The Eugene Water and Electric Board (EWEB) — the largest customer-owned utility in Oregon — serves approximately 80,000 customers. The 10,000 industrial and commercial customers account for almost 60% of the peak 500 MVA system load. Over the past decade, EWEB has observed an increase in the number of customers sensitive to power quality disturbances. To address this phenomenon, the utility has developed a strategy that goes beyond its traditional responsibility as an energy provider and focuses on providing customers with diagnostic services that can help solve PQ-related problems.
EWEB's strategy includes the following goals:
- Install real-time PQ monitoring networks at critical delivery points and make the data readily available on the Internet
- Provide system reliability and disturbance analyses for customers and internal electrical engineering and operations departments
- Improve distribution network reliability and voltage control
- Define and solve compatibility problems between electronic equipment and power systems at customer sites
EWEB's monitoring system consists of power quality meters located at key substations. The meters communicate via a fiber-optic network connected to a server.
Within seconds of a voltage event, some large customers and key EWEB personnel receive an e-mail notification. Within 5 minutes of the event, an automatic report posts the details of the event to EWEB's Web site (see http://eweb.org/energy/powerqual/pqevent/report.ssi). The event data is also recorded in a historic database on the Web site.
The monitoring system collects a variety of data including voltage, current, and power. Since the installation of the system, EWEB engineers have discovered new ways to use the data. For example, they now use current data to pinpoint fault locations on the distribution system. This enables line crews to locate faults quicker and reduce customer outage times.
In one incident, a substation breaker tripped without any visible reason and interrupted service to hundreds of customers. The crew found no damage on the overhead line, and the breaker continued to trip intermittently.
A portable power quality monitor was set up at the substation. The recorded data showed an inrush current typical for transformer magnetization (See Fig. 1). This narrowed the investigation to finding a faulted instrumentation transformer. A later inspection turned up an abandoned customer-owned 12kV current transformer (CT) with insulation damage. During a wind condition, the CT faulted to ground, causing the substation breaker to operate and clear the fault.
Solving Customer Problems
Most of EWEB's diagnostic work for customers requires the installation of portable monitoring equipment at distribution panels or individual pieces of equipment. For more than a decade, EWEB has placed monitoring equipment in customers' facilities to collect data for demand-side management programs. Setting up equipment for PQ analyses was a logical extension and raised few issues that hadn't already been addressed.
EWEB's monitoring system allows their engineers to characterize the quality of the electricity delivered to customers at the cycle-by-cycle range. Knowing the characteristics of delivered power at this resolution allows customers to quantify how their equipment responds to events of different magnitudes and durations. It also reduces the unknowns when a piece of equipment goes offline.
Power quality analyses are usually triggered by telephone calls from customers experiencing unexplainable equipment problems.
The following example outlines the general steps EWEB engineers take to help solve customer PQ problems. In this example, nonlinear equipment operation has produced harmonics. The EWEB engineers will:
- Collect harmonic measurements and system parameters.
- Develop computer models of harmonic sources.
- Determine a system's frequency-response characteristics.
- Estimate harmonic voltage and current levels after applying various mitigation strategies.
- Perform an economic analysis to determine the most cost-effective correction strategy/equipment.
- Obtain cost estimates for the required equipment.
- Implement the chosen strategy/equipment.
EWEB investigators followed this strategy when a local newspaper printing facility's capacitor bank repeatedly failed. The customer had combined switching capacitor banks with a significant number of nonlinear loads. The interaction created a resonance condition that resulted in power equipment malfunctions and, ultimately, equipment damage.
At the newspaper facility, EWEB investigators took harmonic field measurements at multiple locations using power quality monitors that could capture high-speed transients and harmonic trends. The waveforms and harmonic levels at different points are shown in Fig. 2.
The investigators then developed a steady-state harmonic computer model of the facility based on existing loads, transformers, conductors, and field measurements. The frequency scan simulation showed that, in addition to the high level of harmonics, the facility distribution system had a parallel harmonic resonance on the fifth and seventh harmonics (especially the seventh) (see Fig. 3).
Many engineers use passive filters and other passive technologies, such as multiple-phase or phase-shifting methods, to mitigate significant harmonics. However, many passive technologies have a significant limitation — they require balanced 3-phase loads or separate loads of two equal parts.
In the case of the newspaper facility, the simplest solution was to convert an existing capacitor bank to a harmonic filter. But according to the manufacturer's quotation, the cost was almost three times more than replacing it with a new detuned filter.
In addition, the capacitor bank and tuned filter systems were designed for slow switching against dynamically stable loads. When the engineers tried to apply tuned filters, they were left with excessive current, harmonic resonance, and elevated voltage.
In the end, the engineers chose an active harmonic filter (AHF). AHFs don't require the extensive engineering studies that are necessary before and after the installation of a passive system. Active filtering uses variable-speed drive technology to continuously sense and cancel current harmonics. They eliminate nearly all of the potential hazards common to passive filters, and they have the ability to adaptively respond to changes in the electrical environment, such as the addition of new loads.
AHFs also produce leading current electronically without the negative effect of capacitors. They are inherently nonresonating and will not adversely affect a system's resonant point. These characteristics made them well suited for the newspaper facility's printing applications.
At the present time, AHFs have less of a track record than passive filters, but they are a superior technology and their cost is becoming more competitive, especially when complete system costs are taken into account. EWEB personnel were aware of the relatively short track record for AHFs and worked with the product manufacturer to obtain a guarantee for the printing facility, which helped remove some of the risk.
Another example of EWEB's commitment to resolving customer-side power quality problems involves a large semiconductor plant. The plant experienced an internal distribution substation 12kV breaker operation. The breaker protection didn't indicate a fault target, so plant personnel didn't know what caused the breaker trip or how to find the faulted or overloaded equipment.
Fortunately, EWEB's power quality database had captured the event. The waveform was typical for an insulation failure because it occurred at the voltage peak (i.e., the time when the voltage stress or potential to arc to ground was at its maximum) (see Fig. 4). Weaknesses in cable insulation are most likely to arc at the voltage peak.
An investigation by maintenance personnel at the facility found a faulted cable terminal on their distribution system. The recorded data helped reduce the investigation time by steering the customer toward insulation failure instead of an equipment overloading problem.
In addition to helping customer resolve PQ-related problems, EWEB works actively with industrial customers to make production equipment less sensitive to disturbances.
When lightning struck a 500kV transmission line 100 miles away from a large paper mill, the drives tripped and the mill lost production. A subsequent investigation showed that the mill's onsite generator reacted with a time delay and sustained the undervoltage condition longer than the original transmission sag.
By comparing voltage sags from different utility substations with and without local generation support, it was possible for engineers to find the real-time delay constant for the customer. This gave them the information to adjust the settings for their automatic-generator exciter and transformer-load tap changer, which stabilized voltages and improved the drives' ride-through capabilities.
EWEB is finding opportunities to meet and exceed customer expectations by going beyond traditional boundaries to help customers diagnose and resolve power quality problems.
Industrial customers need to know the magnitude, type, and source of disturbances that affect their production. EWEB's practice of sharing information not only offers customers a valuable service, it fosters a problem-solving, collaborative environment, instead of a confrontational one.
In the future, power related standards will require utilities to monitor and document system events. EWEB's power quality monitoring network, which can be expanded to capture distribution as well as transmission events, is a step in that direction.
Alan Fraser is an energy management engineer at the Eugene Water and Electric Board. You can reach him at [email protected].