SYSTEM AND METHOD FOR HIGH RESISTANCE GROUND FAULT
DETECTION AND PROTECTION IN POWER DISTRIBUTION SYSTEMS
BACKGROUND OF THE INVENTION
[0001] The present invcntion relates generally to power distribution systems and,
more particularly, to a system and method for detecting high resistance ground faults in
a power distribution system and protecting the power distribution system from such
ground faults upon detection thereof.
[0002] A typical power distribution system includes a converter, an inverter and a
mechanical load such as a motor. The converter is typically linked to a three phase
source that provides three phase AC power and converts the three phase power to DC
power across positive and negative DC buses. The DC buses feed the inverter which
generates three phase AC power on output lines that are provided to the load. The
inverter controls the three phase AC voltages and currents to the load so that the load
can be driven in a desired fashion. Cables connect the power source to the converter
and also connect the inverter to the load.
[0003] One common type of three-phase power distribution system is an adjustable
speed drive (ASD). ASDs are frequently used in industrial applications to condition
power and otherwise control electric driven motors such as those found with pumps,
fans, compressors, cranes, paper mills, steel mills, rolling mills, elevators, machine
tools, and the like. ASDs typically provide a volts-per-hertz or vector controls and are
adept at providing variable speed andfor variable torque control to an electric driven
motor, such that ASDs have greatly improved the efficiency and productivity of electric
driven motors and applications.
[0004] Power distribution systems such as ASDs require protection from inadvertent
cable and load (e.g., motor) failures which can lead to undesirable ground faults. The
root cause of cable failures is often cable insulation breakdown and therefore most
ground faults occur in the cables between the power source and the converter or
between the inverter and the load. When a ground fault occurs, the results can be
extremely costly. For instance, ground faults often result in power interruptions,
equipment failure and damage, uncoordinated system decisions with potential for
overall plant interruptions, degraded or lost production and overall customer frustration.
[0005] Resistance grounding systems are used in industrial electrical power
distribution facilities to limit phase-to-ground fault currents. Generally speaking, there
are two types of resistors used to tie a power distribution system's neutral to ground:
low resistance and high resistance. High Resistance Ground (HRG) systems limit the
fault current when one phasc of the system shorts or arcs to ground, but at lower levels
than low resistance systems. In the event that a ground fault condition exists, the HRG
typically limits the current to 5-10A, though most resistor manufacturers label any
resistor that limits the current to 25A or less as high resistance.
[0006] HRG systems are commonly seen in industrial applications where continued
operation is important to the process, such as in power distribution systems where any
power source downtime has a dramatic economic cost. HRG systems have gained
popularity in such applications due to their ability to continue operation in lieu of a
single line-ground fault and improved ability to limit escalation of the single lineground
fault into a multi-phase event. Additionally, HRG systems function to suppress
transient line to ground over voltages during a ground fault, eliminate arc flash hazards
with phase to ground faults, and reduce equipment damage at the point of ground fault.
[0007] Since ground fault conditions in HRG systems do not draw enough current to
reliably trigger fault current sensors in an associated motor drive, ground fault
detections systems must be employed to detect HRG faults. Various such ground fault
detection systems and methods have previously been implemented to locate ground
faults. For example, in US Publication No. 2009/0296289, detection of a ground fault
in the HRG system is accomplished by injecting a common mode voltage into the three
phase system and measuring the system response, with the sensed output voltages then
being filtered to determine the HRG fault occurrence. In another example, and as set
forth in US Publication No. 200910080127, detection of a ground fault in the HRG
system is accomplished by measuring the DC bus voltage in the HRG system.
However, while such systems function to detect a ground fault in the HRG system, the
methods employed in those systems are either computationally cumbersome or intrusive
to the system. Additionally, existing ground fault detection systems and methods fail to
locate the ground fault in the HRG system (i.e., identify where and which phase the
fault occurs on). As such, challenges remain in HRG systems with respect to
identifying HRG faults in a cost effective manner and locating the HRG fault in the
system.
[0008] It would therefore be dcsirablc to provide a system and method that provides
a computationally efficient approach to detect an HRG fault in a three-phase power
distribution systcm and identify the HRG fault location in a particular phase.
