voltage source converter basics of investing
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Voltage source converter basics of investing

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Also, the grid may be subject to carrying large amounts of reactive power, which is unintended and gives rise to transmission and distribution losses as well as impedes the flow of useful, active power in the grid. An electric arc furnace is a heavy consumer not only of active power, but also of reactive power.

Also, the physical process inside the furnace electric melting is erratic in its nature, with one or several electrodes striking electric arcs between furnace and scrap. As a consequence, the consumption especially of reactive power becomes strongly fluctuating in a stochastic manner. The voltage drop caused by reactive power flowing through circuit reactances in the electrodes, electrode arms and furnace transformer therefore becomes fluctuating in an erratic way, as well.

This is called voltage flicker and is visualized most clearly in the flickering light of incandescent lamps fed from the polluted grid. The problem with voltage flicker is attacked by making the erratic flow of reactive power through the supply grid down into the furnaces decrease. This is done by measuring the reactive power consumption and generating corresponding amounts in the compact STATCOM and injecting it into the system, thereby decreasing the net reactive power flow to an absolute minimum.

As an immediate consequence, voltage flicker is decreased to a minimum, as well. Important added benefits are a high and constant power factor, regardless of load fluctuations over furnace cycles, as well as a high and stable bus RMS voltage. These benefits can be capitalized as improved furnace productivity as well as decreased operational costs of the process in terms of lower specific electrode and energy consumption and reduced wear on the furnace refractory. To parry the rapidly fluctuating consumption of reactive power of the furnaces, an equally rapid compensating device is required.

This is brought about with the state of the art power electronics based on IGBT technology. With the advent of such continuously controllable semiconductor devices capable of high power handling, VSCs with highly dynamic properties have become feasible far into the MVA range. The function of the VSC in this context is a fully controllable voltage source matching the bus voltage in phase and frequency, and with an amplitude which can be continuously and rapidly controlled, so as to be used as the tool for reactive power control.

At the outputs, the converter is creating a variable AC voltage. This is done by connecting the voltages of the capacitor or capacitors directly to any of the converter outputs using the valves in the VSC. In converters that utilise Pulse Width Modulation PWM , the input DC voltage is normally kept constant when creating output voltages that in average are sinusoidal. The amplitude, the frequency and the phase of the AC voltage can be controlled by changing the switching pattern.

IGBT allows connecting in series, thanks to low delay times for turn-on and turn-off. It has low switching losses and can thus be used at high switching frequencies. Nowadays, devices are available with both high power handling capability and high reliability, making them suitable for high power converters. As only a very small power is needed to control the IGBT, the power needed for gate control can be taken from the main circuit.

This is highly advantageous in high voltage converters, where series connecting of many devices is used. At series connection of IGBTs, a proper voltage division is important. Simultaneous turn-on and turn-off of the series connected devices are essential.

In a two-level converter the output of each phase can be connected to either the positive pole or the negative pole of the capacitor. The DC side of the converter is floating, or in other words, insulated relative to ground. The two-level topology makes two numbers of output voltage combinations possible for each phase on the AC-side. One such converter topology is shown in FIG.

An alternative to series connection of valve positions to achieve the necessary voltage rating is to connect converter cells in series. In this way smoother AC current and AC voltage waveforms are possible to obtain with lower switching frequency and minimal filtering.

One such arrangement is series connection of single phase full-bridge converters, which sometimes are referred to as chain-link cells. A chain-link based converter comprises a number of series-connected cell modules, each cell comprising a capacitor, besides the valves. The DC-capacitor of each such cell module is rather big compared to the above described two-level static compensator, when seen in relation to the total effect of the system.

Each of the three VSC phases consists of a number of chain-link cells, here shown in series in the general diagram of FIG. The phases can also be connected in an Y-arrangement. The number of cells in series in each phase is proportional to the AC voltage rating of the system and can, for high AC voltage systems, consequently include a large number of cells.

