HVDC UNIT-II.pptx
- 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.
- 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 )