Siemens TechTopic | Bus Joint Fundamentals

Proper design of bus bar joints is a necessity for long equipment life. The objectives that a good bolted bus bar joint must fulfill include:

• It must provide good conductivity, so that the bus system will meet the temperature rise requirements in the ANSI standards.

• It must withstand thermal cycling, so that the low resistance joint will be maintained for the life of the equipment.

• The joint pressure should be high (for good conductivity), but not so high that cold flow of the bus material occurs, which would cause the joint to deteriorate with time.

•The joint should have good resistance to corrosion in normal installation environments.

• It must be able to withstand the mechanical forces and thermal stresses associated with short-circuit conditions.

Figure 1: Anatomy of a bolted bus bar joint

Figure 1 shows a bolted bus bar joint, simplified to show two bus bars connected using a single bolt. Except in rare situations, the bus bars are silver plated (standard) or tin plated (optional), to improve the resistance to corrosion. The bolt is a high strength grade 5 cap screw, while the nut is a grade 2 (heavy wall) nut. The joint includes a large diameter, thick flat washer on both sides of the joint, adjacent to the bus bars. A split lock washer is installed under the nut to assure that the joint stays tight over the life of the equipment.

Why do we use a grade 2 nut with a grade 5 bolt? The grade 2 nut is more ductile than the grade 5 bolt, so that when the nut is torqued in place, the threads in the nut will tend to be swaged down and burnished to a degree, which results in a more equal distribution of load on all threads. This spreads the force more evenly and avoids unacceptable stress levels in the bolt and the nut.

Some users request that special non-magnetic hardware be used in bus joints. Historically, particularly in open bus systems exposed to the weather, difficulties were encountered with corrosion, and this may be one reason that some still ask for non-magnetic hardware. Others prefer non-magnetic hardware because of the perception that it results in a lower temperature rise. While these reasons may have had merit decades ago, we feel they are unnecessary today. Non-magnetic hardware (usually stainless steel or silicon bronze) is expensive and difficult to obtain. In addition, the tensile strength and yield strength of non-magnetic hardware is lower than that of high strength steel, so that tightening torques will generally be lower with the special hardware. The net effect of lower torque and pressure may very well counterbalance any slight temperature rise benefit associated with non-magnetic hardware.

We also specify that the flat washers are to have larger diameter and greater thickness than standard washers. The purpose of the washers is to distribute the clamping force of the bolts over a wider area. To accomplish this, we need a washer that is relatively rigid, with a larger diameter than would be normal for the size bolt used. If a normal small diameter, thin washer (or worse, none at all) is used, the joint will deteriorate over time because of cold flow of copper from the high pressure region directly under the bolt head (or the nut).
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Figure 2: Distribution of forces in a bolted bus bar joint

Figure 2 shows the distribution of forces in a bolted bus bar joint. To obtain a low resistance bus bar joint, we must establish and maintain sufficient pressure, and distribute the pressure over a large area. Initially, the two bus bars mate at only a few peaks or high spots. As the bolt is tightened, the bus conductors begin to deform, bringing more of these peaks into contact. At the design pressure, there is a relatively larger contact area, so that there are a multitude of parallel electrical connections between the bars.

As shown in figure 2, the force is concentrated more heavily around the bolt hole. Since the pressure is highest in the vicinity of the bolt hole, the surface irregularities in this area are flattened out as the mating surfaces are forced into more intimate contact. The joint resistance in this area will be lower than elsewhere in the joint. As distance from the bolt hole increases, pressure decreases and joint resistance increases. Beyond the area defined by the washer, pressure decreases rapidly and little effective current carrying capacity results.

From figure 2, we can see how the large diameter washers serve to distribute the clamping force more uniformly over a wider area than would be the case with a smaller washer, or none at all.

A properly designed bolted bus bar joint will allow the bus system to meet the temperature rise limits imposed by the ANSI standards, and will also have the thermal and mechanical capability to withstand the heat generated and forces imposed under the worst case short-circuit conditions.

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SOURCE: T. W. (Ted) Olsen – Manager, Technology | Siemens Power Transmission & Distribution, Inc.

