Over the past few years, the number of power quality (PQ) monitoring systems installed has increased significantly worldwide. In line with this trend, CLP Power in Hong Kong is developing a systemwide PQ monitoring system, based on previous monitoring knowledge and overseas experience. In 2001, 75 recorders were installed in a trial at customers' premises to collect PQ information, including voltage dip and harmonic content. This information was made available to customers on request. The trial confirmed that this PQ information improved customer satisfaction, and subsequently, more recorders were installed. CLP Power also realized the value of PQ monitoring on the power system, which led to additional monitoring installations at each operational voltage level on the power system.

Availability of PQ Data

In the past, CLP Power's monitoring strategy was either focused on the customer side or was performed on an ad-hoc, uncoordinated basis. Therefore, before moving forward to develop a comprehensive PQ system, CLP Power reviewed the existing internal systems within the company as many different parties were using PQ measurement equipment to perform different monitoring tasks. The first phase of the PQ monitoring system used PQ recorders at major customer accounts, fault recorders and remote terminal units (RTUs) at strategic locations on the system, as well as some PQ revenue meters on a trial basis.

A quick way to extend the PQ monitoring system would be to install more recorders. This method would increase accuracy, but the life-cycle costs of adding several hundred recorders would be extremely costly and take a considerable time to implement. After review, CLP Power concluded that the monitoring and control devices in place could be used to capture the PQ data desired.

PQ Data Integration Approach

Following an IT feasibility study, the company proposed to install extra PQ monitors and integrate the collective PQ data. The benefits of this approach are:

• Reduced the capital investment cycle of extending the PQ monitoring system.

• Improved the speed of data collection, analysis and user data exchange.

• Accelerated the benchmarking process with other worldwide power utilities.

• Maximized resource usage by providing multiple communications channels.

• Reduced the need for expensive and proprietary system support and maintenance.

Qualitative Requirements

1. System Handling Size

   Based on overseas experience, monitoring the power system at all voltage levels from transmission to distribution is required to obtain a meaningful representation of the power system performance. With a minimum of one circuit output monitored at every primary substation, the CLP power system will require 150 PQ monitoring positions for benchmarking. Additionally, a further 100 PQ monitoring points will be maintained for monitoring power quality for major accounts. Hence, allowing for system growth, the PQ Data Center should be capable of handling at least 300 monitoring points.

2. Data Integration

   A core function of the PQ Data Center is the integration of the PQ data available that may vary in format from the various systems now in place. The interfacing software should be able to recognize and convert raw data from a discrete system into its own database. This consists of a platform with the latest database management and manipulation technology that is based on a single database to achieve the desired data for analysis, reporting and benchmarking purposes. The PQ Data Center is an integrated suite with a system designed to provide the company with a complete and comprehensive PQ data management center.

3. Storage Space and Processing Power

   The proposed PQ Data Center should be able to store a minimum of three years' online data, which would allow CLP engineers to generate yearly, quarterly, monthly, weekly and daily reports. There are limited choices on the market to satisfy this huge storage requirement and to provide for database growth, processing speed, scalability, I/O performance, capability of sharing and manageability issues.

4. Web-based Data Access and Analysis

   Corporations have greatly improved their ability to share text and graphical data. The PQ Data Center will offer data access via the CLP Power Intranet, providing automated and custom reports that can be retrieved by all interested parties. This ability alleviates the need for managers to request customized reports from frontline engineers and allows multiple report formats to be available from any computer with access to the PQ Web site.

5. Diagnostics Program

   CLP Power also required diagnostics services (a watchdog program) to monitor the system to ensure optimum system operation. Currently, there are no self-monitoring systems of the metering communications channel, so the checking task is manual.

Design Considerations

CLP realized it must address two key issues: What is the input to the Data Center? What is the expected output from the Data Center?

The raw data input and the expected output determine the criteria for designing the system.

To determine the PQ data output, the data received from external systems to be integrated were examined, as there are differences in span of measurement, data log interval, event capture capability, data calculation methodology and accuracy. To make the measured data more useful and to allow for easier data comparisons between systems, CLP Power had to adopt an established industry standard to align PQ data from these systems.

• PQ Standard

  CLP Power adopted the European Standard EN50160 as the measurement methodology and as the basis for PQ benchmarking. This standard gives the main characteristics of the voltage at the customer's supply terminals in public low-voltage and medium-voltage electricity distribution systems. It defines and standardizes the level of characteristics concerning the supply voltage frequency, magnitude, waveform and symmetry of the three-phase voltages.

• PQ Data

  According to the statistical evaluation method and definition in EN50160, PQ data are produced in the form of mean values. This not only fulfills the requirement of EN50160, but it also can be applied to other electrical parameters, such as current, power factor, kilowatt and kilowatt-hour.

  The parameters specified in EN50160, voltage dip, voltage transient and interruption are clearly defined, as shown in Table 2.

• Waveform

  Apart from data logging, waveform capture is another critical item for integration. In accordance with EN50160, CLP Power adopted the minimum sampling rate of 128 (27 samples per cycle for waveform capture). Each waveform capture should include three-phase voltage, three-phase current and neutral current with at least two cycles before the event (pre-trigger) and 12 cycles for post-trigger.

  The requirement in data logging and waveform capture provides the basis to estimate the data volume per site, which addresses the major concern in hardware and data storage design. The standard definitions helped CLP Power liaise with different vendors to ensure its PQ data was available in the specified format. In addition to clarifying the terms and definitions of each PQ parameter, it is necessary to align the site identification, data label, legend, unit, stamp time, triggering threshold in event log and other data to streamline the integration process.

