tiêu chuẩn và phương pháp thực hiện đo phóng điện cục bộ cáp lực cao áp bằng phương pháp điện áp điện áp cộng hưởng tần số tắt dần DACtiêu chuẩn và phương pháp thực hiện đo phóng điện cục bộ cáp lực cao áp bằng phương pháp điện áp điện áp cộng hưởng tần số tắt dần DACtiêu chuẩn và phương pháp thực hiện đo phóng điện cục bộ cáp lực cao áp bằng phương pháp điện áp điện áp cộng hưởng tần số tắt dần DAC
Overview
Background
This guide provides a description of the methods and practices to be used in the application of damped alternating current (DAC) voltages for field testing of shielded power cable systems
DAC voltage testing serves as an effective alternative to traditional AC voltage testing, making it suitable for various cable types, including medium-voltage (MV), high-voltage (HV), and extra-high-voltage (EHV) cables.
DAC test procedure has been used for several years for diagnostic, maintenance and acceptance
(commissioning) tests, it provides a method of evaluation of the insulation condition and helps to fill the need for more complete information on the condition of cable systems
This guide addresses DAC voltage testing in the frequency range from 20 Hz to 500 Hz [B12], [B14],
This guide outlines the methodology, voltage levels, and testing procedures essential for effectively using DAC voltages It also highlights important factors to consider for both withstand testing and other applications.
The numbers in brackets refer to the bibliography in Annex F, which details diagnostic tests For additional information on various field testing methods, please consult the omnibus standard, IEEE Std 400™.
Scope
This guide outlines the testing and diagnostic procedures for shielded power cable systems rated at 5 kV and above, utilizing DAC voltages It is applicable to all power cable systems designed for electric power transmission or distribution The specified test levels are based on the assumption that the cable systems feature either an effectively grounded neutral system or a grounded metallic shield.
Purpose
This guide aims to establish standardized practices for conducting off-line DAC voltage tests on installed shielded power cable systems in the field and to offer guidelines for interpreting the test results As some test parameters and procedures need further examination, this document serves as a foundational resource that can evolve and enhance over time with increased field experience and analysis.
Normative references
The referenced documents are essential for understanding and applying this document, with each citation explained in relation to its content For dated references, only the specified edition is relevant, while for undated references, the most current edition, including any amendments or corrections, should be used.
Accredited Standards Committee, C2-2012, National Electrical Safety Code ® (NESC ® ) 3,4
ASTM D150-11, Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation 5
IEC 60060-3, High Voltage Test Techniques—Part 3: Definitions and requirements for on-site testing 6
IEC 60270, High-voltage test techniques—Partial discharges measurements
IEC 60885-3, Electrical test methods for electric cables;—Part 3: Test methods for partial discharge measurements on lengths of extruded power cables
IEC 61230 Live working—Portable equipment for earthing or earthing and short-circuiting
IEEE Std 4™, IEEE Standard for High Voltage Testing Techniques
2 Information on references can be found in Clause 2
3 The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc
4 IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,
USA (http://standards.ieee.org/)
5 ASTM publications are available from the American Society for Testing and Materials, 100 Barr Harbor Drive, PO Box C700, West
Conshohocken, PA 19428-2959, USA (http://www.astm.org/)
6 IEC publications are available from the Sales Department of the International Electrotechnical Commission, 3 rue de Varembé, PO
Box 131, CH-1211, Geneva 20, Switzerland (http://www.iec.ch/) IEC publications are also available in the United States from the
Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA
IEEE Std 400.3™, IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field
IEEE Std 510™, IEEE Recommended Practices for Safety in High-Voltage and High-Power Testing
NFPA 70E, Standard for Electrical Safety in the Workplace 7
Definitions, acronyms, and abbreviations
Definitions
For the purposes of this document, the following terms and definitions apply The IEEE Standards
For terms not defined in this clause, consult the Dictionary Online A breakdown refers to disruptive discharge along insulation, leading to a failure of the applied voltage A cable system consists of one or more lengths of shielded power cables, rated at 5 kV and above, including accessories such as joints and terminations The charging time, t_c, in seconds, is the duration required at the maximum current, I_C max, to charge the test object's capacitance, C_TO, to the specified test voltage level, V_DAC The circuit's natural frequency, f_r, in hertz, is the reciprocal of the time between two successive peaks of the same polarity, determined by C_TO and L_C, typically expressed as f_r = 1 / (2π√(L_C C_TO)).
DAC excitation involves a comprehensive procedure for testing power cables This process consists of two main parts: Part 1 entails gradually increasing the test voltage to reach the predetermined maximum level, while Part 2 involves applying a damped sinusoidal oscillation that aligns with the circuit's natural frequency and specified damping factor.
A DAC voltage partial discharge (PD) test is a field assessment designed to gather crucial data regarding the existence and behavior of partial discharges within the tested cable section.
The DAC voltage withstand test procedure involves a systematic approach that includes a DAC voltage step phase, where DAC excitations are performed with increasing voltage levels, followed by a DAC voltage hold phase that consists of a series of consecutive DAC voltage excitations at selected voltage levels applied to the power cable under test The damped alternating voltage DAC starts from a maximum voltage level (either negative or positive) and features damped sinusoidal oscillations around the zero level Key characteristics of this voltage include the peak value (V DAC), the circuit's natural frequency (f r), and the damping factor (D f) The damping factor, expressed as a percentage, is calculated by taking the voltage difference between the first and second peaks of the same polarity and dividing it by the voltage value of the first peak.