It is necessary with such high power systems with many cell modules in series to continue operation of the system with failed cell modules in circuit in order to achieve a reasonably high MTTR Mean Time To Repair. To allow for this failure mode of operation the inventors have identified a number of requirements:. To be able to bypass a faulty cell module, it is necessary to provide zero voltage across the AC terminals of the cell. This can be achieved by using a very fast mechanical switch or a solid-state relay bidirectional thyristor or a combination of the two above solutions to allow for low power losses as shown in FIG.

The short circuit device enables safe bridging of a defective submodule. The common features of these methods are that they require additional and controllable components to be introduced which inter alia adds on costs and complexity to the system. Thus, the object of the present invention is to remove the above drawbacks.

The above-mentioned object is achieved by the present invention according to the independent claims. Preferred embodiments are set forth in the dependent claims. A major advantage of the present invention is that non faulty cell components of a cell module are continued to be used without adding on extra circuitry that require active control.

According to the present invention embodiments are disclosed for utilizing a faulty cell module despite a single failure to provide zero voltage at the terminals of the faulty cell module. According to the invention it is assumed that when a fault occurs in the cell it only affects one phase leg of a cell module. Therefore it is possible to operate the non-faulty phase leg in such a way that it can provide zero output voltage across its AC terminals.

According to the embodiments of the present invention there are included a control algorithm for operating the faulty cell module to achieve zero output voltage, the algorithm is discussed in the detailed part of the description. The modular VSC according to the invention may be used for example to control the voltage on the network e. The present invention will now be described in detail by references to the appended drawings.

The static compensator 1 comprises a VSC 2 connected at its DC side to a capacitor 3 and at its AC-side to a power network 8 , also denoted grid. The conventional two-level VSC 2 comprises three phases P 1 , P 2 , P 3 the phases are denoted L 1 , L 2 , L 3 when describing the present invention , each phase consisting of two series-connected valves.

The two valves of phase P 1 are indicated at reference numerals 9 a , 9 b. Each valve 9 a , 9 b in turn comprises a transistor with an anti-parallel diode, or rather, in order to manage high voltages, each valve comprises a number of series-connected transistors, for example IGBTs, each IGBT having an anti-parallel diode.

Each phase, or at least two of them, comprises such phase reactor, starting resistor if needed , switch and circuit breaker. The respective phases are connected to the middle point of the respective phase P 1 , P 2 , P 3 , i. It is possible to reduce the number of components by equipping if needed only two of the phases with the starting resistor connected in parallel with the switch.

Only one phase is described in the following in order to simplify the description, but it is understood that the phases are similar. When the grid-connected VSC 2 is to be energized and started, the circuit breaker 7 is switched so as to provide a current path from the grid 8 through, if needed, the starting resistor 5 , the phase reactor 4 , and through the diodes of the VSC 2 so as to charge the capacitor 3. When the capacitor voltage has reached a predetermined level, the starting resistor 5 is short-circuited by closing the parallel-connected switch 6.

As the starting resistor 5 is short-circuited, the capacitor voltage will increase a bit more and when it is high enough, the valves of the VSC 2 are deblocked and start to switch. The capacitor voltage is then controlled up to its reference value. The stress put on the valves, and in particular the diodes, of the VSC 2 depends on several factors, for example the size of the DC-side capacitor 3 , the size of the phase reactors 4 and on the voltage levels of the power network 8.

The cell module 10 comprises four valves 11 , 12 , 13 , 14 , each valve including a transistor switch, such as an IGBT. A free-wheeling diode, also denoted anti-parallel diode, is connected in parallel with each IGBT. The diode conducts in the opposite direction of the IGBT.

The valves 11 , 12 , 13 , 14 are connected in a full-bridge arrangement with a capacitor unit The present invention will now be further described. As soon as a fault in a cell module is detected this can be a gate unit failure or an IGBT failure , the corresponding phase leg in the cell is blocked, the type of failure is diagnosed using the information provided by the available sensors in the different gate units of the cell and the healthy phase leg is then operated accordingly to provide zero output voltage.

This is achieved under the control of a fault handling control algorithm in synchronism with the phase current and devices are switched at zero current crossover for operating the faulty cell module to achieve zero output voltage. This means in practice that the non-faulty phase leg is switched at fundamental frequency.

