Residual Charge in HV subsea cable 1

[PRincez]

I'm curious to know about residual charge in a subsea HV cable that's about 30km long, how it's generated and how long it takes to dissipate when the upstream supply is switched off or isolated.

During maintenance of equipment downstream the cable, for example a JB, there has to be a means of ensuring that the charge Has dissipated, cable completely free of any voltage, before opening a JB. What is the means of estimating this?

Is there any process during HV switching that highlights this? I'd appreciate any pointer.

 

[crshears]

The presence of residual charge in any cable, subsea or not, depends upon at what point in the cycle the last device removes the cable from potential; where I work we refer to this as a trapped charge.

Trapped charges in a pure cable can be a bear to get rid of, so we do all we can to avoid creating them in the first place, generally by providing an effective galvanic path from the phase conductors of the cable to ground.

Whether or not trapped charges are problematic depends upon power system equipment configuration. Consider the instance of a circuit from A to B that consists of both overhead conductor and cable; when this circuit is removed from potential, the overhead conductor portion of the circuit will serve to drain any trapped charge from the cable portion of that circuit. Here's a quote from one of our instructions on the subject: "Due to the high dielectric quality of cables, the voltage decay rate on pure cables is very slow, as opposed to overhead lines which have a quick decay rate." Depending upon the proportion of overhead conductor to cable, it may take only one span of overhead conductor to serve as an effective drain. In this situation, simply switching the circuit out of service with its terminal breakers to remove it from service poses no problems.

In high-density urban areas where there may not be any overhead conductor in the circuit at all, we take advantage of the way AC devices behave when DC voltages are impressed on them. For example, wye-wound transformers with the star point grounded can be switched out of service with a cable to very effectively drain trapped charges, since the impedance that normally holds the flow of current to negligible values when a source of AC voltage is present disappears when potential is removed, turning the transformer windings into a resistance of low ohmic value. The actual sequence we use in this case is to first unload the circuit using a circuit switcher or, in some cases, a horn-gap-equipped air break switch, provided the loop impedance is not too great. Once the circuit/transformer combination has been unloaded, it is removed from potential by opening all of the secondary circuit breakers of the attached step-down transformer [most of them have pairs of low-voltage windings].

Single-phase wound potential transformers can also serve to drain trapped charges, although it may take longer since the series resistance doing the draining consists of numerous turns of fine wire.

At the time of commissioning new equipment, empirical methods are typically employed to determine how long it actually takes to drain a given trapped charge, and the data collected is then used to write operating instructions for the system controllers and field staff that include the necessary hold times for the trapped charge to be dissipated. This is commonly performed for various scenarios of available and unavailable equipment so that the task can be completed under most of the commonly encountered power system configurations found.

A last resort for draining trapped charges on a pure cable is to close a permanently installed ground switch onto the cable's phase conductors once the cable has been removed from all sources of AC potential. I've had to be the person on the operating handle of the manually operated ground switch being used to accomplish the draining; flash goggles, hearing protection, switching gloves and a sturdy company coat to protect your own clothing from being burned by near-molten spheres of copper are all necessaries for the task. Nerves of steel are useful as well...I never did develop these, so I just had to take a deep breath and do it.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[7anoter4]

If the cable is disconnected both ends but not short-circuited and grounded- as very well it was explained by crshears -the remaining charged voltage [let's say maximum could be sqrt(2)*Vrms -Vrms the voltage between conductor and the shield]- will be discharged only on the insulation resistance.
The time to get a safety voltage [Vsafe] of -let's say 12 V- will be:
t=R*C*Ln(Vini/Vsafe) where R is the insulation resistance ; Vini is the initial voltage
C=epsr/18/Ln(Dsh/dcond)*length[km] microF - single core cable capacity.
R=K*Ln(Dsh/dcond)/length[km]
For XLPE insulation epsr=2.4 K=3.67*ro/10^12 Mohm*km ro=10^12-10^17 ohm.cm
C*R=2.4/18/Ln(Dsh/dcond)/10^6*3.67*ro/10^12*Ln(Dsh/dcond)*10^6=2.4/18*3.67*ro/10^12
[F*ohm]
If Vini=100 kV [rms] and Vsafe=12 V [rms] then if ro=10^12 ohm.cm t=4.4 sec and if ro=10^17 t= 122 hours.
However, this is only a theoretical speculation and it is better to proceed as safety rule requires:
to short-circuit the conductors and against the ground before any contact with the conductors.