7 NFPA publications are available from Publications Sales, National Fire Protection Association, 1 Batterymarch Park, P.O Box 9101,
Quincy, MA 02269-9101, USA (http://www.nfpa.org/)
The IEEE Standards Dictionary Online subscription can be accessed at http://dictionary.ieee.org/ Electrical trees are tree-like formations made up of non-solid or carbonized micro-channels that develop at stress points such as protrusions, contaminants, voids, or interfaces between water trees and dry insulation, particularly when subjected to prolonged electrical stress The presence of an electrical tree signifies irreversible damage to the insulation, and if the voltage stress exceeds the inception voltage, partial discharge occurs, leading to further growth of the tree and potential failure over time A hybrid cable system refers to a cable configuration that combines cables with significantly different dielectric or construction properties, such as extruded dielectric insulation cables alongside laminated insulation cables or cables with varying insulation types.
Insulation weak spots are areas within cable insulation systems where mechanical, chemical, or thermal stresses cause the insulation to fail before the rest of the system, potentially resulting in a breakdown at operating voltage, often referred to as a gross defect Laminated dielectrics are multi-layered insulation materials typically made from fluid-impregnated cellulose paper or polypropylene, commonly found in cable designs such as paper-insulated, lead-covered (PILC) cables and high-pressure pipe-type cables Partial discharge (PD) refers to localized electrical discharges that only partially bridge the insulation between conductors, generating high-frequency current pulses that propagate along shielded power cables Shielded cables consist of insulated conductors enclosed in a grounded conducting envelope, and various testing methods, including off-line testing, are employed to assess cable systems by disconnecting them from the service power source and energizing them with a separate field-test power supply.
From the application point of view, there are three categories of tests:
1) installation test: A field test conducted after cable installation but before the application of joints or terminations
2) acceptance test: A field test made after cable system installation, including terminations and joints, but before the cable system is put into normal service
3) maintenance test: A field test made during the operating life of a cable system b) From the technical point of view, there are three broad sets of tests:
1) diagnostic test: A field test made during the operating life of a cable system to assess the condition of the cable system and, in some cases, locate degraded regions that can result in a failure
2) monitored withstand test: A diagnostic test in which a voltage of a predetermined magnitude is applied according to a withstand test procedure of voltage step and hold phase
Throughout the testing process, various properties of the test object are observed to assess its condition and to decide whether to prolong or shorten the test duration.
3) non-monitored or simple withstand test: A diagnostic test in which a voltage of a predetermined magnitude is applied according to a withstand test procedure of voltage step and hold phase If the test object withstands the test it is deemed to have passed the test voltage level, V DAC , in peak kilovolts: This is the actual test voltage level of the DAC voltage which is equal to the actual selected test voltage level VT as it has to be generated by a DAC system
From a practical perspective, cable test specifications are provided in kVrms, allowing for the determination of V DAC as the value V T 2 in kVrms.
Acronyms and abbreviations
DAC damped ac voltage (for the purpose of this guide: 20 Hz to 500 Hz)
EVH extra-high-voltage f r DAC natural frequency
DF dissipation factor, also referred to as tan delta (tanδ )
MIND mass-impregnated, non-draining
N DAC number of DAC excitations
PDEV partial discharge extinction voltage, V e
PDIV partial discharge inception voltage, V i
PILC paper-insulated lead-covered
U n power cable rms rated voltage, phase-to-phase
U 0 nominal rms operating voltage, phase-to-ground
V T peak test voltage, phase-to-ground
9 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard.
Safety awareness
Ensuring personnel safety is paramount during tests involving hazardous voltage levels All cable and equipment tests must be conducted on de-energized and isolated systems unless explicitly authorized otherwise It is essential to adhere to appropriate safety practices, which include following user safety operating procedures, complying with IEEE Std 510 for high-voltage testing, and observing the National Electrical Safety Code (NESC) Additionally, NFPA 70E guidelines for electrical safety in the workplace and relevant national, state, and local safety regulations must be implemented to protect both utility and customer property.
High-voltage field testing of cable systems involves all of the factors normally associated with working on energized circuits, as well as several unique situations that must be addressed
When testing cable circuits, it's essential to ensure that all remote ends are cleared and guarded for personnel safety Additionally, reliable voice communication must be established between the test operator and all remote locations to facilitate effective coordination during the testing process.
The use of an energized circuit indicator or other suitable device may be used to indicate that the circuit is completely de-energized before application of safety grounds
When testing conductors near other energized lines, it is crucial to ensure sufficient voltage clearance to prevent flashover incidents Using test voltages that exceed the rated operating voltage can increase the risk of electrical arcs between conductors In situations where spacing is limited, implementing additional safety measures is essential to mitigate the potential for flashover.
When discharging cables after high-voltage testing, it is crucial to implement specific techniques to ensure personnel safety Cables possess significant capacitance and dielectric absorption characteristics, which can lead to hazardous charge buildups if they are not properly grounded for an adequate duration post-testing This risk arises from the prolonged time constant linked to dielectric absorption currents Therefore, adhering to the recommended grounding procedures outlined in relevant work rules is essential for preventing potential dangers.
Ensuring personnel safety is crucial during testing procedures, with all cable and equipment tests conducted on de-energized and isolated systems unless explicitly authorized otherwise Adhering to established safety practices is essential and encompasses a range of additional requirements.
Prior to testing, determination of particular safe clearances must consider both the test voltage and voltage of nearby energized equipment:
When isolating a cable circuit using a switch or disconnect device, it's crucial to verify with the manufacturer that the device can withstand the DAC test voltage while ensuring safe voltage isolation during normal operation Key considerations include: monitoring the discharge of long cables, which may take significant time; ensuring adequate clearance distance for isolation when using an air gap; adhering to proper Blocking/Lock-out/Tag-out procedures to prevent unauthorized operation; and wearing appropriate personal protective equipment (PPE) for safety.
