Dynamic performance under fault conditions

The performance and direct-voltage supporting properties of the PD-DVC are demonstrated through fault studies on the ac- as well as the dc-side. These studies are performed on the same five-terminal MTDC grid as described in the previous section, featuring three droop-controlled and two constant-power controlled stations. The objective of the fault study is to compare the performance of the PD-DVC to that of an active-power PI controller that would conventionally be used to ensure constant power flow. As such, two types of MTDC-grid control strategies are tested:

• ”Control Strategy 1”: All stations feature the PD-DVC of Fig. 7.11(a).

• ”Control Strategy 2”: The constant-power controlled stations feature regular PI control

with a rise time that is chosen to be close to the one achieved by the PD-DVC in ”Control Strategy 1”. The other stations are chosen to operate with the proposed PD-DVC in D- DVC mode (selector in position ”0”).

For consistency purposes in both the ac- and dc-side fault scenarios, the following common settings are chosen:

1. The stations are set-up exactly as in Section (7.5.5), with Stations 2 and 4 being in constant-power control mode and the setpoints to all the stations provided as in Table 7.1. 2. The ac-sides of all VSC stations are connected to infinite buses apart from the stations close to which the faults occur. These are connected to an ac grid of Short Circuit Ratio (SCR) equal to 2.

3. DC-choppers have been omitted in order to observe the pure dynamics of the fault phe- nomena.

4. The vector of the reference currents(i(dq)_f )∗

maxto the CC of all stations is limited to 1.0 pu.

5. The reactive power reference is set to zero for all stations.

AC-side fault scenario

The distance of the fault location from the VSC station terminals has a large effect on the response of the station. The closer the fault is placed to the VSC station, the more fault current contribution is bound to come from the station rather than the connected ac-network. In the present simulation scenario, the fault is chosen to be located close to Station 2. Namely, the equivalent grid impedance of the associated ac-network (which has been calculated for SCR=2) is split into two parts in series connection. The first one is equal to the 80% of the grid impedance and is connected to the infinite ac-source while the other part is equated to the rest 20% of the

7.5. Controller offering direct-voltage support in MTDC grids −600 −400 −200 0 200 400 600 Power [MW] P 1 P2 P3 P4 P5 −600 −400 −200 0 200 400 600 Power [MW] P 1 P2 P3 P4 P5 620 640 660 680 υ dc1 [kV]

Control Strategy 2 Control Strategy 1

600 650 700

υ dc2

[kV]

Control Strategy 2 Control Strategy 1

630 640 650

υ dc3

[kV]

Control Strategy 2 Control Strategy 1

630 640 650

υ dc4

[kV]

Control Strategy 2 Control Strategy 1

3 3.5 4 4.5 630 635 640 645 υ dc5 [kV] time [s]

Control Strategy 2 Control Strategy 1

Control Strategy 2

Control Strategy 1

Fig. 7.15 Active-power and direct-voltage response of the five-terminal MTDC grid using the ”Control Strategy 1” and ”Control Strategy 2” schemes. An ac-side fault is applied close to Station 2 at t=3 s.

Chapter 7. Control investigation in Multiterminal VSC-HVDC grids

impedance and is finally connected to the VSC station terminals. A small resistor is connected between the connection point of the two impedances and the earth, through a breaker.

While being in steady-state conditions, the breaker closes at t=3 s and then opens after 50 ms. This causes the voltage at the fault location to drop to approximately 22% of the original 400kV. The power and direct-voltage response of the system for the two different types of control strate- gies is presented in Fig. 7.15. For the ”Control Strategy 2” control mode, the power references of the inverters are closely followed throughout the event, apart from the immediately affected Station 2 which experiences a great power change. The response of the droop-controlled sta- tions is fast and the initial power flow is quickly restored after the fault is cleared. On the other hand, the direct-voltage, at the beginning and the clearing of the fault, exhibits large magnitude deviations followed by relatively poorly-damped high frequency components.

When the ”Control Strategy 1” scheme is used, the power response of all stations is affected. During the fault, the power of the stations seems to change with less severity than in the ”Control Strategy 2” scheme. In fact, the immediately affected Station 2 seems to be able to still export almost 200 MW to its ac-side (rather than only 50 MW in the ”Control Strategy 2”), implying that the droop controlled stations don’t have to significantly alter their contribution. After the fault clearing there is a low-frequency power oscillation until the systems quickly settles again at t=4.2s. This low frequency oscillation is identified to most systems that feature a wide use of direct-voltage droop and reflects the effort of the system to find a new power-voltage settling point, based on the distributed droop curves. Its frequency and magnitude deviation is mostly affected by the droop constantk.

