Patent Description:
For railway systems with frequencies like <NUM>, <NUM> or <NUM>, static frequency converters (SFCs) are already a well-established power supply solution. However, besides power flow control, in the future the SFCs could offer the capability to identify faulty behaviour of the rolling stock or various characteristics of the grid.

Satisfactory coordination between the rolling stock, railway grid and further fixed installations is an important interoperability factor in contemporary railway systems. New vehicles are typically fitted with highly sophisticated and sensitive electric circuitry, including many active components and control loops, and it is therefore crucial for designers to understand their interaction with the grid and ensure compatibility. Poor tuning may lead to instabilities that manifest themselves in the line voltage and current and ultimately to degraded performance. The new vehicles furthermore must undergo rigorous testing before they are released for commercial operation. As one example, the European norm EN <NUM>-<NUM> (Railway Applications - Fixed installations and rolling stock - Technical criteria for the coordination between power supply and rolling stock to achieve interoperability-Part <NUM>: Stability and harmonics) specifies tests for evaluating electric resonance stability, low-frequency power oscillations and robustness to overvoltage caused by harmonics. Another example is the international norm IEC <NUM>.

Testing according to the current proposal <NPL>) includes subjecting the tested equipment to a test signal of the form specified in equation A. <NUM> therein, namely: <MAT> where ωN is a constant fundamental (angular) frequency of the network, ωT is a variable test (angular) frequency, and the six amplitudes Ud<NUM>, Uq<NUM>, Uda, Udb, Uqa, Uqb are real-valued functions of time. The test frequency ωT is varied, in one or more frequency sweeps, to allow recording of a frequency response. The highly nonlinear test signal U(t) shall be generated with testing-grade accuracy, yet with an ability to withstand heavy loading, e.g., when supplying one or more locomotives having a rated power of several megawatts each.

State-of-the-art SFCs fail to adequately address the task of supplying a test signal with the above characteristics.

One objective of the invention is to propose devices for supplying a non-linear test signal to a railway grid. It is a particular objective to propose such devices that enable testing in accordance with EN <NUM>-<NUM>.

These and other objective are achieved by the invention as defined by the independent claim. The dependent claims provide embodiments of the invention.

For conciseness of this presentation, an angular frequency may be referred to simply as a frequency when no confusion can arise.

The aspects of the invention will now be described more fully with reference to the accompanying drawings, on which certain embodiments of the invention are shown.

These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, the embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art.

<FIG> shows an environment <NUM> suitable for performing electric measurements on rolling stock, exemplified by a railway vehicle <NUM> electrically powered by a section 120c of a railway grid. The railway grid, which may be implemented as an overhead line or a third rail, comprises further sections 120a, 120b which are each connected, via supply lines 122a, 122b, 122c, to an SFC <NUM>. The sectioning of the railway grid may be achieved by switches or isolators <NUM>, which define the endpoints of the sections <NUM>. The sectioning of the railway grid may correspond to or overlap with track blocks 110a, 110b, 110c of a railway track on which the vehicle <NUM> runs. The environment <NUM> further comprises measuring equipment <NUM>, 141a, 141b, 141c allowing electric measurements to be performed at different points of the railway grid <NUM> independently. Preferably, each railway grid section <NUM> is provided with one or more sensors 141a, 141b, 141c, such as high-accuracy sensors. The measurements may, as suggested by <FIG>, be available at a central node <NUM>, to allow comparisons or detection of variations between different locations of the railway grid <NUM> or to examine spatial patterns.

Optionally, the environment comprises signalling equipment (not shown) for monitoring and/or controlling rolling stock. The signalling equipment is no essential component of the environment <NUM> unless the measurements or tests to be performed relate to the coordination with the signalling equipment.

The set-up of the test environment <NUM> can be used for various rolling stock tests.

Example <NUM>: Tests of rolling stock's protection, wherein the SFC can control voltage/current amplitude/frequency.

Example <NUM>: Tests of rolling stock's continuous operation capability at different grid frequency. The SFC can control the nominal frequency of the sectioned/island grid to analyse not only the frequency supervision protection functions on board the trains but also the control loop behaviour when the grid frequency deviates. It is therefore easy for the rail operator to check whether the rolling stock is satisfactorily operable in the whole requested frequency range.

Example <NUM>: Measurements of the rolling stock's harmonic spectrum. The SFC can generate low to high frequency sweeps (up to the SFC's modulator limits) in order to check it. The rolling stock has its own harmonic current spectrum based on the way the pulsing is realized for the on-board semiconductors. Therefore, the vehicle design (transformers, modulator technique, filters) must be carefully tested by the rail operator with reference to the given limits defined in norms and standards.

Example <NUM>: Measurements of the rail grid's impedance. Information about the grid impedance can be used for various purposes. For example, one of them is the minimization of overvoltage risk in the grid caused by resonances. The electrical rail power supply system (including transformers, catenary, cables, filters, etc.) contains resonance points, which can be in the range of several hundred Hertz. If such resonance frequencies coincide with the switching frequency of rolling stock's on-board semiconductors, the resonances can be excited and lead to overvoltage.

