Electronics Guide

Causes of Voltage Dips

Faults on the power system are the primary cause of voltage dips. When a short circuit occurs on a distribution line, current flows through the fault, causing voltage drops throughout the connected system. The magnitude and duration of the resulting dip depend on the fault location, fault impedance, and the time required for protective devices to clear the fault. Modern protection systems clear most faults within a few cycles (less than 100 ms), but even brief dips can affect sensitive equipment.

Large load switching, particularly motor starting, produces voltage dips through increased current demand. Large motors can draw six to eight times their rated current during starting, causing significant voltage drop in the supply system. The dip duration depends on the motor acceleration time, typically ranging from a few seconds for small motors to tens of seconds for large industrial drives.

Transformer energization draws high inrush currents that cause voltage dips similar to motor starting. The asymmetric saturation of transformer cores during energization can produce current peaks reaching 10-20 times rated current, though the high-frequency components of inrush current decay relatively quickly.

Causes of Short Interruptions

Short interruptions result from automatic fault clearing and circuit reclosing operations. When a fault occurs, protective devices interrupt the circuit to clear the fault. For temporary faults (such as lightning-induced flashover or tree contact), automatic reclosing attempts to restore service after a brief delay. If the fault has cleared, service is restored; if not, the interruption may become sustained.

Transfer switching between power sources causes brief interruptions during the transfer time. Automatic transfer switches at buildings and industrial facilities switch to backup power during utility outages. Even fast transfer switches typically require 100-200 ms for the transfer, during which equipment experiences an interruption.

Manual switching operations and maintenance activities can cause interruptions of various durations. While planned outages typically allow equipment to be safely shut down, unplanned switching or switching errors can cause unexpected interruptions.

Voltage Variation Terminology

Standardized terminology helps communicate about voltage disturbances. A voltage dip (or sag in American terminology) is a decrease to between 90% and 1% of nominal voltage. A short interruption is a decrease to less than 1% of nominal voltage lasting from one-half cycle to one minute. A sustained interruption lasts longer than one minute. Voltage swell is a temporary increase above 110% of nominal voltage.

Dip magnitude is often expressed as the remaining voltage rather than the depth. A "70% voltage dip" means voltage dropped to 70% of nominal (a 30% reduction). This convention can cause confusion, so careful attention to the definition used in standards and specifications is important.

Voltage Dip Test Levels

The standard defines test levels based on remaining voltage and duration. Typical test points include dips to 70%, 40%, and 0% of nominal voltage with durations ranging from 0.5 cycles to 25 cycles (10 ms to 500 ms at 50 Hz). Longer duration dips and interruptions up to 5 seconds test the equipment's energy storage and recovery capabilities.

The test generator must be capable of producing abrupt voltage changes at any phase angle of the AC waveform. The phase angle at which the dip begins significantly affects the stress on equipment, particularly for dips to 0% voltage. Beginning the dip at voltage zero crossing minimizes the current transient at dip initiation, while beginning at voltage peak creates maximum current interruption stress.

Test Procedure

Equipment is operated normally while voltage dips and interruptions are applied. The test sequence typically includes multiple repetitions at each test point to ensure repeatable results. Monitoring equipment function during and after each event determines compliance with performance criteria.

The performance criteria are similar to those for other immunity tests. Criterion A requires normal performance during the disturbance. Criterion B allows temporary degradation with self-recovery. Criterion C permits loss of function requiring operator intervention, though the function must recover when voltage returns to normal and appropriate recovery procedures are performed.

Preferred Test Levels

Product committees typically select test levels from the ranges defined in the basic standard. Class 2 performance, suitable for most equipment in controlled environments, includes tests at 70% voltage for 25 cycles, 40% voltage for 10 cycles, and 0% voltage (interruption) for 5 cycles. Class 3 performance for industrial environments includes longer durations and additional depth levels.

Equipment intended for critical applications may require extended immunity beyond standard levels. Continuous operation through 500 ms interruptions may be specified for process control equipment. Medical life-support equipment may require even longer ride-through capability or specified graceful degradation behavior.

Power Supply Considerations

Most electronic equipment uses switch-mode power supplies that regulate output voltage over a wide input voltage range. The input voltage range specification indicates the minimum voltage at which the power supply maintains regulation. Below this voltage, output begins to fall, eventually causing system malfunction or shutdown.

