Codeofchina.com is in charge of this English translation. In case of any doubt about the English translation, the Chinese original shall be considered authoritative.
This standard is developed in accordance with the rules given in GB/T 1.1-2009 Directives for standardization - Part 1: Structure and drafting of standards.
This standard replaces NB/T 31015-2011 Technical specification for manufacture of converter of permanent magnet wind turbine generator. In addition to editorial changes, the following main changes have been made with respect to NB/T 31015-2011:
——The standard is renamed from "Technical specification for manufacture of converter of permanent magnet wind turbine generator" to "Technical specification for converter of permanent magnet wind turbine generator";
——The requirement “this standard may be implemented as a reference for converter used for squirrel cage high-speed wind turbine, electrically excited wind turbine and medium/high speed permanent magnet wind turbine” is added (see Clause 1);
——The terms and definitions of nominal current, common-mode voltage, differential mode voltage and du/dt value are added (see 3.24~3.27);
——The mixed cooling type classified according to cooling method of power module is added (see 4.1.1);
——The range of the grid-side voltage grade is modified (see 4.1.2);
——The technical requirements for grid voltage unbalance degree is revised (see 4.2.4.3);
——The requirement that the efficiency be not less than 97% is revised (see 4.3.10);
——The temperature rise requirement of the resistance elements of surge absorber and main circuit is deleted;
——The requirements for over-/under-frequency protection, grid power-off protection and grid current unbalance protection in the protection function are added (see 4.3.13);
——The provisions on liquid cooling type in the protection grade is modified, and the protection grade "not inferior to IP42" is modified as "not inferior to IP54" (see 4.3.18);
——The requirements for sound pressure level greater than 80dB (A) in noise and noise tests are modified (see 4.3.19 and 5.3.19);
——The requirements of 0.6 (0.62) kV grade for electric clearance and creepage distance are added (see 5.3.4.3);
——The relevant provisions on common-mode voltage, differential mode voltage, du/dt value and vibration in "test method" are added (see 5.3.24~5.3.27).
This standard was proposed by China Electrical Equipment Industrial Association.
This standard is under the jurisdiction of NEA/TC1/SC 6 Subcommittee on Wind Power Electrical Equipment of the National Technical Committee on Wind Power in Energy Industry of Standardization Administration of China
The previous edition of standard replaced by this standard is as follows:
——NB/T 31015-2011.
During the process of implementing this standard, the relevant comments and recommendations, whenever necessary, can be fed back to the Standardization Center of the China Electricity Council (No.1, 2nd Lane, Baiguang Road, Beijing, 100761, China).
Technical specification for converter of permanent magnet wind turbine generator
1 Scope
This standard specifies the technical requirements, test methods and inspection rules of converter of permanent magnet wind turbine generator (hereinafter referred to as “converter”) as well as the relevant information of its products.
This standard is applicable to AC-DC-AC voltage source converters connected to the stator winding of permanent magnet wind turbines. This standard may be implemented as a reference for converter used for squirrel cage high-speed wind turbine, electrically excited wind turbine and medium/high speed permanent magnet wind turbine.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 191 Packaging - Pictorial marking for handling of goods
GB/T 2423.1-2008 Environmental testing - Part 2: Test methods - Tests A: Cold
GB/T 2423.2-2008 Environmental testing - Part 2: Test methods - Tests B: Dry heat
GB/T 2423.3 Environmental testing - Part 2: Testing method - Test Cab: Damp heat, steady state
GB/T 2423.4 Environmental testing for electric and electronic products - Part 2: Test method - Test Db: Damp heat, cyclic (12h+12h cycle)
GB/T 2900.33 Electrotoechnical terminology - Power electronics
GB/T 2900.53 Electro-technical terminology - Wind turbine generator systems
GB/T 3797-2016 Electrical control assemblies
GB/T 3859.1-2013 Semiconductor converters - General requirements and line commutated converters - Part 1-1: Specification of basic requirements
GB/T4208 Degrees of protection provided by enclosure (IP code)
GB/T 12668.2-2002 Adjustable speed electrical power drive systems - Part 2: General requirements - Rating specifications for low voltage adjustable frequency a.c. power drive systems
GB 12668.3-2012 Adjustable speed electrical power dive systems - Part 3: EMC product standard including specific test methods
GB/T 13384 General specifications for packing of mechanical and electrical product
GB/T 13422-2013 Semiconductor converters - Electrical test methods
GB 14048.1 Low-voltage switchgear and controlgear - Part 1: General rules
GB/T 15543-2008 Power quality - Three-phase voltage unbalance
GB/T 18451.1 Wind turbine generator systems - Design requirements
GB/T 19963-2011 Technical rule for connecting wind farm to power system
GB/T 20320-2013 Measurement and assessment of power quality characteristics of wind turbines generator systems
JB/T 5777.2-2002 General specification for control and protection panel (cabinet and desk) of secondary circuit of power system
NB/T31051 Test procedure for low voltage ride through capability of wind turbine
NB/T 31054 Test procedure for grid adaptability of wind turbine
NB/T 31111 Test procedure for high voltage ride through of wind turbine
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 3859.1-2013, GB/T 2900.33 and GB/T 2900.53 as well as the following apply.
