Temporary Over-Voltages and System Grounding 20/761
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20/759 Temporary over-voltages and 20 system grounding Contents 20.1 Theory of over-voltages 20/761 20.2 Analysis of ungrounded and grounded systems 20/762 20.2.1 Analysis of an ungrounded system 20/762 20.3 The necessity for grounding an electrical system 20/765 20.4 Analysis of a grounded system 20/765 20.4.1 Solid neutral grounding system 20/766 20.4.2 Impedance neutral grounding system 20/766 20.5 Arc suppression coil or ground fault neutralizer 20/767 20.6 Ground fault factor (GFF) 20/768 20.6.1 An ungrounded or isolated neutral system 20/769 20.6.2 A grounded neutral system 20/769 20.7 Magnitude of temporary over-voltages 20/769 20.8 Insulation co-ordination 20/769 20.9 Artificial neutral grounding of a three-phase three-wire system 20/770 20.10 Grounding of generators 20/771 Relevant Standards 20/772 List of formulae used 20/772 Further Reading 20/772 Temporary over-voltages and system grounding 20/761 20.1 Theory of over-voltages IIgg¢¢ + (–120∞ ) = 3 I g¢ R A three-phase balanced system has all the three phasors Vᐉ I g¢ of voltage and current 120∞ apart, as illustrated in Figure N Y 20.1(a) for a conventional anti-clockwise rotation. These Vᐉ I g¢ phasors are known as positive sequence components. B During a fault, this balance is disturbed and the system becomes unbalanced being composed of two balanced components, one positive and the other negative sequence Cg Cg Cg Vᐉ (Figures 20.1(a) and (b)). For a description of the effects G of these components, refer to (Section 12.2(v)). During I g¢ I g¢ a ground fault, zero phase sequence components also appear, which are single phasor components and combine three equal phasors in phase, as shown in Figure 20.1(c). G This is the residual voltage, V , that appears across the g IIgg = ÷3 ¢ ground circuit, i.e. between the neutral and the ground as illustrated in Figure 20.12. This voltage is responsible for a fault current, Ig · Ig will flow through the grounded Figure 20.2 An ungrounded or isolated neutral system (circuit neutral when it is a three-phase four-wire neutral grounded completing through the ground leakage capacitances) system, as shown in Figure 20.12. It will also flow through a three-phase three-wire artificially grounded system when it is grounded through a neutral grounding transformer (Section 20.9) as illustrated in Figures 20.17 and 20.18. In a three-phase three-wire system, which has neither its own grounded neutral nor an artificially created grounded R neutral, there will be no direct ground fault current. But charging currents through the ground leakage capacitances, N Y particularly on an HV system, may still exist, as illustrated B in Figures 20.2–20.4. These currents may develop dangerous over-voltages across the healthy phases, under certain ground circuit a impedance conditions, as discussed in Section 20.2.1(1). It is thus possible to encounter a ground fault, even when Cg Cg Cg Load Ground leakage the system is not grounded, the fault current finding its capacitances return path through the ground leakage capacitances. While c b an LV system, in view of a far too low ground voltage, G Vg (equal to line voltage, Section 20.2.1(1)), as compared to high ground capacitive leakage reactance, X , would cg G Ig cause a near open circuit (Vg/Xcg being too meagre) and stay immune, leaving the grounded conductor floating at Vg = Vᐉ. Figure 20.3 Case of ground fault within the load B B V V V R V V 3 3 3 N Y V V B R R a G V V Y Y Load Cg Cg Cg (a) (b) (c) Ground leakage c b capacitances Positive sequence Negative sequence Zero sequence or residual quantities G Ig Vr = Vy = Vb = V Figure 20.1 Phasor representation of an unbalanced power Figure 20.4 Case of a ground fault on a power system on system on a ground fault the load side 20/762 Electrical Power Engineering Reference & Applications Handbook 20.2 Analysis of ungrounded and 2 2 = IIg¢¢ + g + 2 ◊ IIgg¢ ◊ ¢ ◊◊∞ cos 60 (as in Section grounded systems 15.4.3) 2 2 2 This is related more to an HV system. = (IIIg¢¢¢ + g + g ) = 3 I (but this current may not be enough to trip 20.2.1 Analysis of an ungrounded system ◊ g¢ a protective relay) This is common to The voltage across the healthy phases is now 3 times or •A three-phase three-wire, star connected isolated 73% more than the phase to neutral voltage under healthy neutral system, conditions (voltage across healthy phases = 3 Vg (on a •A delta connected system, or ground fault Vg = Vᐉ)). •A three-phase four-wire, ungrounded system. Under certain line to ground impedance conditions, when the ground circuit may also contain some inductive An ungrounded a.c. system is more often subject to over- reactance, the voltage of the healthy phases may rise voltages. The reason is that