GB/Z 17625.15-2017Electromagnetic compatibility - Limits - Assessment of low frequency electromagnetic immunity and emission requirements for dispersed generation systems in LV network (English PDF)

3.28 Active feed-in converter

Composed of various technologies, topologies, voltages and capacities, connected between the AC power supply system (line) and the DC side (current source or voltage source)

Self-converting power electronic converters can convert electrical energy in both directions (generation or regeneration), and can control the power factor of the applied voltage or current.

Note: Some of them can add additional control to the applied voltage or current harmonic distortion. Basic topology can be voltage source converter (VSC) or current source rectifier (CSC) Implementation.

5.1 Overview

Compared with other types of generators, the purpose of this section to briefly describe the following different power generation systems is to emphasize the connection to the grid The characteristics of static power supply.

There are three main types of power generation systems connected to the power system, including.

---Induction (asynchronous) generator;

---Synchronous generator;

---Static power converter.

Each type has its typical characteristics in synchronization device, protection function, starting operation, and electrical operation characteristics. The primary energy of the power plant can be So it is internal or external combustion, wind energy, fuel cells, electrochemical batteries, flywheel energy storage systems, small hydropower and photovoltaic cells.

This section describes both current and voltage inverters. Although topologically speaking, most distributed energy inverters are Can be regarded as voltage inverters, but from the point of view of integrated networks, they work in the form of current source control strategies.

This means that it is usually assumed that the line voltage at the DG connection point is regarded as constant, and the required input power is controlled by the output of the inverter. Into the current to obtain.

5.2 Induction (asynchronous) generator

The working principle of the asynchronous generator is the same as that of the AC asynchronous motor, except that during normal operation, its rotation speed is slightly higher than that of the power system.

Synchronous speed. Asynchronous generators are usually only used in power plants that operate in parallel with other power sources (such as public systems).

Asynchronous generators obtain excitation current from the stator, so that they consume reactive power from the grid, which will cause the voltage of the grid to drop and power distribution.

Network loss increases. When the system network loss and voltage sag are very obvious, it is necessary to correct the power factor of the asynchronous generator according to regulations.

Due to the collapse of the excitation voltage during the fault, the asynchronous generator cannot maintain a considerable fault current to the end of the line for a long time. Instead, they A large amount of current will be input in a short time to cause an impact on the grid. Because the asynchronous generator has the above characteristics, its protection and interface It is somewhat different from a synchronous generator.

5.3 Synchronous generator

A synchronous generator (as a voltage source) is a rotary energy converter that can operate as an independent power source (it can operate independently of any power source). source). If they can be correctly synchronized with the power source and appropriate protection/control measures are taken, they can also run in parallel with other power sources (such as Public grid system).

One of the characteristics of the synchronous generator is that it has a complete excitation system, and the excitation control enables it to operate as an independent power source. This pair The DG device is particularly useful, so that it can either supply power to a stand-alone system independently, or it can be connected to the grid. But these devices require additional anti-islanding protection.

In addition, unlike asynchronous generators, synchronous generators need to be accurately synchronized with the grid when they are connected to the grid. This means disconnecting the customer At the moment when the interface of the converter and the main network are connected, the frequency, phase angle, and voltage peak value should be matched to make it strictly controlled within the allowable error range to avoid damage.

Damage or affect the generator or other equipment of the power grid.

The load of the generator is controlled to maintain synchronization. If the generator is out of step and does not disconnect from the grid in time, equipment may occur Damage or power quality problems.

Due to the characteristics of the excitation system of the synchronous generator, it can maintain a longer fault current than the asynchronous generator (assuming that each provides excitation Magnetic source). Therefore, the fault protection of synchronous generators is more demanding than asynchronous generators.

5.4 Static power converter

Static power converter (inverter) is the interface of DC power supply, variable frequency AC power supply and power grid distribution system. Used in power generation system Examples of inverters are. photovoltaic arrays, fuel cells, battery energy storage systems, some types of micro gas turbines and wind generators.

Different from the principle that asynchronous motors and synchronous motors use rotating coils and electromagnetic fields to convert mechanical energy into electrical energy, inverters generally use Power electronic devices convert one form of electrical energy into another form of electrical energy (such as converting DC to AC).

Electronic circuit to control and protect. Once its internal controller finds that the voltage, current, and frequency deviation exceed the maximum allowable value, it will Cut off the power input to the system quickly. It also controls the synchronization system and the startup process.

Although most small conversion units that operate in parallel with the grid can completely rely on their internal protection functions, large and special conversion units The flow unit also requires external control/protection devices.

There are many differences between inverters and rotating electric machines. For example, the inverter has no moving or rotating parts.

The quantity source synthesizes the alternating waveform. In addition, due to the fast response capability of semiconductor switching equipment, once the control protection scheme determines that the energy needs to be interrupted Flow, the inverter can usually stop power supply faster than the rotating motor.

8.2 Harmonics

8.2.1 Harmonic current emission mechanism

The harmonic current of many inverter-based DGs in the PCC is obviously related to the higher harmonic voltage content of the AC side voltage [2]. DG production The influence of the generated harmonic current on the harmonic voltage also depends on the system impedance of the PCC point power supply network, the characteristics of the filter set in the DG equipment and Characteristics of DG control system.

If the control system collects the system voltage as the current reference waveform, the harmonic current generated by the voltage harmonics will tend to increase the voltage distortion.

In addition to the harmonic current caused by the harmonic voltage of the power supply, the DG equipment itself also generates harmonics due to the switching action of its solid-state devices.

