By sion932 
As a crucial national standard in China’s power industry, GB/T 15543-2008, titled “Power Quality – Three-phase Voltage Unbalance”, plays a pivotal role in regulating the operation of AC power systems, ensuring power supply stability, and protecting electrical equipment. Issued on June 18, 2008, and implemented on May 1, 2009, this standard supersedes the previous GB/T 15543-1995 version, introducing significant revisions and supplements to adapt to the evolving needs of the power system. This article elaborates on the core content, technical requirements, and practical significance of GB/T 15543-2008.
1. Scope and Fundamental Objectives
GB/T 15543-2008 specifies the limits, calculation methods, measurement procedures, and value-taking rules for three-phase voltage unbalance. It applies to 50Hz AC power systems under normal operating conditions, focusing on voltage unbalance at the Point of Common Coupling (PCC) caused by negative-sequence fundamental components, as well as voltage unbalance at the PCC in low-voltage systems (nominal voltage ≤ 1kV) caused by zero-sequence fundamental components.
Notably, the standard explicitly excludes instantaneous and temporary unbalance issues. Instantaneous unbalance refers to changes lasting 0.5 to 30 power frequency cycles, while temporary unbalance lasts 30 power frequency cycles to 3 seconds. The allowable unbalance degree of voltage and allowable negative-sequence current value for electrical equipment under rated operating conditions are governed by their respective standards (e.g., rotating electrical machines shall comply with GB 755).
2. Key Terms and Definitions
To ensure uniform understanding and application, GB/T 15543-2008 defines several core terms:
- Voltage Unbalance: A condition where the three-phase voltages differ in amplitude, have phase differences other than 120°, or both.
- Unbalance Factor: A quantitative measure of three-phase unbalance, expressed as the percentage of the root mean square (RMS) value of the negative-sequence or zero-sequence fundamental component of voltage/current to the RMS value of the positive-sequence fundamental component. The voltage negative-sequence unbalance factor, voltage zero-sequence unbalance factor, current negative-sequence unbalance factor, and current zero-sequence unbalance factor are denoted as ε, ε, ε, and ε respectively.
- Positive-sequence Component: The component of an unbalanced three-phase system decomposed by the symmetrical component method that belongs to the positive-sequence symmetrical system.
- Negative-sequence Component: The component belonging to the negative-sequence symmetrical system after decomposition by the symmetrical component method.
- Zero-sequence Component: The component belonging to the zero-sequence symmetrical system after decomposition by the symmetrical component method.
- Point of Common Coupling (PCC): The connection point in the power system where more than one user is connected.
3. Limits for Voltage Unbalance Factor
The standard establishes clear limits for voltage unbalance factors to guarantee the safe and stable operation of the power system:
3.1 Limits at the PCC of the Power System
Under normal operation of the power grid, the negative-sequence voltage unbalance factor shall not exceed 2%; for short-term (3 seconds to 1 minute) conditions, it shall not exceed 4%. For low-voltage systems, the limit for zero-sequence voltage is not specified temporarily, but the voltage of each phase must comply with the requirements of GB/T 12325 “Power Quality – Supply Voltage Deviation”.
It should be noted that the unbalance factor specified in this standard refers to the measured value of the voltage unbalance factor caused by the load during the maximum production (operation) cycle under the minimum (or smaller) normal operating mode of the power system.
3.2 Allowable Values for Individual Users
The allowable value of the negative-sequence voltage unbalance factor at the PCC caused by each connected user is generally 1.3%, and shall not exceed 2.6% for short-term conditions. This allowable value may be appropriately adjusted based on the load conditions at the connection point and the safety operation requirements of adjacent generators, relay protection, and automatic devices, but must comply with the overall limit requirements specified in 3.1.
4. Conversion of Allowable Values
The allowable value of the negative-sequence voltage unbalance factor can generally be converted into the corresponding negative-sequence current value based on the normal minimum short-circuit capacity at the connection point, which serves as the basis for analysis or calculation. For users adjacent to large rotating electrical machines, the negative-sequence impedance of the rotating electrical machines shall be considered during the conversion of the negative-sequence current value. Detailed calculation methods for the unbalance factor are provided in Appendix A of the standard (informative appendix).
