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Baoshan Iron & Steel Co., Ltd.; Baoshan (China) Safe 24V for continuous casting plants High availability, low failure and repair costs: What is being demanded of the whole automation of the new continuous casting plant of the Baoshan Iron & Steel Co., Ltd. in Shanghai applies especially for the 24V supply to the components used in the switch cabinets. Sitop select diagnostic modules are therefore in use as a supplement to the Sitop power packs which switch off faulty circuits quickly, reliably and selectively and enable immediate elimination of errors by reliable diagnostics.
Guarantee for high plant availability With the beginning of the general planning, strategies for partial failures of automation components were already developed. Important aspects here were, on the one hand, the reliable 24V supply of all automation components in the switch cabinet and, on the other hand, the selective disconnection of individual consumers in the event of a fault. Here Siemens offered a totally new perspective regarding reliability and diagnostic facilities with the Sitop power power supplies and the Sitop select diagnostic module. The electronic current limiting of the controlled Sitop power power supplies provides optimum line protection because no impermissibly high currents can occur in the event of a fault. There are no voltage fluctuations on the load side regardless of whether they are caused by fluctuations of the input voltage, overvoltage peaks or secondary side load jumps. The connected consumers are supplied exactly with their rated voltage by the controlled output voltage. This guarantees a maximum life for every electronics consumer and thus an accordingly high plant availability. Discrepancies in the conventional configuration The particular challenge, however, was in the required selectivity in the 24V supply circuit in connection with the modern power supply units. In the event of a fault in a consumer circuit the preceding protective element was to switch off exactly this faulty circuit before the electronic current limiting of the power supply were to become active and the whole 24V supply would break down. The normal solution with medium blow fuses or circuit breakers was therefore ruled out because these do not trip until the current briefly exceeds many times the rated current. The use of fast protective elements was also excluded because these might already be tripped by the inrush peaks of various capacitive and inductive consumers. Clever solution for selective protection In the electronic diagnostic module Sitop select, the planners at Siemens I&S found a solution to the problem which was geared to the future. In every part of the continuous casting plant all 24 V consumers are connected to a central power supply. The consumers are connected in groups and supplied via a monitoring channel of the Sitop select diagnostic module which continuously monitors the size of the flowing current. Sitop select is the first reliable solution for achieving selectivity in the output circuit which allows consumers with high inrush peaks to be connected safely. Ideal trip characteristic Every output is monitored for exceeding the individually set trip current during operation and switched off after 5 s if the current has a value between 101 and 130% of the set value. If the current requirement of one or more outputs exceeds the value of 130% of the respectively set current, the electronics become active. The current flowing through the output is limited to just above 130% for approx. 50 to 100 ms and then switched off providing it does not drop back below 100%. With this characteristic it is possible to start consumers with high inrush peaks without unwanted tripping. In case of consumer short-circuits and even under the worst operating conditions such as long supply cables to the consumer which do not allow the high short circuit current necessary for tripping because of their high impedance, the selective protection or disconnection is guaranteed. When the output voltage drops below 20 V due to exceeding of the total power on the 24 V side, all outputs carrying more than 100% of the individually set output currents at this moment are switched off almost without delay. For all unaffected branches the supply voltage is maintained and safe functioning of the plant is guaranteed. A potential-free sum signal contact enables telediagnosis and therefore easy integration in the whole plant monitoring process. One two colored LED per branch continuously signals the status of the individual branches and enables the operator to locate the cause of the error quickly on the switch cabinet. The defective consumer can be tracked by the switched off branch in this way and identified quickly and reliably. After changing or repairing the consumer the branch concerned is simply switched back on by a reset switch. Solution for retrofitting also There are two screw terminals each for the 24 V and 0 V on the device. This provides the possibility for connecting several modules to one power supply by simply looping the power supply as required. The 24 V are distributed to the various consumer circuits by four output terminals. The trip current of every output can be set between 2 and 10 A with the potentiometers at the front. This reduces the equipment variety and modifications can be made during commissioning without great effort. The electronics of Sitop select are accommodated in a plastic housing which has exactly the dimensions of 4 divisional units for four circuits to be protected. Fast, safe assembly is possible thanks to the simple DIN rail fastening. Sitop select is ideal in its form and concept for retrofitting in existing plants. This may be necessary especially when circuit breakers are installed and problems with the selectivity occur increasingly due to aging plant components. In such cases you can switch over to Sitop select by retrofitting just one 0 V line. In addition each of the four branches of the diagnostic module is equipped additionally with a 15 A flat fuse accessible from the front. By removing or plugging on individual fuses, a graduated plant commissioning, i.e. connection or disconnection of individual consumers, is possibly quickly and easily. The combination of controlled power supply and electronic diagnostic module Sitop select and the fast, reliable fault diagnosis reduce downtimes to a minimum. Therefore the aim of high plant availability demanded by Baosteel is optimally supported. appeared in: elektrotechnik, edition 7-8/03 |
