Series Compensator

The Series Compensator component, shown in Table 1, is a Schematic Editor library block from the FACTS sub-category of the Microgrid core library category. It is a three-phase component that can be used to compensate the physical inductance of transmission lines, thus reducing voltage drop across it. The component model is composed of a capacitor and a Metal Oxide Varistor (MOV) per phase.

Table 1. Series Compensator component in the Schematic Editor core library
component	component dialog window	component parameters
Series Compensator		Property tabs: General MOV Curve Bypass switch

Series Compensator schematic block diagram

The component consists of a capacitor in parallel with a MOV model and a bypass switch, as shown in Figure 1. The MOV is modelled as an association of a diode-leg, voltage sources, and a resistor. The Energy counter block computes the energy that flows through the MOV and sends a signal to close the bypass switch when the defined limit is reached (on the Bypass Switch tab).

Figure 1. Internal structure of the Series Compensator

The voltage sources are set to act as the voltage threshold for MOV conduction. The diode-leg will perform the conduction characteristics of the MOV, while the resistor is determined by the chosen curve slope.

Component dialogue box and parameters

The Series Compensator component dialogue box consists of three tabs for specifying parameters of the component.

Tab: General

In this component tab, general parameters of the Series Compensator can be specified.

Table 2. General parameters description
Parameter	Code name	Description
Series capacitance	C	Value of the capacitance used in the line compensation. [F]
Frequency	fc_sc	Frequency of the system. [Hz]
Execution Rate	execution_rate	Execution rate of the inner signal processing components. [s]

Tab: MOV curve

In this component tab, different MOV curves can be specified. Currently, four types of data inputs are available: One exponential function, Three exponential functions, Manual adjustments, and Coordinates. The MOV preview and Check stability buttons are present are present in all options, and allow you to see the MOV curve from the data entries and to check if the simulation has become numerically unstable. The simulation is run considering the Typhoon HIL linear representation of the curve obtained from the input data and can be seen through the MOV preview button.

Figure 3. MOV curve – One exponential function parameters

Table 3. MOV curve – One exponential function parameters
Parameter	Code name	Description
Vref	vp	Voltage threshold defined for MOV conduction. [V]
Iref	Iref	Current that flows through the MOV when its voltage level achieves Vref. [A]
N	n_exp	Exponent value of the function that determines the MOV curve. This parameter can be changed in order to adjust the curve as desired.

The Series Compensator model "MOV curve – One exponential function" is as follows:

V_{M O V} = V_{r e f} \frac{I^{(\frac{1}{N})}}{I_{r e f}}

Figure 4. MOV curve – Three exponential function parameters

Table 4. MOV curve – Three exponential function parameters
Parameter	Code name	Description
Vref	vp	Voltage threshold defined for MOV conduction. [V]
Iref	Iref	Current that flows through the MOV when its voltage level achieves Vref. [A]
N	n_exp	Exponent value of the function that determines the MOV curve. This parameter can be changed in order to adjust the curve as desired.

The Series Compensator model "MOV curve – Three exponential function" is as follows:

V_{M O V} = V_{r e f} k_{n} \frac{I^{(\frac{1}{α_{n}})}}{I_{r e f}}, n = 1, 2, 3

Figure 5. MOV curve – Three exponential function graph

The transition points from one segment to the other are determined by:

P_{1} = {S_{1} * (\frac{S_{1}}{S_{2}})}^{(\frac{α_{1}}{(α_{2} - α_{1})})}

P_{2} = {S_{2} * (\frac{S_{2}}{S_{3}})}^{(\frac{α_{2}}{(α_{3} - α_{2})})}

here:

S_{n} = \frac{I_{r e f}}{k_{n}^{α_{n}}}, n = 1, 2, 3

Figure 6. MOV curve – Coordinates parameters

Table 5. MOV curve – Coordinates parameters
Parameter	Code name	Description
Voltage vector	v_vector	Vector of voltage values of the MOV curve. [V]
Current vector	i_vector	Vector of current values associated to the values of the voltage vector. [A]

Figure 7. MOV curve – Manual adjustments parameters

Table 6. MOV curve – Manual adjustments parameters
Parameter	Code name	Description
Vref	vp	MOV curve slope [V]
Iref	Iref	Current that flows through the MOV when its voltage level achieves Vref. [A]
MOV curve slope	mov_slope	Percentage slope of the curve where 0 means 0º of inclination and 1 means 89.9999º of inclination, with the angle reference in the break node of the line.

Tab: Bypass switch

The bypass switch is used to protect the MOV from exceeding its thermal rating in case of excessive energy absorption.

Table 7. Bypass Switch parameters
Parameter	Code name	Description
Bypass switch energy limit	bypass_energy	Energy threshold defined for the switch closure. [J]
Bypass resistance	bypass_R	Resistance of the bypass branch. [Ω]
Bypass inductance	bypass_L	Inductance of the bypass branch. [H]

Component Limitations

Numerical instability can occur for some simulation conditions when using this component. The model characteristics can lead the simulation to numerical instability associated with the charging/discharging process of the capacitor due to time step limitations. If the charging/discharging process is substantially fast to the point where information can be lost between time step calculation, the simulation can become unstable. We strongly recommend keeping the relation Tau/simulation_time_step > 2, where Tau is the time constant RC of the component. Stability analysis can be checked by clicking the Check stability button after entering the desired parameters. Evaluation of the stability conditions is also done when compiling the model.

Example model

Overall behavior and control methodologies can be better understood with the use of the given example:

Model name: series compensator example.tse

SCADA interface: series compensator example.cus

Path: examples/models/microgrid/FACTS/series compensator example