BRIEF DESCRIPTION OF THE INVENTION
[0009] Embodiments of the present invention provide a system and method for
detecting HRG faults in a power distribution system and identifying the location of such
ground faults.
[0010] In accordance with one aspect of the invention, a power distribution system
includes a converter-inverter arrangement having an input connectable to a three phase
AC power source and a threc phasc output connectable to an input terminal of a load,
the converter-inverter arrangement configured to control current flow and terminal
voltages in the load. The power distribution system also includes a fault detection and
protection system connected to the converter-inverter arrangement, with the fault
detection and protection system including a plurality of current sensors configured to
measure a current on the three phase output of the converter-inverter arrangement and a
controller configured to measure the three phase current on the three phase output of the
converter-inverter arrangement, extract a fundamental current component for each phase
of the three phase output of the converter-inverter arrangement, extract a third harmonic
component for each phase of the three phase output of the converter-inverter
arrangement, compare the fundamental current component and the third harmonic
component extracted from each phase to a first threshold and a second threshold,
respectively, and detect a ground fault on a phase of the three phase output based on the
comparisons of the fundamental current component and the third harmonic component
to the first and second thresholds.
[0011] In accordance with another aspect of the invention, a method is provided for
detecting a high resistance ground fault in a power distribution system that includes an
AC motor drive in series between an AC powcr source and an AC motor, with the AC
motor drive configured to condition a three phase output to the AC motor. The method
includes measuring current on each of a first phase, a second phase, and a third phase of
the three phase output to the AC motor, extracting a fundamental current component for
each of the first phase, second phase, and third phase, and extracting a third harmonic
component for each of the first phase, second phase, and third phase. The method also
includes comparing the fundamental current component and the third harmonic
component extracted from each of the first phase, second phase, and third phase to a
fundamental component threshold and a third harmonic threshold, respectively and
detecting a ground fault on any of the first phase, second phase, and third phase based
on the comparisons of the fundamental current component and the third harmonic
component on each phase to the fundamental and third harmonic thresholds. If a ground
fault is detected, the method further includes identifying on which of the first phase,
second phase, or third phase the ground fault is present.
[0012] In accordance with yet another aspect of the invention, a system for detecting
a ground fault in a high resistance ground (HRG) power distribution system includes a
plurality of current sensors to measure current on a three phase output of an inverter in
the HRG power distribution system and a controller configured to measure the three
phase current on the three phase output of the inverter, extract a fundamental current
component for each phase of the three phase output, extract a third harmonic component
for each phase of the three phase output, compare the fundamental current component
and the third harmonic component extracted from each phase to a fundamental
component threshold and a third harmonic component threshold, respectively, and
detect a ground fault on a phase of the three phase output based on the comparisons of
the fundamental current component and the third harmonic component on each phase to
the hndamental component and third harmonic component thresholds.
[0013] Various other features and advantages of the present invention will be made
apparent from the following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The drawings illustrate preferred embodiments presently contemplated for
carrying out the invention.
[0015] In the drawings:
[0016] FIG. 1 a schematic of an adjustable speed motor drive (ASD) in a high
resistance ground (HRG) configuration, according to an embodiment of the invention.
[0017] FIG. 2 is a flowchart illustrating a technique for detection of a HRG ground
fault in the ASD of FIG. 1, according to an embodiment of the invention.
[0018] FIG. 3 is a graph illustrating HRG fault characteristics in the ASD of FIG. 1
before and after occurrence of a fault.
[0019] FIG. 4 is a graph illustrating HRG fault harmonics characteristics in the ASD
of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] The embodiments of the invention set forth herein relate to a system and
method for detecting HRG faults in a power distribution system and protecting the
power distribution system from such ground faults upon detection thereof.
[0021] Embodiments of the invention are directed to power distribution systems
encompassing a plurality of structures and control schemes. According to an exemplary
embodiment of the invention, and as shown in FIG. 1, a power distribution system 10 is
provided that may be implanted with embodiments of the invention. In the embodiment
of FIG. 1, power distribution system 10 includes an AC motor drive 11 in the form of an
adjustable speed drive (ASD) is shown in FIG. 1. The ASD 11 may be designed to
receive a three AC power input, rectify the AC input, and perform a DC/AC conversion
of the rectified segment into a three-phase alternating voltage of variable frequency and
amplitude that is supplied to a load. In a preferred embodiment, the ASD 11 operates
according to an exemplary volts-per-hertz or vector control characteristics. In this
regard, the motor drive provides voltage regulation in steady state, and fast dynamic
step load response over a full load range.
