Voltage Dips & Interruptions

An overview of test equipment requirements, importance of test equipment characteristics and changes from IEC 61000-4-11 Ed.2 to IEC 61000-4-11 Ed.3

Voltage dips and short interruptions are events that occur due to faults in a power network by sudden changes of large loads. Voltage variations occur by continuously varying loads connected to the power network. These events can affect electrical and electronic equipment so they need to be simulated in a laboratory environment. This testing is sometimes referred to as power quality testing, although this term might be more suitable when referring to IEC 61000-4-30 tests.

The IEC basic standards outlining the test requirements are:

• IEC 61000-4-11 which applies to electrical and electronic equipment having a rated input current not exceeding 16 A per phase, for connection to 50 Hz or 60 Hz AC networks.
• IEC 61000-4-34 which applies to electrical and electronic equipment having a rated input current exceeding 16 A per phase, specifically voltage dips and short interruptions for equipment connected to either 50 Hz or 60 Hz AC networks, including 1-phase and 3-phase mains. For currents higher than 75 A per phase, IEC recommends in situ measurements, throughout the power system.
• IEC 61000-4-29 which applies to electrical and electronic equipment when subjected to voltage dips, short interruptions or voltage variations on DC power ports.

As with all EMC basic standards, the objective is to establish a common reference for evaluating the immunity of electrical and electronic equipment when subjected to these phenomena. It is the responsibility of product standards to determine the relevance and applicability of the tests outlined in the basic standard. The information herein will be focused on the IEC 61000-4-11 standard.

Test Equipment Requirements

Voltage dips, short interruptions & variations tests can be simulated in laboratories using dedicated test equipment. Voltage variations tests are listed as optional in the IEC basic standards. Below is a list of requirements the test equipment must meet in order to be used for compliance testing:

• Output voltage at no load – the generator output voltage must be within 5% of the defined dip levels in no load condition. The defined dip levels are 0%, 40%, 70% and 80% of nominal voltage.
• Output voltage change with load – the voltage change from no load condition to loaded condition must be less than 5% of the defined dip level.
• Output current capability – the generator shall be capable of carrying current greater than 16A at the defined dip level for a short duration. The harshest condition is at the 40% dip level where the generator needs to carry 40 A for 3 seconds.
• Peak inrush current capability – The peak inrush current capability should not be limited by the test equipment. The maximum peak capability of the generator need not exceed 1000 A for 250 V to 600 V mains, 500 A for 200 V to 240 V mains, or 250 A for 100 V to 120 V mains.
• Voltage overshoot/undershoot – The instantaneous peak overshoot/undershoot of the actual voltage when the generator is loaded with a 100 Ω resistive load should be less than 5% of the defined dip level.
• Voltage rise and fall times – The generator must be capable of rapid switching between 1 µs and 5 µs during an abrupt voltage level change.
• Phase shifting – the generator shall be capable to shift phases between 0° and 360°
• Phase relationship and zero crossing– the generator needs to be able to sense and synchronize with the AC power. The phase relationship of the voltage dips and interruptions event needs to be less than ± 10° of the power frequency. Also, the zero-crossing control of the generator must line up within ± 10° of the mains frequency.

Importance of Rise & Fall Times

When selecting equipment to perform voltage dips and short interruptions, it is important to use test equipment which meet the required rapid rise and fall times in order to avoid significant phase shift during the switch. The 1 µs – 5 µs switch time is a worst-case scenario and simulates a short circuit in the power network in close proximity to the electronic equipment. Therefore, tests performed with this rapid switching can determined the robustness of the equipment being tested in a worst-case scenario. As an example, we will investigate the effect of switch times on a 230V / 50Hz power network. Below is the pure sine wave of this power network:

From the AC power frequency, we can calculate the phase shift for different switch times. If we use the 5 µs slowest switch time limit defined in IEC 61000-4-11, we can see that this corresponds to a phase shift of only 0.09°. A pre-compliance dip generator with a 200 µs switch time adds a phase shift of 3.6° and a 500 µs switch time adds a phase shift of 9°, as shown below:

A side effect of this significant phase shift is a test level decrease. The phase shift effect is even more profound on 60Hz power networks. For example, @60Hz a 200 µs switch time equals a 4.3° phase shift and a 500 µs switch time is equal to a 10.8° phase shift. Taking into consideration that the real dips start angle might also be determined by the generator’s accuracy, maintaining a reduced phase shift due to switch process is of great help. This clearly illustrates the importance of using full compliance test equipment which meet the rise and fall time requirements. In addition, it is very important to use equipment with a high degree of synchronizing accuracy.

