Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 203

Island network

Network fault

Power
system

Wind farm

Consumer
load

Consumer
load

Figure 5.35 Distributed generation protection

the connection of embedded generating plant to the public electricity suppliers'
distribution systems) in the United Kingdom require that embedded generators are
fitted with loss of mains (LOM) detection.
The LOM condition is normally detected using either ROCOF or vector shift
protection relays. ROCOF protection relies on the assumption that, when an
islanding condition occurs, the local generation will not balance the trapped load.
Consequently, the system frequency will change at a rate depending on the power
imbalance and the stored energy of the island network, E island , as previously
expressed in (5.1):

df

dt ¼ D P imbalance f 0

Hz = s

2 E island

When selecting the threshold setting for relay activation, two conflicting fac-
tors must be considered. On one hand, since the power imbalance may be small,
and hence the ROCOF low, the relay should be made as sensitive as possible.
Conversely, if, for example, a large generator is tripped from the system, the system
frequency will initially fall rapidly, as outlined in Section 5.2, causing the ROCOF
protection to activate, exacerbating further the original event (Littler et al. , 2005;
SEI, 2004). Hence, the relay should be made as insensitive as possible. A com-
promise setting is, therefore, required to meet the conflicting requirements, while
ensuring that consumer safety is paramount. The recommended setting in Great
Britain is 0.125 Hz/s, while the equivalent figure for Ireland was increased from
0.125 to 0.5 Hz/s. The significant difference arises because Great Britain, being a
much larger system, has correspondingly greater stored energy relative to a credible
outage. Hence the prospective ROCOF due to a system event is correspondingly
less.

The higher threshold figure for Ireland may not always avoid nuisance trip-
ping. Consider a summer night (low demand) period with a generation capacity of
3,000 MW supplying the system load of 2,500 MW. If the system H value is
4 seconds, and a 350 MW infeed is lost, the initial ROCOF can be calculated as

df

dt ¼ D P f 0

350 50

E system ¼ 3000 ¼ 0 : 73 Hz = s 2 2 4