by Neil O’Sullivan, Noja Power

In power distribution grids, switchgear plays a vital role to:

• Control the flow of power
• Isolate faulty sections
• Maintain system stability
• Allow maintenance of grid equipment

Although non-mechanically ganged switchgear may appear cost effective to manufacture, transport and install, their inherent electrical pole discrepancy introduces several technical risks to the application.

Here’s a brief overview of the technical challenges introduced by un-ganged switchgear on three-phase networks.

Single phasing

“Single phasing” occurs when one phase of a three-phase system is lost. In this case, it is the un-ganged operation of one pole of a three-phase switchgear arrangement, leaving the remaining phases live. Undetected single phasing can rapidly result in unsafe conditions, equipment failures, and costly downtime.

Under phase loss conditions, motors, pumps, blowers, and other equipment draw excessive current on the remaining two phases, which quickly overheats the motor windings. Power output is greatly reduced and starting is not possible in this condition. This might leave the equipment in a 'locked rotor' state, which will overheat and damage the equipment even more rapidly.

Single-phase connected loads may experience brownout conditions, where voltage below nameplate rating is supplied. Single phasing caused by un-ganged switchgear is often compared with a three-phase fuse operation. However, the key difference is that fuses fail open, while not all switchgear does. If a single fuse blows, causing consequential excess current draw on the other phases, the other fuses will also interrupt the current.

Un-ganged switchgear which fails to operate could fail in the closed position, relying on upstream protection to detect and interrupt the fault. This has its own challenges. Voltages and currents in a three-phase system do not typically drop to zero when a phase is lost. Return supply through three phase transformers and the capacitive coupling of power lines to the earth ensure that some voltage remains present.

With these practical effects, measurements yield confusing values that require a great deal of complex analysis to correctly interpret. This problem is resolved by using three-phase ganged switchgear to protect three-phase networks.

Ferroresonance

Ferroresonance is an electrical phenomenon associated with the single-pole switching of certain HV circuits, where network reactance causes large over-voltages when a single phase is disconnected. It occurs commonly during single-phase switching where lightly loaded transformers are connected through a long lines or underground cables.

Under no load, or very light load conditions, the capacitance of the feeder is enough to precipitate ferroresonant behaviour when single phase switching is carried out. If the remaining phases are not quickly interrupted and the phenomenon continues, overvoltage can lead to the breakdown of insulation in the connected components, resulting in their failure.

We can prevent ferroresonance at the design stage by ensuring the deployment of mechanically ganged three-phase switching devices on the network.

Requirements for simultaneity of poles during closing and opening operations

According to the IEC 62271-100 [and SANS 62271 - ed.] circuit breaker standard, when there are no special requirements regarding simultaneous operation of poles, such as for a point on wave switching, the maximum permissible difference of contact separation between all phases during opening shall not exceed 1/6 of a cycle of the rated frequency.

Furthermore, during closing the maximum permissible difference between contact making in all phases shall not exceed ¼ of cycle of the rated frequency. This means during opening, the maximum difference must not exceed 3,33 ms at 50 Hz, and while during closing the maximum difference shall not exceed 5 ms at 50 Hz. These tight time limits are not feasible to achieve and to maintain over the service life of the equipment without the use of the mechanical ganging arrangement between the three poles.

Technical reasons for these requirements to be included in the standard

• In non-effectively earthed neutral systems, if the maximum permissible difference between contact making is not maintained, then in the event of closing onto a three-phase fault, higher peak making conditions may be experienced in comparison with the simultaneous closure case. Equipment connected in series with the switching devices will also be exposed to enhanced peak currents if they occur. For example, for networks with time constant 45 ms the peak making factor is 3,0 instead of 2,5 at 50 Hz rated frequency.

• When an overhead line is switched onto an energised network, a voltage wave is imposed on the line. The imposed wave is reflected at the far end of the line, and when the line is open at the far end (or terminated by a high impedance load for high frequencies), the reflected wave results in doubling the amplitude. An even higher voltage is obtained when the line has a trapped charge before being energized and the switching device happens to close at an instant when the polarity of the network voltage is opposite to that of the voltage that was present on the line. The voltage on the line can, after reflection of the wave, theoretically be up to three times the network voltage. This situation can occur in conjunction with auto-reclosing of a line. Even higher voltages can develop on a three-phase line, when the three poles are not closed simultaneously. A wave on one phase will then generate induced waves on the other phases and under unfavourable circumstances, this can lead to a further rise in voltage on another phase. Ganged three-phase operation mitigates the risk of overvoltage on the other phases.

• The magnitude of the transient recovery voltage following current breaking depend on multiple parameters, including simultaneity of poles. Switching device having contacts intended to open simultaneously will be subjected to greater transient recovery voltages if the contacts lose its synchronicity in service. For example, if a recloser operates to clear a three-phase unearthed fault, the second phase to clear may have to clear against the full phase-to-phase voltage if the contacts of the third phase have not parted prior to current zero. This increase in transient voltage can reach 16% over its design value, which can impose a serious concern about recloser interruption performance, reliability, and service life.

Conclusion
Single phase switchgear is designed for single phase networks. The application of un-ganged single-phase equipment to three-phase distribution networks introduces risks to network safety and operation. While un-ganged switchgear has applications in four wire networks, their use in three-phase networks can create safety issues.

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