DIAGNÓSTICO

4.2

The Offshore AC Frequency

A traditional power system normally has a large inertia depending on the number of generators connected to the grid. The power generation and the load demand must match and be balanced in order to maintain a constant frequency that is equal to the speed of the synchronous generators. The generators in the power system have no control over the load dynamics that can freely change and causes the frequency to deviate from the nominal frequency if generation does not respond accordingly. A generation system would typically adjust the power using speed governors, in order to speed up or slow down the frequency back to the desired reference [140].

In a WF case, where the wind power generation cannot be controlled, then the HVDC export power can be controlled in order to balance the flow and maintain the offshore frequency [141]. However, an AC offshore wind farm network connected by HVDC is islanded and fully decoupled from the onshore grid frequency. The inertia in the network will be very small, and frequency can be subjected to relatively large variations, making it difficult to control without a strong voltage source or fast power response.

As discussed in Section 2.2.5, WTs with Type D configuration are increasingly used for offshore wind farms. Their fully rated frequency converters will fully de- couple the generators from the network frequency. Type D WTs have mechanical inertia but the control of the converters tend to cancel its effect and inertial impact on the network, hence methods are proposed so they can provide “synthetic iner- tia” for supporting traditional power system [142]. However, if all the WTs are of Type D and used in a WF that is only connected by VSC-HVDC, the entire offshore network will have no mechanical inertia and no AC grid frequency coupling with synchronous generators. Therefore, in this case, a voltage source should establish a set frequency and voltage on the offshore AC network, which must absorb abrupt power disturbances in the AC network.

4.2.1

Setting the Master Frequency

The voltage and frequency in the offshore network should be established by the strongest voltage source converter in the network. In this case, it would be the offshore HVDC converter station. The frequency in the master VSC is set in an “open loop” manner by first defining a rotating angle reference frame for the θ input of the dq transformations in equations (4.3) and (4.4). As shown in equation (4.9), the angle reference output θ in radians is derived from the integral of the desired reference frequency f in hertz, which can be easily modified in real-time. To prevent continuous ramping to infinity, the integral output is “wrapped” so the angle signal is reset at every 2π, which will then appear like a sawtooth waveform in the time domain.

θ(s) f (s) =

2π

s (4.9)

Assuming that wind turbines will operate in the usual power (or current) control mode, they will need to track a master AC frequency in order to inject their currents with the appropriate phase shift with respect to the voltage at the point of connec- tion to the AC network. If the converter of the VSC-HVDC system is the only one imposing the frequency to the offshore network, undesirable small signal power os- cillations can easily be avoided while all other converters can be synchronised using phase locked loops (PLL).

In the future, multiple HVDC terminals may co-exist in the same offshore net- work, but one terminal could still be chosen to act as the master VSC to fix voltage and frequency while all the others operate in power control mode. A fixed voltage and frequency set at the master VSC station will act like a slack bus voltage source and will therefore inherently be capable of absorbing power changes in the network. The master role taken by a VSC in this situation, cannot be used when the VSC is connected to a strong grid with a very large inertia and a strong voltage. This is because the limited control effort of the VSC cannot drive the grid voltage to a desired voltage reference.

4.2. The Offshore AC Frequency 93

4.2.2

Variable Frequency Option

If the offshore network is inertia-less, then the variation of the frequency can be very flexible and able to speed up and slow down very quickly to any desirable set reference without much power change. Some studies in literature suggest that the offshore AC network frequency should be artificially coupled with the onshore grid frequency instead of using a fixed frequency at the offshore network [101]. As discussed in Section 2.4.6 (page 54), this would allow the wind turbines to contribute to the grid frequency support. The frequency can then be used as a reference to signal the offshore WTs whether to transmit more or less power. It assumes that the WTs would provide more (or less) power if the frequency decreases (or increase), very much like a conventional power stations. This control mode could be made more flexible with the aid of distributed offshore energy storage and generation capacity reserves. (Please also refer to Section 3.2.3 about the variable frequency networks in Chapter 3).

4.2.3

Phase Locked Loop

The phase locked loop (PLL) allows the converters and distributed power generators to be synchronised to the AC frequency of the network. The PLL is a feedback control system that is able to track the frequency and the space-vector angle or phase of network voltage. The derivative of this angle with respect to time is equal to the AC angular frequency ω. The d-axis of the dq reference frame would conventionally be aligned with this angle of the network voltage vector, and then the q-axis voltage component becomes zero. As a result, the d and q components of the steady-state currents are directly related to the active and reactive power respectively, which is an advantage if they can be controlled independently.

In an ideal simulation model, the exact angle of the voltage space vector with respect to the α-axis can be calculated using the inverse tangent of the ratio of the αβ components from equation (4.1). However, harmonics in a non-ideal model can propagate into the angle for the dq transformation, which leads to noisy signals and unstable control. The filtering effect of the closed-loop nature of the PLL, as shown

in Figure 4.3 can eliminate undesired harmonics in the tracking of the angle of the network voltage. The loop filter is typically a proportional-integral (PI) controller. The frequency (ω/2π) can also be extracted from the PLL.

V_abc Vq Loop Filter θ' θ' abc dq 1 s VCO Δω ω' ω0 Phase detector

Figure 4.3: A block diagram of a generic PLL

A distortion in the grid voltage will lead to an error in the phase angle tracking and contribute to degraded performance of the converter if the PLL is not robust [143]. Gao et al. have presented and compared five different PLL methods in [144], each have their differences in terms of dynamic characteristic response, robustness to distortion and ability to recover from unbalanced faults. This thesis will use the PLL already provided in the PSCAD library for simulation (see Appendix A.2). For small signal analysis, the model of the PLL can be simplified to the transfer function of a simple first order fixed time delay for linearisation.