How are voltage dip generators useful for voltage interruption test

Defining Voltage dip generator
NEMA MG1-16.48 defines voltage dip as the greatest voltage difference from the rated generator output voltage. Inrush currents at motor start-up or large block loads limit engine speed and lower excitation to the main field generate these dips. Because the causes and solutions for instantaneous voltage dips differ from those for block loads, they are measured and analyzed independently. Because of its instantaneous nature, the largest dip caused by motor inrush current occurs within five cycles and can only be monitored with an oscilloscope. Mechanical recorders can detect dips caused by heavy block loads that slow engine speed.

Sustained Dip Confusion
Some gen set brands are difficult to compare since voltage dip is defined differently in company documentation. Instead of instantaneous voltage dip, sustained voltage dip is supplied, which rates the drop at a lower but longer recovery curve.
With a comparison of sub transient reactance of two generators with comparable AVR response times, a meaningful comparison of motor-starting voltage dip may be obtained. When starting the same motor, two machines with identical sub transient reactance will have roughly the same voltage dip.

As a result, providers who use sustained voltage dip as a measure of voltage dip will only provide a flat “yes” or “no” answer as to whether their gen set will match instantaneous voltage dip standards established by other manufacturers.
It is the only method to ensure that you will receive comparable bids on the projects you describe.

Understanding the Transient Response of Generator Sets
There is no need to be concerned about the local utility’s ability to take the load or any transient effects on power quality when a switch sends a few hundred kW out on a circuit. However, these are legitimate issues when power is drawn from a generator set. The amount of load that may be accepted in one step, as well as the magnitude of transitory affects on power quality, varies greatly amongst gen set models.

When a heavy load is applied to a generator set, the engine speed temporarily drops – or dips – before returning to steady-state. When a load is removed, the engine speed temporarily increases – or overshoots. The quality of electrical power is altered because generator frequency is determined by engine rpm. Transient response is the measurement of these transitory speed fluctuations.

The length and % frequency change of a transient reaction are measured (see figure below). The time it takes for the engine to return to steady-state functioning is referred to as recovery time. This can range from one second to twenty seconds. In general, the higher the percentage of dip and the longer it takes the engine to recover, the more weight adds to the bus.

Dips are often more dangerous than overshoots because excessive block loading can cause the engine to stall and the generator voltage to fall. The rotational mass of the generator set aids in frequency maintenance, although inertia must be carefully balanced between the generator and engine. When a larger generator is specified, the frequency drop is reduced, allowing more engine horsepower to be available for recovery. The voltage regulation mechanism of the generator set is the most critical component influencing transient responsiveness. Volt-per-hertz voltage regulation methods control voltage by following frequency proportionally.

As a big block load reduces engine rpm and generator frequency, voltage falls, efficiently unloading the engine and shortening recovery time. This system is used by all Cat gen sets. Constant-voltage regulation systems have a lower percentage of voltage change but a much longer recovery period. When the engine is fully loaded, the danger of an engine stall increases. Some generators employ double-voltage-per-hertz regulation methods. While these methods considerably improve block-loading capabilities or reduce recovery time, they come at a much higher voltage dip. Transient responsiveness is also affected by engine setup.

Most gen set engines are turbocharged to provide additional horsepower – and kW – without requiring a larger engine. The disadvantage of turbocharging is in transient responsiveness. Air becomes a limiting element in hauling scenarios. The longer the transient response of a gen set engine, the more turbocharged it is. Voltage dips and short interruptions are caused by failures in a power network caused by rapid changes in heavy loads. Continuously varying loads connected to the power network cause voltage changes. Because these occurrences can have an impact on electrical and electronic equipment, they must be mimicked in a laboratory setting.

IEC 61000-4-30 tests
• IEC 61000-4-11, which pertains to electrical and electronic equipment with 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 with a rated input current greater than 16 A per phase, specifically voltage dips and short interruptions for equipment connected to 50 Hz or 60 Hz alternating current networks, including 1-phase and 3-phase mains. IEC recommends in-situ measurements throughout the power system for currents greater than 75 A per phase.
• IEC 61000-4-29, which applies to electrical and electronic equipment when voltage dips, short interruptions, or voltage changes on DC power ports occur.
The goal, as with all EMC fundamental standards, is to create a single reference for assessing the immunity of electrical and electronic equipment when subjected to these phenomena. Product standards are responsible for determining the relevance and applicability of the tests stated in the basic standard. The material provided here will be centered on the IEC 61000-4-11 standard.