While testing, one or more cable ends will be remote from the manned testing site, therefore, before testing begins:
Cable ends under test must be cleared and cordoned off
Cables that are de-energized should be grounded when not being tested
Remote cable ends must be marked to indicate that a high-voltage test is in progress
At the conclusion of high voltage testing, attention should be given to the following:
Discharging cables and cable systems including test equipment
Grounding requirements for cables and test equipment to eliminate the after effects of recharging the cables due to dielectric absorption and capacitance characteristics
Cable systems are deemed de-energized and grounded when both the conductor and metallic shield are securely connected to the system ground at the test site, and ideally, at the cable's far end as well.
For effective testing, it is essential to establish a single system ground at the test site, ensuring that the shield or sheath of the cable being tested is securely connected to this ground If this connection is absent, damaged, or removed, it should be promptly replaced Additionally, all test instrument casings must be linked to the system ground via a safety ground cable, and all exposed conductive parts of the test system should be bonded to a common ground point Given that DAC test instruments operate at high voltage, an external safety ground cable must be utilized to ground the cable safely, capable of handling the system's fault current Once the DAC test equipment's test lead is connected to the cable, the safety ground can be detached, allowing testing to begin.
Should a local ground be advisable or recommended for the test equipment, case ground must remain connected to system ground in order to maintain an acceptable single ground potential
Ensuring that all ground connections are secure and cannot be accidentally disconnected is crucial It is advisable to use grounding connections that can be tightly fastened Additionally, utilizing portable ground clamps and grounding assemblies that comply with IEC 61230 standards is recommended for optimal safety and reliability.
Figure 1 —Recommended DAC safety connection diagram
DAC testing
General
Sinusoidal damped ac (DAC) voltage testing, also known as oscillating wave testing, was introduced at the end of the 1980s as an alternative to DC test voltages [B3], [B4], [B5], [B6], [B12], [B13], [B15], [B19],
Since the late 1990s, DAC testing has emerged as a valuable method for onsite assessment of power cable systems, driven by advancements in power electronics and signal processing Currently, several countries utilize DAC for onsite testing, incorporating partial discharge (PD) measurements and dissipation factor (DF) estimation to evaluate the condition of various power cable systems.
DAC voltages are produced by charging a test object to a specific voltage level and subsequently discharging its capacitance through an appropriate inductance The charging phase involves applying a steadily increasing voltage to the test object's capacitance, influenced by both its capacitance and the power supply's current rating In the discharging phase, a DAC operates at a frequency that is determined by the test object's capacitance and the inductance used.
Figure 2 —Schematic overview of the three stages of one DAC excitation
The maximum DAC voltage level is determined by the voltage peak value V T in kilovolts or the voltage rms value V T / √2 in kilovolts
Most current applications for Diagnostic Assessment of Condition (DAC) utilize a combination of voltage withstand testing and advanced diagnostic measurements, such as partial discharges (PDs) and dielectric losses (DF) In voltage withstand tests, a specific number of DAC excitations are systematically applied to evaluate the system's performance.
The withstand test for power cables utilizes damped sinusoidal AC voltage excitations, with the test duration based on the number of DAC excitations applied at a chosen DAC test voltage The maximum DAC withstand voltage is defined by the peak voltage value (VT) in kilovolts or the root mean square (rms) voltage value (VT/√2) in kilovolts of the initial DAC cycle.
[B78], [B79], and [B87], the major advantages and disadvantages of DAC testing can be summarized as follows: a) Advantages:
1) The DAC voltage withstand test by applying a defined number of DAC excitations gives the possibility to produce a breakdown or to initiate PD occurrences in insulation defects [B4],
2) Can give the possibility to detect various defects in the insulation that will be detrimental to the cable system under service conditions, without creating new defects or causing any significant aging of healthy insulation [B18], [B20], [B69], [B81]
3) Gives PD patterns and parameters similarity between the results of DAC tests and continuous power frequency (50/60 Hz) factory tests [B3] [B4], [B15], [B86], [B87]
4) Has low system complexity, is lightweight, and is easy to handle and operate
5) Requires relatively low input power of the DAC test equipment for testing long lengths of cable b) Disadvantages:
1) Due to the charging and decaying characteristic of the voltage, withstand and breakdown
DAC test results can be different from those obtained by continuous ac withstand voltage testing, especially in the case of presence of PD activity (typical for inhomogeneous insulation defects) [B32]
2) The use of fixed inductors with different cable capacitances results in variation of DAC frequency
3) To keep the DAC frequency in the range of 20 Hz – 500 Hz for the case of very short cable lengths, an additional capacitive load is necessary
4) Due to the fact that the recommended test voltages and durations for tests (given in this document for DAC testing, see Annex A) are based on field-experiences as obtained by different users of the DAC technology further data collection and evaluation are necessary and should be part of the next revision process of this document
5) The decay of the DAC voltage depends on the actual dielectric loss behavior of a particular cable section
6) The charging time varies as it is dependent on the cable capacitance, the test voltage level V T and the charging current of the power supply.
Types of DAC testing
Figure 4 illustrates the various DAC field test options for different cable systems Depending on the testing objectives, several approaches can be considered One key option is the DAC acceptance test, which allows for the evaluation of newly installed cable systems using 50 DAC excitations at specified voltage levels, as outlined in Table A.1 Recent practices show that most DAC acceptance tests are conducted with partial discharge (PD) detection As this application continues to evolve, the effectiveness and verification of the DAC voltage withstand test are supported by references [B4] and [B15].
Over the past decade, practical experience has proven valuable, yet additional data collection and evaluation are essential to refine the technique This accumulated knowledge should be incorporated into the upcoming revision of the document.
