Can customer-level voltage tell us about open-phase faults in distribution transformers?

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1. Three-Phase Grid Configuration and Balance

Electric grids are typically configured in a three-phase(3Φ) system, where power is distributed across three conductors(phases). These conductors can be arranged in one of two principal configurations: Wye (Y) or Delta (Δ). Each transformer in a three-phase system comprises three phases on both the primary and secondary sides, with the potential for distinct configurations on each side. The configurations can be classified as Y-Y, Y-Δ, Δ-Δ, or Δ-Y. Figure 1 below shows the two configuration types.

Figure 1: Delta(left) and wye(right) 3Φ configuration types. Source. Each conductor carries alternating current (AC) that is 120 degrees out of phase with the other phases. Balance in this system means that the voltages in all three phases are equal in magnitude and are separated by a 120-degree phase angle (see Figure 2 below).

Figure 2: A balanced 3Φ connection. Voltages in all three phases have equal magnitude and are 120° out of phase with each other. This design ensures stable delivery of electricity to homes, businesses, and industries, and minimizes technical losses ensuring efficiency in transmission and distribution of electricity. It ensures that voltage and current are evenly distributed across all phases, minimizing the risk of voltage fluctuations, phase imbalances, and overheating. This ensures electrical equipment operates optimally. However, when this equilibrium is disrupted, it leads to phase imbalances, which can cause power outages, voltage fluctuations, and equipment damage. One of the causes of phase imbalance is an Open Phase Condition (OPC). An OPC is a fault that occurs when one of the three phases on the primary side of a distribution transformer becomes unintentionally disconnected. This is often due to a broken conductor, loose connections, blown fuses, or faulty circuit breakers — see Figure3.

Figure 3: A 3Φ connection with an open phase OPCs can be challenging to detect, especially at the distribution transformer level, where voltages may still appear balanced under low load conditions. But can monitoring customer-level voltage — typically 230V in homes and businesses — reveal these open-phase faults? Small but significant deviations in voltage at the point of consumption may provide early clues about open-phase faults occurring upstream in the power distribution network. In this post, we explore what an OPC is, how it can manifest at the customer level, and how monitoring voltage fluctuations can provide early clues about these faults hence help improve overall grid reliability.

2. What Happens During an Open Phase Condition?

In a typical 3Φ system, power is distributed across three phases: A, B, and C, each 120° apart in their phase angles. Under normal conditions, these three phases are balanced, meaning the power is evenly distributed.

Figure 4: A 3-Φ transformer drawing showing voltage magnitudes and phase angles on the primary and secondary sides of the transformer. The transformer windings are coils of wire; the primary winding receives input voltage, creating a magnetic field that induces voltage in the secondary winding. The output voltage is determined by the turns ratio of the windings and the input voltage. A transformer’s primary role is to step down high voltages (e.g., 33kV) to low voltages (e.g., 400V line-to-line or 230V phase-to-neutral). When a phase opens on the high side, the transformer reacts based on its configuration (e.g., Y-Δ, Δ-Yg). Let’s say your local distribution transformer supplies each household with a nominal voltage of 230V. If one of the phases on the primary side (say phase A) becomes open, the disconnection of phase A means the power flowing through phase A drops to zero. The magnetic flux generated by phases B and C on the primary side still induces voltage on the secondary windings. However, the absence of phase A alters the current and power distribution across the transformer’s windings, causing imbalanced voltages on the secondary side. The extent to which the imbalance becomes noticeable depends on two main factors: the transformer’s loading conditions and its phase configuration. In a Δ-Yg transformer, for example, when the voltage in the winding associated with phase A drops significantly, the other two phases might still have voltage. The customer ultimately experiences under-voltage (e.g., dropping from 230V to a lower voltage), which affects the operation of appliances and electronics. In transformers with ungrounded primary windings for example, the voltage may drop by half. The imbalance is more noticeable during peak loads because… Figure xx: A flowchart showing the journey from the high side (where the phase opens) through the transformer, down the distribution network, and finally arriving at the customer, highlighting each stage of voltage imbalance.

