Power System Analysis - Renewable Energy in Power Systems - page 166

Types of Nodes (Buses)

PQ Buses
The obvious way of describing the load at a node is by specifying it as an impedance. Simple
uncontrolled loads, such as heaters and incandescent light bulbs, tend to behave as near-con-
stant impedances and thus the power they draw varies with the square of the voltage: a

10%
variation in voltage, which is not uncommon, gives powers ranging from 81 to 121%. Many
loads, however, include some sort of control mechanism such that the power they draw does
remain near constant irrespective of the voltage changes. Most electronic based devices
behave in this way.
Another important example is a distribution network that is supplied via a tap-changing
transformer. Such a transformer senses the voltage on the lower voltage side and changes
the taps so that this is maintained at nearly nominal value. A fi xed load impedance on the
transformer secondary appears on the primary side as a fi xed P and Q demand
irrespective of the transformer ratio. As the taps change the voltage on the primary side of
the transformer may change substantially, but the P and Q are invariant. For example the
tap-changing transformer at the primary substation can be adjusted to achieve a constant
voltage on the 11 kV side irrespective of (modest) voltage changes on the 33 kV side. Thus,
the power drawn by the 11 kV network from the 33 kV network is also held near constant. If
a load fl ow calculation was being performed on the 33 kV network, which would typically
feed several 11 kV networks, it would be reasonable to represent each primary substation
as a constant P and Q . The same is true in load fl ow modelling of the higher voltage
networks.
To summarize, assuming that the voltage at a load bus is kept constant, the load can be
expressed as a fi xed P and Q demand; such nodes are referred to as PQ buses . As will be
explained later, this is a very convenient formulation for the purposes of carrying out the load
fl ow.
Small renewable energy generators also fall in the PQ bus category. Distributed generators
are infrequently called upon to control the network voltage. Instead, they are often confi gured
to operate at near-unity power factor ( Q = 0); in this case, it may be appropriate to label
the node to which they are connected as a PQ node . In the case of fi xed speed wind turbines,
however, the reactive power consumed by the induction generator will be dependent
on voltage, as will the reactive power generated by the power factor correction capacitors.
Many load fl ow software packages include facilities to model induction machines and
related equipment appropriately. The situation is similar with small hydro systems
interfaced to the grid through induction generators. Energy from photovoltaic, wave and
tidal schemes and MW sized wind turbines is fed to the grid through a power electronic
converter. This provides the facility of reactive power injection/extraction at the point of
connection.
To summarize, for relatively small embedded RE generators the P injection depends solely
on the RE source (wind, sun, water) level at the time and the Q injection either on the bus
voltage or on the setting of the power electronic converter. In the latter case the converter
could be regulated to inject active power at a chosen power factor.
According to the generally accepted convention summarized in Figure A.20, at PQ nodes
generators inject active power and so P is positive, whereas for loads, P is negative. The
reactive power Q direction is defi ned similarly.

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