Group 8 hvdc

Simulation of VSC HVDC Transmission System and Fault

Analysis

DEPARTMENT OF EEE,

NATIONAL INSTITUTE OF TECHNOLOGY

TIRUCHIRAPPALLI -620015

Project MembersAaron Saldanha 107111002

Akam Singh Patel 107111011

Mahesh Bolavakar 107111026

John D. Cheerotha 107111038

GROUP 8 ,B.TECH. FINAL YR. , NIT TRICHY 2

Project Aim

Simulation of VSC based HVDC line in MATLAB / SIMULINK

Simulation and Analysis of various DC Fault Condition


This project aims at the implementation of VSC based HVDC systems, which will help bidirectional power flow along with stable operation of the system during disturbances like Faults.

A PWM control system has been designed for the sending-end of the HVDC link.

The control strategy is studied and corresponding performance is observed in MATLAB/ SIMULINK.

The simulation results verify that the PWM controller has good transient and steady state performance.


What is HVDC …?

1. A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems.

2. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses.

3. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle.

4. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links.


Types of HVDC ConvertersLine Commutated Converter (LCC)

− Current Sourced Converter

− Thyristor based Technology

Voltage Sourced Converter (VSC)

− Self Commutated Converter

− Transistor (IGBT, GTO etc.) based Technology


LCC v/s VSC


Why VSC over LCC….?LCC VSC

Use semiconductors which can

turn on by control action

Use semiconductors which can

turn on or off by control action

Requires stronger AC systems Operates into weaker AC

systems

Requires additional equipment

for “Black” start capability

“Black” start capability

Generates harmonic distortion,

AC & DC harmonic filters

required

Insignificant level of harmonic

generation, hence no filters

required

Coarser reactive power control Finer reactive power control

Large site area, dominated by

harmonic filters

Compact site area, 50 – 60% of

LCC site area

Power is reversed by changing

polarity of the converters

Power is reversed by changing

direction of current flow


What is VSC HVDC System…?VSC-HVDC is a new dc transmission system technology. It is based on the voltage source converter, where the valves are built by IGBTs and PWM is used to create the desired voltage waveform. With PWM, it is possible to create any waveform (up to a certain limit set by the switching frequency), any phase angle and magnitude of the fundamental component. Changes in waveform, phase angle and magnitude can be made by changing the PWM pattern, which can be done almost instantaneously. Thus, the voltage source converter can be considered as a controllable voltage source.


Components of VSC - HVDC

a. Physical Structure

b. Converters

c. Transformer

d. Phase Reactors

e. AC Filters

f. DC Capacitors

g. DC Cables

h. IGBT Valves

i. AC Grid


Layout of Project


HVDC allows power transmission between unsynchronized AC transmission systems.

Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power.

HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.


HVDC can transfer power between separate AC networks. HVDC power flow between separate AC systems can be automatically controlled to support either network during transient conditions, but without the risk that a major power system collapse in one network will lead to a collapse in the second.

HVDC improves on system controllability, with at least one HVDC link embedded in an AC grid in the deregulated environment, the controllability feature is particularly useful where control of energy trading is needed.


Part 1 : Three Phase Sinusoidal PWM Rectifier


Supply Voltage data Output of Three Phase Source (Vp-ground) =325kV (AC)

Output of Three Phase Source (Vp line to line) = 3X (Vp-ground)

=563kV (AC)


Supply Voltage (Phase Voltage)


Three Phase Rectifier Unit


Calculations for RectifierInput voltage to the Rectifier (Vp - AC) = 325kV (AC)

Vout = (3 3 × Vp)/π

= 1.654*Vin

= 537.5kV (DC)

This is the Theoretical value.

Output voltage of the Rectifier (Vdc) = 503kV (DC)

This is the Experimental Value.


Output of Rectifier (SIMULINK)


VDC (approx. 500kV)


IDC (approx. 4.325kA)


PDC (approx. 2175MW)


Pulse Sequence Generation


Part 2: Three Phase Inverter


Calculations for InverterInput voltage to the Inverter (Vdc) = 537.5kV

Peak value of output line voltage = 1.10266*Vdc

= (4 × Vp × cos(π/6))/π

= 592.6kV (AC Line-Line)

= 592.6kV/√3

= 342.13kV (AC Line-Ground)

This is the Theoretical value.

Output voltage of the Inverter = 580kV (AC Line-Line)

= 334.8kV (AC Line-Ground)

This is the Experimental Value .


Output of Inverter


Three Phase AC output


Line-Line AC (approx. 580kV)


Line-Ground AC (approx. 320kV)


AC Filter cum Load


DC Faults

The DC Link in subjected to fault at t=0.2 at the mid point of the link.

1) Short Duration Faults . Time Period ≤0.2s

2) Medium Duration Faults. Time Period 0.2 ≤ 0.6s

3) Long Duration Faults . Time Period ≈ 1s


a) Short Duration Fault


b) Medium Duration Fault


c) Long Duration Fault


AdvantagesA long distance point to point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme.

HVDC conversion equipment at the terminal stations is costly, but the total DC transmission line costs over long distances are lower than AC line of the same distance.

HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect.

HVDC transmission losses are quoted as about 3.5% per 1,000 km, which is less than typical AC transmission losses.


Applicationsa) Endpoint-to-endpoint long-haul bulk power transmission, usually to connect a remote generating

plant to the main grid.

b) Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.

c) Power transmission and stabilization between unsynchronized AC networks, with the extreme example being an ability to transfer power between countries that use AC at different frequencies. Since such transfer can occur in either direction, it increases the stability of both networks by allowing them to draw on each other in emergencies and failures.

d) Stabilizing a predominantly AC power-grid, without increasing fault levels.

e) Integration of renewable resources such as wind into the main transmission grid. HVDC overhead lines for onshore wind integration projects and HVDC cables for offshore projects have been proposed in North America and Europe for both technical and economic reasons. DC grids with multiple voltage-source converters (VSCs) are one of the technical solutions for pooling offshore wind energy and transmitting it to load centers located far away onshore.


Conclusion

This project work dealt with the theoretical concepts of the PWM controller scheme implemented in the simulation of VSC based HVDC system with fault analysis.

The controller has been developed and then implemented in MATLAB in order to trigger the VSC. The model used in MATLAB/SIMULINK consists of 6-pulse IGBT VSC based HVDC system. The performance of the system under normal condition and under different types of faults is comprehensively analyzed.


Thank You…!!


Group 8 hvdc

Engineering

Transcript of Group 8 hvdc