Unleashing the Power: Understanding Impulse Testing Solutions

Electrical power engineers must reduce the number of outages and preserve the continuity of service and electrical supply. Therefore, they must focus on protecting electrical apparatus and systems from the main cause of overvoltage conditions, particularly from lightning strikes and switching overvoltages or transients. The danger to electrical systems and apparatus originates from the potential that lightning may produce across the insulation.

Electrical impulse testing is a diagnostic technique used in electrical engineering to assess the insulation integrity and overall condition of electrical equipment such as transformers, cables, switchgear, and motors. Insulation systems are typically provided with a Basic Insulation Level (BIL), which refers to the minimum voltage an insulation system should be able to withstand before a flashover/breakdown occurs or protection devices activate. A commonly used representation of BIL in technical discussions and educational materials is shown below. Similar to how an insulation system protects the equipment from lightning strikes or transient surges, the analogy is made to a dyke protecting land from flooding in the event of a tidal surge.

The principle behind electrical impulse testing involves subjecting the equipment’s insulation system to short-duration high-voltage impulses. These impulses simulate lightning strikes or transient voltage surges the equipment may encounter during operation. By applying these high-voltage impulses, engineers can detect weaknesses or defects in the insulation that might lead to breakdowns or failures under actual operating conditions.

Electrical impulse testing is often conducted following international standards such as IEEE, IEC, or ANSI to ensure consistency and reliability of the test results. Impulse voltage test systems play a critical role in ensuring the safety and performance of electrical infrastructure by identifying potential insulation failures before they can lead to costly breakdowns or accidents.

Impulse voltage test systems

These systems typically consist of impulse generators, capacitive or resistive dividers, measurement circuits, and control units. The impulse generator produces short-duration, high-voltage pulses that simulate lightning strikes or transient surges, while the voltage divider and impulse analyzer ensure accurate voltage measurement.

Generating Impulse Voltages and Currents

The impulse generators can be single-stage or multistage in construction. The single-stage circuits are mainly used for the generation of relatively low voltage impulses, while multistage generators can reach voltages up to 10 MV or even higher. All voltage generators are based on Marx multiplier circuits.

A typical lightning impulse (LI) voltage wave shape is shown below. It is defined by its main characteristics, namely the front or rise time and the decay or tail of the waveform.

The waveform is defined by T1, “front time” or time to reach peak, and T2, “tail time” or time to reach half value. A common standard waveform for lightning impulse is defined as 1.2/50 microseconds (µs), meaning T1 = 1.2 µs and T2 = 50 µs. According to IEC 60060-1, compliance is met when T1 is 1.2 µs ± 30% (0.84 – 1.56 µs) and T2 is 50 µs ± 20% (40 – 60 µs).

A typical switching impulse (SI) waveform is shown below and is defined as having a front time T1 of 250 µs and tail time T2 of 2500 µs. According to IEC 60060-1, compliance is met when T1 is 250 µs ± 20% (200 – 300 µs) and T2 is 2500 µs ± 60% (1000 – 4000 µs).

Another common practice for impulse testing involves what’s known as Lightning Impulse Chopped (LIC), in which the front or tail is deliberately interrupted and causes a rapid collapse of the voltage. The recommended test voltage and the time to chopping are defined in Table 5.6 of IEEE C57.12.00.

Impulse Generator Models

The SGSA impulse generator, the smallest in its range, is highly recommended for medium-voltage equipment and small power transformer manufacturers, as well as for general purpose impulse testing in switchgear, bushings, and more. The basic system offers flexibility for upgrades, enabling users to customize tests for different types of high-voltage objects.

SGSA Features

• Total charging voltage 100 to 1200 kV
• 5 kJ stage energy
• Reliable and accurate triggering by improved Marx circuit
• Handy plug-in resistors and connections
• Unique protective grounding device
• Ease of operation with modern control system
• Impulse wave shape analysis according to the latest IEC and ANSI/IEEE standards
• Compact Design
• Short Configuration times
• Completely SIL 3 compliant with new HVC 300 impulse controls

The SGDA impulse generators offer an ideal solution for conducting impulse tests on high voltage (HV) or ultra-high voltage (UHV) cables, as well as medium voltage (MV) and high voltage (HV) power transformers with either 5 kJ or 10 kJ per stage energy versions. Designed with a unique Delta structure to allow a very small footprint and high stability, the SGDA can be upgraded in the future to meet growing testing demands. This also allows for easy transportation and on-site assembly as a mobile version.

