marcos.pereira@siemens.com

Prospects of the new SVC with modular multilevel voltage source converter

M. PEREIRA, M. PIESCHEL, R. STOEBER Siemens AG, Energy Sector

Germany

SUMMARY Several STATCOM using modern voltage source IGBT converter of modular multilevel technology have been put into operation in the last two years and other several projects are on the way. The equipment in focus is called SVC PLUS and presents a standard line of containerized solutions, which was developed to cover three standardized sizes, up to ±50 MVAr in one container, being already prepared for up to ±200 MVAr with four containers. Each converter phase consists of several cascaded IGBT H-bridges, with isolated bridge capacitors. It is cooled by deionized water, whereby glycol is added if the equipment is to operate at sites where the air temperature can be below zero. The container with the completely assembled equipment is suitable for ground and ship transportation. For higher voltages or power ratings, the converter is available as free standing units to be erected inside a building. Or multiple containerized units can be installed. The modules are controlled so that no more than one module changes its switching status at a time. Therefore the highest voltage step in normal operation is as big as the maximum instantaneous voltage on a single module. This reduces equipment stresses, for instance for the connected cables. And this also limits high frequency interference voltages excited by steep voltage pulses. Although the individual converter modules operate with modest switching frequency, which helps to keep losses low, an "equivalent" pulse frequency of several kHz is achieved at converter terminals due to the plurality of converter levels (e.g. 33 levels for a converter with 16 cascaded modules). As consequence, the converter output approaches a sinusoidal voltage with very small harmonic content in comparison with two or three level converters. Therefore much smaller filters are required. But the waveform produced by the converter is not limited to a sine wave. In fact, it is able to produce a complex waveform, containing several harmonics, provided the control equipment is designed for that. Therefore, the converter can be used to combine reactive power compensation and harmonic reduction according to the

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active filter principles. The highest harmonic order that can be processed depends upon the number of converter modules and the switching frequency. As well known, although equipment based on switching of capacitive or inductive reactance, like the TSC and TCR of the classical Static VAr Compensators (SVC), is able to greatly improve stability and voltage regulation, their benefits during faults or voltage drops are limited, as the current falls proportionally to the voltage. However, the multilevel converter, like other VSC implementations, is also able to still deliver rated current at low voltage levels, e.g. 0.5 p.u. or even less. This characteristic increases the benefits of the equipment in such occurrences. An important characteristic of the application is that the instantaneous voltage produced by the multilevel converter may be driven to any value between the maximum positive and the maximum negative voltage of the converter in very short time, e.g. less than 1 ms – while avoiding excessive voltage slopes. The accuracy (resolution) here is limited by the fact that the converter voltage has discrete values, given by the dc voltage present at the bridge capacitors of the converter modules. However this should have no impact on benefits and can be just neglected if the number of modules is big enough. The advantage here is that this characteristic, associated with fast measuring and suitable control system makes possible very short response times. In few words, one can say that the multilevel converter presents advantages of clean voltage waveform, fast response and controllability, which may be used to improve stability and voltage regulation (based on voltage sensing) and flicker mitigation (generally based on load current sensing), as well as for active filtering of harmonics. Besides, it offers a modular and compact solution. Up to given size (e.g. 50 MVAr per device) it should be built as a ready and tested containerized unit, allowing transportation, easy installation and fast commissioning. The paper discusses the benefits of multilevel converters for FACTS taking the developed equipment presented above as a reference. KEYWORDS FACTS – STATCOM – Voltage Sourced Converter (VSC) – Multilevel Converter – Modular Multilevel Converter (MMC) – Universal Power Flow Controller (UPFC)

