# Grid model

## Introduction

In this page the different network components are described in terms of electrotechnical representation. Each component is identified through a unique ID, and optionally by a name that is easier to interpret for a human. Note that the equipments in the IIDM model may be flagged as fictitious, in order to fine tune the network modelling.

TODO: when we have aliases, add a description here too.

## Network core model

### Network

In IIDM, the network is constituted of substations, which are themselves constituted of voltage levels. All the equipments are then connected to the voltage levels. The network comprises metadata in IIDM:

• a case date: the date and time of the target network that is being modelled
• a forecast distance: the number of minutes between the network generation date and the case date

Available extensions

### Substation

A substation in IIDM represents a specific geographical location with a set of equipments connected to one or several voltage levels. It comprises metadata in IIDM:

• a country: to specify in which country the substation is located. It is an optional attribute, not set on fictitious test networks for example.
• a set of geographical tags: they make it possible to accurately locate the substation
• a TSO information: to track to which TSO the substation belongs

Available extensions

### Voltage Level

A voltage level in IIDM represents a set of equipments connected together with the same nominal voltage, physically close to each other (~ 1-100m). Two voltage levels may be connected through a line (they are then located in different substations) or through transformers (they are then located within the same substation).

A voltage level in IIDM comprises some metadata:

• a nominal voltage (in $$kV$$)
• a low voltage limit and a high voltage limit (in $$kV$$): they are both optional metadata. The voltage should always remain within these bounds, otherwise it means that the equipments will suffer extra wear compared to their normal use
• a topology information: indicates whether the voltage level is described in node/breaker or bus/breaker view

#### Node/breaker topology

TODO: explain the topology.

In node/breaker topology, the voltage level is described with the finest level of detail. See the sketch below for an example. TODO: add sketch of voltage level. The topology is then described as a graph structure, where busbar sections, injections or branches are connected to the vertices. When an equipment is connected to a vertex, in the IIDM descrition it corresponds to a Terminal object. It is not possible to connect two equipments on the same vertex. The edges are constituted of switches or internal connections. See the following sketch corresponding to the previous example: TODO: add sketch of voltage level topology graph.

Busbar section
A busbar section is a non impedant element used in a node/breaker substation topology to connect equipments.

Switch
TODO

Internal connection
An internal connection is a non-impedant connection between two components in a voltage level.

#### Bus/breaker topology

In bus/breaker topology, the voltage level is described with a coarser level of detail. See the sketch below for an example. TODO: add sketch of voltage level in bus/breaker topology. The topology is then described as a graph structure, where the vertices are buses and the edges are switches.

Bus
A bus is a set of equipments connected at the same voltage. When an equipment is connected to a bus, in the IIDM descrition it corresponds to a Terminal object. In IIDM there is thus one Terminal per connected equipment.

Switch
TODO: explain the difference with node/breaker switches

### Injections

An injection in IIDM is any AC equipment with a single connection point to a voltage level. Below are the different types of injections supported by PowSyBl.

#### Generator

A generator is an active equipment that injects active power, and injects or consumes reactive power. It may be controlled to hold a voltage or reactive setpoint somewhere in the network (not necessarily directly where it is connected).

TODO: add a sketch where the sign convention is indicated.

Characteristics

Attribute Unit Description
$$MinP$$ MW The minimal active power
$$MaxP$$ MW The maximum active power
$$TargetP$$ MW The active power target
$$TargetQ$$ MVAr The reactive power target
$$TargetV$$ kV The voltage target
$$RatedS$$ MVA The rated nominal power
$$ReactiveLimits$$ - Operational limits of the generator (P/Q/U diagram)

Specifications

• The minimal active power (in $$MW$$), expected to be lower than the maximal active power. The target $$P$$ is necessarily comprised between the two.
• Setpoints for generators ($$targetV$$, $$targetP$$ and $$targetQ$$):
• They follow the generator sign convention: a positive value of $$targetP$$ means an injection into the bus.
• A positive value for the $$targetP$$ and the $$targetQ$$ means a negative value at the corresponding terminal (which is in passive-sign convention).
• A set of reactive limits can be associated to a generator. All the reactive limits modelings available in the library are described here.
• the rated nominal power (MVA) TODO: explain what it is.