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Paralleling Three-Phase Transformers

Two or more three-phase transformers, or two or more banks made up of three single-phase units, can be connected in parallel for additional capacity.

In addition to requirements listed above for single-phase transformers, phase angular displacements (phase rotation) between high and low voltages must be the same for both.

The requirement for identical angular displacement must be met for paralleling any combination of three-phase units and/or any combination of banks made up of three single-phase units.

CAUTION:
This means that some possible connections will not work and will produce dangerous short circuits. See table 2 below.

For delta-delta and wye-wye connections, corresponding voltages on the high-voltage and low-voltage sides are in phase.

This is known as zero phase (angular) displacement. Since the displacement is the same, these may be paralleled. For delta-wye and wye-delta connections, each low-voltage phase lags its corresponding high-voltage phase by 30 degrees. Since the lag is the same with both transformers, these may be paralleled.

A delta-delta, wye-wye transformer, or bank (both with zero degrees displacement) cannot be paralleled with a delta-wye or a wye-delta that has 30 degrees of displacement. This will result in a dangerous short circuit.

Figure 20 – Delta-Wye and Wye-Delta Connections Using Single- Phase Transformers for Three-Phase Operation.

Note: Connections on this page are the most common and should be used if possible.

Table 1 shows the combinations that will operate in parallel, and table 2 shows the combinations that will not operate in parallel.

Table 1 – Operative Parallel Connections of Three-Phase Transformers

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Table 2 – Inoperative Parallel Connections of Three-Phase Transformers

Wye-wye connected transformers are seldom, if ever, used to supply plant loads or as GSU units, due to the inherent third harmonic problems with this connection. Delta-delta, delta-wye, and wye-delta are used extensively at Reclamation facilities. Some rural electric associations use wye-wye connections that may be supplying reclamation structures in remote areas.

There are three methods to negate the third harmonic problems found with wye-wye connections:

1. Primary and secondary neutrals can be connected together and grounded by one common grounding conductor.
2. Primary and secondary neutrals can be grounded individually using two grounding conductors.
3. The neutral of the primary can be connected back to the neutral of the sending transformer by using the transmission line neutral.

In making parallel connections of transformers, polarity markings must be followed. Regardless of whether transformers are additive or subtractive, connections of the terminals must be made according to the markings and according to the method of the connection (i.e., delta or wye).

CAUTION:
As mentioned above regarding paralleling single-phase units, when connecting additive polarity transformers to subtractive ones, connections will be in different locations from one transformer to the next.

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Guide To Low Voltage Busbar Trunking Systems

Modern electrical desdign and installations are often placing increasing demands on all products of the electrical equipment manufacturer.

Products must have:

• Reliable service life
• Adaptability to new requirements
• Low installation costs
• Low maintenance costs
• Inherent safety features
• Minimal purchase cost
• Energy efficiency

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In today’s market one of the most important elements is cost effectiveness. In an electrical installation, one area where savings can be made and provide the features listed above is in the use of busbar trunking systems. Busbar trunking installations can be categorised into two basic types:

• Distribution
• Feeder

Distribution Feeder

This is the most common use of busbar trunking and is applied to distribute power over a predetermined area.    Busbar trunking can be run vertically or horizontally, or a combination of both. Typical applications would be:

• Supply to large numbers of light fittings
• Power distribution around factories and offices
• Rising main in office blocks or apartment blocks to supply distribution boards serving individual floors.

Power is taken from busbar trunking by the use of tap off units which connect at defined positions along the busbar trunking, and allow power to be taken from the system, usually via a suitable protective device.

Advantages over cable:

• The contractor can achieve savings with respect to material i.e. cable trays and multiple fixings and also labour costs associated with multiple runs of cable.
• Reduced installation time since busbar trunking requires less fixings per metre run than cable.
• Multiple tap-off outlets allow flexibility to accommodate changes in power requirements subsequent to the initial installation (subject to the rating of the busbar trunking).
• Repositioning of distribution outlets is simpler
• System is easily extendable.
• Engineered product with proven performance.
• Type tested to recognised international and national standards.
• Aesthetically pleasing in areas of high visibility.

