Secondary Voltage Control
Secondary voltage regulation has, as its first objective, the automatic voltage control at a power system’s main transmission buses (i.e., the most important load buses) by controlling the largest available reactive power resources on site. Therefore, primary and secondary voltage controls have different and sometimes opposite aims.
Secondary voltage control plays an important role both during normal operating conditions and in front of contingencies:
• In normal grid operation, it ensures:
? Maintenance of network voltages at a specified value and reduction in their changes.
? improve optimization control efficiency;
? optimal distributed online controls of reactive power resources;
? Dynamic performance of first-order type to HV voltage transients, with a dominant time constant of about 50 s.
• Under disturbed conditions, secondary voltage regulation:
? Offers timely controls of generated/absorbed reactive powers in the disturbance area;
? Speedily recovers the disturbance area voltage level;
? Imposes a first-order dynamic response to voltage transients in accordance with PID (PI) control mechanism, with a proposed time constant of about 50 s as well as fast recovery of most of the peak variations resulted from large disturbance
When the pilot node is considered to be the controlled HV bus in figure. The basic principle of SVR is voltage control of a wide HV grid through controlling of a small number of buses—the most important ones each of them able to determine voltage in surrounding buses, so each defining its area of influence. SecVC therefore requires dividing the transmission network into regions, within which the voltage is controlled in the main bus, called the region “pilot node”. A regional controller (which controls the pilot nodes and therefore the region) separately coordinates the generators of a given area by automatically adjusting their reactive powers to control the voltage of the region pilot bus. Analogously to high side voltage control (HSVR), pilot control voltage consists of closed-loop control of the pilot node voltage through a PID control action, which defines an area reactive power level “q”, the reactive powers of all the control power plants in the area. The secondary voltage regulator inputs the instantaneous voltage measure of the area pilot bus and compares it with the pilot node voltage set-point, determining instant by instant the reactive power level to be sent to the control power plants in the area. The reactive power level “q” therefore determines the alignment of each area’s generating units, contributing in proportion to their capabilities to total area reactive power.
The automatic voltage and reactive power control of a transmission network considers the hierarchical structure shown in Fig. 2.3, where the control apparatuses are now apparent:
• In this control structure, the first hierarchical level (the primary level) consists of conventional generator voltage regulators (AVRs). These make it possible to take fast-control action in the face of local perturbations (for instance, short circuits near a generator) and thereby collectively determine the “primary” voltage regulation of the network.
• The second hierarchical level consists of power station CC regulators, which achieve the reactive power required by the CAC or the RVR at a higher hierarchical level, by operating on the primary voltage control set-points.
• The third hierarchical level consists of a slower CAC (or a few RVRs if the grid is subdivided into more than one region: for example, the case of a national dispatcher operating on-field through regional dispatchers), which regulates in an integral way the voltage of the pilot nodes by controlling the reactive power of participating power stations to the second hierarchical level.
The switching of compensating equipment such as capacitor banks and shunt-reactors or the blocking of OLTC tap-changers is part of SecVC control action. It operates at each region on the local switching resources only when needed, according to the region control margin value, given by the difference of the real-time value of region reactive power level “q” with respect to its + 1 or ? 1 p.u. limits. Proper thresholds of the “q” value habilitate area on/off switching according to pre-defined sequences.
B. Tertiary Voltage Control
The basic idea of TerVC derives from the need for a system’s operating security and efficiency to increase through central real-time coordination of the distributed SecVC structure:
• Pilot node voltage set-points must be adequately updated and coordinated online and in real time, with dynamics slower than SecVC, by consideration of the real operating condition of the overall grid and by avoiding pointless and conflicting SecVC inter-area control efforts.
• To this end, pilot node voltage set-points can be computed and updated in real time simply by use of the SVR control system operating conditions that give reliable, synthetic, timely information on what is going on at the overall system: “SecVC controls that are active on the physical process and the pilot node measurement feedback provide, at any instant, an undoubtable figure of the most important essential happenings in the real process”.
• Therefore, pilot node voltage set-points can be optimized in real time to effectively minimize grid losses while still preserving the control margin by simply referring to the “grid equivalent” real-time system model, based on few but very reliable and significant data on control variables the SecV is able to provide to TerVC.
The TVR control level is therefore aimed at optimising nationwide voltages by a suboptimal real-time control . This involves determining moment by moment the pilot node voltage set-point values by minimising the differences of the measured pilot node voltages with respect to their historical references or off-line forecasted values, always maintaining the control margin in each area. Having a proper selection of SecVC areas, this simplified TerVc optimisation is able to achieve a safe and efficient closed-loop system control by a slower than SVR dynamic performance. Therefore, the tertiary loop represents the continuous computing of a wide-area, real-time, updated, optimal voltage plan, applied to the grid through the global coordination of automatic control actions achieved by an SVR. The main TVR objectives are these: (i) the management, at a low speed, of the reactive power flow between the power system areas, accomplished by minimising power system losses; (ii) the increasing of the power system’s controllability and stability.