VTNI • Variable Threshold Neutral Isolator

• Comparison of Dairyland VTNI to Metal Oxide Varistors
• Comparison of Dairyland VTNI to Saturable Reactor
  

Comparison of Dairyland VTNI to Metal Oxide Varistors

The Dairyland VT/NI, used for blocking neutral-to-earth voltage on a utility’s primary neutral from the customer’s secondary neutral, has significant advantages over other technologies, including metal oxide varistors. These include:

Voltage Threshold:
The VT/NI operates at set thresholds that are above normal neutral-to-earth values, but low enough to safely re-bond the primary and secondary grounded neutrals during an AC fault or lightning event. The product differentiates between 60Hz events and higher frequency transients, to present an appropriate voltage blocking level. Metal oxide varistors have a stated voltage breakdown (threshold) level that is usually in the range of several hundred volts, for devices that have adequate lightning surge current capability. As the lightning surge current rating goes up, the threshold associated with that rating also increases. This can result in breakdown levels that are quite high – in the range of 300-700V – in order to have adequate surge capability. The VT/NI has a high lightning surge capability with a relatively low and safe threshold.

AC Fault Ratings:
The VT/NI has been fully tested at power laboratories to verify the product ratings and performance, and is rated for stated AC fault current magnitudes and durations. These ratings represent values in excess of the typical utility distribution system available fault current. Since the isolation device is connected between the primary and secondary neutrals, AC fault current can flow in this bond if the distribution transformer were to suffer a primary to secondary winding fault. Metal oxide varistors are not rated for AC fault current, only lightning surge current, and will fail under AC fault conditions. The typical failure mode is open circuit, with the varistor case rupturing due to the fault energy. In such a case, no further overvoltage protection is offered by the failed product. In contrast, the VT/NI will fail shorted if exposed to current/time beyond the product capability, providing continuing overvoltage protection.

Fail-Safe Design:
Having been verified in power laboratories, the VT/NI will provide fail-safe operation when installed in distribution systems. Test units were subjected to various current/time combinations to determine the product ratings, and to test the products to failure. All failures occur in the shorted mode, thereby re-bonding the primary and secondary grounding systems for safety. Further tests to values well in excess of the failure level on previously failed units resulted in the product remaining as a dead short. The failure mode of metal oxide varistors is predominantly as an open circuit, affording no further protection.

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Comparison of Dairyland VTNI to Saturable Reactor

Threshold:
VTNI: Triggers at about 45V peak for AC events, and at higher values for transients. The purpose for this variable triggering level is to allow the VTNI to block transients of higher magnitudes that would otherwise pass through. These levels were chosen with utility input to be above any typical neutral to earth voltage level on a utility system. The device blocks both DC and AC currents up to the threshold. When the threshold is reached, the device instantly switches to the shorted mode, then reverts to the blocking mode when the event is over.

Saturable Reactor: The impedance changes as the voltage across the winding increases. The blocking voltage is nominally about 12V, after which the device looks like a low impedance path. This may not be adequate for all cases of neutral-to-earth voltage. Further, it allows DC current to flow when blocking AC.

Impedance:
VTNI: In excess of tens of thousands of ohms to both AC and DC. The leakage current is typically measured in tens of microamps.

Saturable Reactor: Upper hundreds to low thousands of ohms, down to 0.5 ohm at 12V, for AC. It offers negligible resistance to the flow of DC current. (Occasionally a farm is affected by a DC-based corrosion prevention system on pipeline systems in the area.) This 0.5 ohm impedance results in a significant voltage drop across the device under fault conditions, which is dictated by the fault current flowing in this path. For this reason, some saturable reactors also have a spark gap arrester in parallel with the winding, to limit over-voltages.

Note:
In Wisconsin, the state PSC has set a limit of current contribution from a utility system, at 1mA. If a device does not present a high enough impedance, this results in inadequate isolation. For this reason, our VTNI is the only device used within the state, and is preferred at numerous utilities elsewhere. We have 7,000+ devices in service around the US and Canada.

Design:
VTNI: Fail-safe construction. If exposed to current in excess of the rating, the device would fail in the shorted mode, bonding the primary and secondary neutrals together for safety. The VTNI has published AC fault ratings adequate for utility distribution systems.

Saturable Reactor: Uses windings instead of switched components.

Testing:
VTNI: Use a multimeter to verify that it is blocking AC voltage. A multimeter can also be used to perform a simple pass/fail test for impedance.

Saturable Reactor: Requires a variable AC source to be applied to the system to verify that the device blocks. The operator looks for a current change as the device threshold is approached. Utilities have advised that the test equipment is bulky and the test inconvenient.

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