SGDA Features

• Total charging voltage 200 to 3,200 kV
• 5 or 10 kJ stage energy
• The lowest internal inductance ever achieved
• Small footprint and compact design
• Reliable and accurate triggering by improved Marx Circuit
• Handy plug-in resistors and connections
• Unique protective grounding device
• Ease of operation with modern control system
• Impulse wave shapes analysis according to the latest IEC and ANSI/IEEE standards
• Short configuration times
• A modular expandable system that grows with your application needs (max. 3,200 kV)
• Easy handling and transportation for on-site testing
• High stability and durability
• Capable of generating up to 20 kA impulse current with an additional accessory
• Completely SIL 3 compliant with new HVC 300 impulse controls

The SGVA series is meticulously designed to meet the demanding requirements of impulse voltage tests in both industry and research settings. Featuring a patented parallel spark gap for reliable triggering and low-impulse circuit inductance, this series ensures accurate results. Additionally, its design facilitates easy access to resistors for rapid stage interchangeability.

SGVA Features

• Total charging voltage up to 10,000 kV
• 20 to 30 kJ stage energy (200 kV per stage)
• Reliable and accurate triggering by improved Marx Circuit
• Handy plug-in resistors and connections
• Rapid stage and resistors access
• Unique protective grounding device
• Integrated base frame carrying generator and charging rectifier
• Internal ladders and platforms for rapid stage access
• Ease of operation with modern control system
• Impulse wave shape analysis according to the latest IEC and ANSI/IEEE standards
• Short configuration times
• A modular expandable system that grows with your application needs (up to 10,000 kV)
• Sophisticated, strong, and flexible design
• Completely SIL 3 compliant with new HVC 300 impulse controls

At a Glance

Impulse Current Generators

Impulse current generators are specialized equipment for conducting impulse current tests on electrical components and systems. These generators are designed to produce high-current impulses of short duration, simulating sudden electrical surges or faults that may occur in real-world operating conditions. Typically comprising a charging system, discharge circuit, and control unit, impulse current generators are capable of delivering precise current waveforms with varying magnitudes and durations. They are commonly employed in the testing of protective devices such as circuit breakers, surge arresters, and grounding systems to ensure their ability to withstand and respond effectively to transient overcurrent events. By subjecting electrical components to controlled impulse currents, these generators help engineers assess their reliability, performance, and safety, ultimately contributing to the resilience and integrity of electrical infrastructure.

The Haefely SSG series of impulse current generators is designed for various applications, including exponential current impulses (ECI) with durations of 1/20 µs, 4/10 µs, and 8/20 µs, as well as lightning current impulses (LCI) lasting 10/350 µs, reaching impulse current amplitudes of up to 200 kA.

SSG Features

• Advanced flexibility
• Long service life
• Fast impulse repetition rates
• Advanced stability
• Automation

Controlling Impulse Generators

The HVC 300, representing a proud lineage of Haefely’s acclaimed impulse control systems, is engineered for seamless integration into high-voltage environments. Its robust construction ensures durability, while its advanced fiber-optic connectivity enables Safety Integrity Level 3 (SIL 3) compliance, safeguarding both operators and equipment in the control room with complete galvanic separation. This system offers versatility, as it can be paired with the HiAS 744 impulse measuring device to create a comprehensive impulse control and measurement setup.

HVC 300 Features

• Fully compliant to Safety Integrity Level 3 (SIL 3)
• Galvanic separation with rugged fiberoptic cables separating operator and test field
• No EMC interference for uninterrupted working in harsh environments
• State-of-the-art software with dark mode user interface for easy setup of automated testing
• Wave shape and polarity preview
• Impulse countdown timer
• Visualization of all failures, alarms, and measuring parameters

Measuring Impulse Voltages and Currents

The Impulse Analyzing System is an advanced diagnostic tool utilized for evaluating the characteristics and responses of impulse signals generated during impulse testing. By precisely measuring parameters such as voltage, current, rise time, duration, and frequency, the impulse analyzer provides valuable insights into the performance and integrity of electrical components and systems subjected to impulse testing. This is useful to detect abnormalities, identify potential weaknesses, and optimize the design and operation of equipment such as transformers, cables, circuit breakers, and surge protectors.

The Highest Resolution Impulse Analyzing System HiAS™ 744, sets a new standard with unparalleled performance. Featuring a front-end solution that delivers up to 16-bit resolution at 250 MS/s and the highest measurement accuracy, Haefely ensures reliability and precision.