3.1
permanent magnet wind turbine converter
AC-DC-AC voltage source static converter matched with permanent magnet wind turbine generator and connected between stator winding and grid
Note: It has the functions of, for example, sending the stator winding energy into the grid, etc. It mainly consists of generator-side converter, DC link, grid-side converter and its control system. Some types of converters may make energy flow in both directions, while others may only make energy flow to the grid side.
3.2
generator-side converter
static converter connected with the stator winding of the permanent magnet wind turbine generator and used for converting alternating current with variable voltage, current amplitude and frequency from the wind turbine into direct current
Note: It is also called stator-side converter or generator-side converter, which may be of full control bridge type or uncontrolled bridge type + boost type, including filters that likely present.
3.3
DC link
DC circuit interconnected between the generator-side converter and the line-side converter
3.4
line-side converter
full control bridge static converter connected with the grid to realize the exchange control of active power and reactive power between the DC link and the grid
Note: It is also called grid-side converter, including filters that likely present.
3.5
wind turbine generator system; WTGS
system converting wind energy into electric energy (excluding grid-connected transformer), hereinafter referred to as "WTGS”
3.6
output of WTGS
connection port between WTGS and grid-connected transformer
3.7
rated operation condition of WTGS
working state of the WTGS operating at rated rotational speed and rated power under the condition of rated voltage and rated frequency of the grid
3.8
rated operation condition of converter
working state of the converter under the rated operation condition of the WTGS, hereinafter referred to as "rated operation condition"
3.9
line-side rated current
root-mean-square value of the maximum fundamental current on the AC side of the line-side converter under rated operation condition
3.10
line-side rated voltage
root-mean-square value of line voltage on the AC side of the line-side converter
3.11
line-side rated frequency
frequency of the fundamental voltage on the AC side of the line-side converter, i.e., the power frequency
3.12
line-side rated capacity
apparent power of line-side converter at line-side rated frequency, line-side rated voltage and line-side rated current
3.13
line-side power factor
ratio of the active power to the apparent power on the AC side of the line-side converter or at the output of WTGS, also called the WTGS power factor
The power factor formula is as follows:
(1)
where,
λ——the power factor;
P——the active power;
S——the apparent power.
3.14
total harmonic distortion; THD
total harmonic distortion is related to the fundamental component of the vector under consideration
the total harmonic distortion (THD) is defined by the following formula:
(2)
where,
Q1——the r.m.s value of fundamental harmonic;
Q——the total r.m.s value, which may represent current or voltage.
The total harmonic contains inter-harmonic. When inter-harmonic exists, the waveform is no longer periodic, and the influence of inter-harmonic may be more complex than that of harmonic.
If inter-harmonic is ignored, Formula (2) may also be expressed as:
(3)
where,
h——the harmonic order, where the number h is even, kTHD is called even harmonic distortion, where h is odd, kTHD is called odd harmonic distortion;
Qh——the r.m.s value of the h-th order harmonic component.