Feeder Trunking

When used for the interconnection between switchboards or switchboard and transformer, busbar trunking systems are more economical to use, particularly for the higher current ratings, where multiple single core cables are used to achieve the current rating and compliance with voltage drop and voltage dip requirements.

Beside this, bunch of cables are increasing possiblity of heating between cables and eventually short circuit.

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Advantages over cable:

• Greater mechanical strength over long runs with minimal fixings resulting in shorter installation times.
• Replaces multiple runs of cable with their associated supporting metalwork.
• Easier to install compared to multiples of large cables with all of the associated handling problems.
• Less termination space required in switchboards.
• Type tested short circuit fault ratings.
• Takes up less overall space, bends and offsets can be installed in a much smaller area than the equivalent cable space.
• Cable jointer not required.
• Busbar trunking systems may be dismantled and re-used in other areas
• Busbar trunking systems provide a better resistance to the spread of fire.
• Voltage drop and voltage dip in the majority of cases is lower than the equivalent cable arrangement.

Tap-Off Units

Tap-off units are of two types, either plug-in or fixed. Plug-in units are designed to be accommodated at tap-off outlets at intervals along the distribution busbar trunking. Fixed tap-off outlets are engineered and positioned during manufacture to suit the specified installation. The tap-off unit usually contains the device providing protection to the outgoing circuit terminated at the unit to distribute power to the required load.

There are various types of protective devices, for example:

1. HRC fuses to BS EN 60269-1 (BS88)
2. Miniature Circuit Breakers to BS EN 60898
3. Moulded Case Circuit Breakers to BS EN 60947-2

HRC fuses may be incorporated into fuse combination units to BS EN 60947-3. The degree of enclosure protection of the tap-off unit is defined by BS EN 60529.

Each tap-off unit contains the necessary safety features for systems and personnel protection, as follows:

• Plug-in units are arranged to be non-reversible to ensure that they can only be connected to give the correct phase rotation.
• Plug-in units are arranged to connect the protective circuit before the live conductors during installation and disconnect the protective circuit after the live conductors while being removed.
• Where units are provided with a switch disconnector or circuit-breaker these are capable of being locked in the OFF position.
• Covers permitting access to live parts can only be removed by the use of a tool and will have any internally exposed live parts shielded to a minimum of IP2X or IPXXB in accordance with BS EN 60529.
• Outgoing connection is achieved by cable terminations in the unit or by socket outlets to BS EN 60309-2 or BS 1363.

Fire Stops

Recommendations for the construction of fire-stops and barriers where trunking penetrates walls and floors classified as fire barriers. Internally the trunking may or may not require fire-stop measures according to the construction; where they are required these will generally be factory-fitted by the manufacturer and positioned according to a schematic drawing for the installation. Compact or sandwich-type trunking does not require internal fire-barriers, as suitability as a fire-barrier is inherent in the design.

However in all cases verification of the performance of the trunking under fire conditions needs to be provided by the manufacturer.

The following information is provided for guidance, and the method used should be agreed with the trunking manufacturer. It is not the responsibility of the trunking manufacturer to provide the specification or detail the rating or construction of the fire-stop external to the trunking.

Protective Earth Condustor Sizes

The sealing external to the busbar trunking (with or without an internal fire barrier) will need to conform to applicable building regulations. This may require filling the aperture around the busbar trunking with material to maintain the same fire proofing as the wall or floor.
Careful consideration needs to be given to the access required to complete the fire- stop. It may be necessary to install sections of fire-stop at the stage of installation of the trunking if access afterwards is impossible e.g. trunking runs in close proximity.

The protective earth connection(s) to the busbar trunking system shall conform to Section 543-01 of BS 7671 (IEE Wiring Regulations Sixteenth Edition).

Low-Noise Earth Systems

A low-noise earth, commonly referred to as a ‘clean earth’, is typically specified when electronic apparatus supplied from the system is sensitive to spurious voltages arising on the system earth. This is particularly true with IT equipment, found in all commercial premises these days, where data processing functions can be corrupted.