• Either the generator is regulating the voltage, and the voltage setpoint is required, or it is not regulating and the reactive power setpoint is required instead.

A generator in IIDM comprises some metadata:

• the energy source, which can be:
• HYDRO
• NUCLEAR
• WIND
• THERMAL
• SOLAR
• OTHER
• The participation to regulation (through a boolean)
• The regulating terminal, which can be local or remote: it is the specific connection point on the network where the setpoint is measured.

Available extensions

A load is a passive equipment representing a delivery point that consumes active and reactive power.

TODO: add a sketch where the sign convention is indicated.

Characteristics

Attribute Unit Description
$$P0$$ MW The active power setpoint
$$Q0$$ MVar The reactive power setpoint

Specifications

• Initial values for loads P0 and Q0 follow the passive-sign convention:
• Flow out from the bus has a positive sign.
• Consumptions are positive.

• The load type, which can be:
• UNDEFINED
• AUXILIARY
• FICTITIOUS

#### Battery

A battery on the electric grid is an energy storage device that is either capable of capturing energy from the grid or of injecting it into the grid. The electric energy on the grid side is thus transformed into chemical energy on the battery side and vice versa. The power flow is bidirectional and it is controlled via a power electronic converter.

Characteristics

Attribute Unit Description
$$P0$$ MW The Constant active power
$$Q0$$ MVar The Constant reactive power
$$MinP$$ MW The Minimal active power
$$MaxP$$ MW The Maximum active power

Available extensions

#### Dangling line

The IIDM network may be connected to other networks for which a full description is not available. In this case, a boundary line exists between the two networks. In the IIDM model of the fully described network, that connection is represented through a dangling line, which represents the part of that boundary line which is known. A dangling line is thus a passive or active component that aggregates a line chunk and a constant power injection, in passive-sign convention. The active and reactive power setpoints are fixed: the injection represents the power flow that would occur through the connection, were the other network fully described.

TODO: add a sketch with the sign convention. TODO: add a link to the Merging documentation. TODO: update the documentation according to Anne’s developments.

Characteristics

Attribute Unit Description
$$P0$$ MW The active power setpoint
$$Q0$$ MVar The reactive power setpoint
$$R$$ $$\Omega\$$ The series resistance
$$X$$ $$\Omega\$$ The series reactance
$$G$$ S The shunt conductance
$$B$$ S The shunt susceptance

Specifications

• $$P0$$ and $$Q0$$ are the active and reactive power setpoints
• $$R$$, $$X$$, $$G$$ and $$B$$ correspond to a fraction of the original line and have to be consistent with the declared length of the dangling line.

In case the line is a boundary, a UCTE Xnode code is defined besides the characteristics of the Table. See the UCTE-DEF documentation page to learn more about this format. This code is actually related to ENTSOE, not only UCTE: it is a key to match two dangling lines and reconstruct the full boundary line.

Available extensions

#### Shunt Compensator

TODO: add a description. TODO: add a sketch with the sign convention. TODO: explain that there are two shunt models: linear and non-linear. Shunt compensators follow a passive-sign convention:

• Flow out from bus has positive sign.
• Consumptions are positive.

Characteristics

Attribute Unit Description
$$bPerSection$$ S The Positive sequence shunt (charging) susceptance per section
$$MaximumSectionCount$$ - The maximum number of sections that may be switched on
$$CurrentSectionCount$$ - The current number of section that may be switched on
$$TargetV$$ kV The voltage target
$$TargetDeadband$$ kV The deadband used to avoid excessive update of controls

TODO: redo the Table and specifications to be up to date with Miora’s work.