3.15
generator-side rated current
root-mean-square value of fundamental current of the generator-side converter under the rated operation condition
3.16
generator-side rated voltage
root-mean-square value of fundamental line voltage on the AC side of the generator-side converter under the rated operation condition
3.17
generator-side rated frequency
frequency of fundamental voltage on the AC side of the generator-side converter under the rated operation condition
3.18
generator-side frequency range
fundamental voltage frequency range on the AC side of the generator-side converter corresponding to the permanent magnet wind turbine generator within the specified rotational speed range
3.19
generator-side converter rated apparent power
apparent power of generator-side converter at generator-side rated frequency, generator-side rated voltage and generator-side rated current
3.20
converter efficiency
ratio of the output active power to the input active power of the converter, which is expressed as a percentage
the converter efficiency formula is:
(4)
where,
η——the converter efficiency;
Pout——the output active power;
Pin——the input active power.
3.21
converter cut in current
maximum peak current generated when the line-side converter is switched on on the AC side
3.22
overload capability
root-mean-square value of the maximum fundamental current supplied within a specified period of time and not exceeding the specified limit under the rated operation condition of the converter
3.23
generator filter
filter consisting of common-mode voltage suppression link, differential mode voltage suppression link and du/dt suppression link, connected between the converter and the generator, and used for reducing the voltage value at the motor side and suppressing the overvoltage (such as high common-mode voltage, high differential mode voltage and the like) at the generator side caused by superposition of long cable reflected waves
3.24
nominal current
root-mean-square value of maximum fundamental current that the converter may continuously supply and that does not exceed the allowable value
3.25
common-mode voltage
average of phasor voltages occurring between each conductor and a specified reference point (generally the earth or rack) or 1/3 the input voltage applied at both measuring terminals of the voltmeter and at the specified common terminal
3.26
differential mode voltage
potential difference between phases
3.27
du/dt value
derivative of voltage with time:
du/dt=ΔUi/Δti=(U90％-U10％)/(t90％-t10％)
where,
du/dt——the derivative of voltage with time;
ΔUi——the voltage change value;
Δti——the time change value;
U90％——90％ of average voltage;
t90%——the time corresponding to 90％ of average voltage.
4 Technical requirements
4.1 Product type and main parameter
4.1.1 Product type
The types of products shall be classified as:
——Low-temperature type and normal-temperature type according to the ambient temperature;
——Air cooling type, liquid cooling type and mixed cooling type according to the cooling method of the power module.
4.1.2 Line-side voltage grade
The following series shall be preferred for the line-side voltage grade of the converter:
0.38 (0.4), 0.6 (0.62), 0.66 (0.69), 1 (1.05), 1.14 (1.2), 2.3 (2.4), 3 (3.15) and 6 (6.3).
Note: The unit is kV. Voltage grades exceeding the above preferred series shall be determined by the user and the manufacturer through negotiation.
4.2 Operating conditions
4.2.1 Environment conditions for normal operation
The converter shall operate under the specified air cooling or liquid cooling environment conditions.
a) Working ambient temperature:
——Normal temperature type: -20℃~45℃.
——Low temperature type: -40℃~45℃.
b) Relative humidity: ≤95% (below 20℃)
c) Altitude: ≤2,000m.
4.2.2 Environment condition for normal test
It mainly includes the operating climatic conditions, and the converter shall be tested under the following atmospheric environment:
a) Ambient temperature: -5℃~40℃.
b) Relative humidity: ≤90% (below 20℃).
c) Atmospheric pressure: 86kPa~106kPa.
4.2.3 Ambient temperature for storage and transport
During storage and transport of the converter, the ultimate temperature of the ambient air shall be -40℃~70℃.
4.2.4 Electrical conditions for normal operation
4.2.4.1 Variation range of grid frequency
Variation range of grid frequency: 47.5Hz~51.5Hz.
4.2.4.2 Fluctuation range of grid voltage
The converter shall operate normally when the grid voltage is within ±10% of the rated value.
Where the grid voltage exceeds the above range, it shall be determined by the user and the manufacturer through negotiation.
4.2.4.3 Grid voltage unbalance degree
According to the requirement in clause 4 of GB/T 15543-2008, the grid voltage unbalance degree shall not exceed 2%, and the short-time voltage unbalance degree shall not exceed 4%.
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Technical requirements
5 Test method
6 Inspection rules
7 Marking, packaging, storage and transport
NB/T 31015-2018, NB 31015-2018, NBT 31015-2018, NB/T31015-2018, NB/T 31015, NB/T31015, NB31015-2018, NB 31015, NB31015, NBT31015-2018, NBT 31015, NBT31015
Introduction of NB/T 31015-2018
Codeofchina.com is in charge of this English translation. In case of any doubt about the English translation, the Chinese original shall be considered authoritative.