Third-Party Systems

CLP Power's PQ Data Center will be able to communicate with the data servers of external systems. An individual data server is responsible for collecting downstream data, and should maintain its own polling scheme and data management activities for the customer's system. It is important to communicate with all system owners in advance and establish agreements to provide reliable PQ data for integration. The agreement defines the demarcation of the project, responsibility and cost impact, as well as detail technical concerns, such as PQ data format requirement, server-to-server connection, data-transfer arrangement and data storage.

• PQ Data Format and Database Structure

  PQ data from external systems need to align with the data provided by associated systems and the format adopted by the PQ Data Center. The data supplied must comply with all critical requirements and state the deviations of noncompliance. A data table with all PQ parameters was prepared and issued to all vendors for customization purposes.

  A further consideration in data integration is the differences in database format. The PQ Recorder and Power Quality Revenue Meter output the data in Sybase database format. The Fault Recorder outputs in text format and RTU (DAS) in SQL database format. To adopt one single database configuration in the data warehouse, the PQ Data Center must be equipped to convert text, Sybase and SQL database into the adopted common database format.

• Server-to-Server Connection

  Four parties are connected with PQ Data Center. The project coordinator is responsible to coordinate with associated systems to assign a time session for individual system data transfer. The assigned time session should avoid collision with the polling sequence, system backup and diagnosis activity of individual systems.

• Data Transfer and Storage

  A Corporate Data Network (Ethernet) was established within CLP Power to share information, which allows PQ data to be transferred at a faster rate with improved reliability. The only issue is transmitting large volumes of data during office hours. The data transmission rate has to be controlled to avoid flooding the data network. The project team studied the bandwidth requirement and estimated the impact on the data network under worse-case scenarios. As expected, the most suitable time for data transfer is during the night.

The storage capacity of an individual external server determines the frequency that the associated system can upload PQ data to the Data Center. CLP Power requires external servers to be able to store the last three days of data, which serves as a temporary buffer in case of system shutdown or data communications failure.

Web Interface and PQ Reports

The Web-based front end is a user-friendly interface, but no application software is required to be installed on the user side. All PQ sites with specified PQ parameters as well as reporting period and type of graphical presentation can be selected online for analysis. All available data in the database can be accessed without limitation. Other requirements include:

• Allow custom setting for Web display.

• Ability to create windows to compare data from different sources.

• A clear layout to show the hierarchy of the installed meters (substation names with corresponding voltage levels help trace the relationship between meters).

• A user-friendly application to add, delete or relocate a PQ site and manage all changes in the database.

• Support the textual or graphical data export function

Apart from the Web requirement, another key input for designing PQ application software is the PQ reporting system. CLP Power designed three types of reports:

• A routine report summarizing the performance of each monitoring point on the power network can be generated on a yearly, quarterly, monthly, weekly and daily basis.

• An ad-hoc report generated by an event trigger, especially after a voltage dip, which shows the dip characteristic of each affected site.

• A PQ Index report, which gathers cumulative voltage dip data and calculates the PQ index for the power system. PQ index is one critical output of this project.

PQ monitoring is evolving from discrete short-term measurements at customers' request to systemwide continuous monitoring for performance benchmarking or power quality baseline establishment. CLP Power is moving toward the development of PQ index and PQ predictive maintenance. Integrating PQ data from existing systems is proving to be a cost-effective approach to systemwide monitoring.

C.W. Li joined CLP Power Hong Kong Ltd. in 1995 and has experience in instrumentation, telecommunications and power quality. He is currently working for the Technical Services Department and is responsible for organizing power-quality services and seminars for customers. Li also is a key engineer for the development of systemwide power-quality monitoring in CLP Power. He graduated from Hong Kong Polytechnic University in electronic engineering and obtained his masters degree from the City University of Hong Kong.
licw7400@clp.com.hk

Cathen Y.K.Ho is the systems engineering manager of CLP Power Hong Kong Limited, responsible for monitoring and enhancement of power quality on the company's supply network and customer installations. He received his BS and MS degrees in engineering from the University of Hong Kong and the Hong Kong Polytechnic University, respectively.
ykho@clp.com.hk

Table 1. PQ statistical data.

Parameters	Statistical data
Frequency	10 sec mean
Voltage	10 min mean
Short-term flicker (Pst)	10 min mean
Long-term flicker (Plt) (IEC61000-4-15)	12 consecutive Pst
Voltage unbalance (Vunbalance)	10 min mean
Harmonic voltage and current (individual)	10 min mean
(IEC61000-4-7)	(up to order 23)
Harmonic voltage and current (THD)	10 min mean
(IEC61000-4-7)	(up to order 40)
The flicker values Pst (perceptibility of the voltage change, short-term) and Plt (long-term) are generated according to IEC 61000-4-15. Observation period of one week with fixed Pst intervals of 10 minutes. A Pst value is considered valid only if the supply voltage is within ±15% of nominal and/or there is no voltage dip ≥15%.

Table 2. Definitions for voltage dip, voltage transient and interruption.

Parameters	Definition
Voltage Dips	Duration	Between 10 ms and 60 sec
	Voltage magnitude	1%Un < U < 90%Un
Transient	Duration	msec to µsec
	Voltage magnitude	< 6 kV peak
Short Interruption	Duration	< 1 sec
	Voltage magnitude	U < 1%Un
Long Interruption	Duration	> 3 minutes
	Voltage magnitude	U < 1%Un