**3. Methodology and calculations of auxiliary services necessary for RES regulation**

The purpose of auxiliary services is provision of steady power balance. On one side there is electricity production starting from traditional sources to RES, on the other side are customers, i.e. final consumers. Production and consumption within the scope of interconnected power system must be in equilibrium at every moment.

A new approach to RES support, embodied in act No. 309/2009 about the support of RES and highly effective combined production [4], brings a quasi new group of producers to the electricity market. Those producers produce electricity not according to market demand but practically any time when climatic (wind, solar flux) conditions allow. Responsibility for sales of produced electricity and for the potential balance from the planned values are transferred primarily to operators of distribution systems who are obliged to buy electricity from RES, but eventually power balanced must be maintained by TSO. Supportive mechanisms for RES are currently only starting and massive capacities of these sources are not installed in SPS yet. The progress in installed capacity of RES in distribution system in 2010 is depicted in Fig. 2. Development trend of installed capacity is soaring confirming relevance of topic of availability and sufficiency of regulative capabilities and possibilities of SPS.

#### **3.1 Methodology of setting necessary auxiliary services**

The setting of necessary range of auxiliary services for securing reliable operation is closely linked to the degree of reliability of the system. The higher the rate of reliability is required, the higher the range of auxiliary services is needed, having a substantial influence on the final electricity price.

When setting the necessary range of auxiliary services important source information include not only expected loads in regulation area but also load diagram for considered interval of time, value of installed capacity of RES and other statistical data associated with system operation. Amount of auxiliary services was calculated according to the methodology published in [1].

Fig. 2. Progress in installed capacity of RES in 2010

#### **3.2 Setting necessary auxiliary services range for RES regulation**

When setting the necessary amount of auxiliary services for SPS needs long term statistics of load and system balance are utilised. Because there is no centralised measurement of power on existing RES, it is not easy to define influence of RES to SPS especially to volumes of auxiliary services. SEPS can only expect that operator of distribution systems will be willing to provide high quality prediction of production from RES and placement of the electricity in the market thus minimising requirements for regulation reserves.

#### **3.3 Impact of powers of RES fluctuations on primary frequency regulation**

The role of primary frequency regulation (PRV) is to avoid the occurrence of impermissible deviation in interconnected power system during few seconds. Resulting from the nature of primary frequency regulation the considerable deviations of balance between production and consumption caused by outages of large electricity sources are compensated within seconds. Overall power reserve of 3 000 MW is necessary to secure functionality of the primary regulation in the interconnected international system RG CE [8].

Primary regulation is of proportional nature and maintains equilibrium of production and consumption in synchronous interconnected area based upon frequency deviation. Aliquot part of the primary regulation reserve of Slovakia for 2010 is *PRV* = ±30 MW. Value of active power is symmetrical, which means 30 MW.

#### **3.4 Impact of powers of RES fluctuations on secondary power regulation**

Secondary power regulation (SRV) maintains equilibrium of production and consumption as well as system frequency in each regulation area taking into account regulation programme without violation of primary regulation, that works concurrently in synchronous interconnected area.

Secondary regulation uses centralised automatic production regulation which maintains setting of active power of production units during seconds up to 15 minutes (typically) after the event. Secondary regulation is based on the secondary regulation reserves controlled automatically. Adequate secondary regulation depends on production sources offered by production companies for disposal for auxiliary services.

Minimal recommended value for secondary regulation reserve within interconnected system RG CE is derived from the expected value of maximum system load in give time period according to the empirical formula [8]:

$$SR\,V\_{\rm RGCE} = \pm \sqrt{a \cdot L\_{\rm max} + b^2} - b \cdot \tag{1}$$

where *a*10 (empirical constant), *b*150 (empirical constant),

*L*max - expected maximum load, *SRV -* secondary power regulation.

The other part of the secondary regulation reserve within the regulation area Slovakia is a component resulting from load changes dynamics of regulation area ( *SRVDYN L*, ). The value of this component can be derived from the statistics monitoring system load during a longer period of time and is within the range of 20 to 40 MW, which means circa 1 % recalculated to average yearly load of SPS.

$$\text{LSR}\,V\_{\text{DYN,L}} = \frac{R\_{\text{sp}}}{2} + \sigma\_{\text{\textdegree}} \tag{2}$$

where *R* – arithmetic average of 10 minute differences of maximal and minimal load values for whole hours,

– standard deviation.

