IEC 60071-1: Benefits, Challenges and Future Trends of Insulation Co-ordination for Three-Phase AC Systems
What is iec 60071 3 phase to phase?
If you are involved in designing, installing or operating electrical systems, you may have encountered the term iec 60071 3 phase to phase. But what does it mean and why is it important? In this article, we will explain what iec 60071 3 phase to phase is, how it works, what are its benefits and challenges, and what are some future trends and developments in this field.
iec 60071 3 phase to phase
iec 60071 3 phase to phase is a part of insulation co-ordination, which is a process that ensures that the insulation of electrical equipment and installations can withstand the voltages and overvoltages that may occur in a system. Insulation co-ordination is essential for ensuring the reliability, safety, efficiency and cost-effectiveness of electrical systems.
iec 60071 3 phase to phase refers to the application of insulation co-ordination to three-phase a.c. systems having a highest voltage for equipment above 1 kV. It is based on the international standard iec 60071-1:2006, which specifies the procedure for the selection of the rated withstand voltages for the phase-to-earth, phase-to-phase and longitudinal insulation of the equipment and the installations of these systems.
In this article, we will follow the procedure of iec 60071-1:2006 and explain each step in detail. We will also provide some examples of applications of iec 60071 3 phase to phase and discuss its benefits and challenges. Finally, we will look at some future trends and developments in this field.
What is insulation co-ordination?
Before we dive into the details of iec 60071 3 phase to phase, let us first understand what insulation co-ordination is and why it is important for electrical systems.
Insulation co-ordination is defined by iec 60071-1:2006 as "the selection of the electric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available protective devices".
In other words, insulation co-ordination is a process that ensures that the insulation of electrical equipment and installations can withstand the voltages and overvoltages that may occur in a system without causing damage or failure. Insulation co-ordination also takes into account the environmental conditions and the protective devices that may affect the performance of the insulation.
Insulation co-ordination is important for several reasons:
It ensures the reliability of electrical systems by preventing insulation breakdowns that may cause interruptions or malfunctions in the operation of equipment and installations.
It ensures the safety of electrical systems by preventing insulation failures that may cause electric shocks, fires or explosions that may harm people, animals or property.
It ensures the efficiency of electrical systems by preventing insulation degradation that may reduce the power quality, increase the energy losses or increase the maintenance costs of equipment and installations.
It ensures the cost-effectiveness of electrical systems by preventing insulation overdesign that may increase the size, weight or cost of equipment and installations without providing any additional benefits.
To achieve insulation co-ordination, iec 60071-1:2006 provides a procedure for selecting the appropriate rated withstand voltages for different types of insulation in a system. The rated withstand voltage is defined as "the specified voltage that insulation must withstand in a standard test". The standard tests are short-duration power frequency and impulse tests that simulate the voltages and overvoltages that may occur in a system.
The procedure for insulation co-ordination consists of several steps, which we will explain in detail in the following sections.
What are the main principles and rules of insulation co-ordination?
The procedure for insulation co-ordination according to iec 60071-1:2006 is based on some main principles and rules that are summarized below:
The procedure applies to three-phase a.c. systems having a highest voltage for equipment above 1 kV. The highest voltage for equipment is defined as "the maximum value of r.m.s. phase-to-earth voltage which can occur across any possible point of an installation or across any open terminal".
The procedure considers three types of insulation: phase-to-earth, phase-to-phase and longitudinal. Phase-to-earth insulation is between a live part and earthed metal or between two live parts with different potentials to earth. Phase-to-phase insulation is between two live parts with different potentials to each other. Longitudinal insulation is along a live part or between two points on a live part with different potentials along its length.
The procedure considers two types of voltages: representative voltages (Urp) and co-ordination withstand voltages (Ucw). Representative voltages are the values of voltages and overvoltages that may occur in a system under normal or abnormal conditions. Co-ordination withstand voltages are the values of voltages that insulation must withstand in order to achieve insulation co-ordination.
