of a distributed generator source then there is a possibility of miscoordination between your upstream and downstream device. Because of this change in the current level and you cannot uniformly say that the current level is always going to increase at the protection point it can actually go in all three ways. So, if you look at then in the case of your parallel connected circuit breakers suppose you add a DG source and then you have a parallel branch which is going into a neighboring facility and that could be a situation similar to this particular circuit breaker too. We saw that the current that needs to be handled by this parallel breaker can potentially go up. So, in parallel with the DG system there can be numerous breakers because you have many loads many possible branches that are there in parallel and there is a possibility that the current level may go above the interoperating of that particular parallel breaker. In which case the that particular device need to be replaced with a higher rated device and then the issues becomes more complicated because the parallel breaker may not be owned by the person who is introducing the DG and then you have the question of who will bear the upgrade cost is it going to be the electricity provider the utility provider or the person who is installing or who is the owner of this parallel branch protective device. So, there are questions like that which arise. So, you also have the situation where say because of the change in current level a recloser which might have been say the coordination settings might have been done to obtain few saving strategy and because of the change in the current levels with the addition of DG now the few saving strategy may not work. So, now there is a direct cost impact on the utility because of the addition of the DG source. Also for example, if you have a sectionalizer sectionalizer is supposed to operate with a upstream recloser. So, if you look at the previous example suppose CB1 was a recloser and CB2 was a sectionalizer then essentially the sectionalizer would open for a case of say fault at bus 4 suppose you have a fault at bus 4 then we saw that essentially what the recloser along with a sectionalizer would do is that the sectionalizer would try to clear a temporary fault over here making use of the characteristics of the recloser. But, now if you have a parallel DG which is sending current through the recloser the recloser will now have to interrupt a non zero current where it was designed to interrupt a zero current level. Also if the current that the DG is providing is now higher than the setting for its counting then the counting of the sectionalizer might get affected. So, there is a possibility that as the sectionalizer might mal operate or might operate at non zero current. So, for a variety of these reasons you would need to address this particular issue and one possibility is to address the issue of miscoordination between protective devices you could think about upgrading your protective devices from the simple time over current type of characteristics to something more complex like what is being used in transmission systems or in meshed network where you can have multiple sources and then you try to protect sections of the network by things like directional relays distance relays etcetera. So, that would be one possibility but then that drastically increases the cost which may not be acceptable for a distribution system. Then the second another possibility is say to increase say to operate your distributed generation source. So, that you rapidly disconnect the DG for any fault on the distribution system. So, in this particular example that you have for any fault in any part of this particular system essentially you need to rapidly operate CB3. So, that you disconnect CB3 rapidly in response to a fault then the feeder goes back to the traditional distribution system operation and then the previous old setting for coordination would work. But, there is always a problem in situation such as this that you have to operate the CB3 extremely rapidly which means that it is more prone towards opening under nuisance conditions. It is circuit breakers have finite number of opening and closing before it needs servicing. So, you would need much higher maintenance for the circuit breaker. So, just being able to rapidly open the breaker may not be again sufficient in all situations. So, another way of addressing this particular situation potentially is instead of having the traditional say voltage behind impedance type of DG. If you can operate your DG such that you have it controlled more like a current source where now your current injection level during fault is almost a load current level then your contribution of fault current from your DG is now much lesser. This is what is more typical in a poly electronic type of DG where often if you look at existing poly electronic systems you have some disturbance coming from the grid your power converter might actually just trip off. So, many times the contribution coming from your power converter might be extremely small. So, if you could have a poly electronic DG rather than a machine type of DG then that might address this issue to some extent. But again this has a cost impact I mean the cost of poly electronics is higher than cost of machines. So, ideally then the challenges can you make the cost of a power converter as low as that of a machine. So, you can see that the variety of challenges in when you connect DG distributed generation source to the distribution system. And there are possible solutions, but it has to be addressed in a clean manner. And