106 Renewable Energy – Trends and Applications

When setting the necessary amount of auxiliary services for SPS needs long term statistics of load and system balance are utilised. Because there is no centralised measurement of power on existing RES, it is not easy to define influence of RES to SPS especially to volumes of auxiliary services. SEPS can only expect that operator of distribution systems will be willing to provide high quality prediction of production from RES and placement of the electricity

The role of primary frequency regulation (PRV) is to avoid the occurrence of impermissible deviation in interconnected power system during few seconds. Resulting from the nature of primary frequency regulation the considerable deviations of balance between production and consumption caused by outages of large electricity sources are compensated within seconds. Overall power reserve of 3 000 MW is necessary to secure functionality of the

Primary regulation is of proportional nature and maintains equilibrium of production and consumption in synchronous interconnected area based upon frequency deviation. Aliquot part of the primary regulation reserve of Slovakia for 2010 is *PRV* = ±30 MW. Value of active

Secondary power regulation (SRV) maintains equilibrium of production and consumption as well as system frequency in each regulation area taking into account regulation programme without violation of primary regulation, that works concurrently in

Secondary regulation uses centralised automatic production regulation which maintains setting of active power of production units during seconds up to 15 minutes (typically) after the event. Secondary regulation is based on the secondary regulation reserves controlled automatically. Adequate secondary regulation depends on production sources offered by

Minimal recommended value for secondary regulation reserve within interconnected system RG CE is derived from the expected value of maximum system load in give time

The other part of the secondary regulation reserve within the regulation area Slovakia is a component resulting from load changes dynamics of regulation area ( *SRVDYN L*, ). The value of this component can be derived from the statistics monitoring system load during a longer period of time and is within the range of 20 to 40 MW, which means circa 1 %

> φ DYN,L <sup>σ</sup> , <sup>2</sup> *R*

<sup>2</sup> *SRV a L b b* RGCE max , (1)

*SRV* (2)

**3.2 Setting necessary auxiliary services range for RES regulation** 

in the market thus minimising requirements for regulation reserves.

**3.3 Impact of powers of RES fluctuations on primary frequency regulation** 

primary regulation in the interconnected international system RG CE [8].

**3.4 Impact of powers of RES fluctuations on secondary power regulation** 

power is symmetrical, which means 30 MW.

production companies for disposal for auxiliary services.

where *a*10 (empirical constant), *b*150 (empirical constant),

*L*max - expected maximum load, *SRV -* secondary power regulation.

period according to the empirical formula [8]:

recalculated to average yearly load of SPS.

synchronous interconnected area.

Resultant value of secondary regulation of regulation area Slovakia then equals to sum of minimal recommended value RG CE and a dynamic component. The value of power is symmetrical.

$$SRV\_{\rm FIN} = \pm \left( SRV\_{\rm RGCE} + SRV\_{\rm DYN,L} \right) \tag{3}$$

With the help of secondary regulation the central controller of regulation area maintains compensation of frequency deviations and compensation of active power balance on the planned level. The value of active power is symmetrical. Minimal offered power for SRV is 2 MW per unit. The whole regulation range must be realised within 15 minutes from the request and has to be symmetrical according to the basic power. Basic power is unit power for DLD coverage determined by provider within the preparation of operation.

Unit has to allow continuous repeated power changes in any direction within offered regulation range for SRV. Offered regulation power has to be available during whole negotiated time period (hour, day, etc.).

When calculating required range of secondary power regulation it is necessary to consider the influence of RES mainly wind and PV plants. RES are causing additional power fluctuations is regulation area. This undesirable phenomenon has a direct impact on secondary regulation reserve increase. Various foreign system analyses proved the fact that after implementing RES dynamic variations and increase/decrease gradient of non-covered load (difference between overall load and RES production) have risen. In view of the secondary frequency regulation mission (whose role is to maintain dynamic unbalance between planned production and expected load, and thus to keep the balance of regulation area) a new component of secondary regulation reserve *SRVDYN RES* , has to be introduced.

Additional components of secondary regulation reserve take into account fluctuation of load and production of RES. As there is neither mutual relation nor dependency between mentioned components, these components cannot be directly arithmetically added. If the arithmetical addition is used the value of overall secondary regulation reserve would rise inadequately. One way how to consider both non-correlating components is the use of the function which calculates geometric sum of the values. Resultant value of secondary regulation reserve is then symmetrical and can be calculated according to the following formula:

$$\text{SRV}\_{\text{VYS}} = \pm \left( \text{SR} \, V\_{\text{RGCE}} + \sqrt{\text{SR} \, V\_{\text{DYN},L}^2 + \text{SR} \, V\_{\text{DYN},RES}^2} \right). \tag{4}$$

Component *SRVDYN RES* , constitutes of two partial components, which consider the influence of wind and PV plants and final value can be calculated according to the following formula:

$$SRV\_{\text{DYNRES}} = \pm \left( SRV\_{\text{DYNMIND}} + SRV\_{\text{DYNPV}} \right). \tag{5}$$

Amounts of secondary regulation reserve for wind plants *SRV*DYN,WIND were specified according to the findings in study [9]. Amounts of secondary regulation reserve for PV plants *SRVDYN PV* , were specified according to the statistical values from PV plants in operation in Czech Republic and Slovakia [23, 24]. However it has to be remarked that considered were only roof applications with low installed capacity and mentioned statistical data did not include long term information.


