Effect of Adjustable-Speed Drives on the Operation of Low-Voltage Ground-Fault Indicators

Transcription

1 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER Effect of Adjustable-Speed Drives on the Operation of Low-Voltage Ground-Fault Indicators Gary L. Skibinski, Barry M. Wood, Senior Member, IEEE, John J. Nichols, Member, IEEE, and Louis A. Barrios, Senior Member, IEEE Abstract Over the years, the petroleum and chemical industry has found increasing favor with 60-Hz low-voltage ( 600 Vac) power systems that utilize a high-resistance grounded (HRG) neutral philosophy. Historically, the older generation of adjustable-speed drives (ASDs) had little or no effect on the normal operation of ground-fault indicators (GFIs) used with the installed HRG systems. This paper focuses its investigation into nuisance GFI alarms that may occur when present-generation ASDs are retrofitted into the existing plant. The paper first reviews possible neutral grounding systems, with emphasis on the types of HRG systems possible and GFI alarm philosophy. The paper then discusses how ASDs may generate zero-sequence high-frequency noise currents in the HRG neutral circuit, which may cause nuisance ground-fault alarms and potentially mask a legitimate ground fault. GFI noise current magnitude is defined for both present and older ASD technologies (e.g., insulated gate bipolar transistor versus bipolar junction transistor drives). The effect this transient zero-sequence noise current magnitude has on GFI operation is described. Mitigation methods used at the drive to reduce ASD noise current magnitude to acceptable nonalarm levels is investigated. Filter solutions located at the HRG/GFI meter that reduce nuisance alarms are also investigated. The pros and cons of at the drive or at the meter filter solutions are supported with laboratory and field test data. Application guidelines are given to help avoid nuisance problems with a plant ground-fault protection scheme, which needs to successfully operate in the presence of multiple ASDs. Index Terms Adjustable-speed drives, common-mode noise, electromagnetic interference filters, ground-fault indicators, highresistance neutral ground, pulsewidth modulation, zero-sequence current. I. INTRODUCTION A. System Grounding Philosophy SYSTEM grounding philosophy for process industry applications is based on providing safe and reliable power distribution, while insuring maximum protection against Paper PID 00 2, presented at the 1999 IEEE Petroleum and Chemical Industry Technical Conference, San Diego, CA, September 13 15, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Petroleum and Chemical Industry Committee of the IEEE Industry Applications Society. Manuscript submitted for review September 15, 1999 and released for publication June 5, G. L. Skibinski is with Standard Drives, Rockwell Automation Allen- Bradley Company, Inc., Mequon, WI USA ( glskibinski@ra.rockwell.com). B. M. Wood is with Chevron Research and Technology Company, Richmond, CA USA ( bamw@chevron.com). J. J. Nichols is with the Wood River Refinery, Tosco Refining Company, Roxana, IL USA ( jnichols@tosco.com). L. A. Barrios is with Equilon Technology, Houston, TX USA ( labarrios@equilontech.com). Publisher Item Identifier S (01) Fig. 1. Typical neutral grounding systems. transient voltages and maintaining uptime availability during ground faults [1]. An advantage of the ungrounded system of Fig. 1 is that a single line-to-ground fault does not require immediate interruption of power flow. A disadvantage is that primary line-toground voltage transients are passed to the secondary without attenuation. Impulse testing of a transformer with an ungrounded neutral shows a 2000-V-peak 1.2 s/50 s impulse test applied line to ground on the transformer primary is virtually unattenuated line to ground on the secondary side. Another disadvantage is that neutral point is capacitively coupled to ground, allowing voltage to float toward the line voltage during transients and overstress the line-to-neutral insulation system. The principal problem with an ungrounded neutral system is that an arcing ground fault can cause escalation of the system line-toground voltages to several times normal line-to-ground voltage. This occurs as the arcing fault alternatively extinguishes and reestablishes itself, successively trapping a higher charge on the system shunt capacitance each time. This causes displacement of the system neutral, which is only connected to ground through the shunt capacitance, to a voltage that is several times normal line-to-ground voltage. Voltages to ground which are sufficient to cause insulation failures, especially in motors, are very possible [2]. An advantage of grounded wye systems in Fig. 1 is a 5 : 1 10 : 1 attenuation of transformer primary line-to-ground voltage transients. Impulse testing of a transformer with a grounded neutral shows a 2000-V-peak 1.2 s/50 s impulse /01$ IEEE

2 1424 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 test applied line-to-ground on the transformer primary had only a 200-V-peak line-to-ground voltage on the secondary side. Also, an neutral solidly connected to earth ground potential reduces the possibility of overstressing line to neutral insulation. A disadvantage is that a line-to-ground fault will interrupt power to the process line. With a solidly grounded neutral system, the problem with line-to-ground voltage escalation due to an arcing ground fault is eliminated, since the neutral is held to ground potential. However, the disadvantage is that a ground fault must now be immediately interrupted due to the higher ground fault current. The load being supplied by this circuit is immediately shut down without warning. The higher ground fault current also has the possibility to inflict more damage due to the higher energy to be dissipated. Other circuits may also be disrupted due to the severe voltage drop caused by the higher fault current. The high-resistance grounded (HRG) system of Fig. 1 adds sufficient neutral-to-ground resistance to limit the ground-fault current to a value greater than or equal to the system capacitive charging current, which is normally in the range of 