Company History

Licht was founded in 1983, initially manufacturing solid state voltage regulators for on-load tap changers. A few years later, digital transducers and indicators were added to our line of products.

From the thousands of devices manufactured by our company, a considerable fraction was installed adjoining power transformers, where environmental and electrical conditions are specially severe. For instance, in a typical transformer panel the ambient temperature may remain at 70 °C indefinitely. This scenario is specially aggravated in tropical installations, where many products developed for temperate climates present performance issues and a much reduced lifetime.

Once powered on, our products operate continuously under such conditions for at least 20 years in order to match a power transformer's typical lifetime. During this period they must be completely reliable because failure in this type of system always brings serious consequences.

To better answer the market's needs, Licht's latest products are microprocessed. Along with top quality components and exhaustively tested software, they also incorporate our knowledge of building equipment able to remain fully operational for large periods of time, even under harsh circumstances.

Licht's Engineering

Introduction

As a manufacturer of electronic products for electrical systems, many of which are installed alongside power transformers, we have had the opportunity to observe in detail the effects of component degradation in products of our own and similar models from other manufacturers.

In order to present fully reliable long term performance over a large range of operating temperatures, special design considerations must be taken. We present below a few premises that we follow in the development of all our products.

Premise #1: Operating Temperature

All effort is taken to ensure that a component's temperature does not rise more than a few degrees above ambient temperature. To accomplish this we have developed our own high efficiency power supplies, ensuring that heat dissipation is minimized or even negligible in comparison to the conducive heat exchange rate. Also, the power supply is usually one of the weakest links due to early component degradation. This is due to the presence of high voltages, currents and transients wich lead to higher temperatures.

Thus, a high efficiency power supply not only reduces component aging over the whole product but also significantly reduces the possibility of failure due to a power component. To ensure this is the case, components are overdesigned with large safety margins. For example, our power supplies typically have nominal ratings 5 times larger than required under normal conditions and our power semiconductors have nominal ratings up to 20 times what they are expected to encounter.

In order to assure adequate heat dissipation, we prevent the crowding of PCBs and design layouts for adequate air convection.

Premise #2: Proper Galvanic Isolation

In order to guarantee the safety of our products and that of others connected to them, certain parts of the circuit must be mutually isolated. In effect they behave as if separate devices with separate supplies, such that surges and transients do not have an electrical path from one circuit to the other. Therefore, transients are restricted to the section of the device to which they arrive. To implement this, we take the following measures:

1. All relays must present adequate galvanic separation between contacts and coil, given that the switching of inductive loads is a significant source of noise. Usually relays with such characteristics are physically larger and have superior costs, resulting in larger equipment.
   Traces leading to such relays must be conveniently separated such that transients cannot arc under all but extraordinary conditions. Separation also reduces EMI pickup (crosstalk).
   Although essential, these measures increment production costs. The relays themselves are more expensive, and larger relays with wide, separated traces work to increase PCB area, therefore increasing the product's size.
   
2. Analog signal generators (current or voltage) must communicate with the processor through optical barriers and there must be no electrical connection between both parts.
   Tracks from this part of the circuit must be sufficiently spaced from any other track. The power supply must also be galvanically separated from any other circuit so that a surge may not cross through it.
3. Digital I/O must be optically coupled. As with analog signals, tracks must be separated from the rest of the circuit and the supply must be galvanically isolated.
4. Apart from pairwise isolation between subcircuits, the power supply's input must be isolated from its output. This is the only way to ensure that any surge will at most damage a single subcircuit, thus protecting other connected equipment.

Premise #3: Redundancy

In absolutely critical applications redundancy mechanisms are recommended. For example, a secondary processor may supervise the primary, performing sanity checks and being able to activate relevant alarms in case abnormal behavior is detected. Redundant power supplies can also be implemented along with alarms in case of failure.

Conclusion

Once the service temperature of a particular component doesn't rise much above ambient temperature, once can adequately predict aging and performance characteristics. As long as the design uses components designed for high temperature applications and has reasonable safety margins, one can expect predictable behavior over time, resulting in satisfactory operation and long term stability.

Considering the several levels of galvanic insulation, transients would usually require going through two layers of separation in order to reach another part of the circuit (and possibly be transmitted along the line to other devices). This is highly unlikely given the limited power and duration of transients. Even in the case of damage due to transients, in most cases the device will remain partially operational because transients would cross peripheral subcircuits and not the processor. Furthermore, redundancy (if implemented) may offer extra reliability.

We have noticed that many functionally similar products are protected exclusively with transient voltage suppressors (such as varistors or zener diodes), which we find unsafe and unreliable. Even though lower costs may be achieved through this practice, it is not a dependable solution. For example,

• TVSs have a limited energy rating, which is proportional to their size. Thus, typically small suppressors will be destroyed even by moderate surges, leaving the system unprotected. Operation at high temperatures worsens the problem by lowering the amount of power the device can safely dissipate.
• Surge suppressors have a minimum response time, under which transients are allowed to pass.
• TVSs usually age considerably, becoming less effective.

In closing, one must always weigh the tasks delegated to digital equipment such as transformer controllers and their responsibility, since they may guarantee electricity to cities, industrial complexes or even whole regions. Therefore, our prime concern is to ensure that our products are conceived under the principles described above, given that failure is an unacceptable outcome.