Energy storage in the power supply bulk capacitors determines ride-through capability during brief interruptions. Larger capacitance provides longer ride-through but increases cost, size, and inrush current. The load current determines how quickly stored energy depletes during an interruption. Low-power equipment may ride through several hundred milliseconds on bulk capacitor energy alone.

Universal input power supplies (85-264 VAC) inherently provide good voltage dip immunity because they are designed to operate at 85 VAC continuously. At nominal 230 VAC input, significant voltage reduction is possible before the minimum operating voltage is reached. At 120 VAC nominal, the margin is smaller, and immunity to deep dips is correspondingly reduced.

Control System Effects

Microprocessors and digital controllers may malfunction during voltage dips even before the power supply output falls below specification. Brown-out detection circuits monitor supply voltage and trigger reset when voltage drops below a threshold. This protective function can cause nuisance resets during voltage dips that would otherwise be tolerable. The brown-out threshold and hysteresis must be carefully set to balance protection against supply voltage droop versus nuisance tripping during dips.

Volatile memory contents are lost during power supply interruptions unless battery backup or other non-volatile storage is provided. RAM-based configuration data, operating state, and transaction data may need to be reconstructed or recovered after power returns. The recovery process adds to the effective duration of the interruption from the user's perspective.

Electromechanical Components

Relays, contactors, and solenoids drop out when coil voltage falls below the holding voltage, which is typically lower than the pull-in voltage. The dropout voltage varies with relay type and manufacturer but is commonly 70-80% of rated voltage. Voltage dips to this level or below cause relay dropout, potentially interrupting control signals or power to loads.

Motor drives and variable frequency drives (VFDs) have varying immunity to voltage dips depending on their design and operating condition. Drives operating at high torque may trip on overcurrent as they attempt to maintain torque with reduced voltage. Many drives include ride-through features that reduce output frequency during dips to maintain motor operation without tripping.

Power Supply Design

Selecting or designing power supplies with wide input voltage range maximizes inherent dip immunity. Power supplies rated for 85-264 VAC input provide substantial margin at nominal voltages. For equipment that must operate from a narrow input range, external voltage regulation can extend the effective range.

Increasing bulk capacitor size extends ride-through time during interruptions. The relationship between capacitor size and hold-up time depends on load current and minimum voltage for regulation. Doubling the capacitance approximately doubles the hold-up time. Supercapacitors can provide extended hold-up times for critical loads.

Active power factor correction (PFC) stages in switch-mode power supplies can improve voltage dip immunity by maintaining regulation to lower input voltages. The boost topology used in most PFC circuits regulates an intermediate DC bus voltage that remains relatively stable during input voltage dips until the input falls below the minimum rectified voltage.

Uninterruptible Power Supplies

Uninterruptible power supplies (UPS) provide the most comprehensive protection against voltage dips and interruptions. Online (double-conversion) UPS systems continuously supply power from the inverter, isolating the load from all input disturbances. Line-interactive UPS systems regulate voltage during dips and switch to battery during interruptions with minimal transfer time.

UPS sizing must account for the load power requirement, desired battery run time, and the need to recharge batteries between events. Oversizing provides margin for load growth and battery aging. For critical applications, redundant UPS configurations (N+1 or 2N) ensure continuous operation even with UPS failures.

Voltage Regulators and Conditioners

Ferroresonant transformers and electronic voltage regulators maintain output voltage during input voltage dips. Ferroresonant transformers use magnetic saturation to regulate output, providing robust performance without active electronics. Electronic regulators using transformer tap changing, buck-boost circuits, or inverter synthesis can regulate over wide input ranges with high efficiency.

The response time of voltage regulators determines their effectiveness during sudden dips. Ferroresonant transformers respond within one cycle. Electronic regulators vary widely, with some responding within milliseconds while others require multiple cycles to adjust. The load's immunity during the regulator response time must be considered.

Software and Control Measures

Software measures can reduce the impact of voltage dips and interruptions that cannot be prevented at the hardware level. Non-volatile storage of critical data enables recovery after power loss. Graceful shutdown procedures protect equipment and data when ride-through is not possible. Automatic restart with validation of operating state minimizes recovery time after interruptions.

Monitoring power quality and logging disturbance events helps identify problems and optimize protection. Power quality monitors track voltage, current, and derived parameters, alarming on excursions beyond defined limits. Event logs help correlate equipment problems with power disturbances and identify patterns that might indicate developing issues.