6.4
Security assessment of AC/DC systems
6.4.1 General framework
The framework outlined hereafter assumes that all TSOs are willing to cooperate, exchange infor- mation, and do their best efforts to ensure the security of the combined AC/DC system. As shown in Fig.6.6, it relies on the existence of a coordinating entity, referred to as “DC TSO”. The DC TSO exchanges information with all connected AC TSOs in order to coordinate them and resolve conflicts2.
model of
area 1 model ofarea N TSO 1 TSO N contingency list
of area 1 contingency listof area N model of
MTDC grid MTDC gridmodel of DC TSO
model of
all AC areas contingency listof MTDC grid MTDC gridmodel of
constraints corr. actions constraints co rr. actions
Figure 6.6: Framework for security assessment of AC/DC system
The AC TSOs have a model of their own AC system and the MTDC grid, and perform security assessment taking into account constraints they receive from the DC TSO about the other AC areas. Instead, the DC TSO has a model of the whole AC/DC system, and it performs security assessment of the complete system, but for the MTDC grid contingencies only. Although the model used for the security assessment by the DC TSO would be significantly larger (i.e. inside boundary C in Fig.6.4), the number of DC contingencies to be investigated would be small. This allows to perform security assessment with a full view of the system without prohibitive increase in computational complexity.
The different cases for AC and DC contingencies are further discussed in the following.
2
6.4.2 AC contingencies
First, the case of an AC contingency is considered. As mentioned before, propagating the effects of a disturbance from one area to the others has to be avoided.
For the i-th TSO, an optimization problem similar to the one described by Eqs.(6.1)-(6.5) can be considered, with the addition of DC grid controls (the subscriptk relative to contingency k is dropped to simplify the notation):
min ∆ui,∆vi,∆ulsi ||∆ui||2Wu+||∆vi|| 2 Wv+||∆u ls i ||2Wls (6.6) subject to: gi(xi, ∆ui, ∆vi, ∆ulsi ) = 0 (6.7) hi(xi, ∆ui, ∆vi, ∆ulsi )≤ Li (6.8) ∆u ls i ≤ ∆u ls i (6.9) |∆vi| ≤ ∆vi (6.10) |∆ui| ≤ ∆ui (6.11)
where∆videnotes a control action on a converter and functionsgiandhi now also involve those
controls. The subscripti denotes a control action calculated by TSO i. The diagonal entries of Wu and Wv are the weighting factors for AC and DC control actions, respectively. The updated
vector Liincludes also the physical and operational bounds of the DC equipment.
Since the MTDC grid is also included in the model of TSOi, the vector ∆vi leads to modifying
the DC grid power flows. If the external systems are simply replaced by Th´evenin equivalents (as in Fig. 6.5a), then the resulting DC grid power flows are only limited by the MTDC grid constraints and the VSC ratings. However, the MTDC grid actions have to be approved by the other TSOs. In accordance with the framework shown in Fig.6.6, TSO i has to inform the DC TSO of its corrective MTDC actions. Based on this information, the DC TSO evaluates whether those actions cause a problem in the other AC areas. If not, the corrective actions are accepted and thei-th AC area is correctively secure with respect to the contingency of concern. Otherwise, the DC TSO computes and sends back to thei-th TSO constraints on the available MTDC grid actions, to be added to its security assessment model, formally:
hthi (∆vi)≤ Lvi (6.12)
where (6.12) includes the VSC limitsLvi set by all external AC areas on the MTDC grid actions available to TSOi, as discussed in Section6.3.2.
6.4. Security assessment of AC/DC systems 143
To avoid performing twice the security assessment for the contingencies requiring actions from the MTDC grid, the DC TSO could update and post the constraints in real time. Therefore each AC TSO could incorporate them directly in its security assessment.
6.4.3 DC contingencies
The assessment of MTDC grid contingencies is performed by the DC TSO. Since it has a full view of the system this analysis is rather straightforward and the corrective actions can be identified by solving the following optimization problem, which minimizes the total control effort over all AC areas: min ∆u,∆v,∆uls||∆u|| 2 Wu+||∆v|| 2 Wv+||∆u ls||2 Wls (6.13)
where∆u = [∆u1 . . . ∆uN] and ∆uls=
h
∆uls
1 . . . ∆ulsN
i
.
The minimization is subject to the following constraints: fori = 1, . . . , N :
gi(xi, ∆ui, ∆v, ∆ulsi ) = 0 (6.14) hi(xi, ∆ui, ∆v, ∆ulsi )≤ Li (6.15) ∆u ls i ≤ ∆u ls i (6.16) |∆ui| ≤ ∆ui (6.17)
and the following constraint for the DC grid control actions:
|∆v| ≤ ∆v (6.18)
The above procedure results in a single set of MTDC actions∆v as well as suggested AC actions ∆ui for each AC area i = 1, . . . , N . The latter are suggested to the AC TSOs, and will not
necessarily be applied. For instance, an AC TSO could opt for other corrective actions taking into account the MTDC grid actions calculated by the DC TSO.
Although it has been assumed that all TSOs are willing to coordinate and respect the recommen- dations of the DC TSO, it is still possible that a disagreement arises between the DC and AC TSOs regarding the DC control actions calculated by the former. In such a case, it should be the respon- sibility of the AC TSO to provide appropriate constraints to the DC TSO. Then, the latter should perform a new security assessment taking into account the updated constraints.
6.4.4 AC-DC contingencies
A clear distinction between AC and DC contingencies was assumed in the previous analysis. However, this distinction might not always be obvious, especially when combined events are con- sidered. An example of such event has been illustrated in Section3.4 where an AC contingency (loss of line and dramatic decrease of SCC) led to instability. Such an event would most probably result in tripping of the VSC or in fast decrease of its power. The latter could emanate from offline analysis and setting up of defense plans. Consequently, such an event would impact the other AC areas since the lost power would be compensated by the remaining VSCs.
This type of event could be treated as an AC contingency. Therefore, the TSO of the AC area in which the initial contingency occurs should ensure that the impact on the other AC areas is between limits. Namely, the change of the powers of the remaining VSCs should be between the limits specified by (6.12). In the example of Section3.4, and assuming that the VSC is connected to an MTDC grid instead of a point-to-point link, the change in the MTDC grid power flows is not due to a corrective action of the TSO, but is rather the natural response of the system. The fast evolution of such events does not leave adequate time for corrective actions; therefore, the TSO should resort to preventive actions by either enhancing its AC grid or by adjusting the power transfer in the VSC.
6.4.5 Discussion
The proposed framework has been described in an abstract and conceptual form to ease presen- tation and maintain generality. In reality, it may be required to also investigate dynamic aspects. If dynamic simulations have to be performed, the complexity of identifying “optimal” actions in- creases significantly. Nevertheless, exchanging constraints between the DC and AC TSOs enables taking advantage of the control capabilities of the MDTC grid, without violating its “firewall” property.
Preventive controls have not been considered in the above analysis. In fact, they are similar to the ones following a DC contingency, in the sense that multiple TSOs may request them simultane- ously. Therefore, in principle the same methodology can be applied, i.e. rely on the DC TSO with a full view of the system to calculate them. To keep the computational burden tractable, the num- ber of contingencies assessed by the DC TSO should be kept as low as possible. Several methods can be contemplated. For example, a filter could be applied so that only the contingencies already assessed as insecure are further considered by the DC TSO.