MSD TM MEDIUM VOLTAGE CAPACITORS ELECTRONICON KONDENSATOREN GMBH GERA GERMANY

Transcription

1 MSD TM MEDIUM VOLTAGE CAPACITORS ELECTRONICON KONDENSATOREN GMBH GERA GERMANY

2 COMPARISON OF CONVENTIONAL MEDIUM VOLTAGE CAPACITORS AND ENGINEERING TECHNIQUES This description and our product range both focus on single capacitors and reactive power compensation systems in the power range from 50 to approx. 6,000 kvar, because the greatest deficit in risk-awareness is present here. In practice, they are often just «parked» somewhere since the dimensions and prices of such equipment are relatively small and they should be close to the load. Protection devices are then often neglected in comparison to larger systems, because the costs appear to be unreasonably high. It is a fact that the protection systems necessary for the reliable operation of a small reactive power compensation system, e.g. 300 kvar, require the same protection relay and a similarly expensive current transformer as a 10 MVAR system. If the total power is divided into 3 or 6 units, even higher costs are incurred. Disparities of up to 80% for the protection component then occur, compared to 5% for larger systems. Serious mistakes are often made especially in the reactive power compensation of small installations, which lead to equipment damage and environmental incompatibility. This unsatisfactory situation was the starting point for the development of a new concept that offers an acceptable relationship between protection costs and total costs, while reducing the ecological risk. Operational stress of Allfilm medium voltage capacitors Capacitors operate at full power immediately after every switching. No-load or low-load periods do not exist. The design is made under economical constraints with high electrical field strengths up to 75 kv/mm and a finite service life, which can be dependent on many influencing factors, and is estimated from statistical data. There are many effects that cannot be detected during durability tests. Summarizing, it can be said that modern power capacitors are very reliable and failure rates greater than 0.2% per year are very rare. However, it must be considered that much higher failure rates occur from early failures, manufacturing faults, dimensioning errors, incorrect application or overload. The effects of such failures must be assessed carefully by means of a risk analysis that also includes the ecological risk, due to the high short-circuit power present in medium voltage installations. Objective of capacitor protection techniques The reduction of damage to the environment is the most important criterion for capacitors, while preventive protection against permanent damage is the prime concern with motors, transformers, inverters, lines, cables and similar components. Breakdown behaviour of Allfilm medium voltage capacitors Allfilm capacitors are constructed with layers of aluminum foil and polypropylene film. These are foils are first rolled together and then flattened to make packets. These packets are connected in series and parallel connection inside a steel enclosure to make a capacitor. Finally, this whole assembly is immersed in synthetic mineral oil (e.g. Jarylec) to complete the construction. In some cases each packet has its own fuse which is meant to disconnect the packet in case of failure. Breakdown of the dielectric is the prime cause of failure. Only this breakdown and the resulting consequential effects are considered here. Every breakdown of a single winding element in a capacitor with several internal serial winding circuits leads to a change of the internal voltage distribution, irrespective of whether this winding is fused individually or whether it is a fuseless design. This leads inevitably to higher voltage stress in the remaining winding elements. Accelerated ageing accompanies the increased voltage stress, which results in further winding breakdowns. Considerable damage to the environment of the capacitor must be expected if the breakdown process is uncontrolled, i.e. operation is continued until an over-current, earth or short-circuit trip responds. This means that, when the maximum permitted energy input into the capacitor casing is exceeded (violation of the typical current-time destruction limit), it can, in the worst case, tear open, and the contents of the casing can be ejected. A considerable shock wave, with ignition of the oil spray and the solid flammable content, are conceivable further consequences.! For medium voltage capacitors, protection normally means the minimization of effects of already existing, irreversible capacitor damage to the direct neighborhood. This occurs in the knowledge that Allfilm capacitors contain a large proportion of ecologically critical, flammable, solid and fluid organic materials, which pose a considerable hazard potential. 1

