EVSE Load Balancing VS Load Shedding 1: Largest number of 30 Amps EVSEs that can be fed as per the code from the 1600A @ 600 volts feeder The schematics shows that the 1600A feeder is split in 7 branches From the electrical code perspective, the limiting factor of each branch is the 225 kva transformer Based on the transformer size, the maximum output current on the 208/120 Volts side is 625 Amps, so the closest standard Distribution panel size would be 600 Amps Each EVSE should be connected between 2 phases (to get 208 Volts) through a dual pole 40 A breaker, and will draw a maximum of 30A (while charging an EV), representing a 6.24 kw load each. To keep everything balanced, every group of 3 EVSEs should be connected as follow o EVSE 1: Between phase A and Phase B o EVSE 2: Between phase B and phase C o EVSE 3: Between Phase C and phase A When fully loaded (80% of 600 amps feeding a perfectly balanced load), the maximum output power we can get from a 3 phases, 600 Amp 208 panel is 172.723 kw So the maximum number of EVSE that can be connected without exceeding the limit as per the electrical code is 27. 27 EVSE being a multiple of 3, this means that in the worst case scenario (27 EV drawing 30Amps), everything will be well balanced. So the maximum number of EVSE that can be connected to each of the 7 branches in full respect of the code is 27 units. For the full site configuration (7 branches), this means a maximum of 189 units can be fed by the 1600A @ 600 Volts AC feeder. The following diagram shows how one of the 7 branches could be set up to feed 27 EVSEs through a 208 600A distribution panel hosting 27 dual pole 40Amps breakers:
2: Putting the electrical code aside, what is the largest number of EVSEs that can reasonably be fed from the same feeder. The limiting factor from this perspective is the amount of energy on average each user will expect when connected for the average amount of time his EV will be connected to the EVSE For a multi-dwelling building the average connection time is from returning home in the evening (6 to 9 PM) to leaving home in the morning (6 to 9 AM), which means 12 hours, to be safe let s reduce that number to 8 hours. In Canada most people live within 30 km from work, meaning that if they don t charge at work, they will deplete their battery for 60 km per day.
Depending on the EV size the driving habits, and the season, the energy consumption is between 100 Watt Hours (Leaf in Summer, average driver) to 300 Watt hours (Tesla S, aggressive driver in winter) per km. Based on an average of 200 Watt Hours per km (which is realistic) and assuming that most drivers will not be able to charge at work (which is not realistic as workplace charging will soon become a trend) the average amount of energy that will be required represents 12 kwh per day per EV (or per EVSE if each one is used) This means that the electrical system must be designed to be able to transfer a minimum of 12 kwh per EVSE per day and this during a period of 8 hours. This means that the average transfer rate is 12 kwh/ 8hours, which means 1.5 kw So based on this, without considering the type of charging solution (level 1 or Level 2 or Smart Level2) the electrical system should be designed for a sustained load of 1.5 kw per EVSE installed This means that each feeders of the clouded area will be able ultimately to feed 172.723 kw/1.5 kw = 114 EVSEs, which means a 4 folds increase compared to the limit imposed by the electrical code 3: Possible options to increase by a factor of 4 the number of 30A EVSE As pointed out, one way of increasing the number of EVSEs is to implement a load control scheme that will time share the total power draw to limit it to an average of 1.5 kw per EVSE. The other way which we promote is to implement a power sharing scheme that will in effect allow 100% of the EVSEs to transfer power at any time, but at a power level which is controlled in real time not to exceed the system limit The following paragraph describe in details how each method can be achieved and what are the implications to make each one compliant to the electrical code. 4: Load control scheme As stated in section 2, to keep the quality of service expected by EV users we need to increase by a factor of 4 the number of EVSEs connected to the 208 distribution panel. This means that the number of individual circuit breakers increase by a factor of 4 (to a total of 128 dual pole 40 A breakers), so a good strategy would be to connect from the main 600A distribution panel, 4 sub panels located at convenient locations to feed, it would be preferable to feed multiple sub panels each located in a convenient area to feed 27 EVSEs To control each individual EVSE, a 40 amps contactor will be needed o For a fail safe operation the Contactor should be Normally open (this