Ungrounding the capacitor bank reduces harmonics and neutral-to-earth voltages.
Harmonic voltage and currents on the distribution system have been largely an industrial issue caused by large motors with variable-speed drives. However, harmonic currents are increasing on the distribution system as a result of growing residential use of non-linear loads, including computers, compact fluorescent light bulbs, TVs, DVD players, variable-speed heating, ventilating and air conditioning components, car chargers and various other single-phase loads. Unlike the large industrial drives, the predominant harmonic current for these single-phase loads is the third harmonic, or triplen harmonic. Managing this harmonic can be a difficult challenge for electric utilities.
Balanced non-triplen harmonic currents cancel out in the neutral of a system and stay on the phase conductors, but since triplens are additive and do not cancel each other out, the neutral carries the sum of the triplen harmonics present in each of the phase conductors. As the neutral current travels back toward the substation, it takes parallel paths. Some of the current travels the neutral conductor back and some travels through earth. The current that travels through earth creates neutral-to-earth voltage (NEV).
Progress Energy was alerted to an issue with stray voltages at a customer's site. One of the main concerns reported was NEV at a natural gas supply point located on a customer's facility across the street from a substation. The utility indicated it had measured voltages of up to 33 V at a relief valve within this substation. During a site visit, these measurements were confirmed. The utility also measured NEV levels of 15 V (maximum) between pole grounds and earth along the feeder located in front of the customer's facility.
Progress Energy line and service technicians checked for voltage across all splices and connections in the neutral. They identified a few issues during the checks, and work to correct these issues was completed in August 2009. NEV was reduced after correcting the identified issues; however, there were still NEVs ranging between 5 V and 7 V at the gas substation relief valve.
In an attempt to further reduce NEV, the substation bus current also was balanced to reduce neutral current flows as much as practically feasible.
Upon measuring the voltage at the natural gas substation again and still obtaining NEVs from 5 V and 7 V with a Fluke 87 voltmeter, technicians then measured voltage with a Fluke 41B meter to determine the harmonic content of the voltage. The remaining NEV was found to be mainly third-harmonic voltage. The substation information on the feeder monitor system (FMS) indicated significant third-harmonic current flow to the substation. It could be seen from the waveform that the dominate frequency of the neutral current was three times the normal 60-Hz fundamental frequency, which is the third harmonic.
Once technicians determined third-harmonic flow on the system neutral was causing the remaining NEV, the question then became how to reduce or control it on the distribution system. Some of the possible solutions were using filters, zigzag transformers or other system design changes.
Capacitor Bank Investigation
While the Progress Energy engineers involved were trying to find a solution to reducing the third harmonic at the substation, another group within the utility was inquiring about the need for a neutral-current sensor to detect blown fuses on a grounded wye-connected capacitor bank. When the group placed the neutral sensors on the capacitor banks, they measured nearly 15 A on the neutral connection, even though there were no blown fuses or bad cells within the capacitor bank.
Since there was no apparent explanation for the high neutral current on the capacitor bank, the team contacted Progress Energy's power-quality (PQ) group. The PQ group surmised that the third harmonic might be the reason behind the 15 A and wanted to test this hypothesis. A line and service technician at the site of the capacitor bank was enlisted to take a harmonic reading of the neutral current flowing through the capacitor bank. From the Fluke 41B measurements, the hypothesis that the triplen-harmonic current was the major contributor to the neutral current of the capacitor bank was confirmed.
While at the site, a test was conducted to see the effects of disconnecting the neutral of the capacitor bank from the neutral of the system. This configuration made it a floating wye capacitor bank. With the neutral disconnected, the third harmonic of the system neutral at the capacitor bank location dropped from 5.3 A to 1.4 A. The PQ group consensus as to why the drop in third-harmonic current occurred when the neutral of the capacitor bank was lifted was that the capacitor banks were most likely in resonance or that some amplification of the third harmonic in the circuit was occurring.
The PQ group wondered if changing the upward of 10 MVAR of capacitance available at the substation to floating wye connections would affect the third-harmonic flow at that substation and thereby reduce the NEV measured at the customer site.