This standard is developed in accordance with the rules given in GB/T 1.1-2009 Directives for standardization - Part 1: Structure and drafting of standards.
This standard replaces NB/T 31015-2011 Technical specification for manufacture of converter of permanent magnet wind turbine generator. In addition to editorial changes, the following main changes have been made with respect to NB/T 31015-2011:
——The standard is renamed from "Technical specification for manufacture of converter of permanent magnet wind turbine generator" to "Technical specification for converter of permanent magnet wind turbine generator";
——The requirement “this standard may be implemented as a reference for converter used for squirrel cage high-speed wind turbine, electrically excited wind turbine and medium/high speed permanent magnet wind turbine” is added (see Clause 1);
——The terms and definitions of nominal current, common-mode voltage, differential mode voltage and du/dt value are added (see 3.24~3.27);
——The mixed cooling type classified according to cooling method of power module is added (see 4.1.1);
——The range of the grid-side voltage grade is modified (see 4.1.2);
——The technical requirements for grid voltage unbalance degree is revised (see 4.2.4.3);
——The requirement that the efficiency be not less than 97% is revised (see 4.3.10);
——The temperature rise requirement of the resistance elements of surge absorber and main circuit is deleted;
——The requirements for over-/under-frequency protection, grid power-off protection and grid current unbalance protection in the protection function are added (see 4.3.13);
——The provisions on liquid cooling type in the protection grade is modified, and the protection grade "not inferior to IP42" is modified as "not inferior to IP54" (see 4.3.18);
——The requirements for sound pressure level greater than 80dB (A) in noise and noise tests are modified (see 4.3.19 and 5.3.19);
——The requirements of 0.6 (0.62) kV grade for electric clearance and creepage distance are added (see 5.3.4.3);
——The relevant provisions on common-mode voltage, differential mode voltage, du/dt value and vibration in "test method" are added (see 5.3.24~5.3.27).
This standard was proposed by China Electrical Equipment Industrial Association.
This standard is under the jurisdiction of NEA/TC1/SC 6 Subcommittee on Wind Power Electrical Equipment of the National Technical Committee on Wind Power in Energy Industry of Standardization Administration of China
The previous edition of standard replaced by this standard is as follows:
——NB/T 31015-2011.
During the process of implementing this standard, the relevant comments and recommendations, whenever necessary, can be fed back to the Standardization Center of the China Electricity Council (No.1, 2nd Lane, Baiguang Road, Beijing, 100761, China).
Technical specification for converter of permanent magnet wind turbine generator
1 Scope
This standard specifies the technical requirements, test methods and inspection rules of converter of permanent magnet wind turbine generator (hereinafter referred to as “converter”) as well as the relevant information of its products.
This standard is applicable to AC-DC-AC voltage source converters connected to the stator winding of permanent magnet wind turbines. This standard may be implemented as a reference for converter used for squirrel cage high-speed wind turbine, electrically excited wind turbine and medium/high speed permanent magnet wind turbine.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 191 Packaging - Pictorial marking for handling of goods
GB/T 2423.1-2008 Environmental testing - Part 2: Test methods - Tests A: Cold
GB/T 2423.2-2008 Environmental testing - Part 2: Test methods - Tests B: Dry heat
GB/T 2423.3 Environmental testing - Part 2: Testing method - Test Cab: Damp heat, steady state
GB/T 2423.4 Environmental testing for electric and electronic products - Part 2: Test method - Test Db: Damp heat, cyclic (12h+12h cycle)
GB/T 2900.33 Electrotoechnical terminology - Power electronics
GB/T 2900.53 Electro-technical terminology - Wind turbine generator systems
GB/T 3797-2016 Electrical control assemblies
GB/T 3859.1-2013 Semiconductor converters - General requirements and line commutated converters - Part 1-1: Specification of basic requirements
GB/T4208 Degrees of protection provided by enclosure (IP code)
GB/T 12668.2-2002 Adjustable speed electrical power drive systems - Part 2: General requirements - Rating specifications for low voltage adjustable frequency a.c. power drive systems
GB 12668.3-2012 Adjustable speed electrical power dive systems - Part 3: EMC product standard including specific test methods
GB/T 13384 General specifications for packing of mechanical and electrical product
GB/T 13422-2013 Semiconductor converters - Electrical test methods
GB 14048.1 Low-voltage switchgear and controlgear - Part 1: General rules
GB/T 15543-2008 Power quality - Three-phase voltage unbalance
GB/T 18451.1 Wind turbine generator systems - Design requirements
GB/T 19963-2011 Technical rule for connecting wind farm to power system
GB/T 20320-2013 Measurement and assessment of power quality characteristics of wind turbines generator systems
JB/T 5777.2-2002 General specification for control and protection panel (cabinet and desk) of secondary circuit of power system
NB/T31051 Test procedure for low voltage ride through capability of wind turbine
NB/T 31054 Test procedure for grid adaptability of wind turbine
NB/T 31111 Test procedure for high voltage ride through of wind turbine
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 3859.1-2013, GB/T 2900.33 and GB/T 2900.53 as well as the following apply.
