Introduction

In many ways, Uninterruptible Power Supplies (UPS) are the quarterbacks of critical power distribution systems. They allow seamless operation of system-critical loads, have scalable levels of stored energy based on external battery strings, and serve as a barrier to most forms of power distortions hazardous to hi-tech facilities. With significant progress in the sampling speed of solid-state electronics, the increase in rectifier and inverter load capability, UPS’s have now become capable of feeding of system-critical broadcast technology.

The invention of the klystron in the 1930’s and its replacement with the much more efficient Inductive Output Tube (IOT) in the 1980’s led to the development of the High Voltage Power Supply (HVPS). Similar to computer power supplies, HVPS’s have a tendency to reject stored energy from their filter capacitors into their loads. This stored energy is rejected at levels of tens of thousands of volts. As IOT’s have very low dielectric resistance to arcing and almost no impedance to constrain the magnitude of an arc fault, they are extremely susceptible to failure due to these spikes. To prevent catastrophic failure, modern transmitter manufacturers install in parallel with the IOT, a modularized crowbar protection circuit. In the event of a potential arc fault, the crowbar device shunts the built up voltage through a high-current thyratron circuit.

Design Considerations

A typical current demand during the crowbar effect may be as high as 4,000 amps and can last as long as 20 milliseconds – or about one cycle. This inrush has a significant effect on three components of the system-critical distribution system: (1) the circuit breakers which must differentiate between a crowbar effect and a ground fault, (2) the emergency generators which must handle the instantaneous current demand, and (3) the UPS system which must maintain a constant and uninterruptible voltage to the critical loads.

1. There are two current spikes inherent to a crowbar effect in transmitter applications. When the thyratron circuit “fires”, the initial spike is the largest and is of principal concern to the instantaneous fault protection characteristics of overcurrent protective devices. A secondary spike occurs when the thyratron is disengaged due to the relationship between the gas-discharge time of the thyratron and the “reloading” of the transmitter. Both these current inrushes and any other critical loads (such as motors) must be taken into account when designing fault protection upstream of the thyratron circuit. Fuses may need to be installed to offer backup protection to the IOT windings. As is true with all systems, extensive trip unit testing is ultimately the best way to insure a safe and effective overcurrent protection system.
2. Generators are easily overlooked when sizing a critical distribution system to sustain the crowbar effect. It is important that the emergency system can supply the current demanded by the simulated ground fault and be available to the UPS bypass while on battery power. One UPS manufacturer recommends generator be sized to three times their nominal load to insure minimal reflection of the impulse demand, but the actual ratio is system dependant and typically in the range of 1.75-2 times the UPS size.
3. The largest paradigm shift associated with supporting a crowbar protection circuit coincides with what is typically the most expensive component of an emergency distribution system. UPS’s are designed for large-scale computing with smaller, low inrush power supplies being deployed. With modern transmitters, UPS’s have to be designed for one very large power supply, and more importantly, the crowbar effect which protects the IOT. The average UPS system has capacity adequate to sustain a 167% momentary overload and a 133% interrupting overload. In order to sustain the enormous overload during the crowbar effect, the UPS system must either be oversized (an extremely costly endeavor) or its controls be integrated with the static bypass switch. During the crowbar effect, the static bypass switch transfers the load from the inverter/rectifier section to the utility via the static bypass switch. Special high speed sensing and controls are required to make this transfer. The utility transformer is capable of riding through a ground fault with minimal reduction in voltage. In order to make this transition appear seamless to the load and without damage to the UPS, the UPS inverter must continually be in synchronism with the utility feed for this to take place.

Conclusion

Therefore the most important component in the distribution system now becomes the static bypass switch. Not only does the switch have to “turn on” swiftly, but it has to withstand extraordinary currents for short durations. The rugged design of this component is the determining factor on whether a properly-implemented UPS system is capable of supplying an HVPS-backed transmitter with reliable and uninterruptible power.