Any repetitive signal, such as three-phase Alternating Current (AC) Voltages and Currents, can be represented as the rotation of a vector around a point. In a balanced three-phase network, the magnitudes of the voltages and currents in all three phases are the same and these phases voltages and currents are shifted symmetrically by 120 degrees to each other. Under this condition, the vector is fixed, and the rate of rotation is constant, and the end of the vector lines will continuously trace a circle. Each pass around the circle represents one complete cycle of the signal.

In the video below, the three-phase waves are displayed with the phase-displacement 120-degrees, but Phase 2 (yellow) voltage is reduced by 6%, which may result in 3^rd harmonics disturbance represented by the thinner solid line (5% 3^rd harmonic) and dotted line representing a 2% 5^th Hamonic.

The harmonic disturbances will cause the fundamental sine wave to be distorted. So, the display is not as accurate as it would be displayed on an oscilloscope.

The three-phases are represented as Phases 1 to 3. In this video, the three-phase vector are not colored as per the IEC standards for the UK & EU and several other countries: Phase 1 = should be Brown but displayed as Red, Phase 2 = should be Black but displayed as Yellow and Phase 3 = should be Grey but displayed as Blue. These are the former colors and still widely used by many countries.

In the vector diagram above, Phase 1 lies along the X-axis with Phase 2 displaced by 120-degrees in a clockwise direction and similarly, Phase 3 is displaced by 120-degrees from Phase 2, also in a clockwise direction. Since the phases would follow a pattern of Phase 1, the Phase 2 and lastly, Phase 3, the normal vector rotation, or more precisely referred to as positive phase sequence, is counterclockwise.

In the vector diagram above, Phase 1 lies along the X-axis with Phase 2 displaced by 120-degrees in a clockwise direction and similarly, Phase 3 is displaced by 120-degrees from Phase 2, also in a clockwise direction. Since the phases would follow a pattern of Phase 1, the Phase 2 and lastly, Phase 3, the normal vector rotation, or more precisely referred to as positive phase sequence, is counterclockwise.

If the magnitudes of the three phases becomes unbalanced or if phases are shifted by something different from the 120 degrees, a negative phase sequence vector is generated. This vector rotates in the opposite direction, which is clockwise.

Because of such voltage unbalances, a common phenomenon found in three-phase power systems which are not well known, additional power losses are being generated. These current and voltage unbalances could damage equipment connected to power system. Again, not always that apparent. In many cases, it happens almost undetected.

Obviously, the greater the unbalances, the greater the risk of damage and more severe it becomes. Another seemingly unknown factor is the substantial financial losses to both distribution network operators and end-customers. This applies to any plant with rotating apparatus and many other user-connected devices. This issue is one of the unrecognisable critical power quality problems which should become a major focal point for utilities and Distribution Generation (DG) industries. But since it is not clearly noticeable, almost no attention is paid to it.

The asymmetry described above, typically appears in the network because of the connection of single-phase customers, which creates uneven load among the phases. This problem, the asymmetry, is further exasperated by large enough single-phase generating devices that are connected to the existing distribution networks. As the non-dispatchable renewable energy systems are being connected to the power systems, the phenomenon of asymmetry will increase to a point where it becomes a major issue, if it is not already the case. Microgrids and charging stations for Electric Vehicles (EVs) are being connected without proper planning or replanning. This has become one of the greatest technical and operation challenges, but hardly any attention is paid to it. Whether that be the so-called third- or first-world countries. Even the best run utilities are not aware of this problem or do not pay enough attention to the phenomenon asymmetry.

It is thus critically important that these issues are mitigated by comprehensive analysis and careful planning. This requires an all-embracing understanding of unbalance propagation and the identification of critical factors that affect such asymmetries.

In a future blog, I will elaborate on the phenomenon of asymmetry and its relation to harmonics.

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Practical imperfections which can result in unbalances are:

• A three-phase equipment such as induction motor with unbalance in its windings. If the reactance of three phases is not same, it will result in varying current flowing in three phases and give out system unbalance.

With continuous operation, motor’s physical environment cause degradation of rotor and stator windings. This degradation is usually different in different phases, affecting both, the magnitude and phase angel of current waveform.

A current leakage from any phase through bearings or motor body provides floating earth at times, causing fluctuating current.

• Any large single-phase load, or several small loads connected to only one phase cause more current to flow from that phase causing voltage drop online.
• Switching of three phase heavy loads results in current and voltage surges which cause unbalance in the system.
• Unequal impedances in the power transmission or distribution system cause differentiating current in three phases.

