current-limiting fuses improve power quality

Current-Limiting Fuses Improve Power Quality

Lj. A. Kojovic S. P. Hassler H. Singh Cooper Power Systems Franksville, Wisconsin

C. W. Williams Jr. Florida Power Corporation

Maitland, Florida, USA

Abstract: Delivering quality power to industrial and commercial customers is of strategic business importance to utilities. In addition, business economics now require utilities to look for the most cost-effective solutions. This paper shows how power quality can be improved for customers with sensitive equipment loads by utilizing current-limiting fuses on distribution feeders and/or laterals. The installation of more expensive power conditioning equipment can be avoided in many situations. Key words: Power Quality, ITI (CBEMA) Curve, Sensitive Equipment, Current-Limiting Fuse, Expulsion Fuse, Distribution System

I. INTRODUCTION Current-limiting fuses (CLFs) are used for overcurrent protection in electric distribution systems. They have many advantages over expulsion fuses. CLFs improve safety and power quality by clearing high current faults much faster than expulsion fuses and by supporting the system voltage during operation. This results in reduced voltage dip duration. CLFs are typically applied to pole and pad-mounted transformers to prevent the consequence of disruptive equipment failures and to reduce fault let-through I2t levels. CLFs are also commonly used at locations where fault levels exceed the interrupting ratings of expulsion fuse-links and fuse-cutouts, or in confined space applications. Paper [1] presented a comparative analysis of the effects of expulsion and CLF operations in distribution systems on power quality. The analysis included field tests of both types on a distribution system in Florida. It also presented computer-modeling methods for CLFs.

This paper is a supplement to paper [1] and presents an extended analysis of CLF fuse operations on distribution system power quality using the same field test results. This paper compares transient overvoltages associated with CLF interruption in actual field installations with those obtained in high power laboratory tests. Transient overvoltages from field tests are significantly smaller and are within the power quality envelope specified by IEEE Std. 1100-1999 [2]. In addition, these overvoltages decrease significantly with distance from the fault location. Power quality improvement can be economically achieved by simply replacing expulsion fuses with CLFs in distribution feeders/laterals that are in the vicinity of substations supplying critical customer(s).

II. CURRENT-LIMITING FUSE OPERATIONAL CHARACTERISTICS Current-limiting fuses typically can interrupt currents up to 50 kA by limiting peak current magnitudes and reducing fault durations as shown in Figures 1 and 2. One characteristic is that their arc voltages are high and quickly established when operating in the current-limiting mode (Figure 2). This arc voltage supports the system voltage.

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Figure 1. 8.3 kV, 20 A CLF Operation

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Figure 2. Current and Voltage Waveforms during an 8.3

kV, 20 A CLF Fuse Operation (Field Test)

Significant improvement in power quality is achieved when fault currents are high (strong source) because CLFs operate in their current-limiting mode with very short voltage dip durations [3,4]. If fault currents are small (weak source), CLFs will wait until a current zero to operate like expulsion fuses. Under these conditions, power quality is not a concern because no appreciable voltage dip is seen in the rest of the system.

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III. FIELD TESTING

Field tests were conducted on an electrical distribution system in a residential area of Florida to determine the relative effects of expulsion fuse and current-limiting fuse operations on the power quality in the system. Several types fuses were tested (12 expulsion and 23 current-limiting fuses) and the results compared. The distribution system selected for field tests is shown in Figure 3. The circuit was a 12470/7200 V grounded wye. All conductors were aluminum with sizes shown in Figure 3. Conductors were installed in a vertical configuration on the pole with 76 cm separation between phases and 152 cm separation between C phase and neutral. Ground faults were initiated by manually connecting phase C to ground. Tests were conducted at locations #1, #2 and #4. Symmetric available fault currents were approximately 6200 Arms, 3000 Arms, and 1700 Arms at locations #1, #2 and #4 respectively.

Voltages and currents were simultaneously recorded at four locations: substation, Sites #1, #2, and #4. Instrument class CTs and PTs were used for all current and voltage measurements. Voltages were also recorded on phase B to determine if fuse operations influenced the voltages of other phases.

