T & D - 28) Enhance Grid Reliability With Hybrid HVDC Breaker

The Hybrid HVDC Breaker by ABB - The hybrid design has negligible conduction losses, while preserving ultra-fast current interruption capability.

Content

1. Introduction to HHVDC
2. How does Hybrid HVDC Breaker work? (VIDEO)
3. Hybrid HVDC Breaker Construction
4. Proactive control
5. Prototype Design Of The Hybrid HVDC Breaker
6. Comparison/Summary

Introduction

The advance of voltage source converter-based (VSC) high-voltage direct current (HVDC) transmission systems makes it possible to build an HVDC grid with many terminals.

Compared with high-voltage alternating current (AC) grids, active power conduction losses arerelatively low and reactive power conduction losses are zero in an HVDC grid.

This advantage makes an HVDC grid more attractive.

However, the relatively low impedance in HVDC grids is a challenge when a short circuit fault occurs, because the fault penetration is much faster and deeper.

Consequently, fast and reliable HVDC breakers are needed to isolate faults andavoid a collapse of the common HVDC grid voltage.

Furthermore, maintaining a reasonable level of HVDC voltage is a precondition for the converter station to operate normally. In order to minimize disturbances in converter operation, particularly the operation of stations not connected to the faulty line or cable, it is necessary to clear the fault within a few milliseconds.

How does Hybrid HVDC Breaker work? (VIDEO)

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Hybrid HVDC Breaker Construction

The hybrid HVDC breaker consists of an additional branch, a bypass formed by asemiconductor-based load commutation switch in series with a fast mechanical disconnector.

The main semiconductor-based HVDC breaker is separated into several sections with individual arrester banks dimensioned for full voltage and current breaking capability, whereas the load commutation  switch matches lower voltage and energy capability.

After fault clearance, a disconnecting circuit  breaker interrupts the residual current and isolates the faulty line from the HVDC grid to protect the  arrester banks of the hybrid HVDC breaker from thermal overload.

Figure 1 - Hybrid HVDC breaker main components

During normal operation the current will only flow through the bypass, and the current in the main breaker is zero.

When an HVDC fault occurs, the load commutation switch immediately commutates  the current to the main HVDC breaker and the fast disconnector opens. With the mechanical switch in  open position, the main HVDC breaker breaks the current.

The mechanical switch isolates the load commutation switch from the primary voltage across the main HVDC breaker during current breaking.

Thus, the required voltage rating of the load commutation switch is significantly reduced.

A successful commutation of the line current into the main HVDC breaker path requires a voltage rating of the load commutation switch exceeding the on-state voltage of the main HVDC breaker, which is typically in the kV range for a 320 kV HVDC breaker.

This result in typical on-state voltages of the load commutation switch is in the range of several volts only.

The transfer losses of the hybrid HVDC breaker concept are thus significantly reduced to a percentage of the losses incurred by a pure semiconductor breaker.

The mechanical switch opens at zero current with low voltage stress, and can thus be realized as a disconnector with a lightweight contact system. The fast disconnector will be exposed to the maximum pole-to-pole voltage defined by the protective level of the arrester banks after first being in open position while the main HVDC breaker opens.

Thomson drives result in fast opening times and compact disconnector design using SF6 as insulating media.

Proactive control

Proactive control of the hybrid HVDC breaker allows it to compensate for the time delay of the fast disconnector, if the opening time of the disconnector is less than the time required for selective protection.

Figure 2 - Proactive control of hybrid HVDC breaker. LCS denotes load commutation switch

As shown in Figure 2, proactive current commutation is initiated by the hybrid HVDC  breaker’s built-in overcurrent protection as soon as the HVDC line current exceeds a certain overcurrent level. The main HVDC breaker delays current breaking until a trip signal of the selected protection is received or the faulty line current is close to the maximum breaking current capability of  the main HVDC breaker.