The low-noise earth is provided by a conductor separated from the protective earth (PE) and from all extraneous earth paths throughout the distribution system.
Many busbar trunking systems provide a ‘clean earth’ conductor in addition to the three phase conductors plus neutral, using the case or an external conductor as PE.

Tap-off units must be specified as ‘clean earth’ for the circuits concerned since the separation of the earths must be maintained and an additional termination will be provided for the load circuit ‘clean earth’ conductor. Sizing of the ‘clean earth’ conductor is not specified in BS 7671 (IEE Wiring Regulations Sixteenth Edition) but the usual practice is to calculate the size in the same way as for the protective earth conductor.

Neutral Sizes/Harmonics

The designer of the electrical network specifies the size of the neutral conductor depending upon the network loading. Typically this tends to be a neutral conductor the same size as the phase conductors (i.e.100% neutral).    As a minimum a 50% neutral may be specified.

The BS 7671 (IEE Wiring Regulations Sixteenth Edition) states “In a discharge lighting circuit and polyphase circuits where the harmonic content of the phase currents is greater than 10% of the fundamental current, the neutral conductor shall have a cross-sectional area not less than that of the phase conductor(s).”

With the increase of non-linear (almost anything electronic) single phase loads connected to a network, for example electronic ballasts in lighting fittings, or switch-mode power supplies (the type found in personal computers and servers) the total harmonic distortion is increased.

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There are two methods for indicating protection relay functions in common use. One is given in ANSI Standard C37-2, and uses a numbering system for various functions. The functions are supplemented by letters where amplification of the function is required. The other is given in IEC 60617, and uses graphical symbols. To assist the Protection Engineer in converting from one system to the other, a select list of ANSI device numbers and their IEC equivalents is given in Figure A2.1.

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MOXA | Video Surveillance in Power Substations

Power substations play a crucial role in delivering electricity to consumers by converting transmission voltage to the lower voltage used in homes and businesses. Since power plants are often located far from the population centers they serve, electricity needs to be transmitted across long distances at a higher voltage.

Power lines deliver electricity from the plant to the power substations where it is converted before being distributed to the local community. It is therefore imperative that power substations are constantly monitored for safety and maintenance as they are often located near or in a populated area. This white paper explains the benefits of real-time video monitoring and IP video technology, as well as factors to consider in deploying an optimal IP video surveillance system for a power substation.

Video Surveillance Benefits

Since power substations are widely distributed and unmanned, remote monitoring is extremely crucial. Real-time video surveillance of power substations offers automatic monitoring and control capabilities in addition to enhancing remote monitoring applications with visual management. These capabilities not only save management costs for manpower, but also realize complete network automation.

Supervisory Control and Data Acquisition (SCADA) systems, which are already deployed in power substations to provide data about the system’s status, can be easily integrated with video surveillance technology. By installing a real-time video monitoring system at power substations, system administrators are able to receive visual data to complement the raw SCADA data. Real-time video monitoring can help ensure normal operations for power equipment, protect against intrusion and tampering by unauthorized personnel, and prevent accidents. For example, intruders, physical obstructions, or smoke indicating a fire can be seen via video so engineers no longer need to visit the site in-person each time to diagnose an anomaly, saving both time and costs.

Remote video surveillance systems can play an important role in monitoring equipment, detecting intruders, and responding to emergency situations. For example, video surveillance can be used to monitor the appearance of the power transformer and relay, fueling and flammable equipment, and the status of the isolation switch. Video surveillance can also monitor the security situation inside and outside the substation by detecting intruders through sound and visual monitoring. In addition, video surveillance can be integrated with the alarm system and RTU (remote terminal unit) over a SCADA system to provide real-time visual information to prevent accidents and assist emergency response personnel in the event of a fire.

Why IP Video?