The procedure considers two types of overvoltages: switching overvoltages and lightning overvoltages. Switching overvoltages are caused by switching operations or faults in a system. Lightning overvoltages are caused by direct or indirect lightning strokes to a system or nearby objects.
How to perform standard withstand voltage tests?
After selecting the rated insulation level, the next step is to perform standard withstand voltage tests to verify that the insulation of the equipment and the installations can withstand the selected rated voltages. The standard tests are short-duration power frequency tests and impulse tests.
Short-duration power frequency tests are performed by applying a sinusoidal voltage with a frequency of 50 Hz or 60 Hz for a duration of 1 min or 4 h, depending on the type of insulation and the application. The test voltage is equal to or higher than the rated short-duration power frequency withstand voltage. The test is successful if no breakdown or flashover occurs during the test.
Impulse tests are performed by applying a standard impulse voltage wave with a front time of 1.2 μs and a time to half value of 50 μs for full waves, or with a front time of 250 μs and a time to half value of 2500 μs for switching surges. The test voltage is equal to or higher than the rated impulse withstand voltage. The test is successful if no breakdown or flashover occurs during the test.
iec 60071-1:2006 provides some general requirements and test procedures for short-duration power frequency tests and impulse tests. Some of them are:
The test voltage should be applied between the terminals of the equipment or installation under test and earth, or between two terminals with different potentials.
The test voltage should be applied gradually and uniformly until it reaches the specified value.
The test voltage should be maintained at the specified value for the specified duration.
The test voltage should be removed gradually and uniformly after the test.
The test should be repeated at least three times for each polarity of the impulse voltage.
The test should be performed under dry conditions, unless otherwise specified.
The apparatus committees are responsible for specifying the detailed test procedures and criteria for each type of equipment, taking into consideration the recommendations of iec 60071-1:2006.
What are the ranges for highest voltage for equipment?
Another important aspect of insulation co-ordination is the classification of highest voltage for equipment into two ranges: range I and range II. This classification has implications for the selection of rated withstand voltages and the performance of standard tests.
Range I applies to systems where overvoltages are mainly caused by switching operations and where lightning overvoltages are relatively low. Range I covers systems with highest voltage for equipment up to and including 245 kV.
Range II applies to systems where overvoltages are mainly caused by lightning strokes and where switching overvoltages are relatively low. Range II covers systems with highest voltage for equipment above 245 kV.
The main difference between range I and range II is that in range I, phase-to-phase insulation and longitudinal insulation are not subjected to impulse tests, whereas in range II, they are. This is because in range I, phase-to-phase overvoltages and longitudinal overvoltages are usually lower than phase-to-earth overvoltages, whereas in range II, they may be higher. Therefore, in range II, phase-to-phase insulation and longitudinal insulation require higher impulse withstand voltages than phase-to-earth insulation.
What are some examples of applications of iec 60071 3 phase to phase?
To illustrate how iec 60071 3 phase to phase is used for insulation co-ordination of electrical equipment and installations, let us consider some practical examples.