5. Measurement and Value-taking of Unbalance Factor
GB/T 15543-2008 elevates the measurement methods, previously included in the appendix, to the main text and adjusts the measurement time and procedures, enhancing their standardization and operability.
5.1 Measurement Conditions
Measurements shall be conducted under the minimum (or smaller) normal operating mode of the power system, with unbalanced loads in normal and continuous operation, ensuring that the maximum operating cycle of the unbalanced loads is included.
5.2 Measurement Duration
For the PCC of the power system, the measurement duration shall be one week (168 hours), and the interval between each unbalance factor measurement may be an integer multiple of 1 minute. For fluctuating loads, in accordance with 5.1, continuous measurement for 24 hours on a normal working day is acceptable, with a measurement interval of 1 minute for each unbalance factor.
5.3 Value-taking Rules
For the PCC of the power system, the 95% probability maximum value of the 10-minute RMS value of the measured supply voltage negative-sequence unbalance factor shall not exceed 2%, and the maximum value among all measured values shall not exceed 4%. For daily fluctuating unbalanced loads, the 95% probability maximum value of the 1-minute RMS value of the measured supply voltage negative-sequence unbalance factor shall not exceed 2%, and the maximum value among all measured values shall not exceed 4%.
For daily fluctuating unbalanced loads, value-taking based on time is also permitted: the cumulative daily time when the unbalance factor exceeds 2% shall not exceed 72 minutes, and the time exceeding 2% within every 30 minutes shall not exceed 5 minutes.
To facilitate practical application, the 95% probability value of the measured values can be obtained by sorting the measured values in descending order, discarding the top 5% of the largest values, and taking the maximum value among the remaining measured values. When taking values based on time, each minute where the 1-minute RMS value exceeds 2% is counted as 1 minute of exceeding the standard.
5.4 Requirements for Measuring Instruments
Measuring instruments for the unbalance factor shall meet the requirements of this standard, with a recording period of 3 seconds and values taken as RMS. Each measurement of the fundamental component of the voltage input signal shall be conducted at intervals of 10 power frequency cycles. For discrete sampling measuring instruments, the calculation shall be performed in accordance with the recommended formula (1) specified in the standard.
6. Key Revisions Compared with GB/T 15543-1995
Compared with the previous version, GB/T 15543-2008 has the following important revisions:
- Updated the standard title from “Allowable Unbalance Degree of Three-phase Voltage” to “Three-phase Voltage Unbalance” to better reflect the comprehensive regulatory scope of the standard.
- Adjusted the English translation of “Power Quality” to align with other power quality standards.
- Added content related to the zero-sequence unbalance degree of low-voltage distribution systems.
- Revised the term “unbalance degree” to “negative-sequence unbalance degree” throughout the standard for greater accuracy.
- Expanded the terminology and definitions, and added “Normative References” (e.g., GB/T 156-2007 “Standard Voltage” and GB/T 12325 “Power Quality – Supply Voltage Deviation”).
- Explicitly specified that the three-phase unbalance degree refers to that of the fundamental component.
- Adjusted the content of “Calculation of Unbalance Degree” in the appendix.
7. Significance and Application
GB/T 15543-2008 was jointly proposed and administered by the National Technical Committee on Voltage Current Levels and Frequencies of Electrical Power Systems, with drafting units including renowned power research institutions and enterprises such as HVHIPOT, China Electric Power Research Institute, and State Grid Wuhan High Voltage Research Institute. The release and implementation of this standard mark a significant step forward in China’s power quality assessment, providing important technical guidance for the design, operation, and management of power systems.
By standardizing the limits, measurement, and calculation of three-phase voltage unbalance, the standard effectively prevents and mitigates adverse impacts of voltage unbalance on electrical equipment, such as equipment damage and reduced operational efficiency. It ensures that electrical equipment operates in a stable and safe voltage environment, thereby improving the overall operational efficiency and economic benefits of the power system. In practical applications, the standard serves as a critical basis for power supply enterprises to assess power quality, for users to optimize load distribution, and for the design and selection of electrical equipment, contributing to the reliable and efficient operation of the entire power grid.
References
[1] GB/T 15543-2008, Power Quality – Three-phase Voltage Unbalance[S]. Beijing: Standards Press of China, 2008.
[2] GB/T 12325, Power Quality – Supply Voltage Deviation[S].
[3] GB/T 156-2007, Standard Voltage[S].