In general, the direct-voltage response is less abrupt and better controlled compared to the one achieved with the ”Control Strategy 2” control. The poorly-damped oscillations experienced previously are now slightly better damped but the major difference is identified at the voltage overshoot at the beginning and the duration of the fault, which is significantly reduced. In the same manner, the voltage overshoot at the moment of fault-clearing is generally reduced with the only exception of Station 3 where the ”Control Strategy 1” scheme features just slightly higher overshoot than the ”Control Strategy 2” control.

Nevertheless, the post-fault power response of the system employing the ”Control Strategy 1” scheme exhibits relatively large oscillations, compared to the system with the ”Control Strategy 2” scheme. It was further found that their frequency is related to the value of the droop con- stantk. Despite the fact that these oscillations are quickly damped (approximately 1 s after the

clearing of the fault), their magnitude is large enough to consider such a power flow behavior as undesired in an actual MTDC. This calls for modifications in the control algorithms.

DC-side fault and disconnection of a station

In this scenario, a fault is applied at t=1.5 s at the point between the upper dc-side capacitor and the positive dc-pole at Station 1, which is connected to earth through a small resistance. The station is provisioned to be equipped with DC-breakers on both of its dc terminals which manage to forcefully interrupt the fault current after 5ms and disconnect the station from the dc grid. For simulation purposes, after the disconnection of the station, the fault location is

7.5. Controller offering direct-voltage support in MTDC grids −500 0 500 1000 Power [MW] P₁ P₂ P₃ P₄ P₅ −500 0 500 1000 Power [MW] P 1 P2 P3 P4 P5 400 500 600 υ dc1

[kV] Control Strategy 2 Control Strategy 1

400 600 800

υ dc2

[kV]

Control Strategy 2 Control Strategy 1

550 600 650 700 750 υ dc3 [kV]

Control Strategy 2 Control Strategy 1

500 600 700 800 υ dc4 [kV]

Control Strategy 2 Control Strategy 1

1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2 550 600 650 700 750 υ dc5 [kV] time [s]

Control Strategy 2 Control Strategy 1

Control Strategy 2

Control Strategy 1

Fig. 7.16 Active-power and direct-voltage response of the five-terminal MTDC grid using the ”Con- trol Strategy 1” and ”Control Strategy 2” control schemes. A dc-side fault is applied close to Station 1 at t=1.5s, followed by the disconnection of the station.

Chapter 7. Control investigation in Multiterminal VSC-HVDC grids

also isolated but the station is kept in operating mode. This has no effect on the system, whose response is the main focus of the fault scenario.

The simulation results are presented in Fig. 7.16. During the fault, the surviving droop-controlled Stations 3 and 5 experience a large inrush of active power when the ”Control Strategy 2” is used, which quickly reaches and slightly exceeds the rated 1000 MW for Station 3. At the same time, the constant-power Station 3 provides a very stiff power control while Station 5 exhibits a poorly-damped power oscillation. In contrast, the power response under ”Control Strategy 1”, features contribution from all stations to the voltage support. Station 3 quickly increases its power but never exceed the rated 1000 MW. Station 2 reduces its power extraction from the grid and imports almost the rated power to the MTDC grid. At the same time, the pre- viously stiff power-controlled Station 4 responds by decreasing its power extraction from the grid. This prevents the converter capacitors of the dc grid to quickly discharge and is evident in all the monitored direct-voltages, which are not allowed to dip excessively right after the fault, compared to ”Control Strategy 2”. This is occurring because the D-DVC part of the proposed controller is operating in all surviving stations (rather than just the pure droop-controlled) and reacts immediately to the change of the direct voltage.

Nonetheless, the long-term direct-voltage response is very similar for both control strategies and in all the remaining stations, mainly characterized by a poorly-damped 53.2 Hz oscillation which is eventually damped after 0.5 s. However for the plurality of the Stations (2, 3 and 4), the direct-voltage overshoot occurring just after the beginning of the fault is always smaller when the ”Control Strategy 1” scheme is used. This becomes important in the cases of Stations 2 and 4 that feature the largest voltage peak and the ”Control Strategy 1”. The sole exception of Station 5 where the ”Control Strategy 1” surpasses ”Control Strategy 2”, in the highest monitored voltage overshoot.