Deregulation is forcing electric utilities to face the prospect of their best customers leaving for 'a better deal'. If utilities don't find new ways to retain or attract key customers, they could be crippled by declining revenues. One electric utility -- whose clients include some of North America's largest automotive manufacturers -- has responded to this challenge by designing new service contracts that entice their large industrial clients into long-term relationships.
Serving Customer Needs
Investor-owned DTE Energy is a pragmatic electric utility with uncompromising standards. Its 10 plants power two million people -- a third of Michigan's population -- but more significantly, American industrial heavyweights such as DaimlerChrysler, General Motors, and Ford tap almost 9 per cent of its peak 10,700 MW system load.
DTE's strategy involves determining what their customers' main interests are, and developing contracts to satisfy those specific needs. Some of DTE's largest users, in the steel industry, are interested primarily in price and have entered into multi-year contracts based on discounted, interruptible rates.
The automotive companies, however, want more than just a good price. Power quality disturbances are especially costly to their manufacturing plants. After an outage or voltage sag, machines have to be unloaded, re-tooled, and re-sequenced, and in-process parts must be scrapped.
Insisting that energy supply contracts focus on power quality, automotive manufacturers joined DTE in an unprecedented effort to quantify the quality of service and set performance targets. The result is a ÒSpecial Manufacturing ContractÓ (SMC). Along with target power quality levels, the deal includes monetary incentives for DTE to perform as required. If the specified levels are not maintained, DTE must pay penalties to the customer.
To uphold DTE's obligations and avoid penalties under an SMC contract, the company needed to monitor customer loads, verify quality levels, and pinpoint disturbance sources. DTE discovered a way to do all of this with a single power monitoring solution.
The company created a real-time monitoring and control scheme using hardware and software from Victoria, BC-based Power Measurement Ltd. The 3720 ACM digital power meters transmit real-time load data to DTE's Operations Center. A second pathway using Power Measurement software supplies RMS voltage variation data to a customized power quality database system. This system, under the responsibility of Planning Engineer Andy Detloff, generates reports that characterize disturbances.
DTE account executives use the reports to inform customers about the company's performance related to the power quality provisions of the contracts. DTE engineers use this same information to investigate quality problems and locate faults, allowing them to prevent further occurrences.
The DTE monitoring and control system, represented in Figure 1, satisfies four SMC contract requirements:
- Power quality monitoring for the 'interruption' and Ôsag' provisions of the contract.
- A direct link between the digital power meters at a customer site and the DTE Operations Center Energy Management System (EMS).
- Involvement of the customer in a load management plan. Each customer has access to a real-time display showing local load data as well as load aggregation of all facilities into a corporate total. The display also provides energy costs, probabilities of interruption, and interruption notices.
- A means to enforce the load management agreement through mandatory load shedding for those unable or unwilling to comply.
Faults Lead to Production Shutdowns and Pay-Outs
For DTE, every disturbance that affects an SMC customer represents a prospective penalty payment. A key responsibility of DTE engineers is to avoid penalties by promptly locating and fixing the sources of problems.
Disturbances are often triggered by faults (short circuits) on a power transmission system. Faults are usually caused by lightning, insulator failure, fallen branches, or animals, and can be cleared within seconds by momentarily interrupting the current. These incidents are characterized by utility protection engineers as 'restore and hold' or 'momentary' incidents. Faults are a major concern for customers with automated manufacturing processes controlled by sensitive electronic equipment.
'Momentaries' are the most common power system problems, yet are the most difficult to locate. Even after a fault is cleared and power restored, it can recur until its source is found and removed. If the source isn't immediately identified, DTE may have to make repeated penalty payments.
Sags, another prominent concern, are the result of fault clearing, lightning, or startup of heavy loads. Sags are short-duration decreases in voltage or current levels, typically between 10 per cent and 90 per cent of normal, lasting from 1/2 a cycle to 1 minute. They wreak havoc on automation equipment like programmable logic controllers (PLCs), causing machines and processes to lose sequence and shut down. Sags also deprive computers of power, leading to unexpected system crashes and lost or corrupted data.
Until recently, faults were considered uneventful, even normal. Every utility engineer can cite statistics on the frequency of faults and the probability that service will be restored successfully. The technology of twenty years ago tolerated an outage of a few seconds reasonably well. But in today's sophisticated manufacturing environments, a one-second outage can have the same detrimental impact as a four-hour outage.
Defining Acceptable Power Quality
The three automotive SMC contracts introduced disturbance penalties in phases. Payments were initially required only for total power interruptions.
Now fully implemented, the SCM contracts require pay-outs for disturbances where the metered voltage falls below 75 per cent of normal, regardless of duration. This extra requirement is in place because total interruptions aren't the only problems that can shut down a highly automated machining plant. Twenty cycles of 75 per cent voltage are probably enough.
For sensitive operations like these, DTE originally looked at emerging standards that could quantify power quality. The most widely recognized benchmark is based on a standard known as the CBEMA curve. This transient voltage tolerance curve was developed by the Information Technology Industries Council (ITIC), formerly called the Computer Business Equipment Manufacturers Association (CBEMA).
The curve plots ÔTime vs. Over-Voltage/Under-Voltage' and is similar to IEEE 469, which includes this measure as part of a power quality standard. The curve is illustrated in Figure 2.
Compared to the ITIC curve, DTE's target power quality levels are more stringent for long-duration disturbances and less stringent for short-duration events. Detloff worked with automotive customers to establish these target levels as a simplified measure for administering the SMC contracts. DTE's payments for under-performance are predicated on outages and voltage sags that fall below 75 per cent voltage. This gives DTE an incentive to keep events above the 75 per cent mark.