many of these problems are actually non-technical like who is the owner of what particular device which is not a clear technical problem. So, you will have to actually look at it in from multiple factors rather than just a technical way of looking at the electrical system. . Now, we will look at any another aspect of distribute the distribution system important again it has implications when DG gets connected and that is grounding. So, if you have any electrical system you would ground it and the primary objective of grounding of a system is actually human safety. Someone should not touch electrical system and get a shock you should not get electrocuted. So, human safety is a very the primary objective if you touch a cabinet you should not get a shock. So, you should have low touch potential and you also want to prevent overheating of conductors after you have a fault you can have over current. And if the current level stays high for a longer duration and the conductors would heat and you can have electrical fires. So, to prevent electrical fires is a important criteria. And to prevent may both that high touch potential lasting for long duration and for ensuring that you do not create overheating you need to actually rapidly disconnect the fault. So, even for there is a short O potential for a fault duration if that duration is very small then the resulting damage can be quite small. Also, another important objective is say prevention of arcing when you have a fault there is always a chance that you might have vaporization of metal very high current levels you have a lot of energy in the arc. And people who might be standing near the arc might get severe burns electrical burn is a very severe category of burn. And to prevent that you want to actually ensure that the energy in the arc is kept minimal. So, you want to prevent electrical the energy in arc faults. There are other factors that are important for example, rapid identification of the fault and its clearance is a important aspect for many equipment ensuring the continuity of electrical service is a important factor. So, if you have a fault in one equipment and if the equipment is critical and if the equipment goes down there might be consequences of shutting down that particular electrical section. So, you might have critical equipment where even after a fault you might need to ensure continuity of a service. So, sometimes the need for continuity of service might conflict with the need for rapidly clearing the fault after identifying it. So, you need to have a definite strategy of what exactly is more important for the portion of network that is being considered. Also, if you look at equipment during fault there is a chance that the phases which are not faulted would potentially experience over voltage. And over voltage can cause damage to insulation and you want to ensure that you do not have over voltage because of the way in which you have grounded. Your grounding also has an impact on equalization of voltage between multiple equipment. Suppose, you have especially when you are looking at communication computer networks etcetera you have equipment in one location trying to communicate to another particular location and you want to ensure the integrity of signals. So, how you ensure what is reference at the two locations would become an important factor for ensuring equalization of voltage and the integrity of communication. Also, things like how you ground your cabinet can have impact on EMI both EMI susceptibility and the emissions can be impacted by the way in which you are grounding. So, overall grounding is a important aspect of any system that you are building or how you operate the system this is a important factor. So, if you look at the grounding of a distribution system you can think about what are the possibilities of at the source end. At the source end you can have a ungrounded system and when you have a ungrounded system you might have a y section of a transformer which is with a insulated neutral or you might have a delta connection at your source. So, ungrounded is one possibility you can also say have solid grounding essentially it means that you can have a y transformer with the neutral solidly connected to ground can have a zigzag transformer. So, with different possibilities you can have very no intentional impedance added between your neutral and the ground is what is a solid grounding is. Then you have the third option which is impedance grounding and when you connect a impedance between your neutral and ground then the question is what is the impedance that you are connecting there are two possibilities one is you have a high impedance or you have a low impedance and then there are further two possibilities whether you are connecting a resistance or a reactance. So, you can have a high resistance grounding low resistance grounding. So, or you can have a high reactance or low reactance grounding high reactance grounding is not that common there are some special cases where people talk about resonant grounding etcetera. But that is only for special cases where you have a lot of control over the parasitic capacitance to ground the common impedance grounding methods or high resistance or low resistance or low reactance. So, we will look at the case of when you have a insulated or ungrounded system and we will assume that you have a delta y transformer and the y point is not grounded and say you are feeding two loads load L 1 and L 2 which in turn are connected through delta 1 delta y transformers and say you have transformers x 1 and x 2 x 2 and x 3 and say you have a fault to ground occurring on