[0022] In an exemplary embodiment, a power source 12 generates a three-phase AC
input 12a-12c is fed to power distribution system 10. According to embodiments of the
invention, the power source 12 that generates the three-phase AC input 12a-12c may be
in the form of a delta or wye source transformer, although other arrangements and
power source configurations are also recognized as being able to provide three-phase
AC input 12a-12c. The three-phase AC input 12a-12c is provided to a three-phase
rectifier bridge 14. The input line impedances are equal in all three phases. The
rectifier bridge 14 converts the AC power input to a DC power such that a DC link
voltage is present between the rectifier bridge 14 and a switch array 16. The link
voltage is smoothed by a DC link capacitor bank 18. The switch array 16 is comprised
of a series of insulated gate bipolar transistor switches 20 (IGBTs) and anti-parallel
diodes 22 that collectively form a PWM (pulse width modulated) inverter 24. The
PWM inverter 24 synthesizes AC voltage waveforms with a fixed or a variable
frequency and amplitude for delivery to a load, such as an induction motor 26. DC link
chokes L1, L2 (indicated in FIG. 1 as 28) are also provided in AC motor drive I I and
are positioned on the positive and negative rails of the DC link 30. The DC link chokes
28 provide energy storage and filtering on the DC link during operation of AC motor
drive 1 1 and motor 26.
[0023] Control of AC motor drive 11 and operation of the inverter 24 is via a
controller 32, which may furthcr be comprised of a plurality of controllers that pcrform
high speed operations such as volts-per-hertz or vector control algorithms, space-vector
modulation, DC link voltage decoupling, and protection, for example. The controller 32
interfaces to the PWM inverter 24 via gate drive signals and sensing of the DC link
voltage and pole currents (by way of a voltage sensor 34 for example) such that changes
in DC link voltage can be sensed. These voltage changes can be interpreted as transient
load conditions and are used to control switching of the switch array 16 of PWM
inverter 24 such that near steady-state load conditions are maintained. Additionally,
controller 32 functions to identify ground current related faults in power distribution
system 10 and protect the power distribution system from such faults, including
protecting the power source transformer 12 such as by preventing transformer winding
insulation degradation. In performing such a fault detection and protection, control
system receives three-phase output current as input, while outputting fault identification
and protection signals responsive to the inputs, as will be explained in greater detail
below.
[0024] According to an exemplary embodiment of the invention, and as shown in
FIG. 1, power distribution system 10 is configured as a High Resistance Ground (HRG)
system that limits phase-to-ground fault currents. That is, the HRG system provides
protection from inadvertent cable and load (e.g., motor) failures that can lead to
undesirable ground faults by limiting the fault current when one phase of the system
shorts or arcs to ground. In order to configure power distribution system 10 as an HRG
system, a resistor 36 (R,,) is placed between the three-phase neutral point of the power
source 12 and ground 38. In a wye connected system, the resistor 36 is placed between
the three-phase neutral point of the power source 12 and ground 38, while in a delta
connected system, an artificial neutral is created by using a zig-zag transformer and then
the resistor 36 is placed between the artificial neutral and ground 38. The resistor 36 is
configured to limit the current to 5-10 amps, for example, in the event that a ground
fault condition exists.
[0025] Also shown in FIG. 1 is the resistance value in the faulted location in ASD
11, identified as a resistor 40 (Rf), with such a fault occurring in the form of a short
through a wirelcable, breaker, or commutator, for cxample. The rcsistancc value of Rf
can vary based on the particular fault that occurs and can range from zero to a larger
value that is typically less than the resistance of resistor 36. The resistance value in the
faulted location in ASD 11 therefore contributes to the overall fault current in power
distribution system 10.
