TRANSMISSION AND DISTRIBUTION The infl uence of HVDC transmission on AC networks Information from Cigré

The use of HVDC links between regions within an AC network is becoming increasing important because of the growing challenge of network development. The lack of public acceptance of new overhead lines delays the process. Therefore, more and more underground connections are considered inside AC networks to replace traditional overhead lines, operating in parallel with existing AC lines.

The first commercial high voltage direct most effective utilisation of these assets HVDC submarine cable started operation in current ( HVDC) installations date back to in transmission networks. The notion of 1954. This system used mercury arc valves, the 1950s. Nowadays, HVDC technologies "embedded HVDC systems" is defined for which required constant maintenance and are in widespread use worldwide and the purpose of this article as follows: rebuilding. With the development of high have long operational experience. ln fact, "An embedded HVDC system is a DC link voltage and high power thyristors in the early this technology exhibits characteristics with at least two ends being physically 1970s, thyristor valves gradually replaced that have already made it attractive connected within a single synchronous AC the mercury arc valves. Thyristor valves over HVAC transmission for specific network. With such a connection, it can increased system reliability significantly applications, such as very long distance perform not only the basic function of bulk with reduced maintenance. This resulted power, transmission, long submarine cable power transmission, but also, importantly, in fast development of the technology and links and interconnection of asynchronous some additional control functions within the building of several new HVDC systems systems, as well as bulk energy transport. the AC network such as power flow worldwide. Today HVDC is a mature, well- The largest and most recent example of control, voltage control, system stability the latter is the installation of a ±800 kV, developed technology. The thyristor-based improvement and the mitigation of system 7200 MW HVDC link in China. HVDC system uses line-commutated cascading failure." converters, because the thyristors cannot Converting an existing AC line to HVDC An illustration of what is regarded as an be switched off by control action. They operation can increase its own transfer embedded HVDC is given in Fig. 1. switch off naturally at current zero crossing, capacity and, in addition, may increase when the current flowing from the anode the reliability-limited loading of parallel or HVDC transmission: state of the art and to the cathode becomes negative. The contiguous lines remaining in AC service. projects description converters are designed to transfer large HVDC systems perform differently to AC amounts of power for long distances, for connections during steady state, dynamic HVDC transmission can provide interesting sea crossings where AC cables cannot and transient conditions. Compared to AC control functions for the surrounding AC be used because of the large capacitive links, embedded HVDC links offer specific network in which they are embedded, functions that can be seen as advantages such as optimal power flow control, current. The thyristor-based HVDC system but with additional costs, complexity and voltage control, system transient stability is economical beyond a break-even sometimes noticeable drawbacks. improvement, low frequency power distance (a few hundreds of kilometers oscillation damping, prevention of system transmission lines). This article deals with the coordination cascading failure, etc. between HVDC links and AC lines in The typical application of this system parallel with an objective to enhance HVDC energy transmission was developed is interconnection between two points. the understanding in the industry for the in the late 1930s and the first commercial The tapping of an HVDC line is difficult with thyristor-based converters. Another application is the back-to-back HVDC, which provides asynchronous interconnection between two AC systems without transmission line. The asynchronous interconnection permits the regulation of power transfer, blocks cascading failures and prevents the increase of short circuit current. The classical HVDC system is based on a well-established and mature technology. The energy availability of most systems (including the , auxiliary system, cooling) is very high, and may be slightly improved with dedicated equipments The recent development of high power Insulated Gate Bipolar Transistors (IGBT) has caused revolutionary changes in HVDC technology and has made the voltage source converter based DC transmission system possible. The first system has been built in 1997. Today, other manufacturers also offer this technology Fig. 1: Examples of different concepts related to embedded HVDC systems and several systems are successfully in and HVDC systems connecting two asynchronous systems. operation.