The harmonics are usually higher than 2kHz. However, due to the defects of the control and solid-state equipment characteristics, there are still small low-frequency components. Therefore DG The amplitude of harmonics generated by the equipment depends on the filter type and component parameters inside the DG equipment.

In addition, the harmonic current will be different under different power generation conditions, such as the inverter input power has a significant difference in the high and low states [3]. Comment This should be considered when estimating the grid connection of photovoltaic power generation based on inverters. For example, in the AC and DC control of solar inverters, when the load is low When using open-loop control and using closed-loop control at high load, harmonics are large at low power and reduced under peak power conditions (see appendix A). Therefore, several load levels are specified in the harmonic current emission test. See Appendix A for more details and measurement results.

8.2.2 Recommended limits and tests for harmonic current emission

Recommendations for harmonic emission limits for distributed power sources interconnected with power systems can be found in IEEE1547 [4].

Even if the harmonic current emission limit in GB 17625.1-2012 (not greater than 16A) and IEC 61000-3-12 (from 16A to 75A)

The derivation considers the load but does not consider the DG, but in terms of harmonic current emission, there are some commonalities between a certain load and distributed generation.

In particular, there are also commonalities between lighting load and distributed power generation below 600W. Lighting and DG are both long-term, and there may be Similar power. All the lighting components of a family can be equivalent to a small inverter in general, while the overall lighting consumption of a small office building is The imbalance caused by DG is directly tested, but voltage fluctuations and flicker may occur, and the following matters should be noted.

Because large centralized power plants use synchronous generators, the system voltage at the outlet of the power plant is usually highly symmetrical. Therefore, one Generally, centralized power generation will not cause imbalance.

However, when small-scale distributed power generation is installed on the user side and appears as an important part of power production, sometimes it is embedded in energy tubes.

Management system, this situation is different. Many of these relatively small units, such as photovoltaic power generation devices, usually use single-phase power electronics The inverter is connected to the low-voltage power grid. The system impedance of the connection point is relatively large (the short-circuit capacity is relatively low), which will cause a greater degree of imbalance compared with the high voltage level.

The impedance of power system components is not exactly the same for every phase. For example, the geometric structure of the overhead line is asymmetric to the ground, which makes the electrical parameters of the line Different. Generally speaking, these differences are very small, and when there are sufficient preventive measures, their impact can be ignored, such as through line transposition.

In most practical situations, load asymmetry is the main cause of imbalance.

GB/T 18039.3-2003 relates to the standard of low frequency conduction compatibility level, the long-term impact of voltage imbalance should be considered, that is, the duration It is 10min or longer, and it is only related to the negative sequence component, which is the interference component that may exist in the equipment connected to the low-voltage power distribution system. the amount. For the neutral point directly grounded system, it is also related to the unbalance of the zero sequence component.

The voltage imbalance caused by the single-phase load connected between the phases is actually equal to the ratio of the load power to the three-phase short-circuit capacity of the system.

Therefore, the imbalance is only considered when larger capacity devices are involved.

8.4 Voltage fluctuation and flicker

8.4.1 Overview

Due to the grid connection and disconnection of the DG unit embedded in the power grid or the user side or due to the randomness of the DG output power change, the distribution network The level of flicker, voltage fluctuations and rapid voltage changes will increase. The worst impacts are often related to winds located in remote villages with weak power grids.

Electric units, or photovoltaic power generation units that are subject to changes in exposure patterns.

When the power supply network is relatively weak, that is, there is a small short-circuit current ratio or a high network power supply impedance, which is caused by the change of DG output power.

Pressure changes may be large enough to cause a rise in consumer complaints. The weak power supply network usually runs at the limit of the specified supply voltage deviation.

For DG equipment with limited power and delivering power to the public power supply network at a high short-circuit capacity connection point, it is unlikely to cause Very large voltage flickers.

However, in a local area with a large number of DG units or several photovoltaic power generation units operating in parallel, flicker may occur.

In winter, conditions such as temperature, clouds and wind may change frequently and suddenly. The rapid changes in power will cause voltage changes. Voltage flicker may occur at the grid connection point.

For DG equipment whose current per phase is 16A and below, GB/T 17625.2-2007 is adopted. Covered by GB/T 17625.7-2013 The grid connection of all equipment with input current of 75A and below per phase is mainly used for low impedance grid connection with high short-circuit capacity.

The flicker level is expressed as Pst (short-term flicker index), measured at the power supply port of the device, and the device contains no power to the public distribution network The DG unit should not exceed Pst=1.0 under normal load conditions.

When DG equipment delivers power to the public power supply network, the DG emission value Pst measured at the equipment power supply port should not exceed 0.5. Measurement The measurement should be carried out under the normal load and steady state of the power supply network, and there is no voltage flicker from the public power supply network. When DG equipment is provided When the electric network transmits power, the contribution to Pst and dmax (the maximum relative voltage change) is the two most important parameters, which represent the The amount of harassment caused by the power grid.

GB/T 17626.15-2011 specifies how to evaluate Pst, Plt (long-term flicker index), dmax and dc (relative steady-state voltage change), It also provides detailed specifications for the evaluation of these directly measured parameters.

The voltage change is usually related to the power receiving, switching and de-loading of the DG equipment, which is expressed as the steady-state electricity immediately before various voltage change events.

The percentage of pressure. These voltage fluctuations are related to the size of the device.

Voltage change limit, especially dc, d(t) (relative voltage change characteristics) and dmax specified in GB/T 17625.2-2007 The number can be used to regulate the operation of DG equipment.