In the past, video surveillance systems such as CCTV networks relied upon analog video cameras. Due to advances in video digitization and compression technologies, high quality digital video images can now be sent over Ethernet TCP/IP networks. By using such devices, system integrators can easily integrate video surveillance applications into their SCADA system. As a result, Internet Protocol (IP) video technology is the current trend in video surveillance systems. The benefits of IP video surveillance include:

One Network - Using the existing IP network saves cabling costs and increases installation flexibility, especially for widely distributed substations. Ethernet TCP/IP networks can accommodate a variety of I/O monitoring and control devices in addition to transmitting data, video, voice, and even power (PoE) over a single network.

One System - Integration with SCADA or alarm systems (such as fire, intrusion, etc.) increases monitoring efficiency and creates an event-driven video surveillance system. This means the video images can be displayed and recorded and real-time responses can be received when an event or alarm occurs.

Constructing an Optimal IP Video Surveillance System

Given the critical role played by power substations in our daily lives, it is important for the IP video solution to be well-designed to ensure that the video surveillance system works properly. System integrators should consider factors such as applicability, reliability, integration, and user-friendliness in order to construct an optimal IP video surveillance system.

Applicability – System integrators need to consider video requirements such as image viewing, recording, and analysis, as well as interoperability with other systems (such as SCADA, Access Control, etc.) when deploying an IP video surveillance system. They also need to know how many cameras are required for the system and whether IP cameras or video encoders are suitable for the application. Network transmission factors such as bandwidth, multicast, and IGMP requirements, and whether the project requires a single network or separate networks for data and video are also important. Central management concerns, including system resources (PCs, servers, cost, etc.), software requirements (pure video or video integrated with another system), storage capability and database management, and whether or not a decoder is required, should also be considered.

Reliability - Since video monitoring is used to ensure safety and security in remote and disperse locations, reliability is a key factor in designing an optimal IP video surveillance system. Factors for reliability include surge protection and fiber transmission to reduce electromagnetic interference. Redundancy, high MTBF (meantime between failures) and IP protection are also important factors to consider for optimal reliability.

Integration - System integrators should consider integrating video surveillance into the central management system, as well as other systems, including SCADA/HMI, remote monitoring, and access control. This not only reduces cabling and network installation costs, but also makes central management and control easier to handle for system administrators. Interoperation with other devices for event-driven video monitoring is another benefit. For example, the system can begin recording video once a card reader or sensor is activated.

User-friendliness - IP video involves new applications and technologies that power system administrators need to learn. For this reason, it is recommended that system integrators choose ready-to-use hardware and software solutions to reduce the time needed to set up an IP video surveillance system. Not only does this simplify the system integrator’s task, but it will also be easier for system administrators to learn and use.

Video Surveillance in Power Substations

VPort Solutions

Moxa’s VPort industrial video networking solutions include video encoders and decoders, IP cameras, and IP video surveillance software designed for mission-critical video surveillance applications. Since most mission-critical application environments are demanding, the rugged design features of Moxa’s VPort solutions are particularly suitable for these kinds of applications.

Video servers – Digital video images require large data files, so video compression (reducing the quantity of data used to represent video images) is required for transmission and storage. Video servers include encoders and decoders. Encoders are used to convert analog video images from cameras into an easy to transfer digital format such as MJPEG or MPEG4. Decoders are used to convert images from compressed formats (MJPEG or MPEG4) back into analog for use with legacy monitors or displays.

IP cameras – IP cameras bypass the need for video encoders because the images are automatically encoded into a digital format (MJPEG or MPEG4) by the camera itself, and are easily transferred via Ethernet/Internet. Moxa’s VPort series of IP camera offers a wide operating temperature range of -40 to 50°C without the need for a fan or heater, IP66-rating for rain and dust protection, one camera lens for both day and night use, up to 30 frames per second at 720 x 480 resolution, and direct-wired power input and PoE (power over Ethernet) for power redundancy. Moxa’s industrial-grade video servers offer 12/24 VDC or 24 VAC redundant power inputs, DIN-Rail mounting and panel mounting accessories, IP30 protection, -40 to 75°C operating temperature range for T models, and RJ45 or fiber optic Ethernet ports.

