Integrate IEDs With OPC Technology

Integrate IEDs With OPC Technology
Integrate IEDs With OPC Technology
Pankaj Bhatt and Roger P. Baldevia, Jr.
Schweitzer Engineering Laboratories, Inc.
Presented at the
5th Annual Clemson University Power Systems Conference
Clemson, South Carolina
March 14–17, 2006
Originally presented at the
7th Annual Western Power Delivery Automation Conference, May 2005
INTEGRATE IEDS WITH OPC TECHNOLOGY
Pankaj Bhatt and Roger P. Baldevia, Jr.
Schweitzer Engineering Laboratories, Inc.
Pullman, WA USA
INTRODUCTION
As technology spawns more intelligent electronic devices (IEDs), the communication protocols
that support them also increase. Today, engineers and technicians are looking for ways to
integrate hardware and protocols on a common platform that will meet the needs of data
consumers in the utility or plant.
Most IEDs specialize in retrieving pertinent data and making it available to various data
consumers. Operators focusing on traditional Supervisory Control and Data Acquisition
(SCADA) data, system planning engineers requiring maintenance information, and managers
gathering economic statistics are some of the data consumers who benefit from tight integration
of IEDs and protocols.
DeviceNet, MODBUS®, DNP3, Profibus, Foundation Fieldbus, IEC 60870, and IEC 61850 are a
few of the protocols available in the utility or industrial field. When selecting the protocol to best
meet integration needs, consider these factors:
• Corporate or personal preference
• Compatibility with hardware and protocols already in place
• Economic feasibility
• Efficiency
• Ease of use
• Functionality of protocol
• Ability of protocol to retrieve data of interest
• Maturity, standardization, and documentation of protocol
OPC, Object Linking and Embedding (OLE) for Process Control, defines a rule set for data
exchange between software applications, much the same way that protocols provide the rules for
data exchange between devices. These software applications include protocol drivers, databases,
and programmable logic, as well as distributed and centralized monitoring, control, and SCADA
systems. The data sources to support these applications are often IEDs, such as meters, relays,
programmable logic controllers (PLCs), Remote Terminal Units (RTUs), or in-service databases.
The goal of the project documented in this paper was to design and test a system using several
protocol drivers that also act as OPC servers for several different protocols within the same
system. This demonstrates that different protocols can coexist and their common OPC interface
will support multiple applications on various hardware platforms representing real-world
applications. We used readily available OPC servers that could easily be used off the shelf,
without software customization.
1
Connecting devices using various protocols that support OPC demonstrates a method that can be
used for making information from disparate protocols available on one common Human Machine
Interface (HMI). We also show how OPC provides a common platform to meet protocol-handling
and hardware-integration situations for other installations.
PROTOCOLS
Protocols are necessary for proper communications and data exchange between IEDs. A protocol
is defined as “a set of conventions governing the treatment and especially the formatting of data
in an electronic communications system.” In other words, protocols provide the translation or
common language for IEDs to communicate and exchange data with one another. There are
numerous protocols that one can use in integration applications, ranging from vendor-specific
(proprietary) to standard protocols. The standard protocols in the following list are readily
available:
• MODBUS Serial and MODBUS TCP (Administered by a vendor-independent standards
group [MODBUS-IDA] and considered a market standard because of its simplicity and
popularity.)
• Distributed Network Protocol, DNP3 and DNP/IP (Administered by a vendor-independent
standards group [DNP3 User Group].)
• DeviceNet (uses Common Industrial Protocol, CIP) (Administered by the Open
DeviceNet Vendor Association, ODVA, an international organization of automation
companies.)
MODBUS Serial and MODBUS TCP
All MODBUS data requests consist of a slave device ID or address, a function code, data, and a
checksum. MODBUS TCP/IP uses TCP/IP and Ethernet to carry the MODBUS messaging
structure. The MODBUS TCP/IP specification is “open,” meaning that it is designed so that the
MODBUS protocol works on networks built from off-the-shelf and existing industrial Ethernet
components and does not require hardware from a specific vendor.
The network at the top of Figure 1 shows a traditional PLC (MODBUS Master) communicating
with MODBUS Slave devices (IEDs) and, in turn, passing the collected data to a personal
computer (PC) supporting the HMI. The network at the bottom of Figure 1 illustrates another
example where the PC directly polls each of the IEDs, performs the HMI function, and supports a
software application that performs the programmable logic that is executed in the PLC in the
network pictured at the top of the figure.