Specifications

• A section of a shunt compensator is an individual capacitor or reactor. A value of bPerSection positive means it is modeling a capacitor, an equipment that injects reactive power into the bus. A value of bPerSection negative means a reactor, an equipment that can absorb excess reactive power from the network.
• The current section count is expected to be greater than one and lesser or equal to the maximum section count.
• Regulation for shunt compensators does not necessarily model automation, it can represent human actions on the network e.g. an operator activating or deactivating a shunt compensator). However, it can of course be integrated on a power flow calculation or not, depending of what is wanted to be shown.
• In case of a capacitor, the value for its Q will be negative.
• In case of a reactor, the value for its Q will be positive.

#### Static VAR Compensator

TODO: add a description with sign convention. TODO: add a sketch with the sign convention. It may be controlled to hold a voltage or reactive setpoint somewhere in the network (not necessarily directly where it is connected).

Characteristics

Attribute Unit Description
$$Bmin$$ S The minimum susceptance
$$Bmax$$ S The maximum susceptance
$$VoltageSetpoint$$ kV The voltage setpoint
$$ReactivePowerSetpoint$$ MVar The reactive power setpoint

Specifications

• $$Bmin$$ and $$Bmax$$ are the susceptance bounds of the static VAR compensator TODO: add the equation that links $$B$$ to $$Q$$, and say that $$B$$ has to be comprised between the bounds.
• The voltage setpoint is required when the regulation mode is set to VOLTAGE.
• The reactive power setpoint is required when the regulation mode is set to REACTIVE_POWER.

• The regulation mode, which can be:
• VOLTAGE
• REACTIVE_POWER
• OFF Note that it is different than the generators’ regulation definition, which is only done through a boolean.
• The regulating terminal, which can be local or remote: it is the specific connection point on the network where the setpoint is measured.

### Branches

A branch in IIDM Grid model is any AC equipment with two or more connection points to the network. Below are the different types of branches supported by PowSyBl.

#### Line

AC Power lines are modeled using a standard $$\pi$$ model with distributed parameters.

With series impedance $$z$$ and the shunt admittance on each side $$y_1$$ and $$y_2$$:

\begin{align*} \begin{array}{lcl} z & = & r+j.x\\ y_1 & = & g_1 +j. b_1\\ y_2 & = & g_2 +j. b_2 \end{array} \end{align*}

The equations of the power line, in complex notations, are as follow:

\begin{align*} & \left(\begin{array}{c} I_{1}\\ I_{2} \end{array}\right)=\left(\begin{array}{cc} y_{1}+\dfrac{1}{z} & -\dfrac{1}{z}\\ -\dfrac{1}{z} & y_{2}+\dfrac{1}{z} \end{array}\right)\left(\begin{array}{c} V_{1}\\ V_{2} \end{array}\right) \end{align*}

Characteristics

Attribute Unit Description
$$R$$ $$\Omega\$$ The series resistance
$$X$$ $$\Omega\$$ The series reactance
$$G1$$ S The first side shunt conductance
$$B1$$ S The first side shunt susceptance
$$G2$$ S The second side shunt conductance
$$B2$$ S The second side shunt susceptance

Available extensions

##### Tie Line

A tie line is an AC line sharing power between two neighbouring regional grids. It is constituted of two half lines. A tie line is created by matching two dangling lines with the same Xnode code. It has line characteristics, with $$R$$ (resp. $$X$$) being the sum of the series resistances (resp. reactances) of the two half lines. $$G1$$ (resp. $$B1$$) is equal to the sum of the first half line’s $$G1$$ and $$G2$$ (resp. $$B1$$ and $$B2$$). $$G2$$ (resp. $$B2$$) is equal to the sum of the second half line’s $$G1$$ and $$G2$$ (resp. $$B1$$ and $$B2$$).