Thus, two design principles are used for the cell construction:. The invention will now be described in particular with references to FIG. Thus, the voltage source converter VSC which is based on a chain-link cell topology comprises one or more phases L 1 , L 2 , L 3 , where each of said phases comprising one or more series-connected chain-link cell modules three in FIG. Each cell module includes four IGBTs, each provided with one gate unit GU , and each cell module is assigned a cell control and protection unit which in turn is connected to the converter control and protection device to which all units are connected.

An output voltage of the voltage source converter is controlled by control signals, generated by the control and protection device, applied to said cell modules. In case of failure of a chain-link cell module that module is controlled, by the control signals, such that zero output voltage is provided at its output voltage AC terminal U AC. The control and protection device responsible for controlling the entire system receives a fault signal from a cell control and protection unit.

The cell control and protection unit receives information from available sensors of the cell. The information includes e. The cell control and protection unit then identifies the type of failure from the sensor signals and information of the type of failure is included in the fault signal. In the figure a GU unit represents the GUs for one cell module and the arrows represent the control of the cell module.

The cell control and protection units include the control and protection units of one phase. As indicated above in relation to FIG. These units are then connected to the converter control and protection device which is responsible for the overall control of the voltage source converter by applying the fault handling control algorithm.

As an alternative the GUs may be directly connected to the converter control and protection device and in that case all functionality of the cell control and protection unit may then be performed by that device. When designing the voltage converter according to the present invention a number of redundant cell modules are needed in order to maintain the system in operation despite one or more faulty cell modules which have zero output voltage.

The output voltage from each non-faulty cell module is controlled such that there is a possibility to increase its output voltage in order to compensate the voltage loss from failing cell modules. By arranging a number of redundant cell modules, where the number is related to known failure probability of a cell module and the output voltage demand, the system is kept operational for the duration of the service interval and the failed cell modules may then be replaced during scheduled maintenance assumed as one year.

In the following different types of cell operation under fault conditions are presented and the requirements for continuous operation of the faulty cell under different fault modes of the IGBT are listed. As an example the maximum used phase current is 1. As soon as a fault is detected by the control and protection device either through receiving an error message from the GU included in a fault signal or establishing by other means that a fault has occurred in an IGBT position, or if a GU does not respond, the phase leg with the faulty position is immediately blocked and the other phase leg is driven to provide zero AC voltage applying fundamental switching frequency.

A new lower DC voltage reference is given to this faulty cell in order to reduce the voltage stresses on the other components of the cell. This safety voltage level should be as low as possible without affecting the energization of the GU. At this moment this DC voltage level is estimated to be in the range of VV.

A fault in a cell can be any one of a number of different types of faults. The initial action of the control and protection unit as a fault is detected is to enter into a diagnostic mode in order to determine which failure type has occurred. The result of the failure type detection indicates which failure mode to be used. These are briefly outlined in the following, which all are related to case 1 above, and with references to FIGS.

In these figures are shown the currents through relevant IGBTs of the cell module to the right in the figures and also the phase current and the transistor switching signal below in the figures. This fundamental switching frequency operation of the phase leg continues until next service period estimated as one year , unless a second failure occurs. Note that this operating mode can also be implemented when one of the GUs are out of function.

With references to FIG. Herein, each valve position is provided with a mechanical bypass switch that may be remotely or manually controlled so that when the switch is closed a faulty valve is bypassed. The bypass switch is closed in response of a bypass switch control signal generated by the GU not shown in the figure thus being responsible for the control of the switch.

Preferably the bypass switch control signal is applied to the bypass switch via an optical cable. An advantage with this embodiment is that the cost of such a closing switch is low due to that no special requirement regarding speed of operation etc. The closing of the switch provides a low loss path for the current through the faulty cell. Since each valve position is provided with a bypass switch, it is also possible to close a second switch allowing the current to bypass the faulty cell completely.

The two switches may be the ones across the lower valves or across the upper valves. The present invention also relates to a method in a voltage source converter based on a chain-link cell topology, said converter comprising one or more phases L 1 , L 2 , L 3. Each of the phases comprising one or more series-connected chain-link cell modules connected to each other.

In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency. HVDC converters can take several different forms. Early HVDC systems, built until the s, were effectively rotary converters and used electromechanical conversion with motor - generator sets connected in series on the DC side and in parallel on the AC side.