1 5 A. Although the neutral is not held at earth ground potential as with the solidly grounded system, this value of resistance will limit the voltages to ground during an arcing ground fault to levels sufficiently low to avoid insulation failures. Secondary attenuation of primary line-to-ground voltage transients depends on the resistor value chosen, which is outside the scope of this paper. The largest advantage of the high-resistance neutral grounding method is that a line-to-ground fault does not require immediate shutdown of the affected equipment. When the ground-fault alarm is initiated, an orderly shutdown of the affected equipment can be organized within a reasonable amount of time period, usually 24 h. The ground fault still must be given attention, to avoid escalation into a higher magnitude phase-to-phase fault if another ground fault should occur on a different phase. A detriment of HRG systems is additional equipment cost over a solidly grounded or ungrounded system. The fault on an HRG system may be more difficult to locate than on a solidly grounded system because there is usually minimal damage and the individual affected circuit is not automatically isolated by circuit breaker or fuse action. The HRG system usually has a current pulsing scheme to enable tracking down the fault with a clamp-on ammeter. However, due to the benefits of providing for continuous operation, minimum ground-fault damage, and avoiding voltage escalation during ground faults, the HRG system, used in conjunction with a ground-fault indicator (GFI) meter, has become the standard practice in the petrochemical process industry. B. GFI Alarm Philosophy Voltage-sensing and current-sensing GFI schemes are shown in Fig. 2. In voltage-sensing GFIs, voltage across -to-ground is sensed with a high-impedance voltmeter. The high input impedance of the GFI meter insures the desired ground-fault current trip point is not desensitized by a parallel current path through the meter. (a) (b) (c) Fig. 2. Ground-fault detection schemes with HRG/GFI systems. (a) Voltage sensing. (b) Current sensing. (c) GFI sensing ungrounded system. In the Fig. 2 current-sensing GFI scheme, current through the grounding resistor passes through a current transformer (CT) and is sensed by a low-impedance ammeter on the CT secondary. The low input impedance of the GFI ammeter insures essentially all of -to-ground voltage is across the resistor and not the meter, so that the desired ground fault current trip point is not desensitized. Various GFI alarm philosophies exist for both GFI schemes. Maximum rms fault current ( ) is typically sized for 1 5 A and is defined by ( ). is the neutral grounding resistor and is rated rms line-to-neutral voltage. Maximum voltage across to ground is, which is 277 V on a 480-V system. In general, it is desirable to set an alarm trip point as low as possible without nuisance tripping. In a balanced three-phase system without ASDs, neutral voltage is theoretically near zero potential, so a GFI voltmeter would normally read 0 V and a GFI ammeter scheme would read 0 A. A relatively low GFI alarm level ( 10% of the maximum voltage or current value) is desirable to provide sensitive ground-fault detection, especially for wye-connected loads where a failure near the neutral point results in relatively little fault current. In practice, line-to-ground 60-Hz capacitive cable charging current, 60-Hz nonlinear loads such as transformer magnetizing current or fluorescent lighting ballasts wired line to ground, and unbalanced loads may exist. These parasitics are discernible on most voltmeters. As a result, a minimum practical voltage trip point is usually in the 10-V range. GFI voltmeters usually contain high-frequency filters or time delays to prevent false alarms during normal system transients that occur, e.g., across the line motor starting. A GFI ammeter approach is somewhat less sensitive in that neutral current for a typical and 10-V -to-ground voltage is barely seen on the 0 5-A scale of the GFI. Historically, a minimum pickup setting of 10% of maximum voltage or current has proven satisfactory for most applications. In a balanced three-phase system with ASDs, neutral voltage is not close to zero potential, so nuisance alarms and masking of legitimate faults on existing standard HRG and GFI system is possible. This paper discusses how ASDs generate transient zero-sequence current, which, in turn, creates a nonzero voltage at the GFI. The effect the ASD transient noise current has on GFI and methods to mitigate the effect of this noise source on existing HRG systems are described.

3 SKIBINSKI et al.: EFFECT OF ASDs ON THE OPERATION OF LOW-VOLTAGE GFIs 1425 Fig. 3. Line-to-ground I cable charging path in a solidly grounded ASD system. II. GENERATION OF ASD ZERO-SEQUENCE CURRENT A. Zero-Sequence Current of Present-Generation Insulated Gate Bipolar Transistor (IGBT) Drives Fig. 3 shows how an ASD generates a repetitive transient zero-sequence current in the neutral circuit of the drive feeder transformer. Transformer secondary ( ) is connected to the ASD ( ) ac input. An ASD input diode bridge converts the ac input voltage to a dc-bus voltage ( ). The ASD three-phase inverter uses six IGBTs semiconductor switches to connect to the line-to-line ( ) ac output. The motor frame is grounded to a plant ground grid at Potential #3 and a motor Power Equipment (PE) ground wire is also connected back to the ASD PE frame ground and plant grid ground at Potential #1. The typical ASD system of Fig. 3 is earth grounded at a steel girder or ground rod arrangement at Potential #4, where the transformer neutral is solidly grounded. The ASD line-to-line output voltage waveform ( ) of Fig. 4 consists of a series of pulses controlled by a pulsewidth modulator (PWM) with peak pulse voltage equal to. Pulsewidth ( ) and dwell time at zero are PWM controlled so a fundamental sine-wave output voltage at a desired variable output frequency ( ) is obtained. Pulse spacing is controlled to maintain a constant V/Hz ratio (460V at 60 Hz or 7.6 V/Hz) to maximize torque production over the range. Output pulse rise/fall times of Fig. 4 are dictated by the ASD semiconductor