[0026] As shown in FIG. 1, control system 32 receives three- phase output current as
inputs, such as by way of current sensors 42,44,46 positioned on the three-phase output
of ASD 11. Based on an analysis of the three-phase current, control system 32
functions to identify HRG faults in power distribution system 10 and protect the power
distribution system from such faults, including protecting against transformer winding
insulation degradation.
[0027] Referring now to FIG. 2, and with continued reference to FIG. 1, an
exemplary embodiment of a technique 50 for detection of a HRG fault in a power
distribution system, as well as for locating such an HRG fault in the system, is shown
according to an embodiment of the invention. The technique 50 may be implemented
by way of a controller associated with an ASD, such as controller 32 connected to ASD
11. As shown in FIG. 2, technique 50 begins at STEP 52 with a measuring of the phase
current from each of the three phases of the ASD output, such as by way of current
sensors 42,44,46. Upon a measurement of the phase currents of the three-phase output,
technique 50 continues at STEP 54, where a filtering of the three-phase current is
performed in order to account forlfilter out a high frequency current ripple that is
present due to IGBT switching in PWM inverter 24. For example, if a switching
frequency of 5kHz is used in the PWM inverter, then a low pass filtering can be
performed to remove the 5kHz and higher frequency current ripple components.
[0028] The measured phase current for each phase is therefore defined as:
where Il is the amplitude of the hndamental current component, 4 is the amplitude of
the third harmonic component (if it exists), and B is the speed at which the load is
commanded to run. According to one embodiment of the invention, the fundamental
component/fundamental frequency is 60 Hz and the third harmonic component is 180
Hz, as is standard in North America, although it is recognized that the fundamental
component and the third harmonic component could be 50 Hz and 150 Hz, respectively,
as is standard in Europe.
[0029] According to an exemplary embodiment, the three-phase output current of the
ASD 11 is measured periodically over a set period of time, with a plurality of current
measurements for each phase being stored in controller 32 and a current value for each
phase being calculated based on the stored plurality of current measurements for each
phase. In a next step of technique 50, the phase current is then analyzed in order to
extract a fundamental current component for each phase of the three-phase output, as
indicated at STEP 56. The fundamental current component for each phase of the threephase
output is extracted according to:
where Il is the amplitude of the hndamental current component, 13 is the amplitude of
the third harmonic component, and B is the speed at which the load is commanded to
run. In extracting the fundamental current component for each phase of the three-phase
output, both [Eqn. 21 and [Eqn. 31 are performed in order to run a self-check, with the
output of [Eqn. 31 being zero when the calculations are performed properly. It is noted
that in extracting the fundamental current component for each phase of the three-phase
output by way of [Eqn. 21 and [Eqn. 31, no total Fast Fourier Transform (FFT) is
employed in the calculations, such that the computational burden imposed on controller
32 is minimized.
[0030] In addition to analyzing the three-phase current in order to extract the
fundamental current component, tcchniquc 50 also analyzes thc three-phase current in
order to extract the third harmonic component for each phase of the three-phase output,
as indicated at STEP 58. The third harmonic component for each phase of the threephase
output is extracted according to:
where II is the amplitude of the fundamental current component, 4 is the amplitude of
the third harmonic component, and 8 is the speed at which the load is commanded to
run. In extracting the third harmonic component for each phase of the three-phase
output, both [Eqn. 41 and [Eqn. 51 are performed in order to run a self-check, with the
output of [Eqn. 51 being zero when the calculations are performed properly. Again, the
third harmonic component is calculated without using a total FFT so as to minimize the
computational burden imposed on controller 32.
[0031] Upon extraction of the fundamental and third harmonic components,
technique 50 then continues at STEP 60, where a determination is made as to whether
the fundamental component and/or third harmonic component extracted from each
phase of the three-phase output exceeds a pre-determined threshold that is set for the
fundamental and third harmonic components. The pre-determined thresholds for the
fundamental component and the third harmonic component will vary depending on the
normal fundamental component and third harmonic component levels (i.e., fundamental
and third harmonic can be 601180 Hz, 501150 Hz, etc.).