However, all HVDC systems built since the s have used electronic static converters. Electronic converters for HVDC are divided into two main categories. Line-commutated converters HVDC classic are made with electronic switches that can only be turned on. Voltage-sourced converters are made with switching devices that can be turned both on and off. Line-commutated converters LCC used mercury-arc valves until the s, [6] or thyristors from the s to the present day.

As of , both the line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are needed, and voltage-source converters used mainly for interconnecting weak AC systems, for connecting large-scale wind power to the grid or for HVDC interconnections that are likely to be expanded to become Multi-terminal HVDC systems in future.

The market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind power , with one particular type of converter, the Modular Multi-Level Converter MMC [8] emerging as a front-runner. As early as the s, the advantages of DC long-distance transmission were starting to become evident and several commercial power transmission systems were put into operation.

The best-known example was the km, Lyon—Moutiers DC transmission scheme in France , which operated commercially from to transmitting power from the Moutiers hydroelectric plant to the city of Lyon. From the s onwards, [6] extensive research started to take place into static alternatives using gas-filled tubes — principally mercury-arc valves but also thyratrons — which held the promise of significantly higher efficiency. Very small mechanical rotary convertors remained in use for niche applications in adverse environments, such as in aircraft and vehicles, as a power conversion method from batteries to the high voltages required for radio and RADAR, until the s and the transistor era.

The term line-commutated indicates that the conversion process relies on the line voltage of the AC system to which the converter is connected in order to effect the commutation from one switching device to its neighbour. Although HVDC converters can, in principle, be constructed from diodes, such converters can only be used in rectification mode and the lack of controllability of the DC voltage is a serious disadvantage.

Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves until the s or thyristors to the present day. In a line-commutated converter, the DC current does not 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 converter. The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or six-pulse bridge , containing six electronic switches, each connecting one of the three phases to one of the two DC terminals.

Normally, two valves in the bridge are conducting at any time: one to a phase on the top row and one from a different phase on the bottom row. The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals. Thus, the DC output voltage at any given instant is given by the series combination of two AC phase voltages. For example, if valves V1 and V2 are conducting, the DC output voltage is given by the voltage of phase 1 minus the voltage of phase 3.

Because of the unavoidable but beneficial inductance in the AC supply, the transition from one pair of conducting valves to the next does not happen instantly. Rather, there is a short overlap period when two valves on the same row of the bridge are conducting simultaneously. For example, if valves V1 and V2 are initially conducting and then valve V3 is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. During the overlap period, the output DC voltage is lower than it would otherwise be and the overlap period produces a visible notch in the DC voltage.

The mean DC output voltage of a six-pulse converter is given by: [13]. In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter. Firing angle control is used to regulate the DC voltages of both ends of the HVDC system continuously in order to obtain the desired level of power transfer. Large filtering components are needed to restore the waveforms to sinusoidal.

An enhancement of the six-pulse bridge arrangement uses 12 valves in a twelve-pulse bridge. Usually one of the valve windings is star wye -connected and the other is delta-connected. For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves make provision for temporary operation with one of the two six-pulse groups bypassed. Early LCC systems used mercury-arc valves , with designs that had evolved from those used on high power industrial rectifiers.

Usually, each arm of each six-pulse bridge consisted of only one mercury-arc valve, but two projects built in the former Soviet Union used two or three mercury-arc valves in series per arm, without parallel connection of anode columns. Mercury arc valves for HVDC were rugged but required high maintenance.

Because of this, most 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 periods of maintenance. Mercury arc valves were built with ratings of up to kV, A. The last and most powerful mercury arc system installed was that of the Nelson River DC Transmission System in Canada , which used six anode columns in parallel per valve and was completed in Mercury arc valves were also used on the following HVDC projects: [24].

Because thyristors have breakdown voltages of only a few kilovolts each, HVDC thyristor valves are built using large numbers of thyristors connected 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 shared uniformly between the thyristors. The thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level.

Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different high potential with respect to earth. The isolation method can be magnetic using pulse transformers but is usually optical. Two optical methods are used: indirect and direct optical triggering. In the indirect optical triggering method, the low-voltage control electronics sends light pulses along optical fibres 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 , [25] although a small monitoring electronics unit may still be required for protection of the valve.

As of , thyristor valves had been used on over HVDC schemes, with many more still under construction or being planned. Two such converters are provided at each end of the scheme, which is of conventional bipolar construction. Because thyristors and mercury rectifiers can only be turned on not off by control action, and rely on the external AC system to effect the turn-off process, the control system only has one degree of freedom — when in the cycle to turn on the thyristor.

This is not a problem supplying additional power to a grid that is already live, but cannot be used as the sole source of power. With other types of semiconductor device such as the insulated-gate bipolar transistor IGBT , both turn-on and turn-off timing can be controlled, giving a second degree of freedom. As a result, IGBTs can be used to make self-commutated converters which are closer to a large inverter in operation.

In such converters, the polarity of DC voltage is usually fixed and the DC voltage, being smoothed by a large capacitance, can be considered constant. 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, and the fact that being self-commutated the converter no longer relies on synchronous machines in the AC system for its operation. Voltage-source converters are also considerably more compact than line-commutated converters mainly because much less harmonic filtering is needed and are preferable to line-commutated converters in locations where space is at a premium, for example on offshore platforms.

In contrast to line-commutated HVDC converters, voltage-source converters maintain a constant polarity of DC voltage and power reversal is achieved instead by reversing the direction of current. HVDC systems based on voltage-source 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. This simplifies the construction of the converter transformer.

However, there are several different configurations of voltage-source converter [28] and research is continuing to take place into new alternatives. The two-level converter is the simplest type of three-phase voltage-source converter [29] and 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 fact that the voltage at the AC output of each phase is switched between two discrete voltage levels, corresponding to the electrical potentials of the positive and negative DC terminals.

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It is possible to convert one DC voltage to another, however, the methods are slightly on the clever side. And no, it does not involve the conversion of DC to AC and back again. As it involves too many steps. Anything that has too many steps is inefficient; this is a good life lesson too.

A boost converter is one of the simplest types of switch mode converter. As the name suggests, it takes an input voltage and boosts or increases it. Also needed is a source of a periodic square wave. As you can see, there are only a few parts required to make a boost converter. It is less cumbersome than an AC transformer or inductor. It was a requirement that these converters be as compact and as efficient as possible.

To understand the working of a boost converter, it is mandatory that you know how inductors , MOSFETs, diodes and capacitors work. With that knowledge, we can go through the working of the boost converter step by step. Here, nothing happens. The output capacitor is charged to the input voltage minus one diode drop.

Also, a magnetic field builds up around the inductor. Note the polarity of the voltage applied across the inductor. So it does not like the sudden turning off of the current. It responds to this by generating a large voltage with the opposite polarity of the voltage originally supplied to it using the energy stored in the magnetic field to maintain that current flow.

If we forget the rest of the circuit elements and notice only the polarity symbols, we notice that the inductor now acts like a voltage source in series with the supply voltage. This means that the anode of the diode is now at a higher voltage than the cathode remember, the cap was already charged to supply voltage in the beginning and is forward biased.

The output capacitor is now charged to a higher voltage than before, which means that we have successfully stepped up a low DC voltage to a higher one! I recommend that you go through the steps once again very slowly and understand them intuitively. These steps happen many thousands of times depending on the frequency of the oscillator to maintain the output voltage under load. By now many of you already have questions about this oversimplified explanation.

There was a lot left out, but it was worth it to make the working of the boost converter absolutely clear. So now that we have that understanding, we can move on to the finer details. The Oscillator. We use our knowledge of inductors to calculate the time required to reach a sensible current one Amp, for example and then configure the on time of the oscillator accordingly.

This results in the inductor current waveform looking like a saw edge, hence the name sawtooth. If you look closely, during step 3, the MOSFET sees a voltage that is the supply voltage plus the inductor voltage, which means that the MOSFET has to be rated for a high voltage, which again implies a rather high on resistance. The maximum output voltage of the boost converter is not limited by design but by the breakdown voltage of the MOSFET.

