Ancillary services in RES: comparisons among different countries

 

 

The forecasted integrations of high penetrations of RES, such as wind and PV, into the electricity supply around the world impose the requirement that they not be detrimental to the overall stability of the electric power systems. One way of ensuring this is to require RES to play a role not only in energy production but also in the delivery of ancillary services that are needed to ensure stability at both the transmission and distribution levels. Definitions of ancillary services can differ significantly Ancillary services in RES: comparisons among different countries. Although some definitions emphasize the importance of ancillary services for system security and reliability, others mention the use of ancillary services to support electricity transfers from generation to load and to maintain power quality. Further, some definitions limit the contribution of ancillary services to the transmission network; others include distribution purposes as well. According to F. V. Hulle et al ancillary services can be defined as all grid-support services required by a transmission or distribution system operator to maintain the integrity and stability of the transmission or distribution system as well as power quality. Further, the amount of ancillary services needed for power systems in the future will increase with high shares of RES.

As grid-connected RES technologies are achieving significant penetration levels, interest in analysing the potential impacts of RES on the electric distribution grid is also increasing. The primary focus of VRE, such as wind and PV, had been on the provision of energy. Early on, because most types of renewable generation were connected to the medium-voltage system, the corresponding distribution grid codes that applied treated these renewables comparable to loads (in this case ‘negative loads’) that had to be disconnected from the grid as fast as possible in the case of disturbances. A gradual change in thinking began in 2001 when the German transmission system operator (TSO) E.ON Netz published a grid code that required wind power plant operators to ride through disturbances and to provide additional functionality (in addition to feeding in power) to support the operation of the conventional power stations. These functionalities included the provision of reactive power, the remote controllability of active and reactive power, the capability to keep operating after grid faults, and the capability to perform frequency down-regulation (e.g., to reduce the active power output in the case of an increase in frequency).



Figure 2.3 Net generation capacity added in 2000–2013

 

Today, such capabilities are usually referred to as ancillary services, and examples can be found in the literature – e.g., the authors of focused on PV production for residential applications, and the authors of analysed intermittent wind resources. Indeed, with an aim towards maintaining continuity and security in electricity supply, most countries with increasing penetration levels of RES have developed Ancillary services in RES: comparisons among different countries specific requirements to connect them to the grid (also known as a grid code). Grid codes aim to avoid undesired disconnection under the presence of disturbances – mainly voltage dips – and they establish rules and limits for the active and reactive power as well as expected performance under frequency and voltage oscillations. These requirements have mainly focused on wind power plants due to their major presence in current power systems. In fact, annual wind power installations in the European Union have increased steadily during the past 12 years – from 3.5 GW in 2000 to 11.2 GWin 2013, with an average annual growth rate of 11%. PV generation has become the third most important renewable source, after hydropower and wind power. Its growth has been considerably high during the last few years. For example, at the end of 2009, cumulative installed PV capacity in the European Union was approaching 17 GW, and one year later it accounted for approximately 30 GW. In 2013, more than 136.7 GW of PV generation was installed globally – an amount capable of supplying at least 77 TWh of electricity every year (Figure 2.3). Figure 2.4 shows the new Ancillary services in RES: comparisons among different countries installed and decommissioned power capacity at the end of 2015 in the European Union, and Figure 2.5 shows the power capacity at the end of 2015 in the world. Nevertheless, and despite the relevant presence of PV power plants in current power systems, few countries have developed specific technical requirements for these installations and their performance under disturbances. Usually these requirements have been quite similar to those for wind power plants, with the exception of the German Grid Code, which refers to technical guidelines and requirements specifically for the interconnection of PV.



Figure 2.4 New installed and decommissioned power capacity at the end of 2015 in the European Union

 

Note that despite the increasing RES installations, the conceptual priority of RES in current grid codes is not to replace conventional generation but primarily to reduce consumption. This may be seen as a question of wording only, but with respect to the requirements this has a significant impact. For a long period of time, ‘negative loads’ such as RES were considered to behave like typical loads connected to a distribution system – i.e., they should limit the impact on the grid by having a predefined power factor range and disconnect in the case of grid events such as relevant changes in voltage magnitude, voltage phase angle, or frequency.



