Stray Current Investigations A Method of Troubleshooting Stray Current on Underground Residential Distribution (URD) Loops Stacey Mighty Malcolm Distribution System Reliability CenterPoint Energy SWEDE 2012 Introduction - What Is Stray Current • Stray Current is defined as "A current resulting from the normal delivery and/or use of electricity that may be present between two conductive surfaces that can be simultaneously contacted by members of the general public and/or their animals. • Common causes of stray current: ▫ Primary and/or secondary return current on the neutral ▫ Breaks in the neutral that causes the current to take an unintended path ▫ Improper grounding practices ▫ Induced voltages from high voltage lines • Stray current is NOT related to power system faults or direct contact with energized lines. Introduction - Utility Design Facts • CenterPoint Energy’s system is designed with 3f 4-wire Wye circuits. • The system is designed with a solidly grounded neutral. • The neutral, from the substation, is grounded multiple times throughout the distribution system, and grounded at every equipment pole in accordance with Section 9 of the National Electric Safety Code IEEE C2-2007 . Introduction - NESC Grounding Requirements CNP system design is in accordance with the NESC, IEEE C2, Section 9: Grounding Methods: LOAD • 092(B)(2) – “Grounding connections shall be made at the neutral of the source. Additional connections may be made, if desired, along the length of the neutral, where this is one of the system conductors.” • 097(B)(2) – “The grounding conductors of primary and secondary may be interconnected.” • 096(C) – Multi Grounded Systems – “The neutral, which shall be of sufficient size and ampacity for the duty involved, shall be connected to a made or existing electrode at each transformer location and at the sufficient number of additional points with made or existing electrodes to total not less than four grounds in every mile of the entire line, not including grounds at individual services.” Introduction – NESC Grounding Requirements An Illustration of What is Permitted • CenterPoint Energy Transformer Connections • In a typical installation, the system neutral is connected to the transformer primary, secondary and also to the earth. Introduction – NESC Grounding Requirements, Cont’d An Illustration of What is Permitted Typical Open Delta Equipment bonding on a pole Connections • All equipment on the pole is bonded together and connected to the neutral. • Connection is made to the grounding electrode conductor and then to the ground rod to earth. All equipment on the pole is bonded to the neutral. Introduction – National Electric Code Customer Requirements for Grounding Single Phase 3 wire Service 2011 NEC Article 250.24(A)(1) • The customer grounding requirements are in accordance with the National Electric Code Article 250. • A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service. • The grounding electrode conductor connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means. Source: 2011 NEC Handbook • 2011 NEC Article 250.26 requires the customer to bond to the utility neutral at their service entrance. Introduction – Typical Grounding at the Service Entrance Service Entrance – Single Family Home Grounding Electrode Conductor connected to the ground rod Introduction – Typical Connections inside a Main Distribution Panel Connections inside of a Main Panel – Single Family Home Un-grounded Conductors Neutral Terminal block Neutral Neutral Main Bonding Jumper Introduction – Typical Grounding at Service Entrance Service Entrance – Single Family Home Grounding Electrode Conductor connected to the ground rod All grounds are bonded to the grounding electrode at the service entrance. Neutral and Grounding Connections and Stray Current • • • • • Stray current can occur: If there is an open connection in the neutral If neutral-ground connections are compromised If there is a failed ground rod clamp If all of the premise grounding electrodes are not bonded together to form the grounding electrode system Note: grounds must be bonded together to eliminate possible difference in potential Stray Current on Underground Residential Distribution Loops • Case: ▫ A residential customer who is served from a URD loop contacts CNP to complain about being “shocked” while swimming in the pool. ▫ The customer complained that after being in the pool, there is a slight sensation of “electricity” by one of the pool lights. ▫ The customer is not aware of what length of time over which this is occurring as it is not evident unless he is swimming for a long period of