Guilherme Cerbatto Schmitt Prym

Brazilian standards ABNT 16149 and ABNT 16150 require that grid-connected photovoltaic inverters meet certain technical operating requirements with regard to power factor. In this article, we will present the concept of power factor and present the technical standards in force in Brazil. Power factor is defined as the relationship between the active and apparent powers of an electrical system, according to the equation: Where: P – is the active power in Q – is the reactive power in S – is the apparent power in A right triangle It is used to relate active, reactive and apparent powers. For the case of a sinusoidal single-phase system, we have: The apparent power in (VA) is the product of the effective values (RMS) of voltage and current. It is so named because its calculation consists of the simple product of voltage and current, by analogy to resistive circuits, but its value does not reveal the portions of active and reactive power present in the system. In electrical installations, the power factor (PF) is an indicator of energy efficiency, relating the active and reactive power present in the circuit. A power factor (PF) close to unity (1,0) indicates low reactive power consumption compared to active power consumption. Since active energy is that which performs useful work, the closer the power factor (PF) is to unity, the greater the energy efficiency of the installation or electrical system. The PF is classified as purely resistive when the electric current and the circuit voltage have the same phase, that is, they are perfectly synchronized and cross zero (horizontal axis) at the same instant, as shown in Figure 2. The power factor can also be inductive (Figure 3) or capacitive (Figure 4), when the current is behind or ahead, respectively, in relation to the system voltage. In Figures 1 to 3 below, the first image shows the information displayed by the Zimmer LMG 671 precision power analyzer and the second shows the waveforms collected from an oscilloscope, the grid voltage and the current injected into the grid by the grid photovoltaic inverter. -tie. Figure 1 - Above: energy meter screen indicating almost unity power factor (resistive). Below: Voltage (yellow) and current (green) waveforms at the output of a photovoltaic inverter connected to the grid, synchronized and in phase Figure 2 - Above: energy meter screen indicating inductive power factor. Below: Voltage (yellow) and current (green) waveforms at the output of a photovoltaic inverter connected to the grid, with the current lagging behind the voltage Figure 3 - Above: energy meter screen indicating capacitive power factor. Below: Voltage (yellow) and current (green) waveforms at the output of a grid-connected photovoltaic inverter, with the current leading the voltage. Power factor control of inverters. Grid-tie photovoltaic inverters have the ability to inject current with a controlled phase shift relative to the voltage into the electrical distribution network. This allows the inverter to control the amounts of active and reactive power that it injects into the electrical grid. In general, photovoltaic inverters are normally programmed to work with unitary PF, that is, to inject purely active power. However, it has been common to use this reactive power injection capacity to make the inverter itself compensate for the installation's power factor, performing a function similar to that of a capacitor bank. It is important to note that there is a limit to the current injection that the inverter can perform. Therefore, if it is programmed to inject a certain amount of reactive power, it will have a reduced capacity to inject active power. Assistance in controlling the stability of the electrical grid can also be requested from inverters through their ability to inject reactive power into the distribution network. This does not yet happen in Brazil, but in the future we will encounter situations in which the electricity concessionaire will be able to send signals to the inverters of distributed generation systems, making them inject reactive power as the needs of the electrical grid require. In other words, the electricity distributor will be able to manage the inverters connected to its distribution network. This already happens, for example, in California, where “California's Interconnection Rule 21” was implemented. In phase 1 of the implementation of this regulation, inverters connected to the grid had to be “smart inverters”, compatible with some control requirements required to support control of the electrical grid. In phase 2, started in 2019, communication requirements for inverters were established, integrating them into an IoT (internet of things) network so that they could be remotely controlled by electricity distributors. In phase 3, still planned for the future, advanced functionalities will be specified, taking advantage of the communication channels created in phase 2. Power factor control tests In the next article (Part II: Normative tests) we will talk about power factor control tests, which are part of the requirements for the approval of inverters in the INMETRO (National Institute of Metrology) certification program , Quality and Technology). Test guidelines and requirements are determined by the following technical standards: ABNT NBR 16150 – Photovoltaic (PV) systems – Characteristics of the connection interface with the electrical distribution network – conformity test procedures; ABNT NBR 16149 – Photovoltaic (PV) systems – Connection interface characteristics with the electrical distribution network. References: California's Interconnection Rule 21: What it means for utilities and customers across the US, https://www.openintl.com/california-rule-21-interconnection/ Booklet “Grid-Connected Photovoltaic Inverters, Test Procedures”, produced by INRI (Institute of Smart Grids) of UFSM (Federal University of Santa Maria)

Guilherme Cerbatto Schmitt Prym

Published articles

Power factor control of photovoltaic inverters