###### Half Line

Characteristics

Attribute Unit Description
$$R$$ $$\Omega\$$ The series resistance
$$X$$ $$\Omega\$$ The series reactance
$$G1$$ S The first side shunt conductance
$$B1$$ S The first side shunt susceptance
$$G2$$ S The second side shunt conductance
$$B2$$ S The second side shunt susceptance

TODO: describe xnodeP and xnodeQ in the java doc and here with a sentence.

#### Transformers

##### Two windings transformer

A two windings power transformer is connected to two voltage levels (side 1 and side 2) that belong to a same substation. Two windings transformers are modeled with the following equivalent $$\Pi$$ model:

With the series impedance $$z$$ and the shunt admittance $$y$$ and the voltage ratio $$\rho$$ and the angle difference $$\alpha$$ and potentially parameters from the current step of a ratio tap changer and/or a phase tap changer, we have:

$\begin{array}{lcl} r & = & r_{nom}.\left(1+\dfrac{r_{r, tap} + r_{\phi, tap}}{100}\right)\\ x & = & x_{nom}.\left(1+\dfrac{x_{r, tap} + x_{\phi, tap}}{100}\right)\\ g & = & g_{nom}.\left(1+\dfrac{g_{r, tap} + g_{\phi, tap}}{100}\right)\\ b & = & b_{nom}.\left(1+\dfrac{b_{r, tap} + b_{\phi, tap}}{100}\right)\\ \rho & = & \dfrac{V_{2nom}}{V_{1nom}}.\rho_{r, tap}.\rho_{\phi, tap}\\ \alpha & = & \alpha_{\phi, tap}\\ z & = & r + j.x\\ y & = & g + j.b\\ V_{0} & = & V_{1}.\rho e^{j\alpha}\\ I_{0} & = & \dfrac{I_{1}}{\rho e^{-j\alpha}}\\ \end{array}$

Using the above notation, the equations of the two windings transformer, in complex notations, are as follow:

$\left(\begin{array}{c} I_{1}\\ I_{2} \end{array}\right)=\left(\begin{array}{cc} \rho\text{²}(y+\dfrac{1}{z}) & -\dfrac{1}{z}\rho e^{-j\alpha}\\ -\rho\dfrac{1}{z} e^{j\alpha} & \dfrac{1}{z} \end{array}\right)\left(\begin{array}{c} V_{1}\\ V_{2} \end{array}\right)$

Characteristics

Attribute Unit Description
$$R_{nom}$$ $$\Omega$$ The nominal series resistance at the side 2 of the transformer
$$X_{nom}$$ $$\Omega$$ The nominal series reactance at the side 2 of the transformer
$$G_{nom}$$ S The nominal magnetizing conductance at the side 2 of the transformer
$$B_{nom}$$ S The nominal magnetizing susceptance at the side 2 of the transformer
$$V_{1\ nom}$$ kV The rated voltage at side 1
$$V_{2\ nom}$$ kV The rated voltage at side 2
$$RatedS$$ MVA The normal apparent power

Specifications

• A ratio tap changer and/or a phase tap changer can be associated with a two windings power transformer.
• For a two windings transformer, the normal apparent power shall be identical at both sides 1 and 2.

Available extensions

##### Three windings transformer

A three windings power transformer is connected to three voltage levels (side 1, side 2 and side 3) that belong to the same substation. We usually have:

• Side 1 as the primary side (side with highest rated voltage)
• Side 2 as the secondary side (side with the medium rated voltage)
• Side 3 as the tertiary side (side with the lowest rated voltage)

A three windings transformer is modeled with three legs, where every leg model is electrically equivalent to a two windings transformer. For each leg, the network bus is at side 1 and the star bus is at side 2.

Characteristics

Attribute Unit Description
$$RatedU0$$ kV The rated voltage at the star bus

TODO: place RatedU0 on the sketch.

Specifications

• A ratio tap changer and/or a phase tap changer can be associated to all three sides of a three windings power transformer. Only one tap changer (either ratio or phase tap changer) is allowed to be regulating on the equipment at a given time.

