System used for Power Quality monitoring in a distribution grid

Cristian ANDREI

Faculty of electrical engineering University Politehnica of Bucharest

Bucharest, Romania Cristian_upb@yahoo.com

Mihai Octavian POPESCU

Faculty of electrical engineering University Politehnica of Bucharest

Bucharest, Romania

In this article I have presented the composition and functioning of a power quality monitoring system used in distribution grids.

I presented an analyzer for power quality monitoring, the ways of transmitting data, a global positioning system clock and a general schema of a power quality monitoring system.

Index Terms - Power quality, Modems, Global positioning system, Monitoring.

I. INTRODUCTION The term "power quality" became particularly frequent

after 1980 and represents a generic covering when considering the influence of a large number of electromagnetic perturbations that may occur in the power grid ( especially medium and low voltage).

Most areas of the industry, but of other nature too, encounter the power quality problem, especially those who have to provide high standard services ( internet, data comunications, factories for semiconductor components).

Power quality can be defined in several ways, each individual having its own perception regarding this concept, reason for which a clear and brief definition of the notion of power quality was necessary and also the standardization of the parameters that interfere in the definition of this notion.

II. CONTENT

Prompt diagnosis of the problems that arise in terms of power distribution grids lead to their prompt correction. Using advanced signal processing techniques and real-time monitoring of disturbances of these systems facilitates troubleshooting and avoiding unforeseen events. Monitoring power supply lines can lead to reliable measures to prevent the occurrence of disturbances.

In this article I have presented the composition and functioning of a power quality monitoring system used in distribution grids.

For power quality analysis,the proposed assembly consists of the following equipment: three-phase analyzer

(fig. 1), global positioning system(GPS) clock, general packet radio service (GPRS) modem and a server data storage and processing. The analyzers are designed to monitor real-time parameters such as: deviations from nominal frequency, overvoltages, harmonic distortion (harmonics and interharmonics), voltage dips and short duration interruptions, long duration interruptions , voltage fluctuations / flicker, unbalance and temporary overvoltage (higher voltage).

Figure 1. Three-phase analyzer

Modern voltage quality measurement devices operate according to the IEC 61000-4-30 standard. This standard defines measurement methods and so provides the user with a basis for comparison. Devices from different manufacturers, which fulfil class A of this standard, must provide the same measurement results. The standard defines two measuring device classes.

Class A measuring devices are used primarily for measurements related to contracts in customer/supplier relationships, while Class B measuring devices can be used for determining statistical quality values. The analyzer complies with the requirements of IEC 61000-4-30 (2008) for class A devices for the following parameters.

Internal memory of the device is 50MB. Among the important technical specifications of such an

analysis I specify: - Measuring the voltage quality according to DIN EN

50160; - Class A device according to IEC 61000- 4-30; - Sampling rate 10.24 kHz; - Fault recorder function up to 20 x nominal current

(100 x In);

978-1-4673-6487-4/14/$31.00 ©2014 IEEE

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- Phase-phase and phase-earth measurements are possible simultaneously;

- Voltage measurement channels for U12, U23, U31, UNE;

- Recording of currents I1, I2, I3, I0; - Determination of over 3000 measurement values; - Freely programmable limit values and outputs using

isolated contacts; - Freely programmable binary inputs to start and stop

measurements remotely; The event recorder stores the messages that denote the

type, time and properties of events in chronological order. All events have the same data structure and contain the

following components - Time stamp : Time at which the event occurred - Identifier : Type of event - Event value : Specific magnitude of event All relevant system processes are registered in events. The event recorder can be seen as a log book that

provides a central, quick history overview for all procedures with minimal memory requirements. These include messages that refer to detailed records for fault events, for example.

The user can apply a configurable event filter to select the messages that are to be stored.

Another requirement of the IEC 61000-4-30 standard is that the analyzer must synchronize its internal clock via a GPS clock (fig. 2).

Figure 2. GPS clock

Once the GPS system is switched on, it first has to receive and evaluate valid data from the satellites in its line of sight. This start-up phase may take as little as 2 – 3 minutes if reception conditions are favourable. However, as long as half an hour may be needed if there is a poor line of sight to the horizon. This start-up phase is also influenced by the configuration the satellites are in at any particular time. It is impossible to fix a position or obtain time information with fewer than three satellites. The red LED “GPS IS WORKING...” flashes while the satellite signals are being received. When the unit subsequently commences continuous operation, this light-emitting diode only flashes when the number of satellites being received falls below three.

It is simple to set up the system and the procedure does not require any particular specialist knowledge. The daylight saving time changes (start and finish) and the time zone can be set using slide switches.

To transmit data between analyzer and database server there are several communication solutions. Of those, I want to remember data transfer by GSM communication type GSM/GPRS or communications using optical fiber(FO). GSM / GPRS communication is performed using a GPRS modem that connects to the analyzer via a serial cable. (fig. 3)

Figure 3. GSM/GPRS modem

The advantage of this solution is the flexibility of placing the equipment, practically the only limitation is GSM signal. The great disadvantage of this type of communication is low data speed transfer.

In case of fiber optic communication / ethernet cable, data transfer speed is superior, the constraint comes from large investment that needs to be made to connect, via FO, the electrical station to the central point where is located the server for storing and processing the data.

The DIN rail-mountable GSM/GPRS modem is specifically designed to meet industrial requirements for remote monitoring and alarm generation. It provides global access to machines and systems via GSM connections. A wide range of security functions, such as adjustable selective call acceptance, connection establishment with password protection, and call back function, protect the system against unauthorized access.

The integrated TCP/IP stack even allows the implementation of simple control systems into the GPRS network.

One particularly useful feature for remote system monitoring are the configurable warning or alarm inputs. If these inputs are activated, the modem calls user-defined numbers and sends stored text messages by fax and/or SMS. Using the switching output additional functions can be controlled via SMS messages.

Power quality analyzers are installed on the secondary circuits of distributor's power stations (fig. 4).

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Figure 4. Connection examples

For data display, graphs and reports users use specific applications through which they can configure the equipments remotely,they can view the information about the operation status of the equiment and the users can process the data received.

Also via specific applications, reports and graphs with processed data can be obtained .

Access to the application is made in a secure manner, on access levels, so the user's access to data is allowed according to the level of access that they have.

Figure 5. View of a specific application

In figure 6, I presented a general schema of a power quality monitoring system. In the case shown, the data transmission between analyzers and database servers was made using optical fiber (FO) technology.

Figure 6. General schema of a power quality monitoring system

ACKNOWLEDGMENT

The results presented in this article were obtained with the support of the Ministry of Labour, Family and Social Protection through the European Social Fund Operational Programme Human Resources Development 2007-2013, Contract no. POSDRU/107/1.5/S/76903.

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