WITH THE GLOBAL DEMAND FOR INCREASED POWER AND IMPROVED QUALITY OF SUPPLY, the development of Portugal has a major impact on the expansion of EDP Distribuição's (Lisboa, Portugal) distribution network. Remote control and monitoring of the distribution network have been key factors in developing the data acquisition and long-distance communication facilities required to reduce the time it takes EDP to restore outages, thereby improving the reliability of the grid.

Typically, the network management system comprises control centers and remote terminal units (RTUs) installed throughout the network, which transfers data through a communication system using specific protocols. The control center provides cyclical RTU interrogation, telecontrol, reception of digital signals (states, alarms) and analog measures from RTUs. This system allows the remote control and state monitoring of medium-voltage (MV) switching devices, a communication protocol based on IEC 60870.5.101, which gives the control center and RTU the ability to exchange data when required.

One of the fundamental components of the remote control of the MV distribution network system is the communication system. From the beginning of the EDP project in 1990 to expand the distribution network, several communication systems were analyzed before the utility selected an FM-based radio-frequency 80-MHz data communication system. The utility experienced some operational problems, including radio-frequency area coverage, frequent communication breakdowns, and atmospheric and external interference. For these reasons, EDP decided to take advantage of the technological evolution by starting to use a cellular-based global system for mobile communications (GSM) technology as an alternative communication system for the remote control of its MV distribution network. The EDP network comprises an overhead line network designed to operate at voltages of 10 kV, 15 kV and 30 kV.

REGULATORY STANDARDS

In 2006, Portugal's Quality of Service Regulation (RQS) established the minimum standards of quality service that must be given by the National Electrical System entities. The RQS defines the continuity of the service and quality of the voltage waveform standards that can vary with the locality and number of customers, in accordance with the following customer zone classifications:

• Zone A covers the district localities with more than 25,000 customers.

• Zone B covers the localities where the number of customers is between 2500 to 25,000.

• THE GSM SYSTEM

  Zone C covers the remaining localities.

The customer supply interruptions, as well as interruptions to the transport and distribution of service, can be attributed to random incidents, system operations, maintenance and customer requests. The indicators for the MV and low-voltage (LV) networks referring to long-duration network outages, which are more than three minutes in duration, shall not exceed the annual values indicated in Tables 1-3.

The GSM technology is an alternative when radio-frequency coverage is a problem. In addition to being low cost, the GSM devices have reduced dimensions, a broad compatibility range and flexibility in setup and installation.

THE GPRS CONCEPT

Then in 2005, EDP began exploring and implementing the general packet radio service (GPRS) technology for remote control of its MV distribution network. It was a natural progression to GPRS, which evolved from an early EDP project on cellular mobile GSM modems. The performance of the GPRS wireless network solved some problems associated with GSM: network congestion in peak traffic situations; data collision on high-level processes and network access management; connection setup time; price (billing is based on the duration of the connection); and band rates.

GPRS SYSTEM ARCHITECTURE

GPRS improves the utilization of the radio resources by offering volume-based billing, higher transfer rates, shorter access times and simplified access to packet data networks.

GPRS is a mobile data service available to users of GSM mobile phones and is often described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused time division multiple access (TDMA). The GPRS is packet-switched, which means multiple users share the same transmission channel, only transmitting when they have data to send. Therefore, the total available bandwidth can be immediately dedicated to users who are actually sending data at any given moment, providing higher utilization where users only send or receive data intermittently. Web browsing, receiving e-mails as they arrive and instant messaging are examples of users who require intermittent data transfers, which benefit from sharing the available bandwidth.

To integrate GPRS into the existing GSM architecture, a new class of network nodes, called GPRS support nodes (GSN), has been introduced. GSNs are responsible for the delivery and routing of data packets between the mobile stations and the external packet data networks. A serving GPRS support node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its service area. Its tasks include packet routing and transfer, mobility management (attach or detach and location management), logical link management, authentication and charging functions. The gateway GPRS support node (GGSN) acts as a door for external data traffic that either originated from within the network and is heading out or originated outside the network and is coming in, such as Internet protocol traffic.

PHYSICAL LAYER

As an upgrade, GPRS has to be integrated into the existing GSM or TDMA network. Every GSM public land mobile network is made up of some combination of the following parts:

• The mobile terminal is the foundational building block of the communications system.

• PHYSICAL PLATFORM

  The base transceiver station (BTS) transmits to and from the mobile terminal. The coverage range provided by the BTS is called a cell.

• The base station controller is responsible for managing any number of BTSs.

• All traffic is routed through the mobile switching center, which is also the administrative hub of the network.

On the physical layer, GSM technology uses a combination of frequency division multiple access and time division multiple access for multiple access. It is used in 850 MHz, 900 MHz, 1800 MHz and 1900 MHz bands all around the world. For example, in 900 MHz, two frequency bands at 45 MHz apart, have been reserved for GSM/GPRS operation: 890 MHZ to 915 MHz for transmission from the mobile station (i.e., uplink) and 935 MHz to 960 MHz for transmission from the base transceiver station BTS (i.e., downlink). Each of these bands of 25-MHz width is divided into 124 single-carrier channels of 200-kHz width. A certain number of these frequencies, the so-called cell allocation, are allocated to a BTS, or in other words, to a cell.