Figure 2.5 Renewable power capacity at the end of 2015 in the world, the 28 member states of the European Union (EU-28), the five major emerging national economies (Brazil, Russia, India, China, and South Africa; BRICS), and the top seven countries.

 

Largely resulting from pressure by TSOs, requirements for RES – even those connected to a distribution system – usually try to limit their impact on the grid. Features such as the capability to support the grid are often referred to as RES being ‘grid friendly,’ and they are Ancillary services in RES: comparisons among different countries necessary to reduce possible impacts on the grid in the case of disturbances; however, note that these features always assume a stable grid consisting of a sufficient number of synchronous generators in operation that ensure basic services such as voltage stability (by providing voltage control and sufficient short-circuit power) and frequency stability (by providing primary control and system inertia). Today’s grid code requirements describing ancillary services are not intended to replace the basic services provided by conventional power stations. Also, today’s RES are neither allowed (by grid codes) nor capable (due to existing control and hardware design) to provide these services. The development of a next generation of RES that may provide such features on a larger scale (not only for a small, islanded system) is only beginning.

 

Active power reserves and frequency control

Frequency control services are related to the short-term balance of energy and frequency of a power system. According to the definition by the Union for the Ancillary services in RES: comparisons among different countries Coordination of the Transmission of Electricity [36], frequency control includes automatic (primary/secondary) and manual (tertiary) frequency regulation and operating reserves, as illustrated in Figure 2.6. This is the main service provided by generators (online for automatic services and online or offline for longer-term activated services). It can also be provided from flexible loads and storage units.



 

Figure 2.6 Principal frequency deviation and subsequent activation of reserves

 

According to the ENTSO-E classification, frequency control reserves are defined as frequency containment reserves, frequency restoration reserves, and replacement reserves; see Figure 2.6. In Denmark, the TSO (Energinet.dk) published an ancillary services strategy for 2011–2015 that classifies the ancillary services into frequency-controlled reserves, secondary reserves, manual reserves, and regulating power and properties required to maintain power system stability (e.g., shortcircuit power, Ancillary services in RES: comparisons among different countriescontinuous voltage control, voltage support during faults, and inertia).

Recently, a demonstration project was conducted by the National Renewable Energy Laboratory in collaboration with Puerto Rico Electricity Power Authority (PREPA) to demonstrate the ability of a PV power plant to operate in spinning reserve mode by curtailment. Puerto Rico’s transmission system consists of 230-kV and 115-kV lines, 38-kV subtransmission lines, and 334 substations. PREPA’s typical summer daytime peak load is approximately 2.8 GW. Total installed generation in the region has a capacity of 6 GW, with 173 MW of wind and PV generation; the rest is based on petroleum and natural gas. AES’s 20-MW Ilumina PV power plant, located in Guayama, Puerto Rico, consists of 40 inverters rated at 500 kWac each. Several solar irradiance sensors are placed strategically within the PV plant, and they are used to forecast the actual output of the PV plant. Based on the real-time forecast of the plant output, supervisory control is used to curtail the output Ancillary services in RES: comparisons among different countries of the PV plant partially by 20% (and subsequently 40%) to operate the PV plant with a spinning reserve of 20% from the maximum available power generation.  The frequency control is demonstrated by the implementation of both the droop control and the automatic generation control. The demonstration projects were implemented successfully, as reported in.

2.2.2 Reactive power control/voltage control

From a system point of view, voltage is the prerequisite for any kind of power transport. By changing the amount of reactive power provided to a grid (by changing the reactive power reference of power stations and active or passive sources), the power flow in the grid can be controlled. In most countries, RES play only a very limited role in controlling the voltage of transmission systems.



