Patent Description:
In some gas turbine systems, a gas turbine may be started and accelerated to a desired speed profile. The speed profile may be one that provides desired starting conditions for the gas turbine. The speed profile may contain details of speed, current versus time, voltage versus time, and/or power versus time, that a system such as a load commutated inverter (LCI) system may provide to a generator along with other details relevant to starting the generator and the gas turbine system. In such applications, a static starter system may be provided in conjunction with an electrical generator acting as a synchronous motor, which may be operatively coupled to a shaft of the gas turbine. During the starting sequence of the gas turbine system, the static starter system may deliver a variable frequency current to drive and control the electrical generator (e.g., by modulating exciter field voltage and/or stator current), which in turn drives the main shaft of the gas turbine into rotation. The static starter system may disengage and electrically disconnect from the generator as the gas turbine enters normal (e.g., self-sustaining) operation. It may be useful to improve fault detection, including ground fault detection, in static starter systems.

<CIT> discloses a ground fault detection system having the features of the preamble of independent claim <NUM> and a corresponding method for detecting a ground fault for use with circuit breakers and relays within <NUM> or <NUM> electrical distribution circuits.

<CIT> discloses a ground fault current limiter, in which reactors are connected in series to a neutral point for compensating for capacitances to ground of a power system, wherein occurrence of a ground fault is detected from a phase difference between a zero-phase sequence current and a zero-phase sequence voltage at time of sampling.

<CIT> discloses a power conversion apparatus in a distributed microturbine power generation system, comprising a DC link reactor, at least one source bridge converter, at least one load bridge and busses therebetween.

"<NPL>, describes on pages <NUM>-<NUM> the protection of gas turbine generators during starting, where the generator is run as a synchronous motor supplied by a static frequency converter (SFC), and mentions on page <NUM> that an independent relay connected at the generator neutral may provide a redundant trip for stator grounds faults.

In one aspect of the invention, a ground fault detection system is provided. The ground fault detection system includes a linear filter configured to receive one or more signals from a static starter system during operations of the static starter system and to produce a linear filter output. The ground fault detection system further includes a rectifier configured to rectify the linear filter output and to produce a rectifier output, and a gain system configured to multiply the rectifier output by a factor to produce a gain output. The ground fault detection system additionally includes a fault indicator system configured to indicate a ground fault based on the gain output, wherein the ground fault detection system is configured to command an action based on the ground fault. According to the invention, the linear filter is a recursive linear filter comprising poles and zeros tuned to allow a selected frequency component from the input, wherein the selected frequency component is between <NUM> to <NUM> times a load frequency, wherein the input comprises a false neutral voltage (FNV) signal, and wherein the linear filter comprises a biquad filter.

A system, not claimed as such, may include a gas turbine, an electrical generator, and a static starter system. The static starter is configured to provide a variable frequency AC signal to drive the electrical generator during a starting sequence of the gas turbine. The system further includes a ground fault detection system configured to receive one or more signals from the static starter system during operations of the static starter system and to linearly filter the one or more signals to produce a linear filter output. The ground fault detection system is additionally configured to rectify the linear filter output to produce a rectified output and to apply a gain to the rectified output to produce a gain output. The ground fault detection system is further configured to derive a mean value from the gain output to produce a mean value output and to detect a ground fault based on the mean value output, wherein the ground fault detection system is configured to command an action based on the ground fault.

In another aspect of the invention, a method is provide, that includes receiving one or more signals from a static starter system during operations of the static starter system, and linearly filtering the one or more signals to produce a linear filter output. The linearly filtering comprises applying a linear filter that is a recursive linear filter comprising poles and zeros tuned to allow a selected frequency component from the one or more signals to produce the linear filter output, wherein the selected frequency component is between <NUM> to <NUM> times a load frequency, wherein the one or more signals comprise a false neutral voltage (FNV) signal, and wherein the linear filter comprises a biquad filter The method additionally includes rectifying the linear filter output to produce a rectified output and applying a gain to the rectified output to produce a gain output. The method also includes detecting a ground fault based on the gain output, and tripping a system based on the detection of the ground fault.