[0032] If it is determined at STEP 60 that the extracted fundamental component and
third harmonic component of any phase of the three-phase current output exceed the
pre-determined thresholds, indicated at 62, then technique continues at STEP 64 where
a HRG fault is declared as being present in power distribution system 10 and an alarm is
activated, such as some sort of audible or visual alarm. In declaring that an HRG fault
is present in power distribution system 10, the location of the fault on a particular phase
is also identified. That is, by determining on which phase of the three-phase current
output the extracted fundamental component and third harmonic component exceed the
current thresholds, the location of the fault on a particular phase is easily achieved.
Appropriate steps can then be implemented to protect components in power distribution
system 10 from damage, such as damage that might occur to the transformer windings
insulation.
[0033] Conversely, if it is determined at STEP 60 that the extracted fundamental
component and third harmonic component of any phase of the three-phase current
output do not exceed the pre-determined thresholds, indicated at 66, then technique
loops back to STEP 52, where additional measuring and analyzing of the phase current
from each of the three phases of the ASD output is performed in order to continue
monitoring for an HRG fault.
[0034] Referring now to FIGS. 3 and 4, HRG fault characteristics and fault
harmonics characteristics in power distribution system 10 (FIG. 1) are illustrated, before
and after an HRG fault occurs. In the example, shown in FIGS. 3 and 4, the power
distribution system is shown as operating normally until a time t=1.4 seconds, at which
time a ground fault occurs in one phase (e.g., Phase C).
[0035] With respect to FIG. 3, the upper window 70 therein is illustrative of the
motor speed 72 and DC link voltage 74, while the middle window 76 is illustrative of
the ASD three-phase output currents 78, 80, 82 and the lower window 84 is illustrative
of the ground current 86. As can be seen in FIG. 3, the motor speed 72 remains in a
steady state before and after occurrence of the ground fault 86 and the DC link voltage
74 remains almost undisturbed. FIG. 3 also shows that the output current on the faulted
phase, such as phase current78, changes in amplitude after occurrence of the ground
fault 86. While this change in amplitude of phase current 78 can be tolerated in the
system, the current contributes to the overall fault condition in the power distribution
system.
[0036] With respect to FIG. 4, the upper window 88 therein is illustrative of the
current components for each phase of the three-phase current output, while the lower
window 90 is illustrative of the ground fault current. As can be seen in upper window
88 of FIG. 4, the amplitude of the fundamental component on the faulted phase,
identified as 92, is greater than the amplitude of the fundamental component on the nonfaulted
phases (e.g., 130 amps versus 110 amps). Additionally, the amplitude of the
third harmonic component on the faulted phase, identified as 94, is greater than the
amplitude of the third harmonic component on the non-faulted phases. More
specifically, the amplitude of the third harmonic component on the non-faulted phases
will be zero, while the amplitude of the third harmonic component 94 on the faulted
phase may be 10 amps, for example. As shown in the lower window 90, the ground
fault current 96.
[0037] Thus, as illustrated in FIG. 4, not only can the presence of an HRG fault in
the power distribution system be detected based on an analysis of the hndamental and
third harmonic components of each phase of the three-phase current output, but the
phase and location on/at which the HRG fault occurs can be readily identified.
[0038] Beneficially, embodiments of the invention thus provide a system and method
of ground fault detection and protection in power distribution systems, including those
incorporating an ASD and having an HRG system configuration. The detection
methods implement already available three-phase output current measurements and
analyze those current measurements in a computationally efficient manner to identify
and locate HRG faults in the system. Embodiments of the invention not only identify a
ground fault condition in a power distribution system, but also determine exactly which
phase out of three motor outputs that a ground fault occurs on.
[0039] A technical contribution for the disclosed method and apparatus is that it
provides for a computer implemented technique for detecting ground faults in a power
distribution system, including systems having an ASD with a HRG configuration. The
technique extracts fundamental and third harmonic current components from each phase
of a measured three-phase current to detcct HRG faults in an ASD, with the t'echniclue
enabling identification of which phase the ground fault is present on.
