Experience in installation of new reactive power compensation on power grid

Capacitor banks in Brittany network

In the past French Transmission System Operator RTE installed 30 Mvar capacitor banks connected to the 63 kV and 90 kV networks. 30 Mvar rated power corresponds to the optimal value to limit the voltage changes when the capacitor bank is switched in, when the short-circuit current (Scc) of the substation is low.

These capacitor banks have an isolated neutral. Indeed, for a 63 kV network, grounded neutral can not be used since the impedance of a 30 Mvar capacitor bank (about 130 ohms capacitive) is rather close to the equivalent zero impedance of a typical 63 kV substation of the South Brittany network (50 ohms inductive).

The connection of the neutral to the ground would change the impedance seen by the protection relay, as well as the equivalent resistance of the fault. In such a case, this could have negative effects on the protection selectivity for fault clearance.

In a substation, a maximum of three standard capacitor banks can be installed. In the 90’s, a damping circuit (DAR) was designed in order to limit the frequency and the amplitude of the voltage oscillations on the bus bar when several capacitor banks are switched on back-to-back.

These oscillations were generating overvoltages on the extremity of the dead end line connected to the busbar.

To avoid such situations, the line circuit breakers cut the current during the first millisecond in the elimination of a fault close to the substation, and to ensure that the transient recovery voltage does not exceeds the standardized gauge.

Besides, in the case of circuit breaker re-striking, the discharge currents of the capacitor in the fault are limited by this damping circuit to avoid high-frequency zero crossing of the current.

These risks must also be taken into account for the new capacitor banks to be installed. Furthermore, the implementation of 540 Mvar (18 capacitor banks) has increased the risk of disturbing the transmission of the signals that are used in France to change the customer’s electricity tariff.

This phenomenon is amplified when plain capacitor banks are installed on the 63kV network. As a rule of thumb we estimate that the apparent Scc is reduced by about 100 MVA when the 10 Mvar capacitor bank is connected to the bus bar.

On Figure 1, we represent a 63 kV busbar supplied by a 1240 MVA Scc source on which are connected in parallel:

• Three capacitor banks (20 + 3×30 Mvar)
• One 63/20 kV and 36 MVA transformer supplying a 35 MVA load which is compensated by a 8.3 Mvar capacitor bank. On the secondary of the transformer a TCFM generator is connected in series,
• Three 36 MVA transformers (108 MVA) supplying a 53 MVA load compensated by a 12 Mvar capacitor bank.