Available extensions

##### Three windings transformer leg

Characteristics

Attribute Unit Description
$$R$$ $$\Omega\$$ The nominal series resistance specified at the voltage of the leg
$$X$$ $$\Omega\$$ The nominal series reactance specified at the voltage of the leg
$$G$$ S The nominal magnetizing conductance specified at the voltage of the leg
$$B$$ S The nominal magnetizing susceptance specified at the voltage of the leg
$$RatedU$$ kV The rated voltage
$$RatedS$$ MVA The normal apparent power

Specifications

### DC components

#### HVDC Line

An HVDC line is connected to the DC side of two HVDC converter stations, either an LCC station or a VSC station.

Characteristics

Attribute Unit Description
$$R$$ $$\Omega\$$ The resistance of the HVDC line
$$NominalV$$ kV The nominal voltage
$$ActivePowerSetpoint$$ MW The active power setpoint
$$MaxP$$ MW The maximum active power

Specifications

• The HVDC line operation depends on a converters mode, which indicates the flow direction. In the specification it is thus mandatory to define ConvertersMode, which can be:
• SIDE_1_RECTIFIER_SIDE_2_INVERTER: the flow goes from side 1 to side 2
• SIDE_1_INVERTER_SIDE_2_RECTIFIER: the flow goes from side 2 to side 1

The flow sign is thus given by the type of the converter station: the power always flows from the rectifier converter station to the inverter converter station. At a terminal on the AC side, P and Q follow the passive sign convention. P is positive on the rectifier side. P is negative at the inverter side.

• The active power setpoint and the maximum active power should always be positive values.

#### HVDC Converter Station

An HVDC converter station converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. Electronic converters for HVDC are divided into two main categories: line-commutated converters (LCC) and voltage-sourced converters (VSC).

##### LCC Converter Station

An LCC converter station is made with electronic switches that can only be turned on (thyristors). Below are some characteristics:

• Use semiconductors which can withstand voltage in either polarity
• Output voltage can be either polarity to change the power direction
• Current direction does not change
• Store energy inductively
• Use semiconductors which can turn on by control action
• Turn-off and commutation rely on the external circuit

Characteristics

Attribute Unit Description
$$PowerFactor$$ - Ratio between the active power $$P$$ and the apparent power $$S$$.

Specifications

• The power factor is equal to $$\dfrac{P}{\sqrt{P^{2} + Q^{2}}}$$ and should be between -1 and 1. Note that at the terminal on the AC side, $$Q$$ is always positive: the converter station always consumes reactive power.
##### VSC Converter Station

A VSC converter station is made with switching devices that can be turned both on and off (transistors). Below are some characteristics:

• Use semiconductors which can pass current in either direction
• Output voltage polarity does not change
• Current direction changes to change the power direction
• Store energy capacitively
• Use semiconductors which can turn on or off by control action
• Turn-off is independant of external circuit

Characteristics

Attribute Unit Description
$$VoltageSetpoint$$ kV The voltage setpoint for regulation
$$ReactivePowerSetpoint$$ MVar The reactive power setpoint for regulation

Specifications

• The voltage setpoint (in kV) is required if the voltage regulator is on for the VSC station.
• The reactive power setpoint (in MVar) is required if the voltage regulator is off for the VSC station.
• A positive value of $$ReactivePowerSetpoint$$ means an injection into the bus, thus a negative value at the corresponding terminal (which is in passive-sign convention). TODO: check the sign convention
• A set of reactive limits can be associated to a VSC converter station. All the reactive limits modelings available in the library are described here.

• The participation to regulation (through a boolean)

In this section, the additional models available in IIDM are described: reactive limits, current limits, voltage regulation, phase and ratio tap changers. They can be used by various equipment models.

### Reactive limits

The reactive limits may be used to model limitations of the reactive power of generators, VSC converter stations and batteries.

#### Min-Max reactive limits

With the min-max reactive limits, the reactive power does not depend on the active power. For any active power value, the reactive power value is in the [minQ, maxQ] interval.

