[A Brief Discussion on Security in AC/DC Hybrid Microgrids] With the widespread application of distributed generation and energy storage, and the rapid growth of DC loads, DC power distribution is receiving increasing attention. DC power distribution systems will become an important component of future power distribution technology. On August 26, 2017, the demonstration project of the National 863 Program, "Key Technologies for High-Density Distributed Energy Access to DC Hybrid Microgrids," was put into operation in Shangyu, Zhejiang Province. This marked the first user-side operation of an AC/DC hybrid microgrid in China. The project involved the modification or access of a total distributed photovoltaic capacity of 1.4MW , two 5kW wind power systems, one 250kW/800kWh lead-acid battery energy storage system, and one AC/DC microgrid power conversion and grid system. The highest AC load within the system was approximately 1.2MW , and the highest DC load was approximately 0.9MW . The following diagram shows the topology of the project.
Figure 1. Engineering topology diagram of key technologies for high-density distributed energy access to DC hybrid microgrids.
As can be seen, this project adopts a single-layer DC bus structure . Approximately 0.85 MWp of photovoltaic system is connected to the microgrid's DC bus via a DC/DC converter, while 1.0 MWp of photovoltaic system is connected to the microgrid's AC bus. An approximately 800 kWh lead-acid battery pack maintains the DC bus voltage stability, ensuring stable off-grid operation of the microgrid. A bidirectional AC/DC converter connects the AC and DC buses, and the DC bus uses the DC/DC converter to transform the voltage to supply power to DC loads such as electric vehicle charging stations, LED lighting, and factory equipment. This demonstration project provides a new model for high-density distributed energy access, clearing technical obstacles for the safe and stable access of new energy sources such as photovoltaic and wind power to the main power grid. It is the first time in China that a hybrid AC/DC microgrid has achieved commercial operation on the user side, meeting the diverse needs of future electricity users.
New technologies bring new electrical safety challenges. DC power distribution systems utilize numerous power electronic conversion devices, resulting in a more complex power environment. Clearly, the low-voltage protection devices commonly used in traditional AC power distribution systems are no longer sufficient. Taking ground fault protection in power distribution systems as an example, residual current protection is generally used for AC power distribution systems employing TN, TT, or IT grounding methods. Traditional electronic AC residual current devices utilize zero-sequence current transformers to detect leakage current. Zero-sequence current transformers generate induced current based on Faraday's electromagnetic induction current and changes in magnetic flux, and can only protect against residual current of the AC component.
Figure 2. Working principle of a household residual current device and its internal zero-sequence transformer.
Similar to AC power distribution systems, DC power distribution systems also have grounding configurations of TN, TT, and IT. Since DC electrical equipment may require either a negative or positive power supply, power distribution systems typically use either a two-wire system with two power lines or a three-wire system with an additional intermediate conductor (M-line). Figure 3 below shows a simplified diagram of the commonly used TT and IT grounding configurations in power distribution systems.
Figure 3. Simplified diagram of commonly used TT and IT grounding systems in DC power distribution systems.
For ground fault protection in DC power distribution systems using TT and TN grounding configurations with power supply-side grounding, residual current protection remains the best approach. The IEC has published IEC TS 63053 : 2017, "General requirements for residual current protective devices for DC systems." This refers to a DC residual current protection device (DC-RCD).
Figure 4 DC-RCD IEC Standard
The standard specifies a minimum protection threshold of 20mA and a maximum of 1A.
In AC/DC hybrid power grids, ordinary AC residual current devices (RCDs) are clearly insufficient to protect the DC load side, necessitating the use of DC-RCDs for DC leakage protection. However, DC leakage protection is not yet widely adopted or commercialized, and there are no relevant standards in China. In reality, leakage current components in AC/DC hybrid microgrids are highly complex. Let's examine ABB's simulation model for AC-side grounding faults in AC/DC hybrid microgrids.
Figure 5 Residual current loop of AC-side ground fault in AC/DC hybrid power grid
As can be seen, if the insulation of an AC load is damaged, the resulting residual current will have not only AC components but also DC components. If we continue to use an AC-type residual current device for leakage protection at the AC load end according to conventional thinking, it is obviously not feasible. Not only will it fail to detect DC leakage, but the superimposed DC leakage will also cause the detection core to be pre-magnetized, resulting in an increased tripping value and destroying the original protection function of the residual current device!
Figure 6 shows the alternating AC residual current protection characteristics with DC components.
Figure 7 Various types of residual current
For residual current circuit breakers with complex residual current components, Type B residual current circuit breakers are required. The corresponding IEC standard is IEC 62423 : 2009, "Type B and Type B residual current operated circuit breakers with and without integral overcurrent protection for household and similar uses," and the corresponding domestic standard is GB 22794 : 2008, "Type B residual current operated circuit breakers with and without overcurrent protection for household and similar uses."
Type B residual current protection devices can not only protect against AC residual current and pulsating DC residual current, but also ensure tripping for sinusoidal AC residual current at 1000Hz and below, AC residual current superimposed with smoothed DC residual current, pulsating DC residual current superimposed with smoothed residual current, pulsating DC residual current generated by two-phase or multi-phase rectifier circuits, and smoothed DC residual current. They are well-suited for use in AC/DC hybrid microgrids.
Magtron's SoC chip solution based on iFluxgate technology provides digital integration for Type B leakage protection, offering a cost-effective Type B leakage protection solution for the technological upgrade of RCCB from traditional Type AC/Type A to Type B, and providing better protection for the electrical safety of charging devices.
References
[1] GB22794-2008 Type B residual current operated circuit breakers (Type B RCCB and Type B RCBO) for household and similar purposes with and without overcurrent protection
[2] Hu Hongyu . Comprehensive protection and product standard analysis of low-voltage DC power distribution system . Building Electrical .
[3]ABB . FaultsinLVDCmicrogridswithfront-endc onverters.