As discussed further below, certain embodiments provide techniques for detecting certain faults (e.g., ground faults), in electrical systems such as static starter systems. Using the techniques further described below, a static starter system such as a load commutated inverter (LCI) system may include a ground detection system that may detect a signal at a virtual load side (e.g., generator side) neutral. For example, a generator neutral may be isolated from ground by opening a breaker/switch connected in series in a grounding arrangement during a static start sequence to protect neutral grounding transformer(s) from a possible direct current (DC) link ground fault on the LCI.

An improved system and/or process is described herein where a ground fault would be identified even with the breaker/switch closed and the generator neutral connected to ground. In certain embodiments, a ground fault on a DC link (e.g., when the breaker/switch is closed) may result in a 3rd harmonic current that flows in a generator neutral via a neutral grounding transformer (NGT). A false neutral voltage (FNV) may be used to detect the ground fault. During load commutation, the voltage signals that make up FNV may be calculated using three signals, e.g., three phase line-ground voltage signals. For example, the three phase line-ground voltage signals may be averaged to obtain the FNV. A trip would be initiated and certain blocks may be applied, for example, to a source Bridge and/or Load Bridge) of the LCI to limit the current flowing through the ground. These techniques may identify the ground fault in a few tens of milliseconds or less and would trip the LCI, for example before a core of the NGT or voltage transformer (VT) becomes saturated, which may result in excessive current flows when saturation of the core occurs. Accordingly, a more efficient ground fault detection for certain electrical systems, e.g., static starter systems, may be provided.

Turning now to <FIG>, the figure is a simplified system diagram showing an embodiment of a turbine-generator system <NUM> that includes a gas turbine <NUM>, a generator <NUM>, and a static starter system <NUM>. Before self-sustaining operation, a static starter may be used to power the gas turbine <NUM>. In self-sustaining operation, combustion of fuel in the gas turbine <NUM> may cause one or more turbine blades of the gas turbine <NUM> to drive a main shaft <NUM> into rotation. As shown, the shaft <NUM> may be coupled to a load <NUM> that may be powered via rotation of the shaft <NUM>. By way of example, the load <NUM> may be any suitable device that may generate power via the rotational output of the turbine-generator system <NUM>, such as an external mechanical load or a power generation plant. For instance, in some embodiments, the load <NUM> may include an electrical generator, a propeller of an airplane, and so forth.

During a starting sequence of the turbine-generator system <NUM> (e.g., when the turbine <NUM> is initially started up from a generally stationary position), the static starter system <NUM> may function as a variable speed AC drive system that drives the generator <NUM> as a synchronous motor. For instance, the static starter <NUM> may include a power conversion module that receives AC power from a source, such as power grid <NUM>, via an AC bus <NUM>, switches <NUM>, and a transformer <NUM> (e.g., isolation transformer dual winding secondary delta or wye providing input power via conduits <NUM>) and provides variable frequency AC power via AC breakers or fuses <NUM> and switches <NUM> to drive the generator <NUM>. A DC link inductor system (e.g., DC link reactor) <NUM> is also shown, which may be used as an inductive bridge further described below. Optional AC output inductors <NUM> may also be used. Accordingly, the generator <NUM> and static starter <NUM> may operate collectively to accelerate the turbine <NUM> in accordance with a desired speed profile. For instance, in one embodiment, a desired starting condition may be one in which the turbine <NUM> reaches a speed such that it is capable of self-sustaining operation independent from the generator <NUM> and static starter <NUM> via its own combustion processes. Once a desired speed is achieved, the static starter system <NUM> may disengage from the generator <NUM> while the turbine <NUM> continues to operate independently of the static starter system <NUM>. As can be appreciated, the use of static starter system <NUM> and generator <NUM> may be beneficial in that it reduces the need for a separate starting device, such as an electric motor or diesel engine, and also reduces the need for torque converters associated with such auxiliary hardware, thus not only reducing overall component cost, but also freeing up space in the vicinity of the turbine unit <NUM> and reducing the overall form factor of the turbine system <NUM>.