way, if a control wire is cut, then the EVSE cannot draw any power) o Some EVSE have a remote control input that could be used to remotely control them, although, this method is not adequate to implement a fail safe load control (the EVSE is allowed to draw full power when the control wire is cut) Two years ago, this was our initial thought on how to get around the code to increase the number of EVSEs, the main issues we discovered with this scheme are the following o The interlock that will avoid to have more than maximum number of EVSE connected will be software based. This means that the controller would need to be UL1998 certified which has many serious implications This certification is very long and expensive to get Software should be re submitted each time it is modified,
o Only 25% of the EVSE will actually be able to charge EVs at the same time, the algorithm that will time share the power transfer will need switch quite frequently (every 30 minutes or every hour) otherwise, people staying connected a short length of time run the risk of getting no energy transfer at all. o Shutting off the AC power of an EVSE while a car is charging creates all kind of issues: Some will horn (like the Chevy Volt) Some others (Nissan Leaf) will turn on their Malfunction Indicator Lamp (MIL) when this happens too frequently. Some will not resume drawing power by themselves (will need to have the connector unplugged and reinserted to continue) Assuming that it would be possible to certify an interlockable load control scheme, a strategy would be to associate one Load controller to each sub panel, and to install each of the 27 X 40 A contactors + the load controller close the subpanel in an enclosure certified for this purpose. At the end the additional hardware required make this solution really expensive The following diagram shows how one of the 7 branches could be set up to feed 108 EVSEs through a 208 600A distribution panel feeding 4 sub panels each hosting 27 dual pole 40Amps breakers, feeding a load enclosure hosting the load controller and the 27 contactor required to connect each group of 27 EVSEs. 5: Power sharing scheme The basis on which is based this scheme is as follow o Our Smart EVSE (CoRe+PS), from the electrical code perspective is an 8 amps load, that can be increased to 30 amps when all conditions are there to safely do it
o When all conditions are not met, then the CoRe+ limits the power draw form any connected EV to 8 amps (1,664 kw), which is a fail safe condition o The controller that allows to increase the power draw beyond 8 amps do it in near real time (every 5 seconds or so) by taking in consideration the actual power drawn of every connected EVs. o So when the maximum power transfer limit is reached, the controller gradually decreases the maximum power set point of the CoRe+PS units feeding EVs drawing power o When connected EVs reach their full charge, the freed up capacity is quickly re assigned to EVs that are still charging, increasing the power transfer (up to the maximum of 6,24 kw) o The end result us an optimized power transfer, and quicker charge time to every EVs This scheme has been developed and proven out: o The CoRe+ PS has been CSA certified during Q1 2014 o Since then we have done several projects representing up to 20 EVSEs which are connected to electrical systems that would have required to be upgraded if regular level 2 EVSEs would have been used, and this in full respect with the electrical code. The following diagram shows how one of the 7 branches could be set up to feed 108 EVSEs through a 208 600A distribution panel feeding 4 sub panels each hosting 27 dual pole 40Amps breakers, feeding a load enclosure hosting the load controller and the 27 contactor required to connect each group of 27 EVSEs. 6: Conclusion
Our power sharing scheme brings the following benefits over developing a load control scheme o Already proven out, certified and compliant with the electrical code o Much lower cost to install o Much lower cost in term of electrical material o Much more scalable, can be deployed in multiple phases. Phase one could represents the installation of the distribution boxes + 50% of the EVSEs, or can represent the installation of 50% of the subpanels, with 100% of the EVSEs Subsequently, additional EVSEs can be added in phase with the demand for the service. o Much better quality of service to users, each EVSE connected to an EV always transfer the highest possible amount of power at all time o Intrinsically safe, each possible failure mode ends up with a power draw that is always below the electrical infrastructure limit. On top of power sharing, we can set up our controller to implement Power Limiting capability to reduce OPEX by taking advantage of Time of Use electricity cost, or to reduce Demand Charges.