Load Shedding Consequences

When poorly maintained switchgear is continually operated, eventually something drastically is bound to happen. But when those extreme event goes unnoticed, the repercussion could have long-lasting outcomes.

Circuit-breakers must be opened when load-shedding starts and closed when ending. If not always, the contacts of these circuit-breakers are predominantly covered. So, when one contact does not make during the closing process, it would not be detected unless an analysis process is instituted, which is highly unlikely.

Negative Sequence Currents

With the one phase “missing”, the system becomes unbalanced, and this causes negative sequence currents. Such a situation can have harmful effects on power system equipment and operation. It can cause increased heating in motors, generators, and transformers, leading to potential damage or reduced lifespan. It can also create mechanical stresses in rotating machinery such as vibrations and torque pulsations. Additionally, negative sequence currents can lead to voltage dips, unbalanced voltages, and increased power losses in the system.

In the picture below, Ideal Voltage Vector is depicted with a Power Factor of 0.9 and at the rated circuit voltage (6.6kV).

However, the Voltage Vector obtained at 03:40:00 on 2023-09-05 was totally different as can be seen in the picture below.

The picture below shows the Ideal Voltage Vector with the recorded Voltage Vector superimposed on top.

The voltages recorded on Phases 1 & 3 were over 7.2kV whilst the recorded voltage on Phase 2 was almost negligible. The currents recorded clearly indicated the unbalanced system.

Consequences

Looking at the picture of the recorded Voltage Vector, one can image what the magnetic fields would look like in electric motors connected to this network. It would be an ellipse which is totally off-center and playing havoc on the bearings and causing a significant vibration.

Negative Phase Sequencing

It is uncertain that negative phase sequencing would ever be detected by operators and maintenance crews. Detecting and measuring negative sequence currents is nevertheless a critical part of maintaining the health of an electrical system. However, this can hardly be done without the use specialized equipment such as a Power Quality Monitor. There are other devices that can be installed on a permanent basis, such as Negative Sequence Current Protection, Differential Protection or Voltage-Based Protection Relays, but on medium voltage networks it is doubtful that this would be done.

Actual Incident

Recently, I came across yet another incident where this happened. After analyzing the results obtained by running my Power Quality Monitor for a week, I noticed rather large discrepancies in the current in the three phases. At low load conditions, the currents appear to have balanced out, but as the load increases, the current in the one phase increased significantly with an increase in the current in the neutral. This bothered me and I started analyzing the data and found that the percentage unbalance also shot up. After further investigation, I found that the voltage on one of the phases became almost negligible.

See how the voltages of two of the phases (Phase 1 & 3) shot up during a swell incident and how Phase 2 (Yellow) became almost negligible as shown in the picture below.

This is supposed to be a 6.6kV feeder with the grey dotted lines depicting the range. But during this incident that lasted 244ms, the voltages of Phase 1 and 3 increased to above 10kV while Phase 3 almost reached 0V. The increase of Phase 1 & 3 was over 51% increase above the rated voltage of the network.

This now requires further investigation to determine the cause of the voltage discrepancy.

Follow-Up Investigations

Whether an investigation will be done and when, is unpredictable.

This work was done for a consultant to determine whether additional load can be added without any upgrading of the network. It was done at a medium-voltage municipal substation with the knowledge and consent of the municipal engineering department. As a courtesy, the results were sent to the municipal engineering department by the consultant, and we must now wait for their reaction.