Example <NUM>: Low-frequency stability tests for rolling stocks as defined in EN <NUM>-<NUM> standard.

Example <NUM>: Tests of track circuits and signalling systems. The objective in this case would be to check the quality of various devices that can be found usually along the rail track. For example, it may be assessed whether a low frequency or transient interference in the grid will falsely operate a track relay. Such set-up will allow the rail grid operator to determine the maximum permissible levels in the track. These levels can be correlated later with the rolling stock performance.

Apparently, the test environment <NUM> is suitable not only for the tests defined in EN <NUM>-<NUM> standard but also for different types of tests. In the latter cases, the SFC <NUM> may have a state-of-the-art control architecture.

Moreover, it is expected that low-frequency oscillations (subharmonic oscillations) occur not only when the rolling stock is running but also when standing still with the control loops active. Considering this, testing may be performed not only on moving rolling stock but also stationary rolling stock. During a test, one or more moving vehicle may be present in the testing environment <NUM>, optionally in combination with one or more stationary vehicles that draw idling or standby power from the grid.

<FIG> is a simplified electric model of a testing environment of the type shown in <FIG>. The first and second railway grid sections 120a, 120b are modelled as serially arranged resistances R and inductances L. The last railway grid section 120c is modelled as a resistance R, inductance L and alternating-current (AC) source connecting to ground. The vehicle <NUM> is modelled as an AC source drawing current from a section of the railway grid through a series of resistance R and inductance L to ground. Obviously, the vehicle <NUM> can move between the railway sections and may therefore experience different impedance at different times.

<FIG> shows example inner workings of the SFC <NUM>. The SFC <NUM> is powered by direct-current (DC) capacitors <NUM>, which may be connected to a utility grid or a different power source. The DC capacitors <NUM> connect via a DC-to-AC converter <NUM>, which is preferably bidirectional to allow regenerative braking, to a transformer <NUM>. An RLC-type high-pass filter <NUM> tuned to draining unwanted highfrequency components to ground is arranged at the output of the SFC <NUM>.

It is emphasized that <FIG> shows an example implementation of the SFC <NUM>. This implementation as well as the railway grid <NUM> and vehicle <NUM> have additionally undergone abstraction and idealization to arrive at a useful mathematical model that is computationally efficient and furthers understanding.

<FIG> shows a control architecture <NUM> of the SFC <NUM>, which comprises a first controller <NUM>, which is configured to provide a fundamental control signal U<NUM>(t) = [Ud<NUM>(t) Uq<NUM>(t)]T for controlling a phase and amplitude of a carrier waveform at a fundamental frequency ωN of the railway grid. For example, the fundamental frequency may be 2π · <NUM>, 2π · <NUM> or 2π · <NUM>. The components of U<NUM> may not be in a one-to-one relationship with the phase and amplitude, but rather correspond to a modulation with orthogonal components {cos ωNt, - sin ωNt}; see also figure A. <NUM> of EN <NUM>-<NUM>.

The control architecture <NUM> further comprises a second controller <NUM>, which is configured to generate a first modulating signal Ud(t) = [Uda(t) Udβ(t)]T and a second modulating signal Uq(t) = [Uqα(t) Uqβ(t)]T as seen in a synchronous reference frame rotating at a modulation frequency ωT. The second controller is further configured to provide a representation, or at least a partial representation, of the first and second modulating signals in a stationary reference frame. The reference frame may be understood as stationary in the sense that the oscillation at the modulation frequency ωT is no longer concealed; as further discussed below. The modulation frequency ωT may be different from the fundamental frequency ωN. For tests of low-frequency oscillation, the modulation frequency ωT is usually lower or significantly lower than the fundamental frequency ωN. For other tests or measurements, the opposite relation may hold.

In the control architecture <NUM>, the first components of the representations of the first and second modulating signals Ud, Uq in the stationary reference frame are summed with a respective first and second component of the fundamental control signal U<NUM> to obtain a composite control signal URef*(t) = [UdRef*(t) UqRef*(t)]T. A modulator <NUM> is controlled by the composite control signal and an indication of the fundamental frequency ωN, so as to provide an output U(t) of the SFC <NUM> accordingly. The output U(t) is preferably a loadable signal that may be fed to the railway grid. In other words, the modulator <NUM> shall have high internal resistance.

