Analysis of HVDC converters

1. UNIT II
ANALYSIS OF HVDC CONVERTERS
• Line commutated converter
• Analysis of Graetz circuit with and without overlap
• Pulse number
• Choice of converter configuration
• Converter bridge characteristics
• Analysis of 12 pulse converters
• Analysis of VSC topologies and firing schemes.

2. Introduction
• Electronics converters for HVDC are divided into two main
categories.
• LCC are made with electronics switches that can only be turned on.
• VSC are made with switching devices that can be turned on and
off.
• LCC used mercury are valves until the 1970’s, thyristors from the
1970’s to the present day.
• VSC which first appeared in HVDC in 1997 use transistor usually
the insulted gate bipolar transistor(IGBT).

3. Line commutated converters:
• The basic configuration of a 3ɸ converter (both LCC and VSC) is
a bridge converter (also called Graetz Bridge) which can be fed
from transformer windings connected in star or delta.
• The converter transformer feeding a Graetz bridge serves the
objectives of providing
1.Galvanic separation between AC and DC Side.
2.Voltage transformation between AC and DC Side.
3. OLTC- Applied voltage can change.
An autotransformer can meet the last two objectives but cannot
provide galvanic separation.

4. 6 pulse converter

5. • In a LCC the switches are made of thyristors valves, whereas in a
VSC, they are made up of IGBT valves ( a series connection of IGBT
and anti-parallel connected diode).
Analysis of Graetz Bridge
• Simplify analysis of greatz circuit have 2 methods.
Analysis of Graetz bridge Without overlap

7. • LCC – SCR Switches.
• VSC – IGBT switches.
Ac side- 3Ø AC Voltage source.
DC side – DC Current source.
S1,S2....S6 SWITCHES(SCR) are used in given
rating.
SCR- A,K,G

8. • SCR- ON by Gate (A +ve and K –ve)
• SCR – OFF by Reversing Voltage.
Current flow is A to K only.
S1,S3,S5 - CURRENT CARRYING TOWARDS “P”
S2,S4,S6 - CURRENT CARRYING FROM“N”
Overlap- Neglected- but it is present due to
Inductance of Transformer.

9. Graetz circuit with overlap
• Due to leakage inductance if the converter transformer and
the impedance in the supply networks the current in a valve
cannot change suddenly and the commutation from one valve
to next cannot be instantaneous.
Mode 1: two and three valve conduction ( u < 60)
Mode 2: Three valve conduction ( u = 60)
Mode 3: Three valve four valve conduction ( u > 60)

10. Choice of converter configuration
1. Pulse number
• The number of pulsations ( cycle of ripple ) per cycle of
alternating voltage.
• The conversion from AC to DC involves switching sequentially
different sinusoidal voltages on to the DC circuit.
• The output voltage of the converter consists of a DC component
and a ripple whose frequency is determined by the pulse
number.
hdc= np
hac= np ± 1

11. 2. Valve and switches
• It can be treated as switch which can be turned on at any instant,
provided the voltage across it is positive.
• A diode is an uncontrolled switch which will turn on immediately
after the voltage becomes positive where as thyristors switching
can be delayed by an angle.
• the opening of the switch (both for diode and thyristor) occurs at
the current zero.

12. 3. Converter configuration
• The configuration for given pulse number is selected in such a
way that both the valve and transformer utilization are maximized.
• converter configuration can be defined as basic commutation
group and the number of groups connected in series and
parallel.

13. 4. Valve rating
• The valve rating is specified in terms of PIV. The ratio of PIV to
the average dc voltages is an index of the valve utilization.
𝑣𝑑𝑜 = 𝑆
𝑞
2𝜋 −𝜋
𝑞
𝜋
𝑞
𝐸𝑚 𝑐𝑜𝑠𝜔𝑡 𝑑𝜔𝑡
• −𝜋
𝑞
𝜋
𝑞
𝐸𝑚 𝑐𝑜𝑠𝜔𝑡 𝑑𝜔𝑡 =2 0
𝜋
𝑞
𝐸𝑚 𝑐𝑜𝑠𝜔𝑡 𝑑𝜔𝑡 𝑝𝑟𝑖𝑜𝑟𝑖𝑡𝑦 𝑖𝑠 𝑒𝑣𝑒𝑛
𝑣𝑑𝑜 = 2𝑆
𝑞
2𝜋 0
𝜋
𝑞
𝐸𝑚 𝑐𝑜𝑠𝜔𝑡 𝑑𝜔𝑡
= 2𝑆
𝑞𝐸𝑚
2𝜋
sin 𝜔𝑡 0
𝜋
𝑞
= 𝑆
𝑞𝐸𝑚
𝜋
𝑠𝑖𝑛 𝜋
𝑞 − 0
= 𝑆
𝑞𝐸𝑚
𝜋
𝑠𝑖𝑛 𝜋
𝑞

14. 5. Peak inverse voltage
• If q is even, 𝑃𝐼𝑉 = 2𝐸𝑚
• If q is odd, 𝑃𝐼𝑉 = 2𝐸𝑚𝑐𝑜𝑠
𝜋
2𝑞
6. Utilization factor
The ratio of PIV to the average DC voltage is known as utilization
factor.
For q is even
𝑃𝐼𝑉
𝑉𝑑𝑜
=
2𝐸𝑚
𝐸𝑚
𝑆𝑞
𝜋
𝑠𝑖𝑛
𝜋
𝑞
𝑃𝐼𝑉
𝑉𝑑𝑜
=
2𝜋
𝑆𝑞𝑠𝑖𝑛
𝜋
𝑞
𝑓𝑜𝑟 𝑞 𝑖𝑠 𝑜𝑑𝑑
𝑃𝐼𝑉
𝑉𝑑𝑜
=
𝜋
𝑆𝑞𝑠𝑖𝑛(
𝜋
2𝑞
)