Before conducting the DAC acceptance test, installation testing can be carried out on new cable sections to ensure installation quality based on user requirements After a cable is terminated or jointed into the overall system and ready for service, installation tests should be replaced by acceptance or maintenance tests For DAC maintenance testing, cable systems in service or following repairs can be assessed using 50 DAC excitations, as outlined in Table A.2, with most tests being monitored through partial discharge (PD) detection and dielectric frequency (DF) measurements Additionally, DAC diagnostic testing allows for periodic condition assessments of in-service cable systems, utilizing PD and DF measurements according to the voltage levels specified in Tables A.1 and A.2, as well as the user's testing procedures.
Figure 4 —General overview of DAC field test possibilities for different testing goals of cable systems.
DAC test circuit and parameters
Overview
To generate DAC voltages, different types of test circuits can be applied [B5], [B9], [B16], [B21], [B29],
The basic DAC voltage generation circuit, illustrated in Figure 5, comprises a high-voltage (HV) source that produces an increasing unipolar voltage, an HV inductor, a capacitive test object, and an appropriate HV switch The capacitive test object may consist of various components, such as power cables or generators, modeled as a lumped capacitor for simplification When the unipolar charging voltage reaches its maximum value (VT), the HV switch is activated, leading to a damped alternating voltage across the capacitive test object The damping factor is influenced by the loss characteristics of both the test circuit and the test object The DAC's natural frequency (fr) is determined by the values of the HV inductor and the capacitance of the test object; if the capacitance falls below a certain threshold, the natural frequency may exceed acceptable limits, necessitating the addition of an HV storage capacitor in parallel with the circuit.
Figure 5 —Schematic overview of a basic DAC test circuit with monitoring:
(a) circuit-charging phase and (b) LC-oscillation phase In case of monitored test, such parameters as PDs and DF can be measured.
DAC test voltage circuit
The fundamental principles of DAC test circuits are illustrated in Figure 5, while Section 6.3 outlines the key parameters for characterizing these circuits The comprehensive process of generating DAC excitation involves three distinct phases, as depicted in Figure 2.
In this phase, the test object is subjected to progressively increasing unipolar voltage, either negative or positive The duration of charging is influenced by the maximum load current available from the voltage supply, the applied test voltage, and the capacitance of the test object.
According to Kreuger (1995), continuous voltage increase until the HV switch is triggered prevents the formation of DC stresses and steady-state conditions in the tested cable Consequently, space charges are less likely to develop in the cable insulation unless the frequency is below 0.01 Hz and the electric stress exceeds 10 kV/mm Research by Dissado et al and Takada indicates that trapped space charge is frequency-dependent, occurring below 0.01 Hz under an applied electric field In contrast, when pure HVDC stress is applied to insulation compared to DAC, the initial voltage distribution is capacitive, gradually transitioning to a resistive distribution, characterized by the time constant of typical XLPE insulation, which is determined by its permittivity and volume resistivity.
In a theoretical scenario involving pure HVDC stress at a constant voltage, the transition time constant would exceed 33 minutes However, the charging phase duration of the DAC remains significantly shorter than this time frame, as indicated by the test voltage levels outlined in Tables A.1 and A.2.
During DAC withstand tests, the electric fields remain below critical values, ensuring that only alternating current (AC) field stresses affect the cable To mitigate potential side effects from unipolar excitation time and space charge development, it is advisable to limit the excitation time to under 100 seconds If this threshold cannot be achieved, increasing the charging supply current can help shorten the charging duration Alternatively, employing a bipolar charging procedure, which involves using both positive and negative voltages with an appropriate high-voltage source, is recommended.
Once the unipolar charging voltage reaches the predetermined maximum DAC test voltage (V T) at a specific voltage ramp rate (dU/dt), the high-voltage switch activates almost instantaneously, typically in under 1μs This rapid switching is crucial to prevent over-voltages and interference during partial discharge (PD) measurements The interplay between cable capacitance and system high-voltage inductance creates an LC oscillating circuit, where the peak DAC current in the circuit is determined by the capacitive load, system inductance, and maximum test voltage.
The frequency of the DAC test voltage equals the natural frequency of the circuit
The DAC attenuation is influenced by the quality factor of the entire resonant circuit, which is inversely related to the circuit's losses These losses encompass the high-voltage switch, the cable system, and the test set inductor.
Due to the symmetrical bipolar ac discharging process, no remaining charges will be introduced to the cable insulation during the oscillating phase
Due to the low inductance of the cable and the absence of transient over-voltages at joints and terminations, the DAC stress on the cable capacitance reflects typical AC stress conditions Consequently, the PD inception voltage (PDIV) is established under these normal operating circumstances.
PD amplitudes, and the PD behavior are comparable to factory testing conditions, and the following references show that a DAC stress is similar for inhomogeneous insulation defects [B54], [B69], [B86],
Applying DAC stress to PD-free insulation yields effects comparable to those observed during factory testing, influencing the pass or fail results of the evaluation.
The DF can be estimated with the decay characteristics of the DAC wave, as shown in Houtepen, et al
[B34], IEEE Std 386™ [B44], and described in Annex B
The measurement of the test voltage should be made with an approved measuring system as described in
According to IEEE Std 4 and IEC 60060-3 standards, the peak value of the test voltage must be measured with an overall uncertainty of ± 5% Additionally, the measuring system's response time should not exceed 0.5 seconds.
DAC parameters
When conducting a DAC test on a power cable circuit, it is essential to consider various parameters associated with the test circuit, procedure, data acquisition, and analysis Understanding these parameters is crucial for establishing the test conditions and ensuring accurate evaluation of the test data for the specific test object.