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1. INTRODUCTION When the idea of FACTS (Flexible AC Transmission Systems) as a way of gaining continuous control on the AC network was introduced in the late eighties [1], several Static VAR Compensators (SVC), based on the use of thyristor controlled reactors and capacitors have been already applied to the transmission system. HVDC, also using thyristor valves, was initially recognized to belong to FACTS due to its ability of moving power in a controlled way between networks or network areas. Few years later the Thyristor Controlled Series Compensator – TCSC was introduced with the capability of controlling the power flow through AC lines and improving network stability [2]. The progress in the power semiconductor technology followed to the development of new devices using turn-off semiconductors like GTO, IGBT or IGCT to produce controlled reactive power. The most applied device of this category is the STATCOM, adding features to the already well accepted SVC. FACTS (Flexible AC Transmission Systems) devices, especially the SVC and the STATCOM are now well established in the electrical power transmission system for improving network performance or energy quality at specific locations. Among other FACTS devices, also the Thyristor Protected Series Compensator (TPSC), the Unified Power Flow Controller (UPFC) and the Grid Power Flow Controller (GPFC) [3] have been frequently mentioned. While SVC and STATCOM are shunt devices and TCSC is a series device mainly inserted in a transmission line, UPFC is a shunt and series connected one. Renewable energy systems as for example wind farms added a new field of application to STATCOM systems. Typically, high power renewable energy systems are strongly fluctuating and may cause voltage variations particularly when connected to a weak network. In this common scenario, STATCOM systems allow to improve power quality and to optimize the power flow in the transmission lines. The structures of an SVC and a STATCOM, both able to produce reactive power in the capacitive and inductive directions, are compared in Figure 1 and Figure 2. In the SVC, inductive reactive power is provided by the Thyristor Controlled Reactor (TCR). Depending upon the switching angle, the power can be continuously controlled from zero to maximal inductive power. The capacitive power is provided by the Thyristor Switched Capacitor (TSC) and can be zero (off) or full capacitive power (on). Continuous control of the SVC in the capacitive range requires switching on both TSC and the TCR and varying the control angle of the latter to cancel part of the TSC current. Of course the SVC can be extended by adding TSC, TCR as well as fixed or mechanically switched capacitors or reactors.

Figure 1 – Example of SVC structure Figure 2 –STATCOM and its equivalent circuit

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The dimensioning of inductive and capacitive operating range is normally custom design. Harmonic filters are always required in combination with the TCR, in order to mitigate the harmonic distortion produced by this one. In case of a noticeable imbalanced network these filters should be designed to dampen 3rd harmonics. Because the thyristors must be switched on at a certain point on the voltage wave, their operation may involve a delay of up to half period. Conversely, once conducting, they cannot accept a new command, which implies a switching-off delay of the same order. This is however acceptable for most applications. Static VAr Compensators are applied in the transmission network since more than three decades and brought big benefits for voltage regulation and stability in the transmission system. In Figure 2 is remarkable that the STATCOM does not require the shunt capacitor bank or reactors of the SVC. The modern STATCOM requires no harmonic filters but just a quite small filter for high frequencies. Therefore it occupies less area than the SVC, and this becomes a more and more important point. This is for instance the case of upgrading new plants, where much free space is normally not available or in cases where the square meter is expensive. Besides, the expenditures and time with transportation and erection are shortened. Additionally, compactness and reduced weight are primordial factors if the equipment should be relocatable. STATCOM injects reactive power in the network not by switching shunt capacitor banks and reactors but by the principle depicted in a simple way in Figure 2, i.e. by varying the amplitude of the voltage produced by the converter, which is essentially in phase with the voltage at the point of common coupling. The reactive power output range is symmetric. Voltage Source Converters (VSC) have been used in all power system applications to cope with this task. This arrangement presents excellent characteristics for flicker reduction and even for mitigation of harmonics, provided the converter output is not limited to a sine wave. In order to ensure the required performance, the converter has not only to exhibit the necessary ratings, but also fast response and accurate voltage wave form. As VSC-based STATCOM operates usually around 0 MVAr and has to generate its full output power only during severe transients or network faults, the losses are smaller than for a thyristor SVC where it frequently has to compensate the reactive power of its passive filters. The Modular Multilevel Converter (MMC) topology brought remarkable benefits for the implementation of STATCOM. They promise big potential for the application in other FACTS devices like the GPFC and UPFC. 2. MULTILEVEL CONVERTERS Converters for FACTS applications present voltage ratings generally starting from about 10 kV rms nominal voltage and going up to some ten kilovolts, being connected to the transmission system by means of a transformer. On the other side, the normally available semiconductors for this purpose belong to a nominal voltage class of 3.3 or 4.5 kV and operate at about half of these values – as the semiconductor blocking voltage is usually higher than the allowed switching voltage. A solution to cope with this is connecting a sufficient amount of semiconductor switches e.g. IGBT and diodes in series. Means to ensure reasonable potential sharing and limiting the voltage peaks of the switching process (snubbering) have to be included in the valve. In this way it is possible to implement a suitable two-level converter. This arrangement is shown in Figure 3. The DC-voltage source is not shown in this figure but can be obtained by a properly dimensioned capacitor. The converter output voltage is obtained by pulse width modulation (PWM) techniques or pre-computed optimised pulse switching patterns. A switching frequency of several times the maximal frequency of the output voltage has to be used in order to produce an acceptable approximation of the desired waveform, e.g. a sine. The voltage steps at the AC terminals have the magnitude of the full voltage between the DC terminals, causing disturbing currents and EMI due to the stray capacitances of the converter. Insulation requirements also depend upon this behaviour. The higher the switching