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Transmission lines with high and extra-high voltage that are untransposed, meaning the conductors are not positioned at the corners of an equilateral triangle and lack an overhead grounding wire (OGW), significantly contribute to the generation of negative-sequence voltages and, to a lesser extent, zero-sequence voltages. As the electrical load increases, the incidence of negative-sequence voltages also rises.

During the late 1970s and early 1980s, Eskom operated only two extensively long 400kV transmission lines that delivered electricity to the Western Cape. One of these lines was transposed, while the other remained untransposed.

At that time, the concept of Negative Phase Sequencing was not widely understood among engineers, nor were the factors that caused it.

Negative Phase Sequencing

Before the practice of live-line maintenance was established, it was necessary to periodically shut down each of the two transmission lines for upkeep.

Confusion ensued at Eskom Western Cape when the transposed line was deactivated for maintenance. Despite an uninterrupted power supply, a major customer’s equipment, which was equipped with highly sensitive control systems, experienced frequent shutdowns.

This prompted an investigation, leading to a consultation with a professor from UCT who provided insights into Negative Phase Sequencing. With this new understanding, I immediately directed the maintenance team to cease work on the transposed line and await further instructions.

We then coordinated with the customer to align our maintenance schedule with their plant’s downtime.

The subsequent introduction of a third Transmission Line to the Western Cape significantly reduced the recurrence of such issues.

To prevent similar incidents, we mandated that the untransposed line should not be the sole power source for the Western Cape.

Automatic Change-Over

Reflecting on past events, it later became clear that Negative Phase Sequencing was behind the numerous unexplained electrical disturbances I had been tasked with resolving.

A notable incident occurred years earlier when a colleague and I were summoned to determine why the standby generator at the undersea cable terminal was erratically activating and deactivating. Observing the voltmeter, we noticed a sudden spike in one phase voltage followed by a swift drop in another. At the time, the phenomenon was baffling, and we failed to link it to upstream occurrences.

Consequences

Click here to read more about a recent incident and an explanation of the consequences of these type of network faults.

In a balanced three-phase network, the magnitudes of the voltages and currents in all three phases are the same and these voltages and currents phase are shifted symmetrically by 120 degrees to each other. If the magnitudes are not the same and phase-shifts are different from the 120 degrees, the network is unbalanced.

Eskom strive to supply a balanced set of voltages at the supply points. Although Eskom may attempt to supply a balanced set of voltages, they do not. This is probably true for the point of supply at the generator terminals, but between the generator terminals and the end user are Transmission, Distribution and often Reticulation Networks that are not maintained or monitored with the same diligence that is required to supply balanced voltages. Often the network operators are oblivious to unbalances occurring on the network. Perhaps there are Negative Protection System installed on the Transmission Networks, but it is doubtful that such protective devices exist on Distribution and Reticulation Networks.

Low Voltage (LV) network unbalances are caused by unequal phase impedances, single-phase laterals and other structural asymmetries of LV networks, uneven allocation of single-phase consumers across the three phases on mostly rural reticulation networks, unbalanced three-phase loads and random variation of consumption over time. In addition, the presence of single- phase distributed generation, for example single-phase solar inverters also contributes to the phase imbalances occurring in LV networks. However, load asymmetry represents the main cause of voltage and current imbalances in LV networks. Asymmetric faults are another source of voltage or current imbalances, but these are usually transient events and are usually cleared quickly from the grid, so it could often be ignored.

In many cases operators and maintenance crews are unaware of the negative consequences that phase imbalances can have on LV networks and electrical equipment. Current imbalances would lead to a reduction in the serviceable loading capacity of LV cables and distribution transformers. Because of the imbalances, some of the phases could carry higher loads while the remaining phase of phases could be lightly loaded but, the limiting factor for the addition of three-phase loads is the current on the highest loaded phase or phases.

Current imbalances can also cause additional heat losses in distribution transformers and LV cables in both the phase and neutral conductors. These types of losses represent a significant part of the total losses occurring in LV networks.

When end users, such as municipalities, are supplied by unbalanced voltages, induction machines and power converters face adverse effects such as reduced efficiency, increased losses, potentially dangerous overheating and, in some situations, premature failures. At severe voltage or current imbalance levels, some types of protection relays could malfunction, leading to miscoordination, nuisance tripping and lack of selectivity.