switching times. Present-generation ASDs use IGBTs with rise/fall times ns. The pulse repetition rate is called carrier frequency ( ) and for IGBT ASDs is user selectable with 1 2 khz on high-horsepower drives and up to 12 khz possible on low-horsepower drives. A transient line-to-ground capacitive cable charging current ( ) is sourced from the ASD,,or outputs of Fig. 3 during every pulse edge of Fig. 4. The magnitude and is between V/ s, depending on system voltage and IGBT rise time. Two paths exist in Fig. 3, one through line-to-ground cable capacitance ( - ) and another through line-to-ground capacitance ( - )ofthe motor stator winding. Peak magnitude is simply defined as - -, and observed as high as 25 Apk for long cable runs and high-horsepower motors. current returns to ASD ground Potential #1 either by the PE ground wire or through the motor-asd ground grid, depending on which is Fig. 4. PWM line-to-line voltage V, line-to-ground I cable charging current, voltage V at earth ground Potential #4, and common-mode (CM) voltage V due to I flowing across Potential #1 to Potential #2 in the ground grid. Fig. 5. Laboratory-measured V and I of 480-V drive at 30 Hz, 300-ft motor cable ( 0 at offset 4 div; 2 ms/div; 2 A/div; 100 V/div). the lower impedance at the oscillation frequency. No physical connection exists between ASD PE ground Potential #1 and the ASD dc bus. Thus, the only remaining path to complete the route back to the ASD is through the feeder transformer neutral and drive input phase wires ( ). The key point is that transient goes through the feed transformer neutral circuit and its magnitude is determined by the sum total of system impedance in the path. Also, the transformer neutral circuit will sum currents from multiple ASDs on the same feeder. The resulting instantaneous neutral current waveform will change, based on the additive or subtractive nature of polarity magnitude and set point of the respective ASDs. A convenient way of investigating all three ASD line-to-ground outputs simultaneously is by establishing a virtual neutral reference node to ground. voltage in Fig. 3 is the voltage generator behind the current sourced from the ASD and is measured from a wye-connected node of three 1 M resistors to ground. As seen in Fig. 5, the waveform is comprised of a low-frequency 180-Hz-amplitude modulated ripple component that is further modulated by the PWM high-frequency switching. The ( ) 180-Hz ripple waveform is due to the three-pulse-ripple voltage measured at the input diode bridge ( ) terminal to true earth ground [3]. Likewise, ( ) 180 Hz ripple is due to three-pulse-ripple voltage measured at the input diode bridge ( ) terminal to ground. Fig. 6 is a time-expanded scale of Fig. 5 showing the higher frequency step-like PWM voltage component. Fig. 6

4 1426 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 Fig. 6. Expanded V voltage and I charging current at f = 4 khz ( 0 at offset 3 div; 50 s/div; 2 A/div; 100 V/div). current waveform is the combined zero-sequence or CM line-toground current from all ASD phases. The 8-Apk current spike of Fig. 6 is typical of systems using solidly grounded transformers. Exact calculation of magnitude is discussed in [4]. Although khz in Fig. 6, the current spike repetition rate into the ground circuit is closer to 20 khz, because every phase switching contributes line-to-ground current transients. Thus, the repetition rate is estimated as. B. Zero-Sequence Current of Past-Generation BJT Drives Previous-generation ASDs used BJT semiconductors with rise/fall times of 1 2 s. For the same system capacitance, magnitude is lower than in IGBT drives because of the lower output. Also, BJT drives had a maximum of 750 Hz 1.5 khz. Thus, reduced current and lower explains why there were fewer problems of nuisance GFI alarms with past-generation drives. III. EFFECT OF ASD ZERO-SEQUENCE CURRENT ON GFIs System grounding philosophy affects the ASD transient zero-sequence current magnitude and circuit path. The ungrounded system of Fig. 1 breaks the return path back to the ASD in Fig. 3. Thus, and CM noise voltage (e.g., ) developed across the ground grid of Fig. 3 does not exist. The grounded wye system of Fig. 3 detrimentally completes a transient zero-sequence noise current return path as previously discussed. current is highest with grounded systems. However, the current is contained totally within the transformer s solidly grounded neutral ( ) and secondary circuit. zero-sequence current cannot flow into the transformer primary circuit of Fig. 3. Thus, the benefit of a solidly grounded transformer of Fig. 3 is that it controls the return path to the ASD and insures that other sensitive equipment or plant HRG unit connected on the primary side are not affected. An HRG system has (1 5 A) in series with the zero-sequence noise current return path which significantly reduces peak current. Thus, CM voltage differences across the ground grid are smaller than with solidly grounded systems. The bottom trace of Fig. 7 shows that in a balanced three-phase system with the ASDs off, the neutral point voltage at is close to zero potential. The top trace of Fig. 7 shows that in a balanced three-phase system with ASDs operating, the neutral point voltage is not close to zero potential, which is due to the ASD zero-sequence current developing a transient voltage Fig. 7. Voltage across R with ASD operating at f =6kHz and with ASD off. Top trace: with ASD operating ( 0 level at offset 4, 50 V/div, 1 ms/div). Bottom trace: with ASD off ( 0 level at offset 1, 50 V/div, 1 ms/div). across the high resistance in to ground. The GFI meter usually has insufficient filter action to prevent false alarms during the voltage spike intervals induced by ASD transients. As a result, the initial desired GFI alarm/trip point must be substantially increased to prevent false or nuisance trips. Consider the single ASD operating waveform of the top trace of Fig. 7 which contains a high-frequency rms voltage of 45 Vrms and voltage spikes up to 175 Vpk across. It is unclear where the GFI alarm must be set. The problem is that the GFI set point may have to be so high that it cannot reliably detect a ground