Siemens 7SR158 Argus, Техническое руководство

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Chapter 7) 7SR158 Applications Guide
©2014 Siemens Protection Devices Limited Chapter 7) Page 9 of 24
2.4 Vector Shift (78)
When a Generator is connected to an electricity supply system, the Generator will respond to any transitory
changes in load to keep both voltage and frequency levels as close to constant as possible. When a LoM event
occurs, however, the Generator is unable to respond instantaneously to the sudden, large-scale change in load
and an instantaneous shift in the phase of the Generator voltage can result (Voltage Vector Shift). The sudden
change in load causes a sudden change in cycle length. The cycle length becomes shorter or longer depending
on whether the newly formed island has a surplus or shortage of generation.
Figure 2.4-1 Vector Shift
This vector shift can be in either direction depending on whether load increases or decreases. Where the DG
capacity is insufficient to supply the local load and power is supplemented from the mains then a LoM event will
cause the DG load to increase.
With an increase in load, the voltage would tend to ‘jump back’; as in Figure 2.4-1. This would occur when a trip
on the mains network has left the distributed generation to try and supply the whole of the downstream load. This
would be the usual condition leading to a LOM tripping.
Alternatively, if the local generation is supplying both a local load and the mains network – such as happens with
some industrial systems, disconnection from the mains network would create an islanding condition in which there
may be an excess of electrical supply and the voltage would tend to ‘jump forward’.
There is an obvious link between the 78 element and the 81R (Rate-of-Change-of-Frequency) element. However
the equivalent vector shift seen for expected ROCOF levels should not result in operation of the 78 element:
The highest 81R pick-up setting is 10 Hz/s
This equates to a ROCOF of 0.1 Hz over the 10ms measuring interval
Giving an equivalent vector shift of (360° / 50 Hz) x 0.1 Hz = 0.72°
At 60Hz system frequency, the equivalent is 0.5° vector shift at maximum ROCOF setting
Attempting to differentiate between the two elements at inordinately high levels of ROCOF would impose a delay
on the operation of the 78 element and so this is considered acceptable. Momentary changes in phase can occur
due to load switching operations. These can be as great as 5° and must be allowed for in the setting of the 78
pick-up setting. A setting of 6° is recommended by the G.59 standard for connection of distributed generation to
an electricity supply system, and settings of 8° to 12° are typical.
When the exported power to the mains network is very low, there is a risk that the drop in load will be insufficient
to cause a vector shift. Or will cause a vector shift of limited extent. Generally it is required that exported power
will be of the order of 10-20% of total power generated before a LOM will always be detected.