What Is Tan δ, Or Tan Delta?

Insulating media are categorized into three main types: gaseous, solid, and liquid, dissipation factor (or Tan Delta) refers to the energy loss caused by the active power dissipation within the insulating medium of electrical equipment under an electric field. Dissipation is usually represented by the loss factor (tan δ), which reflects the phase difference between the leakage current and the capacitive current flowing through the dielectric, in particular, the higher the dissipation factor, the poorer the insulating properties of the material, leading to more energy being converted into heat during operation, over time, this affects the equipment’s lifespan and safety.

The primary purpose of conducting Dissipation Factor (tan δ) tests is to evaluate the insulation condition of the equipment, in order to prevent electrical accidents caused by insulation aging or failure. In practical operations, various types of power equipment used in substations require Tan δ testing, examples include transformers, windings, current transformers (CT), voltage transformers (PT), transformer bushings, cables, and generators, tan δ testing is an indispensable part of maintenance and servicing operations for these devices.

Principle of Tan Delta Test
A pure insulator when is connected across line and earth, it behaves as a capacitor, in an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component, there is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.
In pure capacitor, the capacitive electric current leads the applied voltage by 90°.
In practice, the insulator cannot be made 100% pure, also, due to the aging of insulators, the impurities like dirt and moisture enter into it, these impurities provide the conductive path to the current, consequently, an electric leakage current flowing from line to earth through the insulator has a resistive component.
Hence, it is needless to say that, for a good insulator, this resistive component of electric leakage current is quite low, in another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component, for a good insulator, this ratio would be quite low, this ratio is commonly known as tan δ or tan delta, sometimes it is also referred to as dissipation factor.

1. Dielectric breakdown (BDV) (ASTM D877 & ASTM D1816, IEC 60156)

2. Tan Delta (Dielectric Dissipation Factor (DDF)/ Power factor) (ASTM D924, IEC 60247, IEC 61620, BS 5737, JIS C2101, VDE 0380-2, IS 6262)

3. Resistivity (ASTM D1169, IEC 60247, BS 5737, JIS C2101, VDE 0380-2, IS 6103)

JYC automatic frequency conversion anti-interference dielectric loss tester is used for field anti-interference dielectric loss measurement or laboratory precision dielectric loss measurement,the instrument is an integrated structure, built-in dielectric loss bridge, variable frequency power supply, test transformer and standard capacitor, etc., adopts frequency conversion anti-interference and "Fourier" transform digital filtering technology, fully automatic intelligent measurement, the measurement data is very stable under strong interference, and the measurement results are from large to large, LCD screen display, built-in printer can print out.

JYC Tan Delta Tester Features:

Versatile Measurement Application：

Measures insulation characteristics of power transformers, bushings, circuit breakers, current transformers (CT), potential transformers (PT), capacitor voltage transformers (CVT), cables, surge arresters, and other high-voltage insulation components.

Comprehensive Parameter Measurement：

Simultaneously measures Insulation Power Factor (cos φ), Dissipation Factor (tan δ), Capacitance, Inductance, Excitation Current, Quality Factor (Q), and Power Loss (Watt).

Advanced Anti-interference Technology：

Employs frequency conversion and digital filtering techniques to automatically recognize 50 Hz / 60 Hz system frequency, ensuring accurate results under 200 % interference conditions.

CVT Ratio & Phase Measurement：

Measures CVT transformation ratio, polarity, and phase angle with high precision.

Multiple Operating Modes：

Supports UST / GST configurations, internal / external standard capacitors, and internal / external high-voltage sources. Integrated structure enables all conventional dielectric loss tests without auxiliary devices.

Reverse Connection & Shielding Function：

Provides low-voltage shielding capability, allowing C11 measurement even when the CVT bus is grounded — no need to disconnect wiring.

CVT Self-excitation Measurement Mode：

Allows simultaneous C1 and C2 measurements in a single connection, with automatic compensation for lead capacitance and standard capacitor voltage division.
Automatic Connection Diagnostics：

Offers real-time prompts for poor contact or discharge conditions, helping users verify reliable test connections.
Bridge Simulation Function：

Built-in series and parallel equivalent dielectric loss models simulate Schering Bridge and Current Comparator Bridge, ideal for calibration and verification purposes.
Real-time Monitoring：

Displays test voltage, current, and power factor in real time; can function as a high-voltage meter.
Grounding & Wiring Integrity Check：

Provides automatic alerts for poor grounding or incorrect connections, ensuring measurement safety and reliability.
Power Supply Adaptability：

Operates on both 50 Hz and 60 Hz AC mains, supports generator power, and offers excellent anti-interference capability for field or lab environments.
Data Storage & Communication：

Built-in calendar clock and memory for 100 test records. Includes RS-232 interface and PC software for data processing, report generation, and integration with comprehensive test systems or vehicles.

Operation Features:

Four-channel GST Test Configuration (optional) :

Enables convenient dielectric loss measurement for transformer bushings using four-channel positive wiring mode.
Insulation Resistance Measurement (optional):

Uses the same wiring and shielding configuration as dielectric loss testing—no additional setup required. Simple operation: select 'Resistance / Absorption / Polarization' from the menu to start measurement. High reliability and protection against damage in harsh test environments. Maintains accuracy under up to 5 mA power-frequency interference, with automatic rapid discharge after testing.
CT Ratio and Phase Measurement (optional):

Measures CT transformation ratio, polarity, and phase angle with high accuracy.
Wireless Bluetooth Communication (optional):

Provides wireless remote control and data transmission between the instrument and PC software, improving field operation convenience.
Frequency-by-Frequency Test Function (optional):

Allows user-defined frequency selection from 45 Hz to 65 Hz for dielectric loss measurement.
External Standard Capacitor Interface (optional):

Supports external test power supplies from 30 Hz to 300 Hz. When connected to a high-capacity power-frequency source, enables large-capacitance, high-voltage dielectric loss testing.