3 3-phase medium voltage capacitors These capacitors are frequently built without internal fuses, due to their usually - low power and three-phase design. After a winding breakdown, such a capacitor develops a group shortcircuit. Depending on the number n of internal serial circuits, the capacitance and voltage stress in the affected branch increase by the factor n/n-1. The current increases correspondingly. Thermally, this is barely noticeable due to the low capacitor losses. Further short-circuits can occur, especially if the fault remains unnoticed for a long period.! Since remaining windings can still survive at up to twice their nominal voltage for minutes or even hours, the effects on fuses and feeders under the influence of twice the rated current are to be taken into account. Gas formation need not be expected during this overload phase, since the film layers get welded permanently with each other at the breakdown position and can carry increased current for a long period. The internal pressure does not rise significantly during this period.! Pressure monitors are ineffective in Allfilm medium voltage capacitors. Example: Consider an example of 150kvar/ 6kV/50Hz 1 ph capacitor. When we have a failure situation, the short-circuited capacitor element and all paralleled elements are bridged out of operation. As a result, the effective kvar output of the capacitor increases. This is now 181kvar. When an external star circuit is functioning with 3x 150kvar and one of the capacitors suddenly becomes 181kvar, this causes shifting of the star point and stresses in the circuit. Consider an example with fuse-protected capacitors. In this case, the effective kvar output comes down, but the voltage stress on the functioning packets, in this case, increases by more than 19%. This increased stress is disastrous for the capacitor. 2

4 RISKS AND SHORTCOMINGS OF FORMER COMMON CONCEPTS Various concepts for the protection and switching of medium voltage capacitors are known from practice. The five most usual will be discussed here. Problem 1: Current overload caused by a group short-circuit in the capacitor, or also at resonance with network harmonics, cause a danger that the short-circuit protection fuse trips in the over-current range if the current rating is too low. The arc quenching capability of the fuses can be overtaxed due to the capacitive load. Reverse sparking occurs with very high intensities and ultimately leads to an open arc at the fuse contacts, and to a phase short circuit at the installation position of the fuse. Choice of nominal fuse current Fuses must be chosen for at least 1.6 times the rated capacitor current to withstand the switching current. Since Allfilm capacitors can remain connected to the mains in fault situations (failure of serial groups) for a long period with up to double the nominal capacity, fuses should be rated for at least 2 I N. The risk that fuses rupture due to over-current, but do not quench the arcing is avoided by this method. If fuse ratings of approx. 200 A are exceeded, then an additional problem occurs: In the case of a short-circuit, the fuse throughput energy becomes greater than the energy absorption capacity of the capacitor enclosure. Selection of the rated voltage for capacitor fuses Current limiting fuses have an excellent interruption behaviour during short-circuits, but are usually not able to interrupt capacitive currents at overload. A peak voltage with twice the amplitude occurs at the gap due to the charged capacity, which can result in arcing-back with even higher voltages. One should therefore always employ h.v. h.b.c. general purpose fuses with the next higher nominal voltage than the actual mains voltage. Problem 2: If a protection system for early fault detection, such as asymmetry protection or phase current comparison protection is not employed additionally, then the concept is unsafe.! As a rule, circuit breakers are unsuitable for the disconnection of capacitors with an internal short-circuit, since they do not possess a current-limiting interruption characteristic. The throughput energy before isolation is always much larger than a capacitor enclosure can withstand. 3

5 Problem3: Satisfactory capacitor protection is not possible due to the layout of the fuse and motor protection for the total current. When operated with an undetected group short-circuit in the capacitor, a considerable overvoltage will arise at the motor during the shut-down phase after disconnection from the supply. As a result, the already overloaded residual capacitance may flash over. The next switch-on is then made onto a short-circuited capacitor, which may cause the capacitor to burst and the motor insulation to be damaged.! Fixed compensation systems cannot be satisfactorily protected without additional sensitive protection systems. At least separate fuses matched to the capacitors are necessary. Better would be the application of a highly sensitive additional protection system that trips already at a low fault level. Problem 4: Load unbalance protection circuits detect the circulating current in the neutral branch of a bridge circuit or between two star points of a double circuit which occurs after the breakdown of a winding or a winding group. Even the breakdown of the first winding element in small capacitor systems can reliably be detected and used to trip the associated switch. The other switching functions of this switch are irrelevant, so far as the protection tripping disconnects the capacitor immediately at a low fault level, or in the case of multi-step systems, after a previous alarm signal. Reliable protection against consequential damage is achieved with the correct setting of a pre-defined switch-off threshold at a low fault level.! Remaining problem: High costs and effort for ecological protection and disposal 4