Additionally, the static starter system <NUM> may include a ground fault system <NUM>. The ground fault system <NUM> may detect a ground fault, for example, in the DC link inductor system <NUM>, on a source side, e.g., source AC feeding into the static starter system <NUM>, and/or on a load side, e.g., output side of the static starter system <NUM>. The ground fault system <NUM> may further "trip" the static starter system <NUM> to stop any undesired effects of the ground fault in the static starter system <NUM> or in systems downstream from the static starter system <NUM> (e.g., transformers, load circuitry, and so on). The turbine-generator system <NUM> may also include a control logic or system <NUM>, which may provide various control parameters to each of the turbine <NUM>, the generator <NUM>, and the static starter system <NUM>. For instance, the control logic <NUM> may provide or generate firing commands for solid state semiconductor switching devices, such as thyristors, that may be included in the power conversion module of the static starter system <NUM>. As discussed further below, the control logic <NUM>, in accordance with aspects of the present disclosure, may in some embodiments, work with and/or include the static starter system <NUM> to detect ground faults and to respond to the ground faults.

Referring now to <FIG>, the embodiment of the turbine-generator system <NUM> depicted in <FIG> is illustrated in further detail. Particularly, <FIG> depicts in further detail certain components that may be present in an embodiment of the static starter system <NUM> shown in <FIG>. A static starter system <NUM> or LCI is also known as SFC (Static Frequency Converter). As discussed above with reference to <FIG>, during a starting sequence of the turbine-generator system <NUM>, the static starter system <NUM> may operate as a variable speed AC drive system to provide variable AC power to the generator <NUM>. In the present embodiment, the static starter <NUM> may include a power conversion module <NUM> having two series-connected source bridge converters (e.g., rectifiers) <NUM>, <NUM>, a load bridge converter(s) (e.g., inverter) <NUM>, and the DC link reactor <NUM>. As shown, the source bridges <NUM>, <NUM> feed the load bridge <NUM> through the DC link reactor <NUM> with positive DC voltage, and negative DC voltage is carried through a bus <NUM>. The current techniques are equally applicable when the source side of static starter system <NUM> has a single rectifier bridge. In case of the single rectifier bridge on source side, the isolation transformer <NUM> may have a single secondary instead of two delta or wye secondaries as shown in <FIG>.

The main input power from the AC bus <NUM> to the power conversion module <NUM> may be provided through the isolation transformer <NUM> to deliver three-phase AC input power to each of the source bridges <NUM>, <NUM>. The isolation transformer <NUM>, which may be connected to the AC bus <NUM> by a circuit breaker, may provide correct voltage and phasing to the input terminals of the source bridge rectifiers <NUM>, <NUM>, as well as isolation from the AC bus <NUM>. As shown, three-phase AC power (e.g., from grid <NUM> of <FIG>) is provided along the AC bus <NUM> to a primary winding <NUM> of the isolation transformer <NUM>. The isolation transformer <NUM> also includes two secondary windings, including a secondary winding <NUM>, which feeds the source bridge <NUM>, and a secondary winding <NUM>, which feeds the source bridge <NUM>. In the present embodiment, this arrangement may result in the three-phase AC inputs to source bridge converter <NUM> being offset by <NUM> degrees, and may also reduce unwanted harmonics in the power conversion module <NUM>. Having <NUM> degrees offset in voltages fed to rectifier bridges may also result in reduction of harmonics on the upstream of static starter.

The source bridges <NUM>, <NUM> may be line-commutated and phase-controlled thyristor bridges that, upon receiving inputs from the secondary windings <NUM>, <NUM>, respectively, of the isolation transformer <NUM>, produce a variable DC voltage output to the DC link reactor <NUM>. The DC link reactor <NUM> may provide inductance to smooth the current provided by the source bridges <NUM>, <NUM> and to keep the current continuous over the operating range of the system while also reducing harmonics. In one embodiment, the DC link reactor <NUM> may include an air core inductor. The output of the DC link reactor <NUM> may then be provided to the load bridge <NUM>, which may be a load-commutated or force-commutated thyristor bridge configured to provide a variable frequency AC output, represented here by reference number <NUM>. Accordingly, the static starter system <NUM> may be a load commutated inverter (LCI) system <NUM> suitable for use in starting the turbine system <NUM>.