When evaluating DAC test circuits, it is essential to consider various parameters that define the overall configuration Annex C outlines the critical parameters necessary for determining the specific setup of the test circuit.
The voltage parameters, including the selected test circuit, type and voltage rating of the test object, and applicable test procedures, are detailed in Annex C, which also outlines the fundamental parameters for characterizing the DAC voltage.
Test parameters: Performing a DAC test in accordance with the recommended test parameters can be used to describe the test process (See Annex C)
Performing a DAC test yields various evaluation parameters that are essential for assessing test results These parameters vary based on the test type—such as withstand, monitoring, or diagnostic tests—and offer valuable insights into the outcomes of the tests conducted (Annex C).
Figure 6 —Schematic overview of four different situations of DAC voltage withstand tests:
(a) during selected number of NDAC excitations (dotted lines), no breakdown occurred;
During specific NDAC excitations, no breakdown was observed, although partial discharge (PD) levels exceeded the background noise Additionally, both PD and breakdown events occurred prior to reaching the DAC withstand test level and before the designated number of NDAC excitations was applied.
DAC voltage withstand testing
General
A DAC voltage withstand test involves applying a specific number of DAC excitations at a voltage level typically exceeding the rated voltage This testing method is applicable to all types of cables and accessories, focusing on the insulation's breakdown or non-breakdown results However, in a non-monitored withstand test, there is no insight into how the test impacts the insulation system.
Figure 7 —Schematic overview of DAC voltage application procedure,
PD detection can be performed during step-an-hold phase (monitored withstand test)
The tests can be further subdivided into the following two classes:
The non-monitored DAC withstand test involves applying multiple DAC excitations to evaluate the system's capacity to maintain the maximum DAC voltage without experiencing breakdown, as illustrated by the dotted lines in the accompanying figures.
The purpose of a withstand test is to intentionally induce existing dangerous defects to failure under controlled voltage application, minimizing fault current and ensuring that no system or customers are affected during the process If a failure occurs, it must be identified, repaired, and retested, with outcomes categorized as either "pass" or "fail." Unlike continuous voltage AC withstand tests, this method does not directly correlate the number of DAC excitations to specific voltage levels, meaning that a predetermined number of DAC excitations may not reliably trigger a defect breakdown in a cable.
The monitored DAC withstand test involves applying various DAC excitations while measuring additional attributes to assess the cable's performance The test determines pass or fail status based on these measurements, indicated by black dotted lines for DAC voltage and gray dotted lines for partial discharge (PD) measurement.
Figure 6.) These additional attributes are advanced diagnostic properties, such as PD detection
Temporal stability of the measured property can also be used to monitor the effect of the test on the cable system during voltage application
Although some users perform simple non-monitored withstand testing (see the survey in Annex E), the recommendation is to perform testing monitored by PD detection and recording (see Figure 5)
Monitoring insulation properties through PD detection, along with analyzing the impact of test voltage on diagnostic parameters during a DAC withstand test, enhances the assessment of insulation condition.
For effective testing of shielded power cable systems, it is crucial to maintain consistent voltage levels and the appropriate number of DAC excitations according to the test's purpose The quality and reliability of these systems hinge on two key aspects during field tests and the evaluation of results.
When selecting DAC test parameters, it's crucial to minimize the potential reduction in service lifetime caused by field tests For withstand tests, the intensity must be sufficient to induce a breakdown in defective insulation or surpass a critical monitored property level, while avoiding any degradation of the insulation itself.
The voltage level, number of DAC excitations, and duration of these excitations are critical factors influencing both the testing and post-testing performance of cable circuits Recommended test voltages and excitation counts, as outlined in Annex A, stem from field experiences of various DAC technology users Deviating from these recommended values by arbitrarily increasing voltage or the number of excitations may heighten the risk of premature failures during service.
For quality acceptance tests on new cable systems, the required withstand voltage for routine tests in factory shall be considered adequate.
DAC test parameters and procedures
For DAC voltage testing, five key parameters are essential: the maximum DAC test voltage level (V [kV] T peak), the number of DAC excitations at specified voltage levels, the DAC frequency measured in hertz, the DAC damping percentage, and the DAC charging time in seconds.
Depending on the test objective, a DAC test may consist of two phases (Figure 6 and Figure 7):
During the DAC voltage step phase, the voltage is incrementally raised to a predetermined maximum test voltage in specified steps of ΔV, such as 0.2 U0 Each voltage level is subjected to a minimum of five DAC excitations (N DAC − Step) For further details, refer to Annex A.
During the DAC voltage hold phase, a selected test voltage V [kV ] T peak , for a number of DAC excitations, N DAC − Hold, e.g., NP, is applied to the test object ([B14], [B16], [B45], [B58])
The DAC frequency is influenced by the high voltage inductor (L C) of the testing system and the capacitance of the test object (C TO) Typically, the DAC frequency can be calculated using the formula f r = 1 / (2π L C C TO), in accordance with IEC 60060-3 standards.
DAC damping factor ,D f , is determined by the voltage difference between the first and second peak of same polarity, divided by the voltage value of the first peak.
DAC evaluation criteria
When applying DAC testing for acceptance and/or maintenance testing the following are two possible outcomes:
The complete installation and/or repair have been successfully done, and the cable section is approved and can be used for network operation
The complete installation and/or repair have not been successfully done and the cable section has to be repaired or needs further investigations
For the case of non-monitored DAC voltage withstand test, the evaluation is based on two outcomes:
Pass, in the case of no breakdown during the test
Fail, in the case of a breakdown during the test
For the case of monitored DAC voltage withstand test the evaluation is based on four outcomes (see
For the case of a diagnostic test, the evaluation is based on an application of the knowledge rules (if available) and the asset management (frequently the user’s) expectations.
