Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 92

inductive reactance many times greater than the resistance - the X/R ratio is sig-
nificantly greater than unity. On the other hand, distribution lines may have an X / R
ratio close to unity. Typical values for transmission and distribution lines are given
in Table 2.1.
When real power flows through a resistive element the current is in phase with
the voltage and therefore an in-phase voltage drop occurs across the network. There
is no angular shift between the voltages at the sending end and the receiving end of
the circuit. When real power flows through a purely reactive component, no in-
phase voltage drop occurs but there is an angular shift between the voltages at the
two nodes. The opposite is true for reactive power. These relations are encapsulated
in (2.24).
The conclusion from the above is that the transfer of active power has a major
effect on the voltage profile of low voltage systems, whereas reactive power
transfer is the dominant factor in high voltage systems. Apart from thermal loading
considerations, this is one reason why it is important to connect large wind farms to
higher voltage networks. In all cases reactive power transfer is the dominant factor
in transformer voltage drop (sometimes called regulation).
4.3.1.1 Large wind farms
Consider a 400 MW wind farm connected to a 275 kV network by a direct con-
nection. The network impedance is mainly reactive, viewed from the wind farm.
This allows the exchange of large amounts of active power with relatively little
voltage drop and low losses. Any exchange of reactive power between the grid and
wind farm will affect the voltage. Transmission elements generate and use reactive
power. Thus, in lightly loaded conditions there is a surplus of reactive power on the
network, while in heavily loaded situations a deficit occurs. Generation absorbs
the surplus and at a different time or location may have to generate the deficit of
reactive power, or else the network voltage will be outside an acceptable range.
System planners try to ensure that large wind farms and other embedded generation
can contribute to this regulation of network voltage. (See sample grid code in
Appendix 2, Clause CC.S2.3.2.)
Large generators are specified to have a leading or lagging reactive capability at
full load. Ranges of 0.85 power factor lagging to 0.90 power factor leading at full
load output are common. The required power factor range is generally specified at the
terminals of the stator of the generating unit. Target voltages set on the control system
could be as high as 1.08 pu or as low as 0.94 pu to achieve an even network voltage
measured at various load nodes. The control systems (known as automatic voltage
regulators) vary the reactive power generated to match the set point target voltage.
Often a different range is specified, e.g. 0.95 lagging to 0.95 leading for wind farms
connected to the transmission system or 0.95 lagging to unity or 0.98 leading for
wind farms connected to sub-transmission or distribution systems (see Appendix 2,
Clause CC.S2.3.2 (a) and (b)). Care needs to be taken when comparing a utility's
specified power factor ranges to ensure that adjustment is made for the location of the
assessment point, if a fair assessment is to be made regarding the generator's reactive
power available to manage voltage. The narrower range requirement for power factor