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frequency, the better is the quality of the output voltage, contributing to size-reduction of the harmonic filters. On the other hand, losses in semiconductors and in snubber circuits rise with the switching frequency. A compromise between these aspects has to be considered in the valve design, especially for high voltage converters, such as those for FACTS. A way to reduce the above mentioned effects is using multilevel converter technology. A multilevel converter synthesizes the desired wave form by smaller voltage steps, producing therefore smaller harmonics then a two-level converter with the same switching frequency. Insulation and EMI aspects get then less critical. Figure 4 shows a three-level Neutral Point Diode-Clamped Converter (NPC). This converter is widely known, for instance from traction drive applications. This structure can be extended to 5, 7 or more levels. However, the complexity of the device increases rapidly with the number of levels from both points of view of mechanical construction and connections. Due to this, the number of the levels shall be generally limited to three and the discrete semiconductors for each level shall be replaced by stacks of series connected semiconductors in NPC applications for high voltage. Also the control of the individual switches in order to share the charge among the capacitors gets more complicated as the number of levels increase [4].

Figure 3: Two-level converter topology. The potential at AC terminals jumps between P and N.

Figure 4: Three-level NPC topology. The potential at AC terminals jump between P or N and 0.

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Another well known multilevel converter topology is the converter with capacitor-clamped switches ("flying capacitors") and DC voltage support capacitor [5] [6]. Similarly to the diode clamped converter, the number of levels here shall be limited in high voltage applications. A newly introduced voltage source converter for high voltage application is the Modular Multilevel Converter. 3. THE MODULAR MULTILEVEL CONVERTERS (MMC) The MMC consists of a structure with branches formed by series connected (also called "cascaded") converter modules [7]. Each module contains a semiconductor arrangement and a DC capacitor as well as two terminals for the connection to the neighbour modules of the converter. Such a converter is shown in Figure 5. The voltage at the module terminals can be zero, the capacitor voltage or this one with inverted polarity. Four MMC of this type rated 20 MVA (on a three-phase basis) are used in active AC filters have been commissioned in 2007 at the Neptune HVDC system [8], in the US. A two-terminal converter of this type is able to exchange reactive power with the network and can therefore be used for STATCOM implementations.

The interest in the MMC increased remarkably after introduction of the AC/DC converter with half IGBT bridges [7, 8] and after the HVDC PLUS system based on this converter has been announced [11]. The AC/DC MMC is shown in Figure 6. It is (each phase) a three-terminal device and is able to exchange active and reactive power, thus also suitable for UPFC and GPFC implementations.

