If there is one thing constant about distribution systems, it is that they are always changing. Customers acquire more electric equipment. New accounts come on-line. Customers go out of business. Storms come. The weather heats up. And the utility itself adds equipment, circuits, and homes and businesses to the system.
Another thing that never changes is that customers expect a quality product, delivered 24/7. When there is a voltage disturbance and corresponding load disruption, the electric utility is the first thing they think about. In recent years, this expectation has escalated as more and more businesses and residences depend on power to run their computers, hi-tech machinery, and home and business conveniences. Power quality always has been and always will be everyone's concern.
At CenterPoint Energy (CNP; Houston, Texas) the troubleshooter gets the service voltage and power-quality call when the customer service advisors and the service consultants have not been able to resolve a problem. This normally occurs when the correlation between the customer's disturbance log and the utility's reliability events do not match up. It is the troubleshooter's job to physically evaluate the problem at the service location. CNP has 27 troubleshooters who, along with other personnel during storms, respond to trouble cases generated from an average of 2000 trouble calls a day. Trouble calls can be categorized as either lights out (power outage) or voltage complaint (power quality).
Upon the arrival of a voltage-quality complaint, the troubleshooter first takes voltage and current readings, preferably under load conditions, at the customer's switchbox or main disconnect. These real-time voltage and current measurements are taken with handheld voltmeters and clamp-on ammeters. If the service voltage is out of range on either a continuous or intermittent basis, the troubleshooter will take appropriate steps to track down the origin of the problem and, if possible, take corrective action.
If the readings indicate an overload condition exists, or if the customer has severely fluctuating or pulsating loads with high-magnitude inrush current swells, then the troubleshooter has probably identified the source of the problem. In most of these cases, the customer has added additional loads to his or her service without properly notifying the utility regarding the magnitude or intermittent nature of the added loads. In this scenario, utility engineers are not able to detect and analyze the impact of the load additions in regards to service voltage and power-quality standards until actual field readings are made.
If the initial readings at the switchbox or main disconnect do not identify the source of the problem, then the troubleshooter looks upstream on the electrical service to identify the source of the voltage problem. Based on training and experience, the troubleshooter methodically takes readings at several utility components on the service. These reading points include the self-contained meter, the weatherhead connections, service drops or lateral conductors, the cable tap box, the secondary conductor, transformer bus work and the transformer itself. If necessary, other services in the immediate vicinity may be checked to verify that the utility's supply voltage on the primary distribution system is within regulation limits.
If a problem is found on the utility side, the troubleshooter may be able to take corrective action. A faulty connection (such as corroded or loose cable connectors, compression connectors or splices) may be replaced or a new service drop may be run. If the repair is larger than the troubleshooter can handle alone, he or she may request assistance from another troubleshooter. If the repair job is too big for them, such as transformer or pole replacement, then the job will be turned over to one of CNP's 14 service area organizations. A line crew that specializes in distribution construction will be allocated from the local service area to complete the repair.
Reliability engineers and the Meter Functions department both play an important role in CNP's power-quality testing. If the troubleshooter does not find anything wrong with the service and the customer is still not satisfied with the service, the next step is to install a voltage and current recorder. One of CNP's five reliability engineers will request that a system-specific recorder be installed temporarily on the circuit. A tester and installer from the Meter Functions department will install the recorder for monitoring and data collecting.
These recorders are preprogrammed to trend the steady-state voltage and current over a specified period of time, typically a week, and capture dynamic events such as instantaneous voltage sags, swells and momentary outages. For these applications, CNP typically uses recorders such as the iVS-S (meter socket) or the iV/600 (three phase) from Power Monitors Inc. (PMI; Harrisonburg, Virginia). For small single-phase recorders, CNP uses a recorder that fits into the meter socket. For larger services, a recorder is used that monitors voltage and current inputs obtained from the revenue metering transformers.
The tester and installer initializes the recorders with a laptop computer, and when the monitoring is complete, he or she downloads the data from the recorder and e-mails it to the reliability engineer for analysis. The reliability engineer analyzes the information on a computer using a recorder software platform, which can generate a report on the findings for the customer and other CNP personnel.
The majority of monitoring requests are handled by the installation of PMI monitors. However, for the smaller number of true power-quality cases, where symptoms relating to harmonics or subcycle voltage transients are occurring, CNP uses high-speed recorders such as the Fluke RPM (Everett, Washington) or the Dranetz-BMI PX5 (Edison, New Jersey). These recorders have the high sampling rates necessary to provide harmonic spectrum and event waveform data.
In relatively rare cases, electrical noise (low magnitude, high frequency) may be caused by commutation notching from large rectifiers on the distribution system. These situations make it necessary to use a device with multiple-megahertz digital oscilloscope capabilities. For such applications, CNP uses a Fluke 43 PQ Analyzer to take spot-check readings of the voltage waveform.
Escalating Response Procedures
The diversity of voltage and power-quality scenarios is complex and could potentially compound the problem of identifying the root problem. To more effectively address these problems, an Escalating Response Procedure has been developed that is followed when the CNP call center receives a call or notification.
The call center's first response is typically to route these complaints to a service consultant based at one of the various service centers in the territory. The service consultant speaks with the customer in order to discern if the complaint is related to normal utility events (momentary outages and instantaneous voltage sags). If so, the service consultant can provide the customer with information regarding service reliability and power-quality expectations furnished by the reliability engineer, so that they can take appropriate actions to mitigate problems with sensitive equipment. If no correlation exists between the customer's disturbance log and the utility's reliability events, then a troubleshooter is dispatched, and if necessary, the monitoring steps described above are initiated.
CNP is one of the nation's largest combined gas and electric utilities with a total of 5 million metered customers. The regulated CNP Houston Electric transmission and distribution subsidiary serves approximately 1.9 million metered customers in the Houston metropolitan area. CNP has been proactive in addressing power quality by constantly upgrading and improving the reliability of its system. This investment in state-of-the-art equipment, coupled with its Escalating Response Procedure, has greatly improved both the response time and the workload allocation of each department in the process chain. These initiatives have resulted in higher power-delivery quality and improved customer satisfaction.
Scott Jackson is a consulting engineer in the Distribution System Reliability section at CenterPoint Energy, where he has worked for 24 years. His responsibilities include distribution reliability, power quality and technical support for the service areas. Jackson holds a BSEE degree from the University of Texas at Austin and an MBA degree from the University of Houston-Clear Lake. He is a licensed engineer in Texas. scott.jackson@CenterPointEnergy.com