The invention relates to the field of voltage source converters and especially single phase multilevel converters adapted for compensating for harmonics in railway contact lines. Voltage source converters (VSC) using power electronics including semiconductor switching elements that can be turned off, such as IGBTs (Insulated Gate Bipolar Transistors) have found great use for DC transmission, reactive power compensation, control of active as well as reactive power, being able to create AC voltage out of DC voltage by means of switching control, and for converting AC to DC etcetera.
The multilevel converter technique, employing many voltage levels, wherein each voltage level being individually switched, can be used to create AC voltage from DC in small voltage steps providing a stepped voltage curve much closer to a sinus curve than the previous use of two level and three level converters. The DC voltage can be provided by energy storage means consisting of capacitors, but may also be batteries.
A single phase multilevel converter may be used for reactive power compensation in the contact lines of AC railway systems but the locomotives of the trains generate harmonic currents in the electric system of the railway. Such harmonics can be reduced by harmonic filters, but such harmonic filters weaken the system and may create resonance problems and enhance the voltage in a non-load condition.
As a background some prior art documents (D1-D5) will be discussed in the following. First three documents mainly dealing with three-phase systems (D1-D3), followed by two documents (D4-D5) describing single-phase systems for railway applications. It should be noted that this discussion of prior art documents have been made with knowledge of the present invention and the interpretation of these documents may therefore include novel features. A purpose of the discussion is to describe how systems can be built, especially as they hitherto have been built.
U.S. Pat. No. 6,088,245 (D1) describes a switching control arrangement for multilevel converters that counteract the harmonic content of the converter voltage or current by controlling the switching pattern of the switching devices, e.g. GTO's (see abstract), which switching devices are associated with DC energy sources, or DC sink and source means. Especially, the switching pattern is changed by modifying the timing of the switching of the switching devices (see claim 2 in column 13). The switching of the switching devices provides an output from the multilevel converter, which when connected to the power system, reduce the harmonic distortion of the AC power in the power system.
U.S. Pat. No. 5,532,575 (D2) describes a multilevel converter with means for balancing voltages of capacitors of the converter. D1 describes a multilevel converter primarily intended for use as a static VAr compensator (column 1, line 5-8). The multilevel converter includes three legs, one for each phase, of switching elements (GTO's 30, see FIG. 1), which switching elements (GTO's) are connected to tapping points of capacitors 20 (column 1, line 28-34). The multilevel converter also includes a control system 60 (column 7, line 48-65) that controls the switching of the GTO's. The control system monitors the voltages of the capacitors and (see column 8 line 32-64) if a voltage level of a capacitor is too high or too low, the control system (see abstract) adjusts the timing of the switching of those capacitors that have too low or too high voltage level, but do not change the switching timing of those capacitors that do not deviate. In this way the voltages of those capacitors that do not deviate is not affected (column 8, line 39), whereas the voltages of the deviating capacitors are balanced.
Thus, apart from compensating for reactive current in a transmission line, document D1 and D2 describes two different goals achieved by adjusting the timing of the switching of the switching devices of a multilevel converter, i.e. balancing capacitor voltages and reducing harmonics, respectively. In the multilevel converters described in D1 and D2, the three phases have common energy storage elements, i.e. the three phases share capacitors.
Another known type of multilevel converters, are cascaded multilevel converters having a semiconductor switching element in each switching cell circuit having a half bridge or full bridge configuration. For example, two IGBTs are used in each switching cell in a half bridge configuration with a DC capacitor as energy storage element, and each IGBT is arranged in anti-parallel with its own diode.
In such multilevel converters that have separate energy storage elements for each phase, e.g. capacitors that belong to one phase, sharing of energy between the capacitors within a phase leg, or between capacitors of different phase legs, is difficult to achieve without affecting the power that is transferred to the power network.