Figure 2.7 Voltage deviation and subsequent activation of reserves

At the level of the distribution system, voltage control services focus on maintaining power system voltage within the prescribed bounds during normal operation and during – and especially following – disturbances by keeping the balance between generation and consumption of reactive power. Voltage control includes reactive power supply (injection or absorption), and it can be provided by dynamic sources (generators, synchronous compensators) and static sources (capacitor banks, static voltage controllers, and FACTS1 devices), including network equipment such as tap-changing transformers in the substations and loads. Voltage control has two targets (Figure 2.7):

• Steady-state reactive power/voltage control: The aim is to keep the voltage profile close to the desired profile and within the tolerance band margins within Ancillary services in RES: comparisons among different countries the time frame of hours. This control is commonly achieved by injecting or absorbing reactive power at a voltage-controlled node. The TSO dispatches the reactive power using the active and passive reactive power sources that belong to different levels – generation, transmission, and distribution – using optimal power flow methods. Steady-state requirements are usually defined as power factor or voltage requirements at the level of a wind power plant, with typical response times in the range of seconds up to 1 min. The integration of RES power stations into a power system has usually been facilitated with economic incentives. This is the case of reactive power generation. For example, in Spain in 2004, the Royal Decree 436/2004 introduced an incentive as a percentage of the average reference tariff. In 2007, a new incentive for reactive power was also introduced in Spain, IV.02, which had a scheme similar to that of the previous incentive. It is based on a reference value that is updated based on the consumer price index minus a correction value. In this new scheme, hourly values are used instead of quarter-hourly ones, and dispatches are introduced. This change was because dispatchers seek results at the wind power plant’s point of interconnection, but measurements are performed at the level of the wind power plant’s substation. The values (cent€/kWh) from these two schemes were 7.659 and 7.8441 in 2006 and 2007, respectively. This power factor incentive was suppressed in Spain in 2013.


• Dynamic voltage stability: The aim is to keep the network voltages in a dynamic time frame (seconds to minutes), thus preventing a slow voltage collapse event or limiting the depth and extension of an incident (e.g., loss of a main line, loss of generation unit). Dynamic voltage requirements are usually defined as reactive current response at the turbine level, with typical response times in the range of 30–50 ms.

In traditional power systems, VRE proportion has been considered very small relative to conventional generating units. As the penetrations of RES increase – especially wind and solar – grid codes compel renewable generation to contribute more significantly to power system voltage and reactive regulation.

Starting in 2006, the capability to provide ancillary services, such as (limited) frequency control and voltage control, has become a requirement in grids that have a high penetration level of renewables. Nevertheless, knowledge gaps – especially at the level of distribution system operators – and thus the need for further research have been identified in the area of the provision of ancillary services from RES. For example, according to, in the context of ancillary services delivered from wind power plants, further investigations strengthening system reliability are necessary regarding faster and reliable communication (i.e., among wind power plants and system operator control rooms), dedicated tuning of the control strategies, estimation of available power, and coordination of offshore wind power plants to provide reactive power control or voltage control at their land-based point of coupling. In the context of ancillary services delivered from PV, further investigations are needed on available (regional) power estimation, faster and reliable communication and control within the plants, and improved control strategies.

Grid code design depends a lot on the knowledge of the system operator and the size of the wind power plant. In the United States, large wind power plants interconnected to the transmission system are common, and as a result they are treated as power stations based on synchronous generators using classic (PI) voltage control. Power factor control is usually not allowed. In case the short-circuit power of the grid is considerably higher than the reactive power capability of the wind power plant, pure P-control of the voltage is used (‘voltage static’) instead of a PI control.  This is necessary because the reactive power capability of the wind power plant is too small to really control the voltage, and the controller of the wind power plant would be saturated. (See also as an application in Europe.) In case a wind power plant is too far from the high-voltage grid, voltage excursions can be significantly limited if voltage control is used. In the case of short-circuit ratios (i.e., the grid’s short-circuit power divided by the wind power plant’s apparent power) of 5 and lower, usually voltage control is needed to keep the voltage within the normal operating range. In the United Kingdom, a proportional voltage control (‘voltage static’) is commonly used. In Germany, power factor control had been common, but due to the improved voltage-stabilising capability of voltage control, voltage static at the level of the wind power plant is becoming the preferred method of control for newer, interconnected, medium-voltage wind power plants.