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HVDC Notes CH 2

This document provides an analysis of HVDC converters and their control, focusing on the configuration and operation of six-pulse converters. It discusses key concepts such as pulse number, converter configuration choices, and the analysis of Graetz circuits both with and without overlap. The document also covers the relationship between AC and DC power, as well as the behavior of current in the converters.

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10 views4 pages

HVDC Notes CH 2

This document provides an analysis of HVDC converters and their control, focusing on the configuration and operation of six-pulse converters. It discusses key concepts such as pulse number, converter configuration choices, and the analysis of Graetz circuits both with and without overlap. The document also covers the relationship between AC and DC power, as well as the behavior of current in the converters.

Uploaded by

prabhasshorts
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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UNIT-II

ANALYSIS OF HVDC CONVERTERS AND CONTROL

1.1 Introduction
Electric power transmission was originally developed (Introduction and also list of
assumptions for analysis )

1.2 Pulse Number (P)


Commutation group (q)
Valve groups in series (s)
Valve groups in parallel (r)
P = qsr

1.3 Choice of Converter Configuration


Desired features of converter circuit
• High Pulse Number
• Valve Utilization (PIV/Vdo) should be near unity
• Vdo/E should be high
• Transformer Utilization Factor should be near unity

Secondary RMS Power S


TUF = = t
DC Power Vdo I d

1.4 Analysis of Six-pulse Converter


The conversion from AC to DC and vice versa is done in the HVDC converter stations by
using 3-ph bridge converters.
Analysis of Graetz Circuit without overlap:
At any instant, two valves are conducting in the bridge, one from the upper commutation group
and the second from the lower commutation group. The firing of the next valve in a particular
group results in the turning OFF of the valve that is already conducting. The valves are
numbered in the sequence in which they are fired. Each valve conducts for 120o and the interval
between consecutive firing pulse is 60o in steady state. The following assumptions are made
to simplify the analysis

a. The DC current is constant.


b. The valves are modelled as ideal switches with zero impedance when ON and with
infinite impedance when OFF.
c. The AC voltages at the converter bus are sinusoidal and remain constant.
Average DC voltage:

Figure: DC voltage expression and the valve -1 voltage


❖ Vd = Vdo cos 

3 2
Vdo = VLL

AC Current Waveform:
It is assumed that direct current has no ripple (or harmonics). The AC currents flowing through
the valve (secondary) and primary windings of the converter transformer contains harmonics.

6
• RMS Value of fundamental component: I1 = Id

I1
• RMS value of the hth - harmonics ( h = 6n  1 ): I h =
h
2
• RMS value of the current: I = Id
3
AC Current Waveform:
AC-power supplied to the converter
Pac = 3VLL I1 cos 

The DC power must match the AC power ignoring the losses in the converter.
Pac = Pdc = Vd I d = 3VLL I1 cos 

3 2 6
VLL  cos   I d = 3VLL   I d  cos 
 
❖ cos  = cos 
Analysis of Graetz Circuit with overlap:
The shift of current from one valve to another valve is not sudden duo to leakage inductance
of the converter transformer. The duration when the current is shared by two conducting valves
of a same commutation group is called ‘overlap’ and it’s duration is measured by ‘overlap
angle (u)’.

Analysis of two and three valve conduction mode:


• Two valves conduct for 60-u.
• Three valves conduct for u.
Assume in Graetz circuit valve-3 needs to be fired and valve 1 and 2 are in conduction. The
circuit can be redrawn as

I1 + I 3 = I d

(Check the running Notes for next topics)

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