even when the system is not further, much above the system voltage, due to resonance connected to ground, it makes a ground connection through and ferro-resonance effects discussed hereafter. All this the coupling of the leakage ground capacitances, as in turn may tend to swing the system to an unstable illustrated in Figure 20.2. For all purposes these state. It may also cause arcing grounds* at the supporting capacitances may be regarded as equal and uniformly insulators, leading to voltage surges, travelling in both distributed, providing a balanced system. In the event of directions along the line, and may be enough to cause a ground fault, this circuit is closed as shown and a damage to the line insulators and the terminal equipment. capacitive current, Ig¢ flows back to the healthy phases The magnitude of over-voltage will depend upon the via these capacitances Cg, Analysing the circuit of Figure actual inductive reactance introduced through the ground 20.2, the following may be derived: circuit. The unintentional introduction of an impedance into the ground leakage capacitive circuit is generally a result of If Cg = electrostatic or leakage capacitance to the ground a ground fault in the main equipment such as a generator, a per phase, providing the return path to the fault transformer, or an induction motor and when one or more current Ig, in the event of a ground fault on the of their phase windings form a part of the ground circuit, as system. This current will be capacitive in nature. illustrated in Figure 20.3. Under such conditions, they will Xcg = ground capacitive reactance per phase introduce their own inductive or resistive impedances or a combination of them into the ground circuit and alter the = 1 2 fC parameters of the ground capacitive circuit. Figure 20.5 p ◊ g illustrates the system of Figure 20.2, drawn simply on a Under transient conditions, f becomes surge frequency, single phase basis. The current through the ground circuit fs and will be zero when the system is healthy, the ground leakage capacitances finding no return path (Figure 20.6). It is X becomes, 1 cg 2 fC not necessary that the equipment itself should develop a p sg◊ ground fault and then only its impedance will be inducted where into the ground circuit. It may be introduced into the 1 circuit even when there is a line or a system ground fs = 2 LC fault, as illustrated in Figure 20.4. The ground circuit p ◊ g may give rise to dangerous voltages in the healthy phases, L being the inductance of the circuit. as analysed below. If Ig = ground fault current Vg = ground potential, which is the same as zero (1) When the external impedance is a resistance sequence voltage or residual voltage or a capacitive reactance Vᐉ = line voltage and Vp = phase voltage Referring to Figure 20.5, if Zg¢ is the external impedance introduced into the natural leakage ground circuit, as a then on a ground fault, say, in phase R, the voltage across consequence of a ground fault, as shown in Figures 20.3 this phase will reduce to zero, while the ground potential across the healthy phases will rise to Vᐉ from a zero level earlier. It will cause a fault current Ig¢ in the healthy *Arcing grounds: This occurs during a temporary ground fault on phases also, through the ground leakage capacitances an ungrounded HV system. It causes an arc between the line conductor such that I = V /X (assuming the ground circuit has and the ground, which may be direct or through the flashover of the g¢ I cg line insulators, due to over-voltage. The ground leakage capacitors no other impedance) and the ground fault current through that may be considered as charged with the line voltage are discharged the grounded circuit, to the ground on developing a ground fault. The supply voltage would charge them again and so the process will repeat until the Ig = IIYB + fault exists. The repeated discharges and charges of ground capacitors during the fault produce arcs and are termed arcing grounds, giving = IIgg¢¢ + (both phasors being 60∞ apart) rise to voltage surges. Temporary over-voltages and system grounding 20/763 Y May attenuate at about 8–10 · 2Vl 6 Xcg Zg Vl 5 3 V Z g¢ g¢ 4 l G V 2 Xcg = Ground leakage capacitive reactance 3 Z g¢ = Additional ground impedance (R, Xc or XL) Figure 20.5 Equivalent ground circuit representing the power 2 in terms of system of Figure 20.2 3 3 across the additional impedance R 1 (max) 1 ¢ g N Y V 2 B 0 123456 Ratio of additional impedance Zg¢ to Xcg a 1 Resistive reactance 2 Capacitive reactance Cg Cg Cg Load 3 Inductive reactance c b Figure 20.7 Influence of external impedance Z g in the ground G ¢ circuit on the system voltage in the event of ground fault in an On a healthy system the ground leakage capacitances find no return ungrounded system path and therefore do not activate.