HVDC Transmission

Hi friends as we all know today's life is completely dependent on electricity and power is not generated in every house but we use the power being transmitted from generating station situated far away and connected in a grid. Transmission lines plays most important role between generating station  to distribution substations. there is two methodology to transmit power first is HVAC stands for High voltage AC transmission and other one which is most famous these days for long transmission lines is HVDC which is High voltage DC transmission. Here i ma just talking about some aspects of HVDC transmission.

INTRODUCTION

Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20^th Century, led to greater appeal and use of a.c. transmission giving flexibility of frequency change and ease in changing of voltage level.

D.c. transmission now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV. The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland.

Thyristors were applied to d.c. transmission in the late 1960's and solid state valves became a reality. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme in Brazil. D.c.

WHY USE DC TRANSMISSION?

Now the question arrises, "Why we use d.c. transmission?"

The suitable answer for this question may be given like following.When converters are used for d.c. transmission in preference to a.c. transmission, it is generally by economic choice driven by one of the following reasons:

1.         An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact.

2.         If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible.

3.         Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. This occurs in Japan where half the country is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two together by direct a.c. methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous.

CONFIGURATIONS

The integral part of an HVDC power converter is the valve or valve arm. It may be non-controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series. The standard bridge or converter connection is defined as a double-way connection comprising six valves or valve arms. When electric power flows into the d.c. valve group from the a.c. system then it is considered a rectifier. If power flows from the d.c. valve group into the a.c. system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules.

Figure 1. Standard graphical symbols for valves and bridges

Figure 2. Electric circuit configuration of the basic six pulse valve group with its converter transformer in star-star connection.

Thyristor Module

A thyristor or valve module is that part of a valve in a mechanical assembly of series connected thyristors and their immediate auxiliaries including heat sinks cooled by air, water or glycol, damping circuits and valve firing electronics. A thyristor module is usually interchangeable for maintenance purposes and consists of electric components.

Figure 4. Components of the thyristor modules which make up a valve or quadrivalve.

Substation Configuration

The central equipment of a d.c. substation are the thyristor converters which are usually housed inside a valve hall. other essential equipment in a d.c. substation in addition to the valve groups include the converter transformers. Their purpose is to transform the a.c. system voltage to which the d.c. system is connected so that the correct d.c. voltage is derived by the converter bridges. For higher rated d.c. substations, converter transformers for are usually comprised of single phase units which is a cost effective way to provide spare units for increased reliability.

The secondary or d.c. side windings of the converter transformers are connected to the converter bridges. The converter transformer is located in the switchyard, and  the converter bridges are located in the valve hall, the connection has to be made either of two ways. Firstly, with phase isolated busbars, where the bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating medium or secondly, with wall bushings

Harmonic filters are required on the a.c. side and usually on the d.c. side. A.c. A.c. side harmonic filters may be switched with circuit breakers or circuit switches to accommodate reactive power requirement strategies since these filters generate reactive power at fundamental frequency. A parallel resonance is naturally created between the capacitance of the a.c. filters and the inductive impedance of the a.c. system.

Characteristic d.c. side voltage harmonics generated by a 6 pulse converter are of the order 6n and when generated by a 12 pulse converter, are of the order 12n. D.c. side filters reduce harmonic current flow on d.c. transmission lines to minimize coupling and interference to adjacent voice frequency communication circuits.

D.c. reactors are usually included in each pole of a converter station. They assist the d.c. filters in filtering harmonic currents and smooth the d.c. side current so that a discontinuous current mode is not reached at low load current operation. Because rate of change of d.c. side current is limited by the d.c. reactor, the commutation process of the d.c. converter is made more robust.

Surge arresters across each valve in the converter bridge, across each converter bridge and in the d.c. and a.c. switchyard are coordinated to protect the equipment from all overvoltages regardless of their source. They may be used in non-standard applications such as filter protection. Modern HVDC substations use metal-oxide arresters and their rating and selection is made with careful insulation coordination design.

APPLICATIONS OF HVDC

The first application for HVDC was to provide point to point electrical power interconnections between asynchronous a.c. power networks. There are other applications which can be met by HVDC transmission which include:

1.         Interconnections between asynchronous systems.

2.         Deliver energy from remote energy sources.

3.         Import electric energy into congested load areas.

4.         Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission.