In the depicted embodiment, the generator <NUM> is connected to the outputs <NUM> and include certain capacitors <NUM> and resistors <NUM> that may be grounded. Likewise, the generator <NUM> may also include neutral ground resistor(s) <NUM> and capacitors <NUM> leading to ground. In some embodiments, a neutral grounding transformer (NGT) may also be used. The static starter system <NUM> may include one or more sensors <NUM> that may be communicatively coupled to the ground fault detection system <NUM>. The sensors <NUM> may sense inductance, resistance, capacitance, voltage, amperage, frequency, or a combination thereof. In one embodiment, the ground fault detection system <NUM> may use a linear filter, such as a recursive linear filter, to analyze certain signals during use of the static starter system <NUM> to determine if a ground fault condition is occurring, as further described below.

<FIG> is a block diagram of an embodiment of the ground fault detection system <NUM> that may be used to detect a ground fault in the DC link inductor system <NUM>, on a source side, e.g., source AC <NUM> feeding into the static starter system <NUM>, and/or on a load side, e.g., output side <NUM> of the static starter system <NUM>. In the depicted embodiment, an input <NUM> may include voltage signals, such as a false neutral voltage (FNV) having voltage signals that may contain multiple different frequency components. The voltage signals that make up FNV may be calculated using three signals, e.g., three phase line-ground voltage signals. For example, the three phase line-ground voltage signals may be averaged to obtain the FNV. The input <NUM> is processed by a linear filter <NUM>, a recursive filter. In some embodiments, the linear filter <NUM> is a filter that is tuned to allow approximately between <NUM> to <NUM> times the load frequency. In an exemplary embodiment, the biquad filter allows <NUM> times the load frequency. The output of the linear filter <NUM> is then processed by a full cycle rectifier <NUM>, which takes the absolute value of the linear filter's output and passes it on to a gain system <NUM>. The gain system <NUM> then applies a gain K, e.g., multiplies the absolute value in system <NUM> by a gain K. The gain K may be between <NUM> and <NUM>, and in an exemplary embodiment, the gain K may be <MAT>.

The output of the gain system <NUM> may then be used by a mean value\integrator system <NUM> to derive a mean (e.g., statistical mean) based on a certain number of samples. That is, the two or more outputs of system <NUM> may be stored in memory and then the mean of the stored outputs may then be calculated in system <NUM>. The calculated mean may then be used to derive a fault indication via a fault indicator <NUM>. For example, the calculated mean (block <NUM>) may be compared against a value (or range of values) and if the calculated mean exceeds the value (or range of values) then a fault may be indicated. In some embodiments, instead of using the calculated mean, block <NUM> may integrate the output(s) of system <NUM> over a period (e.g., base frequency) and wait a number of cycles (e.g., wait <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more cycles) to initiate a fault or a trip condition. Thus, in the depicted embodiment a fault indicator (FI) value may be calculated after the gain K <NUM> has been applied. If a ground fault is detected the ground fault may then be used to disconnect, for example, the power conversion module <NUM>, components of the power conversion module <NUM>, input power to power conversion module <NUM>, output power of the power conversion module <NUM>, or a combination thereof. An operator may also be notified of the ground fault.

As mentioned earlier, the linear filter <NUM> is a biquad filter. For the biquad filter, a continuous time transfer function and/or a discrete time transfer function may be used. In an exemplary embodiment, the continuous time transfer function may be <MAT> where d = <NUM>, ω = 2π × <NUM>finv and T is the sampling rate of the circuitry used to sample input <NUM>. In an exemplary embodiment, the discrete time transfer function may be H(z) = <MAT> where <MAT> <MAT> and z is in a z domain (e.g., Z-transform that converts a discrete-time signal, which is a sequence of real or complex numbers, into a complex frequency-domain representation, and can be considered as a discrete-time equivalent of the Laplace transform). The variable 'd' in the transfer function of the biquad filter has an ability to control the amount of delay introduced by the filter <NUM>. Increasing d reduces the selectivity of the filter <NUM> and also reduces the delay introduced by the filter <NUM>. Inversely, decreasing d increases the selectivity of the filter <NUM> and also increases the delay introduced by the filter <NUM>. For calculating the average value of the signal u, a moving average with some window size b may be used. In one embodiment, a more optimal size b, bopt, of the moving average block may dependent on the frequency of the signal at the output of the biquad filter and it is given by <MAT>.