the feeder say with fault impedance Z f and you want to look at what would be the result of such a scenario in this particular system and we are looking at a single line to ground fault which is a common type of fault that can occur in the system. So, to analyze this we will draw the sequence network to analyze this particular fault. So, you have your positive sequence network. So, in this particular case we had assumed that we have a transformer with 5 percent reactance and the zero sequence network is actually now open circuit because it is ungrounded. So, if you look at the there can be no fault current in this I f should be zero in this particular case. So, you have V plus is equal to 1 per unit and V minus is 0 and V 0 is minus V plus minus 1. So, then if you look at your phase voltages using the sequence transform. So, your V a is 0 which is what you would expect because that is the phase that has the fault V b is root 3. So, you can see that phase b and c conductors c line to line voltage on a with respect to ground and but if you look at the loads L 1 and L 2 the loads L 1 and L 2 would not see any disturbance because it is connected through transformers and the transformers block the zero sequence voltage. So, the loads do not see any disturbance, but the feeder would see some the phase b and c would see a increase voltage voltages. And the amount of fault current is actually 0 which means that it can actually continue to operate under that condition for actually a long duration of time. The main concern of this particular situation is that the phase which is which has the fault would typically have some degree of arcing which means that whatever conductor got grounded would have arcs and that arc would periodically act like a switch which is on or off depending on whether there is at the nature of the arcing. And your parasitic capacitances in your feeder along with your physical inductances of your circuit components would potentially have resonant oscillations where with a fairly high Q factor because there are no loads that are now directly seen for this particular zero sequence voltage. And then essentially you would have higher phase voltage which can potentially now be much higher than the root three times the voltage that we saw and you can damage insulation. So, people almost never use a ungrounded systems what would be more common is to use a high resistance grounded grounding system where the resistance would help dampen out the oscillations. And you provide the required damping and also one need to keep in mind the type of over voltage protective devices that are used in the system suppose you have surge arrestors which are rated for line to neutral line to ground voltage of some with some particular margin. Now, if it is seeing 1.7 times the nominal voltage then you need to actually on a longer term basis you need to ensure that your over voltage protective devices are rated for the appropriate level of voltage. The second thing to consider is if you have one fault say occurring on phase A and now you have a second fault now occurring on phase B or phase C or maybe even at a neutral then the resulting fault would be now having much higher fault current levels because it acts like a phase to phase fault and then the over current would immediately trip the breaker. And you do not want to ensure the you want to ensure that the fault which has occurred first is attended to at some particular point before the a second fault occurs in such a system. So, you would like to have some sort of a monitoring system which monitors the voltage and ensure that there is a good availability of I mean one of the attractive features of this particular system was that even after the first fault you had a continuity of service to load 1 and load 2. So, with high impedance grounding people try to make use of the ability for the system to be both available after the first fault. But at the same time you need to have insulation monitoring and residual current measurements etcetera to ensure that some maintenance is being done. So, that before the second fault occurs the first fault is identified and cleared. If you look at the grounding of a typical system one should also not just look at the grounding of the source alone you have to look at both grounding at the source and at the load. This is important because if you look at physical distribution system there is a geographically the source might be a distribution transformer at the end of the street and your loads might be your individual houses all along the street. So, there is a big separation between where the load is and where the source is and unlike say if you have a printed circuit board your power supply and your load on that particular printed circuit board is on the same board and geographically much closer together in distribution system the distance between your source and the load is actually much larger. So, you have to think in terms of both grounding at the source and grounding at the load. So, based on the grounding at the source of the load some of the common ways of nomenclature or configurations for the grounding system one is a i t system where the first word i refers to insulated and the word t refers to tera. So, your source is high resistance grounded or the neutral is insulated from ground and the load frame is connected to earth or tera that is one possibility another possibility is the source is connected to tera through no intentional impedance and the earthing from the source is now your neutral point is distributed to the load and this is this configuration is called a T n configuration also you