Using Voltage Flicker Measurements to Protect Valuable Sub-Station Assets

Hydro Ottawa has prevented serious damage to six on-load tap-changers installed on their substation transformers in the last seven years and Flicker measurements have proven to be an effective early warning system to reduce damage and possibly prevent catastrophic failure.

Many monitoring devices can measure Voltage Flicker as defined by IEC Standard 61000-4-15 and has been valuable for utilities that are susceptible to voltage flicker problems. Hydro Ottawa is not a heavy industry city and has typically not had any serious Flicker problems on their distribution system. They know because their fleet of Schneider® ION revenue meters have been measuring Flicker and their Electrotek Concept’s PQView® software application has been analyzing the data for over 12 years.

During the first identified tap-changer failure, voltage disturbances were recorded by the meter as shown on the graph below (Fig. 1). The loss of current on the secondary bus of the transformer caused the voltage to sag to 89% or nominal voltage for a ½ cycle. Note that this event was recorded on April 21 2010.

Fig.1: Red phase voltage sag event 4/21/2010 06:13:41

Fig. 2 below shows that the red-phase flicker imbalance started on April 19, two days before the first captured voltage disturbance event. Other measurements such as voltage and current distortion also identified similar anomalies for this tap-changer failure but were historically not as predominant as Flicker imbalance on other occasions.

Fig. 2: 3-phase PLT Flicker Trend 4/1/2010 -5/1/2010

Fig. 3 shows the minimal damage that was found when the tap-changer was taken out of service.

Fig. 3 Minimal Tap-Changer Damage

It should be noted that this is not only a Hydro Ottawa phenomenon but is also being experienced by other utilities. The following is a quote from Elder Romero, Lead Engineer from The United Illuminating Company:

“We received several fault event notifications from PQView one day from one of our substations in the early morning (12:30 AM to 5:00 AM). When engineering personnel saw these events later in the day, it was obvious that these were not typical fault events since they didn’t have much fault current and the waveform was not normal. After analyzing the current waveform in PQView, it was seen that the A phase current would go to zero for one or two cycles and then go back to normal. After analyzing the flicker charts in PQView, it was discovered that something was definitely happening and we suspected that it was the Transformer LTC. After draining the oil, a severely damaged stationary contact was discovered. So our power quality monitoring equipment along with analysis in PQView helped us prevent what could have been significant damage or complete failure of the Transformer LTC.”

So our power quality monitoring equipment along with analysis in PQView helped us prevent what could have been significant damage or complete failure of the Transformer LTC. – Elder Romero

Power quality and condition monitoring has come to the attention of electrical utilities in the past 10-20 years for the following reasons:

1.     Assets are getting older and there is a push to reduce maintenance costs and condition monitoring is being used to predict equipment maintenance.

2.     Reduced cost for power monitoring devices. Power Quality Analyzers that cost $30K 20 years ago can now be replaced with devices that only cost $3-4K. Many modern electronic revenue meters and protection relays can now multi-task and monitor power quality as well

3.     Advancements in software applications to store and analyze this data is now available to help quickly identify system anomalies

4.     Lower communication costs has allowed the multi-tasking devices to be connected to the advanced software to bring back almost real-time data to the stakeholders responsible for operating and maintaining this equipment

The high costs of potential catastrophic failure of critical components, expensive environmental oil spills, fire clean up repairs and damage to company reputation because of extended long term outages alone justifies the lower costs to implement condition monitoring.

Keep an eye out for future Current Power Services articles that will highlight other benefits of power quality and condition monitoring including distribution fault locate, improving customer service, etc.

Remember, “You cannot manage what you do not measure.”

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