PV controllable output power (Export Power Control) was first used in large scale projects, because the power factor of inverter is usually designed to be uniformly "1", for some large projects with long-term operation of inductive load (inductive load), it will cause many electric power quality and load balance problems. Therefore, the new generation of inverters, about 500kW or more, are basically equipped with reactive power control system to regulate the output power. I personally predict that intelligent reactive power control and low/zero voltage ride-through will become two mandatory requirements for future inverter standards above 500kW.
In this article, I will present the solution for user inverters up to 30kW. For areas with high PV system penetration or poor grid infrastructure, there are generally certain application restrictions for new PV systems, for example, some Western Australian grids hardly accept any application for installations above 3kW. Why the restrictions? There are two main reasons.
One, because the inverter output is useful work (active power), which can have a significant negative impact on the regional power factor. And poor power factor means that the grid requires inefficient transmission and distribution, which is not economic and is tantamount to disguised energy waste.
Second, for areas with high PV distribution rates, when all inverters are fully loaded with power output to the grid at noon, the grid phase voltage may exceed the standard range and cause appliances, including inverters, to disconnect from the grid.
Case 1.
Ms. A's home is single phase, and she needs a 6kW PV system after estimating the time period of her appliances, but the local grid only accepts applications for systems up to 3kW. If Ms. A purchases only a 3kW system, she will still need to purchase a large amount of electricity from the grid during the PSH period. If a 6kW system is purchased and no load is consumed during the PSH period, the 6kW will be injected heavily into the grid and violate the rules.
Case 2.
Mr. R intends to purchase a 5kW system, however, the local grid does not accept any PV power and requires "zero injection".
Based on this demand, many inverter manufacturers have come up with the "Export Power Control" concept, which is basically a mainstream topology where a third-party control meter is installed between the end of the consumer and the distribution box to communicate with the inverter, while the inverter control program is artificially set to increase the output power. During the PSH period, if the inverter is fully loaded, the consumer will absorb all the PV power; if any consumer is disconnected, the third-party control meter will transmit the output power to the inverter, and if the output power is greater than the set maximum output value, the inverter will limit the DC current through DC/DC Converter, i.e. MPPT, to ensure the output power is always within the specified range. This ensures that the output power is always within the specified range. For case 2, the inverter can be set to input a certain amount of power at a constant level, which means that a certain amount of electricity will be purchased from the grid all the time, thus ensuring 100% zero injection.
There are two points of contention here.
One: In the third-party meter tracking to the inverter determination and adjustment process, according to the test report I currently have, the general first-tier brand machines are controlled within 1.0 to 1.5 seconds (IEC regulations within 2 seconds), but in this interval, there will be electric power injected into the grid, however, how much power is injected? Let's take a 6kW system as an example.
The amount of power injected in 1.5 seconds for a 6kW system is
This means that 0.0025 kWh will be injected into the grid, and the meter inside the distribution box may not even move a bit. However, this 6,000 W of electrical power will indeed have a transient voltage effect and impact on the phase voltage at the customer's end within 1.5 seconds. If this is scaled up to a regional group system of 2,000 households, it is indeed inconsistent with its regulations from the grid's point of view.
Second, what if the third-party meter and inverter communication failure causes the inverter to lose its monitoring function and cannot limit the power?
A more feasible solution is to consider a battery built into the inverter that charges power into the battery via an additional regulator path during the milliseconds of receiving the signal, which replaces the MPPT to regulate the current over time. Or place super capacitors inside the third-party meter to charge and discharge the buffer effect, the rest of the methods are not yet convenient to introduce, but the core is basically centered around energy storage or current draw. At the same time, the inverter needs to be equipped with a second protection device, which means that once it loses the ability to communicate with the third-party meter, it needs to stop working immediately and report an error.
