EMO-L Electrical Topology Management

Grid Connection Management - Grid Code

Grid codes describes safety, security requirements and obligations to be followed for the correct operation of an electrical system interacting with the public grid. They may vary depending on the site, grid operator and country. Grid codes are normally defined in electricity regulations, norms and/or standards. Also, specific grid codes for DER systems must be applied. When one or several generators are intended to operate in parallel with the supply network, the agreement of the utility is required. The utility specifies the conditions of operation of the generators and specific requirements.

The utility generally requires information concerning the generators, such as:

• Level of the short circuit current injected by the generators in case of fault on the supply network.

• Maximum active power intended to be injected in the supply network.

• Operating principle of the voltage control.

• Capability of the generators to control the power factor of the installation.

In case of fault on the utility supply network, the instantaneous disconnection of the generators is generally required. It is achieved by means of a dedicated control functions specified by the utility. This control function may operate according to the below criteria:

• Under-voltage and over-voltage

• Under-frequency and over-frequency

The islanding function generally orders the opening of the main circuit breaker ensuring the connection of the installation to the utility while the generators continue to supply the totality of the internal consumers or a part only if they are not sized for the full power required. In this case, load shedding must be simultaneously executed with the opening of the main circuit breaker.

Considering the power regulation point of view, these grid connection codes are related to the following:

• Active and reactive powers (power factor) at the point of common coupling.

• Grid protection requirements due to bidirectional electricity flows.

• Voltage and frequency stability requirements.

• Islanding procedures.

When considering the grid connection management, the following options are allowed:

• In autonomous mode, EMO-L Microgrid controller monitors and analyzes the grid in real time, detecting when to disconnect or reconnect to the grid based on electrical conditions. A relay manages the switching device at the interconnection point, following grid codes, with reconnection typically requiring operator authorization.

• External trig order: Islanding or reconnection transition is triggered by the grid operator through an external communication link.

• Internal trig order: Islanding or reconnection sequence is triggered by the grid operator through the HMI or the SCADA.

Coupling Microgrid with the Grid

EMO-L helps to synchronize power sources without interruption of the main busbar’s voltage in which the DER are connected, in other words without interruption of the power delivered to the loads. This is also called as close transition.

When targeting a close transition, the power synchronization is compulsory to avoid any electrical voltage and current transitions among the DER which make their protection relays to trip and even destroy the asset. The power synchronization can be achieved during a reconnection to the grid. Generally, a power synchronization occurs in two phases: the voltage synchronization and the interconnection switch closing when voltages and frequencies are synchronized.

Principles of the Power Synchronization

The following two voltage source sides are considered:

• Reference side - Reference voltage source on which the microgrid is synchronized to.

• Slave side (Microgrid busbar) - side to be synchronized, a DER when the synchronization concerns the connection to the island main busbar.

The objective of the power synchronization is to make the amplitude, frequency and phase angle of the three voltages of the slave side matching the closest possible to the reference side. To achieve this, synchro-coupling features are required for the DER which will be synchronized. The available two options are:

• Synchronization by EMO-L. The DER being in (V/F) control mode, EMO-L adjusts the voltage and frequency set points of the DER to align them with the voltage and frequency of the reference target. The interconnection switching device closing order is sent by EMO-L after checking the voltages synchronism along a window. Once the interconnection switching device is closed, the operating mode of DER will be changed to avoid any issue.

• Synchronization by the DER: The synchro-coupling feature is one of the functions of the DER controller. EMO-L launches the synchronization sequence, providing to the DER controller the handle of the interconnection switching device. The voltages synchronization and the closing order is performed by the DER controller. Once the interconnection device is closed, the operating mode of DER will be changed.

Synchro Check Function

The synchro-checking is to verify the voltages on both sides of an opened interconnection switching device are synchronized (voltages gaps in amplitude, frequency and phase angle are in the tolerances of the settings).

A synchro-check relay is a protective function which avoids the closing of interconnection switching device in case of misalignment of the voltages, which would provoke an electrical transient on the DER being connected. A safe closing of the interconnection switching device is possible only when these values are compatible with the pre-set parameters.