Abstract: High Voltage DC (HVDC) technology is the adequate solution for economical power transmission over very long distances and also a trusted method to connect asynchronous grids or grids of different frequencies. Therewith HVDC power transmission is the only realistic alternative to AC technology. There is an increasing demand for high efficiency and high quality of power transmission world wide. In this context the modern (HVDC) gains more importance and utilization in today’s power transmission system. HVDC systems use power electronic converters for the power conversion and power quality control. This paper presents an overview of the status of HVDC systems in the world today. It then reviews the underlying technology of HVDC systems and also presents a comparison of HVDC system with those of an AC system. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s electricity system development and its advantages over AC systems.
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Key Words: VSC, CCC
It has been widely documented in the history of the electricity industry, that the first commercial electricity generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity transmission systems were also direct current systems. However, DC power at low voltage could not be transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems. Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC power at high voltages and over long distances, giving rise to HVDC transmission systems. We will discuss about the main components of HVDC systems and the technologies for HVDC such as the Natural commutated converters, Capacitor Commutated converters (CCC) and Forced commutated converters using Voltage Source Converter (VCS). The fundamental features and characteristics of high power thyristors is discussed with particular reference to its application in high voltage and high current area. Particular focus is imparted to VCS including its working principle, PWM and harmonics factors present in VCS. CCC technology is also being discussed in great detail from future point of view of HVDC Systems.
HVDC Technology and Main Components:
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion:
A] Natural Commutated Converters: Natural commutated converters are most used in the HVDC
systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is possible to build up a thyristor valve, which is able to operate at very high voltages (several hundred of kV).The thyristor valve is operated at net frequency (50 hertz or 60 hertz) and by means of a control angle it is possible to change the DC voltage level of the bridge. This ability is the way by which the transmitted power is controlled rapidly and efficiently.
B] Capacitor Commutated Converters (CCC): An improvement in the thyristor-based Commutation, the CCC concept is characterized by the use of commutation capacitors inserted
in series between the converter transformers and the thyristor valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.
C] Forced Commutated Converters: This type of converters introduces a spectrum of advantages,
Example: feed of passive networks (without generation), independent control of active and reactive
Power, Power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters:
Fig.1: HVDC System.
The GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of them have been in frequent use in industrial applications since early eighties. The VSC commutates with high frequency (not with the net frequency). The operation of the converter is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase 3 angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneously. Thus, PWM offers the possibility to control both active and reactive power independently. This makes the PWM Voltage Source Converter a close to ideal component in the transmission network. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously.
2.1 The components of an HVDC transmission system:
To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented and discussed. The three main elements of an HVDC system are: the converter station at the transmission and receiving ends, the transmission medium, and the electrodes.
Fig. 2: HVDC System and its components.
1.The converter station: The converter stations at each end are replica’s of each other and therefore consists of all the needed equipment for going from AC to DC or vice versa. The main components of a converter station are:
A] Thyristor valves: The thyristor valves can be build-up in different ways depending on the application and manufacturer. However, the most common way of arranging the thyristor valves is
in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a certain amount of series connected thyristors with their auxiliary circuits. All communication between the control equipment at earth potential and each thyristor at high potential is done with fiber optics.
B] VSC valves: The VSC converter consists of two level or multilevel converter, phase-reactors and
AC filters. Each single valve in the converter bridge is built up with a certain number of series connected IGBTs together with their auxiliary electronics. VSC valves, control equipment and cooling equipment would be in enclosures (such as standard shipping containers) which make transport and installation very easy. All modern HVDC valves are water-cooled and air insulated.
C] Transformers: The converter transformers adapt the AC voltage level to the DC voltage level and
they contribute to the commutation reactance. Usually they are of the single phase three winding type, but depending on the transportation requirements and the rated power, they can be arranged in other ways.
D] AC Filters and Capacitor Banks: On the AC side of a 12-pulse HVDC converter, current harmonics of the order of 11, 13, 23, 25 and higher are generated. Filters are installed in order to limit the amount of harmonics to the level required by the network. In the conversion process the converter consumes reactive power which is compensated in part by the filter banks and the rest by capacitor banks. In the case of the CCC the reactive power is compensated by the series capacitors installed in series between the converter valves and the converter transformer. The elimination of switched reactive power compensation equipment simplifies the AC switchyard and minimizes the number of
Circuit-breakers needed, which will reduce the area required for an HVDC station built with CCC.
