Trip Grounding: Issues and Solutions

Bing (Michael) Xia bxia@ualberta.ca

Department of Electrical and Computer Engineering, University of Alberta

Introduction

Grounding worksite is an essential requirement of utility safe work practice. There are, however, some confusion and challenges with respect to this practice. The main issue of trip grounding, therefore, becomes how to strike a comprise between the sensitivity of protection schemes and achievability of small grounding resistance. The goal of this project is to answer the following questions:

Function of Grounding a Worksite

Compatibility of Trip Grounding with Protection

Using Shield Wire for Trip Grounding

Solutions

Grounding worksite is primarily concerned with connecting conductive metallic enclosures of the equipment to the ground system through conductors known as ground conductors. A temporary ground rod is used if the permanent ground grid is not available.

Soil resistivity is one of the main factors that can significantly affect the effectiveness of a grounding ranged. According to the statistical results, most of the grounding resistance values are in the range from 5 to 10 ohms. 95% of the grounding resistance values are less than 51.56 ohms and 87% sites have a resistance less or equal to 25 ohm.

Seasonal Change of Earth Resistance

It can be easily found that 51N has difficulties in detecting fault through a temporary rod, but can operate for more than 87% of the permanent grounding structures according to ATCO grounding data. The 50N is more conservative and it cannot operate for most of the grounding resistance cases. In summary, the commonly used overcurrent protection practice cannot cover many cases involving temporary rods or even some permanent grounding structures.

Based on the results and analysis shown in the previous sections, we can conclude that worksite grounding shall be viewed as a trip grounding practice. The primary goal of grounding is to shorten the fault duration.

• What constitutes an acceptable grounding point for safe work practice?

25kV Increase of Current (Ibase=150A)

Fault MVA Fault I [A] 25Ω 50Ω 100Ω 200Ω

10 693 246% 155% 87% 46%

20 1386 310% 174% 92% 47%

50 3464 355% 185% 94% 48%

100 6928 370% 189% 95% 48%

500 34641 382% 192% 96% 48%

Decrease of Voltage (Vbase=14.43kV)

25Ω 50Ω 100Ω 200Ω

36% 20% 10% 5%

20% 10% 5% 2%

8% 4% 2% 1%

4% 2% 1% 0%

1% 0% 0% 0%

• Grounding a worksite can reduce site voltage and increase fault current in theory;

Review of Rg Requirement

Many utility companies and provincial codes have established requirements on the upper limit of the grounding resistance. But extensive literature search failed to find documents or papers that provide justifications for the grounding resistance thresholds.

In according with the review of various industry practices, it seems that the Rg threshold of 20~25 ohms are less based on the criterion of trip grounding but more on what can be achieved by the existing grounding means or protection experiences.

C22.1-02 Canadian Electrical Code, Part I (Nineteenth Edition)

• Diameter ≥ 15.8mm;

• Length ≥ 3m.

Grounding Resistance Data from ATCO

51N 25Ω 50Ω

MVA Pickup [A] If [A] If [A]

10 280 370 232

20 376 464 260

50 456 532 278

100 485 555 283

500 508 573 288

50N 25Ω 50Ω

Pickup [A] If [A] If [A]

346 370 232

693 464 260

1732 532 278

3464 555 283

17321 573 288

Resistivity

[ohmm]

Length [m]

1 2 3 4 5

30 26 15 11 8 7

100 88 50 35 28 23

1000 881 496 352 275 227

2000 1762 991 704 551 455

Resistance Value of Temporary Rod

Fault and Pickup Current of 51N and 50N in 25kV Systems

Sample Grounding Practice

Current and Voltage Change due to Trip Grounding in 25kV System

National Electrical Code 2011

• Rg ≤ 25ohms;

• Diameter ≥ 15.8mm;

• Length ≥ 2.44m.

51N relay-ground time overcurrent protection

• Rg ≤ 20ohms.

• In reality, the main effect of grounding is on increasing fault current, not reducing the site voltage. The real benefit of grounding a worksite is to cause first fault trip, not voltage reduction;

• The criteria for grounding resistance should be established based on the objective it intends to achieve.

The main concern of using shield wire for trip grounding is the potential damage to the shield wire. The current carrying capability of common shield wires is comparable to the fault currents actually experienced in distribution and transmission systems. So a general conclusion as to the question if shield wire can be used cannot be drawn. The answer depends on the magnitude and duration of the fault and the materials of the shield wire.

Shield Wire Fault Current Capability Transient Current Capacity

of 7/no.8 AWG

Current Capability of Shield

Wire for 0.5s Fault Duration

Short-term Solution is to establish an improved worksite practice if permanent grounding points with assured small grounding resistance is not available.

• Establish a software-based tool like a spreadsheet that can determine if trip grounding can produce meaningful results for a worksite.

Mid-term Solution is to improve the coordination between trip grounding and protection design.

• Develop a systematic Rg measurement/monitoring practice; • Establish an efficient, routine-like decision making system; • Study and build cases for ungrounded practice; • Improve protection coordination practice; • Adopt proven protection schemes; • Investigate, test and adopt new relays, such as 32C.

Long-term Solution is to develop a new protection scheme that causes fast fault trip with a very small or even with a zero fault current using the unique characteristics of worksite energization.

100 200 300 400 500 600 700100

150

200

250

300

350

400

450

500

Fault Current [A]

Ste

p V

olt

age

[V]

Soil Resistivity =100ohmm

10MVA

20MVA

50MVA

100MVA

500MVA

1000MVA

Energized Line

Receiver

Grounded

Equipment

If

Current

Sensor

Ground

Trip SignalRadio Network

Small Grounding Rod

Transimitter

Recloser

Rg= 23 ohms

Rg= 88 ohms

Step Voltage Affected by Fault Current Possible Solution for Trip Grounding

Grounding Rod Requirement

• Can temporary rods, shield wires and other grounding methods meet the requirement?

• If a grounding method is not acceptable, what are the options for grounding, especially for trip grounding?

• What are the strategies to address the problem?