Rectifier filter stage post filtering sense

Transcription

1 TopCon DC power supplies Customer support files Nr _e Topic: Protecting the DC output 1. Abstract TopCon DC power supplies are generally well suited for operation into reactive DC loads. Reactive loads are represented as loads with the ability to store energy in any manner, such as capacitors, inductors, DC motors, focussing coils, magnet coils, superconductors, inverters and converters and so on. Running reactive loads often presents some dynamic effects, which have to be taken into account when designing a load circuit. TopCon power supplies are well protected against a whole number of overcurrent and overvoltage effects, nevertheless it is highly recommended to examine a reactive load with respect to its dynamic behaviour. It is the aim of this paper to explain the main reasons for dynamic effects and how to protect TopCon s output stage against them. 2. Understanding TopCon output stage topology Block diagram of TopCon output stage + - Picture 1 After rectifying, the power signal is filtered to obtain a low-ripple DC. Post-filtering removes HF contents and the sense circuit monitors the actual DC voltage value, either local or remote, if remote sensing is activated. TopCon Output Stage Protection e.doc 1/5 ChR

2 By the nature of a rectified signal, a given polarity of voltage is maintained. Any attempt to reverse polarity will result in a short circuit condition by forward biasing the rectifier circuit. Please note, that filter capacitors represent a certain reservoir of electrical energy. Connecting or switching of a low-impedance load across the output when voltage is present will cause a current peak to flow through the load. Ramping up of the unit instead of hard switching the load can prevent this peak. It should be further noticed, that the voltage sensing circuit is located directly at the DC output, so any overvoltage spike will be captured by the monitoring stage. 3. Reactive energies coming from the load There are several reactive impacts originating from a reactive load. Reactive DC load Free wheeling currents Overvoltage, same polarity Overvoltage, reverse polarity All sorts of inductors, DC motors, AC brushless bridge circuits, reverseable coils etc. drives, regenerative elements DC drives etc 4. Protecting TopCon output stage and considerations 4.1 Free wheeling effects By the nature of the output topology, any freewheeling current is allowed to circulate throughout the output stage. This condition is not dangerous at all as long as the current level is within the nominal current rating of the respective TopCon unit. Therefore it is allowed to switch from a given voltage level freely to another when using inductive loads, even a VOLTAGE OFF command during full operation will result safely in a short freewheeling current circulating between the load and TopCon. Please refer to picture 2 on next page. TopCon Output Stage Protection e.doc 2/5 ChR

3 Picture 2: Free wheeling conditions 4.2 Overvoltage, same polarity Overvoltages across the DC lines may occur by several reasons. Hard switching of inductive circuits or overdriving of DC motors or AC servo drives are typical examples. As mentioned above, TopCon voltage sensing acts firmly at the DC output, so every overvoltage event will be captured immediately. Please note the following rule: Safe DC output voltage level = 110% of TopCon unit nominal voltage Example: A rated 600 VDC unit will handle a permanent 660V DC level without damage. Overvoltage ERROR : Depending on the setting of OVERVOLTAGE limit inside TopCon an OV-ERROR will stop the unit whenever this limit is passed. Resetting this error needs an intervention. In the case of very short and well-known pulses, an appropriate error delay can be programmed by the aid of TopControl service software in order to suppress the error temporarly. This of course decreases the level of OV protection a bit, but will also free the user from repetitive manual reset interventions. Please refer to TopControl manual for details. Note: Standard TopCon units are not actively protected against overvoltages. Although the overvoltage error shuts down the unit immediately, the unit itself cannot dissipate significant energies at high voltage levels. It is within the user s responsibility to avoid too high overvoltages originating from a reactive load. TopCon Output Stage Protection e.doc 3/5 ChR

4 4.2.1 TopCon downprogrammer option DPU A good way to suppress overvoltages and to keep excessive reactive energy is the use of TopCon DPU downprogrammer option. This feature allows for dissipating reactive energy in a quick manner ( switching time < 50 usec ) and therefore to clip pulses and/or steady overvoltages to a safe level. It is highly recommended to contact REGATRON for further details on TopCon DPU Connecting active loads When connecting active loads like batteries, accumulators, loaded capacitors, specially ULTRACAP s, bear in mind that the output of TopCon units contain capacitors, which will be at 0 Volts DC during VOLTAGE OFF. Therefore when connecting, a short load-current spike will flow in opposite direction into the unit peaking-up the filter to the active voltage level. Although this current pulse will not affect TopCon filter capacitors, the external circuit and measurement equipment has to carry this inrush current. The best way in this case is to limit the current by external means like a series resistor or similar, which is bypassed during normal operation. U load I peak U batt Picture 3: Reverse current peak 4.3 Overvoltage, reversed polarity Some types of inverters, reversible drives, reversed DC motors may produce voltages of inversed polarity under certain conditions. Connecting such negative voltages across the DC output will forward bias the rectifier structure and therefore establish a very low impedance across the output. Depending on the type of load and the amount of load energy, significant currents in opposite direction may occur. TopCon Output Stage Protection e.doc 4/5 ChR

5 The current rate cannot be influenced, controlled nor limited by TopCon unit. As long as the current level does not rise above the current rating of the respective unit, no damage will result. Of course, TopCon sense circuits will detect this condition as reverse voltage and current, and therefore a couple of errors will shut down the unit. Nevertheless, the current will continue to flow until the reverse condition is removed. U load Picture 4: Reversed voltage across output As stated above, also this condition is within the operators responsibility! It is best to suppress this condition by appropriate measures. 5. Checklist Freewheeling expected? no further measure necessary Active load voltage? Check polarity : If positive : Within 110% of TopCon unit nominal voltage? If yes: no further measures, possibly OV error delay time settings If no: a) Voltage limiting necessary by external elements b) Ask REGATRON for a DPU unit (optional ) If negative : Avoid this condition by external means, expect low impedance across TopCon DC output high non-regulated current! If not to be avoided check current peak to be within TopCon nominal current range and expect unit shut-down caused by several errors. TopCon Output Stage Protection e.doc 5/5 ChR