IEC TS 61800-8:2010 pdf – Adjustable speed electrical power drive systems -Part 8: Specification of voltage on the power interface.
6 Input converter section
6.1 Analysis of voltages origins
The low frequency grounding potential of the inverter output terminals is determined by the DC link voltage (Vd) and the reference potential of the DC link voltage (VGI) (see Figure 5.)
Grounding potential of the converter output terminals = V01 ± VI2 (18)
When the upper side switch of inverter is switched on. the grounding potential VGI + Vd12 appears at the output of the converter. And if the lower side switch of inverter is switched on. the grounding potential V1 – VI2 appears at the output of the inverter.
6.1.1 The DC link voltage of converter section (Vd)
The DC link voltage is mainly determined by the type of rectifier and by the filtering effect of the impedance at supply line andior DC line and the large DC capacitor. The DC voltage ripple is usually negligible.
The DC link voltage is affected by the following items;
— Type of rectifier (single phase diode, three phase diode, active converter):
— Type of inverter (single phaseithree phase and withlwithout DC brake);
— Line side commutation impedance:
— Load
6.1.2 The reference potential of NP of the DC link voltage
The reference potential VGI of the DC link voltage Is usually very close to the grounding potential, If a TN or IT line side (see Clause 5) grounding system is appl,ed or the neutral point of the DC capacitor Is grounded by some means. Even if a non-grounded (IT) supply system is applied, the average value of V01 may remain close to grounding potential. But It Is also influenced by the grounding impedance of output filter, cable and motor.
The following items may affect the reference potential V01 of the DC link voltage:
— Grounding system of line section;
— Arrangement of input filter and DC reactor;
— Grounding system of converter;
— Grounding impedance of output filter and cable:
— Grounding impedance of motor;
— Switching condition of converter.
6.2 IndIrect converter of the voltage sourc, type, with single phase diode rectifier as line sIde converter
62.1 Voltage source inverter (VSI) with single phase diode rectifier
6.21.1 General
The single phase diode rectifier systems are categorised in the following three supply cases, when line side grounding system is taken into consideration.
Figure 15. Figure 16 and Figure 17 show the configuration of voltage source inverters supplied by L and N for a TN or TT system, supplied by Li and L2 tot TN or TT system and supplied by Li and L2 for IT system, respectively.
7Output converter section (inverter section)7.1General
N-Level inverters have different possibilities of switching strategies, as e.g. applying onlysingle voltage steps or by using redundant switching states with lowest common modevoltage.This might be discussed between system integrator,converter and motor supplier.This document describes in general the worst-case.
7.2Input value for the inverter section
The input value for the inverter section is the averaged DC link voltage vg-vg is determinedby the input converter section (see Clause 6).
7.3Description of different inverter topologies
This section describes the most commonly used topologies of inverters with a DC linkcapacitor bank (or several capacitor banks), i.e. voltage source inverters. The number oflevels N is equal to the number of possible voltages of the output phase to NP-Potential.
7.3.3 N-level inverter7.3.3.1General
Two different approaches are used to achieve multi level voltage source inverters.Twoexamples are shown below. The differences regarding the output section are small. Bothapproaches can be extended to several levels by adding more stages.
Figures 29 and 30 show the simplest way of the idea of the N-Level inverter, which is a threelevel inverter. ln practice three level inverters are typically built as described in Figure 28,where the N-level inverter topologies are used to achieve more levels.