PD measurement using DAC
General
Regarding PD detection, refer to IEEE Std 400.3 for more information about test application Regarding
Partial Discharge (PD) detection using DAC testing is primarily guided by standards such as IEC 60270, IEC 60885-3, and IEC 62478, along with references from Agoris and Cigré These standards are essential for identifying onsite PDs in external energized power cables Typically, one or more PD detection units are connected to the cable terminations or joints Since DAC testing operates on the principle of voltage excitations rather than continuous monitoring, it is crucial to ensure proper synchronization between the DAC voltage source and the PD detection unit for accurate results.
PD detection unit PD detection
PD detection unit unit PD detection unit triggering/synchronization
Figure 8 —Principles of offline PD detection methods as connected to a power cable and its accessories during DAC testing
By applying PD detection during DAC, discharging activity can be detected in both cable insulation and cable accessories (see Figure 8)
Figure 9 —Example of PD patterns at DAC test voltages [B86]: (a) 2D pattern with the PD occurrence q versus the test voltage U on identical time base and (b) 3D pattern with the
PD occurrence represented in discharge magnitude, phase angle, and number of PDs (by grey scale) as generated up to a sselected maximum test voltage level for a number of
PD characteristics
DAC testing is based on single voltage excitations As a result, the PD pattern as generated during one such
DAC excitation (see Figure 9) consists of information about the following:
PD changes as a function of the decaying voltage
DAC testing relies on single voltage excitations, with the determination of Partial Discharge Inception Voltage (PDIV) and Partial Discharge Extinction Voltage (PDEV) derived from the analysis of stored and recorded partial discharge patterns.
For all other PD parameters suitable for onsite PD detection, the references IEC 60270, IEC 60885-3, and
IEEE Std 400.3 can be used
The principles applicable to estimation of the PDIV and PDEV during DAC testing are explained in
Figure 10 —Principles of the determination of PDIV and PDEV voltages during DAC testing
[B86] The PDIV voltage level of the DAC excitation at which the first PD activity has been observed.
PD Evaluation criteria
Utilizing DAC testing for partial discharge (PD) detection yields distinct DAC-specific PD patterns Relevant guidelines for PD detection and parameters can be found in IEEE Std 400.3, particularly in Annex D For additional insights into PD evaluation, refer to Annex D for comprehensive information.
DF (tan δ ) estimation using DAC
General
The dissipation factor (DF) of insulation material can be assessed through a dielectric absorption current (DAC) test, which analyzes the attenuation of the voltage wave applied to the cable, as indicated by its decay characteristics.
According to ASTM D 150-2004, IEC 60141-1, IEC 60141-3, and IEEE Std 1425™, the dissipation factor (DF) of cables increases with aging, making DF measurement a valuable diagnostic tool Additionally, cable capacitance can be determined through a DAC test at a specific test voltage frequency, utilizing the resulting natural frequency and the known air core inductance Furthermore, the dissipation factor (tanδ) can be estimated from the decay characteristics of the damped sinusoidal voltage wave.
Figure 11 —DF estimation during DAC testing; schematic examples of DAC voltage waves as observed for power cables with (a) low DF and (b) higher DF
To accurately estimate the cable losses based on the decay characteristics of the DAC wave, it is essential to understand the losses within the test and measurement circuit, specifically those arising from the air-core inductor.
The high-voltage switch test object can be modeled as a resistance in parallel with a capacitance The resistance of the test object can be determined using the formulas provided in Annex B, allowing for the calculation of the load distribution factor (DF).
Figure 11 illustrates two distinct DAC voltage waveforms: (a) depicts a waveform from a test object with minimal dielectric losses, estimated at less than 0.1% for a newly installed polymeric power cable, while (b) presents a waveform with higher dielectric losses of 0.5%, typical for a service-aged oil-impregnated power cable.
Various DAC voltage waves corresponding to several tanδ values can be seen in Annex B
Table 1 —Examples of discrete DF values are shown representing a non-aged condition of oil-filled (OF) cable insulation Typical accuracy for DF is 0.01%
Test voltage DF phase yellow DF phase red DF phase blue
DF Parameters
Performing DF measurement on a cable sample results in a single DF value in percent, expressed as
N DF × − (with N DF a number for the DF) where the relation is 0.1% 10 10= × − 4 According to
Houtepen, et al [B34], the measuring threshold value of the measurable DF at DAC is 0.1%
Table 1 gives examples of discrete DF values representing a non-aged condition of oil-impregnated cable insulation.
DF Evaluation Criteria
In general, there are no established guidelines for interpreting dielectric loss (DF) values in cable systems, except for the recommendations provided in IEC 60141-1 [B37], which outlines maximum allowable DF values for various insulation types Due to the complexity of cable systems and their numerous components, it is essential to assess the differences in dielectric losses for each type of cable insulation individually This evaluation is crucial for ensuring optimal performance and reliability in electrical applications.
Comparison with other cables (e.g., of the same design and vintage within the same location)
Values when new, or changes as monitored during periodic inspections or under different test conditions, e.g., electrical stresses, different temperatures
Determining acceptable dielectric loss factor (DF) values for cable systems is complex, as these values are influenced by both the quality of the cable system and the technologies of the cables and accessories used Understanding the specific cable and accessory technologies is essential for making accurate comparisons Additionally, since DF is inherently frequency-dependent, it is important to maintain consistent test voltage frequencies when using direct alternating current (DAC) voltage for DF comparisons.
The assessment of condition can be enhanced by analyzing both the absolute value of DF and its increment (Δ DF) at two specific voltages Additionally, the variability of DF values over time at a constant voltage, which can be quantified using standard deviation or interquartile range, provides valuable insights for evaluation.