5.     Power flow control. A.c. networks do not easily accommodate desired power flow control.

6.     Stabilization of electric power networks. Some wide spread a.c. power system networks operate at stability limits well below the thermal capacity of their transmission conductors. HVDC transmission is an option to consider to increase utilization of network conductors along with the various power electronic controllers which can be applied on a.c. transmission.

HVDC Converter Arrangements

HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. Various ways HVDC transmission is used are:

1.         Back-to-Back

2.         Transmission Between Two Substations.

3.     Multiterminal HVDC Transmission System.

4.         Unit Connection. When d.c. transmission is applied right at the point of generation.

5.     Diode Rectifier. It has been proposed that in some applications where d.c. power transmission is in one direction only, the valves in the rectifier converter bridges can be constructed from diodes instead of thyristors.

ENVIRONMENTAL CONSIDERATIONS

The electrical environmental effects from HVDC. transmission lines can be characterized by field and ion effects as well as corona effects. The electric field arises from both the electrical charge on the conductors and for a HVDC overhead transmission line, from charges on air ions and aerosols surrounding the conductor. These give rise to d.c. electric fields due to the ion current density flowing through the air from or to the conductors as well as due to the ion density in the air. A d.c. magnetic field is produced by d.c. current flowing through the conductors. Air ions produced by HVDC lines form clouds which drift away from the line when blown by the wind and may come in contact with humans, animals and plants outside the transmission line right-of -way or corridor. The corona effects may produce low levels of radio interference, audible noise and ozone generation.

Field and corona effects

The field and corona effects of transmission lines largely favor d.c. transmission over a.c. transmission. The significant considerations are as follows:

1.      For a given power transfer requiring extra high voltage transmission, the d.c. transmission line will have a smaller tower profile than the equivalent a.c. tower carrying the same level of power.

2.         The static and steady electric field from d.c. transmission at the levels experienced beneath lines or at the edge of the right-of-way have no known adverse biological effects.

3.         The ion and corona effects of d.c. transmission lines lead to a small contribution of ozone production to higher naturally occurring background concentrations.

Commutation

Rectification or inversion for HVDC converters is accomplished through a process known as line or natural commutation. The valves act as switches so that the a.c. voltage is sequentially switched to always provide a d.c. voltage. With line commutation, the a.c. voltage at both the rectifier and inverter must be provided by the a.c. networks at each end and should be three phase and relatively free of harmonics. As each valve switches on, it will begin to conduct current while the current begins to fall to zero in the next valve to turn off. Commutation is the process of transfer of current between any two converter valves with both valves carrying current simultaneously during this process.

Consider the rectification process. Each valve will switch on when it receives a firing pulse to its gate and its forward bias voltage becomes more positive than the forward bias voltage of the conducting valve. The current flow through a conducting valve does not change instantaneously as it commutates to another valve because the transfer is through transformer windings.

Figure 6. Voltage and current waveshapes associated with d.c. converter bridges.

Voltage dependent current order limit (VDCOL)

During disturbances where the a.c. voltage at the rectifier or inverter is depressed, it will not be helpful to a weak a.c. system if the HVDC transmission system attempts to maintain full load current. A sag in a.c. voltage at either end will result in a lowered d.c. voltage too. The controller which reduces the maximum current order is known as a voltage dependent current order limit or VDCOL (sometimes referred to as a VDCL). Figure below is a schematic diagram of how d.c. transmission system controls are usually implemented.

A.c. voltage control

It is desirable to rigidly maintain the a.c. system and commutating bus voltage to a constant value for best operation of the HVDC transmission system. This is more easily achieved when the short circuit ratio is high. voltage controller is required for the following reasons:

1.         To limit dynamic and transient overvoltage to within permissible limits defined by substation equipment specifications and standards.

2.         To prevent a.c. voltage flicker and commutation failure due to a.c. voltage fluctuations when load and filter switching occurs.

3.         To enhance HVDC transmission system recovery following severe a.c. system disturbances.

4.         To avoid control system instability, particularly when operating in the extinction angle control mode at the inverter.

Hence we find that for long distance transmission HVDC is more economical and environmental friendly.

Thanks for reading this, waiting for your valuable suggestions.