#### Reactive capability curve

With the reactive capability curve limits, the reactive power limitation depends on the active power value. This dependency is based on a curve provided by the user. The curve is defined as a set of points that associate, to each active power value, a minimum and maximum reactive power value. In between the defined points of the curve, the reactive power limits are computed through a linear interpolation.

### Current limits

Some equipments have operational limits regarding the current value, corresponding to the equipment’s physical limitations (related to heating). The current limits may be set in IIDM for lines, dangling lines, two windings transformers and three windings transformers. Current limits are defined by at most one permanent limit and/or any number of temporary limits. The permanent limit sets the current value (in A) under which the equipment can safely be operated for any duration. The temporary limits can be used to define higher current limitations corresponding to specific operational durations. A temporary limit thus has an acceptable duration. The component on which the current limits are applied can safely remain between the preceding limit (it could be another temporary limit or a permanent limit) and this limit for a duration up to the acceptable duration.

### Phase tap changer

A phase tap changer can be added to either two windings transformers or three windings transformers’ legs.

Specifications

A phase tap changer is described by a set of tap positions (or steps) within which the transformer or transformer leg can operate. Additionally to that set of steps, it is necessary to specify:

• the lowest tap position
• the highest tap position
• the position index of the current tap (which has to be within the highest and lowest tap position bounds)
• whether the tap changer is regulating or not
• the regulation mode, which can be CURRENT_LIMITER, ACTIVE_POWER_CONTROL or FIXED_TAP: the tap changer either regulates the current or the active power.
• the regulation value (either a current value in A or an active power value in MW)
• the regulating terminal, which can be local or remote: it is the specific connection point on the network where the setpoint is measured.
• the target deadband, which defines a margin on the regulation so as to avoid an excessive update of controls

The phase tap changer can always switch tap positions while loaded, which is not the case of the ratio tap changer described below.

TODO: check what happens when setting isRegulating to true and FIXED_TAP as regulating mode

Each step of a phase tap changer has the following attributes:

Attribute Unit Description
$$r_{\phi, tap}$$ % The resistance deviation in percent of nominal value
$$x_{\phi, tap}$$ % The reactance deviation in percent of nominal value
$$g_{\phi, tap}$$ % The conductance deviation in percent of nominal value
$$b_{\phi, tap}$$ % The susceptance deviation in percent of nominal value
$$\rho_{\phi, tap}$$ p.u. The voltage ratio in per unit of the rated voltages
$$\alpha_{\phi, tap}$$ $$^{\circ}$$ Angle difference

### Ratio tap changer

A ratio tap changer can be added to either two windings transformers or three windings transformers’ legs.

Specifications

A ratio tap changer is described by a set of tap positions (or steps) within which the transformer or transformer leg can operate (or be operated offload). Additionally to that set of steps, it is necessary to specify:

• the lowest tap position
• the highest tap position
• the position index of the current tap (which has to be within the highest and lowest tap position bounds)
• whether the tap changer is regulating or not; a ratio tap changer always regulates on the voltage
• the regulation value (in kV)
• the regulating terminal, which can be local or remote: it is the specific connection point on the network where the setpoint is measured.
• the target deadband, which defines a margin on the regulation so as to avoid an excessive update of controls
• whether the ratio tap changer can change tap positions onload or only offload

Each step of a ratio tap changer has the following attributes:

Attribute Unit Description
$$r_{r, tap}$$ % The resistance deviation in percent of nominal value
$$x_{r, tap}$$ % The reactance deviation in percent of nominal value
$$g_{r, tap}$$ % The conductance deviation in percent of nominal value
$$b_{r, tap}$$ % The susceptance deviation in percent of nominal value
$$\rho_{r, tap}$$ p.u. The voltage ratio in per unit of the rated voltages

## Going further

TODO: create a tutorial showing how to create the FourSubstationsNodeBreakerFactory network of the tests.