To improve the effectiveness of the dielectric frequency (DF) test for assessing cable degradation, it is essential to monitor DF values over time during repeated diagnostic tests to identify trends A notable rise in DF values compared to earlier measurements may signal insulation degradation.
Data obtained from DAC tests at various voltages and frequencies ranging from 20 Hz to 500 Hz can be effectively compared According to Houtepen et al [B34], these estimations provide valuable insights into the performance characteristics of the tested systems.
The calibration procedure for the DAC system is conducted at various DAC frequencies and testing voltage levels Within the calibrated ranges, the values can be compared and deemed similar Consequently, the acceptable DF limits can be utilized for comparison against traditional constant voltage (AC) standards.
The schematic in Figure 12 illustrates the dielectric factor (DF) measurements at various DAC test voltages, comparing adjacent phases (A, B, C) The analysis reveals that all phases exhibit consistently low DF values, with no significant variation in DF as the test voltage increases.
The schematic overview in Figure 13 illustrates the dielectric factor (DF) measurements across various DAC test voltages by comparing adjacent phases (A, B, C) The analysis reveals notable differences among the specific phases, indicating that not all phases exhibit low DF values.
The schematic overview of dielectric frequency (DF) measurements at varying DAC test voltages reveals no significant differences among the adjacent phases (A, B, C) All phases exhibit elevated DF values, demonstrating a notable increase in DF as the test voltage rises.
The dielectric loss factor (DF) should show minimal variation across different voltage levels for quality cables Notably, the DF of oil/paper insulation tends to decrease with increasing voltage but is more sensitive to temperature compared to extruded insulations An increase in DF at higher voltages may signal issues such as severe partial discharge in paper-insulated lead-covered (PILC) cables or water treeing in extruded dielectrics Additionally, monitoring DF over time at a constant voltage can reveal water treeing, while a decrease in DF may indicate moisture issues in accessories Variations in DF can be as slight as 0.1%, and results should be documented to facilitate further analysis, including trend tracking and comparisons with adjacent cable lengths.
New oil-filled (OF) insulation demonstrates low dielectric losses at power frequencies, with a dissipation factor (DF) not exceeding 0.2% Furthermore, when the test voltage is raised from 0.5 U0 to 2.0 U0, the DF increase should remain below 10×10^−4 Aging effects on the insulation must also be monitored to ensure performance stability.
When the dielectric strength (DF) exceeds 50 10× − 4 (0.5%), there is a significant risk of thermal breakdown in the insulation This breakdown occurs due to various energy dissipation phenomena that take place when voltage is applied to the insulation.
Aging processes in oil-filled (OF) insulation lead to higher dissipation factor (DF) values, making DF a potential indicator of degradation in OF cables As the cables age, the DF tends to increase, signaling a decline in insulation quality.
DF measurements performance on OF cables, the degradation assessment of OF insulation can be made
[B10], [B21], [B22], [B37], [B49], [B57], [B58], [B61], [B70], [B77] DF historical information gathered during maintenance testing can be helpful to give operational recommendations to the cable owner.
Conclusions
DAC testing offers an innovative approach for the evaluation and diagnosis of various distribution and transmission power cable systems This method involves charging and short-circuiting the test object through an inductor, followed by the analysis of damped free oscillation, which is expected to occur within the natural frequency range of 20 Hz to 500 Hz.
This guide provides an overview of DAC non-monitored (simple) and monitored withstand testing, along with diagnostic field testing for installed shielded power cable systems across voltage classes ranging from 5 kV to 230 kV.
Non-monitored and monitored [tangent delta (DF), differential tangent delta, and PD] withstand tests at
DAC are used as diagnostic tools to assess the condition of cable systems
Annex A provides tables of test voltage levels for acceptance and maintenance tests on cable systems up to 230 kV, based on practical experience Recent practices show that most DAC acceptance tests utilize partial discharge (PD) detection for monitoring As this application continues to evolve, further data collection and evaluation are essential to verify the effectiveness of DAC voltage withstand testing, and these efforts should be included in the next revision of this document.
This article explores the benefits and drawbacks of Direct Access Cable (DAC) testing for cables and accessories, highlighting ongoing developments in DAC voltage testing methods and other related testing techniques.
DAC test voltage levels and test procedures
Experience has demonstrated that users typically select applied DAC test voltage levels in alignment with international standards for AC voltage testing, specifically referencing IEC 60502-2, IEC 60840, and IEC 62067 This practice reflects a consensus based on actual feedback and recommendations from various sources.
The number of DAC excitations chosen aligns with those used in previous studies by Aucourt and Farneti et al (1989, 1990) However, due to limited experience, additional verification is required for cables operating above 150 kV.
Table A.1—DAC test voltage levels (20 Hz to 500 Hz) as used for DAC testing
(50 DAC excitations) of new installed power cables Power cable rated voltage U [kV] phase-to-phase U 0 [kV] DAC test voltage level V T [kV peak ] phase-to-ground
Table A.2—DAC test voltage levels (20 Hz to 500 Hz) as used for maintenance testing
(50 DAC excitations) of repaired/refurbished power cables Power cable rated voltage U [kV] phase-to-phase U 0 [kV] DAC test voltage level V T [kV peak ] phase-to-ground
* Test voltage levels given above 230 kV are provisionally and given for study purposes only
The DAC test procedures for acceptance tests and maintenance tests are the following IEC 60060-3, [B3],
Step 1 According to the cable voltage rating select the DAC maximum test voltage level, V T , in peak kilovolts, see Table A.1 and Table A.2
Step 2 For the DAC voltage step phase, fix the test voltage steps, ∆V T , in peak kilovolts or in the unit of
Step 3 Fix the number of excitations N DAC to be applied for the DAC voltage step phase N DAC , e.g., five per each, ∆V T , test voltage step
Step 4 Fix the number of excitations, N DAC , to be applied for the DAC voltage hold phase, e.g., 50
To ensure safe maintenance testing of aged cable systems, it is essential to minimize the number of DAC excitations to the least amount necessary for obtaining valuable diagnostic results This approach helps prevent the potential initiation of electrical trees and allows for informed decision-making regarding any necessary actions before incipient failures occur.