could have power earths earthing points both at the source and at the load this is called a T t configuration where both the source and the load is connected to tera. So, these are important especially considering the secondary distribution network. So, this is the low voltage network which comes from your distribution transformer which might be 11 k v to 415 volts and then coming to the individual homes or establishments and an example of ungrounded secondary distribution network is a i t network where you can see that the neutral is insulated from the ground and the loads have their own local grounds. So, you can have say power earth for say load 1 and 2 might share a power earth load 3 might have its own power earth at a different location. So, these are essentially grounding rods that are connected to the ground. So, essentially load 1's cabinet is connected to power earth load 2's cabinet is connected again to power earth load 3's cabinet is connected to its power earth there are 3 wires plus the neutral from your transformer going to the load. So, your loads might be 3 phase or single phase. So, in this particular situation this corresponds to the situation where if you have a fault say on a line to ground basis then you can potentially continue to operate the loads as we saw in the ungrounded system. But say suppose instead of the fault occurring at this particular point you could also have a situation where say the fault occurs say in load 1 and in which case you need to ensure that the load 1 is disconnected load 1 is disconnected before because there might potentially be a second ground fault may be at some other point in the system. Now, if you want to disconnect load 1 because there is no high current after the first ground fault you need to ensure that you need ground fault detection capability at each particular load. And not just that your circuit breaker should now have the capability to disconnect all 4 conductors. So, it is not a just 3 pole circuit breaker it is a 4 pole type of circuit breaker because even if you disconnect say the 3 phases r y and b for load 1 and say you have a fault in the load where phase r is connected to the cabinet in load 1. Suppose you now have a subsequent fault in load 2 that particular fault can actually propagate through the neutral in through load 1. So, to ensure that the load 1 is fully disconnected you have to actually also disconnect the neutral. So, the type of circuit breaker that you would use in IT network would be different from what you would use in a some other type of grounding network. So, the next possibility is say instead of having an insulated or a ungrounded source you could actually have a solidly grounded source. So, this is an example where you have a delta y transformer which is acting which the y is now your source and the neutral is now connected to ground through no intentional impedance. And for our analysis we will assume that the source side impedance is 0 and whatever is limiting the current is essentially the transformers leakage. And suppose you have a fault between any phase to ground will assume say phase to ground solid fault. Then essentially in this particular case we can then look at what is the resulting voltages and currents. So, we will assume a source through a circuit breaker. So, your sequence network is essentially so your I f by 3. So, your fault current level for a single line to ground fault is 20 per unit. In this particular case we have assumed that your source impedance is 0 in which case your 3 phase fault current and your single line to ground fault current is actually going to be the same. If your source impedance is not 0 if then your single line to ground fault current level with a delta y transformer with the y which is solidly grounded the single line to ground fault current can actually be higher than the 3 phase fault current. Because the 0 sequence impedance back at the source is not captured in the 0 sequence network. So, sometimes people might consider adding neutral grounding impedance for the transformer even in this particular case. But to look at the analysis of this particular situation we can then look at what is the resulting voltage your V plus. So, your V plus turns out to be 0.67 angle 0 your V minus. So, we can calculate our phase voltages V a is 0. So, we can see that in this particular case when you have solid grounding your phase B and C does not see over voltage. But you can see that your fault current level is quite large and in our example it is 20 per unit can cause arcing faults. . However, you can see that even though the fault current is quite large the current level is large implies that your circuit breakers would act on an instantaneous basis. So, the duration of fault current would be shorter. So, your fault is cleared on a short time. So, the thermal damage may not be that bad, but you can have arcing for your short duration when there is a high current flowing when there is a single line to ground fault. So, if you look at then an example system where you have solid line to ground I mean solid grounding of the source an example is the T n network where the y point of your transformer is solidly connected to power earth and neutral is then distributed to all the loads. So, the neutral is now expected to be at low voltage at the ground voltage because it is connected in a tight manner to power earth. So, your loads load 1 and load 2 cabinet is also now connected to the distributed neutral. So, you have a situation where you can say for example, you might have a fault in load 1 and you now have a large current flowing between your phase conductor