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connecting large AC capacitive filters at the converter stations. For a common LCC- HVDC link, the large filters not only increase the costs, but also occupy large amounts of space of the converter stations. Besides, they also contribute to the temporary overvoltage (TOV) and low-order harmonic resonance problems of the HVDC link when connected to a weak AC system. Another well-known problem of the LCC-HVDC system is the occurrence of commutation failures (especially on the inverter side) caused by disturbances in the AC system. Either depressed voltage magnitude or phase-angle shift of the alternating voltage may reduce the extinction volt-time area of the inverter Fig. 2: Graetz bridge circuit. valve. If the extinction angle of the inverter valve is smaller than 5 to 6°, the previously conducted valve will re-conduct, which will end up with a commutation failure. Commutation failures are common phenomena of LCC-HVDC systems. A single commutation failure generally does no harm to either the converter valves or the AC system. However, a number of repeated commutation failures may force the HVDC link to trip. While the above two problems can be mitigated by relatively easy measures, the third problem is more fundamental, and can become a limiting factor for LCC-HVDC applications. For LCCs, the successful commutation of the alternating current from one valve to the next relies on the stiffness of the alternating voltage, i.e., the network strength of the AC system. If the AC system has low short-circuit capacity relative to the power rating of the HVDC link, problematic interactions Fig. 3: 3-level neutral-point-clamped (NPC) voltage-source converter. between the AC and the DC systems are to be expected. Besides, the SCR of the AC system also imposes an upper limitation on the HVDC power transmission, which HVDC transmission using line-commutated less than 90 degrees, the direct current is is often described by the well-known current-source converters flowing from the positive terminal of the DC maximum power curve (MPC). The major problem with mercury-arc circuit, thus the power is flowing from the As mentioned before, an LCC-HVDC technology, used in the past, was arc AC side to the DC side; if the firing angle is link normally requires reactive-power back failure which destroyed the rectifying greater than 90 degrees, the direct voltage compensation by connecting large AC property of the converter valve and changes polarity, thus the direct current is filters at the converter stations. These consequently triggered other problems. In flowing from the negative terminal of the create additional problems in weak AC the late 1960s, thyristor valve technology DC circuit. The power is then flowing from systems as described below. One such was developed which overcame the the DC side to the AC side. An HVDC link problem is the aforementioned TOV issue. problems of mercury-arc technology. is essentially constructed by two Graetz In case of a sudden change in the active Converters based on either mercury bridges, which are interconnected on the power, or the blocking of the converter, valves or thyristor valves are called line- DC sides. The interconnection could be an the large capacitors at the converter commutated converters (LCC). The basic overhead line, a cable, or a back-to-back station together with the high inductance module of an LCC is the three-phase full- connection. of the AC system cause a temporary wave bridge circuit shown in Fig. 2. This The application of LCC-HVDC technology overvoltage before the protection system topology is known as the Graetz bridge. has been very successful and the disconnects the capacitors. TOVs can Although there are several alternative installations of LCC-HVDC links are also lead to saturation of the converter configurations possible, the Graetz bridge expected to grow at least in the near transformer or close to the DC has been universally used for LCC-HVDC future. However, the LCC technology station. Another problem related to weak- converters as it provides better utilisation suffers from several inherent weaknesses. AC-system connections is the low-order of the converter transformer and a One problem is that the LCC always harmonic resonance. The high inductance lower voltage across the valve when not consumes reactive power, either in of the AC system and the large filters of the conducting. mode or in inverter mode. Depending HVDC link create a resonance of which the The Graetz bridge can be used for on the firing angles, the reactive power frequency tends to be lower for weak AC transmitting power in two directions, i.e., consumption of an LCC-HVDC converter systems. Generally speaking, the lower the the rectifier mode and the inverter mode. station is approximately 50 to 60% of resonance frequency, the greater the risk This is achieved by applying different firing the active power. The reactive-power for harmful interaction with the converter angles on the valves. If the firing angle is consumption requires compensation by control system.

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independent of the AC system. Many different topologies have been proposed for VSC. However, for HVDC applications, they have been so far limited to three major types: two-level converter, three- level converter, and modular multilevel converter (MMC). Two-level voltage-source converter