fault when the ASDs are operating. Also, unless precautions are taken in rating the neutral grounding resistor, the power dissipation of the ASD high-frequency component in the resistor may exceed the power rating, especially when combined with the added fundamental frequency component during a ground fault. IV. METHODS TO REDUCE CURRENT IN THE NEUTRAL There are a few methods that minimize, but may not eliminate, ASD zero-sequence current and its effect on the ASDgenerated voltage developed across the neutral grounding resistor. Lower the carrier frequency. Reduce the number of ASDs on a system transformer. Operate the ASD closer to 60 Hz. Add CM chokes to drive output. Add output line reactor. Add an electromagnetic interference (EMI) filter to ASD input. Add CM capacitors to each ASD dc bus. Investigate lighting surge capacitors in plant. Use shielded or armor cable. Use a drive isolation transformer. Lowering the carrier frequency to the minimum allowable value (typically 2 khz) will increase the time between transient current spikes in Figs. 6 and the top trace of 7, so that the GFI meter responds to a lower effective RMS noise voltage. Section VII-D test results of a single ASD drive show the ASD-generated voltage read on the GFI meter is 1/4 1/6 lower at khz as compared to 6 khz. Decreasing the number of ASDs on a transformer is impractical and may not reduce the -to-ground voltage to acceptable GFI levels. The GFI meter readout obtained under single drive operation may not necessarily increase as the number of

5 SKIBINSKI et al.: EFFECT OF ASDs ON THE OPERATION OF LOW-VOLTAGE GFIs 1427 Fig. 8. Zero-sequence current generated during switching interval with and without CM cores added. Fig. 9. Addition of drive EMI filter to reroute I back to ASD input and away from the HRG unit in the Xo neutral circuit. ASDs on the transformer is increased. This result is verified and discussed in Section VIII-A using actual field test data. In any event, unacceptable GFI readouts still occur as increased numbers of ASDs are added to the system. Section VII-D test data show ASD-induced -to-ground voltage decreases as is increased from 0 to 60 Hz. At low speeds, the PWM controller typically gates two or three phases on simultaneously, which tends to double and triple the total transient magnitude into ground. As increases, the higher motor counter EMF (CEMF) voltage per phase tends to reduce magnitude in the - branch circuit to ground in Fig. 3. Thus, the total value (and, therefore, -to-ground voltage) tends to decrease as speed increases. As increases past 45 Hz, the PWM controller enters the pulse-dropping mode which continuously reduces the actual number of output carrier frequency pulses as speed is increased toward 60 Hz. Thus, as speed is further increased toward 60 Hz, the quantity of transient current spikes into ground is reduced as well as the rms value and -to-ground rms voltage. However, placing restrictions on the ASD operating to reduce the GFI -to-ground voltage is impractical. Adding CM inductors to each drive output may reduce peak transient current (6 Apk reduced to 2 Apk in Fig. 8), but is usually not enough to get acceptable GFI meter readings. Adding low-impedance output line reactors (3% 5%) to each drive will reduce peak transient current to lower levels, but at substantial size, cost, weight, and output voltage drop penalties. An acceptable 10-V GFI reading with an ASD operating with output reactors and a 1-A neutral grounding resistor results in only 35-mA rms allowed in the neutral circuit. Thus, there is no guarantee that GFI meter readings will be acceptable with output reactors. The drive EMI filter of Fig. 9 may reduce transient magnitude to very low values in the neutral circuit. The filter s high-frequency bypass capacitors ( ) from line to ground reroute transient away from the grounding resistor and back to the ASD inputs ( ). Series filter inductors ( ) present high impedance to the filter input and further reduce the possibility of flowing in the neutral circuit. CM capacitors added on each drive from to ground and to ground act as high-frequency bypass capacitors which reroute transient current back to the ASD dc bus in Fig. 3 and away from the grounding resistor in the trans- Fig. 10. Use of a solidly grounded drive isolation transformer to reroute I back to ASD input and eliminate any possibility of I flowing in the HRG unit. former neutral. The difficulty lies in choosing a capacitor that is an effective solution for every possible system configuration. Surge capacitors connected line ( ) to ground in Fig. 9 may be part of a plant s lightning protection scheme. These capacitors may inadvertently reduce GFI -to-ground voltages, since they act as high-frequency bypass capacitors to transient current. Shielded tray cable or continuous aluminum sheath armor cable with a PVC jacket will help prevent interference with other sensitive equipment when applied to the drive input and output, and when the shields are grounded on both ends, [5, Fig. 20]. Transient thus tends to flow on the cable shield and not in the ground grid. Line-to-shield capacitance of the input cable may return some of back to the ASD and reduce -to-ground voltage at the GFI. However, shielded cable will not completely solve the GFI problem, since the grounded shield at the HRG unit will allow current to reenter the neutral circuit and transformer secondary. Use of a solidly grounded drive isolation transformer, in addition to the existing main transformer and HRG unit, will totally eliminate the GFI problem as shown in Fig. 10. Transient flows only in the isolation transformer neutral circuit and not in the plant HRG neutral circuit. Section VIII-E field test data show the validity of this approach. The isolation transformer also beneficially prevents high-frequency current from flowing in the remainder of the plant electrical system ground. Although the isolation transformer solves the problem of the high-frequency current on the neutral ground resistor, the application must permit immediate tripping of the motor for a ground fault due to the solidly grounded neutral, thus negating one of the benefits of the HRG system.

