Temporary overvoltages in ungrounded neutral HVDC - VSC systems:

a question of uncertainties

HVDC international workshop - Venice, March 28-30, 2017

M. Marzinotto, F. Palone, M. Rebolini TERNA - Engineering Dept.

The neutral in a HVDC transmission system In conventional HVDC – LCC systems, which normally have a solid connection to the ground, the definition of the neutral point is straightforward: the neutral correspond to the grounded terminal of the converter bridge:

The neutral in a HVDC transmission system At present time, modular multilevel converters HVDC – VSC systems are built in a symmetrical monopole arrangement, so that there is no direct ground connection to any point of the dc circuit. This choice is mainly dictated by the use of zero sequence harmonics overmodulation of the converter bridge. The ground reference for measuring and control purposes is generally performed by means of: • an high impedance star point reactor, connected between the converter

transformer, valve side, (delta winding) and converter bridge, • Or, alternatively, using an high impedance connected between the star point of

the converter transformer, valve side, (wye winding) and the ground.

The neutral in a HVDC transmission system The new IEC TS 61936-2 “Power installations exceeding 1 kV a.c. and 1,5 kV d.c. - Part 2: d.c.” defines the neutral point of an HVDC system as follows: “D.C. neutral point common point of two monopoles forming a bipole converter or the earthed point of a monopole converter” The method of neutral earthing of a system is important with regard to the following: • selection of insulation level; • characteristics of surge arresters; • fault current magnitude; • selection of protective relays.

What is the neutral in a HVDC transmission system? The following are examples of d.c. neutral earthing methods: • isolated neutral (balanced systems); • high impedance earthing (typically used in last generation of MMC VSC

systems with symmetrical monopole arrengement); • high resistive earthing; • solid (low impedance) earthing (used in most LCC systems). The choice of the type of d.c. neutral earthing is based on the following criteria: • local regulations (if any); • continuity of service required for the network; • limitation of damage to equipment caused by earth faults; • selective elimination of faulty sections of the network; • detection of fault location; • touch and step voltages; • inductive interference; • operation and maintenance aspects; • overvoltage mitigation;

Ungrounded neutral operation of HVDC VSC systems: advantages, drawbacks and uncertainties: PROs: • Negligible converter short circuit current (useful in half bridge modules) • Possible use of zero sequence overmodulation (losses and cost reduction) CONs: • Higher overvoltages on the healthy pole due to ground faults. • DC stresses on the bridge side ac equipment (e.g. converter transformer, arm

reactors, bushings, etc.) during pole to ground faults. • Higher and longer DC stress on cables

Uncertainties: • Effects on DC equipment • Effects on AC equipment • Ageing

Overvoltages due to ground faults in ungrounded VSC systems.

In case of single-pole to ground faults on the dc side, the IGBTs are blocked during the first hundreds of μs. Current conduction however continues through the free-wheeling diodes or bypass thyristors. The converter then acts as a 6-pulse rectifier until ac breakers open after about 100 ms, leading to significant temporary overvoltages on the healthy pole. At a first glance, as an order of magnitude and neglecting the cable discharge transient, the prospective TOV can be evaluated as:

𝑇𝑇𝑉[𝑝.𝑢.] =𝑈𝑚𝑈𝑑𝑑

∙1𝑇 ∙

3 2𝜋

Being Um the maximum continuous ac system voltage, Udc the rated dc voltage and T the voltage ratio of the converter transformer. For typical, 320 kV-class HVDC systems

1,15 ≤ 𝑈𝑚𝑈𝑑𝑑

∙ 1𝑇≤ 1,3

so that the TOV can attain 1,55 ≤ 𝑇𝑇𝑉𝑝.𝑢. ≤ 1,76

COUNTERMEASURES for DC TOV amplitude.

Surge Arresters (SAs) are generally selected to limit transient, rather than temporary overvoltages. As consequence, their SIPL is generally higher than the expected TOV. However, SAs can be selected and designed with an higher energy rating, to cope with temporary overvoltages, leading to an higher cost and footprint. However, even considering the use of multiple-column construction, the ratio between SAs SIPL and continuous operating voltage Uc cannot be arbitrarily reduced, so that this countermeasure can only limit the TOV to about 1,5 p.u. The energy rating of the SAs is mainly dictated by the opening time of the AC circuit breakers, so that protection relay delays shall be carefully evaluated in order to avoid thermal runaway.

Overvoltages due to ground faults in ungrounded VSC systems: AC side An example of the phase voltages on the converter transformer bushing (valve side) during a pole to ground fault: DC voltage component arises. No factory tests with DC voltages are considered for VSC transformers (such transformers are considered standard transformers!).

Overvoltages due to ground faults in ungrounded VSC systems: DC side

AC breakers opening

Abroupt Grounding

Overvoltages due to ground faults in ungrounded VSC systems: DC side

The following figure shows the pole voltages during discharge: • Red curve R= 0Ω • Blue curve R= 5Ω • Green curve R= 65Ω

Consequences

Different equipment are subjected to overvoltages not considered in relevant standard: • Transformer valve hall windings • AC Reactors • AC Bushings • Insulators • DC reactors • DC bushings • Voltage transducer • Surge arrestors • Cables DC side insulations are probably the most stressed due to preexisting

space charges

Troubles The overvoltage wave shape is far from the standardize wave shape: Time to front: 0.5 – 3 ms Permanence to peak: 150 – 300 ms Chopping tail time :10 – 50 ms No agreement on the wave shape among involved parties Hard to reproduce in labs (specific equipment shall be installed

increasing the cost of the test)

Uncertainties. Can standardized wave shapes be in any case equivalent under a stress point of view? Difficulties in finding only one waveshape representing the situation: shape strongly depends on system components and system reaction Shall equipment insulation be properly selected in order to not increase the ageing rate? The effect of such overvoltages is the same for all the stressed equipment? For sure cable systems are the most stressed by overvoltages: beside

ageing effects (common to all stressed equipment) the “enlargement effect” shall be properly taken into account.

Facts

Nobody knows the effect of such overvoltages on different kind of insulation: it is sometimes presumed, assumed, under/overestimated…. Waveshapes and peak values of such overvoltages usually come out at a late stage: after system study once contract awarding Responsibilities are difficult to be addressed Tests are expensive and not easy to be arranged Out of contract tests can relieve or tense up the situation Dissatisfaction can prevail if such troubles are not predicted and

arranged in advance.

Conclusion

HVDC – VSC systems with ungrounded neutral are affected to overvoltage waveshapes far from the standardized ones (both impulsive and temporary) AC and DC components subjected to such kind of overvoltages are not tested Since no standard takes into account such problem, TSO can impose specific tests Scientific community is encouraged to deepen such aspects

Thank you for your attention