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3x400kvar

Tested Fuse

Closing Switch

#2 #3 #4

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Load Load Load Load

Load s/s

Section Conductor size#1-#2 3-795 Ph, 1/0 N#2-#3 3-795 Ph, 1/0 N#3-#4 3-1/0 Ph, #4 N

Figure 3. Distribution System in which Fuses were Tested

ABC

N

Source Side(from substation)

Load Side(to Sites #1, #2, #3, and #4)

Closing Switch

Tested Fuse

CT #1

CT #2

IlIs

If

Recorder

PT #2 PT #1

Figure 4. Voltage and Current Instrumentation used in

Testing at one Test Location

Two types of results are reported in the following sections for tests performed at sites #1, #2, and #4: a) C-phase voltage during fuse operation, recorded

simultaneously at all test locations by PT #1 (Figure 4), b) C-phase current during fuse operation, recorded

simultaneously at all test locations by CT #2 (Figure 4). 3.1. Tests at Site #1

Expulsion Fuse Operation. Expulsion fuses of the following ratings were tested: 12K, 40K, 50K, 65K, 80K and 100KS. For all testing locations, the lower rated expulsion fuses interrupted currents at the first current zero, while some higher rated fuses interrupted currents at the second current zero. After the current was interrupted by the expulsion fuse the system was subjected to a transient recovery voltage (TRV).

Figure 5a shows recorded voltages on C phase during a 65K expulsion fuse operation. This fuse caused a voltage dip duration of almost 15 ms because it interrupted current at the second current zero. The largest TRV of 12 kVpeak was recorded at Site #1. No overvoltages were recorded on B phase. The 10,500 A peak value of the second let-thru current loop was recorded at the substation (Figure 5b). The second current loop was higher than the first current loop. This is the result of the fault incidence angle. It is noticeable that load currents are significantly higher after the fault than before the fault, which indicates that the long voltage dip caused by expulsion fuse operation affected the load operation.

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F au lt at S ite #165 K , E xp u lsion F u se

T est #9695_04

Figure 5a. System Voltages at Substation, Sites #1, #2, and

#4 for 65K Expulsion Fuse Operation at Site #1

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Site #4

C urrentm u ltipliedby 10

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Figure 5b. Currents at Substation, Sites #1, #2, and #4 for

65K Expulsion Fuse Operation at Site #1

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The voltage dip duration for a 12K expulsion fuse operation was 12 ms even though the fuse interrupted current at the first current zero. A TRV of 1.43 p.u. was recorded at Site #1, 1.37 p.u. at the substation and 1.45 p.u. at Site #4. The peak value of let-thru current at the substation was 9600 Apeak. Recorded load currents were also significantly higher after the fault than before. The same effect on load currents was recorded at all other testing locations.

Current-Limiting Fuse Operation. CLFs of different ratings and manufacturers were tested. This paper shows results of CLF operation referenced in [5]. The main characteristics of the CLF operations were the following: short duration of the voltage dip, no influence on subsequent load currents, overvoltages with magnitudes that are less than CBEMA limits. The highest overvoltages were at the fault location. The overvoltages at locations nearer to the source were significantly lower. Let-thru currents were much smaller when compared with expulsion fuses.

Figures 6a and 6b show system voltages at the substation

and Sites #1, #2, and #4 for a 20 A CLF operation at Site #1. The voltage dip duration is only 1 ms. The highest overvoltage due to the fuse operation at Site #1 was 1.7 p.u. of the system voltage. The overvoltage at the substation was 1.65 p.u., and at Sites #2 and #4 was 1.7 p.u. and 1.5 p.u., respectively.

All CLF tests confirmed the transient overvoltages associated with CLF interruption were smaller than those obtained in high power laboratory tests. Figure 6c shows a comparison between a 20 A CLF tested in a laboratory and one that was tested at Site #1. The peak arc voltage in the laboratory was 23.3 kV or 2.28 p.u., while at Site #1 it was only 1.7 p.u. The peak arc voltages at all other locations were below 1.7 p.u.

Figure 6d shows the CLF let-thru current of 2800 Apeak at the substation, which is much smaller than expulsion fuse let-thru currents. The load currents were the same before and after the fault, which proves that the CLF operations did not affect the load operation. Oscillatory currents during the fault is due to the capacitor bank discharge.

Figure 7a shows system voltages at the substation and

Sites #1, #2, and #4 for a 65 A CLF operation at Site #1. Even in this case the voltage dip duration was only 2 ms. The highest overvoltage due to the fuse operation was again at Site #1 and was similar to the 20 A CLF operation. Figure 7b shows a comparison between a 65 A CLF tested in a laboratory and one tested at Site #1. The peak arc voltage in the laboratory was 23.4 kV or 2.3 p.u., while at Site #1 it was only 1.7 p.u., which was also significantly smaller than when tested in a laboratory. Figure 7c shows the 65 A CLF let-thru current of 5800 Apeak at the substation.

