Standard AC/DC installation for villa
Use of the next3 and bypass schemes

Summary

This document is an aid for system installers. It is intended to explain the various bypass circuits that are possible when the inverter needs to be shut down or serviced, the overvoltage protection for the PV and the special parameters that must be programmed during commissioning and could be required by the DSO. 

Basic concept

The next3 is an all-in-one hybrid solar inverter (with battery) with backup functionality. This device is connected between the grid and the house, directly after the introduction and the main meter. The grid is connected to the AC-source terminal, the loads to the AC-Loads and AC-Flex terminals. An internal relay in the unit next3 opens in case of mains failure and the terminals AC-Loads and AC-Flex can be supplied from the batteries in case of main failure. This internal relay complies with the norms for the grid (double relay with contact monitoring, 4 pole break). The detection follows the requirements of the grid (voltage, frequency,  anti-islanding,...). When the grid is back, the next3 synchronizes to the voltage and closes is internal relay when the voltage is well in phase, avoiding any voltage jump.

The AC Flex output is a second output of the inverter that can be controlled (internal relay in the unit) or simply used to measure consumption separately (next3 monitors/records all current flows). 

A bypass should be installed to provide redundancy in case of failure. A double bypass can be installed if AC-Flex is used.

Example of a next3 integration to an installation with existing electrical cabinet:

• an added bypass box/table

• of surge arresters and DC protection devices for solar installations

In the (Swiss) regulations, DC and AC must be separated. The DC wiring must have is own electrical panel. Precautions must also be taken so that the unit can be stopped easily and clearly in case of problems.

Technical principle diagram of the unit 
schematic representation of the next3

Note: The relay that disconnects the feeder complies with the grid directives. The functionalities concerning the grid interaction have been certified (anti-islanding , NA protection, frequency behavior, ...). The grid standard must be selected by the installer during commissioning according to the requirements of the DSO. For Switzerland, the network operators usually require the German standard VDE-ARN4105 corresponding to the requirements of the RR-IPE/ NR7 2020.

Introduction
 

The next3 has connections and a transfer capacity (max. current flowing through the unit between AC source and AC loads) of 80Aac. This is usually more than the ampere rating of a standard house connection. So the next3 is protected by the input fuse. However, it is good to install a circuit breaker and bypass to isolate the unit in case of maintenance. Thus, the AC source input can be isolated by this circuit breaker and the AC load output can be isolated by the bypass.

The bypass comes in different types: 

• automatic or manual

• single (for AC-Load) or double (for AC-Load and AC-flex)

Note that a bypass must always pass through a fully off position and implies a switching delay and thus a micro-interruption.

The neutral conductor can be left continuous (solid neutral) or interrupted, but this must be communicated to the unit during programming, as it must manage a correct neutral-earth connection. This connection neutral-earth must comply with the installation standards of the respective country. See below for comments and diagrams.

A simple, full automatic bypass box is available from Studer-Innotec or can be made by an electrician or installed in the main distribution box of the house if space is available.

Simple manual bypass

In this case, the source switch must have a zero position that guarantees a separated position.

Examples of components that can be used for this manual source selector switch :

• HIM404 (Hager) 4P 40A

• HIM406 (Hager) 4P 63A

• HIM408 (Hager) 4P 80A

• OT40F4C (ABB) 4P 40A 
  

• … non-exhaustive list

Manual double bypass

The same type of manual double bypass can be realised with two switches if the AC-FLEX output is used. Indeed the output AC-flex and AC-Loads should not be connected together :

Automatic bypass

In this case, an automatic bypass is realised by relays controlled by the presence or absence of voltages. 

As with all source inverters, a mechanical interlock is required to ensure that no short circuit is possible. An electrical interlock of the controls guarantees a defined state. The priority command to the next3 is given by a relay K3. When the next3 provides a voltage, it is activated and switches off the network. 

In this case, the priority command is given to next3 via a K3 relay. When the next3 supplies a voltage, it is activated and switches off the mains.

Delays provided by K4 on the switching on of K2 (changeover to inverter) and K5 on the switching on of K1 (changeover to inverter) K5 on the switching on of K1 (switch to grid) guarantee real interruptions which allow loads to be demagnetised and phase jumps to be avoided. This is for cases where the inverter was running but not synchronised to the grid. The 100ms delay is sufficient.

Manual forcing on the grid is possible with a circuit breaker F2 that interrupts the  coils of K2 and K3:

An automatic bypass box of 25A/40A/60A/80A can be ordered as an accessory directly from Studer-Innotec.