The inductor. Inductors used in boost converters should be able to withstand the high currents and have a highly permeable core, so that the inductance for a given size is high. So we store some energy in the inductor from the input and transfer that same energy to the output though at a higher voltage power is conserved, obviously.

This happens many thousands of times a second depending on the oscillator frequency and so the energy adds up in every cycle so you get a nice measurable and useful energy output, for example 10 Joules every second, i.

As the equation tells us, the energy stored in the inductor is proportional to the inductance and also to the square of the peak current. To increase output power, our first thought might be to increase the size of the inductor. Of course, this will help, but not as much as we think!

However, since energy is proportional to the square of the maximum current, increasing the current will lead to a larger increase in output energy! So we understand that choosing the inductor is a fine balance between inductance and peak current.

To begin with, we need a thorough understanding of what our load requires. It is highly recommended from experience that if you attempt to build a boost converter at the beginning it is very important to know the output voltage and current independently, the product of which is our output power. Once we have the output power, we can divide that by the input voltage which should also be decided to get the average input current needed.

This new value is the peak input current. Also the minimum input current is 0. For example, if valves V1 and V2 are initially conducting and then valve V3 is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. During the overlap period, the output DC voltage is lower than it would otherwise be and the overlap period produces a visible notch in the DC voltage.

The mean DC output voltage of a six-pulse converter is given by: [13]. In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter. Firing angle control is used to regulate the DC voltages of both ends of the HVDC system continuously in order to obtain the desired level of power transfer. Large filtering components are needed to restore the waveforms to sinusoidal.

An enhancement of the six-pulse bridge arrangement uses 12 valves in a twelve-pulse bridge. Usually one of the valve windings is star wye -connected and the other is delta-connected. For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves make provision for temporary operation with one of the two six-pulse groups bypassed.

Early LCC systems used mercury-arc valves , with designs that had evolved from those used on high power industrial rectifiers. Usually, each arm of each six-pulse bridge consisted of only one mercury-arc valve, but two projects built in the former Soviet Union used two or three mercury-arc valves in series per arm, without parallel connection of anode columns. Mercury arc valves for HVDC were rugged but required high maintenance. Because of this, most 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 periods of maintenance.

Mercury arc valves were built with ratings of up to kV, A. The last and most powerful mercury arc system installed was that of the Nelson River DC Transmission System in Canada , which used six anode columns in parallel per valve and was completed in Mercury arc valves were also used on the following HVDC projects: [24].

Because thyristors have breakdown voltages of only a few kilovolts each, HVDC thyristor valves are built using large numbers of thyristors connected 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 shared uniformly between the thyristors.

The thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level. Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different high potential with respect to earth. The isolation method can be magnetic using pulse transformers but is usually optical. Two optical methods are used: indirect and direct optical triggering. In the indirect optical triggering method, the low-voltage control electronics sends light pulses along optical fibres 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 , [25] although a small monitoring electronics unit may still be required for protection of the valve. As of , thyristor valves had been used on over HVDC schemes, with many more still under construction or being planned. Two such converters are provided at each end of the scheme, which is of conventional bipolar construction.

Because thyristors and mercury rectifiers can only be turned on not off by control action, and rely on the external AC system to effect the turn-off process, the control system only has one degree of freedom — when in the cycle to turn on the thyristor. This is not a problem supplying additional power to a grid that is already live, but cannot be used as the sole source of power. With other types of semiconductor device such as the insulated-gate bipolar transistor IGBT , both turn-on and turn-off timing can be controlled, giving a second degree of freedom.

As a result, IGBTs can be used to make self-commutated converters which are closer to a large inverter in operation. In such converters, the polarity of DC voltage is usually fixed and the DC voltage, being smoothed by a large capacitance, can be considered constant. 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, and the fact that being self-commutated the converter no longer relies on synchronous machines in the AC system for its operation.

Voltage-source converters are also considerably more compact than line-commutated converters mainly because much less harmonic filtering is needed and are preferable to line-commutated converters in locations where space is at a premium, for example on offshore platforms.

In contrast to line-commutated HVDC converters, voltage-source converters maintain a constant polarity of DC voltage and power reversal is achieved instead by reversing the direction of current. HVDC systems based on voltage-source 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.

