Monitoring large electric motor terminations prevents failure

December 10th, 2014, Published in Articles: Energize

 

It is often the case that failure of the cheapest component can bring about failure of the whole system, and in many cases condition monitoring is applied effectively to larger system components but not as effectively to the less significant ones. In the case of large electric motors, failure of the terminations can result in the failure of the whole system and effective monitoring of these seemingly insignificant parts is important.

According to Phumlani Khumalo, the chief technologist at Eskom, the failure of a large motor in a critical part of a power station, such as a fan motor in the primary air feed, can cause the shut down of a large portion of the generating plant. Khumalo says that a significant number of unplanned capability loss factors (UCLF) are due to the failure of the motor terminations, and the primary cause is overheating of connections. Overheating of the air in the sealed termination box causes ionisation, which results in short circuits and arcing between terminals, leading to the ultimate destruction of the power connection to the motor. Energy and arc plasma may also find their way into the motor, causing serious damage.

Causes of damage to terminations

Damage is caused by short circuits between terminals inside the box, which can result in arcing. Short circuits develop across the air insulation gap between terminals. The primary cause is believed to be heating of the air due to development of hot spots on the connectors within the termination box.

Fig. 1: Damage to motor connection box due to overheating (photo: Phumlani Khumalo).

Fig. 1: Damage to motor connection box due to overheating (photo: Phumlani Khumalo).

The breakdown voltage of air decreases with an increase in temperature according to the expression [1]

Vt = Va * Ta/Tp    (1)

Where Vp = breakdown voltage at temperature Tp (°K)

Va = breakdown voltage at ambient temperature Ta (°K)

Taking 300°K (27°C) as ambient with a breakdown voltage of 20 kV/cm this reduces to 10 kV/cm at 600°K (327°C)

Heat from a contact terminal is the cause of breakdown. The sealed connection box does not allow convection of air and the whole box does not need to be heated to this temperature – the air in the gap between contacts will be hotter than air in the box due to the restricted captive air gap, and can result in breakdown of path between contacts. Fig. 1 shows the damage that can occur to a termination block.

Heat from the contact may also cause vapourisation of the material of the box and insulation material which could also contribute to breakdown. If an arc develops vapourised metal can also be present. Many manufacturers have fitted pressure relief devices to evacuate arc gases and relieve pressure in junction boxes.

Monitoring of the temperature of the contacts provides an early warning of problems. “The other advantage would be the use of the system during commissioning or re-installation tests” says Khumalo. “Currently we do resistance balance tests on the windings and cables to check for bad joints, but they don’t always reveal bad connections”.

Monitoring the temperature rise of connection points during the commissioning phase can reveal potential failure points far more accurately than other methods. Heating of connections can be caused by a number of factors:

  • Loose or badly made connections: Resulting in connections working loose over time.
  • Inadequate contact material to carry the current: This may be due to bad design, manufacturing defects, poor installation practices such as over-tightening, creepage of contact material, etc.

Current maintenance procedures

“At the moment thermograhic analysis is used to detect hot spots and connection problems,” says Khumalo, “checks are performed at three monthly intervals, but this is too long to catch faults that develop. The procedure is applied to the motor in general and not on the terminal box, so unless an experienced thermographer is instructed to check the box, it may be missed”.

Motors are direct online start with no VFD equipment, and problems often occur at motor start up with a high inrush current.

Possible solutions

The following three methods were considered:

  • Thermography
  • Internal air temperature monitoring – continuously on line. The assumption behind this method is that a hot connection raises the temperature of the air in the box.
  • Terminal temperature monitoring – continuously on line, directly measures the temperature of the terminal.

“We first thought of monitoring the air temperature inside the connector box, but this would not give an adequate indication of the state of the connectors themselves”, says Khumalo. “An artificial hot connection was set up to compare the three methods, and the temperature difference between the hot connection and the others was measured.”

An experiment was conducted to compare three methods of monitoring. A hot, but tight joint was introduced on one of three power terminals (V-phase). The motor  (6,6 kV, 3,72 MW) was run at no-load
(112 A) for 1 h, and the change in temperature indicated by the three methods recorded and monitored. Fig. 2 shows the results of those measurements.

The results showed that terminal temperature monitoring gave the most accurate indication of problems. IR thermography scans on the closed terminal box showed no change in skin temperature over the test duration. Internal air temperature change was the lowest and second least representative of the hot connection on one of the terminals. The change in nominal temperatures of the three terminals indicated a defect within 10 minutes of motor operation and throughout the routine test period.

On-line continuous recording of all terminal temperatures is the most useful strategy that can be adopted. Monitoring and alarming on temperature imbalance is the quickest and most effective method for detecting hot connections to enable elimination of any during initial motor start-up, well before thermal damage occurs.

Fig. 2: Temperatures and change in temperatures measured (P Khumalo).

Fig. 2: Temperatures and change in temperatures measured (P Khumalo).

Temperature measuring system

The system chosen uses passive wireless surface acoustic wave (SAW) sensors. The sensor is powered by the burst of RF from the transceiver, which sets up an oscillation in the quartz crystal. After the RF burst is removed the sensor continues to oscillate, generating an RF signal back to the transceiver by a reverse piezo-electric effect. The dimensions of the sensor, and hence the information carried back on the return burst, are dependant on temperature, and the received signal is decoded, after being retrieved from the returned signal, by the transceiver. The use of wireless SAW devices has several advantages:

  • No power is needed – no need to change batteries
  • Remote access is possible
  • The technology is well developed

The readings from the transceiver are forwarded to a SCADA system which monitors the recorded temperature on a continuous basis and detects any abnormal rise in temperature. The monitoring regime adopted is based on nuclear plant maintenance strategy.

“The system was originally developed for monitoring switchgear temperatures”, says Mario Kuisis, the MD of Martec, a company which is developing the system together with Eskom. “They are widely used in the switchgear and distribution industry.” Motor connections are not the only ones that develop temperature problems. Any connection that carries high current and can be subject to rapid changes in current or high inrush currents for short periods can develop this problem. That fact that temperature monitoring of both connections and contacts in switchgear is a regular practice shows the extent to which the problem exists. They are often found in substations, ring main units, etc.

Reference

[1]    H Uhm, et al: “Influence of gas temperature on electrical breakdown in cylindrical electrodes”, Journal of the Korean Physical Society, Vol. 42, February 2003.
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