2
MODBUS
HMI
IED
IED
IED
IED
IED
IED
IED
IED
PLC
MODBUS
OPC Server
/ HMI
Figure 1
Two MODBUS Network Examples
DNP
Distributed Network Protocol (DNP) is also known as DNP3. DNP3 is a nonproprietary protocol
maintained by a users group of vendors and end users, rather than a proprietary protocol
maintained by just one vendor. DNP3 was designed specifically to support typical SCADA
communications over low-bandwidth communications channels. Its support of several dataacquisition methods, including the report-by-exception method, which transfers only the data that
has changed since the last poll, makes DNP very efficient. In addition, all data are referred to by
labels that represent objects, such as analog value, status value, and alarm value objects, rather
than by labels that represent memory locations, as in MODBUS. Users can request each class of
objects (analog, status, alarm, etc.) separately without needing to know how or where the data is
stored in the source IED. Unlike MODBUS, DNP3 provides event-time-stamping of the data, so
that time-tagged change-of-state and analog rate-of-change data from the IED can be transferred
with the appropriate time stamp.
DNP3 supports many communications media and architectures, including both serial and Ethernet
connections. The DNP3 protocol continues to evolve with Ethernet applications via DNP over IP.
DeviceNet
DeviceNet is an open, low-level network that connects industrial IEDs to higher-level devices
such as PLCs and computers. The DeviceNet network uses Common Industrial Protocol (CIP) for
control, configuration, and data acquisition. DeviceNet interoperates with multiple vendor IEDs.
ODVA manages DeviceNet technology built on CIP, and develops DeviceNet specifications.
This protocol is not intended to be vendor specific and is considered nonproprietary. However,
because it relies on specific technology to create the networks, it is not an open network. ODVA
promotes the worldwide adoption of DeviceNet products in industrial automation.
3
OPC
As mentioned previously, OPC defines a rule set for data exchange between software
applications. Leading worldwide automation suppliers collaborated with the Microsoft®
Corporation to develop the standards specifications known as OPC. Their goal was to provide
data to a variety of end users from assorted data sources. OPC is the common link between
applications acting as data sources and applications acting as data consumers; examples of both
source and consumer include HMIs and information storage/retrieval systems. OPC was designed
to work with a single computer or between applications on multiple computers networked
together. New technologies, such as wide area networks (WANs) and faster network connections,
make OPC very effective over distributed networks.
Before OPC technology, the distinction between hardware developers and software developers
was clearly defined. Software developers developed their own applications and drivers (software
interface) to various hardware devices. For example, an HMI developer not only created the HMI
package but also provided drivers to a PLC or some other IED acting as the data source. The
development of OLE/component object model (COM) brought the software and hardware realms
together. Today, software developers can create applications without worrying about
compatibility issues with hardware devices. Now, hardware developers need develop only one set
of component software for a device that supports applications from many software developers. A
COM enhancement, distributed component object model (DCOM) provides access for client
applications to remote OPC servers. Figure 2 and Figure 3 illustrate the concept of OPC
technology. Figure 4 illustrates the physical layout of implementing OPC technology. This figure
shows data from field devices being converted to OPC data via the OPC server and made
available to OPC applications via OPC tags.
Figure 2
Data Exchange via Unique Protocols on Direct Connections for Each Device
4
Figure 3
Data Exchange via Distributed OPC Applications on Shared Logical Network
Windows Workstation
With HMI OPC
Servers With DCOM Gateways
Windows Workstation
With OPC Apps
Ethernet Switch
Device 1
Device 2
Communications
Processor
Device 3
Figure 4
OPC Data Server
Device 4
Data Exchange via Distributed OPC Applications on Shared Network;
Other Ethernet Traffic Is Not Affected by OPC Traffic
5
OPC is widely used because its open standards support open connectivity. The OPC Foundation
promotes interoperability through creation and maintenance of open-standard specifications, and
by adapting and creating standards to meet evolving industry needs.
At present, there are nine OPC standards existing or under development:
OPC Data Access
Moves real-time data from PLCs, Digital Control Systems (DCSs),
and other control devices to HMIs and other display clients.
