GridStab News: Regular posts to demystify power system dynamics and stability

Topic 2: Voltage dynamics and control

While frequency (discussed in previous article, see here) is a wide-area indicator of grid balancing which is influenced by the active power, voltage magnitude is a local indicator, different from one node to another and is mainly influenced by the reactive power.

Note that in AC electric power systems, the AC voltage is a waveform that can be represented by a sinusoidal signal, and hence is defined by a frequency (e.g., 50 Hertz in Europe), a magnitude and a phase angle.

Did you say "reactive power"?

The reactive power is a power that cannot be converted into useful work. It results from the alternating (AC) nature of current and voltage. Indeed, in AC power systems, there are inductive (L) and capacitive (C) components that exchange magnetic and electric energy creating flows of reactive power.

Classic power systems are dominated by High Voltage (HV) transmission lines, which are "inductive" (L) by nature. It means that the reactance (X) of HV Transmission Networks is usually much higher than the resistance (R). In these situation, the load flow equations can be simplified and show that the active power flows mainly causes an increase of voltage phase angle while the reactive power flow is mainly responsible for the drop of the voltage magnitude. Therefore, even if it is considered as a "useless power", reactive power needs a careful attention.

Why do electric power systems need voltage control?

In order to operate properly, equipment connected to the electrical network need the voltage in an acceptable range of values (around nominal), depending on the grid voltage level. Some equipment are more inductive by nature, which means that they consume a given amount of reactive power, which creates reactive power flows and hence voltage (magnitude) drop. If reactive power flow is important and/or if it is transmitted over a long distance (it means over a large impedance), it can results in prohibited under-voltages (note that the same reasoning applies to capacitive equipment and the risk of over-voltage).

In traditional electric power systems, voltage is mainly controlled by classic thermal units (e.g., gas, nuclear,...) and their Synchronous Generators (SGs). SGs are able to absorb and produce significant amount of reactive power which allow to decrease and increase the voltage magnitude locally. Indeed if you produce the reactive power locally, close to an inductive load, you reduce the amount of reactive power that needs to flow through the grid to feed this inductive load, and thus you reduce the voltage (magnitude) drop. More specifically, most of SGs connected to HV grids have an Automatic Voltage Regulator (AVR) which is able to accurately control the value of the voltage magnitude at the Point Of Connection (POC) by adjusting the reactive power output of the SGs. If the voltage decreases, the injection of reactive power will increase (resp. if the voltage increases, the injection of reactive power will decrease). Thanks to their AVR, SGs are able to impose their voltage reference to the grid. In addition, during a short-circuit leading to sudden and large voltage decrease, SGs are able to inject without any delay large short-circuit current which will support the voltage locally and avoid the propagation of the fault hence limiting the area of the grid that will be impacted by the short-circuit.

It is then understood that a sufficient amount of SGs reduces the variation of voltage from one node to the other and contains the impact of a short-circuit on the grid. SGs are of tremendous importance today to guarantee the voltage stability of the power system.

Evolution of the electric power system and impact on voltage dynamics

While SGs can efficiently control the voltage at their POC, standard Renewable Energy Sources (RES) connected to the grid through Power Electronic (PE) converters, cannot. In addition, almost all existing PE converters rely on a Phase Locked Loop (PLL) to track the voltage (magnitude and angle) evolution at their POC and to inject the appropriate active and reactive current. This means, in order to operate properly, PE converters need a "strong" voltage reference, mainly provided by SGs.

With the current evolution of electric power systems (i.e., more and more RES and less and less SGs), it is then understood that voltage will become more and more volatile, due to lack of voltage control capability. Furthermore, RES will have more and more difficulties to efficiently track the voltage evolution due to the voltage reference that is becoming weaker and weaker. Beyond a certain point, the risk is that RES will not be able to inject a proper current and will lose synchronism with the power system, leading to their disconnection. With more and more power provided by RES, it is clear that this is a critical issue. In addition, PE converters have a limited overcurrent capability and cannot provide a large amount of short-circuit current during a fault, hence also reducing voltage support in the event of such scenarios and increasing the fault propagation hence impacting more equipment.

The progressive replacements of SGs by RES is also disrupting the way power is flowing through the grid. Indeed, historically, large thermal power plants have been built close to the main load centers and the grid has been designed around that, which was limiting the distance over which power was transmitted. Now large RES (e.g., offshore Wind Parks) are connected far from main load centers which makes that power is now transmitted over longer distances, exacerbating voltage drop and phase angle difference which brings he system closer to its voltage stability limit.

Finally, electric power systems are also facing more and more penetration from small Distributed Energy Sources (e.g., rooftop PVs) connected to the distribution network which are significantly reducing the net load seen from the transmission system. This combined with more and more capacitive (i.e., that produce reactive power) HV cables (due to a lot of public opposition to build new transmission lines) makes that power systems are also facing more and more over-voltage issues, highlighting once again the need for additional voltage controllability.

All in all, it can be understood that voltage control and stability will become a big challenge with the evolution of the electric power system.

What now?

With the progressive decommissioning of thermal power plants, It should be now understood that devices that are able to provide strong voltage references to the system are needed to improve voltage controllability and support.

Multiple alternative exists to compensate for the lack of SGs. While it has been presented in this article as a way to provide additional inertia, synchronous condensers are good candidates to also improve voltage control and stability, as there are basically SGs not coupled with a turbine and hence only able to provide reactive power (no active power).

Historically the so-called Flexible AC Transmission System (in short FACTS) devices like Static Var Compensator (SVC) or STATCOM were used to support the voltage, as they can provide or absorb reactive power very quickly in response to voltage variation. The problem is that these devices also rely on PLL to work properly and therefore can only operate in a system with sufficient amount of SGs.

To cope with that issue, new generation of STATCOMs rely on grid-forming control which does not require an external strong voltage reference to operate properly. In general, progressively switching to RES with PE converters relying on grid-forming control would in theory improve the voltage stability of the power system. Unfortunately, the maturity of this technology is still low. A future article will discuss in more details the Grid-Forming control.