Talking about the Safety in AC/DC Hybrid Microgrid

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With the large-scale application of distributed generation and energy storage and the rapid growth of DC load, DC power distribution has attracted more and more attention. DC power distribution systems will be an important part of future power distribution technologies. On August 26, 2017, the National 863 Project “Key Technologies for High-density Distributed Energy Access DC Hybrid Microgrid” was put into operation in Shangyu, Zhejiang, to realize the first user-side operation of the AC/DC hybrid microgrid in China, engineering transformation or access. Distributed PV total capacity 1.4MW, 2 sets of 5kW wind power generation system, 1 set of 250kW/800kWh lead-acid battery energy storage system, 1 set of AC/DC microgrid power conversion and grid system, the maximum load in the system is about 1.2MW, DC The maximum load is about 0.9MW. The figure below shows the topology of the project.

It can be seen that the project uses a single-layer DC bus structure, about 0.85MWp photovoltaic system is connected to the micro-grid DC bus through the DC/DC converter device, 1.0MWp photovoltaic access micro-grid AC bus, and about 800kWh lead-acid battery pack maintains DC The bus voltage is stable, ensuring stable operation of the microgrid off-grid. A bidirectional AC/DC converter is connected between the AC bus and the DC bus, and the DC bus is used to supply power to the DC load of the electric vehicle charging pile, LED lighting, plant equipment, etc. through the DC/DC converter. The demonstration project provides a new mode of high-density distributed energy access, clearing the technical barriers for the safe and stable access of new energy sources such as photovoltaics and wind power to large power grids, and commercializing the user-side operation of AC/DC hybrid microgrid for the first time in China. It can meet the needs of the diversification of power users in the future.

The new technology will bring about new power safety problems. The DC power distribution system uses a large number of power electronic conversion equipment, and the power environment is more complicated. It is obvious that the low-voltage protection appliances commonly used in the original AC power distribution system cannot. Continue to use, taking the ground fault protection in the power distribution system as an example. For AC power distribution systems using TN, TT, and IT grounding methods, residual current protection is generally used. The traditional electronic AC residual current protector uses a zero-sequence transformer to detect leakage. The zero-sequence transformer is based on the Faraday electromagnetic induction current, and the magnetic flux changes to generate an induced current, which can only protect the residual current of the AC component.

Like the AC power distribution system, the grounding form of the DC power distribution system is also divided into TN, TT, and IT. Since the DC power supply equipment may have a negative power source or a positive power source for the power supply system, there are two power sources in the power distribution system. Two-wire system of the line or a three-wire power distribution system with an intermediate conductor (M line). Figure 3 below is a simplified diagram of the system in the form of TT and IT grounding commonly used in power distribution systems.

For the ground fault protection of DC power distribution systems with TT and TN grounding with power supply grounding, the use of residual current protection is still the best way. The IEC has issued IEC TS 63053:2017 "General requirements for residual current protective devices." For DC systems. The DC residual current protection device (DC-RCD) standard specifies a minimum protection threshold of 20 mA and a maximum of 1 A.

In the DC load side of the AC/DC hybrid power grid, it is obvious that the ordinary AC type residual current protector cannot complete the protection. DC-RCD needs to be used for DC leakage protection. However, the current DC leakage protection has not been promoted and marketed, and there is no relevant in China. standard. In fact, in the AC-DC hybrid microgrid, the leakage component is very complicated. Let us look at ABB's simulation model for the AC-side ground fault in the AC-DC hybrid microgrid.

It can be seen that if the AC load is damaged by insulation, the residual current generated has not only the AC component but also the DC component. If the AC-type leakage protector is still used for the leakage protection at the AC load end according to the inertia thinking, it is obviously not possible. Detecting DC leakage, even superimposed DC leakage will cause the magnetic core to be pre-magnetized, resulting in an increase in the trip value and destroying the original protection function of the residual current protector!

Type B and type residual current operated circuit-breakers with and without integral overcurrent protection for household and similar to IEC standard Uses, corresponding to the domestic standard GB22794:2008 "B-type residual current operated circuit breaker without and with overcurrent protection for household and similar purposes"

The B-type residual current protector not only protects the AC residual current and the pulsating DC residual current, but also superimposes the smoothed DC residual current and the pulsating DC residual current on the 1000 Hz and below sinusoidal AC residual current and AC residual current. The pulsating DC residual current generated by the current, two-phase or multi-phase rectifier circuit and the smooth DC residual current ensure tripping, which can be very well applied in the AC-DC hybrid microgrid.

Magtron's overall SoC chip solution based on iFluxgate technology digitally integrates B-type leakage protection, providing a cost-effective B-type leakage solution for RCCB's upgrade from traditional AC/A to B-type technology. It provides better protection for the safety of the charging equipment.

【references】

[1] GB 22794-2008 Type B residual current operated circuit breakers (B-type RCCB and B-type RCBO) without and with overcurrent protection for household and similar purposes

[2] Hu Hongyu. Comprehensive protection and product standard analysis of low-voltage DC distribution system. Building electrical.

[3]ABB.Faults in LVDC microgrids with front-end converters.