A solution to this balancing problem exists for three phase inverters. Document WO2010/145706 (D3) provides a solution for balancing voltages of the energy storage elements of a delta connected multilevel converter, having serially connected switching cells with a corresponding energy storage element, arranged in three phase legs. In more detail, D3 describes a multilevel converter having delta connected phase legs and wherein the DC voltages of the switching cells of each of the phase legs are balanced by means of a balancing current circulating between the phase legs, and distributing energy between the energy storage elements of the phase legs. D3 describes an arrangement for exchanging power in a shunt connection with a three phase power network, which arrangement comprises a voltage source converter having three phase legs in a delta connection, wherein each leg comprises a series of switching cells (see abstract of D3). The electrical conditions of the three phases of the power network and the converter are measured and a control unit (19) is configured to determine if the phases are unbalanced. The control unit (19) determines a zero sequence current that indicates such an unbalance and uses this determined zero sequence current to control the switching cells to add a circulating current to the currents in the phase legs to counteract such an unbalance (see claim 1 of D3). The circulated current is driven inside the delta of the converter legs and moves energy inside the delta, between the legs without negatively affecting the power network, and avoids creating harmonics in the power network (see D3 page 4, lines 24-29).
In such a delta connected multilevel converter the phase legs handles the phase voltage and comprises a sufficient number of levels to handle the voltage level between the phases.
A multilevel converter having a single phase leg that use currents to move energy between the energy storing elements affects the power transmission network or contact line of a railway, because the leg do not provide a closed circuit like the phase legs in a delta connected multilevel converter do.
Tan, P. C. et al. “Application of multilevel active power filtering to a 25 kV traction System”, Australasian Universities Power Engineering Conference (AUPEC), Monash University, Melbourne, Sep. 29-Oct. 2, 2002 (D4 or Tan 2002) describes filtering of harmonics in single phase railway power systems.
Section 2 of D4 discusses four classes of multilevel inverter topologies; diode-clamped, flying-capacitor, cascaded H-bridges and hybrid inverters. Each of these inverters are combined with a passive filtering of harmonics.
Section 3 of D4 suggests using a hybrid inverter for active filtering of harmonics, which hybrid inverter (paragraph 3.1 and FIG. 2) is used to provide reactive power compensation and harmonic compensation (of the 3rd, 5th and 7th harmonics). FIG. 2 also describes a DC bus voltage control that maintains a constant voltage of the DC bus.
Thus, D4 describes using a power control system for a railroad including a multilevel converter of a hybrid class type adapted both for active filtering of harmonics and reactive power compensation, and which power system compensates the DC voltage level in the DC bus of the multilevel converter.
D4 do, however, not describe if or how any of the remaining three types of multilevel inverter topologies; diode-clamped, flying capacitor or cascaded H-bridges can be used for active filtering.
Lee Y. K. et al. “The High Power Active Filter System for Harmonic Compensation of Electric Railway”, (Order no. T4.3.1.1), Proceedings of the 7th World Congress on Railway Research (WCRR), Montréal, Canada, Jun. 4-8, 2006 (D5 or Lee 2006) describes filtering of harmonics. D5 suggests (see abstract and introduction) using a cascaded multilevel H-bridge inverter for active filtering of harmonics in an electric power system of a railway.
D5 combines passive filtering of the 3rd and 5th harmonics (§2.2 and FIG. 5) with active filtering of higher harmonics by means of the multilevel converter. D5 do not describe compensation of DC voltages.
A problem that may arise when such an H-bridge inverter or converter, having capacitors isolated from each other by switching means, is used for filtering harmonics, which harmonics are received through the energy storing capacitors, is that the voltage over individual energy storing capacitors become too large or too low. Such voltage imbalances cannot be compensated for without adding current to the contact line. During time periods when there is no need for power compensation e.g. when no train is supplied with energy from the contact line, the voltage levels of the capacitor cannot be balanced without simultaneously inducing a current in the contact line.
D5 do not address the problem of balancing DC voltage levels, and therefore do not describe how such DC voltages can be adjusted. In the system of D5 it is presumably possible to balance the DC voltage levels of the capacitors by creating a current from the capacitors of the converter to the passive filter, so that no current needs being transferred further along the contact line.