time. Typical Underground Residential Distribution (URD) Loop • A typical CNP URD Loop is served from two overhead sources with a normal open point to split the load on the loop. • The configuration allows for heightened reliability and can reduce the impact of extended outages through switching. Typical Electrical One-line of a URD Loop • A typical CNP URD Loop is served from two overhead sources with a normal open point to split the load on the loop. URD Padmounted Transformer Single Phase URD padmounted transformer 19.9 kV Line to Ground Primary – 120V Secondary URD – Padmount Transformer – Typical Connection X-Feed • Concentric neutral along the primary Y-Feed • Concentric neutral along the primary Secondary-Feed • Neutral is grounded in PMT Ground Rod All Neutral conductors are grounded in the transformer Measuring current along the loop Current measurements are taken along every URD transformer at both primary and neutral connections. Method of Investigation – Initial Check • In order to determine the cause/source of the stray current the entire loop and the customer’s service has to be checked. Utility Initial Checkpoints Customer Initial Checkpoints • Ensure that the URD transformer is properly grounded and that there are no loose connections. • Ensure that there is a proper ground at the customer’s service entrance. • Disconnect customer’s service to check if the stray current still exists. • Ensure that the terminal poles at both ends of the loop have good neutral/ground connections. • Ensure that grounding is in compliance with Article 250 of NEC. • All grounding electrodes shall be tied together to form grounding electrode system. Note: Article 250.53(A)(2) Measure resistance on customer’s ground to ensure that no supplemental grounding electrode/rod/pipe is required (i.e. resistance must be less than 25 ohms. Investigation – Measurements • The initial check of the customer/utility showed no apparent causes for the stray current in the pool. • Additional Measurements had to be taken in order to detect the source of the stray current: Measure voltage at the pool light in reference to ground. Measure current along the primary and neutral at the terminal poles and at every transformer in the loop. Current measurements other than a fundamental frequency of 60Hz may not be a electric utility source. The sum of the current measurements along the loop should approach zero. Investigation – Data Gathering Current measurements were taken at all URD transformers terminal poles Location TIME PX PY NX NY TP X 9:30 16.2 PMT 1 9:43 15.4 9.4 PMT 2 9:50 6.5 4 PMT 3 10:05 7.2 1.2 5.4 0.9 PMT 4 10:08 0 0.3 1.4 1.1 PMT 5 10:22 0 1.8 0.9 0.6 PMT 6 10:30 3.9 6.2 2.1 1 PMT 7* 10:40 4.8 5.5 0.4 2.9 PMT 8 10:55 3.7 8.1 1.1 0 PMT 9 11:14 10.7 15.6 1.7 3.8 PMT 10 11:22 17.2 17.1 2 1.7 PMT 11 11:30 13.3 14.4 1.2 2.3 PMT 12 11:41 15.7 13.5 4.1 3.1 PMT 13 11:52 18.3 18.7 3.7 3.5 PMT 14 11:59 16.5 18.5 4.5 6.6 TP Y 12:04 11.3 19.1 10 6.8 2.4 8.2 Analysis of Field Findings Normal Open Point Location of Pool • We noticed that there was a change in the current along the neutral on the x and y of the transformer cable. Analysis of Field Findings, Continued Normal Open Point Location of Pool • Changes in the neutral and primary current around transformer 7 showed unexpected results. • The data along the neutral between transformers 6-7 and 7-8 showed that there was a higher change in the current than the other spans of the loop. Corrective Action Taken • Direct buried cable along three sections of the loop was replaced and placed in conduit ▫ Cable between PMT 6-7 ▫ Cable between PMT 7-8 ▫ Cable between PMT 8-9 Follow Up Measurements • The same measuring method was used to detect if there was any stray current at the pool light. • No evidence of stray current existed at the time. • If the customer complains of stray current in the pool again, cable in the entire loop needs to be assessed for replacement. References • • • • Power Distribution Engineering, James J Burke NFPA 70, 2011 National Electric Code IEEE C2, 2007 National Electric Safety Code “Impact of Transmission Lines on Stray Voltage”, IEEE Nagy Abed, Sasan Salem, and James Burke • CNP Overhead Distribution Standards • “Dangers of Stray Voltage and Stray Current”, Donald W Zipse, P.E. • “Electrical Shock Hazard due to Stray Current”, Donald W Zipse, P.E. Appendix – National Electric Code Customer Requirements for Grounding 3 Phase 4-wire Wye Service Source: 2011 NEC Handbook 3 Phase 4-wire Delta Service Source: 2011 NEC Handbook 2011 NEC Article 250.26 requires the customer to ground the utility neutral at their service entrance.