In conventional GSM, a channel is permanently allocated for a particular user during the entire call period (whether data is transmitted or not). In contrast, in GPRS, the channels are allocated when data packets are sent or received, and they are released after the transmission. For burst traffic, this results in a much-more-efficient usage of the scarce radio resources. With this principle, multiple users can share one physical channel.

The main constraint of this project was that the development should be totally transparent for the existing MV remote control system. So, if a RTU needed to transmit a frame, the GPRS modem should automatically receive the frame and send it instantaneously to the other modem installed in the control center, which received the frame and redirected it to the control center front end through RS232. The RS232 interface allows data communication between the GPRS modem and the RTU or the control center.

The GPRS terminal applied was chosen to be a versatile device that can be used in a wide range of telemetry and telemetric applications that rely on the remote exchange of data, and also voice, short-message service or fax through the GSM cellular network. This electrical interface is split between a standard serial command interface, compatible with RS232 PC modem communication, and a configurable electrical interface, which meets a variety of interface requirements.

FIELD RESULTS

The GPRS development package provides a powerful support environment designed to facilitate the development of cost-effective wireless machine-to-machine or man-to-machine (M2M) applications. It enables the developer with innovative new applications to embed them directly onto the modem using specifically designed development tools. The package includes a radio device, and software and hardware development tools. The script language is based on ANSI C and JAVA. The software development tools include application function libraries and reference applications, while the package also includes the hardware elements needed for operation.

In order to secure the integrity of supervisory control and data acquisition data, a secure over-the-air Internet protocol virtual private network was created through the mobile operator, ensuring the GPRS traffic always travels inside the private network, Access Point Name, protected against external attacks. In fact, these outgoing packets, originated in a mobile station, are received always by another MS and only use the SGSN and GGSN for encapsulate, routing and delivery functions between the stations.

The GPRS module is optimized for M2M communications, and it has been implemented with a user datagram protocol/Internet protocol stack, which allows the effective use of GPRS communications.

User datagram protocol is simpler, faster and cheaper than transmission control protocol. User datagram protocol is connectionless and unreliable, which means it does not establish a virtual circuit-like transmission control protocol, nor does it demand an acknowledgement; it merely sends out the message. In fact, it is EDP's IEC-based proprietary communication protocol that has the transport-layer reliability of checking the transmit frame by transmitting acknowledges in both ways, the error bits with the frame check sequence and checksum of the frame.

Like all real-time data change systems, EDP's MV communication system needs to transmit data not only in an efficient way but with considerable speed. The User datagram protocol prioritizes speed over reliability (in EDP's case, this reliability is guaranteed by the IEC-based communication protocol), so the data is not worth the overhead that establishing a transmission control protocol connection demands.

EDP started a test pilot in the center region of Portugal. Table 4 lists the response times obtained for several types of commands executed by the operators in the control center. The GPRS project was an important step in the evolution of EDP's MV communication network platform. All targets were achieved, and this system is working on more than 800 RTUs on the EDP network in Portugal. The overall performance has increased by a factor of 10, and this project has proven EDP to be a leading utility in the application of GPRS technology.

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Rui Pena (rui.pena@edp.pt) received his degree in electrotechnical engineering (information technology) in 1989 from the University of Coimbra in Portugal, and in 1990, he joined Portugal Telecom as an engineer. In 1992, he joined EDP to work in communications and SCADA systems. He was involved in projects and the implementation of control centers, distribution automation, HV/MV protections, digital control and remote control projects. Pena is now vice-director in the Development and Organisation Direction, responsible for the Quality of Information areas, and is EDP's coordinator of mobility and workforce management projects.

NETWORKControl

Joaquim Sousa (joaquimisidoro.sousa@edp.pt) received his degree in electrotechnical engineering (telecommunications) in 2004 from the University of Coimbra in Portugal and then joined EDP Distribution as an engineer working on remote control and SCADA systems. From 2005 to 2006, Sousa's main activities were centered on distribution automation and telecontrol systems, from medium-voltage automation equipment and communication protocols to high-voltage digital control, intelligent electronic devices, and CCTV systems for substations. Currently, Sousa is a member of the project management team for medium-voltage distribution automation and telecontrol systems.

Table 1. Maximum reliability indicators for MV and LV voltage networks as a function of customer zones.

Reliability indicator	Voltage	Customer zone classification	Maximum values
TIEPI (hour) - In Portuguese, the duration of an interruption and the installed capacity time	MV	A B C	2 4 10
SAIFI (number) - System Average Interruption Frequency Index	MV	A B C	3 6 8
	LV	A B C	3 6 8
SAIDI (hour) - System Average Interruption Duration Index	MV	A B C	3 5 10
	LV	A B C	4 7 12

Table 2. The maximum number of long-duration outages per year.

Customer zone	HV	MV	LV
A	8	8	12
B		21	21
C		21	30

Table 3. The maximum total duration of long-duration outages in hours per year.

Customer zone	HV	MV	LV
A	4	4	6
B		8	10
C		16	20

Table 4. Response times obtained in GPRS verses GSM tests executed in RTUs and CC.

Actions/Units	GPRS	GSM
Network access / call establishment	1 to 2 sec.	30 sec.
Request for RTU state (all signals)	17 to 20 sec.	30 sec.
One state signal send from RTU	6 sec.	30 sec.
Request for RTU state (two units in simultaneous)	34 sec.	60 sec.