Open

Description

The battery energy storage system shown in Figure 1 features a battery connected to an inverter via a second order DC filter. The inverter is connected to the grid via a RL choke filter. The battery model was implemented from a Matlab/Simulink model , its parameters are described in Mask and Parameters. The DC filter must be designed to limit the battery current ripple and the DC Link voltage ripple. The inverter is responsible for controlling the active and reactive power at its point of connection, which is normally the secondary winding of a transformer. The control modes of the BESS are explained in the Modeling Details section. The inverter model used is an average converter. The choke filter must be designed to limit the total harmonic distortion of the BESS current injected into the grid at the point of common coupling (PCC).

The BESS must be connected to an external step up transformer to the grid. It is recommended to use a star-delta transformer with a base power of 1.2 times the nominal power of the BESS.

Figure 1 BESS components schematic 

Open

Mask and Parameters

System Parameters

Control Loop Parameters

Battery Parameters

Ports, Inputs, Outputs and Signals Available for Monitoring

Ports

Inputs

Outputs

None.

Sensors

Modeling Details

The BESS has several operating modes, as shown in Figure 2. The model operates in either the grid following (grid connected) mode, or grid forming mode (islanded). Within the grid following mode, it can operate in active and reactive power reference regulation or active power and voltage regulation mode. Each mode provides the current references to the current regulator. Each regulator and operation mode is explained in the following sections. 

Figure 2 BESS control diagram with mode selection

Current Regulators

The BESS performs the control of its output currents in the dq reference frame. The structure of current regulator is shown in Figure 3. I_dq  and V_dq  are the grid currents and voltages, respectively, in the dq reference frame at the PCC while V_dqi  are the inverter output voltages. The d axis current corresponds to the active power and the q axis current corresponds to the reactive power. 

Figure 3 Current regulator

The BESS has two modes of operation in terms of its synchronization: 1) the grid-following mode; and 2) the grid-forming mode. For the grid-following mode, the synchronization angle is obtained via a phase locked loop (PLL) on the PCC. Which means the converter follows the frequency set by the grid voltage at the PCC or secondary of the transformer, if externally added. In the grid-forming mode, the synchronization angle of the dq-reference frame is produced by an internal virtual oscillator of the converter. Which means the converter sets the frequency of the distribution grid and acts as a slack bus. The angle selection based on the BESS mode of operation is shown in Figure 4. 

Figure 4 Synchronization angle selection for the dq reference frame

Each mode is explained in the following subsections.

The Grid Following Mode(s)

In this mode of operation, the inverter synchronizes itself to the voltage at the PCC. The d axis current references are generated to follow the active power references provided to the inverter. The active power regulator is shown in Figure 5, where the regulator output is limited to respect the current limit defined. 

Figure 5 Active power regulator

The q axis current references are generated to follow the reactive power references provided to the inverter. The reactive power regulator is shown in Figure 6, where the regulator output is limited to respect the current limit defined.

Figure 6 Reactive power regulator

In Figure 6, the reference for reactive power, Q_Gref , can be either directly provided, or can be provided by a voltage regulator as shown in Figure 7. In this figure, V _­ is the positive sequence voltage at the PCC. Qlim_up and Qlim_lo is the upper and lower limit on the reactive power of the converter. 

Figure 7 Positive sequence voltage regulator

The Grid Forming Mode

In this mode of operation, the inverter synchronizes itself to the virtually generated angle to form the frequency of the grid. The d and q current references are generated to control the voltage at the PCC to be equal to V_dqref , For the active and reactive powers at the transformer secondary to directly correspond to the d and q currents respectively, the references selected are V_d = Vref and V_q =0 pu. The voltage regulator in the grid forming mode is shown in Figure 8. The regulator is limited such that the current references respect the current limit defined.

Figure 8 Grid forming voltage regulator.

References