HVDC

Slide 1 : Presented by M.Venkateshkumar Lecturer , Dept of EEE AVIT, Chennai. HVDC Transmission

HVDC : HVDC 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 systems. For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.

HVDC : HVDC The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden at ASEA. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 10-20 MW system between Gotland and mainland Sweden in 1954.[1] The longest HVDC link in the world is currently the Inga-Shaba 1,700 km (1,100 mi) 600 MW link connecting the Inga Dam to the Shaba copper mine, in the Democratic Republic of Congo.

The major advantages are: : The major advantages are: If the cost of converter station is excluded, the dc overhead lines and cables are less expensive than ac lines and cables. A dc link is asynchronous. The corona loss and radio interference are less. For dc line, reactive power compensation is not needed. However, reactive power support will be required at both ends as explained later. The line length is not restricted by stability.The interconnection of two separate ac systems via a dc link does not increase the short-circuit capacity, and thus the circuit breaker ratings of either system. The dc line loss is smaller than for the comparable ac line.

The major disadvantages are: : The major disadvantages are: The converters generate harmonic voltages and currents on both dc and ac sides and therefore filters are needed. The converter consumes reactive power.The dc converter stations are expensive.The dc circuit breakers are difficult to design.

Types of HVDC Configuration : Types of HVDC Configuration 1. Monopole and earth return 2. Bipolar 3. Homopolar

HVDC links : HVDC links

Monopole and earth return : Monopole and earth return

Monopolar : Monopolar Monopolar link has a one conductor ( ground or return) A Metallic return can also be used where concerns for harmonics and or / corrosion exist. In application with DC cables (ie HVDC light) A Monopolar link is normally operated with negative polarity.

Bipolar : Bipolar

Bipolar : Bipolar Under normal load, negligible earth-current flows, as in the case of mono polar transmission with a metallic earth-return. This reduces earth return loss and environmental effects. When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode. Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating. In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.

Homopolar link : Homopolar link

Homopolar : Homopolar In this type of link two conductors having the same polarity (usually negative) can be operated with ground or metallic return. Due to the undesirability of operating a dc link with ground return, bipolar links are mostly used. A homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweigh the advantages.

Back to Back : Back to Back High Voltage DC technology enables the interconnection of two asynchronous AC networks. It also enables interconnection between two networks with completely different frequencies; e.g. 50Hz and 60Hz. An HVDC system takes electrical power in an alternating current (AC) system and converts it into high voltage direct current (DC) using a converter station.  It then transmits the DC to a remote system, where it is converted back again to AC by another HVDC converter station.

Slide 15 : A back-to-back HVDC arrangement is used when two asynchronous AC systems need to be interconnected for bulk power transmission or for AC system stabilization reasons. In an HVDC back-to-back station there are no overhead lines or cables separating the rectifier and the inverter, hence the transmission electrical losses on the DC side can be neglected. In a back-to-back HVDC, the DC current can be kept high and the DC voltage low. In order to optimize the costs of the thyristor valves as well as the DC side equipment, the DC current is kept as high as the standard thyristor rating and valve cooling system can handle safely. This means that the DC voltage can be kept fairly low, and is thus chosen in relation to the rated DC power.

Diagram : Diagram

Tower model : Tower model

Cost of HVDC : Cost of HVDC

Applications of DC Transmission : Applications of DC Transmission Underground or underwater cables Long distance bulk power transmission Asynchronous interconnection of ac systems Stabilization of power flows in integrated power system

Analysis of HVDC Converters : Analysis of HVDC Converters

INTRODUCTION : INTRODUCTION A HVDC system requires an electronic converter for its ability of converting electrical energy from ac-dc or vice versa. There are basically two configuration types of three-phase converters possible for this conversion process

Types of Converters : Types of Converters Current Source Converter (CSC), Voltage Source Converter (VSC).

Current Source Converter (CSC) : Current Source Converter (CSC)

Voltage Source Converter (VSC) : Voltage Source Converter (VSC)

Cont…. : Cont…. During the period (about) 1950-1990s, HVDC systems used the CSC configuration almost exclusively. The traditional CSC utilized the mercury-arc valve from the early 1950s to the mid-1970s, and thereafter, the thyristor valve as its fundamental switching device. From about 1990 onwards, the alternative VSC became economically viable due to the availability of new self-commutating high- power switches (such as GTOs and IGBTs) and the computing power of DSPs to generate the appropriate firing patterns.

Cont… : Cont… Modern HVDC transmission systems can utilize either the traditional Current Source Converter (CSC) or the Voltage Source Converter (VSC) as the basic conversion workhorse. The two converters are actually duals of one another.