Requirements for Test Equipment
Dedicated test equipment can be used in laboratories to replicate voltage dips, short interruptions, and variability tests. The IEC basic standards provide voltage variations tests as optional. The following are the standards that test equipment must meet in order to be utilized for compliance testing:

• No-load output voltage – the generator output voltage must be within 5% of the set dip levels when no load is applied. Dip levels are specified as 0%, 40%, 70%, and 80% of nominal voltage.
• Change in output voltage with load – the voltage change from no load to loaded must be less than 5% of the defined dip level.
• Output current capability – the generator must be capable of carrying current more than 16A for a short period of time at the required dip level. The most difficult circumstance is at the 40% dip level, when the generator must handle 40 A for 3 seconds.
• Peak inrush current capability – The test equipment should not limit the peak inrush current capability. The generator’s maximum peak capability must not exceed 1000 A for 250 V to 600 V mains, 500 A for 200 V to 240 V mains, and 250 A for 100 V to 120 V mains.
• Voltage overshoot/undershoot – When the generator is loaded with a 100 resistive load, the instantaneous peak overshoot/undershoot of the actual voltage shall be less than 5% of the set dip level.
• Voltage rise and fall times – The generator must be capable of switching between 1 and 5 seconds during a sudden voltage level shift.
• Phase shifting – the generator must be capable of shifting phases between 0 and 360 degrees.
• Phase relationship and zero crossing- the generator must be able to detect and synchronize with alternating current power. The voltage dips and interruptions event’s phase relationship must be less than 10° of the power frequency. In addition, the generator’s zero-crossing control must be within 10° of the mains frequency.

Importance of Rise & Fall Times
It is critical to employ test equipment that meets the required rapid rise and fall times while performing voltage dips and short interruptions to avoid major phase shift during the switch. The switch time of 1s – 5s is the worst-case scenario and replicates a short circuit in the power network near the electronic equipment. As a result, tests using quick switching can assess the durability of the equipment being evaluated in the worst-case situation. We will look at the effect of switch timings on a 230V / 50Hz power network as an example.

We can determine the phase shift for various switch timings using the AC power frequency. We can see that the 5 s slowest changeover time limit established in IEC 61000-4-11 translates to a phase shift of only 0.09°. A pre-compliance dip generator with a switch time of 200 s adds a phase shift of 3.6° and a switch time of 500 s adds a phase shift of 9°.

A drop in test level is a secondary effect of this considerable phase shift. On 60Hz power networks, the phase shift impact is even more pronounced. A 200 s switch time, for example, represents a 4.3° phase shift at 60Hz, while a 500 s switch time equals a 10.8° phase shift. Given that the true dips start angle may also be dictated by the generator’s precision, keeping a decreased phase shift owing to the switch process is quite beneficial.

Importance of Inrush Current Capability
When you connect electronic equipment to a power network, inrush current rushes into the equipment, which might cause harm. Most electronic equipment is designed with a circuit to limit this inrush current. When the power network recovers after a voltage dip or short interruption, the same inrush current flow resumes, but the protective circuit may be disengaged. To minimise equipment damage during a voltage dip or brief interruption, the dip generator must give sufficient current while without limiting the inrush current.

The voltage dips and short interruptions test equipment should ideally meet the peak inrush current driving capacity. If the test equipment meets this requirement (at least 1,000A for 250V – 600V mains, 500A for 220V to 240V mains, and 250A for 100V – 120V mains), measuring the EUT’s peak inrush current is unnecessary, saving time. If the observed inrush current of the EUT is less than 70% of the reported inrush drive capability of the test equipment, IEC 61000-4-11 allows a workaround of using a generator with a lower inrush current. Because both characteristics must be measured prior to the test, it increases time and expense.

Changes between IEC 61000-4-11 Ed.2 and Ed.3
IEC 61000-4-11 Ed.3 was issued in 2020 and replaces the previous IEC 61000-4-11 Ed.2 from 2004. The key modifications in the standard are a more explicit description of rise and fall time and a reiteration of the strong requirement to use a generator with rise and fall times ranging from 1s to 5s for compliance testing.

The standard’s over/undershoot requirements were unclear in Edition 2, leading to misunderstanding regarding which parameters needed to be measured during calibration/verification. According to some interpretations, overshoot and undershoot should be recorded both when a level transition occurs and when the level transition is finished.

Overshoot and undershoot are now explicitly defined as effects that occur after switching, rather than before switching. This indicates that a falling edge undershoot merely requires measurement, but a rising edge overshoot requires measurement. When measured with a 100 resistive load, the overshoot or undershoot must be less than 5% of the actual voltage.

FAQs
Why does the voltage drop occur?
A voltage dip happens when the supply voltage (UF) drops below a threshold set at 90% of the stated supply voltage (Uc). A voltage dip occurs on a polyphase system when at least one of the voltages falls below the threshold and ends when all of the voltages are equal to or above the threshold.

What exactly is a voltage dips and interruptions test?
Voltage dips and short interruptions are caused by failures in a power network caused by rapid changes in heavy loads. Continuously varying loads connected to the power network cause voltage changes.

What exactly is a voltage interruption?
A voltage interruption occurs when the URMS(1/2) voltage falls below the designated interruption level. Typically, the interruption threshold is set significantly lower than the voltage dip level. The interruption begins when the URMS(1/2) voltage falls below the interruption threshold value and ends when the URMS(1/2) voltage equals or exceeds the interruption threshold value plus voltage hysteresis.

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