Challenges of High Voltage Measurement System
When establishing measurement systems, there are instances that present unique challenges. The author once encountered an unusual situation when tasked with setting up a system for recording vibrations and strain in a contractor operating at a daunting 25 kVAC. However, a significant challenge arose, primarily centered around the power supply for this measurement system. With an energy demand of approximately 30W, the feasibility of using batteries was ruled out, given the need for prolonged operation spanning many hours.
The conventional approach of employing an isolating transformer seemed logical at first, but here’s where it gets tricky. A voltage rating of 25 kVAC corresponds to a peak voltage nearing 40 kV, not to mention the necessity of incorporating a safety margin. Furthermore, any equipment connected to high voltage lines should be capable of withstanding the rigors of lightning strikes. As a result, the isolation had to be resilient enough to endure a testing voltage of 150 kV, which places a substantial demand on the isolating materials.
Challenges in Finding Suitable Transformer
The search for an appropriate transformer rated at 50 W, featuring a 230 V primary, 12 V secondary, and an isolation of 25 kVAC proved to be an arduous one. Despite extensive research, no supplier could provide such a specific transformer. Consequently, an alternative dynamic system had to be devised, albeit with a drawback of increased wear and tear. This makeshift system comprised a 50 W 3-phase motor connected to a 30 W generator via an isolating drive-shaft. Remarkably, the generator utilized in this setup was a 3-phase servo motor repurposed as a generator. This makeshift generator powered the data logger and its associated electronics.
Simplified Secondary Supply System
Utilizing a 3-phase generator had its advantages, particularly after full-wave rectification (achieved through D1 and D4 to D8), which yielded a reasonably stable voltage. This stability was further enhanced by the generator’s relatively high revolutions per minute. Consequently, the secondary supply design could remain relatively uncomplicated. The primary 9 VDC supply was regulated by IC3, an LM317T. This regulated voltage was then channeled into several small DC/DC modules (IC1, IC4, IC5), generating +5 V, +30 V, and -9 V outputs vital for various parts of the circuit. Additionally, IC2, an LM566 voltage-controlled oscillator, facilitated the flashing of LED D2 to indicate the presence of the supply voltage.