The two-level bridge is the simplest topology that can be used in order to build up a three-phase self-commutated VSC bridge. The bridge consists of six valves and each valve consists of a switching device and an anti-parallel free-wheeling diode. For an HVDC link, two VSCs are interconnected on the DC side. For high voltage applications, series connection of switching devices is necessary. The operation principle of the two-level bridge is simple. Each phase of the VSC can be connected either to the positive DC terminal, or the negative DC terminal. By adjusting the width of pulses, the reference Fig. 4: Modular Multilevel voltage-source converter. voltage can be reproduced. (a) One MMC module. (b) One phase topology. Three-level converter The three-level VSC shown in Fig. 3 is also called the neutral-point-clamped (NPC) converter. The key components that distinguish this topology from the two-level converter are the two clamping diodes in each phase. These two diodes clamp the switch voltage to half of the DC voltage. Thus, each phase of the VSC can switch to three different voltage levels, i.e., the positive DC terminal, the negative DC terminal and the mid-point. Consequently, voltage pulses produced by a three-level VSC match closer to the reference voltage. Additionally, the three- level NPC converter has lower switching losses. Compared to two-level VSCs, three- level NPC VSCs require more diodes for neutral-point clamping. However, the total number of switching components does not necessarily have to be higher. The reason for this is that, for HVDC applications, a valve consists of many series-connected switches. In the two-level case a valve has Fig. 5: Typical P/Q capability for VSC converters. to withstand twice as high voltage than in the three-level case. Accordingly, the total number of switches is approximately An improved topology of the LCC-HVDC HVDC transmission using self-commutated equal. system to overcome part of the above voltage-source converters mentioned problems is the capacitor- The NPC concept can be extended to Voltage-source converters are a commutated converter (CCC)-HVDC higher number of voltage levels, which new converter technology for HVDC technology, where AC capacitors are can result in further improved harmonic transmission. The first commercial VSC reduction and lower switching losses. inserted in series between the valves HVDC link with a rating of 50 MW was and converter transformers. The series- However, for high-voltage converter commissioned in 1999 in Gotland island of applications, the neutral clamped diodes connected capacitors not only supply Sweden, close to the world's first LCC-HVDC complicate the insulation and cooling the reactive power consumed by the link. Voltage-source converters (VSCs) utilise design of the converter valve. Therefore, valves, they also improve the dynamic self-commutating switches, e.g., gate NPC concepts with a number of voltage performance of the HVDC system. turn-off thyristors (GTOs) or insulated-gate levels higher than three has never been However, the major drawback of the bipolar transistors (IGBTs), which can be considered for HVDC applications The CCC concept is that the series capacitors turned on or off freely, contrary to the LCC recently proposed modular multilevel increase the insulation costs of the valves. where the thyristor valve can only be turned converter (MMC) concept attracts Thus, the CCC-HVDC technology has been off by reversed line voltages. Therefore, significant interests for high-voltage so far only applied to back-to-back HVDC a VSC can produce its own sinusoidal converter applications. Fig. 4 shows the links, where the voltage level of the valves voltage waveform (using for instance MMC topology for one phase. Compared is much lower. pulse-width modulation (PWM) technology) to the above two topologies, one major

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the LCC-HVDC technology. In addition, it supports the AC system with reactive- /consumption independently from the active power order (except for highest values). Fig. 5 represents typical P/Q capabilities for VSC converters.

Main features of embedded HVDC systems Modern HVDC transmission systems do not only allow for electrical power transmission from one area to another but also offer several advantages over conventional HVAC transmission in the form of advanced control functions. Though they may not be exclusively provided by HVDC (FACTS or some HVAC equipments can also be used to a certain extent to fulfil some of those benefits), these different advantages are briefly described here.

Power flow optimisation Compared to AC infrastructures, HVDC lines provide the capability to set and control active power flow going through them. The Fig. 6: African Power Pool with integrated HVDC schemes active power set-points have an impact with lumped generation representation. on the global grid power flows (AC and DC). Many existing links are exploited with the objective of optimisation of the static power flow, aiming at fulfilling different feature of the MMC is that no common the increase in number of switches. On criteria: controlling and limiting the AC capacitor is required at the DC side. the other hand, the reduction of switching system load, minimising the consequences Instead, the DC capacitors are distributed losses and savings on filtering equipment of contingencies, minimising the overall into each module, while the converter is of the MMC can justify its application for losses, etc. built up by cascade-connected modules. HVDC transmission. VSC-HVDC technology overcomes most of the weaknesses of For the HVDC links embedded in the Modular multilevel converter The MMC concept is especially attractive for high-voltage applications, since the converter can be easily scaled up by inserting additional sub-modules in each arm. If considerable amounts of sub- modules are cascaded (a few hundreds of sub-modules would be common for HVDC applications), each sub-module theoretically only needs to switch on and off once per period, which greatly reduces the switching losses of the valves. However, in the existing projects slightly higher switching frequencies have been found to be more cost-effective (up to 150 Hz). With MMC, the harmonic content of the voltage produced by the VSC is so low that additional filtering equipment is almost The World’sorld’s Most Efficient HighHigh-Capacity Capacity unnecessary. An additional benefit of the Transmission Conductor MMC is that the control system has an extra freedom in dealing with faults at the DC side. The DC capacitors are not necessarily Twice the Capacity of ACSR discharged during faults. Thus, the fault 30-40% Reduction in Line Losses recovery can be faster. Compared to the other two topologies, the major drawback Reduce Line Losses of the MMC topology is that the required switching components are doubled since only one of the valves of each sub-module contributes to the phase voltage when the submodule is inserted. In addition, Distributors for South Africa & SADC region the design and control of the MMC are Chris: +27 11 439 0000 generally more complex at least than the [email protected] Clinton: +27 31 910 0200 two-level converter. However, since the www.arb.co.za switching frequency of the MMC can be kept very low switches with higher blocking voltages may be used, which in turn limits

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transmitted DC power is transiently varied and adds to or subtracts the power from the actual power depending on the power direction of the HVDC. Fig. 7 shows a typical Power Oscillation Damping function. The final transfer functions including time constants, scaling factor and power limits will be defined by a system stability and modulation study during the detailed design stage of the project.