3.1
permanent magnet wind turbine converter
AC-DC-AC voltage source static converter matched with permanent magnet wind turbine generator and connected between stator winding and grid
Note: It has the functions of, for example, sending the stator winding energy into the grid, etc. It mainly consists of generator-side converter, DC link, grid-side converter and its control system. Some types of converters may make energy flow in both directions, while others may only make energy flow to the grid side.
3.2
generator-side converter
static converter connected with the stator winding of the permanent magnet wind turbine generator and used for converting alternating current with variable voltage, current amplitude and frequency from the wind turbine into direct current
Note: It is also called stator-side converter or generator-side converter, which may be of full control bridge type or uncontrolled bridge type + boost type, including filters that likely present.
3.3
DC link
DC circuit interconnected between the generator-side converter and the line-side converter
3.4
line-side converter
full control bridge static converter connected with the grid to realize the exchange control of active power and reactive power between the DC link and the grid
Note: It is also called grid-side converter, including filters that likely present.
3.5
wind turbine generator system; WTGS
system converting wind energy into electric energy (excluding grid-connected transformer), hereinafter referred to as "WTGS”
3.6
output of WTGS
connection port between WTGS and grid-connected transformer
3.7
rated operation condition of WTGS
working state of the WTGS operating at rated rotational speed and rated power under the condition of rated voltage and rated frequency of the grid
3.8
rated operation condition of converter
working state of the converter under the rated operation condition of the WTGS, hereinafter referred to as "rated operation condition"
3.9
line-side rated current
root-mean-square value of the maximum fundamental current on the AC side of the line-side converter under rated operation condition
3.10
line-side rated voltage
root-mean-square value of line voltage on the AC side of the line-side converter
3.11
line-side rated frequency
frequency of the fundamental voltage on the AC side of the line-side converter, i.e., the power frequency
3.12
line-side rated capacity
apparent power of line-side converter at line-side rated frequency, line-side rated voltage and line-side rated current
3.13
line-side power factor
ratio of the active power to the apparent power on the AC side of the line-side converter or at the output of WTGS, also called the WTGS power factor
The power factor formula is as follows:
(1)
where,
λ——the power factor;
P——the active power;
S——the apparent power.
3.14
total harmonic distortion; THD
total harmonic distortion is related to the fundamental component of the vector under consideration
the total harmonic distortion (THD) is defined by the following formula:
(2)
where,
Q1——the r.m.s value of fundamental harmonic;
Q——the total r.m.s value, which may represent current or voltage.
The total harmonic contains inter-harmonic. When inter-harmonic exists, the waveform is no longer periodic, and the influence of inter-harmonic may be more complex than that of harmonic.
If inter-harmonic is ignored, Formula (2) may also be expressed as:
(3)
where,
h——the harmonic order, where the number h is even, kTHD is called even harmonic distortion, where h is odd, kTHD is called odd harmonic distortion;
Qh——the r.m.s value of the h-th order harmonic component.
