#######
 Design
#######


Network object is the overall container
=======================================

The ``pypsa.Network`` is an overall container for all network components;
components cannot exist without a network. It is also the object on which
calculations, such as power flow and optimal power flow, are performed.


Buses are the fundamental nodes
===============================

The bus is the fundamental node to which all loads, generators, storage units,
lines, transformers and links attach. You can have as many components attached
to a bus as you want. The bus's role is to enforce energy conservation for all
elements feeding in and out of it (i.e. like Kirchhoff's Current Law).


.. image:: img/buses.png


Energy flow in the model
========================

Energy enters the model in generators, storage units or stores with
higher energy before than after the simulation, and any components
with efficiency greater than 1 (e.g. heat pumps).

Energy leaves the model in loads, storage units or stores with higher
energy after than before the simulation, and in lines, links or
storage units with efficiency less than 1.



Data is stored in pandas DataFrames
===================================

To enable efficient calculations on the different dimensions of the
data, data is stored in memory using pandas DataFrames.

Other power system toolboxes use databases for data storage; given
modern RAM availability and speed considerations, pandas DataFrames
were felt to be preferable and simpler.


To see which data is stored for each component, see :doc:`components`.


Static component data
=====================

For each component type (line, transformer, generator, etc.), which
must be uniquely named for each network, its basic static data is
stored in a pandas DataFrame, which is an attribute of the network
object, with names that follow the component names:

* ``network.buses``
* ``network.generators``
* ``network.loads``
* ``network.lines``
* ``network.transformers``

These are all pandas DataFrames, indexed by the unique name of the
component.

The columns contain data such as impedance, capacity and the buses to
which components are attached. All attributes for each component type
are listed with their properties (defaults, etc.) in :doc:`components`
and are accessible from the network object in
e.g. ``network.components["Bus"]["attrs"]``.


Network components cannot exist without a network to hold them.



.. _time-varying:

Time-varying data
=================

Some quantities, such as generator ``p_set`` (generator active power
set point), generator ``p`` (generator calculated active power), line
``p0`` (line active power at ``bus0``) and line ``p1`` (line active
power at ``bus1``) may vary over time, so PyPSA offers the possibility
to store different values of these attributes for the different
snapshots in ``network.snapshots`` in the following attributes of the
network object:

* ``network.buses_t``
* ``network.generators_t``
* ``network.loads_t``
* ``network.lines_t``
* ``network.transformers_t``

These are all dictionaries of pandas DataFrames, so that for example
``network.generators_t["p_set"]`` is a DataFrame with columns
corresponding to generator names and index corresponding to
``network.snapshots``. You can also access the dictionary like an
attribute ``network.generators_t.p_set``.

Time-varying data are defined as ``series`` in the listings in  :doc:`components`.


For **input data** such as ``p_set`` of a generator you can store the
value statically in ``network.generators`` if the value does not
change over ``network.snapshots`` **or** you can define it to be
time-varying by adding a column to ``network.generators_t.p_set``. If
the name of the generator is in the columns of
``network.generators_t.p_set``, then the static value in
``network.generators`` will be ignored. Some example definitions of
input data:


.. code:: python

    #four snapshots are defined by integers
    network.set_snapshots(range(4))

    network.add("Bus", "my bus")

    #add a generator whose output does not change over time
    network.add("Generator", "Coal", bus="my bus", p_set=100)

    #add a generator whose output does change over time
    network.add("Generator", "Wind", bus="my bus", p_set=[10,50,20,30])

In this case only the generator "Wind" will appear in the columns of
``network.generators_t.p_set``.

For **output data**, all time-varying data is stored in the
``network.components_t`` dictionaries, but it is only defined once a
simulation has been run.


Internal use of per unit
===========================

Per unit values of voltage and impedance are used internally for
network calculations. It is assumed internally that the base power is
1 MVA. The base voltage depends on the component.

.. _unit-conventions:

Unit Conventions
=================

The units for physical quantities are chosen for easy user input.

The units follow the general rules:

.. list-table:: Title
   :widths: 25 75
   :header-rows: 1

   * - Quantity
     - Units
   * - Power
     - MW/MVA/MVar (unless per unit of nominal power, e.g. generator.p_max_pu
       for variable generators is per unit of generator.p_nom)
   * - Time
     - h
   * - Energy
     - MWh
   * - Voltage
     - kV phase-phase for bus.v_nom; per unit for v_mag_pu, v_mag_pu_set, v_mag_pu_min etc.
   * - Angles
     - radians, except transformer.phase_shift which is in degrees for easy input
   * - Impedance
     - Ohm, except transformers which are pu, using transformer.s_nom for the base power
   * - CO2-equivalent emissions
     - tonnes of CO2-equivalent per MWh_thermal of energy carrier

.. _sign-conventions:

Sign Conventions
================


The sign convention in PyPSA follows other major software packages,
such as MATPOWER, PYPOWER and DIgSILENT PowerFactory.

* The power (p,q) of generators or storage units is positive if the
  asset is injecting power into the bus, negative if withdrawing power
  from bus.
* The power (p,q) of loads is positive if withdrawing power from bus, negative if injecting power into bus.
* The power (p0,q0) at bus0 of a branch is positive if the branch is
  withdrawing power from bus0, i.e. bus0 is injecting into branch
* Similarly the power (p1,q1) at bus1 of a branch is positive if the
  branch is withdrawing power from bus1, negative if the branch is
  injecting into bus1
* If p0 > 0 and p1 < 0 for a branch then active power flows from bus0
  to bus1; p0+p1 > 0 is the active power losses for this direction of
  power flow.

AC/DC Terminology
=================

AC stands for Alternating Current and DC stands for Direct Current.

Some people refer to the linearised power flow equations for AC
networks as "DC load flow" for historical reasons, but we find this
confusing when there are actual direct current elements in the network
(which also have a linearised power flow, which would then be DC DC load
flow).

Therefore for us AC means AC and DC means DC. We distinguish between
the full non-linear network equations (with no approximations) and the
linearised network equations (with certain approximations to make the
equations linear).

All equations are listed in the section :doc:`power_flow`.


Set points are stored separately from actual dispatch points
============================================================

Dispatchable generators have a p_set series which is separate from the
calculated active power series p, since the operators's intention may
be different from what is calculated (e.g. when using distributed
slack for the active power).