For More than a Decade, Industrial Personal Computers have Controlled High-Voltage DC Installations all Over the World, processing not only the inverter signals but also the surrounding dc station control equipment. A few years ago, Vattenfall (Stockholm, Sweden) installed one of the first HVDC Light installations (50 MW, 80 kV) on Gotland, an island in the Baltic Sea.

One of the rectifier terminals was installed in close proximity to an existing ac transformer station, where several signals would be common to both ac and dc control. The utility took action to establish whether the dc control computers could also control the ac bays that were vital to the converter station. Following the installation of specialized software for ac protection and control, the system performed satisfactorily. Thus, Vattenfall decided to subject this innovation to a full-scale test using existing ac substations on the island.

THE INSTALLATIONS

HVDC Light, which Vattenfall purchased from ABB (Zurich, Switzerland), is a fully integrated system in which all controls are on standardized hardware and software platforms. By collecting all the signals in the substation in one I/O rack, a single control computer could handle protection, control, fault recording and power-quality registration, for example.

The first computer-based integrated system formed part of the Gotland HVDC Light installation, commissioned in 1998. This included a double computer system for both the HVDC Light and ac substation, and now there are two double computer stations and two single computer stations on the island. A double computer station means that all vital software exists on two computers, and all indications and measured values have a single connection, but the signals can be used several times in the computer. Although this simplifies the physical connection, it requires more programming.

The development environment for the hardware systems is vendor specific, but the software functions are available from other vendors. An external manufacturer created a program that has been integrated into the Gotland system, which includes circuit-breaker monitoring functions registering the trip current, scaled in primary values. The ability to see the current that the protection functions registers, speeds up testing and fault analysis. The trip current is visible in the control room immediately after a disturbance.

The latest installation has taken place in the 130/40-kV Ulricehamn substation, situated 100 km (63 miles) east of Gothenburg, Sweden. This substation is equipped with six 130-kV bays, three lines, a capacitor bank and two 130/40-kV, 63-MVA transformers. The station has a peak demand of 85 MW and eight 40-kV bays for the six outgoing circuits and the two transformers.

The existing main protection and control system was installed when the substation was first commissioned in the 1960s. For a substation of this capacity and importance, duplicate computers were considered necessary. In view of the number of algorithms required, each system comprised two computers.

The new substation control system provides the same functionality as the former system, but has several new functions such as transient fault recorders (TFRs), event recorders, distance protection at 40 kV, bus bar protection and power-quality measurement. The system has functionality for automatic closing and delayed automatic reclosing for 40 kV (and may also be used for 130 kV).

The equipment consists of redundant systems in cabinets for the 130-kV and 40-kV switchyards, as well as an additional cabinet containing equipment for substation monitoring, an event database and remote control.

The computer operating system is a real-time kernel in combination with Windows XP. System software and application programs for the different protection and control functions are run on top of the operating system. The different computers are connected to the communication network, connecting to the station control and monitoring system that includes an industrial PC with a keyboard and monitor. The computer has a database for the storage of events and a communication protocol converter (gateway workstation) for connection to Vattenfall's control center system.

Within the substation, there is an operator workstation for local supervision, fault management, data processing and a printer. The control center has an identical system. Each workstation contains Windows XP Professional Edition, HiDraw, InTouch HMI software and Reval software for disturbance data analysis. HiDraw is a combined presentation, fault analysis and documentation program, which can also be used as a development tool.

FIELD EXPERIENCE

The new substation control system has a protection action matrix (PAM) that can configure the protection algorithm to trip a specific circuit breaker, a facility that makes it easy to change settings. For example, Vattenfall changed the settings at one of Gotland's main substations to allow for a large-scale maintenance job, which required several outgoing feeders and transformers to be supplied by a single circuit breaker. It took just 5 minutes to remotely reconfigure the changes required in the PAM, instead of a week to locally rewire the relays.

Because all the signals and measurement values are collected in one system, all the values can be made available online and viewed directly on a computer screen, locally or remotely. Also, it is easy to check blocking functions online.

For conventional protection relays, it takes considerable time to verify all protection functions, ensuring that the redundant protection does not operate and cause inadvertent trip operation. With the PAM system, testing is simplified because it is possible to control the trip signals — an improvement that is easy and fast even for complex systems.

A valuable benefit of the new system, proven during project development, was the ability to add new protection functions. The delivery time for these new functions is the time it takes to start the development tool, open a menu, click on the appropriate function blocks and, with no wiring required, add them to the design drawing and the connection of the appropriate signals. Another benefit is that the system can be tested from an office-based computer using a debug mode before live operation.

The system includes a TFR that is able to record four measurement values for every incident on some 15 to 20 outgoing circuits. Therefore, you can see what happened on the faulted circuit, as well as the remaining healthy circuits, making it possible to check complete system response to a grid fault. The system uses a GPS for the time synchronization of events and the TFR. The millisecond accuracy of the GPS is valuable for disturbance analysis and performance comparisons between substations.

The first pure ac installation was a double-system application installed in 1999. At that time, the computers did not have the capacity to run all the functions, so the system was installed without some. In 2003, the computers were replaced with high-speed units, complete with all the functions. Installation of the main system took two days, and the redundant system took one day. The substation was fully protected during the installation as, apart from the computers, the physical connections were not at all affected.

