13 Figure 4 – Example of test generator verification load...9 Figure A.1 – Unbalanced voltage vectors ..... This immunity test should be included in product, product family or generic st
Test generators
The generator shall have provisions to prevent the emission of disturbances which, if injected in the power supply network, may influence the test results
The output voltage shall be adjusted to ±1% of U N and the phase to ±0,3°
Table 2 – Characteristics of the generator
Output current capability Sufficient to supply the EUT under all test conditions
Overshoot/undershoot of the actual voltage, generator loaded with 100 Ω resistive load
Less than 5 % of the change in voltage
Voltage rise (and fall time) during voltage changes, generator loaded with 100 Ω resistive load
Total harmonic distortion of the output voltage Less than 3 %
1° between any two phases Frequency accuracy 0,5 % of f 1 (50 Hz or 60 Hz)
Verification of the characteristics of the test generators
It is recognized that there is a wide range of EUTs and that consequently test generators with different output power capabilities may be used, as required
The test generator must be verified to meet the specifications outlined in Table 2 Its performance will be assessed using resistive loads that draw an RMS current not exceeding the generator's output capacity.
The generator must demonstrate an output current capability that achieves a crest factor of at least 3 when the nominal voltage (U N) is applied to a single-phase load, with the rms current not exceeding the generator's output capacity Each output phase of the generator should be individually verified, and a suitable verification load example is illustrated in Figure 4, specifically a 230V/16A load.
To ensure optimal performance, select resistor \$R_a\$ so that the total series resistance, which includes the additional resistor \$R_a\$, wiring resistance \$R_{wire}\$, internal resistance of the two conducting diodes \$R_{diodes}\$, and internal resistance of the capacitor \$R_c\$, is maintained at 92 mΩ (± 10%).
Figure 4 – Example of test generator verification load
The test will be conducted with the Equipment Under Test (EUT) connected to the test generator using a manufacturer-specified supply cable If the manufacturer does not specify a cable length, the shortest suitable length for the EUT will be used The length of the cable will be documented in the test report.
Figure 3 shows a schematic drawing for the generation of voltage unbalance (amplitude or phase change) using a generator with power amplifier
Generators with transformers and switches need to have variable transformers on at least two phases
The ports of the EUT shall be connected to appropriate peripherals as defined by the manufacturer If appropriate peripherals are not available, they may be simulated
Laboratory reference conditions
In order to minimize the impact of environmental parameters on test results, the tests shall be carried out in climatic and electromagnetic reference conditions as specified in 8.1.1 and 8.1.2
The laboratory's climatic conditions must adhere to the limits set by the manufacturers for both the Equipment Under Test (EUT) and the test equipment, unless the responsible committee specifies otherwise.
Tests shall not be performed if the relative humidity is so high as to cause condensation on the EUT or the test equipment
It is essential to inform the committee responsible for this standard if there is substantial evidence indicating that climatic conditions influence the effects of the phenomenon addressed by this standard.
The electromagnetic conditions of the laboratory shall not influence the test results.
Execution of the test
The EUT shall be configured for its normal operating conditions
The tests shall be performed according to a test plan that shall specify
− ports to which the test shall be applied;
− representative operating conditions of the EUT;
The power supply, signals and other functional electrical quantities shall be applied within their rated range If the actual operating signal sources are not available, they may be simulated
For each test level, a succession of at least three unbalance sequences shall be applied, with an interval of a least 3 min between each (see figure 2)
The applied test levels shall be rotated as follows:
First sequence: U a to L 1 , U b to L 2 , U c to L 3 ;
Second sequence: U a to L 2 , U b to L 3 , U c to L 1 ;
Third sequence: U a to L 3 , U b to L 1 , U c to L 2 where
U a , U b and U c (see table 1) are the voltages of the generator and
L 1 , L 2 and L 3 are the inputs of the EUT
Changes in supply voltage shall occur at zero crossings of U a The output impedance of the test generator shall be low in steady state and during transition periods
Performance degradation must be documented for each test, and the monitoring equipment should display the operational mode status of the Equipment Under Test (EUT) during and after testing A comprehensive functional check is required after each group of tests.
Test results will be categorized based on the equipment's loss of function or performance degradation compared to the manufacturer's defined performance level or an agreed standard between the manufacturer and purchaser The classification includes: a) normal performance within specified limits; b) temporary loss of function that resolves automatically after the disturbance; c) temporary loss of function requiring operator intervention for correction; and d) irrecoverable loss of function or performance due to hardware or software damage or data loss.
The manufacturer's specification may define effects on the EUT which may be considered insignificant, and therefore acceptable
This classification serves as a valuable guide for committees developing performance criteria for generic, product, and product-family standards It also provides a framework for establishing performance criteria agreements between manufacturers and purchasers, particularly in cases where appropriate standards are lacking.
