The house diagram is used to calculate the power flow from a generator connected to an infinite bus supplying a load. A house diagram plots the frequency-power relationships of the generator and bus on the same axes, with positive bus power on the negative x-axis and positive generator power on the positive x-axis. The intercept of the plots indicates the power PG supplied by the generator to the bus and load. If the load demand is greater than PG, additional power Pbus is drawn from the bus such that Pload = PG + Pbus. For the generator to supply positive power, its no-load frequency must be greater than the system frequency.

1. The House Diagram
Consider the diagram below, where a generator is
connected to an infinite bus which is supplying a load.
In order to calculate how much power from the generator
or bus flows to the load, a house diagram is used. A
house diagram is constructed by taking the frequency-
power plots of two power supply systems and plotting
them on the same diagram, each with its own positive
power axis.
In the diagram above, positive bus power is plotted on the
negative x-axis, positive generator power on the positive
x-axis. Since both systems are operating at the same
frequency, the intercept of the power-frequency plots
indicates how much power is flowing from the generator.
In the diagram above, the generator supplies PG to the

2. combined bus and load. Since the load power demand is
greater than the power supplied by the generator,
additional power Pbus is taken from the bus. The bus
power can be found using
Pload=PG+Pbus
In the case that the generator is supplying more power
than is required by the load, the excess power will flow to
the bus and the value of Pbus in the above equation will
be negative.
It is important to note in the diagram above that the
generator no load frequency is greater than the system
frequency.
fnl>fsys
As the generator frequency falls when the power flow is
increased, the no-load frequency must be greater than the
system frequency in order for the generator to supply
positive power. In the diagram below, the no load
frequency is too low and the generator power is negative,
i.e. the synchronous machine is operating like a motor.
The effect illustrated above is the reason why the
frequency of an oncoming generator must be higher than
the existing system frequency when connecting
generators to a power grid. Most generators automatically
detect negative power flow and disconnect from the grid
when this situation occurs.

3. ASWAN UNIVERSITY
FACULTY OF ENGINEERING
ELECTRICAL ENGINEERING DEPARTMENT
engineer: Ahmed Amin Hassan
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" house diagram concept "