Beware of Asymmetry – Causes, Consequences, Measurement and Control

Published by Sonel


  • Three-phase motors are overheating!
  • Overcurrent breakers are triggering without a reason!
  • Energy bills are higher than expected!
Undesirable Asymmetry

The economic benefits of energy suppliers and their users are strongly dependent on reliability, safety and efficiency of the power system. One of the phenomena that is strongly related to the efficiency of the power system is an asymmetry.

Current and voltage asymmetry degrades the efficiency of the power system. It reduces the efficiency of generation, transmission and distribution of electricity. Ultimately, this results in an increase in the price of electricity. The electricity consumer will also incur additional costs due to a decrease in the efficiency of electrical equipment with the appearance of voltage asymmetry.

Figure 1. Asymmetry in power network causes higher costs.

About Asymmetry

There are three types of asymmetry states in the power grid. It is the current asymmetry, voltage asymmetry and the simultaneous occurrence of both current and voltage asymmetry. 

With regard to three-phase systems, voltage asymmetry is defined as a state in which the effective values of the three phase voltages are not the same and/or the angles between them differ significantly from 120 ° as can be seen in figure 2b. Any three-phase system of voltage or current vectors can be decomposed into a sum consisting of three components: zero sequence, negative sequence and positive sequence component. Coefficients that are scaled to the positive sequence can be calculated and are used to describe the quantitatively phenomenon of voltage and current asymmetry.

Figure 2. Phasor graph in a) reflects perfect symmetry in 3 phase power system. All vectors corresponding to 3 phases have the same magnitude and there is 120° between two adjacent vectors. Phasor graph in b) reflects asymmetry in power system. The vectors have different length and the angular shifts are different than 120° between adjacent phases.

Sources of Asymmetry

Voltage asymmetry and current asymmetry are two different types of asymmetry in the power system. The source and nature of these asymmetries are different. Voltage unbalance results from the structural asymmetry of generators (variations in internal construction), transformers, and transmission and distribution lines. In addition, asymmetry can be caused by a voltage drop on the system impedance by asymmetrical currents. In turn, the main source of current unbalance is load imbalance, caused by a single phase load in the distribution system or a fault on the load side.

Voltage asymmetry can also cause asymmetry in the supply current. This is particularly evident in the current of induction motors supplied with asymmetric voltage. For example, a 1% asymmetry in the supply voltage can cause several times greater current unbalance in induction motors.

Negative Effect of Asymmetry

Asymmetry of the current, i.e. the occurrence of the asymmetrical component, causes the dissipation of energy in the elements of the power system in the form of heat. As a result, current asymmetry reduces performance when generating, transmitting and distributing electricity. Therefore, the distribution system cables and wires must be selected taking into account the level of asymmetry.

The negative voltage asymmetry component will contribute to the creation of a magnetic field with the opposite direction in induction motors. It’s like accelerating and braking at the same time. For example, the occurrence of voltage asymmetries of up to several percent is able to significantly increase the temperature of the motor winding and reduce the winding life by more than half. Therefore, the load on motors should be reduced accordingly to compensate heating losses resulting from asymmetry.

Figure 3. High asymmetry caused overheating windings of the engine and fire. 

Asymmetry also has a negative effect on three-phase rectifiers and inverters. Voltage unbalance causes asymmetry of the supply current, which increases the temperature of the rectifier diodes and disrupts the operation of the safety devices. Other negative effects occur during transient states mainly caused by faults in the power system.

Transient current asymmetry occurs, for example, due to phase-to-phase faults. In this case, extreme levels of current asymmetry will occur, which will last only for a few seconds. However, this may lead to instability and system failure if the causes are not eliminated on time.

Methods of Mitigating Asymmetry
  1. Adoption of appropriate standards regarding acceptable levels of current and voltage asymmetry and their control.
  2. Applying regulations and standards to the equipment and transmission line.
  3. Structural modifications of single-phase loads.
  4. Single-phase voltage regulators.
Parameters and Devices for Measuring the Asymmetry

The asymmetry is related to the presence of positive sequence and zero sequence components and parameters u0  and  u2 (equations shown in the table) are commonly used in measurements in power engineering:

u0=U0/U1 × 100%
u2=U2/U1 × 100%

where:
u0 –  zero sequence asymmetry parameter,
u2 – asymmetry of the opposite parameter,
U0 – zero sequence component,
U1 – positive sequence component,
U2 – negative sequence component.

Commonly used devices for measuring power network parameters, including asymmetry, are power quality analyzers. Series of devices named PQM is the series of power quality analyzers by Sonel. All analyzers in the PQM series have the ability to measure asymmetry parameters.

A Few Simple Measures to Perform the Measurement and Diagnostics of Asymmetry:
  1. Connect any PQM Sonel series analyzer according to the manufacturer’s instructions and set the voltage and/or current asymmetry measurement mode.
  2. After the appropriate measurement time, download the data to the computer and use the Sonel Analysis program to plot the time graph of the course of the asymmetry parameters.
  3. If the level of the asymmetry parameter exceeds the threshold of EN 50160 or other standards, take action to mitigate the adverse effects.

Figure 4. Electrician is configuring connected PQM for measurement of asymmetry.


Source: https://www.sonel.pl/en/knowledge-centre/press-articles/power-quality-analysis/beware-of-asymmetry-causes-consequences-measurement/

Published by PQTBlog

Electrical Engineer

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