OPC Alarms & Events
Provides alarm and event notifications on demand (in contrast to the
continuous data flow of OPC Data Access). These notifications
include process alarms, operator actions, informational messages, and
tracking/auditing messages.
OPC Batch
Carries the OPC technology to the specialized needs of batch
processes. This standard specifies interfaces for exchanging
equipment capabilities (corresponding to the S88.01 Physical Model)
and present operating conditions.
OPC Data eXchange
Transports data from client/server to server-to-server with
communication across Ethernet networks. This standard provides
multivendor interoperability, remote configuration, and diagnostic
and monitoring/management services.
OPC Historical Data
Access
Provides access to data that is already stored (in contrast to OPC Data
Access, which provides access to real-time, continually changing
data). From a simple serial data logging system to a complex SCADA
system, historical archives can be retrieved in a uniform manner.
OPC Security
Specifies client control access to plant process servers in order to
protect this sensitive information and to guard against unauthorized
modification of process parameters. All the OPC servers provide
information that is valuable to the enterprise and, if improperly
updated, could have significant consequences to plant processes.
OPC XML-DA
Provides flexible, consistent rules and formats for exposing plant
floor data using eXtensible Markup Language (XML), leveraging the
work done by Microsoft and others on XML Simple Object Access
Protocol (SOAP) and Web Services.
OPC Complex Data
Allows servers to expose and describe more complicated data types
such as binary structures and XML documents. This standard
specifies a companion specification to Data Access and XML-DA.
OPC Commands
Specifies a new set of interfaces that allow OPC clients and servers to
identify, send, and monitor device control commands.
6
SOFTWARE AND HARDWARE REQUIREMENTS FOR OPC
One of the advantages of OPC is flexibility in implementation. This section gives an overview of
software and hardware requirements.
Software
OPC technology is based on Microsoft COM/DCOM, which is an open, published protocol that
links software applications. These components are readily available in Microsoft operating
systems from Windows® 95 to Windows XP. Any OPC server that is COM/DCOM compliant
can be installed in these Microsoft operating systems. The choice of an operating system depends
on OPC package support and security issues.
Hardware
There are many vendors providing OPC server and client packages; each package has a set of
minimum requirements that guides hardware selection. Hardware selection also depends on the
environment in which the OPC software operates. Three widely used hardware types are
home/office personal computers, industrial computers, and rugged computers.
Home/Office Personal Computers: Home/office PCs range from personal desktop computers to
laptops that are being used in an office or at home. These types of computing machines are
readily available from computer retail stores or mail order and generally run Microsoft Windows
operating systems. These computers are ideal for testing automation applications in wellcontrolled environments.
Industrial Computers: Unlike the home/office PC, industrial computers are built to withstand
moderately harsh environments that may include vibration, wide temperature range, and large air
contaminants. Industrial computers provide better reliability than PCs. These computers have a
design similar to home/office PCs with the addition of components that perform better in harsh
environments: redundant power supplies, additional fans, air filters, and shock-mounted drive
cages. Industrial computers often support Microsoft Windows, embedded Microsoft, or
embedded Linux operating systems. Embedded operating systems have smaller, specific feature
sets, can lock out unwanted features, and need less memory, which leads to more compact and
more power-efficient applications.
Rugged Computers: Unlike PCs and industrial computers, rugged computers are built to
withstand extremely harsh environments such as excessive vibration, extreme temperature ranges,
moisture, and small particulate air contaminants. Rugged computers are used for mission-critical
applications where reliability and low failure rates are essential. These computers have a very
different design from PCs and industrial computers. They are designed for extremely harsh
environments with special purpose power supplies, protection and isolation on power and
communications connections, redundant serial, USB, and network connections, diagnostic LEDs,
no moving parts, and no vent holes. In addition, components can be selected and placed to avoid
adverse reaction to vibration. Rugged computers support Microsoft Windows, embedded
Microsoft, or embedded Linux operating systems.
7
COLLECTING DATA FROM OPC SERVERS
OPC servers are software applications that run in a Windows or embedded operating system,
transmit a protocol through a communications connection, and translate the data received into
OPC. These applications also accept OPC data from other applications and translate it into
appropriate protocol messages.
Some common OPC servers communicate with IEDs that support DNP, MODBUS, and
DeviceNet, and then create tag databases. Tag is a term used to describe an element in a software
application that represents all the data associated with a data point, including present and past
values, time stamps, and attributes of the point and the data.