Example 1: A three-phase transformer with a highest voltage for equipment of 72.5 kV
A three-phase transformer has a highest voltage for equipment of 72.5 kV. It belongs to range I and has an effectively earthed system. The representative voltages (Urp) for phase-to-earth, phase-to-phase and longitudinal insulation are assumed to be:
Urp (phase-to-earth) = 80 kV (rms)
Urp (phase-to-phase) = 140 kV (rms)
Urp (longitudinal) = 20 kV (rms)
The co-ordination withstand voltages (Ucw) for phase-to-earth, phase-to-phase and longitudinal insulation are determined by multiplying the representative voltages by a factor of 1.3. Therefore:
Ucw (phase-to-earth) = 104 kV (rms)
Ucw (phase-to-phase) = 182 kV (rms)
Ucw (longitudinal) = 26 kV (rms)
The required withstand voltages (Urw) for phase-to-earth, phase-to-phase and longitudinal insulation are determined by adding a safety margin of 10% to the co-ordination withstand voltages. Therefore:
Urw (phase-to-earth) = 114.4 kV (rms)
Urw (phase-to-phase) = 200.2 kV (rms)
Urw (longitudinal) = 28.6 kV (rms)
The rated insulation level for phase-to-earth, phase-to-phase and longitudinal insulation are selected from the lists of standard rated withstand voltages provided by iec 60071-1:2006. The selected values should be equal to or higher than the required withstand voltages. Therefore:
Rated short-duration power frequency withstand voltage (phase-to-earth) = 140 kV (rms)
Rated impulse withstand voltage (phase-to-earth) = 325 kV (peak)
Rated short-duration power frequency withstand voltage (phase-to-phase) = 200 kV (rms)
Rated impulse withstand voltage (phase-to-phase) = not applicable
Rated short-duration power frequency withstand voltage (longitudinal) = 30 kV (rms)
Rated impulse withstand voltage (longitudinal) = not applicable
The standard tests for phase-to-earth, phase-to-phase and longitudinal insulation are performed according to the general requirements and test procedures of iec 60071-1:2006. The test voltages should be equal to or higher than the rated withstand voltages. Therefore:
Test voltage for short-duration power frequency test (phase-to-earth) = 140 kV (rms)
Test voltage for impulse test (phase-to-earth) = 325 kV (peak)
Test voltage for short-duration power frequency test (phase-to-phase) = 200 kV (rms)
Test voltage for impulse test (phase-to-phase) = not applicable
Test voltage for short-duration power frequency test (longitudinal) = 30 kV (rms)
Test voltage for impulse test (longitudinal) = not applicable
Example 2: A three-phase circuit breaker with a highest voltage for equipment of 420 kV
A three-phase circuit breaker has a highest voltage for equipment of 420 kV. It belongs to range II and has an effectively earthed system. The representative voltages (Urp) for phase-to-earth, phase-to-phase and longitudinal insulation are assumed to be:
Urp (phase-to-earth) = 460 kV (rms)
Urp (phase-to-phase) = 800 kV (rms)
Urp (longitudinal) = 100 kV (rms)
The co-ordination withstand voltages (Ucw) for phase-to-earth, phase-to-phase and longitudinal insulation are determined by multiplying the representative voltages by a factor of 1.3. Therefore:
Ucw (phase-to-earth) = 598 kV (rms)
Ucw (phase-to-phase) = 1040 kV (rms)
Ucw (longitudinal) = 130 kV (rms)
The required withstand voltages (Urw) for phase-to-earth, phase-to-phase and longitudinal insulation are determined by adding a safety margin of 10% to the co-ordination withstand voltages. Therefore:
Urw (phase-to-earth) = 657.8 kV (rms)
Urw (phase-to-phase) = 1144 kV (rms)
Urw (longitudinal) = 143 kV (rms)
The rated insulation level for phase-to-earth, phase-to-phase and longitudinal insulation are selected from the lists of standard rated withstand voltages provided by iec 60071-1:2006. The selected values should be equal to or higher than the required withstand voltages. Therefore:
Rated short-duration power frequency withstand voltage (phase-to-earth) = 680 kV (rms)
Rated impulse withstand voltage (phase-to-earth) = 1425 kV (peak)
Rated short-duration power frequency withstand voltage (phase-to-phase) = 1170 kV (rms)
Rated impulse withstand voltage (phase-to-phase) = 2500 kV (peak)
Rated short-duration power frequency withstand voltage (longitudinal) = 150 kV (rms)
Rated impulse withstand voltage (longitudinal) = 325 kV (peak)
The standard tests for phase-to-earth, phase-to-phase and longitudinal insulation are performed according to the general requirements and test procedures of iec 60071-1:2006. The test voltages should be equal to or higher than the rated withstand voltages. Therefore:
Test voltage for short-duration power frequency test (phase-to-earth) = 680 kV (rms)
Test voltage for impulse test (phase-to-earth) = 1425 kV (peak)
Test voltage for short-duration power frequency test (phase-to-phase) = 1170 kV (rms)
Test voltage for impulse test (phase-to-phase) = 2500 kV (peak)
Test voltage for short-duration power frequency test (longitudinal) = 150 kV (rms)
Test voltage for impulse test (longitudinal) = 325 kV (peak)
What are the benefits of iec 60071 3 phase to phase?