6 Problem 5: Variant 5 is similar to an asymmetry protection scheme; it can avoid consequential damage from premature tripping. Network asymmetries or starting asymmetries can lead to erroneous tripping more easily than with variant 4. The sensitivity which may be achieved by phase comparison protection is lower than with an asymmetry protection circuit. Apart from that, the problems of variant 4 remain. Course of damage at capacitors with winding fuses Depending on type, winding fuses are able to interrupt currents at network frequency, with current limitation at mains voltages up to several kv. At higher voltages and with several internal series connections, the voltage at the remaining windings continues to increase with decreasing number of paralleled winding elements, until the disconnection ability of the winding fuse is overtaxed. The damage arising here can lead to the destruction of the insulation between the phases or towards the enclosure. The consequences are depending on the through-put energy of the associated over-current protection and more or less identical to those occurring at capacitors without fuses. Capacitors can be considered partially safe if they contain no more than two internally fused, series-connected winding groups between the phases, e.g. star-connected 3.6 kv capacitors or singlephase capacitors up to 3.6 kv in furnace systems. During the development of a highly advanced state of fault, there is not only a rising risk of winding breakdowns occuring, but also damage to parts of the internal insulation, which may lead to a short-circuit at a high fault level. The risk of consequential damage is significantly higher than in systems using asymmetry protection. Basic rule The most important criterion for a timely disconnection without consequential damage is early detection and, if possible, rapid disconnection of capacitor groups after the failure of windings or winding groups. This ensures that fuses do not trip due to increased current and that rapid short-circuit trips do not occur, both of which can have the very serious consequences described above. This applies especially to three phase capacitors. Other criteria must be taken into account in the case of large compensation powers, which shall not be discussed here. A network short-circuit is improbable due to multiple external series circuits. Correct handling of fault tripping One repeatedly observes that fault trips are reset or re-closing attempts are made by operating staff, without the cause of the trip being clarified and cleared. Enormous consequential damage is observed and documented from such incorrect conduct. 5

7 MSD TM SELF-HEALING MEDIUM VOLTAGE CAPACITORS IN DRY TECHNOLOGY Increasing safety awareness resulting from stricter product liability, the unsatisfactory situation with fusible links and high cost for asymmetry protection or phase comparison protection have motivated us to seek a safe, simple and economic solution- MSD TM - the ecologically safe and self healing medium voltage capacitor. Our MSD TM capacitors are filled with solid materials, i.e. dry, instead of combustible liquid as with Allfilm medium voltage capacitors. High-quality insulation between the active elements and the enclosure is achieved using a special process, which is designed and tested to suit the requirements for the nominal insulation voltage of the capacitor. This special insulation is of crucial importance for the safe operation of the internal pressure monitor: self-healing capacitors are not (yet) covered by current standards for medium voltage capacitors, such as IEC 60871, however, our MSD TM series fulfils all electrical and safety requirements of these standards. It has to be noted that the MSD TM capacitors like any self healing capacitor do not produce short circuits and can therefore not be disconnected from the system by internal or external blow-out fuses. This task is performed by integrated over-pressure switches as described in the standards applicable to capacitors for power electronics and for inductive heating. (VDE-EN and VDE-EN 60110) Design and characteristics The MSD TM technology is based on the logical development of proven selfhealing technology for lowvoltage power cap. It also permits the economic manufacture of medium voltage capacitors without employing inflammable and environmentally critical fluid oil fillings. The actual active capacitor element consists of a large number of high-quality, self-healing round MKP elements, which are wired to each other and installed in a stainless steel enclosure. The windings are round and thermally stabilized, unlike the Allfilm packets which are physically pressed into rectangular shape. MSD TM round windings are more stable and ensure a longer life. Stretching Wrinkles Above: The pressing operation of the Allfilm construction causes stretching of the outer layers of the packet. Left: Round Windings with heat stabilisation, with no uneveness in the construction. This is extremely essential for excellent self healing behaviour. 6

