ABB Relion 670 series, Техническое руководство

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• Limitation of earth fault current at stator ground fault
• Damping of transient overvoltages in the generator system
• Required insulation level to withstand overvoltage in “healthy” phases at single phase ground
fault
• Δ or Y coupled stator windings
• Connection of a single generator via a step up transformer or connection of more than one
generator via a common step up transformer
Normally the generator system has some kind of high resistance grounding, giving ground fault
current within the range 5 – 20 A, thus preventing serious damages in case of stator ground faults.
Direct grounding will give too high ground fault current level. Isolated generator system will give
risk of transient overvoltages.
Below some alternatives for generator system grounding are shown;
A B C
IEC110000066-1-en.vsd
IEC11000066 V1 EN-US
Figure 224: Generator grounding alternatives
A High-resistance grounding with a neutral point resistor
B Effective high-resistance grounding via a distribution transformer
C High-resistance grounding via a grounded wye-broken delta transformer
These earthing alternatives are characterized by the following properties:
A: High-resistance grounding with a neutral point resistor
This grounding method utilizes a high resistance in the primary circuit by inserting resistor R
N
between the generator neutral and the ground. The actual resistance value of R
N
is generally in
order of kΩ. Such high resistance is required in order to limit the primary ground fault current to a
quite small value (i.e. always < 20A and quite often < 10A primary). Actual primary ground fault
current can be calculated as follows:
Section 7 1MRK 502 066-UUS B
Impedance protection
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Technical manual