TECHNICAL WHITE PAPER STRUCTURAL PRINCIPLE OF A DART CIRCUIT

Transcription

1 PROCESS AUTOMATION TECHNICAL WHITE PAPER STRUCTURAL PRINCIPLE OF A DART CIRCUIT This document explains the function of DART Technology and describes the individual elements and principle structure of a DART circuit. The white paper was written specifically for technical decision makers and technicians interested in using DART Technology in their applications and equipment. Prepared by: Michael Trautmann Product Portfolio Manager, Pepperl+Fuchs

2 STRUCTURAL PRINCIPLE OF A DART CIRCUIT Pepperl+Fuchs is a leading company in intrinsically safe explosion protection for the process automation industry. The company s product portfolio extends from conventional interface technology and I/O systems to fieldbus infrastructure. DART (Dynamic Arc Recognition and Termination) represents another important step towards greater power and reliability in technical processing plants: When an electrical current circuit closes or opens and generates a spark, a distinctive, easily recognizable change to the current and voltage is perceived. DART detects this change and switches off the current circuit within a matter of microseconds (µs) to prevent ignitable sparks from occurring, even at high power outputs. On dynamic systems such as DART, the cable length must also be taken into consideration. Information about a change in current (di/dt) and the subsequent triggered shutdown of the power supply is transmitted from the fault location to the power supply and back through the cable at a speed of approximately 160,000 km/s in the form of a guided wave. With longer cables, this transmission time has a significant influence on the overall shutdown time and, therefore, makes the cable length an important safety aspect.

3 TABLE OF CONTENTS Table of Contents 1 Introduction Hazardous Areas: Zones vs. Class/Divisions Zones Classes/Divisions Ignition Potection Concepts Explosion transfer prevention Spatial separation of ignition source and potentially explosive atmosphere Preventing an effective ignition source Ignition Potection Concepts: Classifying DART DART Functionality DART Topologies Point-to-Point applications with DART Multidrop applications with DART Components of the DART Current Circuit Structural principle of a DART Power Supply Structural principle of a decoupling circuit Increased safety isolation Start-up circuit Current and di/dt limitation Start-up circuit of the downstream unit Field Cables Outlook /9

4 1 INTRODUCTION 1 Introduction The concept of intrinsically safe explosion protection was born on the day of a mining disaster in South Wales in 1931, where an electrical current circuit ignited mine gas and the subsequent explosion killed 439 people. In the wake of the disaster, an inquiry concluded that energy-restricted electrical equipment and associated electrical accessories could have prevented the accident. In plain terms, this means that arcs, sparks, or thermal effects that could ignite an explosive mixture, should not occur during normal plant operation or in the event of faults. DART Technology follows this basic principle the only difference is that higher power outputs than ever before can now be supplied to electrical equipment. The two areas (Divisions 1, 2) indicate the probability of an explosive mixture developing. 3 Ignition Potection Concepts The explosion triangle displays the three elements that can cause an explosion when combined under the right conditions. For this reason, at least one of the elements (ignition energy, oxidant, combustible substance) must be avoided in hazardous areas to exclude the risk of an explosion. 2 Hazardous Areas: Zones vs. Class/Divisions Two main, globally recognized systems for categorizing hazardous areas currently exist: Zones and Classes/Divisions. The USA and Canada mainly use the Class/ Division concept, whereas the rest of the world mainly categorizes hazardous areas into zones. 2.1 Zones The zone concept is currently based on Standard IEC/EN and makes a distinction between three areas of focus for gases/vapors and dust. Gases are categorized into Zone 0, 1, 2 and dust into Zone 20, 21, 22. Figure 1: Explosion triangle In principle, a distinction is made between three basic ignition protection methods: 3.1 Explosion transfer prevention This is the only method where an explosion is allowed to take place. The effects of this explosion are restricted to precisely defined areas to prevent propagation into the surrounding atmosphere. The flameproof enclosure (Ex d) ignition protection class is based on this method. 2.2 Classes/Divisions The Class/Division concept is based on standards NEC 500-x and CEC J18-0xx, which categorize flammable substances into three classes: Class I Gases or vapors Class II Dust or powder Class III Bound or suspended state 3.2 Spatial separation of ignition source and potentially explosive atmosphere With this method, an attempt is made to spatially separate potential ignition sources (electrical components, hot surfaces) from explosive mixtures. The pressurizing system (Ex p), sand enclosure (Ex q) 2/9