ON ON

OFF OFF

C uDC

OFF OFF

ON ON

C uDC

OFF ON

ON OFF

C VDC-VDC

ON OFF

OFF ON

C VDC+ VDC

Capacitor charging/discharging

Capacitor bypassed“Off“ State

“On“ State

ON ON

OFF OFF

C uDC

OFF OFF

ON ON

C uDC

OFF ON

ON OFF

C VDC-VDC

ON OFF

OFF ON

C VDC+ VDC

Capacitor charging/discharging

Capacitor bypassed“Off“ State

“On“ State

Figure 5 – MMC with 2-terminal arms with H-bridges for AC application

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Both configurations are adequate for converters up to 100 kV or more, depending of the number of modules of the converter branches. Converters for FACTS applications will have typically more than 10 modules per branch, i.e. 21 levels, using 3.3 or 4.5 kV IGBT at present time. The first advantage for the application is that the output voltage is built from small voltage steps, of about the half of the IGBT rated voltage – that is the capacitor voltage. An example of MMC output is illustrated in Figure 7. This brings two important gains: The first is that the harmonic distortion of the converter voltage is extremely low, even with modest IGBT switching frequency. The second is that the disturbing current spikes caused by the voltage steps on the stray capacitances of the semiconductors and busbars keep corresponding low levels even in FACTS application with higher voltages (e.g. valve voltage of 36 kV or more). For this reason, the produced EMI values are also very small. These aspects make the valve design uncritical, if compared with the other aforementioned converter topologies. Another benefit of MMC for FACTS is that no snubber circuits are required as each semiconductor handles the voltage of the associated bridge capacitor. And the switching frequency of the individual switches can be low, for instance around 150 Hz, without loss of voltage quality. Both characteristics are favourable in the sense of reduction of losses. The modularity of the converter allows limiting the damages due to semiconductor fault to the faulty module, which can be bypassed while the converter remains in operation, similarly to the well established strategy in HVDC and thyristor-based FACTS devices. The MMC output can be driven to any value inside its voltage range in very short time, e.g. < 1 ms. A combination of fast measuring and suitable control system makes possible very short response times. All these characteristics lead to a compact, modular, scalable and reliable solution. As a result, FACTS devices using this technology gain competitiveness also in costs, which has been a challenging issue for VSC applications in the past.

Figure 6 – MMC with half bridges and three terminal per branch for AC/DC application

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Figure 7 – Example of MMC output voltage

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4. MMC APPLICATION IN FACTS DEVICES An example of STATCOM implementation is the SVC PLUS® [3] presented in Figure 9. The MMC occupies the central room of the container while the pumps and accessories of the cooling system are assembled in the room at the left side. The control equipment is located at the end. The valves are accommodated in a three level rack, being one phase per level. The MMC shown in the figure comprises 16 modules per phase to deliver ±35 MVar, whereby up to two modules per phase can be automatically bypassed in case of failure without stopping operation (redundancy). The container for ±50 MVAr looks just the same but it has 22 modules per phase. The valve modules are cooled by a closed de-ionized water system.

The layout of an installation using the equipment described here is exemplified in Figure 10. Besides the container, the figure shows the external heat exchanger of the cooling system, the air core reactors which are connected in series with each converter branch, the step-down transformer and ancillary equipment. The whole equipment occupies an area of 25 x 21 m, which is less than 50% of the area required for an SVC of same range. Figure 11 shows the power module used for the SVC PLUS implementation. The biggest block is the DC-capacitor. The smaller block at the front side is the IGBT H-bridge assembly with the automatic device to bypass permanently the module in case of failure and the interface for the fibre optics of communication channel to the control system. Standard IGBT devices are used in the valve module. These are available from several semiconductor manufacturers and have been characterized according to the specifications for this application. A critical point regarding to the MMC is the destruction of the IGBT by sudden discharge of the DC-capacitor due to failure in the semiconductors. The world-wide experience with converter valves shows that this possibility cannot be neglected. The design of the MMC modules has to ensure that a semiconductor explosion remains confined to the module and no damage to neighbour equipment occurs in this case. Furthermore it is envisaged that the damaged module will be automatically bypassed so that operation will continue without forced outage as long as enough redundant modules are available.

Figure 9: ±35 MVar STATCOM container: cooling system assembly, MMC and control room.

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Two units of this equipment have been energized at Thanet wind park (UK) in 2009. Another three followed for the Greater Gabbard wind park (UK) in 2010. Two units are connected to a tertiary winding of a transformer at Kikiwa station (NZ) and operating since 2010. Currently, four SVC PLUS are commissioned for the London Array wind farm and one for LINCS, both in the UK. One device will be delivered for a transmission application in Rio Branco (BR). One SVC PLUS in open-rack configuration with ±60 MVAr is scheduled for commissioning at one of the Cook-Strait HVDC stations (NZ). Three SVC PLUS open-rack with ±100 MVAr each are planned for commissioning in Australia. In this application the SVC PLUS is not solely used to stabilise the network but also to compensate voltage imbalances which are caused by railroad traffic. An open-rack SVC PLUS for mitigation of flicker caused by an electric arc furnace is planned for commissioning at the end of 2011.

Figure 10: STATCOM layout Figure 11: MMC module

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