Photovoltaic system grid connection technical requirements (GB/T 19939-2005)
1 Scope
This standard specifies the grid connection method, power quality, safety and protection and installation requirements for photovoltaic systems.
This standard applies to photovoltaic systems connected to the grid by means of static converters (inverters) in a low-voltage manner.
Photovoltaic systems can also refer to this standard for relevant parts that are connected to the grid in medium or high pressure mode.
2 normative references
The clauses in the following documents have been adopted as references to this standard. For dated references, all subsequent amendments (not including errata content) or revisions do not apply to this standard, but the parties to the agreement are encouraged to study whether the latest versions of these documents can be used. For undated references, the latest version is applicable to this standard.
GB/T 2297-1989 Solar photovoltaic energy system terminology
GB/T 12325-2003 power quality supply voltage tolerance
GB 2894-1996 safety signs (neq ISO 3864:1984)
GB/T 14549-1993 Power Quality Public Power Harmonics
GB/T 15543-1995 power quality three-phase voltage allow unbalance
GB/T 15945-1995 Frequency tolerance of power quality power system
GB 16179-1996 Guidelines for the use of safety signs
GB/T 18479-2001 General description and guidance of surface photovoltaic (PV) power generation systems (idt IEC 61277:1995)
SJ/T 11127-1997 Overvoltage protection for photovoltaic (PV) power generation systems - Guidelines
3 Definitions
The following terms and definitions apply to this standard.
3.1
PV system PV system
A system that includes all inverters (single or multiple) and associated BOS (Balanced System Components) and solar cell arrays with one common connection point.
3.2
Grid grid
A combination of various devices and equipment for transmission, distribution, substations, power lines, or cables. It connects the power plants and users distributed in a wide area into a whole and distributes the concentrated power to a large number of dispersed power users.
This standard specifically refers to the power transmission subsystem secondary output to the user's end of the power transmission network.
3.3
Grid protection device
Monitoring of the technical status of PV grid-connected power grids, and disassembling the photovoltaic system and power grid safety in the event of an index over-limitation.
3.4
Grid interface grid -interface
Interconnection between photovoltaic system and grid distribution system.
Refers to the decoupling point between the power generation equipment and the power grid.
3.5
Islanding islanding
When the grid voltage is lost, the photovoltaic system still maintains the state of power supply to some part of the missing piezoelectric network.
3.6
Inverter
Static power converter (see Note 1).
A device that converts direct current into alternating current. Transform the DC power of PV system into AC equipment. Electrical equipment for converting electrical power to one or more forms of electrical power suitable for use on a power grid.
NOTE 1 Any static power converter with control, protection and filtering functions for the interface between the power supply and the grid. Sometimes referred to as a power regulation subsystem, a power conversion system, a static converter, or a power conditioning unit.
3.7
Emergency power supply system
A power system that can continue to supply power for a specific load when the power grid is outage for any reason. It generally includes an inverter, a protection switch, a control circuit, an energy storage device (such as a battery), and a charging device with a charging control circuit.
4 grid connection
According to whether the photovoltaic system is allowed to transmit power to the high-voltage grid through the transformer in the power supply area, it is divided into a grid-connected mode with reversible flow and irreversible flow.
5 power quality
The quality of the photovoltaic system to provide electrical energy to the local AC load and to send electrical energy to the grid should be controlled. Practical requirements and standards should be met in terms of voltage deviation, frequency, harmonics and power factor. When there is a deviation from the standard, the system should be able to detect these deviations and safely disconnect the PV system from the grid.
Unless otherwise required, it should be ensured that all power quality parameters (voltage, frequency, harmonics, etc.) can be measured at grid connection of grid-tied photovoltaic system.
5.1 Voltage deviation
In order for the local AC load to work properly, the output voltage of the inverter in the PV system should match the grid.
During normal operation, the allowable voltage deviation at the interface between the photovoltaic system and the grid shall comply with the provisions of GB/T 12325. The allowable deviation of the three-phase voltage is ±7% of the rated voltage, and the allowable deviation of the single-phase voltage is +7% and -10% of the rated voltage.
5.2 frequency
When the photovoltaic system is connected to the grid, it should run synchronously with the grid. The rated frequency of the power grid is 50Hz. The allowable deviation of the frequency after the photovoltaic system is connected to the grid shall comply with the provisions of GB/T 15945, ie, the deviation value shall be ±0.5Hz.
5.3 Harmonic and Waveform Distortion
The harmonic levels of low currents and voltages are desirable; higher harmonics increase the likelihood of detrimental effects on the connected equipment.
The permissible level of harmonic voltage and current depends on the characteristics of the distribution system, the type of power supply, the connected load/equipment, and the current regulations of the power grid.
