This phenomenon can only occur in multi-circuit rights of way, when one circuit is de-energized while a parallel circuit remains energized.
Figure 3-5 shows a typical double circuit tower with one circuit in service (I) and another circuit out-of-service (II).
Due to inter-circuit capacitive coupling, voltage is induced in an open (not earthed) line if the parallel circuit is energized. The normal induced voltage in the de-energized circuit (Ucircuit_II) can be estimated as:
_ = _ +
Eq. 3-1
where Cs is the inter-circuit capacitance between circuits I and II and Cp is the capacitance to ground of circuit II
(A) One stuck circuit breaker pole during Busbar + Line Energisation
(B) Two stuck circuit breaker poles during Busbar + Line Energisation Pole fails to close
A B C
Energised phase
A B C
˜ ˜
˜
Energised phase
Substation-A
Substation-B
Busbar Shunt-Reactors
A B C
Energised phase
A B C
˜ ˜
˜
Substation-A
Substation-B
Busbar Shunt-Reactors
Pole fails to close Pole fails to close Disconnected phase
Disconnected phase Disconnected phase
Figure 3-5 Capacitances in Double-Circuit Transmission Line
This normal induced voltage in the de-energized circuit is typically just a small fraction of the inducing voltage. As an illustrative example, the normal induced voltage in the 500kV double circuit referred to in Section 3.1.1 is approximately 16.3% of the inducing 500kV source, assuming Cs = 1.05 nF/km and Cp = 5.39 nF/km.
However, the installation of shunt reactors can introduce resonant conditions at (or near to) power frequency for certain operating topologies and degrees of shunt compensation. Under resonance (or near resonance) conditions, the induced voltages on the de-energized circuit are several orders of magnitude higher than those calculated with Eq. 3-1 and can over-stress the line connected equipment. Early identification of these topologies will allow implementation of cost-effective mitigation solutions at the design stage. When assessing these scenarios, it is essential to consider both aspects related to the resonant overvoltages: (i) amplitude and (ii) duration.
Figure 3-6 illustrates three operating scenarios in which resonance can be observed in a shunt-compensated de-energized circuit, for certain size of shunt reactors.
c) Case 1 reproduces a possible situation where one circuit is energized while the parallel circuit is out of service. A resonant circuit can be formed in the de-energized circuit depending on the size of the installed shunt reactors. This is a steady-state condition – i.e. the resonant condition will be present in the de-energized circuit as long as the parallel circuit is de-energized.
d) Case 2 reproduces a possible situation where a fault occurs in the energized circuit while the parallel circuit is out of service. A resonant circuit can be formed in the de-energized circuit depending on the size of the installed shunt reactors. This is a temporary condition excited by the fault in the parallel circuit– i.e. the resonant condition will be present until the fault is cleared.
e) Case 3 reproduces a scenario where there is a fault in the de-energized circuit while the parallel circuit is in service. This scenario could arise as follows:
o During the maintenance outage of one circuit with the other parallel circuit still in service (or energized), earths are applied to the disconnected circuit. A resonant circuit can be formed if one or two phases of the earthing switch fail to close (i.e. effectively creating a SLG or LLG fault on the de-energized circuit) resulting in high overvoltages on un-earthed phase(s) of the disconnected
C
SC
Pcircuit. The resonant condition will last as long as the unbalanced earthing remains or as long as the parallel circuit is energized.
or
o During normal operation of both circuits, a SLG or LLG fault occurs in one of them and it is cleared by three-phase tripping – i.e. the faulted circuit is now de-energized. A resonant circuit can be formed resulting in high overvoltages on the healthy phase(s) of the disconnected circuit. This is a temporary condition – i.e. if resonance occurs, high overvoltages will be present in the de-energized circuit only for the duration of the fault (i.e. until extinction of secondary arc) or until the auto-recloser brings the circuit back into service.
An example of typical amplitude and location of resonant overvoltages on a 765kV double circuit construction is presented in Figure 4-34 for the three cases described above as a function of the shunt compensation degree.
Figure 3-6 Risk of resonance in shunt compensated double-circuit lines
The scenarios shown in Figure 3-6 can electrically arise under various network topologies, other than the standard double-circuit construction with shunt reactors directly connected to the line. A few examples are illustrated in the following subsections. These examples may seem unrealistic during normal operating conditions, but they can arise as a result of extraordinary switching operations during commissioning, maintenance of equipment or during emergency situations as part of a blackstart restoration path. Identification of these critical topologies is essential to guarantee that the equipment is not overstressed.
Closed
Risk of resonance for shunt compensation degrees of
Risk of resonance for shunt compensation degrees of
Risk of resonance for shunt compensation degrees of
60-100%
Resonant condition for duration of fault
3.2.2.1 Busbar Shunt Reactors + Double Circuit Tr ansmission Line
This example presents two credible topologies leading to resonance in a double-circuit transmission line due to the interaction with busbar shunt reactors. The dangerous topology arises when the busbar (with the shunt reactor) and one of the circuits are de-energized while the parallel circuit remains energized from a remote end, thus coupling energy to the reactor + de-energized circuit combination.
Figure 3-7 Double-Circuit Line and Busbar Shunt Reactors
Topology 1:
Figure 3-7 (a) shows a busbar section in substation B with two line feeders and one shunt reactor connected to it.
Cct ii is energized from substation A and open at substation B. A resonant circuit can be formed upon opening the parallel cct#1 circuit breaker in Substation A. This topology effectively leaves the busbar shunt reactor directly connected to the de-energized circuit (cct#1). Resonance occurs between the busbar shunt reactor and the capacitance of the de-energized circuit (cct#1), with energy coupled from cct ii, via inter-circuit capacitive coupling.
Topology 2:
Figure 3-7 (b) shows another situation where resonance can occur in a similar network topology. In this case, cct#2 is energized from substation A and open at substation B while cct#1 is connected to Substation B (without voltage) but open at Substation A. A resonant circuit can be formed upon closing the shunt-reactor circuit breaker. The resonant circuit is identical to the previous topology.
3.2.2.2 Power Transformer, Tertiary Shunt Reactors and Double Circuit Transmission Line
This example presents two possible topologies leading to resonance in a double-circuit transmission line due to the interaction with shunt reactors connected to the tertiary winding of a power transformer. The dangerous topology arises when the transformer (with the tertiary shunt reactor) and one of the circuits are de-energized while the parallel circuit remains energized from a remote end, thus coupling energy to the transformer/reactor + de-energized circuit combination.
Similar to the example described in section 3.2.2.1 for busbar shunt reactors, Figure 3-8 shows the network topology where a resonant circuit can be formed. The description of the switching scenarios and topologies is the same as in section 3.2.2.1, with the circuit reactance arising from the series combination of tertiary reactors and power transformer reactance.
cct #1
cct #2
cct #1
cct #2
Figure 3-8 Double-Circuit Line and Transformer Tertiary Shunt Reactors