By following the procedure of iec 60071 3 phase to phase for insulation co-ordination, electrical equipment and installations can enjoy several benefits, such as:
Reliability
By selecting the appropriate rated withstand voltages and performing the standard tests, the insulation of electrical equipment and installations can be verified to withstand the voltages and overvoltages that may occur in a system under normal or abnormal conditions. This reduces the risk of insulation breakdowns that may cause interruptions or malfunctions in the operation of equipment and installations.
Safety
By selecting the appropriate rated withstand voltages and performing the standard tests, the insulation of electrical equipment and installations can be verified to withstand the voltages and overvoltages that may occur in a system under normal or abnormal conditions. This reduces the risk of insulation failures that may cause electric shocks, fires or explosions that may harm people, animals or property.
Efficiency
By selecting the appropriate rated withstand voltages and performing the standard tests, the insulation of electrical equipment and installations can be verified to withstand the voltages and overvoltages that may occur in a system under normal or abnormal conditions. This reduces the risk of insulation degradation that may reduce the power quality, increase the energy losses or increase the maintenance costs of equipment and installations.
Cost-effectiveness
What are the challenges and limitations of iec 60071 3 phase to phase?
Despite the benefits of iec 60071 3 phase to phase for insulation co-ordination, there are also some challenges and limitations that need to be considered, such as:
Complexity
The procedure of iec 60071 3 phase to phase for insulation co-ordination involves several steps, factors and calculations that may be complex and time-consuming. For example, determining the representative voltages and overvoltages requires a detailed knowledge of the system characteristics, operating conditions and fault scenarios. Selecting the co-ordination withstand voltages requires a trade-off between reliability and cost-effectiveness. Performing the standard tests requires special equipment and procedures that may not be readily available or feasible.
Variability
The procedure of iec 60071 3 phase to phase for insulation co-ordination is based on some assumptions and simplifications that may not reflect the actual situation of a system. For example, the representative voltages and overvoltages are based on statistical data that may not cover all possible cases or variations. The co-ordination withstand voltages are based on standard values that may not match the exact requirements of a system. The standard tests are based on ideal conditions that may not represent the actual environment or stress of a system.
Uncertainty
What are some future trends and developments in iec 60071 3 phase to phase?
As the electrical systems evolve and become more complex and interconnected, iec 60071 3 phase to phase may also need to adapt and improve to meet the new challenges and opportunities. Some of the possible future trends and developments in iec 60071 3 phase to phase are:
New standards
iec 60071-1:2006 is currently under revision and a new edition is expected to be published soon. The new edition may include some updates and changes to reflect the latest developments and practices in insulation co-ordination. For example, it may address some issues such as the impact of distributed generation, renewable energy sources, smart grids, power electronics and harmonics on insulation co-ordination. It may also provide some guidance on how to deal with non-standard voltages and overvoltages that may occur in some systems.
New technologies
New technologies may offer some new solutions and possibilities for insulation co-ordination. For example, new materials and devices may improve the performance and durability of insulation. New sensors and monitoring systems may enhance the detection and diagnosis of insulation faults. New simulation and analysis tools may facilitate the design and optimization of insulation co-ordination. New communication and control systems may enable the coordination and integration of insulation protection devices.
New methods
New methods may provide some new approaches and perspectives for insulation co-ordination. For example, probabilistic methods may complement or replace deterministic methods for insulation co-ordination by taking into