Units-related bugs

[luke-kiernan]

There's a number of bugs that have come up that could be prevented by better units handling. I'm using this discussion to keep track of them, so that as we prototype units-aware things, we structure things so as to better prevent and detect such things.

Bad arithmetic

• FixedAdmittance load calculation in system checks #1618: P = Re(V^2/Y), when it should be P = Re(V^2 * Y). Context: calculating total load (power) from fixed admittance elements, assuming V = 1.0 p.u..

Oddities of a stateful unit system

• with_units_base bug #1590: the implementation of with_units_base on a component inadvertently broke the association between the component's unit setting and the system's unit setting.
• Disallow zero base power #1575: when base power is zero, internal conversion arithmetic can silently set the power to x/0.0 = Inf or 0.0/0.0 = NaN.

Implicit units

• Power flows: solving Ax = b for a DC power flow, where A is in p.u. and b comes from calling get_active_power on the components. Will give incorrect results when the system's unit setting isn't p.u.
• CATS system: parsing a spreadsheet implicitly in p.u., when the data is in natural units.