Converter At the heart of an
HVDC converter station, the equipment that performs the conversion between AC and DC is referred to as the
converter. Almost all HVDC converters are inherently capable of converting from AC to DC (
rectification) and from DC to AC (
inversion), although in many HVDC systems, the system as a whole is optimized for power flow in only one direction. Irrespective of how the converter itself is designed, the station that is operating (at a given time) with power flow from AC to DC is referred to as the
rectifier and the station that is operating with power flow from DC to AC is referred to as the
inverter. Early HVDC systems used electromechanical conversion (the
Thury system) but all HVDC systems built since the 1940s have used electronic converters. Electronic converters for HVDC are divided into two main categories: • Line-commutated converters • Voltage-sourced converters
Line-commutated converters Most of the HVDC systems in operation today are based on line-commutated converters (LCCs). The basic LCC configuration uses a three-phase
bridge rectifier known as a
six-pulse bridge, containing six electronic switches, each connecting one of the three phases to one of the two DC rails. A complete switching element is usually referred to as a
valve, irrespective of its construction. However, with a phase change only every 60°, considerable
harmonic distortion is produced at both the DC and AC terminals when this arrangement is used. An enhancement of this arrangement uses 12 valves in a
twelve-pulse bridge. The AC is split into two separate three-phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, and the other a delta secondary, establishing a 30° phase difference between the two sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30°, and harmonics are considerably reduced. For this reason, the twelve-pulse system has become standard on most line-commutated converter HVDC systems built since the 1970s. With line commutated converters, the converter has only one degree of freedom the
firing angle, which represents the time delay between the voltage across a valve becoming positive (at which point the valve would start to conduct if it were made from diodes) and the thyristors being turned on. The DC output voltage of the converter steadily becomes less positive as the firing angle is increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages. The practical upper limit for the firing angle is about 150–160° because above this, the valve would have insufficient turnoff time. Early LCC systems used
mercury-arc valves, which were rugged but required high maintenance. Because of this, many mercury-arc HVDC systems were built with bypass switchgear across each six-pulse bridge so that the HVDC scheme could be operated in six-pulse mode for short maintenance periods. The last mercury arc system, at the
HVDC Inter-Island link, was decommissioned on 1 August 2012. The
thyristor valve was first used in HVDC systems in 1972. The thyristor is a solid-state
semiconductor device similar to the
diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. Because the voltages in HVDC systems, up to 800 kV in some cases, far exceed the
breakdown voltages of the thyristors used, HVDC thyristor valves are built using large numbers of thyristors in series. Additional passive components such as grading
capacitors and
resistors need to be connected in parallel with each thyristor in order to ensure that the voltage across the valve is evenly shared between the thyristors. The thyristor plus its grading circuits and other auxiliary equipment is known as a
thyristor level. between the North and South Islands of
New Zealand. The man at the bottom gives scale to the size of the valves. Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different (high) potential with respect to earth. The command information to turn on the thyristors therefore cannot simply be sent using a wire connection it needs to be isolated. The isolation method can be magnetic but is usually optical. Two optical methods are used: indirect and direct optical triggering. In the indirect optical triggering method, low-voltage control electronics send light pulses along optical fibers to the
high-side control electronics, which derives its power from the voltage across each thyristor. The alternative direct optical triggering method dispenses with most of the high-side electronics, instead using light pulses from the control electronics to switch
light-triggered thyristors (LTTs). In a line-commutated converter, the DC current (usually) cannot change direction; it flows through a large inductance and can be considered almost constant. On the AC side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the AC network. For this reason, a line commutated converter for HVDC is also considered as a
current-source inverter.
Voltage-sourced converters Because thyristors can only be turned on (not off) by control action, the control system has only one degree of freedom – when to turn on the thyristor. This is an important limitation in some circumstances. With some other types of semiconductor devices such as the
insulated-gate bipolar transistor (IGBT), both turn-on and turn-off can be controlled, giving a second degree of freedom. As a result, they can be used to make
self-commutated converters. In such converters, the
electric polarity of DC voltage is usually fixed and the DC voltage, being
smoothed by a large capacitance, can be considered constant. For this reason, an HVDC converter using IGBTs is usually referred to as a
voltage-sourced converter. The additional controllability gives many advantages, notably the ability to switch the IGBTs on and off many times per cycle in order to improve the harmonic performance. Being self-commutated, the converter no longer relies on synchronous machines in the AC system for its operation. A voltage-sourced converter can therefore feed power to an AC network consisting only of passive loads, something which is impossible with LCC HVDC. HVDC systems based on voltage-sourced converters normally use the six-pulse connection because the converter produces much less harmonic distortion than a comparable LCC and the twelve-pulse connection is unnecessary. Most of the VSC HVDC systems built until 2012 were based on the
two-level converter, which can be thought of as a six-pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes and the DC smoothing reactors have been replaced by DC smoothing capacitors. Such converters derive their name from the discrete, two voltage levels at the AC output of each phase that correspond to the electrical potentials of the positive and negative DC terminals.
