The Chroma 19055 Series Hipot Analyzer is designed for Hipot test and analysis. It has a maximum output power of 500VA, a maximum AC output of 5kV/100mA, and Floating output for EN50191 requirements. (Please refer to the application notes for more detail information.)
The 19055-C includes ACW/DCW/IR tests, but also has a new measurement technology; Corona Discharge Detect ion (CDD). In addition, the Breakdown Voltage (BDV) Analysis can detect faults as follows;
- Corona discharge Start Voltage (CSV)
- Flashover Start Voltage (FSV)
- Breakdown Voltage (BDV)
The Chroma 19055 series also has both High Frequency Contact Check (HFCC) and Open Short Check (OSC) functionality. This increases the test reliability and efficiency significantly through application of the contact check during the Hipot test.
The Chroma 19055 series is equipped with a large LCD screen which is convenient for the user to operate and judge. In addition, a GFI human protection circuit and Floating output design prevents exposing users to electrical hazards.
Dielectric Withstand Test - Corona Discharge / Flashover / Breakdown Detection
What does dielectric withstand fail mean? Most of the regulations state: "During the test, no flashover or breakdown shall occur." Nowadays, the study of the insulation failure and the electrical discharge is very important for insulation materials and the high voltage components. Because the electrical discharge and insulation capability are interrelated, discharge level detection is not only a safety issue but also crucial to product quality. The electrical discharge can be categorized into 3 groups; Corona discharge, Glow discharge, and Arc discharge, each according to the material discharge characteristic.
When the voltage between two electrodes increases, the electrical field becomes stronger. If the strength of the electrical field; produced by the current, is greater than the ionization potential of air, there will be temporary ionization of the air near the surface of the insulation materials. When this ionization occurs, visible light is produced and the temperature rises around the discharge area. Long term Corona discharge and heat may cause a Qualitative Change of the material, insulation deterioration, and finally insulation failure. Corona discharge, which is shown in Figure 1, is a transient discharge with a high frequency which can be detected by using high frequency measurements.
▲ Figure 1: Corona Discharge
Glow Discharge and Arc Discharge
When a high voltage is applied to an insulation material, a portion of the material may have electrical discharges. The high voltage may make the insulation material lose its insulation capability and cause transient or discontinuous discharges. This can form a carbonized conductive path through the insulation, or damage the product. As Shown in Figure 2, Glow discharge or Arc discharge cannot be detected by monitoring only the leakage current, but can be detected by Flashover detection - which is monitoring the change rates of the test voltage or the leakage current. This can be used for sorting out defective products. Flashover detection is one of the most indispensable test items in electrical safety testing.
▲ Figure 2: Flashover Waveform
The Chroma 19055 Series provides Breakdown Voltage (BDV) Analysis, which includes Corona Discharge Detection (CDD, 19055-C only), Flashover (ARC) detection, and Breakdown detection. Breakdown Voltage (BDV) Analysis is the best tool for research & development and quality assurance testing.
▲ Figure 3: Discharge Level Analysis (DLA)
Breakdown Voltage (BDV) Analysis
The dielectric withstand voltage of the passive components depend on the insulation materials and the manufacturing processes. To improve the insulation capability, the discharge level, which combine Corona discharge, Flashover, and Breakdown, should be defined and analyzed. The Chroma 19055 series has added Breakdown Voltage (BDV) Analysis which allows users to program the start voltage, end voltage, test time, test steps, test limits and more for analysis.
Breakdown Voltage (BDV) Analysis has three levels of judgment. These are Corona limit, Flashover (ARC limit) and Breakdown (high limit). When discharge occurs during the test, the withstand voltage, which depends on the limits from the different levels, is determined through the Breakdown Voltage (BDV) Analysis. The withstand voltage represents Corona discharge Start Voltage (CSV) for Corona limit fail, Flashover Start Voltage (FSV) for ARC limit fail, and Breakdown Voltage (BDV) for high limit fail. R&D personnel are able to research and improve the insulation capability by collecting the test results from Breakdown Voltage (BDV) Analysis.
Contact Check - High Frequency Contact Check (HFCC) & Open Short Check (OSC, PATENT #: 254135)
High Frequency Contact Check (HFCC) is a new measurement technology for the contact check function. HFCC can be performed during the AC/ DC Hipot test. The HFCC test frequency, which is around 500 kHz, has greatly improved the accuracy of the contact check function and production efficiency.
