11.07.2019 Products and ServicesProjects

PQ provides high voltage inrush current limiting resistor for rectifier transformer

In rural areas, far away from power plants, the power distribution and trans-mission network are rather weak with respect to the short circuit power level. Because of that, these kind of networks are very sensitive in terms of transient events (e.g. switching events).

The production of chemical products and the production processes demanding very often for powerful electrolyzer systems. These electrolyzer units are mainly operated with a DC voltage below 1000 V and operating currents ranging between a few kA up to 3 digit kA values. Due to these parameters the power consumption of electrolyzer plant are typically in a range of several MW´s. The network connection is realized with powerful rectifiers and associated rectifer transformer.

In the specific project case the customer built up a complete new eletrolyzer line with a nominal power of 57.2 MVA. Once all the equipment have been installed, the first energization of the rectifier transformer was scheduled. During the energization sequence the peak inrush current of the rectifier transformer was unexpectedly high and reached values of around 3000 A at the 75 kV busbar. This value was more than five-times higher than allowed. As one of the next steps, the customer has changed the circuit breaker from a standard breaker to a point-of-wave breaker type. But even this measure brought not the expected results. Finally the customer consulted MR-PQ for further support.

MR-PQ´s starts the investigation of the network with a comprehensive study in order to reproduce the inrush behaviour of the transformer using the measurement results done by the client. After finishing, it has been decided installing a 75 kV Inrush Limit-ing Resistor system with a by-pass breaker in serial to the rectifier transformer.

Challenging for the project were the limited space at site and the clear request of the customer not to modify the existing 75 kV busbar.

MR-PQ could solved the problem in a way that the whole solution fits into the existing feeder yard. So even the initially intended spare breaker for connecting the by-pass breaker could be saved.

As the customer has only 2-3 slots/year for shutting off the 75 kV busbar for mainte-nance work, the timeframe was very tough. MR-PQ was awarded in November 2018 with the clear task to set the whole system into operation by begin of May 2019. The scope of supply were not only the resistor units. Moreover MR-PQ had to provide the whole detail engineering with layout drawings, wiring books and coordination of a sub-contracted local third party at site (responsible for the civil and installation work).

The years of experiences of the MR-PQ engineering team for complex projects were finally the key to success and ensured that the whole project have been delivered and installed on time.

Before the rectifier transformer could be energized again, MR-PQ´s commissioning engineers have checked the whole installation (mechanical/electrical) and installed a measurement device in order to record the current values during the energization sequence. Finally the measurement results confirm the performance of the solution, as the re-sulting peak inrush current has been reduced from initial ~ 3000 A down to 500 A. Because of that fact the voltage drop in the 75 kV network could reduced as well.

Finally the customer was very satisfied with MR-PQ´s work and got the release for operation of the new rectifier line from the local utility.