The overvoltage at the substation was 1.6 p.u., and at Sites #2 and #4 was 1.67 p.u. and 1.5 p.u., respectively.

T est #9695_07

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Figure 6a. System Voltages at Substation, Sites #1, #2, and

#4 for 20 A CLF Operation at Site #1

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Figure 6b. System Voltages at S/S and Sites #1, #2, and #4

for 20 A CLF Operation at Site #1 (zoomed from Figure 6a)

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F ield test, V = 17.4 kV(S ite #1)

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Figure 6c. Comparison of 20 A CLF Overvoltages Tested in

Laboratory and Site #1

T est #9695_07

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Figure 6d. Currents at Substation, Sites #1, #2, and #4 for

20 A CLF Operation at Site #1

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T im e [s] Figure 7a. System Voltages at Substation, Sites #1, #2, and

#4 for 65 A CLF Operation at Site #1

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Figure 7b. Comparison of 65 A CLF Overvoltages Tested in

Laboratory and Site #1

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Figure 7c. Currents at Substation, Sites #1, #2, and #4 for

65 A CLF Operation at Site #1 3.2. Tests at Site #2

Figure 8a shows system voltages at Sites #1 and #2 for a 20 A CLF operation at Site #2. The voltage dip duration was only 1 ms and the overvoltage due to the fuse operation at Site #2 was 17.4 kV or 1.7 p.u. The overvoltage at Site #1 was 1.5 p.u., and at the substation the overvoltage was only 1.45 p.u.. Figure 8b shows the 20 A CLF let-thru current of 2300 Apeak at Site #1. The load currents were the same before and after the fault, which again indicates that the CLF operation did not affect the load.

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Figure 8a. System Voltages at Sites #1 and #2 for 20 A CLF

Operation at Site #2

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Figure 8b. Currents at Sites #1 and #2 for 20 A CLF


Figure 9a shows system voltages at Sites #1 and #2 for a 65 A CLF operation at Site #2. The voltage dip duration was 7 ms and the overvoltage due to the fuse operation at Site #2 was 17.4 kV or 1.7 p.u. The overvoltage at Site #1 was 1.5 p.u., and at the substation the overvoltage was only 1.4 p.u. Figure 9b shows the CLF let-thru current of 4500 Apeak at Site #1. Pre-fault load currents were not recorded so no conclusion could be made about the effects on the load operation.

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Figure 9a. System Voltages at Sites #1 and #2 for 65 A CLF


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3.3. Tests at Site #4

Figure 10a shows system voltages at Sites #1 and #4 for a 20 A CLF operation at Site #4. Voltage dip duration was 3 ms. The overvoltage due to the fuse operation was 1.4 p.u. at Site #4. No overvoltage was recorded at Site #2. Figure 10b shows the 20 A CLF let-thru current of 1300 Apeak at Site #1. The load currents were the same before and after the fault, which again indicates that the CLF operations did not affect the load.

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Figure 10a. System Voltages at Sites #1 and #4 for 20 A

CLF Operation at Site #4

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Figure 11a shows system voltages at Sites #1 and #4 for a 65 A CLF operation at Site #4. The voltage dip duration was 5 ms. The overvoltage due to the fuse operation was 1.5

p.u. at Site #4. No overvoltage was recorded at Site #1. Figure 11b shows the 20 A CLF let-thru current of 3300 Apeak at Site #1. Pre-fault load currents were not recorded so no conclusion could be made about the effects on the load operation.

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Figure 11a. System Voltages at Sites #1 and #4 for 65 A CLF Operation at Site #4

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Maximum transient overvoltages associated with the CLF interruptions always occur at the fault location in the field and are therefore selected for comparison with those obtained in high power laboratory tests. Table 1 shows that maximum transient overvoltages recorded in the field were 25% lower than compared to overvoltages recorded in the laboratory tests, irrespective of the CLF rating. Table 1. Maximum Overvoltages for CLFs Tested in

Laboratory and at Sites #1, #2, and #4 CLF

Rating Lab. Tests

[p.u.] Site #1 [p.u.]

Site #2 [p.u.]

Site #4 [p.u.]

20 A 2.3 1.7 1.7 1.4 65 A 2.3 1.7 1.7 1.5

The field tests also show that the CLF generated

overvoltages decreased rapidly with distance from the fault. Figure 12 shows overvoltage propagation through the test system for 20 A and 65 A CLFs. Overvoltages were highest at the fault (fuse) locations, and significantly lower on both source and load sides.

