CONSIDERATIONS FOR GROUND-FAULT PROTECTION FOR LOW-

VOLTAGE VARIABLE-FREQUENCY DRIVE CIRCUITS SUPPLIED BY HIGH-

RESISTANCE-GROUNDED POWER-DISTRIBUTION SYSTEMS

Copyright Material IEEE Paper No. PCIC-2013-13

Merv Savostianik, P.Eng. Michael Noonan, P.Eng. Ross George, P.Eng. Mike Vangool, P.Eng. Member, IEEE Member, IEEE Member, IEEE Member, IEEE Littelfuse Startco Suncor Potash Corp. Littelfuse Startco 3714 Kinnear Pl P.O Box 2844 P.O. Box 509 3714 Kinnear Pl Saskatoon, SK S7P 0A6 Calgary, AB T2P 3E3 Saskatoon, SK S7K 3L6 Saskatoon, SK S7P 0A6 Canada Canada Canada Canada

Abstract - Many low-voltage (LV) adjustable speed drives

(ASD’s) have a built-in ground-fault function that is not capable of detecting a low-level ground fault and may require the use of a suitable supplemental protection relay for reliable detection. While high resistance grounding (HRG) has become the norm, over recent decades, in many industrial facilities the surge in application of ASD’s has challenged designers and end users to manage the attendant risks associated with achieving the intended safety and reliability of LV HRG systems, while managing the inevitable growth of loads that incorporate high-speed electronic switching equipment. Using a zero-sequence current transformer (ZSCT), connected to a ground-overcurrent relay, to detect small values of fault current in ASD circuits is accepted as a reliable approach; but where to locate the CT, upstream or downstream of the drive, is a matter of some debate. This paper reports the unsettling discovery in an industrial installation that the ground-fault-protection of low-voltage ASD’s was incompatible with the HRG systems, discusses appropriate HRG design considerations, describes the selection of a retrofit- and new-installation solution, and offers a suggestion for the optimal ZSCT location.

Index Terms — Ground fault, earth fault, earth leakage, variable-frequency drive, adjustable-speed drive, variable-voltage variable-speed drive, high-resistance grounding, VFD, ASD, VSD, VVVFD, HRG, NGR.

I. INTRODUCTION The IEEE accepted definition of “High Resistance Grounding”

implies that, through purposely introducing resistance that limits the flow of ground-fault current, a system can survive the faulted condition for some time without exacerbating damage [1]. Traditionally speaking, this approach was well suited to industrial loads, the majority of which comprised fixed-speed motors operating on pure sinusoids. HRG systems appear to have performed well through earlier generations of solid-state machine control. However, the application of more recent ASD technology, incorporating faster switching power semi-conductors, demands more careful attention to HRG and system response to the dynamic behavior of switching and conversion elements during both faulted and unfaulted conditions. More than a decade ago, IGBT (insulated-gate bi-polar

transistor)-based PWM (pulse width modulation) drives supplanted LV products that featured slower devices, resulting

in cost effective life-cycle improvements to all aspects of the development, operation, and maintenance of industrial facilities. But what impact have these improvements made on the reliability of HRG systems, and how does the plant balance the need to selectively identify and isolate phase-to-ground faults against the risk of greater loss when such faults are left unchecked? This paper reviews factors that may affect ground fault

detection with some LV ASD products in HRG systems, and makes reference to industrial solutions that have been recommended and implemented by others. The experiences of a user and manufacturer in implementing a ZSCT-based selective ground fault detection scheme is described, including field observations, and laboratory testing and simulations that led to an acceptable solution. Finally, the rationale for optimum ZSCT location and component frequency response is presented.

II. BACKGROUND ASD’s have replaced many traditional approaches to driving

loads in industrial facilities. New developments in motor and drive technology permit users to replace high RPM fixed speed motors, coupled to maintenance-intensive geared drive trains, with simple, adjustable, slow speed solutions. A good example of this evolution is conventional cooling tower fans. The reliability of cooling tower equipment is critical to the production of a refinery or petro-chemical facility. Many traditional cooling tower fan applications that demanded speed adjustment made use of a NEMA induction motor designed for two-speed operation. ASD – motor combinations can be much more efficient than their fixed-speed counterparts, resulting in reduced life cycle costs. Consequently, the benefits of retrofitting such installations can be substantial [6, 7]. A recent case involved the ASD retrofit and upgrade of nine

150-HP two-speed machines in cooling tower service with 480-V 250-HP induction motors. The decision to retrofit this system was based on improved process control, soft starting, enhanced system efficiency, increase in process throughput, and reduced mechanical maintenance. Fig. 1 illustrates the system single line of the proposed installation. The neutrals of the WYE-connected LV secondary’s of the transformers in Fig. 1 were high-resistance grounded through a

neutral grounding resistor (NGR). Fault current was limited to 5 A. Notable features of this system included the following:

� Motor branch feeder long lead lengths � Parallel asymmetric 3-conductor branch feeders � 6 – pulse PWM drives

� Carrier frequencies > 2500 Hz � NGR rated for 277 Volts, 5 Amps - continuous � NGR filters not considered � Motors located in Class 1 Div. 2 hazardous location � 9 drives on a 480V secondary selective system

Fig. 1 Retrofit Single Line

III. SYSTEM FAILURES

The system developed the following ground fault issues:

1. Arcing ground faults downstream of the drive outputs that were not detected by the LV ASD ground-fault-detection schemes.

2. Undetected phase-to-ground faults resulted in damage beyond repair in some cases. Refer to Figs. 2 and 4.

3. Under various operating scenarios and loading conditions, phase-to-ground faults caused the system NGR to overheat. The resistor was not sized for a traditional AC system. Refer to Fig. 3 and Section V.

4. Phase-to-phase faults, and trips, developed from faults that originated as phase-to-ground faults that went undetected.

Fig. 2 Ground Fault in Motor Terminal Box

Fig. 3 NGR Thermal Damage

Fig. 4 Effects of Phase-to-Ground Fault - Stator

IV. IGBT-BASED PWM ASD BEHAVIOR AND RELATED EFFECTS

A number of factors relating to conventional ASD’s will influence the performance of an overall HRG system. Some of these are described as follows:

� Fast switching IGBT-based PWM drives have a significant advantage over the relatively low-frequency switching techniques available in the older, more traditional products. Because the output can be at a much higher frequency, harmonic distortion is moved to higher frequencies, making filtering easier. The lower-order harmonics from the output of a PWM inverter can be reduced by increasing the number of pulses per half-cycle. This is achieved through a reduction in the pulse width, which was not previously achievable using older technologies such as bi-polar junction transistors (BJT’s) [8, 9]. Under normal operating conditions, the induction motor load will attenuate the higher order harmonics, resulting in a near sinusoidal motor current [10].

� Unfortunately, increasing the carrier frequency to reduce pulse width is achieved at the expense of creating a high-frequency zero-sequence (ZS) voltage which is not attenuated when the system is subjected to a ground fault. The system capacitive reactance is attributable to the phase-to-ground cable capacitance, the presence of any system phase-to-ground filter capacitors, and the phase-to-ground capacitance of the motor stator coils. Since the system capacitive reactance is inversely proportional to the applied frequency, the combined effects of very fast switching and high inverter carrier frequencies will result in the flow of, zero-sequence currents through the system NGR, even when a ground fault is not present. This effect can be more pronounced for longer cables, and if multiple ASD’s are served from the same system, the neutral current will be influenced by the instantaneous values, positive or negative, of ZS current produced by each circuit [3, 5].

� Another source of ZS current flow is influenced by cable cross sectional geometry. The motor branch feeder cable asymmetry can result in induced ground current in the cable ground conductor due to the asymmetric arrangement between the three phase conductors of a cable and a single ground conductor. The ZS current increases with cable length, and, if parallel feeders are installed, an even greater ZS current contribution will result under normal operating conditions [11].

� A voltage difference referred to as Neutral Shift can result in ASD systems, wherein a voltage difference develops between source neutral and motor neutral. Some constant current source inverters can produce very high neutral displacement values with motor phase-to-neutral values as high as 3.3 per unit. This condition may also contribute to ZS current flow through the NGR [4,12].

� If isolation transformers are used to mitigate high carrier-frequency effects at the inverter output,

transformer magnetic material (core steel) must have better high frequency characteristics for the PWM circuit, unless these high frequencies are filtered before the transformer [8].

V. INFLUENCE OF ASDs ON NGR SIZING

� NGR sizing is partly determined by estimating or measuring ZS current flow through the NGR. NGR sizing must also consider the fact that the RMS dc link rail-to-rail voltage is typically 35% higher than the ac rms input voltage. For a 480V system with 480V ac applied to the ac-to-dc converter input, the resulting open-circuit dc voltage at the converter output is nearly 650V [4]. With a zero-impedance dc-bus fault, the NGR will dissipate significantly more power, in the form of heat, than it will with a zero-impedance line-side phase-to-ground fault. Fig. 3 illustrates the effects of thermal damage to an NGR assembly as a result of a phase-to-ground fault at one of the motors encountered in the retrofit case. The NGR was sized without consideration for ASD effects.

� Unfaulted ZS current flow through the NGR may exceed predetermined alarm set points, causing nuisance alarms and masking legitimate alarms.

� Further complicating the NGR sizing problem is the need to take into consideration future additions and modifications to system loading and load characteristics. For instance, modifications to the carrier frequency for a group of drives served from a common bus may have a profound effect on the ZS current flowing through the NGR. NGR sizing should also attempt to address the effects of secondary selective operating scenarios on high frequency current flow through the NGR such as when two busses are served from a single transformer source with the secondary tie closed.

VI. INPUT – OUTPUT ASD ISOLATION A popular misconception that was dealt with during LV ASD laboratory testing is the notion that an ASD isolates the load from the supply. The most common approach to ASD design involves the use of a dc link between the input power converter (rectifier) and output power converter (inverter). A capacitor or inductor is usually placed between these two stages. The ac-to-dc conversion and dc-to-ac conversion of the ASD can be controlled independently; however, ignoring converter losses, the law of conservation of energy must apply. In the case of the ASD the average energy that flows through the input and output stages must be equal. If the inverter is 100% efficient, the average input power equals the average output power. While the input and output energy delivery can be at different frequencies, and with differing waveforms, the dc components must be the same. The differences between the instantaneous input and output energy must be absorbed or delivered by an energy storage element. This load-balancing element is located on the dc link, and can be a capacitor, an inductor, or both, depending upon the drive topology [8]. This

understanding supports the notion that a ZSCT placed on the line side of the ASD can detect ground faults within the drive and on the load-side cable and motor. A description of the laboratory tests and a summary of the resulting supporting data are discussed in later sections of this paper.

VII. COMPENSATING FOR ASD BEHAVIOR – “TO TRIP OR NOT TO TRIP”

A number of proven solutions for managing the adverse

effects of ASD’s on system grounding, NGR energy dissipation, ZS voltage mitigation, and others have been developed. Exploring these solutions is beyond the scope of this paper [3,4,5,7,8,10]. Implementing one or several of the proposed solutions will depend on the unique circumstances encountered in each case, and on the level of expertise and resources available to the end users. In the case described earlier, the authors and end users evaluated and implemented a solution that addressed the needs of the operating facility. The end users determined that system conditions, design variables, and factors contributing to the difficulty in detecting and isolating ground faults on ASD circuits warranted a solution that would deliver selective and immediate ASD circuit identification and isolation. In this case the advantages of allowing a ground fault to persist on ASD circuits were considered of marginal benefit based on the pre-retrofit operating experience, the inherent redundancy in critical ASD circuits, the excess cooling capacity of the cooling water facility, the cost of maintenance, and the potential for losses arising from undetected phase-to-ground arcing faults in critical assets and hazardous locations. The details of the final solution are discussed in subsequent sections of this paper.

VIII. LV ASD GROUND-FAULT DETECTION Many ASD’s are designed for use on solidly grounded

systems and their ground-fault detection feature often requires a low-impedance system ground in order to function at all. If an ASD has a built-in ground-fault-protection function, it is primarily used to detect a load-side (motor or cabling) ground fault to protect the drive from an overcurrent condition that would not be quickly interrupted by overcurrent protection (IEEE Device 50 or 51). Although some models have an adjustable pickup level, the pickup level is typically fixed at a large fraction (say, one third or one half) of the ASD nominal current rating—often higher than the HRG prospective ground-fault current. While researching this topic, the authors discovered that the details of the ground-fault-protection function for many ASD’s are not readily available in manufacturer documentation and customer-support portals.

IX. HIGH-RESISTANCE GROUNDING A high-resistance-grounded (HRG) system includes a

current-limiting neutral-grounding resistor (NGR) that is installed in the system neutral-to-ground path. The neutral is created by the wye-wound connections of a three-phase transformer secondary or generator winding, with a zigzag transformer, or the like. The several benefits of HRG systems, including improved system reliability through the control of transient overvoltages, reduced insulation voltage stress, enhanced ground fault detection and clearing, reduced common-mode noise, limiting transferred earth potentials, regulatory compliance, improved selective coordination [2], lower probability of shock and arc flash, and reduced equipment damage due to ground faults [1,3,4], have made this the grounding system of choice in many industries and geographies. The HRG system should add sufficient neutral to ground resistance to limit the ground fault current to a value equal to or greater than the system capacitive charging current [1,5]. The ground-fault-current limitation that is the heart of an HRG system—5-A NGR’s are common in 480- and 600-V systems—may inhibit the built-in ground-fault-detection feature of many ASD’s by ensuring that not enough ground-fault current can be present to be detectable by the ASD. (Note that for a small drive, less than 10 hp, a 5-A ground fault is a detectable overcurrent or current-unbalance condition.) Table 1 shows ground-fault protection pickup levels for several ASD manufacturers and models. If ground-fault protection is required for such an ASD circuit, a supplemental protection device must be used—a ground-fault protection relay that is optimized for the rigors of an ASD HRG application—one that can reliably detect low-level faults in an electrically noisy environment.

TABLE 1 ASD GROUND-FAULT PROTECTION LEVEL

Vendor ASD Model Power Rating (HP)

GF HRG Compatible

A 1 > 15 No

2 > 15 No

B 1 > 15 No

2 > 15 No

3 > 15 No

4 > 15 No

C 1 > 15 Yes

2 > 15 Unknown

D 1 < 15 Yes

2 > 20 No

3 < 40 Yes

4 > 50 No

5 > 75 No

E 1 > 3 No

2 > 5 No

5 > 7.5 No

F 1 > 15 No

2 > 15 No

3 > 7.5 No

4 > 2 No

X. LV ASD TOPOLOGY

Fig. 5 shows a typical six-pulse LV ASD component layout,

connected to an HRG supply and a motor. The ASD does not isolate the line supply from the driven load.

Fig. 5 Six-Pulse LV ASD with Motor and HRG Supply

Switch pairs (S1, S2), (S3, S4), and (S5, S6) connect motor phases A, B, and C to the +DC or –DC bus. Semiconductor devices such as insulated-gate bipolar transistors (IGBTs) typically act as fast-functioning switches, and the switching action is in a PWM or vector format. Depending on the drive’s waveform-generation method, the high-frequency components may be fixed or spread over a wide range. In most ASDs, the high-frequency spectrum changes based on operational parameters. A. LV ASD Motor-Terminal Ground Fault

Consider a motor-terminal ground fault on phase A. Current is switched by S1 and S2. When S1 is closed, current flows through the NGR and through diodes D1, D3, and D5. In this case, the fault current is defined as positive. The fault current is not pure dc because it will contain a 180 Hz ripple (3 pulses per cycle). When S2 is closed, current flows through the NGR and through diodes D2, D4, and D6. In this case, the fault current is defined as negative. As in non-ASD applications, the current is a function of the NGR and system phase-to-neutral voltage for a ground fault on the load side. Without an effective output filter, the load-side ground-fault current contains the following components: 1. The PWM carrier or spread-spectrum of the ASD, with the instantaneous current defined by the NGR resistance and instantaneous phase-to-neutral voltage. 2. A ripple component, which is a function of the rectifier design and supply frequency. For a standard 6-pulse ASD with a 60-Hz supply, the ripple is 180 Hz. Ripple reduces as

higher-pulse systems are used. The 180-Hz ripple component is approximately 15% of the total. 3. A drive-output-frequency fundamental component, the magnitude of which is a function of the modulation level and the NGR value. (A pulse-width-modulated signal consists of a carrier and a modulation signal. For the ASD, the modulation level controls the degree to which the pulse width changes and this defines the fundamental magnitude.) The fundamental component magnitude is a function of output frequency and is a maximum at high output frequency or speed. Since the motor impedance decreases at low frequency, fundamental-output voltage magnitude is reduced by the ASD to avoid motor saturation—a constant volts-to-hertz ratio is approximately maintained. As a result, the drive-output ground-fault current will be less at low-frequency output as compared to high frequency. Fig. 6 shows the load-side phase-to-ground voltage on a typical LV ASD running at 33 Hz. The waveform includes the 33-Hz fundamental component, the 180-Hz ripple from the 6-pulse rectifier, and the PWM carrier signal. The 33-Hz modulation of the output drive frequency is difficult to see because of the time scale.

Fig. 6 LV ASD Output Voltage to Ground

Fig. 8 shows the same voltage measurement as Fig. 6 with a 500-Hz low-pass filter applied.

Fig. 8 LV ASD Filtered Output Voltage to Ground

If a ground fault is applied, the ground-fault current will contain these same components. Figs. 7 and 9 show the waveforms of both the voltage and current at the point where a fault resistance is applied.

Fig. 7 LV ASD Voltage to Ground and Ground Current

Fig. 9 shows the same two waveforms as Fig. 7 with a 500-Hz low-pass filter applied.

Fig. 9 Filtered Voltage to Ground and Ground Current

XI. SUPPLEMENTAL GROUND-FAULT

PROTECTION

While other ground-fault detection methods such as ASD dc-bus voltage analysis have been proposed [13], ZS-current sensing remains the method that can detect low-level faults across the full ASD-output-frequency band while providing the ability to identify or automatically de-energize the faulted circuit. A window-type zero-sequence current transformer (ZSCT) placed around the system phase conductors, and connected to an appropriate ground-fault protection relay (IEEE Device 50G or 51G) can add the required protection to an LV ASD. A standard ZSCT and ac-current-sensing protection relay will respond to frequencies down to about 20 Hz and can include all higher frequencies or incorporate a filtering method to exclude response to high-frequency common-mode and harmonic content. Specialized AC/DC ground-fault relays (IEEE Device 50G or 51G and 76G) are available that can detect 0-Hz and higher-frequency faults using ZSCT’s. The location of the ZSCT determines the protection zone—placement on the ASD

load side detects a ground fault in the motor or cabling from the ASD to the motor, while placement on the ASD line side includes the ASD in the protected zone. In the latter case, an AC/DC-sensing relay will also detect a dc-bus or rectifier ground fault, and in both cases such a device is better suited to detecting a ground fault when the ASD is operating at a very low frequency.

The authors built and instrumented a laboratory simulation to show that a LV ASD does not isolate the load from the supply and that a load-side ground fault can be detected with a ZSCT placed at either location, or at the NGR. Figs. 10 and 11 illustrate the test configuration.

Fig. 10 LV ASD HRG GF Test Setup During laboratory testing, ground-fault relays were placed at

the three locations, labeled GFx in Fig. 10, and current- and frequency-defined ground faults were imposed. The responses of the relays under test were recorded by instrumenting their analog outputs. A recording digital power analyzer was used to display and capture voltage and current waveforms at the point of fault and at the NGR.

Fig. 11 LV ASD HRG GF Laboratory Setup

On a grounded system, a ZSCT will detect a phase-to-

ground fault that is downstream (load side) of its location. (It will also detect the charging current of the downstream feeder when a ground fault occurs elsewhere on the system, but that topic is beyond the scope of this paper.) A CT measuring NGR current will detect a phase-to-ground fault anywhere on the connected system (provided there is no power transformer

between the supply and the ground fault)—effectively the entire system is downstream of this CT. A. Test Results

Tests at various ASD-output frequencies in the range of 10 to

60 Hz were performed using ground-fault relays with both 80- and 400-Hz low-pass filters to demonstrate ground-fault-detection performance with each filter. Figs. 12 and 13 show the point-of-fault and NGR voltage and

current signatures during a 60-Hz 300-mA ground-fault test. The former shows the wide-band signal and the latter is 500-Hz low-pass filtered.

Fig. 12 60-Hz 300-mA Fault, wide-band Filter

Fig. 13 60-Hz 300-mA Fault, 500-Hz Low-Pass Filter The traces in both Figs. 12 and 13 are, respectively, faulted-

phase-to-ground voltage, faulted-phase-to-ground current, supply-neutral-to-ground voltage, and neutral current. It can be seen from the similarities in the waveforms at the two locations that ASD-load-side ground-fault current flows to the supply neutral. Table 2 shows the test results for phase-to-ground faults

placed at location GF1, the motor terminals. Power-analyzer measurements are labeled PA1.1, PA1.2, PA2.1, and PA2.2, corresponding to the ground-fault location and NGR-current measurement, and to a wide-band and 500-Hz filter selection. The protection relays each had a selectable Discrete Fourier

transform (DFT) filter with a 32 to 86-Hz response, and a peak-detection filter with a 20 to 410-Hz response. The relay analog-output measurements, scaled to correspond to CT-primary ZS current, are shown in Table 2 as GFR1.1, GFR1.2 to GFR3.1 and GFR3.2 showing measurements upstream and downstream of the ASD and at the NGR, and to the narrow- and wide-filter pass bands. (Test runs with the relays set to the DFT and peak filter selections were conducted separately.)

TABLE 2 LV ASD GF LABORATORY TEST RESULTS

PA 1.1

PA 1.2

PA 2.1

PA 2.2

GFR 1.1

GFR 1.2

GFR 2.1

GFR 2.2

GFR 3.1

GFR 3.2

Power Analyzer (mA) Ground-Fault Relays (mA)

Point of Fault NGR ASD Line ASD Load NGR

Test Frequency

(Hz)

Wide

Band

500

Hz

Wide

Band

500

Hz DFT Peak DFT Peak DFT Peak

60 255 254 251 -- 243 -- 251 --

294 247 296 247 -- 264 -- 263 -- 268

50 373 258 375 259 260 -- 260 -- 260 --

315 219 318 219 -- 265 -- 253 -- 268

40 535 302 537 305 270 -- 270 -- 270 --

369 208 371 208 -- 268 -- 258 -- 268

30 762 337 763 337 330 -- 300 -- 330 --

452 202 452 202 -- 260 -- 270 -- 260

20 No Data

696 224 699 223 -- 312 -- 312 -- 325

10 No Data

1030 224 1030 224 -- 400 -- 380 -- 400

A high level of correspondence between the ac-sensing

ground-fault relays, and between the ground-fault relays and the power analyzer is observed, with decreasing agreement as the ASD fundamental frequency is reduced below the ground-fault relay low-end frequency response (here, 32 Hz for the

DFT filter and 20 Hz for the peak filter). This clearly illustrates that an ASD does not isolate load-side ZS current from the supply. B. AC/DC Ground-Fault Protection

On ASD circuits that often operate near or below the typical

ac ground-fault relay frequency-detection limit, consideration should be given to choosing a supplemental protection relay that will respond to the full operating-frequency range of the ASD,such as those applied in the retrofit case. Table 2 shows the both decreasing signal-to-noise ratio in

the power analyzer ground-fault current at low frequencies (due to the constant carrier-frequency component and decreasing fundamental component to maintain the volts/hertz ratio) and a decreased ac ground-fault-relay accuracy at low frequencies (due to decreased ZSCT sensitivity). An AC/DC-sensitive device is required. The frequency-response characteristics of a particular product is shown in Table 3. This device can use one or two ZSCT’s with pickup settings up to 5,000 mA.

TABLE 3 AC/DC SENSITIVE EARTH-LEAKAGE RELAY

FREQUENCY RESPONSE CT

Input 1

CT Input 2

Filter A Filter B Filter C Filter D

3 dB Freq Response

(Hz) 0 to 90 20 to 90

20 to 3,000

20 to 6,000

190 to 6,000

Pickup Range (mA)

30 to 5,000

30 to 5,000

30 to 5,000

30 to 5,000

30 to 5,000

XII. CONCLUSIONS There are substantial benefits to implementing high-

resistance grounding and adjustable-speed drives in industrial power utilization systems. Both are considered essential elements of reliable and efficient process facilities, and when they are both used together, the combined effects must be evaluated, presenting challenges for the engineer, the drive manufacturer, and the end user. Like many engineered solutions, each requires an analysis of the total effect on system performance. . The maximum ground-fault current might be larger than

expected—dc-bus voltage exceeds system voltage. The ground-fault protection or detection devices required in an HRG system might not be adequate when ASD’s are also used—harmonic frequencies can lead to false operation. The built-in ground-fault protection/detection function of many low-voltage ASD’s is not compatible with HRG power-distribution systems—their detection level is often higher than the prospective ground-fault current. When ground-fault protection for the feeder supplied by the drive is required, a supplemental ZS-current-sensing ground-fault protection relay can be used. The choice of ground-fault protection-relay characteristics includes an evaluation of the expected operating-frequency range of that drive. The decision to locate the ZSCT upstream or downstream of the drive determines whether the drive will be included in the ground-fault monitored zone. The decision to trip, or not, under ground-fault conditions

must include an evaluation of the impact of leaving the faulted motor in service.

XIII. REFERENCES [1] IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, IEEE Std. 142-1991. [2] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Std. 141-1993. [3] J. P. Nelson, P.K. Sen, “High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry”, PCIC – 1996 - 3 [4] J.C. Das, R.H. Osman, “Grounding of AC and DC Low-Voltage and Medium-Voltage Drive Systems”, Pulp and Paper PID 97-19 [5] G.L. Skibinski, B.M. Wood, J.J. Nichols, L.A. Barrios, “Effect Of Adjustable Speed Drives On The Operation Of Low Voltage Ground Fault Indicators”, PCIC-99-04 [6] R. McElveen, K. Lyles, B. Martin, W. Wasserman, “A New More Reliable Solution For Cooling Tower Drives”, PCIC-2009-13 [7] M.J. Melfi, “Quantifying The Energy Efficiency Of Motors Fed By Adjustable Frequency Inverters”, PCIC-2009-9 [8] J.G. Kassakian, M.F. Schlecht, G.C. Verghese, Principles of Power Electronics. Massachusets Institute of Technology: Addison Wesley, pp. 170-191, 235-238, 1992 [9] S. Nasar, “Electric Machines and Power Systems, Volume I – Electric Machines”, McGraw Hill, Inc., pp. 345-389. 1995. [10] C.W. Lander, “Power Electronics”, McGraw Hill, pp.207-221, 378-405. 1993 [11] G.L. Skibinski, B. Brown, M. Christini, “Effect of Cable Geometry On Induced Zero Sequence Ground Currents With High Power Converters”, PCIC-2006-19 [12] “Motors & Generators”, National Electrical Manufacturers Association, NEMA MG1-2006 Part 31 [13] Lixiang Wei; Zhijun Liu, "Identifying ground fault location in High Resistance Grounded systems for Adjustable Speed Drive at low speed," Energy Conversion Congress and Exposition (ECCE), 2012 IEEE , vol., no., pp.3609,3616, 15-20 Sept. 2012

XIV. VITAE

Mervin J. Savostianik has a Bachelor of Science degree in

Electrical Engineering from the University of Saskatchewan and is a registered Professional Engineer who has been with Littelfuse Startco since 1997. In his capacity as Sales Engineering Manager he has worked with many system designers and end users to find solutions for electrical safety including ground-fault relaying and neutral-grounding-resistor applications. Mervin is a member of the Institute of Electrical and Electronics Engineers (IEEE) and co-authored the previously published paper; “Why Neutral-Grounding-Resistors Need Continuous Monitoring. He can be reached at msavostianik@littelfuse.com. Michael Noonan is a University of Calgary Electrical

Engineering graduate and an IEEE member. He has been engaged in domestic and international oil & gas, mining, and electric utility industries for 35 years in roles including: electrical engineer, contract consultant, project manager, and superintendent. He currently leads the electrical discipline in Suncor Energy’s Corporate Technical Standards Group in Calgary, Alberta where he provides standards development and technical support services for Suncor’s domestic, international, and offshore businesses. He can be reached at mnoonan@suncor.com Ross George holds a Bachelor of Science degree from the University of Saskatchewan in Electrical Engineering. He is a registered Professional Engineer and is a member of the Institute of Electrical and Electronics Engineers. He has worked with low and medium voltage adjustable speed drives in oilfield, mining and refining applications throughout his career. He can be reached at ross.george@potashcorp.com Michael P. Vangool received his B. Sc. (Electrical

Engineering) Degree for the University of Saskatchewan in 1978. He worked as a Research and Development Engineer at the University of Saskatchewan and developed a shaft-voltage monitor for turbo-generators used by the Saskatchewan Power Corporation. In 1979 he joined Startco Engineering Ltd., where he has been involved in the development of more than thirty products including single-function ground-fault relays, multi-function motor- and feeder-protection relays, and solid-state motor starters. He is currently a Senior R&D Engineer at Littelfuse Startco. He can be reached at mvangool@littelfuse.com.

ATTACHMENT A

LARGE-FORMAT IMAGES OF FIGURES 6, 7, 8, 9, 12, AND 13

WAVEFORM SCREEN CAPTURES

Fig. 6 LV ASD Output Voltage to Ground, wide band

Fig. 8 LV ASD Output Voltage to Ground, 500-Hz low-pass filtered

Fig. 7 LV ASD Voltage to Ground and Ground Current, wide band

Fig. 9 LV ASD Voltage to Ground and Ground Current, 500-Hz low-pass filtered

Fig. 12 60-Hz 300-mA Fault; point-of-fault and NGR voltages & currents, wide band

Fig. 13 60-Hz 300-mA Fault; point-of-fault and NGR voltages & currents, 500-Hz low-pass filtered