With VSC converters there is no need to compensate any reactive power consumed by the converter itself and the current harmonics on the AC side are related directly to the PWM frequency. Therefore the amount of filters in this type of converters is reduced dramatically compared with natural commutated converters.
E] DC filters: HVDC converters create harmonics in all operational modes. Such harmonics can create disturbances in telecommunication systems. Therefore, specially designed DC filters are used in order to reduce the disturbances. Usually no filters are needed for pure cable transmissions as well as for the Back-to-Back HVDC stations. However, it is necessary to install DC filters if an OH line is used in part or all the transmission system the filters needed to take care of the harmonics generated on the DC end, are usually considerably smaller and less expensive than the filters on the AC side. The modern DC filters are the Active DC filters. In these filters the passive part is reduced to a minimum and modern power electronics is used to measure, invert and re-inject the harmonics, thus rendering the filtering very effective.
Technology used in HVDC systems:
Voltage source converters for HVDC:
The world of converters may be divided in to two groups that are to be distinguished by their operational principle. One group needs an AC system to operate and called as line commutated converters. Conventional HVDC systems employ line commutated converters. The second group of converters does not need an AC system to operate and is therefore called as self commutated converters. Depending on the design of the DC circuits this group can be further divided in to current source converters and voltage source converters. A current source converter operates with a smooth DC current provided by a reactor, while a VSC operates with a smooth DC voltage provided by storage capacitor. Among the self commutated converters it is especially the VSC that has big history in the lower power range for industrial drive applications.
Fig. 3: VSC based HVDC system using IGBT.
Basic Working Principle:
The basic function of a VSC is to convert the DC voltage of the capacitor into AC voltages. Fig 2 illustrates the basic operating principle. The polarity of the DC voltage of the converter is defined by the polarity of the diode rectifier. The IGBT can be switched on at any time by appropriate gate voltages. However if one IGBT branch is switched on, the other IGBT must have been switched off before to prevent a short circuit of storage capacitor. Reliable storage converter inter lock function will preclude unwanted switching IGBT. Alternating switching the IGBT’s of one phase module as shown successively connects the AC terminals of the VSC to the positive tapping and negative tapping of the DC capacitor. This results in a stair stepped AC voltage comprising two voltage levels +Vdc/2 and -Vdc/2. A VSC as shown is there fore called a 2 level converter.
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Due to switching frequency, that is considerably higher than the AC system power frequency the wave shape of the converter AC current will be controlled to vary sinusoidal. This is achieved by special Pulse Width Modulation. Besides the 2 level converters, so called 3 level converters have been used for high power applications.
A three level VSC provides significant better performance regarding the total harmonic voltage distortion (THD).However, the more complex converter layout resulting in the larger footprint and higher investment costs makes 2 level technology the preferred solution for HVDC from today’s point of view.
Pulse Width Modulation:
A converter is used for interconnecting two electric networks to transmit electric power from one network to the other, each network being coupled to a respective power generator station. The converter, having an AC side and a DC side, includes a bridge of semiconductor witches with gate turn-off capability coupled to a control system to produce a bridge voltage waveform having a fundamental Fourier component at the frequency of the electric network coupled to the AC side of the converter. The control system includes three inputs for receiving reference signals allowing controlling the frequency, the amplitude and the phase angle of the fundamental Fourier component with respect to the alternating voltage of the network coupled to the AC side of the converter. Through appropriate feedback loops, the converter may be used to maintain at a predetermined level the power flowing there through or to keep at a preset value the voltage across the DC terminals of the converter and, in both
Fig. 4: PWM Waveform.
cases, to maintain the frequency synchronism between the fundamental Fourier component and the alternating voltage of the network coupled to the DC side of the converter.
Characteristic of VSC – HVDC:
The principal characteristic of VSC-HVDC transmission is its ability to independently control the reactive and real power flow at each of the AC systems to which it is connected, at the Point of Common Coupling (PCC). In contrast to line-commutated HVDC transmission, the polarity of the DC link voltage remains the same with the DC current being reversed to change the direction of power flow. The 230 kV, 2000 MVA AC systems (AC system1 and AC system2 subsystems) are modeled by damped L-R equivalents with an angle of 80 degrees at fundamental frequency (50 Hz) and at the third harmonic. The VSC converters are three-level bridge blocks using close to ideal switching device model of IGBT/diodes. The relative ease with which the IGBT can be controlled and its suitability for high-frequency switching has made this device the better choice over GTO and thyristors.