On one occasion, when a computer failed, an alarm was sent to the control center. But following a restarting of the computer, everything worked correctly again. Also in 2004, a computer installed in 1998 suffered from a hardware problem and failed before being replaced by a new computer. To date, Vattenfall has five ac installations, two with single computers and three with double systems; to date, no complete double-system faults have been recorded.

ALTERNATIVE CONFIGURATIONS

Based on the experience gained by Vattenfall for small single-transformer substations with loads less than 20 MW and a few outgoing circuits, a single computer connected to a single communication network and one set of I/O racks for the process connection is sufficient. Fault-protection backup for the outgoing lines can simply be handled by the I/Os. Depending on the system configuration, protection from feeding stations could be used for backup, and it is possible to add separate backup protection for the high-voltage system.

The high-end solution for transmission system substations contains a system with completely duplicated components including total backup for the control functionality. The two protection and control systems could include more than one PC depending on how many functions they should handle as in the Ulricehamn substation.

In the double computer system, all key programs run in both computers. The difference is that one computer is the master. If functions are nonvital for security, they may exist in only one computer, but protection functions are always active in both systems and both can trip the circuit breaker.

The layouts of the majority of substation installations are somewhere between the basic and high-end solutions. Vattenfall believes the standard layout should be a system with duplicated main computers and single I/O units, because such a solution fulfills all the basic requirements. The new control panels installed in the Ulricehamn 130/40-kV substation are shown in the photo on page 48.

NEW FUNCTIONALITY

The introduction of new functionality into the totally integrated control and protection system at Ulricehamn was fairly easy. Completely new and requested functionality was developed to test the work process for adding new functionality and to train personnel in the operation and upgrading of the system.

The first function, “presentation of data” is used to export information from the system, in this case parameter settings for all the protection relays in the substation. The exported data may be read, customized and displayed by almost any external software. Vattenfall uses Microsoft Excel. In a spreadsheet, the protection relay parameters are presented in addition to their alternative settings for the active and standby systems. Each time a parameter is changed, a historical file of the previous settings is saved, and these values are also presented on the spreadsheet. With the push of a single button, any deviation in settings (of some 16,000 values) and the historical settings can be displayed.

The second function, “communication between substations” is designed to implement interaction of the protection between two adjacent substations using the TCP/IP protocol. This was achieved by installing a PC at the adjacent 130-kV substation. A line-differential protection system was developed and installed in the two substation computers. Measured real-time data is sent between the two substations over a common communication link to the line-differential algorithm, which calculates the differential current and sends the tripping signal to the PAM as soon as any internal line fault is identified.

The basic system software and the application software were prepared and tested in the laboratory and distributed as an add-on package that can be “dropped” over the system software. Implementing the functions on-site was easy because no wiring was required. Therefore, upgrading the system software was completed without interrupting the protection system.

FUTURE WORK

Currently, the dc software Vattenfall has been working with is optimized to handle converter control but not ac transformer stations. According to the manufacturer, further development is need before it can be introduced as a general PC solution.

A standardized process bus is a prerequisite for the development of a commercial product. The introduction of the process bus IEC 61850-9-2 will open up the possibility to use the same I/O modules for several systems. This increases the flexibility and allows for introducing additional systems.

In the future, the time it takes to replace an existing integrated system with a new integrated system will most likely be much shorter than it takes today. Less reorganization of cables and the possibility to reuse the existing I/O card will shorten the installation period.

The possibility to upgrade the application and system software remotely is a useful application, but there is a concern regarding IT security. This needs to be handled with care, with a possible solution being an annual visit to each substation with a CD update.

OPERATING EXPERIENCE

The experience within Vattenfall is that PC-based integrated protection and control systems have several advantages compared to conventional systems. The most important advantage is the flexibility to modify and add functions in the substation protection and control system. Furthermore, enhanced functionality can be introduced in small substations at a low cost.

Different configurations have been used in different installations, and these have satisfied the requirements on the availability and reliability set up for each of these installations.

Compared with the aged, mostly analog technique that the majority of Sweden is still using, it is that obvious a computerized system requires knowledge in modern information technology. This is not specific to PC-based integrated protection and control systems, but also for other computerized systems. Therefore, information and training is important to ensure that personnel accept and familiarize themselves with the new technology.

Finally, the increased use of standardized industrial computers and standardized I/O modules will allow for competition, giving several suppliers the opportunity to offer equipment.

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Per Norberg is a senior adviser at Vattenfall Eldistribution AB. His professional engineering career started with system planning and asset management. For the past decade, Norberg has held several positions in the transmission and distribution sector in Sweden. As a CIGRÉ member, he has authored and co-authored a number of international power engineering articles. per.norberg@vattenfall.com

Ulf Johansson was a senior research engineer at Vattenfall Research and Development AB during this project, working in the department for Power Engineering Technologies R&D on power quality, measuring techniques and standardization issues. Johansson's expertise is centered on substation protection, fault monitoring and management systems. Johansson currently works for The Swedish National Electrical Safety Board, Elsäkerhetsverket. ulf.johansson@elsakerhetsverket.se

Anders Johnsson is a senior research engineer at Vattenfall Research and Development AB. His professional experience includes power system monitoring and control with special focus on IT and communication. More recently, Johnsson has been working on IEC standardization projects. He earned his MSEE degree from the Royal Institute of Technology (Stockholm, Sweden). anders.johnsson@vattenfall.com

Axel Fogelberg is an engineer with Gotlands Energi AB. For the past eight years, he has been engaged with the study of soft protection and control systems. axel.fogelberg@geab.vattenfall.se