Once the MODBUS server is launched, the channel is selected. The channel represents the device
connected on the Ethernet port of the PC. In this case, the channel is assigned the name CM, and
the MODBUS Ethernet driver is also selected (see Figure 5).
Figure 5
Selecting the Channel Name and Driver
8
After channel selection, the device is selected and assigned a name, in this case, Circuit (see
Figure 6). The device is also assigned an IP address, along with the “model” (driver). The TCP/IP
port assigned is 502.
Figure 6
Selecting the Device Name and Address
After device selection, various tags are created. Figure 7 shows the window for defining the tag
name, (Current_Ia).
Figure 7
Selecting the Tag Name, Address, and Other Properties
9
Figure 8 illustrates a screen display of communications settings used by the OPC Server so that it
can communicate with the IED. Each device has a MODBUS address or index number, which is
obtained from the device manual. The other communications settings for each IED include
Address Locations Being Polled, Data Type, and Scan Rate.
Figure 8
MODBUS OPC Server Configuration Screen Illustrating Address
Locations Being Polled, Data Type, and Scan Rate
Creating tags in an OPC server varies from one vendor to another, however, there are basic
similarities. A tag is composed of Tag Identification and Data Properties. Tag Identification
provides information about the tag: tag name, address of the source from which the tag derives its
data, and description. Data Properties provides the data type (such as word, float, Boolean, etc.),
the access type (either read/write or read only), and the scaling properties of the value of the tag.
Once configured in the project, each protocol OPC server collects data from the respective
devices, and then makes these data available as OPC tags to the OPC HMI client software. The
HMI software presents the data as visual information. The link between OPC Servers and HMI is
the OPC Link, an application that falls under the category of protocol gateway or converter. In
our application it serves as a bridge between the OPC server and the Wonderware (HMI)
Suitelink protocol. However, because it is accessing an OPC server, it also functions as an OPC
client. OPC Link provides information on how to access the OPC server whether the server
resides on a local or a remote computer. This information is known as the Topic Definition.
Figure 9 illustrates the topic definitions that were created to access the various OPC servers. This
figure also shows the status of the communications channel between the HMI client and the OPC
Server via the OPC Link, the number of tags (items) being accessed on the OPC server, and
whether there are any errors during the operation of OPC Link.
The serial port expander converts the USB port on the computer into four serial ports. This device
enumerates itself as standard communications ports. The maximum baud rate on these ports is
115 kbps.
Two scenarios were developed for this paper. The first one involved installing the OPC Client
(HMI) and the OPC Server on a single computer. The second option, discussed later, involved
having the OPC Client installed on a separate computer.
10
Figure 9
OPC Link Communication Diagnostics
Figure 10 illustrates a stand-alone OPC Server with a built-in HMI. This application is considered
stand-alone because it performs both functions. The screen capture in Figure 11 shows the single
HMI for various devices with different protocols.
Distribution Relay With Communications Processor
DNP Protocol
With DNP Protocol
Differential Relay With
MODBUS Protocol
Meter With
MODBUS Protocol
Circuit Monitor With
MODBUS Protocol
Motor Relay With
DeviceNet Protocol
USB to
Serial Port
Expander
MODBUS/MODBUS TCP
Gateway
DeviceNet Card
Hub
MODBUS OPC Server/
DeviceNet OPC Server/
DNP OPC Server/HMI
LEGEND
Ethernet Cat V Cable
Serial Cable
DeviceNet Cable
4-wire EIA-485 Cable
Figure 10 OPC Server and HMI on the Same Computer
11
Figure 11 Devices with Disparate Protocol Connected to a Single HMI
NETWORKING OPC
OPC becomes very powerful in a networking environment, which allows installation of the OPC
server on one computer and an OPC client, such as an HMI, on another. The first implementation
in this paper illustrated a stand-alone OPC system where server and client were on the same
computer. This type of installation is useful in substation automation applications or in any
application in which an HMI (OPC client) is needed near the field devices (IEDs). However,
there are many applications where field devices are scattered across a plant or where field device
environments are too harsh for HMI computers. In these applications, Distributed Component
Object Model (DCOM), mentioned previously, becomes important.