3.15
generator-side rated current
root-mean-square value of fundamental current of the generator-side converter under the rated operation condition
3.16
generator-side rated voltage
root-mean-square value of fundamental line voltage on the AC side of the generator-side converter under the rated operation condition
3.17
generator-side rated frequency
frequency of fundamental voltage on the AC side of the generator-side converter under the rated operation condition
3.18
generator-side frequency range
fundamental voltage frequency range on the AC side of the generator-side converter corresponding to the permanent magnet wind turbine generator within the specified rotational speed range
3.19
generator-side converter rated apparent power
apparent power of generator-side converter at generator-side rated frequency, generator-side rated voltage and generator-side rated current
3.20
converter efficiency
ratio of the output active power to the input active power of the converter, which is expressed as a percentage
the converter efficiency formula is:
(4)
where,
η——the converter efficiency;
Pout——the output active power;
Pin——the input active power.
3.21
converter cut in current
maximum peak current generated when the line-side converter is switched on on the AC side
3.22
overload capability
root-mean-square value of the maximum fundamental current supplied within a specified period of time and not exceeding the specified limit under the rated operation condition of the converter
3.23
generator filter
filter consisting of common-mode voltage suppression link, differential mode voltage suppression link and du/dt suppression link, connected between the converter and the generator, and used for reducing the voltage value at the motor side and suppressing the overvoltage (such as high common-mode voltage, high differential mode voltage and the like) at the generator side caused by superposition of long cable reflected waves
3.24
nominal current
root-mean-square value of maximum fundamental current that the converter may continuously supply and that does not exceed the allowable value
3.25
common-mode voltage
average of phasor voltages occurring between each conductor and a specified reference point (generally the earth or rack) or 1/3 the input voltage applied at both measuring terminals of the voltmeter and at the specified common terminal
3.26
differential mode voltage
potential difference between phases
3.27
du/dt value
derivative of voltage with time:
du/dt=ΔUi/Δti=(U90％-U10％)/(t90％-t10％)
where,
du/dt——the derivative of voltage with time;
ΔUi——the voltage change value;
Δti——the time change value;
U90％——90％ of average voltage;
t90%——the time corresponding to 90％ of average voltage.
4 Technical requirements
4.1 Product type and main parameter
4.1.1 Product type
The types of products shall be classified as:
——Low-temperature type and normal-temperature type according to the ambient temperature;
——Air cooling type, liquid cooling type and mixed cooling type according to the cooling method of the power module.
4.1.2 Line-side voltage grade
The following series shall be preferred for the line-side voltage grade of the converter:
0.38 (0.4), 0.6 (0.62), 0.66 (0.69), 1 (1.05), 1.14 (1.2), 2.3 (2.4), 3 (3.15) and 6 (6.3).
Note: The unit is kV. Voltage grades exceeding the above preferred series shall be determined by the user and the manufacturer through negotiation.
4.2 Operating conditions
4.2.1 Environment conditions for normal operation
The converter shall operate under the specified air cooling or liquid cooling environment conditions.
a) Working ambient temperature:
——Normal temperature type: -20℃~45℃.
——Low temperature type: -40℃~45℃.
b) Relative humidity: ≤95% (below 20℃)
c) Altitude: ≤2,000m.
4.2.2 Environment condition for normal test
It mainly includes the operating climatic conditions, and the converter shall be tested under the following atmospheric environment:
a) Ambient temperature: -5℃~40℃.
b) Relative humidity: ≤90% (below 20℃).
c) Atmospheric pressure: 86kPa~106kPa.
4.2.3 Ambient temperature for storage and transport
During storage and transport of the converter, the ultimate temperature of the ambient air shall be -40℃~70℃.
4.2.4 Electrical conditions for normal operation
4.2.4.1 Variation range of grid frequency
Variation range of grid frequency: 47.5Hz~51.5Hz.
4.2.4.2 Fluctuation range of grid voltage
The converter shall operate normally when the grid voltage is within ±10% of the rated value.
Where the grid voltage exceeds the above range, it shall be determined by the user and the manufacturer through negotiation.
4.2.4.3 Grid voltage unbalance degree
According to the requirement in clause 4 of GB/T 15543-2008, the grid voltage unbalance degree shall not exceed 2%, and the short-time voltage unbalance degree shall not exceed 4%.
Contents of NB/T 31015-2018
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Technical requirements
5 Test method
6 Inspection rules
7 Marking, packaging, storage and transport
