Negative sequence currents are defined as unbalanced currents that occurs in three-phase electrical systems.
In a balanced three-phase system, the currents in each the three phases are equal in magnitude and have a phase difference of 120 degrees. This may change due to certain network abnormalities. Amongst the primary causes to such imbalances are single-phase type faults (broken conductors or line-to-line faults), unbalanced loads, or the presence of unbalanced impedances – more about unbalanced impedances later.
Negative Phase Sequencing is therefore a vector spinning in a clockwise rotation.
In a Positive (“normal”) Sequence, the currents in the three phases flow in the order of Phase A, Phase B, and Phase C. In the video and figure below, it is indicated by Red, Yellow, and Blue.
The vector diagram below shows how Phase 1 is followed by Phase 2 with a phase-shift angle of 120-degrees behind Phase 1. Similarly, Phase 2 is followed by Phase 3 also with a phase-shift angle of 120-degrees.
Both the images above represent a perfect condition. Also see the video below.
In contrast, in a Negative Sequence situation, two of the phases are swapped around. The currents therefore flow in the order of Phase A (or 1), Phase C (or 3), and Phase B (or 2), see below.
In the video and figure below, it is indicated by Blue, Yellow, and Red.
In this case, Blue is followed by Yellow which is followed by Red.
In the video below, I am showing what a three-phase voltage wave may look like with a 6% voltage reduction in one of the phases and a 10% 3rd Harmonic Disturbance in all three phases. Pay attention to the Yellow and Blue Phases and how it is rises above the “standard” voltage levels.
NPS_Three-Phase_3rd_5th_Harmonics_Sinewave
Effects of Negative Sequencing
Negative sequence currents 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.
Heating and Overloading
The increased heating is caused by the negative sequence currents that produce a rotating magnetic field in the opposite direction to the normal rotation, resulting in additional losses and overheating of the equipment.
Voltage Imbalance
The presence of negative sequence components can lead to unbalanced voltages across the phases, affecting the quality and stability of the supply. Voltage imbalances can result in decreased system efficiency, increased reactive power consumption, and reduced motor performance. Furthermore, excessive voltage imbalances can lead to equipment malfunctions, flickering lights, and voltage sags or swells.
Unwanted Torques and Vibrations
In rotating machines such as motors and generators, negative sequence currents generate additional torques in the opposite direction to the normal rotation. These unwanted torques can cause mechanical vibrations, increased stress on bearings and shafts, and potential damage to the mechanical components of the machines.
Loss of System Stability
Excessive negative sequence currents can lead to voltage instability, voltage collapse, and even system-wide blackouts. The unbalanced conditions caused by negative sequence currents can disrupt the normal operation of protective relays, voltage regulators, and control systems, potentially compromising the overall stability and reliability of the system.
Interference with Protection Systems
Negative sequence currents can interfere with protective relays and other protective devices. The presence of unbalanced currents can cause incorrect operation or miscoordination of protective relays, leading to ineffective fault detection and inadequate fault clearing. This can impact the overall system protection and increase the risk of prolonged outages.
Detecting and Measuring Negative Sequence Current
Detecting and measuring negative sequence currents is a critical part of maintaining the health of an electrical system. To do this, we use specialized equipment such as a Power Quality Monitor. There are other devices that can be installed on a permanent basis.
Harmonic Content
When negative sequences are present, they interact with other currents in the system and produce higher harmonics, leading to overheating torque pulses and reduced efficiency.
Temporary Characteristics
In some instances, negative sequence currents can appear and disappear or increase and decrease either in long- or short-durations.
Mitigating the Effects
Protective relays and devices can be installed to detect and mitigate the effects of negative sequence currents. These devices can monitor the magnitude and phase angle of the currents and take appropriate actions, such as tripping circuit breakers or activating alarms, to prevent further damage or mitigate the unbalanced conditions.
Some common protection measures used to mitigate the effects of negative sequence currents:
Negative Sequence Current Protection Relays
Protective relays specifically designed to detect and respond to negative sequence currents can be employed. These relays monitor the current flowing through each phase and compare the magnitudes and phase angles to identify unbalanced conditions. When excessive negative sequence currents are detected, the relay can trip the associated circuit breakers to isolate the fault or unbalanced condition.
Differential Protection
Differential protection schemes can be utilized to detect faults and unbalanced conditions caused by negative sequence currents. By comparing the currents entering and leaving a particular component, such as a motor or transformer, differential relays can identify any imbalances and initiate protective actions.
Voltage-Based Protection
Negative sequence currents are associated with voltage imbalances. Voltage-based protection schemes can detect and respond to abnormal voltage conditions, which can indicate the presence of negative sequence currents. Voltage relays and monitors can be utilized to monitor the voltages across the phases and activate appropriate protection measures when voltage imbalances exceed predetermined thresholds.
Generator and Motor Protection
Negative sequence current protection is particularly important for generators and motors, as they are most susceptible to the detrimental effects of unbalanced conditions. Generator and motor protection relays often include functions to detect negative sequence currents and initiate appropriate protective actions such as tripping the circuit-breaker or reducing the output power.
System Grounding and Neutral Protection
Proper grounding and neutral protection measures are essential to mitigate negative sequence currents. Effective grounding practices, such as solid grounding or low-impedance grounding, can limit the propagation of negative sequence currents and reduce their effects. Additionally, neutral grounding devices like neutral grounding resistors or reactors can help detect and limit the flow of negative sequence currents during ground faults.
System Analysis and Maintenance
Regular system analysis, including load flow studies and fault analysis, can help identify potential sources of negative sequence currents. By understanding the system characteristics and monitoring the operation, appropriate maintenance and corrective actions can be implemented to minimize the occurrence of unbalanced conditions and negative sequence currents.
Consulting Service
The Detection and Measuring of Negative Sequence Currents and Voltages require sophisticated instrumentation and is a specialist service. Click here to order this service.
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