The Open Short Check (OSC) function can check for any open circuit (poor connection) or short circuit (Shorted DUT) that occurs during the test. The DUT may not be judged correctly if an open circuit occurs during the test. Furthermore, a shorted DUT can be sorted out prior to damage to the fixture, saving test costs.
The capacitance (Cx) of a product (DUT) could be tens of pF to several μF under Hi-pot test in normal conditions. If the connection with the product is poor or the connecting cable is broken, a small capacitance (Cc; shown in Figure 4.2), which is formed in the gap between the surfaces of the poor connection, is usually lower than 10pF. This small capacitance (Cc) makes the equivalent capacitance (Cm) of the load lower than the normal value of the DUT capacitance (Cx). When the product is short or almost short, the equivalent capacitance (Cm) of the load is higher than the normal value of the capacitance (Cx). Thus the high/low limit of the capacitance (Cx) can be used to identify contact issues on the production line.
▲ Figure 4.1：Normal Condition
▲ Figure 4.2：Open Circuit Cm = Cc * Cx / (Cc + Cx) << Cx
▲ Figure 4.3：Short Circuit Cm >> Cx
Operator Protections – Floating Output & Ground Fault Interrupt (GFI)
The purpose of electrical safety testing is to protect users. In addition, the operators need to be protected by the test equipment during the test operations. The Chroma 19055 series has two types of protection mechanisms; Floating output and Ground Fault Interrupt (GFI).
For operators to manipulate the tester safely, Chroma has developed a brand new technical protection: Floating output. This protection complies with EN50191; Equipment Safety Standard. Because earth leakage current (iH) from the Floating output is less than 3.5mA, no matter which terminal the operator touches during Hipot test, the operator will not be injured by the electricity. (Shown in Figure 5)
Ground Fault Interrupt (GFI), developed by Chroma, is another human body protection for operators. The currents (i1 & i2) can be measured by the current meters (A1 & A2). When the current difference iH (iH=i1-i2), which is the difference between i1 and i2, is too large, the GFI protection cuts output power immediately to protect the human body (operators) from getting an electrical shock. (Shown in Figure 6)
▲ Figure 5: Floating output
▲ Figure 6: Floating output
The Chroma 19055 Series Hipot Analyzers have functions for Corona Discharge Detection (CDD) to detect Corona discharge (19055-C Only) and Breakdown Voltage (BDV) Analysis to find the CSV, FSV and BDV. These analyzers are capable of providing useful data for verifying product insulation capability and the reliability of the manufacturing process.
Corona Discharge Issue
Transformer: When the primary side of a transformer has poor insulation, Corona discharge occurs on the primary winding under normal usage (Shown in Figure 7.1). The insulation capability of the primary side will be decreased after Corona discharge has occurred over time. For instance, most power transformers reserve an auxiliary coil in the primary side for other circuits to use (Shown in Figure7.2). When the peak voltage (Vpk) between pin 1 and pin 5 is 750 volts, if the manufacturing process was bad (For example, bad taping with insulation tape or a bad wire sleeve), Corona discharge occurs continuously under usage. The insulation capability of the primary winding will be decreased, and eventually the power transformer will burn out due to the enameled carbonization.
▲ Figure 7.1：Corona discharge
▲ Figure 7.2：Primary winding fail cause insulation failure
Motor: Electric rotating machineries (For example, industrial motors, electric vehicle motors, etc.) are often used for a long time under environments with large variations in temperature and humidity, so high durability and reliability are required. Temperature and humidity are key factors that influence insulation capability. When Corona discharge occurs in turn-to-turn or turn-to-ground over time, it will cause the temperature to rise and the material quality to change, and lead to insulation deterioration. Adding Corona Discharge Detection (CDD) into the Hipot test improves the quality requirement on insulation, and also sorts out products with poor insulation to reduce defective rates due to long-term use.
▲ Figure 8：Corona discharge in motor
Discharge Issue for Capacitor, Photocoupler, Insulation Material: Breakdown Voltage (BDV) Analysis is often used to verify the withstand voltage for high voltage capacitors, safety capacitors, photo-couplers and insulation materials. When gaps or voids caused by manufacturing processes are produced in the insulation medium, different electric fields - which are formed inside the DUT during Hipot test - cause Corona discharge to occur. The insulation medium changes and the insulation quality issues appear due to long-term use.
▲ Figure 9：Void discharge