Case Studies
Procedural Culprit
Sector: Mining and Exploration
Sometimes procedure is the culprit.
While testing a 2000 kW diesel generator, PowerLens™ captured a peak transient of 964 V. This represented 246% of rated voltage suddenly appearing in the system. After further investigation, the source of the problem was the opening of the main generator breaker to the load bank before all the cooling fans were shut off. The generator breaker interrupted the inductive current to the fans and this sudden change caused the inductive reactance of the motors to try and maintain the inductive current flow.
Thus, a switching procedure created a damaging voltage that could fail motors and/or other equipment in the system. A simple operational procedure change was the solution: first remove all fans from service before opening the main generator breaker.
Transient recovery voltage from generator breaker switching.
Static Var Compensator Commissioning
Sector: Theme Park Industry
The importance of commissioning.
In many cases, sophisticated power equipment is not set to run “as-is” after installation and proper commissioning and troubleshooting can be needed to get equipment operating properly.
Theme parks are constantly in search of new and intense thrill rides, and with this often comes even greater power requirements. In this case study, Acacia’s simulation work determined a ride’s reactive power requirement was too great and necessitated a static var compensator (i.e. real-time var compensation) to keep the ride from creating excessive voltage flicker across the power distribution system. After installation, Acacia monitored the equipment to ensure it was providing the specified vars within the specified time response.
Due to the fast load and unload profile of the ride, the static var needed to operate in an open-loop control mode, matching var for var to create a net unity power factor load. During commissioning it was discovered the response was inadequate and the source inverter modules were producing excessive harmonics. Vendor investigation uncovered a need for significant updates in the controls and logic. Unfortunately, that initial update caused a failure in the static var which blew all fuses on the input. Powerlens™ real-time captured waveforms of the fault allowing the vendor’s engineer to quickly determine the problem’s source and implement an effective solution.
Static var compensator response (Q SVC) to load var demand (Q Lift).
Wind Farm Transformer Failure
Sector: Renewable Energy
The inter-operability of equipment.
Detailed investigation of pad mount transformer (PMT) failures led to discovering wind turbine inverter induced high frequency noise being applied to the low voltage line-to-neutral windings. This was not expected, and significant effort was made to ensure this noise was truly present and not a sympathetic response of the data acquisition hardware to radiated inverter switching noise. Measurements were moved from inside the turbine tower to outside on the direct secondary of the PMT. The key issue with this noise is that these transformers were not of any special design to accommodate the high frequency content, and its creation of high peak voltage stress. As shown in the figure below, peak voltage stress due to the noise exceeded 1.5 per unit of the rated secondary line to neutral voltage (398.37 VLN).
High peak voltage stress from inverter high frequency switching noise.
Filter Tuning and Rectifier Misoperation
Sector: Petroleum and Chemical
Running HOT! and didn’t know it.
Acacia was brought in to evaluate a 5th harmonic filter that had failed catastrophically. The client’s initial conclusion was they added capacitors as per drawings but incorrectly tapped the reactor. Acacia determined the reality of the situation was significantly different.
First, measurements of the filter tuning reactor at various tap positions revealed that the manufacturer’s drawings of tap vs. inductance were in error. They did not present the tuning reactor correctly nor did they construct it for its intended purpose of handling two plant operating conditions with two different arrangements of capacitor units. From this knowledge, the filter reactor was then set correctly per measured inductance, desired power factor correction needs and desired tuning.
To be thorough, Acacia also measured the currents produced by both sides of their 12-pulse rectifier system. The delta side was operating correctly with symmetrical and balanced currents as shown below. The wye side however was misoperating and generating an unhealthy amount of non-characteristic harmonics as shown below. These harmonics were the major contributing factor that lead to the harmonic filter failure. The filter was then left off-line until the rectifier system was repaired. The waveform imbalance was also causing excessive overheating of the transformer core (measured to be 185o C) due to saturation. Recommendations were made to curtail the process load until the rectifier was repaired.
Delta side balanced current waveform & expected 6-pulse characteristic harmonics.
Wye side un-balanced waveforms & unheathy harmoimc spectrum.
Mysterious Feeder Faults
Sectors: Utility & Co-ops / Petroleum & Chemical
Don’t stop until you catch the problem in the act.
An oil pumping station utilized a long overhead feeder to transfer prime power generation from diesel generators to a pumping station. This line had a series of mysterious faults and subsequent trips with no apparent cause. To investigate this issue, instrumentation was set to capture faults at one end of the line. After capturing the first fault (see figure below), data was analyzed to determine the fault location on the feeder. That location was inspected and again yielded no conclusion of the cause. After capturing 4 to 5 faults, evaluating the fault location and performing multiple inspections, still no root cause had surfaced.
Voltage & current waveforms for captured line-to-ground fault on overhead line.
The next step was visually monitoring locations along the line until another fault occurred. Eventually, it was noted that wasps were building mud nests on the insulators. As shown in the figure below, this mud contamination covered significant insulator creep. It was only a matter of time until this mud nest would flashover on the insulator and be vaporized leaving no evidence of its role in the problem. Since there is no solution that can control the nest building activities of these insects, and any material added to the insulator would compromise its insulating capability, the best and only option was replacing the overhead line with an underground cable.
Insulator contaminated with mud wasp nest
Harmonics Solution with 519 Violation
Sector: Utility & Coop
The effectiveness of system simulation.
A distribution system was noted as having a serious problem with telephone interference. During hours when a dredge operation was running, the local phone system would shut down. Without any on-site measurements, a network model was constructed to simulate the distribution network, complete with all overhead charging and pole mount capacitor banks.
Simulations showed that the dredge (utilizing a 6-pulse drive system) created significant, but not necessarily severe voltage distortion on the system. Further analysis evaluating TIF and IT Product showed clearly that the harmonic currents flowing in the system were however producing very high levels of telephone interference. To mitigate the issue, existing pole mount capacitor banks were relocated at an optimal location to absorb the most relevant frequencies (i.e. frequencies in the audible range) produced by the dredge. Upon relocation of the bank, the telephone system worked fine with no interference issues. One side effect however was that the feeder voltage distortion increased up above 5% THD, the IEEE 519 limit for voltage distortion on medium voltage feeders. The utility decided that the excess level of distortion was completely acceptable given the phone system was now fully functional. In this case, exceeding IEEE distortion limits mitigated a more serious issue.
Prime Power Gen Failure
Sector: Petroleum and Chemical
Sometimes the damaging unit is not the damaged unit.
A prime power generator station was damaging generators with no discernible root cause. The facility had five parallel connected generators, had damaged multiple replacement units, and needed answers fast. Powerlens™ was connected at the sight to several of the generators and revealed a sudden increase in 3rd harmonic neutral current when one particular generator was connected. Operators were instructed to immediately remove that unit from service to avoid damage.
Neutral current in generator with 13/18 winding pitch.
Further investigation revealed this particular generator had a different winding pitch (13/18) than all of the other operating generators (2/3). Since this location had multiple solidly grounded paths, the winding pitch difference allowed the 3rd harmonic voltage on the odd unit to drive 3rd harmonic current in the system unhindered. One final plot clearly shows the third harmonic going away as soon as the odd pitched generator was removed from service. The solution: only add generators in parallel with the same winding pitch.