The output of the PV system should have low current distortion to ensure that it does not adversely affect other equipment connected to the grid.
The total harmonic current should be less than 5% of the rated output of the inverter. Each harmonic should be limited to the percentages listed in Table 1 and Table 2.
Even harmonics in this range should be less than 25% of the low odd harmonic limit.
5.5 Voltage imbalance
When the photovoltaic system is connected to the grid (only for three-phase output), the unbalance of the three-phase voltage at the grid interface shall not exceed the values ​​specified in GB/T 15543. The permissible value is 2% and the short-term must not exceed 4%.
5.6 DC component
When the photovoltaic system is connected to the grid, the direct current component fed from the inverter to the grid should not exceed 1% of its AC rated value (the inverter power system and the grid should be isolated and connected through a dedicated transformer).
6 Security and Protection
When photovoltaic systems and grids are abnormal or faulty, they must have corresponding grid-connected protection functions to ensure equipment and personal safety.
6.1 Over/Under voltage
When the voltage at the grid interface exceeds the voltage range specified in 5.1, the photovoltaic system shall stop sending power to the grid. This requirement applies to any phase in a multiphase system.
All system voltages referred to in this standard refer to the local nominal voltage.
The system should be able to detect abnormal voltage and react. The rms value of the voltage is measured at the grid interface and should meet the conditions in Table 3.
6.2 Over/Under frequency
When the frequency at the grid interface exceeds the frequency range specified in 5.2, the over/under frequency protection should act within 0.2 s to disconnect the PV system from the grid.
6.3 Anti-islanding effect
When the grid of PV systems fails to lose power, the PV system must be disconnected from the grid within the specified time limit to prevent the islanding effect.
At least one active and passive anti-islanding protection should be set.
Active anti-islanding protection methods mainly include frequency deviation, active power variation, reactive power variation, and impedance fluctuation due to current pulse injection.
The passive anti-islanding protection methods mainly include voltage phase jitter, 3 voltage harmonic changes, and frequency change rate.
When the grid loses pressure, anti-islanding protection should act within 2 seconds to disconnect the photovoltaic system from the grid.
Note 1: The disconnection of the photovoltaic system from the power grid does not include the main control and monitoring circuits used to monitor the status of the power grid.
6.4 Restoring the grid connection
Since the over-limit state causes the photovoltaic system to stop sending power to the grid, the photovoltaic system should not transmit power to the grid within 20s to 5 min after the grid voltage and frequency return to the normal range.
6.5 Lightning and Grounding
Lightning protection and grounding of photovoltaic systems and grid-connected interfaces shall comply with SJ/T 11127.
6.6 Short Circuit Protection
Photovoltaic system should set short-circuit protection to the power grid. When the power grid is short-circuited, the over-current of the inverter should not exceed 150% of the rated current, and the photovoltaic system should be disconnected from the power grid within 0.1 s.
6.7 Isolation and Switching
Manual and automatic disconnect switches shall be provided in the switchgears connected between the photovoltaic system and the grid, and the disconnect switches shall be mechanically switchable using visual breakpoints in principle. Electronic switches must not be used unless the local power supply company agrees.
6.8 Reverse Power Protection
The system works in a grid-connected mode with irreversible flow. When the secondary current at the power transformer is detected as 5% of the rated output of the inverter, the reverse power protection should disconnect the photovoltaic system from the power grid within 0.5 s-2 s.
7 installation requirements
7.1 Electrical Connections and Reference Diagrams
The electrical connection mode of the photovoltaic system and the grid should be the same as the grid. Refer to Appendix A for the electrical connection reference diagram.
7.2 Energy Meter f and Counterflow Detection
Grid-connected photovoltaic systems should be equipped with energy metering devices.
For the irreversible grid connection method, a counterflow detection device should be installed at the output end of the power transformer.
7.3 Security Identification
The special low-voltage switchgear connecting the photovoltaic system and the grid should have a prominent logo. The logo should indicate such warning words and symbols as "warning" and "dual power". The shape, color, size, and height of the logo are referenced to GB 2894 and GB 16179.
7.4 Additional Requirements for Emergency Power Systems
7.4.1 Overview
The emergency power system shall meet the requirements of 7.1-7.4 and shall also meet the additional requirements listed in this section.
7.4.2 Circuit Arrangement
The emergency power system should supply power to one or more specific load circuits.
7.4.3 Security Requirements
When the PV system is used as an emergency power supply for a specific load, it should be ensured that it has completely disconnected from the switchgear circuit in Figure A.2 of Appendix A.
7.4.4 Warning Identification
When the PV system is operating in the emergency power mode, a warning sign should be set on the switch cabinet because at this time, even if the power grid has been powered off, the neutral or ground wire of the power supply line may still be charged.
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