DTE and other utilities may eventually apply the ITIC curve as an industry-standard measure of power quality. More and more energy consumers with sensitive processes are examining the effects of power disturbances on their profits and are considering standards like ITIC. For faults that are within the curve, energy consumers won't collect penalties from the utility but may be able to go to equipment suppliers for compensation.
Tools for Transmission and Subtransmission Systems
The majority of DTE's SMC customers are fed directly by 120 kV transmission systems, but 35 percent are fed by 24 kV and 40 kV subtransmission systems. When DTE first began implementing the SMC contracts, some tools were readily available for locating faults on 120 kV transmission lines. They included fault recorders and microprocessor-based relays. Although they weren't sufficient to fully monitor all customer areas, at least they covered some very sensitive sites.
By contrast, tools for identifying subtransmission faults were meager; few subtransmission switching stations had any fault recorders, digital relays, or monitoring equipment. This was a problem, because sites fed by subtransmission lines are more exposed to disturbances. Most of these lines are constructed next to public roadways, not limited-access corridors like transmission-level lines. Plus, a greater number of subtransmission lines are connected to buses, causing more substations and customers to be affected by faults. And fault clearing takes much longer at the subtransmission level, increasing the probability of performance contract violations.
DTE had to find the right fault locating tools for subtransmission systems before recurring faults became costly.
Locating Transient Faults on Subtransmission Lines
DTE Protection Engineer Gus Gazepis found a solution for locating subtransmission faults. It was based on previous work done with Voltage Control Engineer Jeff Erard. To picture the setup, refer to Figures 3 and 4.
Before SMC contracts came into existence, Gazepis and Erard had developed a method for locating faults on 'all cable' subtransmission trunk lines. They first recorded the voltage sag at the source bus and assumed a station source impedance. Then they estimated the distance to the fault by predicting the length of cable impedance needed to produce the same sag as the one at the source station.
For this approximation to work, Gazepis and Erard needed two things: normal operation of all lines and equipment in the area of the fault, and accurate voltage sag measurements. The model required normal system conditions because it didn't allow adjustments for out-of-service lines or breakers, which could dramatically alter voltages and currents.
Sufficiently-accurate voltage sag data could be retrieved directly from 40 kV transient voltage monitors, or predicted from fault recorders at nearby transmission stations. Occasionally, microprocessor-based transmission line relays provided enough information. Accuracy would have improved if fault current and tripping time were also measured, but this data was simply not available for most subtransmission faults.
The introduction of SMC contracts presented Gazepis and Erard with an incentive to improve their process. Their mandate was to accurately identify all faults affecting SMC customers.
Enhancing the Fault Location Process
The limitations in the fault location process meant that it didn't work as well for tie lines. These links to neighboring stations are prevalent in subtransmission systems. Complications arise out of in-feed from the opposite ends of tie lines, along with other effects of the interconnected system. In 1997, Gazepis was able to add some important elements to the original process, extending its usefulness beyond radial lines to tie lines and making it remarkably precise.
Gazepis introduced Aspen One Liner software, which offered several advances in fault analysis. First, it applied complex line impedance models instead of simple source impedance assumptions. Second, its interactive graphic user interface allowed Gazepis to incrementally slide a fault along a line and observe the corresponding changes in theoretical site voltage. When the theoretical voltage matched the voltage measured by 3720 ACM meters, Gazepis could quickly correlate the fault's position in the model with actual geographical information about the line. That determined the precise physical location of the fault.
This method is now highly successful in locating zero-impedance faults, or as protection engineers call them, Ôbolted' faults. In rarer cases, though, a match between the theoretical and actual voltages cannot be obtained for any fault position. In these circumstances, resistance is present in the fault through arcing, high ground resistance, or a circuit branch.
To locate these 'arcing' faults, Gazepis has devised a different solution. He plots each measured voltage sag as a function of the fault's resistance and possible position. The voltage plots intersect at a single point, which is the location of the fault. This same technique can determine the location of phase-to-ground resistive faults. The only change would be in plotting voltage sags with 'zero sequence' voltages.
Subtransmission Monitoring Plan
DTE has achieved great success in locating faults using information about voltage sags from the power meters. The meters may also reduce the need for line patrols and 'walk downs' in deep right-of-way, which would yield further savings.
DTE now has justification for improving all voltage monitoring at subtransmission buses. Engineers are planning new 40 kV bus monitoring digital meters, chosen specifically for their large data story capacity and high waveform sampling rate. One of these meters on each 40 kV bus will provide all essential data.
DTE's Distribution Monitoring Plan
DTE has already equipped a considerable number of 13.2 kV substations with over-current devices to capture fault current. However, it appears that these current measurements alone are not sufficient to accurately find faults on distribution systems. It is also necessary to record voltage sags.
Thus, the addition of meters on the transformer feeds to distribution substation buses has been proposed. For DTE, this change is not monumental. DTE substations already use meters for feeder monitoring. Moreover, with modest software changes, the metering units will be able to share the common communications processor using DNP 3.0 or Modbus protocol in the automation RTU/Gateway.
Technology for Power Quality
The automotive industry provided DTE with a new sensitivity to power quality. To administer the power quality portions of Special Manufacturing Contracts, DTE installed a monitoring and analysis system, which proved to be a valuable tool in more ways than one. The system helped pinpoint the cause of the problems that it was installed only to monitor.
The digital power meters supplied voltage disturbance data that was key to locating faults on subtransmission lines. DTE has further optimized this process by integrating a new fault study software system.
DTE's fault location technique promises to be an effective and low-cost way to improve customer satisfaction. The technology, currently used for subtransmission systems, is equally valuable for distribution systems, and can easily be included in the DTE automation plan.
Jim Evans is with DTE Energy. ET