ISOLATION TRANSFORMERS AGAINST NOISE AND TRANSIENTS

Tuesday, October 18, 2011

Isolation Transformer is a special type of transformer, wherein the primary and secondary windings are physically separated through a so-called double insulation. Also, an isolation transformer with electrostatic shields is commonly employed as power supplies for sensitive devices like computers, laboratory measurement instruments and medical equipment.

The leakage inductance of isolation transformers is the primary feature that electrically isolates people and equipment from the hazards of power quality problems such as transients and high-frequency noise. Subsequently, they have a lower leakage current than those that use a shield. In addition, isolation transformers can prevent transfer of DC signals from one circuit to the other, as well as block interference due to ground loops. However, they permit AC power or signals to pass. 
Isolation Transformer
Isolation Transformer
Applications and Benefits

Basically, isolation transformers are used to protect people against the hazards of electric shocks, and isolate electrical equipment from the negative effects of transients and high-frequency noise. Some of the applications are discussed below.

Medical Transformers

Medical transformers are basically isolation transformers, which are designed to isolate hospital patients and operators from electric shock, and to protect the equipment from power surges or faulty components. They are used to offer a safer environment by minimizing the potential hazards caused by touch voltages and by avoiding loss of power even under single fault conditions.

Electronics

Isolation transformers are used as a safety precaution in electronics troubleshooting and testing. Grounded things near the device under test (lamp, oscilloscope ground lead, etc.) may have a dangerously high potential difference relative to that device, since the neutral wire of an outlet is directly connected to ground. With the use of an isolation transformer, the bonding is removed and the shock hazard is contained within the device.

Ships

Metal ships are subject to corrosion due to use of grounded power from shore when anchored. This is because galvanic currents flow through the water between shore ground and the hull. To avoid this problem, an isolation transformer with the primary and case connected to shore ground, and the secondary floating should be employed.

In addition, a metal safety screen between the primary and secondary of the transformer is connected to shore ground. Consequently, when a short circuit occurs in the primary, it will cause the fault current to trip a shore-based circuit breaker rather than making the hull live.

Separately Derived System

An isolation transformer is used to establish a new neutral point (ground reference) or a separately derived system. For example, when installing electrical equipment in a grid without a neutral point, the solution is to install an isolating transformer. Also, several applications require the neutral point for controlling purposes.

Furthermore, other benefits and uses of isolation transformers are the following:

  • Capacitor-switching and lightning transients can be attenuated, thus preventing nuisance tripping of adjustable-speed drives and other equipment.
  • Improve power quality by reducing high-frequency noise currents.
  • Complete isolation from the input AC line.
  • Combined with surge protective devices, it offers continuous filtering of power line noise in all modes.
  • Active transformer filtering provides common-mode noise rejection with no wearable parts, exceptionally reducing surges in the worst of power environments to harmless levels.
  • Limit voltage notching due to power electronic switching.
  • Eliminate neutral-to-ground voltage and noise, which cause reliability problems for microprocessor-based electronics. 

Construction and Design

Isolation transformers are normally designed with consideration to capacitive coupling between the transformer windings. This is because excessive capacitance could couple AC current from the primary to the secondary. Therefore, a double isolation is required to solve such problem.
Isolation Transformer Diagram
Isolation Transformer Diagram
Double Isolation

There are two methods to set up double isolation:

1.    Placing grounded metallic safety or Faraday shields between the primary and secondary windings.

The electricity simply flows to ground if the isolation breaks, providing the required safety. The shield will attenuate transients and will reduce the coupling of common-mode noise by decreasing the capacitive coupling.

2.    Reinforced insulation

This is made up of multiple layers, which pass the tests required by established standards. If one layer gets damaged, the next layer gives the required safety.

Reference:
De Potter, P. (2011). Isolation Transformers

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.