DCOM provides the means of connecting OPC clients to remote OPC servers by configuring the
DCOM security settings for each component, both client and server. DCOM is based on
providing permissions for a user or group of users to access/launch a particular component that
resides on a remote computer. Using DCOM, which communicates through an Ethernet network,
may require the assistance of the network administrator, especially for networks with security
devices, such as firewalls. Figure 12 is a simplified illustration of how DCOM works.
12
OPC Client A
DCOM
OPC Client B
DCOM
OPC Client C
DCOM
DCOM
Authenticates
and Permits
OPC Server
Figure 12 DCOM Security
When configuring an OPC server for remote access, follow the specific vendor’s recommended
DCOM security settings. In some cases, the server software will actually ask, during installation,
whether you would like to modify the DCOM settings so that it can be accessed across a network.
The following section discusses general settings in DCOM and accessing DCOM. We also share
our experiences with DCOM.
DCOM Configuration on OPC Server PC and OPC Client PC
To configure DCOM for general settings in Windows NT/2000 and XP, first launch the DCOM
application called “dcomcnfg.exe” The following Microsoft Knowledge Base articles provide
detailed information about launching this particular application for other Windows operating
systems.
176799—INFO: Using DCOM Config (dcomcnfg.exe) on Windows NT
182248—How To Use DCOM Config (dcomcnfg.exe) with Windows 95/98/Me
After launching the DCOM application you should have a screen similar to Figure 13.
Figure 13 Component Services Screen
13
The settings can be configured globally, which changes the access settings for all components, or
on a component level. Configuring the security on a component level allows particular servers to
be accessed remotely. The security properties for each component (OPC server) fall into two
categories: Access Permissions and Launch Permissions. With these categories, you can make the
OPC server available to everyone or to a particular user or group of users.
The security settings, Authentication and Impersonation, allow remote clients to access the
computer where the OPC server resides before the application/component (a DNP OPC server or
MODBUS OPC server). As indicated in the following tables, combinations of these two settings
can result in no security checking or every access/request checking.
These tables summarize the settings used to configure the DCOM of the OPC server.
Computer Properties
Authentication
Impersonation
Comments
None
Anonymous
No security checking
Connect
Identify
Verification only on the initial connection
Call
Identify
Security check on every call during connection
Application Properties
Security Launch
Customize
Security Access
Comments
Customize
Specify user or groups
DCOM uses Microsoft Windows networking technology of usernames and passwords, so
consider whether logging in as a local user account or a domain user account will provide the
most appropriate access privileges.
Migrating Stand-Alone OPC to Networked OPC
The OPC installation described earlier in this paper was a stand-alone application in which the
OPC client and server resided on the same computer. To convert this stand-alone application to a
network application, we transferred the OPC client application (HMI) to a remote computer and
connected them via an Ethernet network. This new application demonstrated remote OPC
connectivity by connecting two separate remote field offices; one in Belleville, IL and one in
Franklin, TN.
Both computers belong to the same domain and are authenticated by a domain controller. We
used the same username (belonging to a User or Power User group) and password on both
computers, which ensures that the applications that are installed and running on both machines
(OPC server and OPC client) have similar permissions and access. This system restricts these
applications so that they can only be launched and accessed by this particular user. Authentication
is already completed because both computers are on the same domain.
14
Next, we configured the client computer to refer to a remote node (remote OPC server) instead of
a local host (i.e., stand-alone application). Figure 14 illustrates the configuration of the OPC
client for a remote application.
Distribution Relay Communications Processor
With DNP Protocol
With DNP Protocol
Differential Relay With
MODBUS Protocol
Meter With
Circuit Monitor With
MODBUS Protocol MODBUS Protocol
Motor Relay With
DeviceNet Protocol
USB to
Serial Port
Expander
MODBUS/MODBUS TCP
Gateway
DeviceNet Card
Hub
MODBUS OPC Server/
DeviceNet OPC Server/
DNP OPC Server/HMI
WAN
LEGEND
Ethernet Cat V Cable
Serial Cable
DeviceNet Cable
4-wire EIA-485 Cable
HMI
Figure 14 OPC Server and HMI on Different Computers Connected Across WAN
Connecting to remote OPC servers can be an easy process or a very technical one, depending
upon your application. The implementation involves mainly determining the type of network to
use, the type of security, and the network structure. Although DCOM is readily available in
Microsoft Windows operating systems, there are third-party applications that can make this
remote connectivity simple and there are network practices (tunneling, VPN) that can assist in
strengthening your security issues.
