Denilson Lima

In recent years, photovoltaic plants have grown significantly, especially mini-generation connections. By definition, minigeneration refers to a generating plant with installed power greater than 75 kW and up to 5 MW for dispatchable sources and up to 3 MW for non-dispatchable sources. A source is considered dispatchable when the plant has a storage system that corresponds to 20% of the monthly generation capacity. Most of the time, this type of connection needs to be made in the SDMT (Medium Voltage Distribution System), which operates at voltages between 1 kV and 69 kV, requiring the construction or modernization of a substation to enable the injection of generation into the electrical system. For these connections, energy concessionaires require that the user prepare, among several studies, forms and documents, a coordination and selectivity study of the protection system. In this study, the user must present the calculation and suggested values for implementation in the protection relay of their primary cabin, in order to meet the criteria established by the concessionaire regarding the selectivity of the protections. It is important to remember that protection must be adjusted according to the criteria established by the distributor. In this sense, PRODIST (Distribution Procedure), specifically Module 3, deals with access to the concessionaire's distribution system. This document establishes detailed instructions and requirements for connection to the electricity distribution system, as set out in Resolution 1.000/2021, which deals with the Rules for Provision of the Public Electricity Distribution Service. The main protection functions at the interface with the distributed mini-generation connection network are indicated in Table 1-A of PRODIST. Table 1 - Protection functions at the microgeneration or minigeneration interface In addition to the functions listed in the table above, the distributor may propose additional protections or waive some protection, as long as it is technically justified, based on specific characteristics of the accessed distribution system, at no cost to the distributed microgeneration. It is important to emphasize that the parallelism of photovoltaic plant installations with the distributor system cannot cause technical or safety problems to other users, to the accessed distribution system and to the personnel involved in its operation and maintenance. Below are listed the main functions for connecting photovoltaic plants required by PRODIST and the supply standards of the concessionaires. These protection functions do not necessarily have to be performed by the same device, some of them are applied, for example, in the frequency inverter and others in the protection relay, which can act on the medium voltage circuit breaker. Undervoltage protection function (27): This function is responsible for monitoring phase undervoltages. It is agreed that undervoltage occurs when the nominal voltage drops to values below 80%, thus establishing a minimum acceptable phase-to-phase voltage. Overvoltage protection function (59): This function is responsible for monitoring phase overvoltages. It is agreed that an overvoltage occurs when the nominal voltage rises to values above 115%, thus establishing a maximum acceptable phase-to-phase voltage. Underfrequency and overfrequency (81U/O): This function disconnects the Photovoltaic Plant from the utility grid when the grid frequency fluctuates outside the working range of the inverters. It acts when the system frequency deviates from the nominal values for a certain time, at a certain value and/or rate of variation. Phase current imbalance protection function (46): This function monitors phase currents to prevent phase imbalances from occurring to the point of generating load imbalance. Voltage reversal and unbalance protection function (47): This function monitors the order in which the voltage phasors reach their peak value. This function does not imply adjustment values, only that the ideal phase sequence for the application is enabled in the relay, so that it operates on the circuit breaker if the sequence changes, preventing the operational logic of the other protection functions from being compromised. Instantaneous phase overcurrent (50): This function monitors the instantaneous phase current, taking into account the inrush current, as it is not desirable for it to act on the circuit breaker every time the transformer is energized. Instantaneous neutral overcurrent (50N): This function monitors the instantaneous neutral current. Phase timed overcurrent (51): This function monitors the phase current in steady state, with an operating time longer than that of the instantaneous unit. Neutral time overcurrent (51N): This function monitors the neutral current in steady state. Synchronism (25): This function monitors the synchronism between the Photovoltaic Plant inverters and the utility grid. Reconnection timeout function (62): This function monitors the system after a “disconnection” due to an abnormal grid condition, the distributed generation system cannot resume supplying power to the power grid (reconnection) for a period stipulated by the utility company. Neutral overvoltage (59N): This function monitors the neutral voltage in steady state, widely used to eliminate defects related to single-phase faults in isolated systems, high-impedance single-phase faults, phase loss or severe voltage unbalance. Phase directional overcurrent (67): This function monitors the directionality and intensity of short-circuit currents in the phase conductors. Neutral directional overcurrent (67N): This function monitors the directionality and intensity of short-circuit currents in the neutral conductor. Directional power (32): This function monitors directional overpower in the direction from the photovoltaic plant to the utility company. Normally the maximum injection power has a tolerance of 5% above the nominal power. Earth fault time overcurrent (51GS): This function monitors the earth fault current in steady state. Overcurrent with voltage restriction (51V): This function is used to shift the curve of the overcurrent functions when there is a reduction in the voltage measured by the VTs (Potential Transformers). This function aims to accelerate the tripping of the overcurrent function according to the voltage reduction. Conclusion It is important to highlight the great variation in the requirements of the different dealerships in Brazil. To obtain more precise details of the required protection functions and the parameters to be followed, it is essential to consult the specific standards of each concessionaire. This makes it possible to obtain the information needed to define the appropriate configuration for the substation and perform specific protection adjustments, both for modernizing existing primary cabins and for new installations. Article published in the 24st edition of the MagazineCanal

Denilson Lima

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