Subcriterios del Capital Relacional

For enhanced reliability and functionality as well as minimising conversation losses and

saving costs, HVDC network can also be interconnected to form a multi-terminal

network. Theoretically, this can either be achieved by either connecting the converters in

series or in parallel[24], however only the parallel scheme has been practically proven.

As shown in Figure 2.15, four converters are connected in parallel; comprising two

rectifiers and two inverters thus given more flexibility and reliability, and improved

power transfer capacity.

In parallel operation (Figure 2.15), all parallel converters are connected to the same bus.

Therefore, the DC link voltage is a common parameter for all converters, but with

25

all converters connected in parallel are provided with voltage and current control

capabilities. However, as there is only one DC link voltage, only one converter and often

the inverter with the largest capacity controls the DC voltage [24] whilst the other

converters control their respective DC currents.

Fig. 2-15 Multi-terminal HVDC system [24]

Generally, controlling either the current or voltage has some technical implications and

limitations and can be achieved by either of the two HVDC converter technologies viz:

the line commutated converter (LCC) and the voltage source converter (VSC)

technologies but with each having its own relative advantages and disadvantages.

Fig. 2-16 Types of HVDC converters[2]

LCC are based on thyristors whilst VSCs are based on Insulated Gate Bipolar Transistor

26

point-to-point transmission based HVDC systems. In practice, they are usually fitted with

a large DC side inductance connected in series with the line (Figure 2.16), and as such

are robust to DC side faults since the DC side inductance can limit the rate of rise of fault

current (di/dt) during DC side short circuits[25].However, a major disadvantage of the

LCC for use in MT-HVDC system is that power reversal can only be achieved by

reversing the voltage polarity, hence making it impossible to operate each converter

station independently as either a rectifier or an inverter without changing the polarity of

the DC pole voltages. This capability is a key requirement for interconnecting several

converter stations (or regional countries or cities with HVDC links), thus resulting in the

so-called HVDC grids; which can either be radially or ring connected (Figure 2.17). For

this reason, the use of LCC is limited to parallel HVDC networks (Figure 2.5) but not

applicable for use in DC grids (Figure 2.17) since reversing the power will reverse the

voltage polarity of the DC bus. Generally, unlike the multi-terminal HVDC networks

shown in Figure 2.15, each converter station in a DC grid must have the capability of

operating as either an inverter, or a rectifier

VSC converters have numerous advantages over the conventional thyristor based Line

Commutated Converters (LCC) such as black start capability and the ability to

independently control active and reactive power. Furthermore, the power flow in VSC -

HVDC systems can be independently controlled without changing the voltage polarity

thus allowing for a common bus to be used to interconnect several VSCs as well as

enabling the use of Cross Linked Polyethylene (XLPE) cables[26]. Generally, as VSC

does not require polarity inversion, there is no problem of decrease in the insulation

27 2.4.1 Key drivers for DC grids

In general, the primary motivation for the development of HVDC grid is the need to

interconnect multiple HVDC links and to enable power transfer between all DC terminals.

Some of the benefits include better utilisation of assets, better reliability and security of

power transfer, better efficiency, and enhanced power trading and increased operational

flexibility. In light of these numerous advantages as well as its envisaged capabilities,

VSC technology is the only viable option for the development of future HVDC grids.

Key advantages of DC grids are:[3][6] [27][28].

▪ Better management and integration of renewable energy generation ▪ Increased grid security

▪ Enabling cross border energy trading

▪ Provides more efficient network by reducing reliance on thermal generation ▪ Enabling the integration of large scale renewable energy sources (RES)

Integration

▪ Enabling the Internal Electricity Market

▪ Ensuring supply security by complementing micro grids

▪ Provision of multiple DC lines for the power delivery within the system thus enhancing system redundancy and reliability

▪ Creating export opportunities for European technology

▪ Increased availability of the grid, and the reduced construction and operation costs since existing network can be expanded to realise HVDC grid

28 AC 2 A _B C AC 1 AC 3

(a) Radial Connection

AC 2 A _B C AC 1 AC 3

(b) Ring (or Meshed)

Connection AC 2 A _B C AC 1 AC 3 C AC 4 (c) Hybrid Connection - Converter Station

29 2.4.2 The European Super Grid

The transmission grids will be more international, crossing different economic zones and

national borders. It will also have to be operated and regulated by a mixture of

international agencies, national agencies as well as system operators. The grid, which is

generally referred to as “European Super Grid” would help to create a secure, cost- effective and sustainable power system. A conceptual configuration for the European

Super grid interconnecting different regions and countries is shown in Figure 2.18 [29].

Fig. 2-18 A conceptualized European Super grid [29].