The PQ group decided to perform a test on the system and measure the third-harmonic flows on the system. The test team selected a day when the load would not vary drastically over the course of the test. The plan was to de-energize all the capacitor banks fed from the substation and monitor the third-harmonic current flow using the FMS and a Megger PA9, which would be set locally at the substation to monitor neutral current. Then, upon completion of the test, the PQ group would analyze the data and draw a conclusion about the benefits of changing the capacitor bank configuration to reduce the amount of third harmonics on the substation.
The capacitor banks were de-energized for each feeder, one feeder at a time, and a snapshot of the system was taken with the FMS. Earlier in the morning, a PA9 had been set to record the current values on a 1-minute basis to give a higher resolution of the current flow throughout the test. The Megger also gave the team the ability to do a harmonic spectrum analysis on the neutral current returning to the substation.
The test data indicated a reduction of approximately 30% to 40% in harmonic current flowing in the substation system neutral. From the test, it was surmised that reconfiguring the capacitor banks into a floating wye configuration would remove the path current could take to the neutral connection and would provide a simple, economical solution to the potential problem of capacitor banks resonating or amplifying the third harmonic.
Operations and Specifications Changes
Before permanently changing the capacitor banks to floating wye configurations, certain specification changes had to be made. The existing 15-kV switches had to be replaced with 27-kV switches, because switches under a floating wye connection could be exposed to voltages in excess of 15 kV as a result of a shorted (failed) capacitor unit. The fuse sizes also were reduced per the manufacturer's recommendations. Finally, the grounded capacitor buss connection to the system neutral was removed.
As a field safety measure, operations personnel were provided training on the differences between floating wye operation and grounded wye operation. The change was minimal, involving a smaller fuse size required for maintenance replacements, as needed, and a different switch if one was required for replacement.
By the end of August 2009, the work to convert the capacitor banks from a grounded wye to a floating wye configuration was completed. Upon completion of the work, the measured NEV at the gas distribution substation was below 2 V.
Actual current flow recordings at the substation before and after the connection change showed about a 40% to 50% reduction in third-harmonic current, which is slightly better than what the original test data indicated. The average neutral current dropped from approximately 90 A to around 55 A. Most all of the current reduction was in third-harmonic flow.
A Possible Solution for the Future
Reconfiguring the existing capacitors from a grounded wye bank to a floating wye bank reduced the third-harmonic flow, reduced NEVs and maintained volt-ampere-reactive control for the future. Reviewing all of the options and knowing that the distribution load on the system is very dynamic, the floating wye capacitor bank seems to be the best solution. It also allows the use of existing equipment with minimal changes.
At present, the Aberdeen distribution system is the only Progress Energy application of floating wye capacitor banks. To date, no operational issues have been seen as a result of the changes in the capacitor bank configuration. Progress Energy plans to continue monitoring the substation and possibly use this technique on other substations in the future, should the need arise.
Another possible third-harmonic reduction method that was reviewed was the use of third-harmonic filters on the system. Some of the issues associated with this solution were the introduction of new equipment onto the system for line and service personnel, high cost and limits that could be placed on control and configuration of the system due to the possible detuning of the filter with the addition of more line capacitors. Given current smart grid initiatives underway at Progress Energy, this solution was not compatible with the control and direction of that strategy.
G. Scott Peele (firstname.lastname@example.org) is a power-quality engineer for Progress Energy, where he has worked for 30 years. He works to resolve power-quality issues involving large industrial and governmental customers. Peele has degrees in HVAC, electronics and computer engineering. He maintains a commercial refrigeration contractor's license and is a licensed electrical contractor in North Carolina. Peele also is a registered professional engineer in North Carolina.
Don Guinn (email@example.com) is a lead engineer in distribution standards with Progress Energy. He has held various distribution engineering and operations positions at Progress Energy during his 34-year career. Guinn holds a BSEE degree from North Carolina State University. He is a registered professional engineer in North Carolina and South Carolina.
Joseph Grappé (firstname.lastname@example.org) has worked at Progress Energy for seven years and currently works with commercial, industrial and governmental customers to resolve power-quality issues. After serving four years in the Marine Corps, Grappé earned a BSME degree from North Carolina State University. He is a registered professional engineer in North Carolina as well as a certified energy manager.
Progress Energy progress-energy.com