Power flow

Introduction

The power flow is a numerical analysis of the flow of electric power in an interconnected system, where that system is considered to be in normal steady-state operation. Power-flow or load-flow studies are important for planning future expansion of power systems as well as in determining the best operation of existing systems. The principal information obtained from the power-flow study is the magnitude and phase angle of the voltage at each bus, and the real and reactive power flowing in each line. In this page we’ll go into some details about what are the inputs and outputs of a load flow simulatio, what is expected from a power flow result, how the validation feature of PowSyBl works, what power flow implementations are compatible with PowSyBl, and how to configure PowSyBl for the different implementations.

Inputs

The only input for a power flow simulation is a network.

Outputs

The power flow simulation output consists of a network, which has been modified based on the simulation results, and some metrics regarding the computation: whether or not it has converged, in how many iterations. Depending on the loadflow implementation the content of these metrics may vary. In the network, the modified variables are the active and reactive power at the terminals, and the voltage and angle at all buses. If the load flow calculation has converged, PowSyBl also identifies the slack bus(es) used by the simulator: the bus at which the power balance was done for each synchronous component.

Validation

Expected results

A load-flow result is considered acceptable if it describes a feasible steady-state of a power system given its physics and its logics. More practically, generations of practitioners have set quasi-standard ways to describe them that makes it possible to define precise rules. They are described below for the different elements of the network.

Buses

The first law of Kirchhoff must be satisfied for every bus for active and reactive power:

\[\begin{align*} & \left| \sum_{branches} P + \sum_{injections} P \right| \leq \epsilon \\ & \left| \sum_{branches} Q + \sum_{injections} Q \right| \leq \epsilon \\ \end{align*}\]

Branches

Lines and two windings transformers are converted into classical PI models:

TODO: make a proper sketch

    V1*exp(j*theta1)     rho1*exp(j*alpha1)             r+j*x              rho2*exp(j*alpha2)   V2*exp(j*theta2)
        (P1,Q1)->      ____O/O__________________________-----__________________________O/O_____     <-(P2,Q2)
                                            |           -----           |
                                  g1+j*b1  |_|                         |_| g2+j*b2
                                            |                           |
                                           _|_                         _|_
                                            _                           _
                                            .                           .
  • Power flow results:
    • \((\|V_1\|, \theta_1)\) and \((\|V_2\|, \theta_2)\): Magnitude (kV) and angle (\(°\)) of the voltage at the sides 1 and 2, respectively.
    • \((P_1, Q_1)\) and \((P_2, Q_2)\): Active power (MW) and reactive power (MVAr) injected in the branch on each side.
  • Characteristics:
    • \((\rho_1, \alpha_1)\) and \((\rho_2, \alpha_2)\): Magnitude (no unit) and angle (\(°\)) of the ideal transformers ratios on each side.
    • \((g_1, b_1)\) and \((g_2, b_2)\): Complex shunt impedance on each side (S).
    • \((r, x)\): Complex series impedance \((\Omega)\).

Thanks to Kirchhoff laws (see the line and 2-winding transformer documentation), estimations of powers are computed according to the voltages and the characteristics of the branch:

\[(P_1^{calc}, Q_1^{calc}, P_2^{calc}, Q_2^{calc}) = f(\text{Voltages}, \text{Characteristics})\]

Three-windings transformers

To be implemented, based on a conversion into 3 two-windings transformers.

Generators

Active power

There may be an imbalance between the sum of generator active power setpoints \(\text{targetP}\) on one side and consumption and losses on the other side, after the load flow optimization process. Note that, if it is possible to modify the setpoints during the computation (for example if the results were computed by an Optimal Power Flow and not a Power Flow), there should be no imbalance left.

In case of an imbalance between the sum of generator active power setpoints \(\text{targetP}\) on one side and consumption and losses on the other side, the generation \(P\) of some units has to be adjusted. The adjustment is done by modifying the generation of the generators connected to the slack node of the network. It may also be done by modifying the loads connected to the slack node. The slack node is a computation point designated to be the place where adjustments are done.

This way of performing the adjustment is the simplest solution from a mathematical point of view, but it presents several drawbacks. In particular, it may not be enough in case of a large imbalance. This is why other schemes have been developed, called “distributed slack nodes”.

Generators or loads are usually adjusted proportionally to a shift function to be defined. Three keys have been retained for the validation (\(g\) is a generator): Usual ways of defining this function, for each equipment that may be involved in the compensation (generator or load), read:

  • proportional to \(P_{max}\): \(F = f \times P_{max}\)
  • proportional to \({targetP}\): \(F = f \times targetP\)
  • proportional to \(P_{diff}\): \(F = f (P_{max} - targetP)\)

\(f\) is a participation factor, per unit. For example, a usual definition is: \(f\in\{0,1\}\): either the unit participates or not. The adjustment is then done by doing: \(P <- P \times \hat{K} \times F\) where \(\hat{K}\) is a proportionality factor, usually defined for each unit by \(\dfrac{P_{max}}{\sum{F}}\), \(\dfrac{targetP}{\sum{F}}\) or \(\dfrac{P_{diff}}{\sum{F}}\) depending on the adjustment mode (the sums run over all the units participating to the compensation).