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12.47/7.2 kVs/s #1

3x400kvar#2 #3 #4

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n/a - not recorded

Fuse Fault Overvoltages for Current-Limiting Fuses [p.u.]Size Location s/s #1 #2 #4

20 A#1#2#4

1.651.45n/a

1.71.51.0

1.71.7n/a

1.5n/a1.4

65 A#1#2#4

1.61.4n/a

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1.5n/a1.5

Figure 12. CLF Overvoltage Propagation through the System

IV. POWER QUALITY REQUIREMENTS

Figure 13 shows the CBEMA curve (revised by ITIC in 1996) [2], which defines a criterion for acceptable power quality. Although the voltage tolerance envelope defined by this curve was intended for single-phase (120/240 V) sensitive electronic equipment, it has grown in stature to a widely used measure of power quality performance. The CBEMA voltage tolerance envelope primarily includes the following types of disturbances: voltage dips/sags, voltage swells, and transient overvoltages.

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Figure 13. CBEMA Voltage Envelope and CLF Operation

Voltage Dips/Sags. Expulsion fuse operation can

produce longer voltage dip/sag durations than specified by the CBEMA envelope. Field tests also demonstrated that even when expulsion fuses operate within the CBEMA envelope, load operation is affected. CLFs force current zero and have very short voltage dips/sags, usually 1-2 ms in duration. CLFs meet CBEMA requirements under all operational conditions and do not affect load operation.

Overvoltages. Both CLFs and expulsion fuses cause transient overvoltages when interrupting fault currents. Overvoltages due to expulsion fuse operations are due to the recovery voltage of the circuit (TRV) and are usually lower than the peak arc voltages generated during CLF operations. However, field tests show that the voltage support provided by CLF during a fault current interruption minimizes the influence of the fault on the subsequent load operation (the load currents do not change magnitudes). At the same time the CLF’s arc voltage that provides the voltage support does

not exceed the acceptable power quality criterion as per CBEMA curve [2].

As demonstrated during CLF field tests, the CLF peak arc voltages at the fault location were always significantly lower than those from laboratory testing. Also, CLF test results show the overvoltage associated with CLF operation rapidly decreases with distance from the fault. Therefore, laboratory test overvoltages are not a good indicator of the power quality impact of CLF operation in distribution systems.

V. CONCLUSIONS Transient overvoltages associated with CLF interruption from field tests are significantly smaller than when tested in high power laboratory. These overvoltages also rapidly decrease with distance from the fault location. Overvoltage magnitudes do not vary with CLF rating and meet CBEMA power quality requirements. CLFs improve power quality by supporting system voltage during faults and reducing voltage dip duration. CLFs improve safety since they operate without the noise, flame, and violence associated with a normal expulsion fuse operation. References [1] Lj. A. Kojovic, S. P. Hassler, K. L. Leix, C. W. Williams, E.

E. Baker, “Comparative Analysis of Expulsion and Current-Limiting Fuse Operation in Distribution Systems for Improved Power Quality and Protection”, IEEE Transactions on Power Delivery, Volume: 13 Issue: 3, July 1998, Pages: 863–869.

[2] IEEE Std 1100-1999, “IEEE Recommended Practice for Powering and Grounding Electronic Equipment”, IEEE Emerald Book.

[3] K. L. Leix, Lj. A. Kojovic, M. B. Marz, G. C. Lampley, “Applying Current-Limiting Fuses To Improve Power Quality and Safety”, IEEE T&D Conference, Volume 2, April 1999, Pages: 636–641.

[4] Lj. A. Kojovic, S. Hassler, "Application of Current Limiting Fuses in Distribution Systems for Improved Power Quality and Protection", IEEE Transactions on Power Delivery, Volume: 12 Issue: 2, April 1997, Pages: 791–800.

[5] “ELF™ Current-Limiting Dropout Fuse”, Cooper Power Systems, Bulletin 240-66, August 1996.

Biographies Ljubomir A. Kojovic (SM ’94) is the Chief Power Systems Engineer for Cooper Power Systems at the Thomas A. Edison Technical Center. Stephen P. Hassler is Project Manager for Cooper Power Systems at the Thomas A. Edison Technical Center. Harinderpal Singh (SM ’97) is a Senior Power Systems Engineer for Cooper Power Systems at the Thomas A. Edison Technical Center. Charles W. Williams Jr. (SM ‘88) is a Staff Engineer for the Power Quality and Reliability Department for Florida Power-Progress Energy Company.

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current-limiting fuses improve power quality

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