6 1428 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 (a) Fig. 11. Input impedance and phase angle versus frequency of GFI voltmeter. The above methods attempted to reduce the ASD zero-sequence current in the -to-ground circuit, so that ground resistor voltage is reduced and lower GFI alarm set points are allowed. Each method has a performance versus cost expense associated with it that must be evaluated. The following sections investigate allowing system ASD current to flow, but solving the GFI readout problem with filtering at the GFI meter. V. CHARACTERIZATION OF VOLTAGE AND CURRENT GFIS Prior to designing a high-frequency zero-sequence noise filter between the HRG and GFI, it is necessary to characterize the frequency response of the respective GFIs. A. Characterization of Voltage-Sensing GFI Fig. 11 plots input impedance versus. frequency of a typical GFI voltmeter shown in Fig. 2(a). The meter looks like a simple lag filter with 60-Hz impedance of 620 k and corner cutoff frequency of 20 khz. Meter input impedance at 60 Hz is high, as compared to, and avoids desensitizing the 60-Hz ground-fault alarm set point. The 20-kHz corner frequency implies an 8- s filter time constant. Thus, some filtering of the rise-time component exists, but the ASD zero-sequence voltage at will overload the meter, causing GFI readout problems. B. Characterization of Current-Sensing GFI Frequency response of a typical current-sensing GFI system of Fig. 2(b) was evaluated at a field site. System voltage was 480 Vac. was rated at 130 so that 60-Hz ground-fault current was limited to 2 Arms. The CT had a unity primary-tosecondary ratio (5 A : 5 A) and fed a digital GFI with a 1-A alarm setting and 3-s delay. The GFI digital meter read 0 A,even with 15 PWM IGBT ac drives powered on the feed transformer. Fig. 12 shows an ASD-induced 75-Vpk voltage developed across the neutral grounding resistor (0.5 Apk in CT primary), while the CT secondary response read zero current. The CT iron lamination thickness, chosen for 60-Hz frequency, could not respond to the high-frequency ASD transient current. Thus, some GFI current-sensing schemes may inadvertently solve the GFI readout problem under ASD operation by their inherent high-frequency rejection characteristics. (b) Fig. 12. Neutral grounding resistor voltage and CT secondary current of current-sensing GFI system measured at field site with multiple PWM IGBT drives operating between Hz with f = 2:5 khz. (a) Voltage across neutral grounding resistor ( 0 at offset 5, 25 V/div). Time scale: 200 s/div. (b) CT secondary current ( 0 at offset 2, 0.2 A/div). Time scale: 2 ms/div. Fig. 13. GFI meter plus high-frequency bypass capacitor. VI. METHODS TO REDUCE ZERO-SEQUENCE VOLTAGE AT GFI The advantages and disadvantages of the following solutions used at the GFI meter location to reduce the effect of zero-sequence voltage are discussed: high-frequency bypass capacitor across grounding resistor and GFI meter; low-pass filter between grounding resistor and GFI voltmeter; impedance-buffered filter between grounding resistor and GFI meter. Fig. 13 shows a high-frequency bypass capacitor placed in parallel with the grounding resistor and GFI meter. There is a

7 SKIBINSKI et al.: EFFECT OF ASDs ON THE OPERATION OF LOW-VOLTAGE GFIs 1429 Fig. 14. GFI voltmeter with L C low-pass filter. low-frequency and high-frequency circuit design requirement. The two capacitors in series allow continued operation in the event of a shorted capacitor. The capacitors are fused to handle the rms value and a 60 Hz fault current component. Requirement 1 : Requirement 2 : Hz (2) Requirement #1 implies an effective lowering of parallel - to-ground impedance at and higher frequencies, so that the ASD-induced zero-sequence voltage across the parallel combination is substantially reduced. An 8- F capacitor is required to obtain tolerable GFI voltage readings for an ASD operating at khz and grounding resistor values between The filter does not reduce, but just bypasses through the capacitor. Requirement #2 implies 60-Hz bypass filter impedance be 10 greater than the neutral grounding resistor value, so that a 60 Hz ground-fault current is not totally bypassed around the grounding resistor. The10 factor still desensitizes the original 60-Hz fault setting by 10%. Requirement #2 implies F, while Requirement #1 implies F to reduce the ASD-induced high-frequency voltage. Thus, design tradeoffs are required. The main circuit disadvantage is that a GFI nuisance alarm, occurring as a result of ASD-induced GFI voltage at frequencies, can only be lowered at the expense of desensitizing the desired 60-Hz set-point value. Another disadvantage is that high rms ripple current goes through the capacitor. Field measurement may be required for multiple ac drive systems to determine the effective capacitor current rating required. Advantage of the bypass filter circuit is that it is a quick low-cost solution, where a desensitized 60-Hz trip value is tolerable. It may be used for applications with a low number of ASDs in the system, which have low rms capacitor current and short output cables. Circuit test results are discussed in Sections VII-D and VIII-B. Fig. 14 shows a low-pass filter inserted between the grounding resistor and the GFI voltmeter. The filter is used to attenuate the ASD-induced zero-sequence voltage occurring at (1) Fig Hz. GFI meter plus impedance-buffered low-pass filter with cutoff at and greater. Low-pass input to output filter attenuation is ( ) 40 db per decade for every decade past the cutoff frequency. Requirement #1 implies there are infinite combinations of values that satisfy the cutoff frequency. A cutoff frequency of 200 Hz is required to obtain ( ) 40-dB attenuation at ASD khz. Requirement 1 : Requirement 2 : cuto frequency (3) Requirement #2 implies filter and impedances are series connected and frequency sensitive. Ground-fault current flowing has parallel paths through the series filter branch and grounding resistor. Thus, the series filter branch must have a 60-Hz impedance 10 greater than the parallel grounding resistor, so that the original GFI alarm current set point is not desensitized. It can be shown that selecting values to meet both requirements is not possible. For example, an mh and F with cutoff frequency of 200 Hz has a 60-Hz impedance of. This