HiAS 744 Features

• Models featuring 11-bit resolution at 125 MS/s, 50 MHz analog bandwidth or 16-bit resolution at 250 MS/s, 100 MHz analog bandwidth
• Optically decoupled front-end solution
• 2000 Vpk down to 5 Vpk analog input range with LEMO 4S connector
• 1 or 2-channel digitizer unit and can be expanded up to 4 channels
• Exceeds latest IEC 61083-1,-2, IEC 60060-2, IEEE Std. 4 and related standards
• 4th Generation digitizer from Haefely
• Mains powered
• The highest measurement accuracy in the market
• Excellent interference immunity and safe operation
• Integrated solution, no additional divider necessary, compatible with any divider ratio
• Synchronous multi-channel record
• Compliant, advanced, state-of-the-art solution
• Software and solution proven by many hundreds of users
• No battery pack or recharge needed

Impulse Voltage Dividers

Impulse voltage dividers are specialized components designed to accurately measure and scale down the high voltages generated during impulse testing to levels that are safe and manageable for instrumentation and analysis. Typically consisting of resistive and capacitive elements arranged in a specific configuration, impulse voltage dividers ensure precise voltage division while minimizing distortion and transient effects.

Damped Capacitive Voltage Dividers offer high dynamic behavior and are suitable for measuring various impulse voltages, including lightning impulses (LI), switching impulses (SI), and AC voltages, meeting the requirements of IEC 60060-2 for accuracy and step response. Equipped with a mobile base for easy maneuverability, these dividers have a secondary unit at the bottom for connection to control and measuring systems. The CS models are ideal for voltages up to 1000 kV (LI), 750 kV (SI), and 300 kV (AC), while the CR models suit voltages up to 4000 kV (LI), 3300 kV (SI), and 1000 kV (AC).

Resistive Voltage Dividers are chosen for their superior response parameters and are primarily used to measure full and front-chopped lightning impulses (LI) or other impulse voltages with rapid wavefronts. They are preferred when additional capacitance in the test circuit is undesirable due to its effect on rise time.

Reference Voltage Dividers serve the purpose of comparative measurements during the calibration of measuring systems. Type RCZ reference dividers are specifically engineered for calibrating voltage dividers across AC, DC, full- and tail-chopped impulse voltages, with the flexibility to be utilized for standard measurements as well. On the other hand, Type RT reference dividers are tailored for calibrating voltage dividers intended for full and front-chopped lightning impulse voltages, while also being suitable for standard measurements.

Impulse System Options and Accessories

Chopping gaps are designed to chop or shape the leading and trailing edges of impulse signals to simulate specific types of transient voltage events that electrical equipment may experience during operation. By altering the waveform characteristics, chopping gaps enable engineers to replicate various scenarios such as lightning strikes, switching surges, or fault conditions for testing the resilience and performance of electrical components and systems.

KFS Horizontal & Vertical Sphere Gaps, constructed from highly polished copper with tight tolerances, come in horizontal or vertical designs. Horizontal models, like KFS H, handle chopping voltages up to 340 kV, while vertical models, such as KFS V, manage up to 715 kV. Adjustments are facilitated by precision scales and motorized drives controlled by the impulse generator control HVC 300.

The MAFS Multiple Chopping Gap, a patented Haefely design, efficiently chops lightning impulses, ensuring precise chopping times without waveform distortion. With an additional secondary unit, it can function as a voltage divider in select applications. Unlike rod gaps and sphere gaps, which draw pre-discharge current,s causing voltage drop, MAFS maintains waveform integrity. Its highly polished copper spheres with tungsten inserts reduce burn-off, and precise sphere distance adjustment is automated by the Haefely impulse generator control HVC 300. Models are available rated up to 3600 kV impulse voltage.

Low-Voltage Impulse Solutions

A recurrent surge generator is a specialized device used for simulating repetitive voltage surges or transients that can occur in power systems. The recurrent surge generator is designed to generate and reproduce these surges accurately in a controlled laboratory environment for testing the resilience and performance of electrical equipment and systems.

The Recurrent Surge Generator 482, a bench-top device, is akin to a low-voltage version of a high-voltage impulse generator. Its versatile applications range from testing models to analyzing voltage distribution in high-voltage windings under impulse voltage conditions and determining circuit parameters for impulse test setups. The device allows for quick and reliable testing without posing risks to personnel. By adjusting selector knobs, users can vary impulse circuit elements and waveform characteristics over a wide range.

RSG 482 Features

• Small, bench-top model (also rack-mountable) with 500 V output
• Easy access to control knobs and input/output jacks
• Various impulse waveforms can be created by easily adjusting series and parallel capacitance or resistance, and inductance
• Internal chopping with broad limits
• Analysis of impulse waveform on any oscilloscope
• Ideal for training purposes and R&D experimentation