3.5 Application of HVDC transmission using VSC
HVDC Light is a recent technology that utilizes Voltage Source Converters (VSC) rather than line commutated converters. HVDC Light offers advantages due to the possibility to independently control both active and reactive power HVDC Light employs Insulated Gate Bipolar transistors (IGBTs), plus other important technological developments are:
1. € High voltage valves with series-connected IGBTs
2.€ Compact, dry, high-voltage dc capacitors
3.€ capacity control system
4.€ Solid dielectric DC cable
In the HVDC Light transmission schemes, the switching of the IGBT valves follows a pulse width modulation (PWM) pattern. This switching control allows simultaneous adjustment of the amplitude and phase angle of the converter AC output voltage with constant dc, PWM pattern and the fundamental frequency voltage in a Voltage Source Converter. With these two independent control variables, separate active and reactive power control loops can be used for regulation. With these two independent control variables, separate active and reactive power control loops can be used for regulation.
Thyristor Valves Modules:
To optimize the cost of the thyristor valves (and the dc side equipment), the nominal dc current is optimized to be as close to the limit of the thyristor current rating at the permitted valve cooling limit. This permits the rated dc voltage to be kept low to achieve the rated dc power. A low dc voltage rating is beneficial for compact modular valve housing as air clearances can be kept small. The thyristor valves are air-insulated at atmospheric pressure and installed in modular valve housings. Each valve module contains two single valves, i.e. three modules for a 6-pulse converter. The thyristor valves are suspended from the ceiling and easily accessible for maintenance purposes. The surge arresters across the valves are also included in the housings.
To minimize the cost of the valves and dc equipment, the dc current is kept as high as possible within the thermal capabilities of the thyristor and cooling system. This means that for a given dc power, the dc voltage can be maintained low permitting a small air clearance requirement resulting in a compact modular design for the valve housing. The thyristor valves are air insulated at atmospheric pressure and installed in modular valve housings. Each valve module contains either two or more single valves, which implies up to 6 valve modules per 12 pulse converter. The thyristor valves are suspended from the ceiling and are easily accessible for maintenance purposes. The housing also contains the surge arresters connected across the valves.
Capacitor Commutated Converters:
The CCC is characterized by having capacitors inserted in between the converter transformer and the converter valves. Thus, this capacitor is in series with the leakage impedance of the transformer and the main valves. This has a two-fold effect:
1. The capacitor provides a forced commutation facility to the main valves (as explained earlier in another chapter), and
2. The capacitor compensates for the leakage inductance (or reactive power demand) of the converter transformer.
Sizing of the commutation capacitor, therefore, becomes a very important criteria as it impacts on the above two effects. A too-small capacitor will cause a large overvoltage across the capacitor (and valves), and not compensate sufficiently for the leakage inductance to result in a lagging current
drawn from the ac bus. A too-large capacitor will result in low over voltages and over-compensate for the demanded reactive power and might even draw a leading current from the ac system.
The major motive for using the converter concept with series capacitors is to provide additional commutation voltage and for the reduction of reactive power. In contrast to HVDC-converters using the conventional design, with switchable shunt-capacitor banks on the primary converter-transformer side, here the capacitors are connected in series directly between the transformer and the converter-bridge. Due to their electrical position, the series capacitors directly take influence on the voltage of the commutation circuit and therefore to the commutation process itself.
In this paper, we have presented the analysis of High voltage DC transmission using VSC, the number of advantages associated with implementing VSC-based designs for HVDC applications that result in systems with high reliability and superior operating performance; these benefits including economic, environmental or technical aspects. Of particular note today is the ability to control power flow and prevent propagation of severe disturbances, thus limiting blackout extension. This ability to maintain in dependence of interconnected networks can be of prime importance when the two systems have different regulatory procedures, notably if two counties, and also technically if the load frequency control regimes are not compatible .These properties are further enhanced by using HVDC Light which gives independent control of reactive power at both stations, in addition to active power flow control.
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