Step 5 According to the parameters as defined in Step 1 through Step 3, start the test procedure of the
DAC voltage step phase (see also Figure 7)
Step 6 For a PD-monitored test, evaluate the PDIV In the case of PD occurrence, evaluate the harmfulness of the PD source, e.g., external or internal, and take the appropriate action
Step 7 After the DAC maximum test voltage level, V T , has been achieved, start the DAC voltage withstand test according to the parameters of Step 1 and Step 4
Step 8 For a PD monitored test, evaluate the PD inception For a PD occurrence, evaluate the harmfulness of the PD source, e.g., external or internal and take the appropriate actions
Step 9 Evaluate the results of the test in accordance with Figure A.1 and in 7.3
Breakdown during DAC voltage withstand test
No breakdown during DAC voltage withstand test
Breakdown during DAC voltage withstand test
No breakdown during DAC voltage withstand test
Case: large homogenous defect(s) Case: weak-spot(s)
Case: large inhomogenous defect(s) Case: inhomogeneous defects
Breakdown during DAC voltage withstand test
No breakdown during DAC voltage withstand test
Breakdown during DAC voltage withstand test
No breakdown during DAC voltage withstand test
Case: large homogenous defect(s) Case: weak-spot(s)
The evaluation criteria for partial discharge (PD) monitored dielectric withstand tests of high-voltage (HV) power cables indicate that both new and maintained cable circuits must be free from PD Even in the absence of breakdown, any detected PD in the cable necessitates its rejection.
The DAC test procedure for a diagnostic test is:
To determine the appropriate DAC maximum test voltage level (V T) in peak kilovolts, refer to the cable voltage rating outlined in Table A.2 Additionally, based on the maintenance and operational history, adjust the maximum test voltage to a level that anticipates no adverse effects from previously consumed lifetime.
Step 2 For the DAC voltage step phase, fix the test voltage steps ∆V T in in peak kilovolts or in the unit of U 0 ,e.g., 0.2 U 0
Step 3 Fix the number of excitations N DAC to be applied for the DAC voltage step phase N DAC , e.g., five per each ∆V T test voltage step
Step 4 According to the parameters as defined in Step 1 through Step 3, start the test procedure of the
DAC voltage step phase (see also Figure 7)
Step 5 For a PD monitored test, evaluate the PDIV In case of PD occurrence, evaluate the harmfulness of the PD source, e.g., external or internal, and take the necessary actions
Step 6 For DF measurement at selected DAC test voltage levels the actual DF values have to be determined
Step 7 Evaluate the results in accordance with 7.3, Clause 8, and Clause 9
DF estimation for DAC voltages
The DF represented by tanδ parameter is an important diagnostic parameter to assess the condition of oil- impregnated paper insulation [B10], [B18], [B20], [B22], [B27], [B37], [B38], [B44], [B48], [B49], [B57],
The primary sources of losses in insulation systems include conductive losses due to the finite bulk resistance and leakage currents, friction between dipoles and the insulation material leading to polarization losses, and local field enhancements at interfaces of materials with differing permeability Additionally, the presence of partial discharge (PD) activity can further elevate the dissipation factor (DF) in these systems.
For healthy paper oil insulation at power frequencies around tens of hertz, the dissipation factor (DF) should be low, ideally below 0.2% (20 x 10^-4), and exhibit minimal dependence on electric stresses, remaining under 0.1% as the test voltage increases from 0.5 U0 to 2.0 U0 Values exceeding 0.5% DF at nominal voltage, particularly when combined with aging effects and temporary thermal over-stresses, could indicate an increased risk of thermal breakdown.
An increase of electric stress above PDIV accompanied with high intensity PD activity, e.g., PD-levels in the range of nano-Coulombs [nC] may also increase dielectric losses
Figure B.1 shows a circuit model for the DAC system, essentially a series RLC circuit
Figure B.1—DAC circuit model that can be used for the DF estimation
R L denotes the total internal resistance of the system, which changes with the amplitude and frequency of the applied voltage R C indicates the losses within the test object, while L C represents the inductance of the air-core inductor.
C TO is the capacitance of the test object It should be noted that C TO and R C may also be a function of voltage and frequency
The decay characteristics of the oscillating voltage in a DAC test setup can be utilized to measure the DF, as demonstrated in the studies by Houtepen et al and Wester et al (2007).
The DAC voltage waveform depicted in Figure B.2 illustrates the test system configuration with an inductance of 2 H and a capacitance of 1 µF, highlighting variations in dielectric loss factors A detailed view of the eighth DAC cycle on the right side emphasizes the differences in damping characteristics.
A DAC voltage wave can be described by
U 0 is the voltage at the end of the unipolar voltage charging time β is the attenuation coefficient ϕ is the phase shift
2 f ω= π tanδ can be calculated from the decay characteristics of the wave
To estimate the dissipation factor (DF) of the test object, it is essential to understand the losses in the DAC test setup, which can be quantified as a resistance \( R_L \) in the entire DAC test circuit, excluding the test object's losses The losses associated with the test object can be modeled as a parallel resistance \( R_C \) This resistance \( R_C \) can be calculated numerically using measurement results and the formulas provided by Houtepen et al or Wester et al (2007).