This simplifies the construction of the converter transformer. However, there are several different configurations of voltage-source converter [28] and research is continuing to take place into new alternatives. The two-level converter is the simplest type of three-phase voltage-source converter [29] and 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 fact that the voltage at the AC output of each phase is switched between two discrete voltage levels, corresponding to the electrical potentials of the positive and negative DC terminals. The two valves corresponding to one phase must never be turned on simultaneously, as this would result in an uncontrolled discharge of the DC capacitor, risking severe damage to the converter equipment.

The simplest and also, the highest-amplitude waveform that can be produced by a two-level converter is a square wave ; however this would produce unacceptable levels of harmonic distortion, so some form of pulse-width modulation PWM is always used to improve the harmonic distortion of the converter. Several different PWM strategies are possible for HVDC [31] but in all cases the efficiency of the two-level converter is significantly poorer than that of a LCC because of the higher switching losses.

Another disadvantage of the two-level converter is that, in order to achieve the very high operating voltages required for an HVDC scheme, several hundred IGBTs have to be connected in series and switched simultaneously in each valve. In an attempt to improve on the poor harmonic performance of the two-level converter, some HVDC systems have been built with three level converters. A common type of three-level converter is the diode-clamped or neutral-point-clamped converter, where each phase contains four IGBT valves, each rated at half of the DC line to line voltage, along with two clamping diode valves.

In this latter state, the two clamping diode valves complete the current path through the phase. In a refinement of the diode-clamped converter, the so-called active neutral-point clamped converter, the clamping diode valves are replaced by IGBT valves, giving additional controllability. Another type of three-level converter, used in some adjustable-speed drives but never in HVDC, replaces the clamping diode valves by a separate, isolated, flying capacitor connected between the one-quarter and three-quarter points.

Both the diode-clamped and flying capacitor variants of three-level converter can be extended to higher numbers of output levels for example, five , but the complexity of the circuit increases disproportionately and such circuits have not been considered practical for HVDC applications. Like the two-level converter and the six-pulse line-commutated converter, a MMC consists of six valves, each connecting one AC terminal to one DC terminal. However, where each valve of the two-level converter is effectively a high-voltage controlled switch consisting of a large number of IGBTs connected in series, each valve of a MMC is a separate controllable voltage source in its own right.

Each MMC valve consists of a number of independent converter submodules , each containing its own storage capacitor. In the most common form of the circuit, the half-bridge variant, each submodule contains two IGBTs connected in series across the capacitor, with the midpoint connection and one of the two capacitor terminals brought out as external connections. Each submodule therefore acts as an independent two-level converter generating a voltage of either 0 or U sm where U sm is the submodule capacitor voltage.

With a suitable number of submodules connected in series, the valve can synthesize a stepped voltage waveform that approximates very closely to a sine-wave and contains very low levels of harmonic distortion.

The MMC differs from other types of converter in that current flows continuously in all six valves of the converter throughout the mains-frequency cycle. The direct current splits equally into the three phases and the alternating current splits equally into the upper and lower valve of each phase. A typical MMC for an HVDC application contains around submodules connected in series in each valve and is therefore equivalent to a level converter.

Consequently, the harmonic performance is excellent and usually no filters are needed. The MMC has two principal disadvantages. Firstly, the control is much more complex than that of a 2-level converter. Balancing the voltages of each of the submodule capacitors is a significant challenge and requires considerable computing power and high-speed communications between the central control unit and the valve.

Secondly, the submodule capacitors themselves are large and bulky. A variant of the MMC, proposed by one manufacturer, involves connecting multiple IGBTs in series in each of the two switches that make up the submodule. This gives an output voltage waveform with fewer, larger, steps than the conventional MMC arrangement. Another alternative replaces the half bridge MMC submodule described above, with a full bridge submodule containing four IGBTs in an H bridge arrangement, instead of two.

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Power system HVDC

technology including water pumping which requires high investment and is limited by can improve the operation of microgrids through VSC control. This is especially true with the lag in transmission investment and the separation in ownership of generation and transmission assets. HVDC and FACTS. An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa.