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4.5% Individual Current Harmonics in Percent of IL

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4.5% Individual Current Harmonics in Percent of IL

Column CM Column CN

harm_volt

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Individual Voltage Harmonics in Percent of the PCC voltage

Column CT Column CU

POWER :15MVA

Ie (Estimated Waveform) Harmonic Calculator WEG

Voltage at PCC (rms) Volts

Case Study: Company Name

Motor Voltage (rms) IL 1137.2 Amp

Motor Rated Current (rms)THDv1.70% Isc / IL14.4

6 pulse 12 pulse

18 pulse

VFD Rectifier Estimated Results

Motor

Graphs (PCC)

ctrl thdv0 ctrl tdd0 soma ctrl0 0 amarelo 0 vermelho 0

Harmônicas de Corrente

Ie - spectrumVpcc - spectrum

Vpcc - spectrum

Gráficos

Corrente Isc_IL 14.42308 Tensão Pulsos 6 Pulsos 6 harmAmp% of ILLimite IEEEharmAMP_V

Joable Andrade Alves & Paulo José Torri WEG Automação S.A.

Introduction

Figure 1 - six-pulse rectifier Figure 2 - Voltage distortion caused by currents drained by a non-linear load.

Where: h : harmonic order

 IEEE-519 Recommended Practices for Applications with

Variable Frequency Drives

The indirect frequency converters or variable frequency drives (VFDs) are usually composed of an input diode rectifier stage. The three-phase full-wave bridge rectifier, also known as Graetz Bridge or six-pulse rectifier (Figure 1) is one of the most used configurations for the input stage of VFDs. The currents drained by this converter are of pulsed-type, containing low-order harmonics that may cause voltage harmonic distortion at the electric power supply system where VFDs are connected. The voltage distortion is related to the magnitude of the current harmonics of the electrical equipment and also to the network impedance, i.e., to the network capacity and to the ratio between the installed drives capacity and the network capacity.

In several applications, the six-pulse variable frequency drive with an input line reactor or a DC reactor may perfectly meet the IEEE-519 recommendations. When it is not possible, the following options are available for reducing the harmonic currents: 1) Increase the number of pulses of the input rectifier using 12, 18 or even 24-pulse; 2) Use an active front-end rectifier (regen-drive).

The greater the number of pulses of the rectifier, the lower the current harmonic content at low frequencies. The characteristic harmonics can be represented as follows:

 n = 1, 2, 3, 4, ∞

P : number of rectifier pulses

At the same time that the harmonic currents at low frequencies are reduced, there are other disadvantages in increasing the number of rectifier pulses such as: the need of using phase-shifting transformers and increase on the cost of the configuration. Particular attention needs to be given to the increase on the equipment complexity and to the reduction on the efficiency and reliability of the whole system (due to the use of a larger number of components).

D1 D2 D3 D4 D5 D6

Multipulse Configuration

The characteristic harmonics of the six-pulse rectifier are given by: The magnitudes of these harmonics, in respect of the fundamental, are given by:

Figure 3 - Theoretic harmonic spectrum of the input current for a six-pulse rectifier.

In the industry, the term “multipulse”, when related to frequency converters, means the association, in series or in parallel, of six-pulse three-phase rectifiers and the mandatory use of phase-shifting transformers to feed the rectifiers. The fundamental idea of the multipulse configurations can be understood as the interconnection of six-pulse rectifiers so that the characteristic harmonics generated by these rectifiers are cancelled by the harmonics generated by other sets of rectifiers. This mitigation is performed by appropriate design of the phase-shifting transformer with multiple secondaries. The harmonics of the six-pulse rectifier that are present on the secondary of the transformer will be cancelled and will not appear at the primary of the transformer that is connected to the utility.

The following figure shows the spectrum of these harmonics and their magnitude (as a percentage of the fundamental).

The increase in the number of the rectifier pulses and the use of an appropriate phase-shifting transformer allow the mitigation of the harmonic components at the primary of the transformer. Figure 4 shows the expected typical waveforms of the input currents and their respective harmonic spectrum to the 6, 12 and 18-pulse configurations.

It is important to emphasize that the harmonics multiple of the 3rd are cancelled in a balanced three-phase system (zero sequence).

In practice, however, the non-characteristic harmonics usually present a magnitude different from zero. This is due to the pre-existing imperfections of the electrical system, such as, voltage unbalance, pre-existing harmonic distortion, etc. Another factor that contribute to the appearance of non-characteristic harmonics is the transformer imperfections, which blocks the mitigation of non-characteristic harmonics at the primary of the transformer. The data presented in the following figures were obtained considering an ideal system. In the numerical simulation the system was modeled as: power electric system, input impedance, rectifier, capacitive filter and load (current source).

6-pulse 12-pulse

Figure 4 - Typical input currents for the 6-pulse recifier or at the primary of the phase-shifting transformer input (12 and 18-pulse) and their respective harmonic spectrum (harmonic components expressed as a percentage of the fundamental).

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