Pulse-width modulation (PWM) is usually used to improve the harmonic distortion of the converter. Some HVDC systems have been built with
three-level converters, but today most new VSC HVDC systems are being built with some form of
multilevel converter, most commonly the
modular multilevel converter (MMC), in which each valve consists of a number of independent converter submodules, each containing its own storage capacitor. The IGBTs in each submodule either bypass the capacitor or connect it into the circuit, allowing the valve to synthesize a stepped voltage with very low levels of harmonic distortion.
Converter transformers , are shown on the left. The line-winding bushing projects vertically upwards at center-right At the AC side of each converter, transformers—often a bank of three single-phase transformers—isolate the station from the AC supply to provide a local earth and to achieve the correct DC voltage. Converter transformers for LCC HVDC schemes are quite specialized because of the high levels of harmonic currents that flow through them, and because the secondary winding insulation experiences a permanent DC voltage, which affects the design of the insulating structure inside the tank. In LCC systems, the transformers also need to provide the 30° phase shift required for harmonic cancellation. Converter transformers for VSC HVDC systems are usually simpler and more conventional in design than those for LCC HVDC systems.
Reactive power A major drawback of HVDC systems using line-commutated converters is that the converters inherently consume
reactive power. The AC current flowing into the converter from the AC system lags behind the AC voltage so that, irrespective of the direction of active power flow, the converter always absorbs reactive power, behaving in the same way as a
shunt reactor. The reactive power absorbed is at least under ideal conditions and can be higher than this when the converter is operating at higher than usual firing or extinction angle, or reduced DC voltage. Although at HVDC converter stations connected directly to
power stations some of the reactive power may be provided by the generators themselves, in most cases the reactive power consumed by the converter must be provided by banks of shunt
capacitors connected at the AC terminals of the converter. The shunt capacitors are usually connected directly to the grid voltage but in some cases may be connected to a lower voltage via a tertiary winding on the converter transformer. Since the reactive power consumed depends on the active power being transmitted, the shunt capacitors usually need to be subdivided into a number of switchable banks (typically four per converter) in order to prevent a surplus of reactive power being generated at low transmitted power. The shunt capacitors are almost always provided with tuning reactors and, where necessary, damping resistors so that they can perform a dual role as
harmonic filters. VSCs, on the other hand, can either produce or consume reactive power on demand, with the result that usually no separate shunt capacitors are needed (other than those required purely for filtering).
Harmonics and filtering All electronic power converters generate some degree of harmonic distortion on the AC and DC systems to which they are connected, and HVDC converters are no exception. With the recently developed MMCs, levels of harmonic distortion may be practically negligible, but with line-commutated converters and simpler types of VSCs, considerable harmonic distortion may be produced on both the AC and DC sides of the converter. As a result, harmonic filters are nearly always required at the AC terminals of such converters, and in HVDC transmission schemes using overhead lines, may also be required on the DC side.
Filters for line-commutated converters The basic building block of a line-commutated HVDC converter is the
six-pulse bridge. This arrangement produces very high levels of harmonic distortion. It is very costly to provide harmonic filters capable of suppressing such harmonics, so a variant known as the
twelve-pulse bridge, consisting of two six-pulse bridges in series with a 30° phase shift between them, is nearly always used. The task of suppressing harmonics from this arrangement is still challenging, but manageable. Line-commutated converters for HVDC are usually include combinations of harmonic filters designed to deal with the 11th and 13th harmonics on the AC side, and 12th harmonic on the DC side. Sometimes, high-pass filters are included to deal with 23rd, 25th, 35th, 37th... on the AC side and 24th, 36th... on the DC side. Sometimes, the AC filters provide damping at lower-order,
noncharacteristic harmonics such as 3rd or 5th harmonics. The task of designing AC harmonic filters for HVDC converter stations is complex and computationally intensive, since in addition to ensuring that the converter does not produce an unacceptable level of voltage distortion on the AC system, it must be ensured that the harmonic filters do not resonate with some component elsewhere in the AC system. Detailed knowledge of the
harmonic impedance of the AC system, at a wide range of frequencies, is needed in order to design the AC filters. DC filters are required only for HVDC transmission systems involving overhead lines. Voltage distortion is not a problem in its own right, since consumers do not connect directly to the DC terminals of the system, so the main design criterion for the DC filters is to ensure that the harmonic currents flowing in the DC lines do not induce interference in nearby open-wire
telephone lines. With the rise in digital mobile
telecommunications systems, which are much less susceptible to this type of interference, and
fiber optic communication which is immune, DC filters are becoming less important for HVDC systems.
Filters for voltage-sourced converters Some types of voltage-sourced converters may produce such low levels of harmonic distortion that no filters are required. However, converter types such as the
two-level converter, used with
pulse-width modulation (PWM), still require some filtering, albeit less than on line-commutated converter systems. In two-level converters, the harmonic spectrum is shifted to higher frequencies compared to line-commutated converters. The dominant harmonic frequencies are
sidebands of the PWM frequency and multiples thereof. In HVDC applications, the PWM frequency is typically around 1 to 2 kHz. The higher frequencies allow the filter equipment to be smaller. ==Configurations==