To extend the time before the self-protection function of the main HVDC breaker trips the hybrid HVDC breaker, the main HVDC breaker may operate in current limitation mode prior to current breaking.

The main HVDC breaker controls the voltage drop across the HVDC reactor to zero to prevent a further rise in the line current.

Pulse mode operation of the main HVDC breaker or sectionalizing the main HVDC breaker as shown in Figure 2 will allow adapting the voltage across the main HVDC breaker to the instantaneous HVDC voltage level of the HVDC grid.

The maximum duration of the current limiting mode depends on the energy  dissipation capability of the arrester banks.

On-line supervision allowing maintenance on demand is achieved by scheduled current transfer of the line current from the bypass into the main HVDC current breaker during normal operation, without disturbing or interrupting the power transfer in the HVDC grid.

Fast backup protection similar to pure semiconductor breakers is possible for hybrid HVDC breakers applied to HVDC switchyards.

Due to the proactive mode, over-currents in the line or superior switchyard protection will activate the current transfer from the bypass into the main HVDC breaker or possible backup breakers prior to the trip signal of the backup protection.

In the case of a breaker failure, the backup breakers are activated almost instantaneously, typically within less than 0.2 ms. This will avoid major disturbances in the HVDC grid, and keep the required current-breaking capability of the backup breaker at reasonable values. If not utilized for backup protection, the hybrid HVDC breakers automatically return to normal operation mode after the fault is cleared.

Prototype Design Of The Hybrid HVDC Breaker

The hybrid HVDC breaker is designed to achieve a current breaking capability of 9.0 kA in an HVDC grid with rated voltage of 320 kV and rated HVDC transmission current of 2 kA. The maximum current breaking capability is independent of the current rating and depends on the design of the main HVDC breaker only.

The fast disconnector and main HVDC breaker are designed for switching voltages exceeding 1.5 p.u. in consideration of fast voltage transients during current breaking.

Figure 3 - Design of 80kV main HVDC breaker cell

The main HVDC breaker consists of several HVDC breaker cells with individual arrester banks limiting the maximum voltage across each cell to a specific level during current breaking. Each HVDC breaker cell contains four HVDC breaker stacks as shown in Figure 3.

Two stacks are required to break the current in either current direction.

Each stack is composed of up to 20 series connected IGBT (insulated gate bipolar transistor) HVDC breaker positions.

Due to the large di/dt stress during current breaking, a mechanical design with low stray inductance is required.

Application of press pack IGBTs with 4.5 kV voltage rating [6] enables a compact stack design and ensures a stable short circuit failure mode in case of individual component failure. Individual RCD snubbers across each IGBT position ensure equal voltage distribution during current breaking.

Optically powered gate units enable operation of the IGBT HVDC breaker independent of current and voltage conditions in the HVDC grid. A cooling system is not required for the IGBT stacks, since the main HVDC breaker cells are not exposed to the line current during normal operation.

For the design of the auxiliary HVDC breaker, one IGBT HVDC breaker position for each current direction is sufficient to fulfill the requirements of the voltage rating.

Parallel connection of IGBT modules increases the rated current of the hybrid HVDC breaker. Series connected, redundant IGBT HVDC breaker positions improve the reliability of the auxiliary HVDC breaker.

A matrix of 3×3 IGBT positions for each current direction is chosen for the present design. Since the auxiliary HVDC breaker is continuously exposed to the line current, a cooling system is required.

Besides water cooling, air-forced cooling can be applied, due to relatively low losses in the range of several tens of kW only.

Comparison

Existing mechanical HVDC breakers are capable of interrupting HVDC currents within several tens of milliseconds, but this is too slow to fulfill the requirements of a reliable HVDC grid.

HVDC breakers based on semiconductors can easily overcome the limitations of operating speed, but generate large transfer losses, typically in the range of 30 percent of the losses of a voltage source converter station.

To overcome these obstacles, ABB has developed a hybrid HVDC breaker described above.

The hybrid design has negligible conduction losses, while preserving ultra-fast current interruption capability.