Conclusions In spite of the constantly growing number of HVDC schemes worldwide, those which comply with the definition of embedded HVDC in an AC system are more limited. Fig. 7: Power oscillation damping (POD) function for HVDC application. Embeddd HVDC, like FACTS are active equipments which provide supplementary control over the AC network in which they are installed. This gain in flexibility comes Southern African Power System, a Grid operated either in voltage control mode or at the expense of complexity. Depending Master Power Controller has been in reactive power control mode. With this on the underlying DC technology, different implemented to control power flows kind of control, the insertion of a VSC-HVDC technical issues arise, along with different between the parallel DC and AC paths. link into an AC network may lead to new features and performances. Hence, each This controller is especially important during issues related to the coordination of both project leads to different choices, such as: contingency situations. Similarly, with the ends (to avoid circulating reactive power  Static power dispatch on hybrid AC/DC existing AC connection, the choice of through the AC network, for instance), transmission corridors HVDC for the Kii channel connection in and with other voltage or reactive power  Design of an automatic system security Japan was driven by the ability to control supporting equipments. the power flow through the entire system. control

System security enhancement  Contribution to voltage or reactive Voltage and reactive power management functionalities support Depending on its technology, an HVDC link Control functions associated with  System stability enhancement for will offer more or less flexibility to provide embedded HVDC systems also provide transient stability and recovery (change voltage and reactive power management dynamic capabilities because of their in active/reactive current priority during to the AC system under steady state reaction speed. Automatic active power voltage dips, dedicated control for fast conditions. Line commutated converter controls can be used for response during power recovery after an AC fault) (LCC) stations do not provide close control and following severe disturbances in the of the amount of reactive power exchanged  Power oscillation damping AC system, improving the security and the with the AC network. Indeed, the phase shift  Supplementary controls activated stability of the power system in the post- between the current and voltage waveform under specific conditions (such as an disturbance period. This type of control depends on the firing angle of the thyristor automatic frequency control in case could be of great interest in scenarios valves. For such converters, tan j remains of islanded portions of the AC network) where the HVDC is located in parallel with roughly constant (tan j = 0,55). Capacitor AC lines which are sensitive to the changes The wide range of features provided by banks are therefore required for each in HVDC active power. Run-up/run-back HVDC links requires additional attention converter station to maintain reactive controls as well as fast power reversal can for its integration into an AC network power absorption close to zero. As a side with existing equipment. Moreover, the effect, those filters not only increase the be implemented in order to suppress nearby AC overloadings and to improve operation of such a device with regards costs, but also occupy large amounts of to other HVDC, FACTS or PST requires a fault recovery and system stability. space of the converter stations. Besides, global view that certainly goes beyond they also contribute to the temporary An HVDC system can be used for power the usual operation of power transmission overvoltage (TOV) and low-order harmonic oscillations damping in the AC system, systems. Thus, the insertion of an HVDC resonance problems of the HVDC link when when oscillations can be observed at link into an AC system results in different connects to a weak AC system. least at one end of the HVDC transmission coordination needs (at the local, national, system. A damping function can be and trans-national levels), and in different With voltage source converter (VSC) provided within the HVDC controls using constraints, compared with the installation technology, each converter offers a the measured frequencies at each end of an AC line, and these need to be possibility of providing or absorbing of the HVDC system. This mechanism considered during the complete process, reactive power independently of the can be used to damp oscillations in from the planning phase, to testing and transmitted active power. A converter early operations, and during subsequent parallel AC transmission lines but also for normal operation. can also work as a STATCOM when no oscillations caused by machines located power is transmitted through the HVDC close to an HVDC converter station. Acknowledgement link or even in the case of unavailability The Power Oscillation Damping (POD) of the cable or of the other converter. The function becomes active automatically This article was published in Electra, ability of VSC technology to operate in the under emergency conditions or major April 2013, and is republished here with four quadrants of the P/Q plane is shown disturbance of the AC system. The permission. in Fig. 5. This capability is used through additional power is calculated based Contact Rob Stephen, two separate and exclusive controls: on the temporary frequency difference Eskom, Tel 031 563-0063, each end of a VSC-HVDC link is basically of the connected AC systems. The total [email protected]

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