Photovoltaic export power control was initially used in large-scale projects. Because the power factor of inverters is typically designed to be uniformly "1", this can cause numerous power quality and load balancing issues for large projects with long-term inductive loads. Therefore, the new generation of inverters, generally models above 500kW, are equipped with reactive power control systems to regulate output power. I personally predict that intelligent reactive power control and low-voltage/zero-voltage ride-through will become two indispensable mandatory requirements in future standards for inverters above 500kW.
This article will introduce solutions for user-installed inverters under 30kW. In areas with high photovoltaic (PV) penetration or poor grid infrastructure, there are generally restrictions on new PV system applications. For example, in parts of Western Australia, the grid almost never accepts any installation applications exceeding 3kW. Why are there these restrictions? There are two main reasons:
Firstly, since inverters output active power, this has a significant negative impact on the regional power factor. A poor power factor means that the power grid needs inefficient transmission and distribution, which is uneconomical and tantamount to a waste of energy.
Secondly, in areas with high photovoltaic distribution rates, when all inverters are fully loaded and outputting power to the grid at noon, the grid phase voltage may exceed the standard range, causing electrical appliances, including inverters, to be disconnected from the grid.
Case 1:
Ms. A's home has single-phase electricity. Based on her estimated electricity usage during peak hours, she would need approximately 6kW of solar power. However, the local grid only accepts applications for systems not exceeding 3kW. If Ms. A purchases only a 3kW system, she will still need to buy a large amount of electricity from the grid during peak hours (PSH). If she purchases a 6kW system, and there is no load consumption during PSH, then a large amount of electricity will be injected into the grid, violating regulations.
Case 2:
Mr. R planned to purchase a 5kW system, but the local power grid did not accept any photovoltaic power, requiring "zero injection".
Based on these needs, many inverter manufacturers have proposed the concept of "Export Power Control." The mainstream topology typically involves installing a third-party control meter between the appliance and the distribution box to communicate with the inverter. Simultaneously, the inverter's control program is manually programmed to maximize output power. During PSH (Power Seekers Hibernate) periods, if the inverter is fully loaded, the appliance will consume all the photovoltaic power. If an appliance disconnects, the third-party control meter will transmit output power to the inverter. If the output power exceeds the set maximum output value, the inverter will limit the DC current through the DC/DC converter (MPPT) to ensure the output power remains within the specified range. In Case Two, the inverter can be set to a constant input power, meaning it will continuously purchase a certain amount of electricity from the grid, thus ensuring 100% zero power injection.
There are two points of contention here:
Firstly, regarding the process of third-party electricity meters tracking and adjusting the inverter, according to my current test reports, most top-tier brand machines control this process within 1.0 to 1.5 seconds (IEC specifies within 2 seconds). However, within this range, some electrical energy is injected into the grid. But how much energy is injected? Let's take a 6kW system as an example:
The amount of electricity injected by a 6kW unit in 1.5 seconds is:
In other words, it would inject 0.0025 kWh of electricity into the grid, which might not even cause the meters in the distribution box to move. However, this 6000 W of electrical power would indeed have a transient voltage effect on the phase voltage at the user end within 1.5 seconds. If this is scaled up to a regional system with 2000 households, from the perspective of the power grid, it is indeed inconsistent with its regulations.
Secondly, what if a communication failure between the third-party electricity meter and the inverter causes the inverter to lose its monitoring function and thus be unable to limit power?
A feasible solution currently is to consider building a battery inside the inverter. During the milliseconds of receiving a signal, an additional regulator path would charge the battery, thus replacing the MPPT (Multi-Level Testing) for current regulation. Alternatively, a supercapacitor could be installed within the third-party meter to provide a buffer effect for charging and discharging. Other methods are not convenient to discuss at this time, but their core principles all revolve around energy storage or current diversion. Simultaneously, the inverter needs to be equipped with a secondary protection device; that is, if communication with the third-party meter is lost, it should immediately stop operating and report an error.
