High-power DC switchgear is still under development with few mature commercial products. The traditional mechanical structured CB used in AC systems cannot be applied due to the slow fault isolation speed and the requirement of zero-crossings in the fault current. Therefore, power electronic devices are used to quickly block fault currents, such as IGBT and gate turn-off thyristors (GTOs). Generally, CBs of this kind are called solid-state CBs (SSCBs). One option includes a paralleled mechanical switch Sp as an auxiliary switch for lower loss during normal operation, a
metal-oxide-varistor surge arrester MOVCB, and power electronic blocking device PECB, Figure 5.5(a). The PECB block can be a parallel or series topology. Figure 5.5(b)
and 5.5(c) realise bi-directional current block functions. Sometimes, a series inductance LCB and a switch Ss are used as a fault current limiter and to provide an
obvious electrical isolation point for the network operator, i.e. as a disconnector. This CB topology can be seen as device redundancy to enhance reliability, as a comparison with topology redundancy which will be discussed in Chapter 6.
The technical challenges for high-power DC CBs are the current isolation capability of power electronic devices, their high costs and their losses. Although some fault tolerant converters can reduce the allocation of CBs, as long as the development of this DC network includes traditional VSCs, reliable system protection relies on DC
Chapter 5 Protection Coordination of Meshed VSC-HVDC Transmission Systems for Large-Scale Wind Farms 132
CBs. At this stage, in terms of device number, it is still economical to allocate DC CBs. Multi-IGBT devices could be used in series or parallel connection to increase the voltage or current ratings, particularly for high-overcurrent situations.
The DC switchgear allocation is illustrated in Figure 5.6 for the six-node test system. Uni-directional current-blocking DC CBs are used. This is a trade-off option considering both economic costs and function. The CB at each cable end has only one IGBT for fault current blocking but the two CBs can cooperate to isolate faults that occur between them on the cable. This requires only half the number of power electronic devices for fault current cut-off compared to the fully functioned bi-directional CBs, with half the loss but with a reduction in functionality. This CB allocation and configuration will influence the coordination strategy design. If bi-directional functionalised CBs are used, the multi-loop coordination strategy of the AC system can be applied to this DC loop protection analysis [5.6], [5.7].
LCB PECB Sp Ss MOVCB CB Parallel PECB Series PECB (a) (b) (c)
Figure 5.5: A DC CB option: (a) DC CB configuration; (b) parallel connected bi-directional PE
block; (c) series connected bi-directional PE block.
(1) (3) (5) (2) (4) (6) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Wind Farm 3 Wind Farm 1 to AC Grid 1 to AC Grid 2 Wind Farm 4 Wind Farm 2
(*) Node number [*] CB number
f1
f2
f3
Chapter 5 Protection Coordination of Meshed VSC-HVDC Transmission Systems for Large-Scale Wind Farms 133
The operating state of the wind farm depends on the wind resource conditions. The power acquired from a large wind turbine is variable but normally varies over at time period of seconds. Simulation of variable wind speed conditions have been performed using the wind profile shown in Figure 5.7. In the simulation, the whole wind farm is exposed simultaneously to the wind profile which is the most severe case of power flow and current variation on the cable. The wind model applied is from PSCAD/EMTDC with gusts, noise and a rated speed of 12 ms−1. The shear and tower effects which result in a flicker power quality problem [5.17] are not considered. This will not influence the protection system operation.
The results in Figure 5.7 show that: 1) There can be steep current increase and decrease; 2) DC-link voltage fluctuation is not as dramatic as under fault conditions. With many distributed wind-turbines aggregation reduces the fluctuation effect. Hence the power fluctuation due to changes in wind conditions will not influence the relay system performance as long as the rate of change of current is not utilised for fault detection. The DC fault currents are always extreme where overcurrent occurs in milliseconds and is distinct enough from normal fluctuations for fault identification.
Figure 5.7: DC cable current and voltage responses under wind speed fluctuation: (a) wind speed
(ms−1); (b) cable current (p.u.); (c) inverter DC-link voltage (p.u.). (a)
(b)
Chapter 5 Protection Coordination of Meshed VSC-HVDC Transmission Systems for Large-Scale Wind Farms 134
Figure 5.8: DC cable current and voltage responses under sudden power increase: (a) cable currents
(p.u.); (b) inverter DC-link voltage (p.u.).
The power flow calculation for this linear system will obey basic physical principles, which will not need a specific algorithm for the convergence of results, like those commonly used for nonlinear AC systems. In this DC system, all the power sources can be calculated separately to estimate current flow according to the superposition theorem. Therefore the theoretical power flow results can be calculated almost instantaneously which is helpful for the real-time decision process. Figure 5.8 shows a power increase due to a change in system operation. There are high rates of change of current in some cables which reinforces the need to not use rate of change of current in the decision making process.
Another issue is the exclusion of current harmonics due to the modulation method of the converters [5.15]. The harmonics are with known high-order frequencies and can be eliminated from the method used to detect faults via frequency detection. Hence, only in the low frequencies given in Table 5.1, current and DC-link voltage amplitude and direction changes will the signals be used to detect fault conditions.