Transformer Tan Delta Tester （JYC） control panel and test wiring diagram：

TECHNOLOGY SPECIFICATION

Item	JYC Tan Delta Tester
Capactance (Cx)	Cx: ±(1% of reading+2pF)
Dissipation Factor (tan δ)	tan δ: ±(Reading×1%+0.00040)
Anti-interference Performance	Equipped with frequency conversion and digital filtering; accuracy maintained under 200% interference level
Capacitance Measurement range	Internal HV source: 3pF～60,000 pF/10kV, 60pF～1µF/0.5kV
	External HV source: 3pF～1.5µF/10kV, 60pF～30µF/0.5kV
	Resolution: Max. 0.001pF, 4 valid digits
Dissipation Factor (tan δ) Measurement Range	No limit, resolution:0.001%. Automatic identification of capacitive, inductive, and resistive loads
Test Current Range	5µA-5A
Internal HV Source	Output Voltage Range: 0.5～10kV (continuously adjustable)
	Max. Output Current: 200mA
	Voltage Regi;atopm: Continuous and smooth adjustment
	Voltage Accuracy: ±(1.5% of reading+10V)
	Voltage Resolution: 1V
	Frequency Options: 45,50,55,60,65Hz (Single Frequency)
	45/55Hz, 55/65Hz, 47.5/52.5Hz (Dual Frequency))
	Frequency Accuracy: ±0.01Hz
External HV Source Mode	Maximum test current (GST): 5A,  40～70Hz
	Maximum test current (UST): 10kV/5A,  40～70Hz
CVT Self-excitation low voltage output	Output voltage: 3～50V, output current: 3～30A
Measurement time	Approximately 15s (depending on test mode)
Power Supply	180V～270V AC, 50Hz/60Hz±1%, Mains or generator
Environmental Temperature	-10℃～50℃
Relative Humidity	≤80% RH, (non-condensing)
Volume / Weight	L430mm*W320mm*H330mm / 29 kg

What are the Different Modes of the Tan Delta Test?

When it comes to tan delta test, there are essentially three modes of power factor testing. Those are

GST Guard – This calculates the amount of current leakage to the ground. This method eliminates the current leakage through red or blue leads. Whereas in UST, the ground is termed to be guard because grounded edges are not calculated. When the UST method is applied on the device, then the current measurement is only through blue or red leads. The current flow through ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.
UST Mode – This is employed for the calculation of insulation in between ungrounded leads of the equipment. Here the individual portion of isolation has to be separated and analyze it having no other insulation connected to it.
GST Mode – In this final mode of operation, both the leakage pathways are calculated by the test apparatus. The current, capacitance values, UST, and GST guards, loss in watts need to be equal to the GST test parameters. This provides the entire behavior of the test.
When the summing value of GST Guard and UST is not equal to the GST parameters, then it can be known that there is some crashing in the test set, or might the test terminal are not correctly designed.

FAQs:

• What does the Tan Delta value indicate?
  The Tan Delta value provides information about the quality of the insulation. Higher tan δ values suggest poorer insulation, while lower values indicate better insulation condition.
  
• When is the Tan Delta Test typically performed?
  The test is performed during the manufacturing, commissioning, and maintenance of electrical equipment, as well as during routine diagnostic assessments to monitor the condition of insulation over time.
  
• What are the applications of the Tan Delta Test?
  The Tan Delta Test is commonly used for assessing the condition of power cables, transformers, bushings, circuit breakers, and other high-voltage equipment with insulating materials.
  
• What are the advantages of the Tan Delta Test? Early Detection: It can detect insulation problems in their early stages, allowing for timely maintenance and preventing catastrophic failures.
• What are acceptable Tan Delta values?
  The acceptable Tan Delta values vary depending on the type of equipment and insulation material. Generally, lesser values than 0.7% indicate better insulation quality. Acceptable values should be defined by equipment standards or manufacturer guidelines.
  
• Is it necessary to perform the Tan Delta Test regularly?
  Regular testing is recommended, especially for critical equipment, as it helps to track the insulation’s condition and plan maintenance activities accordingly.
  
• Can the Tan Delta Test be used for oil-filled equipment?
  Yes, the Tan Delta Test is commonly used for assessing the insulation condition of oil-filled transformers and bushings, where it helps identify issues like moisture or contamination in the oil.
  
• How this Test is conducted on site?
  On-site testing involves using specialized test kit that applies high voltage at a specific frequency to the equipment under test, measuring the phase angle difference between current and voltage. The results are then compared to reference values.
  
• Can this Test predict the remaining lifespan of equipment?
  While the Tan Delta Test provides valuable data about insulation condition, it cannot predict the exact remaining lifespan of equipment. It helps in making informed decisions regarding maintenance and repair.
  
• Is it safe to perform this Test on energized equipment?
• No, it is not safe to perform on energized equipment. The equipment being tested should be properly de-energized and grounded to ensure the safety of the testing personnel.

Kingrun Transformer Instrument Co.,Ltd.

More Transformer Testers from Kingrun

turn ratio tester, transformer test instrument, winding resistance tester, transf