8 Benefits of the new MSD TM technology Short-circuit currents are not possible due to the high-resistance fault characteristic of the self-healing dielectric. Special, short-circuit current limiting capacitor fuses are not necessary. Functional switching devices are sufficient for tripping. The costs of the monitoring circuit are very low. It is sufficient to control the tripping function of the switch via the normally closed contact of the pressure monitor. Every enclosure is monitored separately. Any number of individual capacitors can be grouped together for protection purposes. As a consequence of the different partial failure mode of MSD TM capacitors compared to that of Allfilm capacitors, the possibility of current imbalance in the three phases can be ruled out almost completely. Hence monitoring of the star point is unnecessary which leads to further reduction of installation cost Oil sumps are unnecessary due to the dry design; no oil which could pollute the local environment. No disposal problems at the end of useful service live. It makes these capacitors specifically useful where the consequences of leakage can be huge. reliability during applications in de-tuned and tuned filters and double filters by long term stability of capacitance significantly smaller temperature sensitivity of capacitance with values of approx x 10-4 compared to a 60% higher value for All Film dielectrics very small capacitance tolerances up to ±2.5% Comparison MSD TM vs. Allfilm capacitors ( construction and application) MSD TM ALLFILM (traditional style) Filling/hazard Solid resin / no leakage risk Synthetic oil / risk of leakage and fire spread Asymmetry Impossible possible Dielectric breakdown Self-healing 1st breakdown provokes total failure of capacitor Safety device Pressure sensor star-point monitoring requires 1ph design or special terminal and separate relay; internal fuses, but big portions of capacitance are disconnected. Overvoltage monitoring Not required (self-healing dielectric) Necessary to prevent dielectric failure Capacitance drift Close to zero Capacitance drifting with temperature 3ph design Standard only available without asymmetry protection 7

9 Economic viability The pure capacitor manufacturing costs maybe slightly higher than former Allfilm capacitors as a result of the employment of especially high quality materials and a special production process. If the total system costs including electrical protection systems such as asymmetry protection, fire protection and environmental protection measures are also considered, then significant cost benefits are achieved, especially in the small to medium power range. The new technology offers decisive advantages in the range from 50 to approx 6,000 kvar. Larger powers are also generally possible in the voltage range up to 12 kv. Oil-filled capacitors are not permitted at locations with special fire risk, e.g. in mines, in protected water catchment areas or drinking water pump stations, so that other alternatives are often unavailable. Application Fixed motor and transformer compensation, automatic capacitor banks, mobile sub-stations, de-tuned and tuned filter circuits, in double filters, audio frequency links and other applications in critical areas for application in the 1.9 to 12kV range. For detuned and tuned filters it is necessary to select the voltage rating of the capacitors properly. Detuned capacitors can in general be operated in any mains. In any case, they are a safer choice than non-detuned capacitors and future-proof under the conditions of more and more deteriorating power quality in modern mains. We strongly advise to conduct a comprehensive mains analysis, including measurement of the harmonic content, before designing and installing your power factor correction equipment. In cases, however, where such analysis is not possible, cautious and conservative assessment of the situation to be expected shall be made. For tuned capacitor bank care should taken of carefully calculating the harmonic currents through the capacitor since at medium voltage both upstream and downstream harmonics can be expected. It is expected that the designer will have the system response characteristics at his disposal before designing such a system. ELECTRONICON MSD TM capacitors are easy to use and mount. These capacitors can easy connected in parallel on a common bus. Being highly resistant to vibration and these are widely used in such environments. MSD TM capacitors are designed for use in indoor applications- for outdoor applications please contact ELECTRONICON. 8