In the United States, voltage magnitude variations are limited to ±5% and in most European countries it is limited to ±10%. For the rest of the world, these limits are not much different.

The question is: is Eskom aware what is happening on the Distribution and Reticulation Networks? It is highly unlikely that they do. In a recent unrelated “survey”, I came across a 10-minute averaged voltage unbalance of 327% between Phase 2 and Phase 1.

The second question is: would this be restricted to just the one substation? It is highly unlikely. In another post I will talk about and incident where the Negative Phase Sequence occurred on the Transmission Network which affected the entire region.

In a balanced three-phase network, the magnitudes of the voltages and currents in all three phases are the same and these voltages and the phases voltages and currents are shifted symmetrically by 120 degrees to each other. If the magnitudes are not the same and phase-shifts are different from the 120 degrees, the network is unbalanced.

In the picture below, the Voltage Vector for the ideal network is shown with the magnitudes of the voltages being equal and the phases shifted symmetrically by 120 degrees to each other, while the solid lines showing the results recorded. The magnitudes of the actual voltages are not equal since the Yellow/White Phase 2 is almost negligible and the phases are not shifted symmetrically.

Eskom strive to supply a balanced set of voltages at the supply points. But they do not always do that. This is probably true for the point of supply at the generator terminals, but between the generator terminals and the end user are Transmission, Distribution and often Reticulation Networks that are not maintained or monitored with the same diligence to supply voltages with the same magnitudes and phase-shifts as mentioned above.

Often the network operators and maintenance crews are oblivious to unbalances occurring on the network. Perhaps there are Negative Protection System installed at the Generating Stations and on the Transmission Networks, but it is doubtful that such protective devices exist on Distribution and Reticulation Networks.

Low Voltage (LV) network unbalances are caused by unequal phase impedances, single-phase faults and other structural asymmetries of LV networks, uneven allocation of single-phase consumers across the three phases on mostly rural reticulation networks, unbalanced three-phase loads and random variations of consumption over time. In addition, the presence of single-phase distributed generation, for example, single-phase solar inverters also contribute to the phase imbalances occurring in LV networks. However, load asymmetry represents the main cause of voltage and current imbalances in LV networks. Asymmetric faults are another source of voltage or current imbalances, but these are usually transient events and are usually cleared quickly from the grid, so it could often be ignored.

In many cases operators and maintenance crews are unaware of the negative consequences that phase imbalances can have on LV networks and electrical equipment. Current imbalances would lead to a reduction in the serviceable loading capacity of LV cables and distribution transformers. Because of the imbalances, some of the phases could carry higher loads while the remaining phase of phases could be lightly loaded but, the limiting factor for the adding more three-phase loads is the current on the highest loaded phase or phases.

Current imbalances can also cause additional heat losses in distribution transformers and LV cables in both the phase and neutral conductors. These types of losses represent a significant part of the total losses occurring in LV networks.

When end users, such as municipalities, are supplied by unbalanced voltages, induction machines and power converters face adverse effects such as reduced efficiency, increased losses, potentially dangerous overheating and, in some situations, premature failures. At severe voltage or current imbalance levels, some types of protection relays could malfunction, leading to lack of coordination, nuisance tripping and lack of selectivity.

In the United States, the voltage magnitude variations are limited to ±5% and in most European countries it is limited to ±10%. For the rest of the world, these limits for voltage magnitude variations are quite similar.

The question is: is Eskom aware what is happening on the Transmission, Distribution and Reticulation Networks? It is highly unlikely that they do. When looking at Eskom’s Quality of Supply document, there is a lot being said about the consequences of unbalanced voltages, but I cannot find the voltage magnitude variations limits.

In a recent unrelated “survey”, I came across a voltage magnitude variation unbalance of 327% between Phase 2 and Phase 1. The instrument was set to record the values at 10-minute intervals and the 327% is the average over that period. See the picture below.

Since the medium-voltage supply comes directly from an Eskom substation which is probably about 20-metres away, one wonders what is going on at the Eskom substation since it is highly unlikely that the voltage unbalance is as a result faulty equipment at the municipal substation. It is as if one phase is completely missing. The same “missing” phase also show an abnormal high current. The neutral current which is supposed to be at or close to zero is also very high.

The second question is: would this be restricted to just the one substation? It is highly unlikely. In another post that is due to come out soon, I will talk about and incident where the Negative Phase Sequence occurred on the Transmission Network which affected the entire region.