[0040] According to one embodiment of the present invention, a power distribution
system includes a converter-inverter arrangement having an input connectable to a three
phase AC power source and a three phase output connectable to an input terminal of a
load, the converter-inverter arrangement configured to control current flow and terminal
voltages in the load. The power distribution system also includes a fault detection and
protection system connected to the converter-inverter arrangement, with the fault
detection and protection system including a plurality of current sensors configured to
measure a current on the three phase output of the converter-inverter arrangement and a
controller configured to measure the three phase current on the three phase output of the
converter-inverter arrangement, extract a fundamental current component for each phase
of the three phase output of the converter-inverter arrangement, extract a third harmonic
component for each phase of the three phase output of the converter-inverter
arrangement, compare the fundamental current component and the third harmonic
component extracted from each phase to a first threshold and a second threshold,
respectively, and detect a ground fault on a phase of the three phase output based on the
comparisons of the fundamental current component and the third harmonic component
to the first and second thresholds.
[0041] According to another embodiment of the present invention, a method is
provided for detecting a high resistance ground fault in a power distribution system that
includes an AC motor drive in series between an AC power source and an AC motor,
with the AC motor drive configured to condition a three phase output to the AC motor.
The method includes measuring current on each of a first phase, a second phase, and a
third phase of the three phase output to the AC motor, extracting a fundamental current
component for each of the first phase, second phase, and third phase, and extracting a
third harmonic component for each of the first phase, second phase, and third phase.
The method also includes comparing the fundamental current component and the third
harmonic component extracted from each of the first phase, second phase, and third
phase to a fundamental component threshold and a third harmonic threshold,
respectively and detecting a ground fault on any of the first phase, second phase, and
third phase based on the comparisons of the fundamental current component and the
third harmonic component on each phase to the fundamental and third harmonic
thresholds. If a ground fault is detected, the method further includcs identifying on
which of the first phase, second phase, or third phase the ground fault is present.
[0042] According to yet another embodiment of the present invention, a system for
detecting a ground fault in a high resistance ground (HRG) power distribution system
includes a plurality of current sensors to measure current on a three phase output of an
inverter in the HRG power distribution system and a controller configured to measure
the three phase current on the three phase output of the inverter, extract a fundamental
current component for each phase of the three phase output, extract a third harmonic
component for each phase of the three phase output, compare the fundamental current
component and the third harmonic component extracted from each phase to a
fundamental component threshold and a third harmonic component threshold,
respectively, and detect a ground fault on a phase of the three phase output based on the
comparisons of the fundamental current component and the third harmonic component
on each phase to the fundamental component and third harmonic component thresholds.
[0043] The present invention has been described in terms of the preferred
embodiment, and it is recognized that equivalcnts, alternatives, and modifications, aside
from those expressly stated, are possible and within the scope of the appending claims.
WO 20131184332
We Claim:
1. A power distribution system comprising:
a converter-inverter arrangement having an input connectable to a three
phase AC power source and a three phase output connectable to an input terminal of a
load, the converter-inverter arrangement configured to control current flow and terminal
voltages in the load; and
a fault detection and protection system connected to the converterinverter
arrangement, the fault detection and protection system comprising:
a plurality of current sensors configured to measure a current on
the three phase output of the converter-inverter arrangement; and
a controller configured to:
measure the three phase current on the three phase output
of the converter-inverter arrangement;
extract a fundamental current component for each phase
of the three phase output of the converter-inverter arrangement;
extract a third harmonic component for each phase of the
three phase output of the converter-inverter arrangement;
compare the fundamental current component and the third
harmonic component extracted from each phase to a first threshold and a second
threshold, respectively; and
detect a ground fault on a phase of the three phase output
based on the comparisons of the hndamental current component and the third harmonic
component to the first and second thresholds.
2. The power distribution system of claim 1 wherein the controller is
configured to declare a ground fault when one phase of the three phase output has a
fundamental current component that exceeds the first threshold and a third harmonic
component that exceeds the second threshold.
3. The power distribution system of claim 2 wherein the controller is
configured to identify which phase of the three phase output the ground fault is present
on.
4. The power distribution system of claim 2 wherein the controller is
configured to generate an alarm when a ground fault is declared.
5. The power distribution system of claim 4 wherein the controller is
configured to define the phase current on each phase of the three phase output as:
where Il is the amplitude of the fundamental current component, 13 is the amplitude of
the third harmonic component, and 0 is the speed at which the load is commanded to
run.