CONCLUSIONS
1) OPC technology is a powerful tool for integrating a variety of devices into one common
platform.
2) OPC technology is easy to implement and configure, and has endless options. It is possible to
integrate several protocols into one device and use OPC to provide information to assorted
data consumers.
3) OPC provides the means of transporting field data to computing systems through various
communication infrastructures via Ethernet LAN (local area networks) or WAN (wide area
networks).
15
4) OPC communicating over a WAN requires an assessment of network and security issues.
Following vendors’ requirements may or may not provide remote access.
5) OPC servers vary from vendor to vendor but, in general, all provide the same result. Use
vendor demonstrations to determine the server that meets your specifications: ease of use,
protocol compatibility, etc.
6) First time OPC users should refer to the OPC Foundation website (www.opcfoundation.org)
for a listing of OPC products, vendors, and various documentation.
SOURCES
Ken Curtis, DNP Users Group, “A DNP3 Protocol Primer,”
http://www.dnp.org/files/dnp3_primer.pdf
“Introduction to OPC and other OPC Documentation,” OPC Task Force, OPC Foundation,
“OPC Overview,” http://www.opcfoundation.org
“What is OPC? Basic Principles and Advantages,” OPC Foundation, http://www.merzsw.com
“DCOM for OPC,” Data Layers Limited, http://www.opcware.com/DCOM.html
“DNP IO Server Instruction Manual for DNP IO Server,” http://www.ioserver.com
“DeviceNet Technical Overview,” OpenDeviceNet Vendor Association (ODVA),
http://www.odva.org/10_2/05_tech/PUB00026R1.pdf
“Kepware OPC Instruction Manual for KepServerEX,” Kepware Technologies,
http://www.kepware.com/Support_Center/helpfiles.htm
“Designing and Configuring a DeviceNet Network Using RSNetWorx for DeviceNet
Software,” Rockwell Automation Student Manual, Rockwell Automation, 2004.
www.rockwell.com
“SST DeviceNet PCMCIA Card Instruction Manual for SST DeviceNet PCMCIA Card,”
Woodhead Industries, http://www.mysst.com/tech/content.asp
“Square D Instruction Manual for PowerLogic Series 4000 Circuit Monitor (CM4000) and
Ethernet Gateway (EGX400),” Square D, http://www.squared.com
“Instruction Manual for the Futura Meter,” Electro Industries/Gauge Tech,
http://www.electroind.com
“InTouch 8.0 Application and Documentation,” Invensys Systems,
http://www.wonderware.com
Modbus-IDA organization. http://www.modbus.org
Figures 4 and 5 reprinted with permission from OPC Foundation 2004.
16
BIOGRAPHIES
Pankaj Bhatt has a B.S. in Electrical Engineering from M.S. University of Baroda in India in
1984. He earned his M.E. in Electrical Engineering from City College of City University of New
York in 1987 and his M.S. in Electric Power Engineering from Rensselaer Polytechnic Institute in
Troy, New York in 1989.
Mr. Bhatt is an international integration application engineer with Schweitzer Engineering
Laboratories. Before joining SEL in 2003, Mr. Bhatt designed, configured, and installed control
systems for fossil power plants and gas turbines using Distributed Control System (DCS) at
Siemens Westinghouse.
Prior to that he was involved with system integration including load shedding, demand limiting,
and power monitoring projects for SquareD PowerLogic. As an employee of New York State
Electric and Gas, he developed engineering designs and coordinated start-up of DCS,
programmable logic controllers, 5 kV-class switchgear, and motor control centers for various
power plant operations. He obtained his Professional Engineering license in New York in 1994.
He is a member of both IEEE and ISA.
Rogelio P. Baldevia, Jr. earned his BS in Electrical Engineering from Seattle University in 1993.
That same year, he started working for Guam Power Authority’s engineering department as a
systems planning engineer and later transferred into operations as a SCADA engineer. His
experience encompasses communication technologies, systems planning studies, and energy
management systems.
Mr. Baldevia joined SEL in 2003 as an integration application engineer. His responsibilities
include providing technical support, as well as assisting and training SEL customers on their
integration and automation applications. Mr. Baldevia obtained his Professional Engineering
license in 1997 from the U.S. Territory of Guam.
Copyright © SEL 2005
(All rights reserved)
20050328
TP6193-01
17
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