circuit is not recommended, since it desensitizes the actual ground-fault 60-Hz component going through a grounding resistor by 70%, with most of the 60-Hz ground-fault current component bypassed through the lower impedance filter. This circuit would also allow a higher ground-fault current, which may not be acceptable on a system where a ground fault causes an alarm, but is not immediately tripped. Fig. 15 shows an impedance-buffered low-pass filter between the grounding resistor and the GFI voltmeter to attenuate the zero-sequence ASD-induced voltage at and greater. The filter is 95 Hz with input output attenuation of ( )40dB per decade beyond. The main difference is that the filter impedance is impedance buffered by an input autotransformer. With the autotransformer, the HI LO filter input terminals have a referred impedance at 60 Hz which meets the desired requirement. This GFI filter is analyzed and tested in Sections VII and VIII. (4)

8 1430 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 (a) Fig. 17. Measured input output attenuation of GFI filter versus frequency. (b) Fig. 16. (a) Top trace: applied 60 Hz voltage across HI LO filter input terminals; middle trace: voltage across T 1 terminals H H ; bottom trace: voltage across T 1 terminals X X. (b) Applied 60-Hz voltage across filter input terminals HI LO and filter output terminals HI LO. VII. IMPEDANCE-BUFFERED LOW-PASS FILTER DESIGN A. Conceptual Design of the Filter The input section of Fig. 15 consists of a voltage step-down autotransformer. A standard 480-V/120-V control transformer is reconnected as an autotransformer with a 5 : 1 ratio. Fig. 16(a) shows 60-Hz voltage applied to the filter input terminals, with 80% of the input voltage across of and 20% across of. Voltage across is applied to a low-pass filter section consisting of,, and. Low-voltage capacitor and inductor components rated for 600 Vac may handle much higher transient voltages across the grounding resistor because of 1:5 step-down ratio. Transformer provides impedance buffering and is critical in determining the low-pass. Low-pass section components along with the parameters set a cutoff frequency Hz. Filter input output attenuation follows the standard 40 db per decade past. Capacitor voltage is applied to voltage step-up autotransformer, which is a standard 120-V/480-V control transformer reconnected as an autotransformer with a 1 : 5 ratio. Transformer also acts as an impedance buffer, in case the output terminals are shorted together. Fig. 16(b) shows there is no input-to-output attenuation of the 60-Hz component through the filter. Voltage applied to the HI LO filter input terminals is stepped downed, passed through the low-pass section, and stepped up back to the original input voltage magnitude at the filter output terminals HI LO. The sealed filter box physical dimension is 9 in 6in 4 in and may be placed at the HRG location. B. Filter Input Output Attenuation Versus Frequency Fig. 17 is a plot of filter input-to-output attenuation versus frequency. Fundamental frequencies between Hz are not attenuated. Attenuation of ( ) 40 db per decade exists beyond the Fig. 18. Measured input impedance magnitude and phase angle versus frequency of the impedance-buffered low-pass GFI filter. 95-Hz cutoff frequency up to 1000 Hz. Third harmonic (180 Hz) voltage in the transformer neutral is attenuated by ( ) 14 db, if it exists. Attenuation greater than ( ) 40 db/decade occurs for frequencies 1000 Hz due to high-frequency skin effects, substantially increasing the ac resistance of the inductor and transformer coils. The ambient noise floor [( ) 85 db] is reached at 6 20 khz. At 20 khz and greater frequencies, the inductance of the iron components is reduced to air core values. A high-frequency magnetic skin-effect phenomenon reduces the effective permeability of the iron lamination to the permeability of air. Beyond 20 khz, the gradual decrease in attenuation versus frequency is due to parasitic winding capacitance of the magnetic components becoming predominant along with the coil skin-effect ac resistance. The following discussion identifies the critical ASD noise frequencies, which

9 SKIBINSKI et al.: EFFECT OF ASDs ON THE OPERATION OF LOW-VOLTAGE GFIs 1431 TABLE I GFI METER READINGS WHEN USED ALONE, WITH A BYPASS CAPACITOR FILTER CIRCUIT, AND WITH AN IMPEDANCE-BUFFERED LOW-PASS FILTER CIRCUIT (a) Fig. 20. Single ASD operating in laboratory at f =6 khz, f =3 Hz with 0.05 F added line to ground on drive outputs. Top trace: Xo-to-ground voltage ( 0 level at offset 4, 250 V/div, 200 s/div); bottom trace: Filter output ( 0 level at offset 1, 50 V/div, 200 s/div). TABLE II GFI Vrms OUTPUT IN FIELD VERSUS LABORATORY WITH NO SOLUTION APPLIED (b) Fig. 19. (a) Single 2-hp ASD operating at f = 6 khz, f =30Hz, 30-ft cable. Top trace: Xo-to-ground voltage ( 0 level at offset 4, 50 V/div, 1 ms/div); bottom trace: filter output ( 0 level at offset 1, 50 V/div, 1 ms/div). (b) Time scale expansion of (a) but at 50 s/div. need to be attenuated by the filter before entering the GFI meter. Corresponding filter attenuation at these frequencies is obtained from Fig. 17. Conversion of decibel-to-attenuation ratio may be done using. The predominant frequency of ASD-induced voltage across the grounding resistor that corrupts the GFI readout is the carrier frequency and frequency components of Fig. 6. Results of Fig. 17 show a 80-dB to 85-dB reduction (1 : ) and total removal of these components prior to the GFI meter connection. A second ASD frequency component developed across the grounding resistor is that due to the IGBT ns rise times shown in Fig. 4. Equivalent frequency ( ) relative to rise time is obtained from Fourier analysis of the PWM pulse and defined by. The MHz frequencies are also greatly attenuated by the filter at ( )40dBto( )50dB from Fig. 17 and verified Section VII-D. A third ASD frequency component developed across the grounding resistor is that due to a kHz oscillation following the IGBT rise time of Fig. 8. These components are also greatly attenuated by the filter at ( )60dBto( )70dB from Fig. 17 and verified in Section VII-D. Thus, the impedance buffered low pass filter is effective in removing ASD frequency components that might corrupt and falsely trigger a GFI alarm. It also protects the GFI meter from transient voltages that might occur in the system. C. Input Impedance of the GFI Filter The filter Hz input impedance value is critical when accurate ground fault detection is required. Fig. 18 is a plot of measured input impedance and phase angle with the filter output