Voltage and reactive power

If the voltage regulation is deactivated, it is expected that:

\[\left| targetQ - Q \right| < \epsilon\]

If the voltage regulation is activated, the generator is modeled as a \(PV\) node. The voltage target should be reached, except if reactive bounds are hit. Then, the generator is switched to \(PQ\) node and the reactive power should be equal to a limit. Mathematically speaking, one of the following 3 conditions should be met:

\[\begin{align*} |V - targetV| & \leq && \epsilon && \& && minQ & \leq & Q \leq maxQ \\ V - targetV & < & -& \epsilon && \& && |Q-maxQ| & \leq & \epsilon \\ targetV - V & < && \epsilon && \& && |Q-minQ| & \leq & \epsilon \\ \end{align*}\]

Loads

To be implemented, with tests similar to generators with voltage regulation.

Shunts

A shunt is expected not to generate or absorb active power:

\[\left| P \right| < \epsilon\]

A shunt is expected to generate reactive power according to the number of activated sections and to the susceptance per section \(B\): \(\left| Q + \text{#sections} * B V^2 \right| < \epsilon\)

Static VAR Compensators

Static VAR Compensators behave like generators producing 0 active power except that their reactive bounds are expressed in susceptance, so that they are voltage dependent.

\(targetP = 0\) MW

  • If the regulation mode is OFF, then \(targetQ\) is constant
  • If the regulation mode is REACTIVE_POWER, it behaves like a generator without voltage regulation
  • If the regulation mode is VOLTAGE, it behaves like a generator with voltage regulation with the following bounds (dependent on the voltage, which is not the case for generators): \(minQ = - Bmax * V^2\) and \(maxQ = - Bmin V^2\)

HVDC lines

To be done.

VSC

VSC converter stations behave like generators with the additional constraints that the sum of active power on converter stations paired by a cable is equal to the losses on the converter stations plus the losses on the cable.

LCC

To be done.

Transformers with a ratio tap changer

Transformers with a ratio tap changer have a tap with a finite discrete number of position that allows to change their transformer ratio. Let’s assume that the logic is based on deadband: if the deviation between the measurement and the setpoint is higher than the deadband width, the tap position is increased or decreased by one unit.

As a result, a state is a steady state only if the regulated value is within the deadband or if the tap position is at minimum or maximum: this corresponds to a valid load flow result for the ratio tap changers tap positions.

Implementations

The following power flow implementations are supported:

Configuration

You first need to choose which implementation to use in your configuration file:

loadflow:
  default-impl-name: "hades2"

or

loadflow:
  default-impl-name: "OpenLoadFlow"

Then, configure some generic parameters for all load flow implementations:

load-flow-default-parameters:
    voltageInitMode: DC_VALUES
    transformerVoltageControlOn: false
    specificCompatibility: true 

The parameters may also be overridden with a JSON file, in which case the configuration will look like:

{
  "version" : "1.0",
  "voltageInitMode" : "PREVIOUS_VALUES",
  "transformerVoltageControlOn" : true,
  "phaseShifterRegulationOn" : false,
  "noGeneratorReactiveLimits" : true,
  "specificCompatibility" : false,
  "extensions" : {
    ...
  }
}

Available parameters

noGeneratorReactiveLimits
The noGeneratorReactiveLimits property is an optional property that defines whether the load-flow is allowed to find a setpoint value outside the reactive limits of a generator or not.

phaseShifterRegulationOn
The phaseShifterRegulationOn property is an optional property that defines whether the load-flow is allowed to change taps of a phase tap changer or not.

specificCompatibility
The specificCompatibility property is an optional property that defines whether the load-flow runs in legacy mode (implementation specific) or not.

transformerVoltageControlOn
The transformerVoltageControlOn property is an optional property that defines whether the load-flow is allowed to change taps of a ratio tap changer or not.

voltageInitMode
The voltageInitMode property is an optional property that defines the policy used by the load-flow to initialize the voltage values. The default value for this property is UNIFORM_VALUES. The available values are:

  • UNIFORM_VALUES: v=1pu, \(\theta=0\)
  • PREVIOUS_VALUES: use previous computed value from the network
  • DC_VALUES: preprocessing to compute DC angles

Default configuration

The default values of all the optional properties are read from the load-flow-default-parameter module, defined in the configuration file.

Open LoadFlow configuration

TODO

Hades2 configuration

Once you have chosen Hades2 as a load flow implementation, you have to provide the path to your Hades2 installation:

hades2:
    homeDir: <PATH_TO_HADES_2>

In this section, you may also choose the option debug: true to tell PowSyBl not to erase the temporary folder created by Hades2 for the calculation. Thismakes it possible to check what happened on the Hades2 side for debugging purposes.

Then, you need to configure the load flow calculation itself, because the sensitivity analysis of Hades2 relies on an initial load flow calculation. For example:

hades2-default-parameters:
    balanceType: PROPORTIONAL_TO_LOAD
    computedConnectedComponentNumber: 0
    reactiveCapabilityCurveWithMoreThanThreePoints: THREE_POINTS_DIAGRAM
    withMinimumReactance: false
    minimumReactancePerUnit: 0.0007
    anglePerte: false
    remoteVoltageGenerators: true
    dcMode: false
    hvdcAcEmulation: false

The complete list of available parameters for the Hades2 load flow is available here.

Going further

To go further about the power flow with PowSyBl, check the following pages: