A new web-based distributed power quality monitoring system is being tested in the USA. The system promises to make database management easier for utilities as well as reduce costs.
Christopher Melhorn, EPRI Corporation, USA and William Brumsickle, SoftSwitching Technology, USA.
With the proliferation of embedded processors and other sensitive digital loads, the need for increased understanding of power quality (PQ) event characteristics in electrical power systems has never been greater. EPRI’s Distribution Power Quality Project (DPQ) in the early 90s led to the concept of installing system-wide power quality monitors with a central database that is maintained by expert personnel. Over the last decade, several vendors have developed systems for power quality monitoring and many EPRI member utilities have installed system-wide power quality monitors. While this market transformation led by EPRI’s DPQ study has resulted in viable PQ monitoring systems, the installed cost for these systems and the on-going cost for maintenance, download, and database management has been a bottleneck in wide spread deployment of these types of systems.
The I-Grid system
A recent development by SoftSwitching Technologies has resulted in a service based model for power quality monitoring systems that enables utilities to deploy a large number of single-phase and three-phase PQ sensors at a fraction of the cost of traditional systems. This system is known as the I-Grid.
The I-Grid is an innovative, web-based distributed power quality and reliability monitoring and notification system that eliminates the need of downloading and managing individual databases since the data from each sensor is collected at a central server location and the data analysis and presentation is provided over the web. This model has the potential to considerably reduce the total system cost of installing and managing a power quality monitoring system. If wide scale deployment of such sensors can be achieved, then the data collected can be used to conduct a nationwide PQ and reliability benchmarking effort. Figure 1 illustrates the main page of the I-Grid web site.
The I-Grid system uses low-cost sensors to collect data. These sensors come in two varieties, single-phase and three-phase. Power disturbance and steady state data are captured by these I-Sense monitors, transmitted using an internal modem via the internet to a central database, and displayed on the I-Grid website. The I-Grid system has the ability to:
- Record, report time/ voltage profiles for events including sags, swells, interruptions, and outages of various durations
- Deliver prompt automatic notification of outages, restorations, PQ events, including event data and timestamp.
- Display RMS voltage, as well as waveform data on the I-Grid web site
- Generate summary reports of events and steady-state voltages recorded by I-Sense monitors, including event clustering, aggregation, voltage regulation, and voltage imbalance.
- Use shared telephone line, typically through local ISP.
In addition to single event information, the I-Grid system has the capability to aggregate from multiple I-Senses as illustrated in Figure 3 which is a plot of events recorded by all I-Senses on the I-Grid system.
Figure 1. The I-Grid web site
Although data is available from all sites, the owner and location of the data is kept confidential from the casual visitor to the site. In fact, the only way to see actual single site data is to either be the owner of the I-Sense that recorded the data or be explicitly granted viewing permission by the owner.
Figure 2. Example of PQ disturbance captured by an I-Sense and displayed on the I-Grid web site
EPRI’s project opportunity
In 2003, EPRI is offering a tailored collaboration project to provide member utilities with field experience and to demonstrate the feasibility of such low-cost monitoring systems. A secondary purpose is to participate in the data analysis and data presentation scheme design that is currently in the early stages of development by the vendor. The project has five main objectives:
1. Improve the capabilities of the existing monitoring systems to the extent feasible without sacrificing the total per sensor cost.
2. Limited customization of the data analysis and presentation based on participant requirements.
3. Develop a scheme for site characteristics evaluation.
4. Deploy single-phase and three-phase sensors at key customer locations (this activity will be carried by the members).
5. Obtain one-year field operational experience regarding the performance of the monitors and the data management and analysis functions.
Figure 3. Magnitude-duration plot for the I-Grid system from 05/15/2003 ” 05/16/2003
EPRI members participating in the project will receive a number of benefits including:
- First hand operational experience of a new technology for PQ monitoring.
- Ability to influence the design and data analysis methodology during this early stage of technology development.
- A low cost method of deploying a large number of sensors for evaluating power quality levels on their system and at key accounts;
- Ability to notify utility departments and customers of outages and power restoration in real-time.
- Opportunity for participation in any follow-up activity on nationwide PQ benchmarking efforts after large penetration of such sensors has been achieved.
- Opportunity to view data from other sensors/meters that might be deployed by other parties such as customers in their service territory.
The I-Grid system stores data from all I-Sense monitors in a shared database. Web access to detailed data is restricted by user account. The anonymity of individual monitor sites is maintained, yet the database can be made available for data mining. SoftSwitching also offers private I-Grid systems, wherein data ownership and security can be directly controlled by the utility. A direct benefit of the central database system is the ability to integrate relevant data with a variety of utility business processes.
An example of regional benchmarking is demonstrated by the I-Grid data in Table 1, recorded by 22 monitors in a Southeastern US state. The data show that the percentage of outages versus voltage sags in this region remains nearly constant between a one month and six month period.
Table 2 shows information on a specific voltage sag event from one multi-megawatt manufacturing site. Figure 3 shows the rms voltage profile throughout the event. The reported event duration is 13.8 cycles (230 ms). It is apparent from examination of Figure 3 that the fault that caused the sag was cleared after 4 to 5 cycles (70 to 80 ms), yet restarting of large motors on the system kept the voltage depressed for 9 cycles (150 ms) more.
The abrupt return to near-nominal voltage following a voltage sag is known to have damaging effects on motors and many power supplies.
The ability to review historical data is critical for evaluating solutions to PQ&R problems. For each individual event, a point for each worst case phase voltage is plotted, even if only one phase was affected in the event, hence some clustering of events around 0.9 to 1.0 per unit is shown. The total number of distinct three-phase events recorded over this period is 57, encompassing two monitors on site.
Figure 4. Cycle-by-cycle rms voltage profile during sag event: Phase A (blue), Phase B (orange), Phase C (red) (Summer 2002)
It is notable that no voltage swells or long interruptions occurred during this 6-month period. Brief voltage transients, less than 50 à‚µs in duration, are not captured by the monitors. The need for coordinated surge protection is well established, and many plants include this protection routinely. The need for sag protection is much more site-dependent. For instance, this site, typical of many industrial locations, validates that a voltage-sag-correction device would have provided adequate process protection.
The I-Grid system has been in operation for two years, with over 700 monitors installed. By utilizing the internet for data communication, the I-Grid is readily expanded to international locations: pilot locations in Canada, Brazil, Australia, Malaysia, and Singapore are already operating. A previous EPRI study (TR-1001651) has verified the accuracy and performance of the I-Sense monitors and the I-Grid system.
Low cost monitors finally allow wide-scale deployment for grid-wide PQ and reliability monitoring, while central data storage enables advanced analysis, trending, and integration with a variety of utility business processes. The EPRI tailored collaboration project will help develop this promising technology.