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When switchgear is poorly maintained and operated continuously, it is only a matter of time before something significant occurs. However, if these extreme events go unnoticed, the consequences can be long-lasting.

During load-shedding, circuit-breakers should be opened when load-shedding starts and closed when load-shedding ends. Unfortunately, the contacts of these circuit-breakers are often concealed. As a result, if one contact fails to connect during the closing process, it may go undetected unless a thorough analysis is conducted, which is unlikely.

Negative Sequence Currents

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.

1. Phase 1 & 3 Voltages:

• The voltages recorded on Phases 1 and 3 were over 7.2 kV.
• This means that the voltage levels for these phases exceeded 7,200 volts.

2. Phase 2 Voltage:

• In contrast, the recorded voltage on Phase 2 was almost negligible.
• Negligible voltage implies that it was significantly lower than the voltages observed in Phases 1 and 3.

3. Currents and System Balance:

• The currents recorded will provide additional insight.
• The unbalanced system is evident from the significant difference in voltages between the phases.

In summary, the voltage readings indicate an unbalanced system, with Phases 1 and 3 having high voltages while Phase 2 remains at a much lower level.

Consequences

1. Voltage Vector:

• The voltage vector represents the voltages across the three phases (Phase 1, Phase 2, and Phase 3) in an electrical system.
• When we visualize this vector, we can imagine it as an ellipse due to the varying magnitudes and phase angles of the voltages.

2. Off-Center Ellipse:

• In this case, the ellipse is described as off-center.
• This means that the voltages in the three phases are not balanced or symmetrical.
• An off-center ellipse indicates that the system is experiencing unbalanced voltages.

3. Impact on Electric Motors:

• When electric motors are connected to such an unbalanced network, several issues arise:
  • Bearing Stress: The off-center ellipse causes uneven magnetic forces within the motor windings. As a result, the bearings experience **uneven stress** due to the varying magnetic fields.
  • Vibration: The uneven magnetic forces lead to vibration in the motor. This vibration can be significant and detrimental to the motor’s performance and longevity.

In summary, the off-center voltage ellipse can wreak havoc on electric motor bearings, causing vibration and potential damage. Engineers and maintenance personnel should address this unbalanced system to ensure optimal motor operation and prevent premature wear and tear.

Negative Phase Sequencing

1. Negative Phase Sequencing:

• Negative phase sequencing refers to the incorrect order of phases in a three-phase electrical system.
• Detecting this issue is crucial because it affects the overall health and stability of the system.

2. Why It Matters:

• While operators and maintenance crews may not always notice negative phase sequencing, it can lead to serious consequences.
• Negative sequence currents can cause motor overheating, increased losses, and reduced efficiency.
• Addressing this issue promptly helps prevent equipment damage and ensures reliable operation.

3. Challenges in Detection:

• Detecting negative sequence currents is challenging without specialized equipment.
• Power Quality Monitors are essential for accurate measurements. These devices analyze voltage and current waveforms to identify imbalances.
• Other protective devices, such as Negative Sequence Current Protection, Differential Protection, or Voltage-Based Protection Relays, can be installed permanently.
• However, on medium voltage networks, the installation of such protective devices may be doubtful due to cost considerations or other factors.

In summary, while negative phase sequencing may go unnoticed by operators, investing in specialized equipment and protective relays is crucial for maintaining a healthy electrical system.

Actual Incident

Recently, I encountered another incident related to power quality. After analyzing the results from my Power Quality Monitor over the course of a week, I observed significant discrepancies in the current across the three phases. Under low load conditions, the currents seemed balanced, but as the load increased, one phase experienced a substantial increase in current, along with a corresponding rise in neutral current. This discrepancy concerned me, prompting further investigation. Upon closer examination, I discovered that the voltage on one of the phases had dropped to nearly negligible levels.

The image below illustrates how the voltages of Phases 1 and 3 surged during a swell incident, while Phase 2 (indicated in yellow) became almost negligible.

During a recent incident lasting 244 milliseconds, the voltages on Phase 1 and Phase 3 of what was supposed to be a 6.6 kV feeder exceeded 10 kV. Meanwhile, Phase 3 nearly reached 0 V. This substantial increase — over 51% above the rated voltage of the network — raises concerns and warrants further investigation to identify the cause of this voltage discrepancy

Follow-Up Investigations

Whether an investigation will be carried out 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.

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