The pieces for offshore DC grids are almost in place. There are no technical barriers. Erik Koldby,...

The pieces for offshore DC grids are almost in place. There are no technical barriers.

Erik Koldby, ABB A/S, Bo Westman and Mats Hyttinen, ABB AB, EWEA 2012-04-17

The DC breaker is often seen as a barrier to DC gridsBut DC switches have been in service since the 1970’s!

A typical mechanical DC circuit breaker (MRTB) 4000 A, 125 kV Breaking time 40 ms

Transfer breakers developed for bipolar LCC

Today the solution is standard SF6 AC circuit breakers with a passive or an aktive parallel LC resonanse circuit

Principle for an active resonance mechanical HVDC circuit breaker

But the breaking time make these solutions insufficient for DC grids

UDC

Cigré 13-06 1976

© ABB Group Slide 3PowDoc id

Fast: Breaking times of less than 2ms

Powerful Current breaking capability of 16kA

Efficient Transfer losses are less than 0.01%

Modular Easily adapted to actual voltage & current ratings

Reliable Protective current limitation, functional check while in service

Proven Power electronic design similar to converter technology

DC breakers are no longer a showstopper for large HVDC grids

Hybrid DC breaker is well suited for HVDC grids

© ABB Group Slide 311MR0339


Hybrid DC BreakerFast breaking within time delay of selective protection

Normal operation: Current flows in low-loss bypass

Proactive control: Load commutation switch transfer current into Main Breaker switch, the Ultra Fast Disconnector opens with very low voltage stress

Current limitation: Main Breaker switch commutates fault current into parts of the sectionalized arrester bank

Fault clearance: Main Breaker switch commutates fault current into arrester bank



A possible development: First Regional HVDC gridsCan be built today with proven technology

A typical regional DC grid is defined as a system that constitutes of one protection zone for DC earth faults.

To temporarily and rarely lose the whole HVDC system has a limited impact on the overall power system.

Fast restart of the faultless part of the system

HVDC breakers are not needed Normally radial or star network

configurations Limited power ratingRegional DC grid with

optimized voltage level.



Line fault handling in regional DC grid DC earth faults vs other faults

DC earth faults All AC breakers (X) open

DC switches (-) open and isolate the faulty part

Start-up of none-faulty part

Faults in controls, auxiliaries etc cause a trip of the converter:

The rest of the system continues without interruption

In point-to-point transmission, regional HVDC grids and interregional HVDC grids

One protection zone for DC earth faults

=~

=~

=~

P1

P2 x

x

x

=~

=~

=~

P1

P2 x

x

x

Several protection zones for faults in

controls, auxiliaries



Later step: An interregional DC grid

An interregional HVDC grid is defined as a system that needs several protection zones for DC earth faults.

Developments focus: HVDC breakers and fast protections Grid Power flow control/Primary

control: automatic control Master control: start/stop, re-

dispatching On-going Cigré WG B4.52 ”HVDC Grid

Feasibility study”


Example: 420 kV circuit breakers

1960s: Air blast 1970s and 1980s: Minimum oil 1980s and 1990s: SF6

You know there will be development – you just do not know how and when – but behave like they did in the 50’es: Act on what is available. Europe needs the new grids! The lack of a DC Circuit breaker is no longer a valid excuse

What will the future bring?Inspiration: Look back on AC circuit breakers

The pieces for offshore DC grids are almost in place. There are no technical barriers. Erik Koldby,...

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