Software – Moxa’s SoftDVR Surveillance Software, which includes SoftDVR Lite (4-ch) and SoftDVR Pro (16-ch), is designed for IP-based video surveillance systems. The client/server-based network infrastructure makes it easy to build a user-friendly video surveillance system. SoftDVR offers multi-screen viewing, event-driven recording, easy to use search and playback, data storage to network hard disks, scheduling feature for recording and alarm activation, and remote access by web browser.

SDK – Most video surveillance systems require customized video management functions, or must be integrated with other applications such as SCADA, access control systems, and fire alarms. For this reason, a user-friendly SDK (software development kit) is a good tool to have available for building customized video management systems. Moxa’s VPort SDK, which includes CGI Commands, ActiveX, and a C library, is available free of charge to system integrators and third-party software developers. Learning to use the Moxa VPort SDK is easy, and detailed documentation and sample code is provided for quick reference.

Summary

Transmitting video, voice, and data simultaneously over Ethernet/Internet is becoming a standard feature due to the ever-increasing popularity of IP networks. Versatile and advanced video digitizing and compression technologies, such as MJPEG and MPEG4, are also making it possible to migrate analog CCTV surveillance systems to IP-based platforms.

Power substations play an instrumental role in delivering electricity from power plants to end-users, so managing and ensuring the safety and security of these installations through an optimal video surveillance system is imperative. Since power substations are unmanned and widely distributed installations, video surveillance grants system administrators visual management capabilities in addition to data management provided by existing central control systems that only provide raw quantitative data. The versatility of IP video technology and its ability to be integrated with existing central control systems make it an attractive option for remote video monitoring in power substation applications.

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SOURCE: MOXA, Harry Hsiao, Product Manager at Moxa; www.moxa.com; harry.hsiao@moxa.com

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Testiranje performansi IEC 61850 GOOSE poruka

Jedan od čestih zahteva za uređaje relejne zaštite je podrška za IEC 61850 standard. U okviru standarda predviđene su i poruke za brzu razmenu informacija između releja tzv. GOOSE (Generic object-oriented substation event). U pitanju su uglavnom trip, interlocking, breaker failure i slični signali. Vreme transfera ovih signala je kritično, njihovo kašnjenje može prouzrokovati neželjeno isključenje potrošača ili oštećenje opreme.

U ovom radu istražićemo koja softverska arhitektura je najpogodnija da se ostvare tražene performanse. Softver za slanje/prijem GOOSE poruka može se nalaziti u real time (RT) ili user space prostoru operativnog sistema. Razmotrićemo user space i RT implementacije na dve različite mikroprocesorske arhitekture – ARM9 i PowerPC.

Do degradacije performansi može doći iz 2 razloga:

• Zaštitna funkcija ima najviši prioritet. Najmanje 500 μs tokom svake milisekunde GOOSE task će biti uskraćen za procesorsko vreme.
• U slučaju čiste user-space implementacije operativni sistem će prekidati GOOSE task na potpuno nedeterministički način.

User Space Test

Za testiranje performansi GOOSE poruka u user space-u razvijeno je okruženje bazirano na ARM7 arhitekturi:

• ARM7 sa integrisanim Ethernet-om za slanje, prijem i time-stampovanje poruka.
• PC aplikacija za setovanje parametara i prikupljanje rezultata.

Suština testa je sledeća: ARM7 ploča lansira niz poruka i beleži odlazno vreme za svaku poruku. ARM9 i PowerPC ploče su podešene da odmah po prijemu GOOSE poruke odgovore sa identičnom porukom sa istim rednim brojem.

ARM7 ploča registruje odgovor i pomoću rednog broja uparuje poruku sa originalnom porukom i računa proteklo vreme.

Na gornjoj slici može se videti analiza utrošenog vremena. A i B su zanemarljivi. Zbog prirode testa može se precizno meriti 2C+D ali ne možemo tačno znati koliko iznose C i D pojedinačno. No, u krajnjoj liniji to i nije bitno sa stanovišta standarda. Pogledajmo rezultate testa. ARM7 ploča lansira niz GOOSE poruka u razmaku od 100ms. Rezultati se mere i prikazuju u Excel-u.