5 4 IGNITION POTECTION CONCEPTS: CLASSIFYING DART and cast enclosure (Ex m) ignition protection classes are based on this method. 3.3 Preventing an effective ignition source This method limits thermal effects such as hot surfaces. Electric sparks are either generally prevented [ increased safety (Ex e)] or limited to a safe level [ intrinsic safety (Ex i)]. Ignition sources can also be restricted under certain fault conditions. 4 Ignition Potection Concepts: Classifying DART The energy cannot simply ignite on creation; it must be concentrated at a single point (such as a spark) and exceed a critical value. This ignition energy is categorized as follows: Table 1: Characteristic values for energy limitation Type Typical gas Ignition energy [µj] I Methane 280 IIA Propane > 180 IIB Ethylene IIC Hydrogen < 60 The values are derived from the ignition limit curves of IEC/EN as follows: DART Technology was defined and developed based on EN60079 in collaboration with the PTB. The current state of development includes use in Zones 1, 2, 21, and 22. Use in the USA or categorization according to Class I, II, III/Division 1/2 will become an objective as soon as the installation and design requirements for DART devices become a standard part of IEC/EN DART Technology is, therefore, categorized in the intrinsic safety class (Ex i) and prevents effective sources of ignition from developing. However, unlike classic intrinsic safety, the power supplied in the field is not strictly limited. Sparks are detected the moment they occur and the power source is then switched off before a state of ignitability is reached. By quickly deactivating the ignition source, DART limits the amount of energy that the power supply feeds into a spark. In terms of intrinsic safety, this occurs at very high power outputs. 5 DART Functionality The principle of intrinsic safety is based on the limitation of power and energy to prevent the explosive atmosphere inside the endangered area from igniting. Figure 2: Ignition limit curves for power limitation An ignition spark is generated when a current circuit is opened or closed and can occur as a result of a planned procedure (such as closing a terminal) or unintentionally (for example, if mechanical influences cause a lead to break). When a spark of this kind is created, a characteristic curve is produced as shown in the following diagram: 3/9

6 5 DART FUNCTIONALITY Figure 3: Curve for a generated spark As long as a current flows within a closed cable with a very small cable diameter Δ L, it can be assumed that the decreasing voltage U falls to 0 V and the converted power P equals almost 0 W. If the cable is opened at this point, a spark begins to form. One characteristic aspect in the formation of this spark is the rapid decrease of current IF over the duration and an increase of voltage UF via the broken cable. The following power conversion occurs (in the spark) as a consequence: P ( t) = U ( t) I ( t) F F F The energy contained in this spark can be expressed as follows: ( t) I ( t)dt EF = U F (for t ~ [5 µs 2 ms]) F If you then wish to limit the energy to a specific value so that ignition cannot occur, the integral calculated from the product of UF and IF must be smaller than the ignition limit energy for the spark duration (see left section in Figure 2 on page 3). With classic intrinsic safety, the selected UF and IF values are so small that a potential spark could not trigger an ignition during the entire burning period. In DART Technology, the strict limitation of UF and IF is no longer required. Instead, the time tf of the generated spark is reduced sufficiently to allow effective limitation of the energy in the spark (see Figure 4 on page 5). As a consequence, the prompt suppression of a spark is known as a DART event. The triggering element of a DART event is the steepness of the gradient F di, which is a characteristic dt feature of a generated spark. 4/9

7 6 DART TOPOLOGIES Figure 4: DART switches off the current circuit before a spark can occur. The time t, within which a generated spark must be extinguished, depends on the current IF within the cable and is between ~5 and ~30 µs. di Information about the variation in current F dt conveyed from the location where a spark originates along the cable to the DART Power Supply (see Figure 5 on page 5). The DART Power Supply detects the change and switches off the power supply. The residual energy is fed to the generated spark before it ultimately extinguishes. is 6.1 Point-to-Point applications with DART The point-to-point topology has the same classic structure as a transmitter power supply or a solenoid driver with a corresponding load in the hazardous area. Power outputs up to 50 W are possible with a cable length of 100 m. Figure 6: Point-to-point topology with DART 6.2 Multidrop applications with DART Figure 5: When a spark is generated and extinguished 6 DART Topologies The multidrop topology is used in fieldbus applications. Here, several devices are connected to a master cable supplied from a DART Power Supply. In DART applications, a distinction is essentially made between two topologies: Point-to-point connections, where a field device is supplied from a power supply. Multidrop applications, where several field devices are connected to a master cable. Figure 7: Multidrop topology with DART 5/9