3. Analyzing Household-Level Voltage Data for OPC Detection

nLine has recently released a comprehensive dataset containing household-level voltage data from Accra, Ghana, collected over a five-year period. This dataset features voltage measurements recorded at a two-minute resolution from sets of households connected to the same distribution transformer. This enables the analysis of voltage patterns within individual households and comparing them with those of other households connected to the same transformer. This comparative analysis enables us to identify anomalies and fluctuations in voltage, providing a basis for exploring the possibility that such data can indicate the presence of OPC. The nominal voltage for the Ghanaian electrical grid is 230V. This section takes a systematic approach to explore and determine the presence of OPC using the Accra dataset. We begin by performing a time series analysis to understand normal voltage patterns, which enables us to identify anomalies when they occur. We then compare the voltage behaviors of households connected to the same transformer to ascertain whether multiple households exhibit similar voltage drops, or experience fluctuations. Additionally, we investigate other indicators, such as outage events after the suspected OPC. By taking this systematic approach, we are able to build a case around whether the phenomena observed in household-level voltage data indeed points to an OPC.

3.1. Understanding the normal voltage patterns

Understanding the normal voltage patterns is essential in identifying when an anomaly presents itself in the data. For the case of the Accra dataset, phase voltage as measured at the household level generally stays within +/- 10% of the nominal voltage 230V, as shown in Figure 5.

Figure 5: Voltage time series showing normal voltage for three sensors deployed under the same transformer. Further, the voltage values fluctuate in a regular drop-and-rise manner through different hours of the day, as indicated in Figure 6. Hour-of-week voltage profiles are shown for different households where the data was collected. These voltage drops are mainly during peak load, but the voltage largely stays within the nominal range of +/-10% of nominal voltage. However, household 1 is peculiar as it experiences voltage values consistently below the nominal range.

Figure 6: hour of week voltage profile to show periodic voltage drops during peak load. On further investigation of this household by plotting the time series voltage data over a period, there is an observed sudden and sustained drop in the voltage value from the nominal range to a significantly lower value. This peculiarity, shown in Figure 7, is only observed in this household.

Figure 7: An example of a possible OPC in household 1 This phenomenon points to a possible case of OPC. To build the case for an OPC here, other features of the data need to be analyzed as described in sections 3.2. and 3.3.

3.2. Comparison Across Multiple Households

In an OPC, we expect certain behavior of customers connected to the transformer. If only one household is experiencing a voltage drop, as is the case in this context, the household might be connected to a different phase than others. The “victim” household is possibly connected to the phase that is open on the primary side of the transformer. However, if multiple households connected to the same transformer show similar voltage drops, it shows that these households are possibly on the same phase of the transformer. We could also see different households under the same transformer being affected differently (e.g. experiencing voltage fluctuations rather than voltage drops).

3.3. Other Possible OPC Indicators in Voltage Data

Apart from sudden sustained drops in the customer-level data, there are other indicators that may strengthen the argument that a distribution transformer is experiencing an OPC in one of its primary phases. First, looking out for an outage event just before the voltage drop is restored to normal is a good indicator. Utilities typically switch off the transformer when they go in to rectify the OPC conditions. This translates to a period of outage just before normal voltage is restored, observed in the voltage data as 0V periods. Furthermore, if we have the additional information on what phase each customer is connected to, we can track for voltage imbalance. OPC causes voltage imbalance that manifests in the voltage magnitude and phase angles in the different phases. Finally, we can check for major event days on the grid. OPC often occurs due to physical damage (e.g., broken conductor, loose connection). Checking if any major weather events (storms, high winds) or other incidents correlate with sustained voltage drops or fluctuations can indicate a possible OPC in the transformer.

4. Conclusion

Analyzing household-level voltage data can provide valuable insights into the health of the electrical distribution system. Sustained voltage drops, especially those that end in an outage before being reset, can be early indicators of an Open Phase Condition (OPC). By systematically monitoring and visualizing customer-level voltage data, utilities can detect potential faults in addition to relying solely on transformer-level protection systems, preventing equipment damage and reducing outage times.