Da bi rezultat bio što realniji uključena je prekostrujna zaštita. Na Y osi prikazano je vreme u milisekundama a na X osi redni broj GOOSE poruke.

Slika 3. ARM9 100ms (X osa – redni broj poruke, Y osa vreme transfera)

Vidimo da tokom 20 sekundi vreme odgovora osciluje oko 2 milisekunde. Sledeći korak je bio da se uključi još nekoliko zaštitnih funkcija tako da tokom 1 milisekunde zaštita troši 700 μs. Očekivano je da pri većem opterećenju GOOSE performanse opadnu.

To se zaista i dešava kao što možemo videti na sledećoj slici:

Slika 4. ARM9 100ms, 700μs (X osa – redni broj poruke, Y osa vreme transfera)

Vreme sada osciluje oko 7 milisekudi. Iako je očekivano da će performanse opasti, ipak je postignuti rezultat iznad očekivanja. 7 milisekundi je i dalje dovoljno za neke aplikacije. Ovo su rezultati sa ARM9 platformu. PowerPC platforma se pokazala nešto bolje, jer ima skoro 2 puta veću procesorsku snagu. Na sledeće 2 slike vidimo rezultate.

Slika 5. PowerPC 100ms (X osa – redni broj poruke, Y osa vreme transfera)

Slika 6. PowerPC 100ms, 700μs (X osa – redni broj poruke, Y osa vreme transfera)

Pri manjem opterećenju vreme osciluje oko 0.8 ms a pri većem oko 2.5 ms. Kao što vidimo izmerena vremena se kreću u okvirima koji nude solidan dijapazon primena. Na žalost ova vremena važe samo u slučaju da je GOOSE task jedini aktivni task. U slučaju postojanja drugih taskova – na primer disturbance recorder, event recorder, embedded web server, IEC 61850 MMS server itd… vremena postaju nepredvidiva i mogu ići i do 80ms, što je naravno neprihvatljivo.

Real Time Test

Slika 7. Test konfiguracija za real time test

Mada je real time GOOSE nešto teži za implementaciju, nudi neke značajne prednosti kako ćemo videti. Test okruženje za real time je značajno drugačije. Za testiranje je korišćen mrežni analizator. Program se može besplatno skinuti sa Interneta (1). Suština testa je sledeća: zaštitni relej je podešen da osluškuje poruke koje emituje laptop računar i da momentalno odgovori sa istom vrednošću dataseta koja je u dolaznoj poruci. Kada analiziramo niz poruka u mrežnom analizatoru doći ćemo do momenta kada relej i laptop emituju identičnu vrednost.

Vreme između momenta kada laptop počinje sa emitovanjem i momenta kada relej počne da emituje istu vrednost kao i laptop je traženo vreme.

Na sledećoj slici možemo videti rezultate prikazane u mrežnom analizatoru.

Slika 8. Ethereal mrežni analizator

Slika 9. Niz GOOSE poruka sa vremenima prijema, mrežnim adresama i oznakom protokola

Poruku broj 42 emituje laptop,a poruku 43 relejna zaštita. Ako oduzmemo vremena prijema: 3,757 – 3,753 = 4msec. Pri ponovljenim merenjima rezultat osciluje oko 4ms. Razlog za to je što su taskovi za slanje i prijem podešeni da se bude na svake 2 milisekunde.

Zaključak

Na prvi pogled real time i user space implementacije operišu u sličnim vremenskim okvirima. Ali, postoji ozbiljna razlika. Kod RT implementacije GOOSE task može da deli procesor sa proizvoljnim brojem ostalih taskova kao što je disturbance recorder i sl. Takva arhitektura u mnogome smanjuje krajnju cenu uređaja i daje korisniku više funkcionalnosti. U suprotnom bi GOOSE softver morao da obitava na zasebnom hardveru.

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AUTOR STRUČNOG TEKSTA:

Veljko Milisavljević | ABS Control Systems, Srbija

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