The ground fault detection system <NUM> may be implemented using hardware (e.g., suitably configured circuitry), software (e.g., via a computer program including executable code stored on one or more tangible computer readable medium), or via using a combination of both hardware and software elements. For example, the ground fault detection system <NUM> may be implemented as a circuit operatively and/or communicatively connected or included in the control system <NUM>. Similarly the ground fault detection system <NUM> may be implemented in software executable via the control system <NUM>. Additionally or alternatively, the ground fault detection system <NUM> may be implemented as a combination of circuitry and software that may be operatively and/or communicatively connected to the control system <NUM>.

<FIG> is a flowchart of an embodiment of a process <NUM> suitable for deriving a ground fault in certain machinery, for example, in the static starter system <NUM>. The process <NUM> may be implemented as circuitry and or computer code, for example, via the ground fault detection system <NUM>. In the depicted embodiment, the process <NUM> may receive (block <NUM>) signals representative of machine operations, such as operations of the static starter system <NUM>. As mentioned above, the signals may include signals from the inputs <NUM>, DC link reactor <NUM>, bridge converters (e.g., rectifiers) <NUM>, <NUM>, load bridge converter (e.g., inverter) <NUM>, bus <NUM>, outputs <NUM>, or a combination thereof. The signals may be sensor <NUM> signals representative of inductance, resistance, capacitance, voltage, amperage, frequency, or a combination thereof.

The process <NUM> then applies (block <NUM>) a filtering analysis to the signals. The linear filter <NUM> (biquad filter) is used to filter out the signals. The result of the filtering (block <NUM>) is a filtered frequency signal, such as a filtered load frequency signal. In certain embodiments, the filtering allows <NUM> times the load frequency. The process <NUM> then processes (block <NUM>) the results of the filtering analysis. In one embodiment, the filtering may be further processed (block <NUM>), for example, by taking an absolute value of the filtering output, then applying a gain k as described above to the absolute value, and then further finding a mean of two or more absolute values.

The process <NUM> then derives (block <NUM>) the existence of a ground fault. In certain embodiments, the ground fault may be derived (block <NUM>) by using a fault indicator (FI) value resulting from the application of the gain k. If the FI exceeds a threshold value then a ground fault may be found. The process <NUM> then acts on the derivation of a ground fault, for example, by tripping (block <NUM>) certain equipment, such as opening switches to turn off the inputs <NUM> and/or outputs <NUM>. By applying a filtering analysis and processing, the techniques described herein may find a ground fault more quickly and minimize or eliminate false positives/negatives.

Claim 1:
A ground fault detection system (<NUM>), comprising:
a linear filter (<NUM>) configured to receive as input one or more signals from a static starter system (<NUM>) during operations of the static starter system (<NUM>) and to produce a linear filter output;
a rectifier (<NUM>) configured to rectify the linear filter output and to produce a rectifier output;
a gain system (<NUM>) configured to multiply the rectifier output by a factor to produce a gain output; and
a fault indicator system (<NUM>) configured to indicate a ground fault based on the gain output, wherein the ground fault detection system (<NUM>) is configured to command an action based on the ground fault;
characterized in that
the linear filter (<NUM>) is a recursive linear filter comprising poles and zeros tuned to allow a selected frequency component from the input, wherein the selected frequency component is between <NUM> to <NUM> times a load frequency, wherein the input comprises a false neutral voltage (FNV) signal, and wherein the linear filter (<NUM>) comprises a biquad filter.