and returning back to the source through this neutral wire and the current level is quite high and one thing is it is it can be quite difficult to distinguish between now unbalanced load current flowing through the neutral and ground fault current which is now coming through the neutral because ground conductor is being shared. Also, if you look at the network suppose you have a considerable say distance between your source and your load then your impedance of the neutral conductor might be not negligible which means that if you have a high current which is now flowing through this particular neutral conductor then person who is actually having his hand on say a cabinet will see a high touch potential because and the touch potential is now seen across the network. But it is typically for a short duration because we saw that the current level is quite large which means that your instantaneous strips would operate. So, for a short duration you could have elevated touch potentials. So, one issue was this neutral current is not being differentiated from your load current. So, one way of getting around that particular problem is to have something called the TNS grounding where the neutral and the power earth are now separated as conductors. So, your unbalanced load currents will flow through your neutral conductor and only your fault current will flow through your ground conductor. So, this is TN with separate ground and neutral conductors here it is now possible to differentiate between a case of unbalance and ground current. But here again you can see that immediately the cost is high for distributing a power in 3 phase now you have 5 conductors. So, then you can say maybe you can have some combination of this where maybe for some distance you will have a common say conductor and further behind it you can have separate conductors for your network. So, you might have somewhere this is called a TNCS network where there is TN combined for some section and then separate for the rest of the section. So, as long as the fault current is being monitored say at different locations where you have the separation then it is possible to look at the sum of the currents in your phases and along with your neutral to identify whether there is a ground fault. But then you do not have to distribute your 5 conductors all the way it is combined to some extent and then it is separated. In this particular case unlike the case where we saw in the IT type of distribution networks it is now sufficient to have your circuit breakers on a 3 pole basis because of your solid grounding each phase any of the phase which would have a ground fault would see the elevated current and then it would get disconnected from the source and your neutral in way is solidly connected to ground. So, a 3 pole circuit breaker would be sufficient in a TN type of network unlike the IT type of network. So, next possibility of having a grounding in your network is to have a impedance between your neutral and ground. So, you intentionally add a resistance or a reactance to the particular network and then we look at the resulting situation what would happen in this particular case. So, in this particular example we look at a case where you are adding a reactance of J point 1 J point 1 per unit. So, the reactance is quite small. So, this is low reactance type of grounding which means that the ground current in this particular case is high. You might select the reactance that it is high just high enough to trigger your instantaneous protections, but it is not too high that you might have severe arcing damage. So, you get the advantage of the high currents at allowing your device protective devices to trip rapidly, but at the same time the currents not becoming so large that you would have dangerous arcing at the point of fault. So, your fault damage can be reduced in this particular case. So, if you look at the resulting sequence network. So, we look at impedance grounding and we look at an example of J point 1 per unit as we mentioned. So, if we look at what would be the resulting sequence network when there is a fault. So, the positive sequence network is this is the negative sequence network and we will assume Z f to be 0 and the current here is I f by 3. So, you can see that your fault current level has come down when we had solid grounding it was 20 per unit. Now, I f is 6.6 per unit and then we can look at the resulting voltage your v plus and then you could calculate what your phase voltages are and that works out to be nearly 0 v b is works out to be about 1.45 per unit at angle minus 143 degrees v c is. So, you can see that the resulting voltage levels is somewhere in between what you would get in a solid grounded situation and a ungrounded system. So, in a ungrounded system you have 1.7 per unit voltage here in a solid grounded case you have 1 per unit here you have 1.5 per unit as your voltage that is seen on a phase to ground basis. And if you look at your fault current level and if you look at your fault current level. So, your peak fault current level is reduced. So, we see that you can have advantages of the two cases when you are now connecting a impedance to your neutral. Again depending on what is the particular way your application you might have different ways of connecting your grounds and we will continue with this discussion of grounding in the next class. But you can see that this is a important aspect of any system design or even equipment design to ensure that whatever your designing is actually safe and can be used in manner which would not cause any danger to the potential user of the equipment. Thank you.