The test report shall contain all the information necessary to reproduce the test In particular, the following shall be recorded:
– the items specified in the test plan required by clause 8 of this standard;
– identification of the EUT and any associated equipment, e.g brand name, product type, serial number;
– identification of the test equipment, e.g brand name, product type, serial number;
– any special environmental conditions in which the test was performed, e.g shielded enclosure;
– any specific conditions necessary to enable the test to be performed;
– performance level defined by the manufacturer, requestor or purchaser;
– performance criterion specified in the generic, product or product-family standard;
– any effects on the EUT observed during or after the application of the test disturbance, and the duration for which these effects persist;
– the rationale for the pass/fail decision (based on the performance criterion specified in the generic, product or product-family standard, or agreed between the manufacturer and the purchaser);
– any specific conditions of use, for example cable length or type, shielding or grounding, or
EUT operating conditions, which are required to achieve compliance
Figure 1 – Example of unbalanced three-phase supply voltage (Test 3)
←⎯⎯⎯⎯→←⎯⎯⎯⎯⎯⎯⎯→ minimum unbalance of 180 s sequence of duration t
Figure 2 – Succession of three unbalance sequences of the test
NOTE These figures apply to 50 Hz systems
Figure 3 – Schematic diagram of test instrumentation for unbalance
Sources, effects and measurement of unbalance
The predominant cause of unbalance is single-phase loads
In low-voltage networks, single-phase loads are primarily connected phase-to-neutral and are evenly distributed across the three phases In contrast, medium-voltage and high-voltage networks allow single-phase loads to connect either phase-to-phase or phase-to-neutral Significant examples of single-phase loads include a.c railway supplies and single-phase induction furnaces Additionally, certain three-phase loads, such as arc furnaces, can create an unbalanced operating regime.
Short-term high levels of unbalance are usually due to faults in the electrical network, primarily affecting the low-voltage network, although medium- and high-voltage networks can also be impacted.
Depending on the characteristics of the protection equipment and the impedance of the network, these faults result in different fault conditions as described in table 1
Under unbalanced conditions, a three-phase induction machine exhibits low-impedance characteristics similar to its starting state, leading to a significant increase in current draw—up to ten times the steady-state current This results in an unbalanced current that is several times greater than the unbalanced supply voltage, causing considerable differences in the three-phase currents The increased heating in the phase(s) with higher current is only partially mitigated by the reduced heating in other phases As temperatures rise, the risk of disconnection in one phase increases, which can quickly lead to the machine's destruction.
Larger and more costly motors and generators are often equipped with protective systems designed to identify supply imbalances and disconnect the equipment When the unbalance reaches a critical level, the "single-phasing" protection mechanism can detect the unbalanced currents and subsequently trip the machine to prevent damage.
Polyphase converters, where each input phase voltage sequentially contributes to the direct current (d.c.) output, can be negatively impacted by an unbalanced supply This imbalance leads to an unwanted ripple component on the d.c side and introduces non-characteristic harmonics on the alternating current (a.c.) side.
Control equipment can be adversely affected, especially when the design relies on a balanced supply network Additionally, for cost efficiency, sensors are frequently installed on just one or two phases This can result in control and regulation errors, potentially causing significant performance losses.
The following method of symmetrical components is presented with reference to three-phase systems, but also applies to polyphase systems
A three-phase supply system is deemed unbalanced when the vectors representing voltage or current differ in magnitude or when the phase angles between them are not 120° To address unbalanced conditions in these circuits, the method of symmetrical components is utilized, simplifying the calculation of power system unbalanced faults, loads, and stability limits in three-phase power systems.
This method transforms three unbalanced vectors (U_a, U_b, and U_c) into three sets of balanced vectors (U_{1a}, U_{1b}, U_{1c}; U_{2a}, U_{2b}, U_{2c}; U_{0a}, U_{0b}, U_{0c}) Each set consists of vectors that are equal in magnitude and spaced at either 0° or 120° These sets represent symmetrical components of the original unbalanced vectors, categorized as positive-sequence, negative-sequence, or zero-sequence vector systems This concept is applicable to rotating vectors, such as voltages or currents, as well as non-rotating vector operators like impedance or admittance, with a focus on voltage rotating vectors.
In a fault condition, symmetrical vectors exhibit specific amplitudes and phases During normal operation, a system experiencing unbalanced conditions typically shows that voltages U₀ and U₂ are only a small percentage of Uₙ.
U 0c a) Positive-sequence voltage b) Negative-sequence voltage c) Zero-sequence voltage
Figure A.2 – Components of the unbalanced vectors in figure A.1
The three sets of component vectors share a counter-clockwise rotation direction, consistent with the original unbalanced vectors While the negative sequence does not rotate in the opposite direction to the positive sequence, its phase sequence is indeed opposite to that of the positive-sequence set The phase sequence refers to the order of maximum values occurring in the time domain.
A.3.2 Negative and zero unbalance factors
To assess the degree of negative-sequence voltage unbalance in an unbalanced voltage system, the unbalance factor (k u2) is calculated using the ratio of the negative-sequence component (U 2) to the positive-sequence component (U 1) This relationship is expressed as k u2 = U 2 / U 1.
U 2 is the negative-sequence voltage;
U 1 is the positive-sequence voltage
Negative-sequence voltages experience significant attenuation when moving from lower to higher voltage networks Conversely, when propagating from higher to lower voltage levels, the degree of attenuation is influenced by the presence of three-phase rotating machines, which help to balance the voltages.
The negative-sequence voltages in a network mainly result from the negative-sequence currents of unbalanced loads flowing in the network
In addition, the degree of zero-sequence voltage unbalance can be determined by the ratio of the zero-sequence component to the positive-sequence component, the unbalance factor (k u0 ): k u0 = U 0 /U 1 where
U 0 is the zero-sequence voltage;
U 1 is the positive-sequence voltage
The propagation of the zero-sequence unbalance voltage is stopped by the delta-connected transformers