Importance of Inrush Current Capability

Inrush current flows into electronic equipment when you connect it to a power network which can result in damage to the equipment. To prevent damage, most electronic equipment is designed and built with a circuit to limit this inrush current. The same inrush current flow occurs when the power network recovers from a voltage dip or short interruption but the protection circuit may be deactivated. To prevent damage to equipment after a voltage dip or short interruption, the dip generator needs to supply sufficient current and not limit the inrush current.

Ideally, the voltage dips and short interruptions test equipment meets the peak inrush current drive capability. If the test equipment meets this capability (at least 1,000A for 250V – 600V mains, 500 A for 220V to 240V mains, and 250A for 100V – 120V mains), it is not necessary to measure the EUT’s peak inrush current which saves time. IEC 61000-4-11 does allow a workaround to use a generator with a lower inrush current, as long as the measured inrush current of the EUT is less than 70% the measured inrush drive capability of the test equipment. Since both of these parameters need to be measured prior to the test, it adds time and cost. 

Changes from IEC 61000-4-11 Ed.2 to Ed.3

IEC 61000-4-11 Ed.3 was released in 2020 and supersedes the old IEC 61000-4-11 Ed.2 published in 2004. The main changes in the standard refer to a more precise definition of rise and fall time and reiteration of the strict necessity to use a generator with 1 µs– 5 μs rise and fall times for compliance testing.

Edition 2 of the standard was unclear about its over/undershoot requirements leading to confusion which parameters need to be measured during calibration/verification. Some interpretations were that both overshoot and undershoot were to be measured at the instant a level transition occurred and at the instant the level transition completed. Edition 3 now clearly defines overshoot and undershoot as an effect after switching, not before the switch. So, this means for a falling edge undershoot only needs to be measured, and for a rising edge overshoot only needs to be measured. The overshoot or undershoot must be less than 5% of the actual voltage when measured with a 100 Ω resistive load. 

Test Equipment Recommendations

The IMU series of immunity generators from EMC PARTNER are the ideal solution for testing voltage dips and short interruptions on single phase and three phase EUTs up to 690VAC / 75A per phase. Voltage variations are also possible on EUTs drawing up to 16A. These generators meet or exceed all requirements from the IEC standards and therefore can be used for full compliance testing. Beneficial features include:

• 1 µs – 5 µs  full-compliant switch times
• 500A or 1000A inrush currents as required by the standards. No need for time-consuming measurements prior to testing
• 2 dips in one cycle – IMU generators can trigger 2 dip events within one cycle. One voltage dip in the positive half cycle and one in the negative half cycle
• Special dip mode for testing electricity meters in accordance with IEC 62052-11 which cannot be met with a typical dip generator
• Multiple variac options – 5A internal variac is very practical for testing EUT’s drawing less than 5A current. 16A variac option to cover EUTs up to 16A. Dip level is user programmable in 1% increments which exceeds standard requirements (0%, 40%, 70% & 80%)
• Tapped transformer option – Economic and very reliable solution for 16A dips. EMC PARTNER serviced only one dips switch circuit since the PFS / SRC product line was launched in 2005
• Full range of accessories available – VERI-DIPS inrush current verification device and DIPS100E purely resistive 100 Ω load for calibration of dips switch time
• IMU generators are user configurable and up to 10 IEC standard tests can be performed with one test system

If you would like to learn more about EMC Partner voltage dips, interruptions and variations test equipment, please contact our EMC sales team at (703) 365-2330 or emcsales@hvtechnologies.com