Reactive Power Control in Utility-Scale PV Plants

Reactive-power control can be considered as one of the least explored problems in photo-electric industry, at the same time it can provide the key to considerable profit increase for proprietors of commercial solar power-stations. In this article we will review methods of voltage control within systems of transmission and distribution of electric power. Also we will describe consequences and effects of reactive-power surplus on distributed systems of electric power generation and electricity transmission, and also we will discuss some preventive measures on that.

Monetization of reactive-power control

Reactive currents backstreaming in power supply system is not only harmful but also expensive for financial purposes. Parasite currents in both electricity transmission lines and equipment of solar power-stations cause financial losses because of converting electric energy to thermal (thus making generation amount lower, that decreases profits of a solar power-station proprietor) and increase equipment deterioration (decreasing non-failure operating time and increasing maintenance budget).

In addition, reactive energy generation into a national grid results in penalty charges, which means direct financial losses. So adjusting and compensation of reactive power is an obvious method to increase profitability of a power-station.

Methods of reactive-power control

Reactive-power management is an integral part of control process related to voltage level in any electrical power system. When load is small, system generates reactive-power, that should be absorbed. At the same time at large loads it consumes plenty of reactive energy that needs to be produced.

Traditional synchronous generators, widely used in domestic networks, are perfectly able to support balance of reactive power, however expenses on this method of compensation are relatively high. In order to maintain grid parameters companies frequently use static compensative machinery, which (depending on a current situation) connects either capacitors or inductance coils to electrical transmission lines.

Main disadvantages of this method are following:

• Low response time (from few seconds to few minutes);
• Discretisation of elements, that does not provide full compensative influence;
• Voltage and current surges because of transitional processes during turning on or shutting down discrete modules. In order to solve the problem designers of electric power systems install synchronous compensators, static var compensators (SVCs) and static synchronous compensators (STATCOMs) in every point of a network, where rapid and smooth control of reactive-power is required.

Synchronous compensators are lightweight synchronous engines with no-load operation. In this mode they generate reactive-power, while active-power generation is zero. SVCs and STATCOMs, unlike synchronous compensators, structurally are not synchronous engines, but they are categorized as dynamic devices because quick response time and variable output power.

SVC are actually packs of ordinary capacitors and inductors with high-speed commutation lines (usually semiconducting regulated semistors), what allows to adjust current more fluently, than static batteries do. STATCOMs are solid-state power electronic devices, such as solar inverters, but out of array of a solar power-station. They are able to absorb and generate reactive energy, converting voltage that it becomes more or less that common bus voltage. Taking into account that inverters cost decreases three-four times quicker than prices of traditional compensators of reactive power, their application for these tasks grows every year and in prospect they will completely force out other compensative options.

Distributed generation of electric power

Many photovoltaic inverters, connected to common bus, consist a structural part of a solar photovoltaic station. As we said earlier, each of them can either absorb reactive power component, preventing voltage boosts in connection point, or generate it, preventing voltage falls.

High integration of solar power-stations and energy distributing networks enables construction of systems to supress voltage surges, that inevitably occur in any large network. Sensitiveness to luminosity fluctuations (and generation amounts recpectively) on separate areas of a network also will be minimal.

Thus, creation of revolutionary instrument (distributed system) is possible. Such system will be able to minimize thermal losses from reactive currents and even to compensate reactive amounts from external consumers. However, many operational issues for similar distributed systems are not decided yet, so their implementation is a matter of future time.

Inverter level and enterprise level of ajustment

When planning an adjusting and control system of a solar station, developers can provide regulation at the level of every separate negator or at the level of a whole solar power-station. Negator level ajustment presumes that a device is programed to change its operational mode depending on generation amounts and voltage across terminals.

At power-station level central controller manages rate of reactive-power, power-factor or voltage in the point of interconnection of an enterprise network and external electric networks. Except it, central controller makes possible to use additional equipment for adjusting of reactive component of current (for example, capacitor batteries).

Modern inverters can have a few embedded functions enabling following strategies at inverter level:

• Keeping a constant power-factor. Usually inverters operate with power-factor equal to 1, but they can be programed to keep lower ratio (on the condition that parameter values must be within operational range of the model);
• Keeping a constant reactive-power – regardless of active-power, produced by the system;
• Voltage feedback. This mode means that ratios of active and reactive current are regulated by central controller on the basis of current data from voltage sensors.
•  Dymanic control of operating values. Inverter adjusts operating values for power-factor or reactive-power rate according to operator’s commands.

Industrial solar power-stations as integral complex systems can consist of dozens or even hundreds of photo-electric generators. Central controller allows to coordinate operations of separate inverters and further turns them into a single virtual generator. Thus commands to the controller are transmitted through SCADA software or through another cheaper channel (for example, via remote terminal, RTU).

Such management mode is especially useful for energy transmission between interrelated solar power-stations that must work in voltage control mode: depending on specific scenario central controller will permanently adjust reactive-power of inverters, keeping voltage at constant level.

When planning a system, which regulates an enterprise-level (or power-station level) grid, it is necessary to understand that central controller will be critically important equipment.

Its breakdown or software failures results in total control loss over inverters or transporting system of electric power once at enterprise scale. And that is why it is critically necessary to duplicate all systems: server and its power subsystem, data communication equipment, etc. And whenever possible it is necessary to design local management subsystems that will be able to perform at least some functions of central processing unit under conditions of its breakdown or inaccessibility.

It is possible to use other types of equipment for control and adjusting of reactive-power except inverters, especially if inverters application will result in reduction of power output. Thus developers of a system can choose commuted capacitors and compensators. Such replacement will demand ROI analysis and calculation on correlation between electric power prime price changes and amount of capital investments.

But before consideration of such options a project designer should think about possible combination of static and dynamic devices. It may well be that economically best choice is rough adjusting of reactive power by commuted capacitor batteries and fine tuning by inverters. Pulse switching circuit will result in increase of parasite transitions at moments of connection and disconnection of capacitor batteries, that will demand more powerful and quality systems of dynamic control. But decrease of total prime price of whole project (static devices usually cost considerably cheaper, than dynamic with similar specifications) will allow to purchase dynamic systems of higher quality.

Solar power plants are one of the key competencies for companies belonging to the Avenston group. We provide all necessary services for solar energy projects, starting from the support of the development or pre engineering stages to maintenance of already built photovoltaic power plants or systems. List of our solar projects includes a large number of designed and built solar PV power plants of any type and size.