<FIG> shows an example structure of the second controller <NUM>. A first composition element 321a receives the components Udα(t), Udβ(t) of the first modulating signal Ud in a synchronous reference frame rotating at ωT. This is to say, the oscillation at frequency ωT is implicit from the representation. The first composition element 321a forms the first modulating signal Ud and supplies it to a first dq-to-αβ converter 322a. The first dq-to-αβ converter 322a transforms the first modulating signal Ud into a stationary reference frame, which renders explicit the oscillation at frequency ωT which was hitherto absorbed in the basis vectors. The dq-to-αβ transformation is given by <MAT> and corresponds, with ω = ωT, to the action of the matrix <MAT> Alternatively, the first dq-to-αβ converter 322a may be regarded as a modulator configured to a generate an output signal that is a linear combination of {cos ωTt, - sin ωTt} as weighted by the components of the first modulating signal Ud.

A first decomposition element 323a downstream of the first dq-to-αβ converter 322a receives the components of the transformed first modulating signal Ud and extracts the first component (α component), which forms one of the outputs of the second controller <NUM>. It is not necessary for the first dq-to-αβ converter 322a to provide the second component (β component) which is not used here. Accordingly, it suffices to generate the transformed first modulating signal as <MAT> where * denotes an arbitrary or absent element. It is recalled that the time dependence of the amplitudes Udα(t), Udβ(t) corresponds to the modulation over time. By an analogous processing chain, comprising a second composition element 321b, second dq-to-αβ converter 322b and second decomposition element 323b, a transformed second modulating signal is obtained: <MAT>.

The first and second dq-to-αβ converters 322a, 322b may be implemented in software executed by a general-purpose computer, as application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or as a non-programmable chip or chipset. The first and second dq-to-αβ converters 322a, 322b may be general-purpose circuitry that is not dedicated to the specific signalling processing tasks occurring in an SFC <NUM>.

Returning to <FIG>, the composite control signal URef* obtained by the summations is given by <MAT> The composite control signal URef* controls the action of the modulator <NUM>, which forms a linear combination of components {cos ωNt, - sin ωNt}. This yields, in a stationary reference frame, <MAT> An SFC output of this form is equal to the signal required by equation A. <NUM> of EN <NUM>-<NUM>.

Alternatively, the output of the modulator <NUM> may be considered as the first component (α component) of a dq-to-αβ transformation Tω applied to URef* with ω = ωN.

<FIG> is flowchart of a method <NUM>. The method facilitates automatic control of a phase and amplitude of an SFC. It comprises a first step <NUM> of obtaining a fundamental control signal [Ud<NUM> Uq<NUM>]T for controlling a phase and amplitude of a carrier waveform at a fundamental frequency ωN. In a second step <NUM> of the method, a representation is obtained in a synchronous reference frame rotating at a modulation frequency ωT, of a first modulating signal [Udα Udβ]T and a second modulating signal [Uqα Uqβ]T. In a third step <NUM>, each of the first and second modulating signals is transformed into a stationary reference frame. In a fourth step <NUM>, there is provided a representation, in a synchronous reference frame rotating at the fundamental frequency, of a composite control signal for controlling the phase and amplitude of said output of the SFC. In the composite reference signal, a first component is a sum of a first component of the fundamental control signal and a first component of the transformed first modulating signal, and a second component is a sum of a second component of the fundamental control signal and a first component of the transformed second modulating signal. In an optional fifth step <NUM>, the composite control signal is transformed into a stationary reference frame, whereby the oscillation at frequency ωN is rendered explicit.

Claim 1:
A static frequency converter, SFC, comprising:
a first controller (<NUM>) configured to provide a fundamental control signal (U<NUM>(t) = [Ud<NUM> Uq<NUM>]T) for controlling a phase and amplitude of a carrier waveform at a fundamental frequency (ωN) wherein the components of the fundamental control signal (U<NUM>) correspond to a modulation with orthogonal components {cos ωNt, -sin ωNt};
a second controller (<NUM>), which is configured to:
obtain a representation, in a synchronous reference frame rotating at a modulation frequency (ωT), of a first modulating signal (Ud(t) = [Udα Udβ]T) and a second modulating signal (Uq(t) = [Uqα Uqβ]T);
transform each of the first and second modulating signals into a stationary reference frame; and
provide a representation, in a synchronous reference frame rotating at the fundamental frequency (ωN), of a composite control signal URef*(t) = [UdRef* UqRef*]Tfor controlling the phase and amplitude of an output of the SFC, wherein:
a first component of the composite control signal is a sum of a first component of the fundamental control signal and a first component of the transformed first modulating signal: <MAT> and
a second component of the composite control signal is a sum of a second component of the fundamental control signal and a first component of the transformed second modulating signal: <MAT>
the SFC further comprising a modulator (<NUM>) configured to receive the composite control signal and to provide, based thereon, said output of the SFC: <MAT>
wherein the SFC is configured to feed a railway grid (<NUM>) for electrically powering rolling stock (<NUM>), and,
the SFC is configured to be used in a testing environment (<NUM>) comprising measuring equipment (<NUM>, 141a, 141b, 141c) configured to perform electric measurements at different points of the railway grid (<NUM>) independently, and to detect variation between different locations of the railway grid (<NUM>).