8 7 COMPONENTS OF THE DART CURRENT CIRCUIT 7 Components of the DART Current Circuit 7.1 Structural principle of a DART Power Supply The structure of a DART Power Supply can be divided into five basic function blocks: The power supply (e.g., 19 to 30 V) is processed by a step down regulator (1) and transmitted through the galvanic isolation (3) by a push-pull stage (2). The maximum output voltage is then limited by a crowbar (4), for example. galvanic isolation and voltage limitation are included in EN/IEC The dimensions are modified only to enable greater power transmission. The actual DART current circuit (5) is used as an additional function block, which is responsible for the identifier described above and switches off the power whenever required. Another central element of DART current circuits is the start-up circuit: It checks the connected line for faults (lead breakage or short circuit) beforehand to make sure the DART current circuit starts safely. This test is performed using a conventional intrinsically safe current circuit. This structure is the same as a classic transmitter power supply. All known requirements relating to Figure 8: Function blocks of a DART Power Supply Figure 9: Block diagram, DART identification and shutdown 6/9

9 7 COMPONENTS OF THE DART CURRENT CIRCUIT In order to assemble this current circuit, the connected unit must have a corresponding decoupling circuit (see 7.2 Structural principle of a decoupling circuit). The circuit draws only a very small amount of current at start-up. The DART Power Supply checks whether the controlling voltage is stable. Then, the high power output of the DART Power Supply is enabled (see Figure 9 on page 6). In case of a failure (lead breakage / short circuit) a spark will be detected by the di/dt sensing. Thereupon, the IS voltage will be deactivated by the start-up circuit. Failure signalization/leds will indicate such a DART event accordingly. Furthermore, the DART Power Supply comes with diagnosis circuits for voltage detection and current limitation. By means of the start-up circuit, they also restrict the energy sourced into the hazardous area in case of a failure. The decoupling circuit in the unit determines whether the DART Power Supply is in start-up or DART operation by monitoring the control voltage (start-up voltage << DART voltage). 7.2 Structural principle of a decoupling circuit The decoupling module was designed to isolate the electronics of an application from the DART Power Supply and the associated cable in the field, while simultaneously enabling the DART supply to start-up. The complexity of the decoupling module depends on the behavior of the downstream electronics. Figure 10 shows the structure that can be used for purely passive ohmic and/or inductive loads. Figure 10: Decoupling circuit for purely passive ohmic and/or inductive loads 7/9

10 8 FIELD CABLES Figure 11: Decoupling circuit for active and/or capacitive loads Figure 11 displays a structure which is suitable for active and/or capacitive loads. An decoupling module comprises these main components: Increased safety isolation A start-up circuit for the DART Power Supply A current and di/dt limiter For active/capacitive loads, an additional circuit component that provides the required start-up current for the downstream unit Increased safety isolation Current and di/dt limitation Di/dt and current limitation ensures that the DART supply starts up in a defined sequence without triggering a DART event (for example, loading the downstream C, significant load changes, etc.) Start-up circuit of the downstream unit This circuit component ensures that di/dt limitation of the DART supply does not interfere with start-up of the connected unit and makes sure that only one buffer capacitor is activated before the unit is enabled. Increased safety isolation prevents the supply of current back to the cable in the event of a fault and uses the RLC network to generate a familiar input impedance on the four-pole decoupling circuit Start-up circuit The start-up circuit regulates activation of the DART Power Supply. As soon as the power supply switches from conventional Ex i operation to DART operation (higher voltage), the start-up circuit sets the decoupling circuit to low-resistance operation. 8 Field Cables The cable is the connecting element between the DART Power Supply and the decoupling circuit of the field device. In order to shut down promptly as shown above in Figure 5 on page 5, information about the line fault must first be transmitted to the power supply, then detected, deleted, and the line deenergized. The propagation speed of the relevant signal along the cable, therefore, plays a decisive role. 8/9

11 9 OUTLOOK 9 Outlook The next important step is the inclusion of DART technology in EN60079, which is currently being masterminded by the PTB in collaboration with partner companies as part of a Technical Specification (TS) under the working title Power-I. Figure 12: Relationship between output current and delay time The propagation speed can be determined using the inductance and capacitance parameters of a cable. The runtime trun along the cable (each direction) is calculated as follows: Inductive cable parameter: L in [H/km] Capacitive cable parameter: C in [F/km] The subsequent steps will describe integration in American standards and the definition of a rapid, generally accessible (not proprietary) communication protocol for the DART line. This creates an opportunity to fully exploit the advantages of DART Technology on a global scale. The power supply to the field devices can be combined into a single cable with a high power output and powerful signal transmission. The maximum overall response time of the system consisting of a supply device, cable, and decoupling circuit can be calculated in line with the current in the cable, where the supply device contributes approx. 3 µs and the decoupling circuit a negligible delay. The following cable lengths are possible, depending on the output power of the supply devices. Table 2: Possible cable lengths Voltage Vout Power Pout Possible cable length 24 VDC Approx. 22 W 100 m 30 VDC Approx. 12 W m These specifications may vary depending on the quality and parameters of the cable. 9/9