Battery

Description

This component models a rechargeable, electrochemical battery.

Secondary electrochemical batteries (i.e. rechargeable batteries) are of great importance in power systems because they provide a means for storing small quantities of energy that is easily accessible if needed. Some of the main uses for batteries are:

Uninterruptable Power Supplies (UPS)
Battery Energy Storage Systems (BESS) installed in power grids with the purpose of compensating active and reactive power (in this sense they are an extension of the SVC, and therefore are sometimes referred to as SWVC)
Electric vehicles

There are many types of batteries and many factors that affect battery performance. To predict the performance of batteries, different mathematical models exist. None of them are completely accurate, nor do any include all necessary performance effecting factors. This battery model is based on the method proposed in reference [26]. This method is a general approach, in which an ideal controlled voltage source, in series with a resistance, is used to model the battery.

Battery Equivalent Circuit

At every time step, the voltage of the controlled source is computed, based on the state of charge of the battery using two different methods. The first method is based on a non-linear equation (shown below) that uses the actual state of the battery to calculate the no-load voltage. The value of the resistance is assumed to be constant.

Non-Linear Battery Model

Where:

E	No-load voltage [V]
E0	Battery constant voltage [V]
K	Polarization voltage [V]
Q	Battery capacity [Ah]
	Actual battery charge [Ah]
A	Exponential zone amplitude [V]
B	Exponential zone time constant inverse [1/Ah]

The battery voltage equation can be modified as follows in order to be expressed in terms of SOC instead of it:

(BATT-1)

This model is based on a few simplifying assumptions and has some limitations

Assumptions:

During the charge and discharge cycles, the internal resistance is assumed to be constant.
The amplitude of the current does not have any effect on the internal resistance
The discharge characteristics curve of the battery is used to derive the battery parameters, since the discharge and charge characteristics are assumed to be the same.
The amplitude of the current does not have any effect on the capacity of the battery (No Peukert effect).
Temperature does not change the model’s behavior.
Self-discharge of the battery is not represented.
Charge and discharge history does not affect battery characteristics (i.e. No hysteresis)

Limitations:

The battery voltage cannot be negative and the maximum battery voltage is not limited.
The capacity of the battery cannot be negative and the maximum capacity is not limited.

PSCAD General Reference [26]

Input Parameters

ConfigurationConfiguration

Battery Name	Text		Name for the battery (optional). A name should be entered here to avoid compilation warnings.

Data Entry	Choice		Select Shepherd Model or Tabular Data.

Nominal Voltage	REAL	Constant	Enter the nominal voltage of the battery [kV].

Rated Capacity	REAL	Constant	Enter the rated capacity of the battery [kAh]

Loss of Capacity at Nominal Current in an Hour	REAL	Constant	The nominal discharge current, for which the discharge curve has been measured [%].

Initial State of Charge	REAL	Constant	Enter the initial state of charge of the battery (%)

State of Charge	REAL	Output	[%]

Shepherd ModelShepherd Model

Nominal Capacity	REAL	Constant	Enter the nominal capacity based on rated capacity [pu].

Resistive Drop	REAL	Constant	Enter the voltage drop due at nominal discharge current, as a per unit value of the nominal voltage [pu].

Voltage at Exponential Point	REAL	Constant	Enter the voltage at exponential point as a per unit value of nominal voltage [pu].

Capacity at Exponential Point	REAL	Constant	Enter the discharge capacity at exponential point based on rated capacity [pu].

Fully Charged Voltage	REAL	Constant	Enter the fully charged voltage based on nominal voltage [pu].

Discharge Curve of Battery VoltageDischarge Curve of Battery Voltage

State of Charge (SOC) #	REAL	Constant	Enter the state of charge (SOC) of this point in the battery voltage curve [%].

Voltage (V) #	REAL	Constant	Enter the voltage of this point in the battery voltage curve [kV].

Discharge Curve of Internal ResistanceDischarge Curve of Internal Resistance

State of Charge (SOC) #	REAL	Constant	Enter the state of charge (SOC) of this point in the battery internal resistance curve [%].

Resistance (R) #	REAL	Constant	Enter the resistance of this point in the battery internal resistance curve [kV].