Comparison of VSC and CSC : Comparison of VSC and CSC On AC side CURRENT SOURCE CONVERTER Act as a constant voltage source Require a capacitor as its energy storing device Requires large Ac filters for harmonic elimination Requires reactive power supply for power factor correction

Voltage sources converter : Voltage sources converter On AC side Act as a constant current sources Requires an inductor as its energy storing devices Requires only a small AC filter for harmonics elimination Reactive power supply is not required as a converter can operate in any quadrant.

on DC side : on DC side CSC Acts as a constant current sources Requires an inductor as its energy storing devices Requires DC filters Provides inherent fault current limiting features

On DC side : On DC side VSC Acts as a constant voltage sources Requires a capacitor as its energy storing devices Energy storage capacitor provide DC filtering capability at no extra cost Problematic for DC line side faults since the charged capacitor will discharge into fault

switches : switches CSC Line commutated or forced commutated with series capacitor Switching occurs at line frequency Lower switching losses

On AC side : On AC side VSC Self commutated Switching occurs at high frequency High switching losses

Rating range : Rating range CSC 0 – 550MW per Converter Upto 600 KV

Rating range : Rating range VSC 0 – 200 MW per Converter Upto 100 KV

Commutation : Commutation Within the context of HVDC converters, the definition of commutation is the transfer of dc current from one valve to another in the same row is termed “commutation”.

Comparing characteristics of CSC and VSC : Comparing characteristics of CSC and VSC

Graetz Bridge : Graetz Bridge Six pulse bridge and Twelve pulse bridge(two stage of six pulse bridge ) is called Graetz Bridge

Current source converter : Current source converter

CSC : CSC Where Id---- DC current Lc --- leakage inductance Ld--- smoothing reactor Case 1----no overlap period Case 2---- with overlap less than 60 degrees

Slide 40 :

Waveform of line to line and phase voltage : Waveform of line to line and phase voltage

Line to neutral emfs of the three phase AC source : Line to neutral emfs of the three phase AC source ea = Em cos(wt + 600 ) eb = Em cos(wt - 600 ) ec = Em cos (wt - 1800 ) Line – Line emfs Eac = ea - ec = 1.732 * Em cos (wt + 300 ) Eba = eb - ea = 1.732 * Em cos (wt - 900 ) Ecb = ec – eb = 1.732 * Em cos (wt +1500 ) Vd0 = ideal no load direct voltage Em = peak AC voltage , ELL – rms L-L AC voltage .

CSC : CSC

Voltage Source Converter : Voltage Source Converter

Introduction to VSC : Introduction to VSC The commercial availability of high-power and high-voltage GTO and IGBT valves in the 1990’s offered the viable operation of VSCs in HVDC schemes.

PWM generation : PWM generation Periodical sampling (using D –type Flip flop) Hysteresis band (frequency is not fixed) Triangular carrier (required PI controller)

Advantages of the VSC : Advantages of the VSC Rapid control of active as well as reactive power, It provides a high level of power quality, Minimal environmental impact, and Ability to connect to weak ac networks, or even dead networks.

Application VSC : Application VSC Low power (less than 250 MW) HVDC transmission (commercially referred to as “HVDC Light”), VAR Computation (SVC and STATCOM), and Active Filters.

Operation of VSC : Operation of VSC The operating principles of a VSC are evident. The dc side capacitor Cd and ac side inductor Lc are necessary elements of the VSC. The dc voltage Vd is monitored and compared to a reference value Vref to generate an error signal which controls the PWM controller. When the dc current Id is positive, the VSC acts as a rectifier; the dc capacitor is discharged as it feeds the dc load, and the control system will modify the firing angle to import power from the ac system.

Contd… : Contd… When the dc current Id is negative, the VSC acts as an inverter; the dc capacitor is charged from the dc source, and the control system will modify the firing angle to export power to the ac system. The VSC can also modulate the firing of the valves to control the reactive power so that a unity power factor (or any other value, for that matter) can be obtained

Waveform VSC : Waveform VSC Four quadrant operation of the VSC Rectifier operation at unity power factor Inverter operation at unity power factor Purely reactive with leading current Purely reactive with lagging current.

Slide 52 :

Result VSC waveform : Result VSC waveform

Terms and Definitions : Terms and Definitions Rectifier operation ( less than 900 ) Inverter operation (more than 900 ) Bridge converter connection Converter Arm ( forward direction ) Commutation ( transfer current b/w two paths ) Forward direction ( cathode to anode) Forward voltage ( voltage applied b/w An to Cat) Conduction state ( valve exhibits low R)

Contd… : Contd… Non conducting state (high resistance to flow of current) Gate pulse(firing pulse ) Phase control( cycle AC wave) Valve blocking (inhibiting the gate pulses) Valve (thyristor) Multiple valve unit ( two or more valve in single stru..) Reference book title HVDC and FACTS Controllers Application of static Converters in power system BY Dr. Vijay K.Sood LONDON