T & D - 9) Using HVDC Technology For Transmitting Electricity

An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current. Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice.

Thyristors are controllable semiconductors that can carry very high currents and can block very high voltages. They are connected is series to form a thyristor valve, which allows electricity to flow during the positive half of the alter-nating current voltage cycle but not during the negative half.

Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation.

HVDC transmission lines can either be single pole or bipolar, although most are bipolar, that is, they use two conductors operating at different polarities such as +/-500 kV.

HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation, cross-linked polyethylene.

Although synchronous HVAC transmission is normally preferred because of its flexibility, historically there have been a number of applications where HVDC technology has advantages:

1 The need to transmit large amounts of power (>500 mW) over very long distances ( >500 km), where the large electrical angle across long HVAC transmission lines (due to their impedances) would result in an unstable system.

Examples of this application are the 1,800 mW Nelson River Project, where the transmission delivers the power to Winnipeg, Canada, approximately 930 km away; the 3,000 mW system from the Three Gorges project to Shanghai in China, approximately 1,000 km distant; and the 1,456 km long, 1,920 mW line from the Cabora Bassa project in Mozambique to Apollo, in South Africa. In the United States the 3,100 mW Pacific HVDC Intertie (PDCI) connects the Pacific Northwest (Celilo Converter Station) with the Los Angeles area (Sylmar Converter Station) by a 1,361 km line.

2 The need to transmit power across long distances of water, where there is no method of providing the intermediate voltage compensation that HVAC requires. An example is the 64 km Moyle interconnector, from Northern Ireland to Scotland.

3 When HVAC interties would not have enough capacity to withstand the electrical swings that would occur between two systems. An example is the ties from Hydro Quebec to the United States.

4 The need to connect two existing systems in an asynchronous manner to prevent losses of a block of generation in one system from causing transmission overloads in the other system if connected with HVAC. An example is the HVDC ties between Texas and the other regional systems.

5 Connection of electrical systems that operate at different frequencies. These applications are referred to as back-to-back ties. An example is HVDC ties between England and France.

6 Provision of isolation from short-circuit contributors from adjacent systems since dc does not transmit short-circuit currents from one system to another.

With the deregulation of the wholesale power market in the United States, there is increasing interest in the use of HVDC technology to facilitate the new markets.

HVDC provides direct control of the power flow and is there-fore a better way for providing contractual transmission services. Some have suggested that dividing the large synchronous areas in the United States into smaller areas interconnected by HVDC will eliminate coordination problems between regions, will provide better local control, and will reduce short-circuit duties, significantly reducing costs.

HVDC PLUS – Maximum power in the smallest space

HVDC PLUS is an advanced and flexible solution for power transmission in fields where space is at a premium. The innovative technical converter concept allows power transmission from remote offshore platforms and wind farms to the onshore grid.

Advantages of HVDC

As the technology has developed, the breakeven distance for HVDC versus HVAC transmission lines has decreased. Some studies indicate a breakeven distance of 60 km using modern HVDC technology.

Some of the advantages identified are:

• No technical limits in transmitted distance; increasing losses provide an economic limit;
• Very fast control of power flow, which allows improvements in system stability;
• The direction of power flow can be changed very quickly (bi-directionality);
• An HVDC link does not increase the short-circuit currents at the connecting points. This means that it will not be necessary to change the circuit breakers in the existing network;
• HVDC can carry more power than HVAC for a given size of conductor;
• The need for ROW is much smaller for HVDC than for HVAC, for the same transmitted power.

Disadvantages of HVDC

The primary disadvantages of HVDC are its higher costs and that it remains a technology that can only be applied in point-to-point applications because of the lack of an economic and reliable HVDC circuit breaker.

The lack of an HVDC circuit breaker reflects the technological problem that a direct current system does not have a point where its voltage is zero as in an alternating current system. An HVAC circuit breaker utilizes this characteristic when it opens an HVAC circuit.