محولات التأريض (Zigzag ZN) ومقاومات تأريض المحايد (NGR): تحديد المقاس وتحمل العطل

إنشاء نقاط محايد اصطناعية في أنظمة دلتا غير المؤرضة، وحساب ممانعة المركبة الصفرية (Z0)، وتصنيف مقاومات NGR الحراري لمدة 10 ثوانٍ، والحماية الحساسة من الأعطال ال

1. Artificial Neutral Grounding in Delta Networks

Ungrounded medium-voltage systems (delta windings) experience severe transient overvoltages during arcing ground faults. Grounding transformers with Zigzag (ZN) or Wye-Delta (Yd) windings establish an artificial neutral point.

2. Neutral Earthing Regime Comparison

Grounding MethodGround Fault Current (Ig)Transient Overvoltage (Vph-g)Protection Selectivity
Ungrounded Delta< 10 A (capacitive)High (> 2.73 × Vph arcing ground)Hard to locate faulty feeder
Solid GroundedHigh (> 10 kA)Low (1.0 × Vph)Immediate trip, severe equipment damage
Low-Resistance Grounding (NGR)Controlled (100 A - 1000 A)Low (≤ 1.73 × Vph)Fast selective relay trip, minimal damage
High-Resistance Grounding (HRG)5 A - 10 ALow (≤ 1.73 × Vph)Alarm only, system remains in service

3. Neutral Earthing Resistor (NGR) Resistance Formulation

For a desired ground fault current Ifault (typically 200 A - 400 A):

RNGR = (ULL / √(3)) / (Ifault)

Example for 33 kV Grid, 400 A Fault Limit:

RNGR = (33000 / √(3)) / (400) = 47.63 Ω

4. 10-Second Thermal Withstand Rating

NGRs are rated for 10 seconds full current duty per IEEE 32:

Ethermal = Ifault2 · RNGR · tfault [Joules]

Stainless steel grid elements withstand temperature rises up to 760°C without deformation.

*Reference: IEEE Std 142-2007 (Green Book); IEEE Std 32-2020; IEC 60076-6.*