Microgrid Solution

Author: Elena

2023-01-09 11:11

Technology is rapidly driving the transition to smart grids. At the same time, micro-grid has been the new trend, driven by technological improvements and the aim of transition to the clean energy, in order to improve reliability and resilience of electrical grids. This article explains what is micro-grid is, and introduces the GoodWe solution with grid-tied inverter and AC coupled inverter.

 

What is Micro-grid

A Micro-grid is a small-scale group of interconnected loads and distributed energy resources that acts as a single controllable entity with respect to the grid. It can operate in grid-connected or island mode depending on the connection state to the grid.

A micro-grid generally operates while connected to the grid, but it can also operate on its own and have the capability of keeping electricity producing for the systems connected to it in case of the grid power outage. A micro-grid might have number of connected distributed energy resources, such as solar, energy storage, wind turbines or traditional generator.

Advanced micro-grids enable the local power generation system to keep the local grid running, even if there is an outage of the utility grid. In addition, for some areas, where there is no connection to the utility grid, a micro-grid can still provide the power production. 

 

Background

Solar power as the most abundant renewable energy resource can be converted to electrical or thermal energy. Solar inverter is used to convert the DC energy produced by a solar panel into AC electricity. However, the commonly used grid-tied inverter is grid-dependant and work in accordance with the grid. The inverter will stop working when the grid is out of power. Regular grid failures can last up to long time, which requires to use alternative forms of energy. To date, diesel generators have been a traditional preferred backup solution. A hybrid micro-grid system utilizes a diversity of PV, diesel generator is an optimum way for saving fuel, reducing emissions and maintenance. And energy storage inverters can be integrated into the micro-grid system as a zero-emissions substitute for diesel generator.   

 

Advantage of Micro-grid in PV+Storage System

Nowadays, the growing environmental problems and energy policy changes make the energy storage inverter very popular. Especially in some residential scenarios, the energy storage inverter will be used for the retrofit or expansion project to upgrade existing PV system to storage system.

The AC coupled inverter and grid-tied inverter can form a typical micro-grid. In this Micro-grid system, when the grid fails, the grid-tied inverter can still work under the micro-grid provided by AC coupled inverter’s off grid output. It helps to balance out supply and demand and make the system more flexible and reliable.

 

GoodWe Micro-grid Solution (GoodWe grid-tied inverter with third-party AC-coupled inverter)

Take GoodWe DNS and Tesla Powerwall as the example. Since Powerwall is an AC coupled device, it is compatible with typical inverters installed on a home energy system.

As shown in the system diagram (figure 1), the DNS can operate under both on-grid and island (off-grid) modes.The ATS is used to switch “on-grid” and “island” modes in the microgrid system. As a general rule, the ATS switches seek connection to the primary utility power source by default and will only connect to the alternate power source when grid failure or requested by operator command to do so . In addition, the ATS will send the signal to DNS inverter to notice it whether the AC supply is grid or AC coupled inverter’s off-grid output now.

 

Grid connected mode

When grid is available, the system works like a traditional AC Retrofit project. The Tesla Powerwall is working under on-grid mode, and the ATS switches utility grid source and DNS can work properly. PV power from DNS supports the loads first, then charge the battery through Tesla Powerwall, the surplus power will be able to export to grid which could be limited. The battery will discharge if PV power is insufficient to support the home loads.

Solar supply priority: Loads→Battery→Export

 

Island mode

When the grid is down, the Tesla Powerwall will switch to backup mode with an off-grid output. At this time, the ATS will automatically connect the off-grid output, which provides AC supply to DNS inverter. Then the DNS can continue to work. The PV will still support the load and then charge the battery. However, The surplus power cannot be exported to the utility. To prevent overcharging the batteries as well as overloading the inverter, it is necessary to regulate the PV power. In addition, the export power limit function should be disabled during the island mode.

- Frequency Shifting and Freq-Watt response function

In island operation mode, the grid-tied inverter is no longer connected to utility grid, the frequency is then imposed by the storage inverters’ off grid output. When the battery reaches the bulk charge voltage or the battery SOC is high, the Powerwall’s frequency will be ramped up linearly and the DNS will respond by curtailing power according to the frequency- Watt Curve setting, until the battery enters a low-power discharge state. The configuration must meet the following rules.

1.          The Solar inverter and AC-coupled inverter should choose the same safety code (in this case, we choose Germany Safety code)

2.          The PV inverter regulates the battery charge of the AC-coupled inverter using Frequency shift, which exploits the active power reduction versus frequency feature of PV inverter (respond to the Frequency-Watt curve in the figure below)

3.          When the frequency is shifted to over 51.5Hz, PV inverter will shut down.

 

- Export power limit function should be disabled

As the system diagram shows, the CT is connected to on grid side. When the grid is available, the output limit function will work properly. When grid is failure in spite of the Tesla have off-grid output, the export power limit function should be disabled.

When grid is outage, the current detected by the CT at the on grid side will be zero. If the power limit function still works, the inverter will continue to export power as the CT cannot reach the setpoint. However, the output of grid-tied inverter should be controlled by the frequency of the Tesla’s output in island mode. The output power limit function will impact the output of DNS. Therefore, it should be disabled in the event of grid failure.

The system should have a signal to notice DNS that the AC is grid or micro-grid to change the state of the export power limit function. The ATS will send the signal to DNS through the remote shutdown terminal in sync with the switch state. The configuration must meet the following rules.

1.          DNS must upgrade the DSP and ARM to the special firmware, which import the logic for distinguishing of the state of the remote shutdown terminal.

2.          If using micro-grid mode, the DNS do not have remote shutdown function.

3.          Port 2 and Port3 of the remote shutdown(in the following) terminal will be used.

(1)        When Port2 and Port3 is short circuit, the inverter is in on-grid mode.

(2)        When Port2 and Port3 is open circuit, the inverter is in is-land mode, and the export power limit function will be disabled.

4.          A relay will be used on site to control the state of circuit according to the signal from ATS.

 

In addition to the micro-grid solution with GoodWe grid-tied inverter and third-party AC-coupled inverter, GoodWe provides the solution with A-BP (GoodWe AC coupled inverter) for US market. Please refer to the relevant document by visiting GoodWe Solar Community. 

 

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