10 1432 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 (a) (b) Fig. 21. (c) (d) Waveforms at Field Refinery Site #1a with No Filter Solution and a High-Frequency Bypass Capacitor Solution across the HRG for an ASD operating at fc = 2 khz and fo = 60 Hz. (a) Xo-to-ground voltage with No Filter Solution. (b) Xo-to-ground voltage using 13-F Bypass Filter Cap Solution across HRG. (c) Spectrum of No Filter Solution (a) showing 52 Vrms at a fundamental frequency content of 2 khz and 2-kHz harmonics. (d) Spectrum of Bypass Filter Cap Solution (b) showing 5 Vrms at a fundamental frequency content of 2 khz and 2-kHz harmonics. terminals open circuited and short circuited. The open-circuit case simulates operation with the GFI meter s 620-k input impedance. The filter 60-Hz input impedance with its output open circuited is The 60-Hz ground-fault current component, when it exists, will divide between the grounding resistor and filter input impedance. The GFI trip level is only desensitized by 6% and is an improvement as compared to other filters discussed. The 20-kHz resonant point in Fig. 18 is where the filter-input impedance is maximum. Beyond 20 khz, input impedance drops and input phase angle becomes negative, due to capacitive effects in the windings of the magnetic components. The filter input impedance does not change much with the output terminals short circuited, so that there are no large current inrushes into the filter when there is an accidental short circuit on the GFI meter terminals. D. Laboratory Test Results of Impedance-Buffered Low-Pass Filter A single 480-V 2-hp ASD was connected to an input HRG transformer with and to a motor with 30 ft of unshielded cable. The GFI voltmeter reading was recorded for ASD operation at 5, 30, and 60 Hz and with two carrier frequencies settings of khz and khz. Tests were repeated with an 8- F Bypass Capacitor Filter of Fig. 13 and Impedance-Buffered Low-Pass Filter of Fig. 15 connected to the GFI meter. Table I results show that with no filter solution, the false GFI readings increase with higher carrier frequency and tend to decrease with increasing ASD output frequency. GFI readings are generally 20 V for this single drive application with a low-capacitance cable of short length. Both filter circuits of Fig. 13 and Fig. 15 functioned properly when connected, in that they allowed the GFI meter to accurately read the desired 60-Hz zero-voltage value. Fig. 7 shows the input waveform to Fig. 15 filter for the corresponding test conditions described and the ASD operating at khz, and Hz. Filter output voltage recorded was 0 V. Fig. 19(a) shows the Fig. 15 filter input and filter output voltage waveforms for the ASD now operating at khz, and Hz. Fig. 19 shows that the grounding resistor voltage peaks are lower at the higher fundamental output frequencies. Filter output voltage at the desired zero-voltage value is still achieved under this operating condition. Fig. 19(b) is a time-scale expansion of Fig. 19(a) showing the step-like modulation voltage across the neutral grounding resistor. The next test condition attempted to simulate the appreciable cable line-to-ground capacitance of a long shielded or armor cable or a motor with a large line-to-ground capacitance. This was accomplished by adding F CM capacitors on each ASD output between line and ground. The results of Table I show large erroneous readings when the GFI meter is used alone with no filter solution. Table I shows that ASD operation at low fundamental output frequency is the worst case, as well as ASD operation at higher carrier frequency. The 8- F Bypass Capacitor Filter circuit reduces the worst case 220-V GFI reading down to 20 V in Table I, but did not reduce it to zero. This suggests that this circuit is probably best suited for single drive operation with short cables and is subject to system analysis for every drive application condition. The Impedance-Buffered Low-Pass Filter circuit reduces the worst case 220-V GFI reading down to the desired 0-V value for any ASD operating condition and provides both a technical and cost-effective filter solution. Fig. 20 shows input and output filter waveforms with the CM output capacitors added. The higher ASD output capacitance changes the -to-ground voltage waveform as compared to Fig. 19(b). It is seen that the -to-ground voltage in Fig. 20 does not fully decay to zero before the next switching instant. Peak-to-peak voltage excursions

11 SKIBINSKI et al.: EFFECT OF ASDs ON THE OPERATION OF LOW-VOLTAGE GFIs 1433 (a) (b) (c) (d) (e) Fig. 22. Waveforms at Field Refinery Site #2 with no solution, a GFI filter solution, and a solidly grounded drive isolation transformer solution applied. (a) Line-to-line output voltage of 200-hp ASD (500 V/div, 2 ms/div). Zero-sequence I current at ASD output (10 A/div, 2 ms/div). (b) Xo-to-ground voltage with No Solution showing 150-Vpk spikes and step-like modulation (50 V/div, 200 s/div). (c) GFI voltage with Impedance-Buffered Low-Pass Filter Solution (10 V/div, 10 ms/div). (d) Xo-to-ground I current in the ASD isolation transformer when a Drive Isolation Transformer Solution is added to the plant (10 A/div, 50 s/div). (e) GFI voltage across the HRG unit of the main plant transformer when a Drive Isolation Transformer Solution is applied (50 V/div, 200 s/div). on the transformer neutral -to-ground voltage can swing from ( ) 750 Vpk to ( ) 750 Vpk depending on system conditions. The GFI filter of Fig. 15 attenuates these repetitive high-frequency high-magnitude spike voltages and correctly outputs a zero-voltage value to the GFI meter. The filter output waveform in Fig. 20, with the added CM capacitors, show a small 5-V peak spike that decayed to zero in 10 s. The 8- s filter time constant in the GFI meter can effectively handle this 5-Vspike and allowed the GFI meter to record the desired zero-voltage value. VIII. FIELD TEST OF GFI FILTER SOLUTIONS The GFI voltmeter results taken before and after filter solutions were installed are presented. Results were taken at separate refinery sites during running of the standard process line. Field Site #1a): This is a 480-V HRG unit of 300 installed on a 750-kVA transformer which supplied a single 200-hp ASD and other fixed-speed loads. The ASD was connected to the motor with 600 feet of 600 V, nonshielded continuous welded armor cable. ASD operation was at khz and Hz. Field Site #1b): This is a 480-V HRG unit of 304 installed on a 1-MVA transformer with a total bus loading of 400 A. Two 25-hp ASDs with output line reactors were installed and connected to a motor with 700 ft of continuous welded armor cable. ASD operation was at khz and Hz. Field Site #2): This is a 480-V HRG unit of 250 installed on a 1-MVA main transformer which supplied a single 200-hp ASD and other plant fixed-speed motor loads. The ASD was connected to a motor with 600 ft of three single conductors plus