6. The power distribution system of claim 5 wherein the controller is
configured to extract the fundamental current component without using a total fast
fourier transform (FFT), the hndamental current component being extracted according
to :
where II is the amplitude of the fundamental current component, 13 is the amplitude of
the third harmonic component, and B is the speed at which the load is commanded to
run.
7. The power distribution system of claim 5 wherein the wherein the
controller is configured to extract the third harmonic component according to:
where I, is the amplitude of the fundamental current component, I3 is the amplitude of
the third harmonic component, and 6' is the speed at which the load is commanded to
run.
8. The power distribution system of claim 1 wherein the third harmonic
component is zero on phases where there is no ground fault.
9. The power distribution system of claim 1 fbrther comprising a resistor
positioned between a neutral point of the three phase AC power source and earth
ground, such that the power distribution system comprises a high resistance ground
(HRG) system.
10. The power distribution system of claim 9 wherein the converter-inverter
arrangement comprises an adjustable speed drive (ASD), and wherein the controller is
further configured to filter out switching frequency components from the measured
three phase current generated by switching of a plurality of switches in the ASD.
11. A method for detecting a high resistance ground fault in a power
distribution system that includes an AC motor drive in series between an AC power
source and an AC motor, with the AC motor drive configured to condition a three phase
output to the AC motor, wherein the method comprises:
measuring current on each of a first phase, a second phase, and a third
phase of the three phase output to the AC motor;
extracting a fbndamental current component for each of the first phase,
second phase, and third phase;
extracting a third harmonic component for each of the first phase, second
phase, and third phase;
comparing the fundamental current component and the third harmonic
component extracted from each of the first phase, second phase, and third phase to a
fundamental component threshold and a third harmonic threshold, respectively;
detecting a ground fault on any of the fxst phase, second phase, and third
phase based on the comparisons of the fundamental current component and the third
harmonic component on each phase to the fundamental and third harmonic thresholds;
and
if a ground fault is detected, identifying on which of the first phase,
second phase, or third phase the ground fault is present.
12. The method of claim 11 further comprising declaring a ground fault
when a phase of the three phase output has a fundamental current component that
exceeds the fundamental component threshold and a third harmonic component that
exceeds the third harmonic threshold.
13. The method of claim 1 1 further comprising defining the phase current on
each of the first phase, second phase, and third phase as:
where II is the amplitude of the fundamental current component, I3 is the amplitude of
the third harmonic component, and Q is the speed at which the load is commanded to
run.
14. The method of claim I 1 wherein extracting the fundamental current
component comprises extracting the fundamental current component according to:
where II is the amplitude of the fundamental current component, 4 is the amplitude of
the third harmonic component, and 8 is the speed at which the load is commanded to
run.
15. The method of claim 11 wherein extracting the third harmonic
component comprises extracting the third harmonic component according to:
where I1 is the amplitude of the fundamental current component, I3 is the amplitude of
the third harmonic component, and 8 is the speed at which the load is commanded to
run.
16. The method of claim 11 further comprising filtering out switching
frequency components from the current measured on each of the first phase, second
phase, and third phase, so as to further isolate the fundamental current component and
the third harmonic component.
17. A system for detecting a ground fault in a high resistance ground (HRG)
power distribution system, the system comprising:
a plurality of current sensors to measure current on a three phase output
of an inverter in the HRG power distribution system; and
a controller configured to:
measure the three phase current on the three phase output of the
inverter;
extract a fundamental current component for each phase of the
three phase output;
extract a third harmonic component for each phase of the three
phase output;
compare the fundamental current component and the third
harmonic component extracted from each phase to a fundamental component threshold
and a third harmonic component threshold, respectively; and
detect a ground fault on a phase of the three phase output based
on the comparisons of the fundamental current component and the third harmonic
component on each phase to the fundamental component and third harmonic component
thresholds.
18. The system of claim 17 wherein the controller is further configured to
declare a ground fault when one phase of the three phase output has a fundamental
current component that exceeds the fundamental component threshold and a third
harmonic component that exceeds the third harmonic component threshold.
19. The system of claim 17 wherein the controller is configured to identi@
which phase of the three phase output the ground fault is present on based on the
extraction of the fundamental current component and the third harmonic component
extracted from each phase.
20. The system of claim 17 wherein the third harmonic component is zero on
phases where there is no ground fault.