12 1434 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 5, SEPTEMBER/OCTOBER 2001 a ground wire which are unshielded and installed in rigid steel conduit. ASD operation was at khz and Hz. Field Site #3): This is a 480-V HRG unit of 130 installed on a 1.5-MVA transformer with a total bus loading of 40% contribution from 15 installed ASDs and 60% from other plant fixed-speed motor loads. The 15 ASDs each had 3% input line reactors and ranged in horsepower from 7.5 to 150 hp. Continuous welded armor cable was installed on the drive input and output with lengths ranging from 100 to 700 ft. ASD operation was at khz and Hz. (a) A. Test Results With No Filter Solution Field Site #4) had a single 4-kHz IGBT ASD, connected to a motor with armor cable, and operating on a transformer with an HRG unit using No Filter Solution at the GFI meter. Table II field results verify the previous laboratory data taken from Section VII-D and show erroneous GFI voltmeter readings decrease when ASD output frequency is increased. Field Site #1) data also show a similar trend for and frequency. Field Site #1a) -to-ground voltage across the grounding resistor with No Filter Solution applied is shown in Fig. 21(a). The waveform is similar in appearance to the drive output voltage waveform of Fig. 6. The ring up voltage on every drive output switching step is due to reflected wave interaction of the long output cable and motor line-to-ground capacitance. The HRG waveform has a peak voltage of 150 Vpk and a cycle that repeats at the expected 2-kHz (500 s) carrier frequency. Fig. 21(c) shows the voltage harmonic spectrum taken across the HRG/GFI unit that corresponds to the time window of Fig. 21(a). The 2-kHz 150-Vpk waveform has a fundamental component of 52 Vrms. This voltage gives false GFI readings. Due to the step-like modulated waveform, the appearance of (8 khz) (12 khz) harmonics in Fig. 21(c) are also seen as previously discussed in Section II. The long armor cable adds significant line-to-ground capacitance, so that the GFI 52-Vrms readout is close to the Table I 60-Hz laborarory data entry labeled GFI meter alone with CM output capacitors. Field Site #2) -to-ground voltage across the grounding resistor with No Filter Solution applied is shown in Fig. 22(b). The waveform is also similar in appearance to the drive output voltage waveform of Fig. 6. The HRG waveform also has a peak voltage of 150 Vpk and a cycle that repeats at the expected 2-kHz (500 s) carrier frequency. In this case, the large line-to-ground capacitance of the 200-hp motor is predominant over the smaller cable-to-conduit capacitance in determining the neutral voltage waveform. Field Site #3) -to-ground voltage across the grounding resistor with No Filter Solution applied is shown in Fig. 12. The waveform is not at all similar in appearance to previous Field Site cases. The HRG waveform, which had voltage peaks to 150 Vpk, now has an occasional peak voltage of only 75 Vpk with 25 Vpk being more common. This is due to the instantaneous ( ) and ( ) peaks of each ASD waveform adding and subtracting in the neutral circuit. The instantaneous sum of all 15 ASD currents tends to reduce the high peak values and create more of a white noise average waveform at a lower rms value. This is interesting effect, since many of the 15 drives investigated had currents of 25 Apk to ground on the ASD output. Fig. 23. (b) Waveform at Field Refinery Site #1b with ASD Output Line Reactor Solution and Impedance-Buffered Low-Pass Filter Solution for a single ASD operating at fc = 2 khz and fo = 10 Hz. (a) Xo-to-ground voltage with ASD Output Line Reactor Solution installed. (b) Voltage across GFI (output of Impedance-Buffered Low-Pass Filter) with ASD Output Line Reactor Solution and Impedance-Buffered Low-Pass Filter Solution installed. B. Solution at GFI Using a High-Frequency Bypass Capacitor Field Site #1a) -to-ground voltage across the grounding resistor with a parallel 13- F High-Frequency Bypass Capacitor Solution applied is shown in Fig. 21(b). The HRG peak voltage is now reduced by a factor of ten from 150 Vpk with No Solution to less than 10 Vpk, with a waveform cycle that repeats at the expected 2-kHz carrier frequency. The capacitor addition causes increased high-frequency noise ringing on every step edge due to interaction with transformer leakage inductance, cable inductance, or possibly some amount of motor stator leakage inductance. Fig. 21(d) shows the voltage harmonic spectrum taken across the HRG/GFI unit that corresponds to Fig. 21(b). The 2-kHz 10-Vpk waveform has a fundamental component of a 5 Vrms, which is an acceptable GFI reading The step-like waveform with the increased high-frequency ringing leads to the to harmonics being higher in magnitude. C. Solution at the ASD Using Output Line Reactor Field Site #1b) -to-ground voltage across the grounding resistor is shown in Fig. 23(a) when output line reactors were installed and one of the 25-hp ASDs shut off. The waveform is not similar in appearance to the previous No Filter Solution voltage waveforms of Fig. 21(a). The line reactor interacts with line-to-ground capacitance to smooth out the waveform. The HRG waveform has a peak voltage of 20 Vpk. In this case, it is seen that the line reactor can reduce, but not eliminate the ASD induced -to-ground voltage across the grounding resistor. D. Solution at GFI Using Impedance-Buffered Low-Pass Filter Field Site #1b) conditions of Fig. 23(a) were modified to insert an Impedance Buffered Low Pass Filter Solution between