Table 2. Hourly changes of PV plants power

The basis for the necessary range of the secondary regulation reserve *SRVDYN PV* , determination is hourly sample of expected time changes of overall PV plants' power taking into account hourly power shown given in Table 2. Additional values for secondary regulation reserve were calculated according to the method of *SRVDYN PV* , calculation.

Based on statistics, performed calculations and after adaptation of the values for the conditions in Slovakia the expected volumes of secondary regulation reserve for RES *SRVDYN RES* , were determined and are shown in Table 3.


Table 3. Expected volumes of secondary regulation reserve for RES

#### **3.5 Impact of powers of RES fluctuations on tertiary power regulation**

Tertiary regulation (TRV) uses tertiary reserve, which is usually activated manually by TSO in the case of actual or expected secondary regulation activation.

Tertiary regulation is principally used for secondary reserve's release in the balanced state of the system, but it is also activated as a supplement of the secondary reserve after larger outages for system frequency restoration and following release of primary reserve within the whole system. Tertiary regulation is typically performed within the responsibility of TSO.

The nature of tertiary regulation differs from that of secondary regulation. While secondary regulation maintains dynamic unbalance between planned production and expected consumption, tertiary regulation corrects errors in production programme introduced by larger imperfections in consumption prediction and sources outages – Fig. 3.

This regulation affects the change of active power of generators in the whole range up to their withdrawal or connection into operation. It reacts to overall state of given power system and acts after the secondary power regulation or cannot be activated at all. The sources covering tertiary regulation of active power can use their whole regulation range or its parts for it. When starting from the zero power they have to supply power to the electric system equivalent to the basic regulation range of tertiary regulation (technical

plants *SRVDYN PV* , were specified according to the statistical values from PV plants in operation in Czech Republic and Slovakia [23, 24]. However it has to be remarked that considered were only roof applications with low installed capacity and mentioned statistical

*P*inst (kW) 10 20,1 62,4 68,3 84 *P P* hour inst % 2,8 2,4 1,8 1,7 1,4

The basis for the necessary range of the secondary regulation reserve *SRVDYN PV* , determination is hourly sample of expected time changes of overall PV plants' power taking into account hourly power shown given in Table 2. Additional values for secondary regulation reserve were calculated according to the method of *SRVDYN PV* , calculation. Based on statistics, performed calculations and after adaptation of the values for the conditions in Slovakia the expected volumes of secondary regulation reserve for RES

*P*inst RES (MW) 300 400 500 600 800 1000 1200

*SRV*DYN,RES 43 50 58 65 80 95 109

*SRV*DYN,RES (% of *P*inst RES) 14,29 12,57 11,53 10,84 9,98 9,46 9,11

Tertiary regulation (TRV) uses tertiary reserve, which is usually activated manually by TSO

Tertiary regulation is principally used for secondary reserve's release in the balanced state of the system, but it is also activated as a supplement of the secondary reserve after larger outages for system frequency restoration and following release of primary reserve within the whole system. Tertiary regulation is typically performed within the responsibility of

The nature of tertiary regulation differs from that of secondary regulation. While secondary regulation maintains dynamic unbalance between planned production and expected consumption, tertiary regulation corrects errors in production programme introduced by

This regulation affects the change of active power of generators in the whole range up to their withdrawal or connection into operation. It reacts to overall state of given power system and acts after the secondary power regulation or cannot be activated at all. The sources covering tertiary regulation of active power can use their whole regulation range or its parts for it. When starting from the zero power they have to supply power to the electric system equivalent to the basic regulation range of tertiary regulation (technical

data did not include long term information.

Table 2. Hourly changes of PV plants power

*SRVDYN RES* , were determined and are shown in Table 3.

Table 3. Expected volumes of secondary regulation reserve for RES

in the case of actual or expected secondary regulation activation.

TSO.

**3.5 Impact of powers of RES fluctuations on tertiary power regulation** 

larger imperfections in consumption prediction and sources outages – Fig. 3.

minimum of source). Reserve of tertiary regulation of active power can be secured with different activation times.

Fig. 3. Impact of RES operation on tertiary regulation reserve

Tertiary regulation *TRV30MIN* provides coverage of load changes caused by temperature, uncertainty in load estimates, outages of sources and electricity demand.

Necessary power reserve for tertiary power regulation coverage *TRV30MIN* can vary for both regulation directions and thus is split to positive and negative reserve.

Positive tertiary power regulation *TRV30MIN+* is calculated according to [1] and consists of several components:

Inaccuracy of load estimation and influence of temperature

$$TRV\_{\text{no}} = \text{NP}\_{\Phi} \cdot \text{MAX} \,/\, 100 \tag{6}$$

(*NP* – inaccuracy of load estimation, *MAX* – maximum load). Stochastic load change:

$$TRV\_{\text{nx}} = \text{NP}\_{\Phi+} \cdot \text{MAX} \,/\, 100 \,\, \text{J} \tag{7}$$

(*NP*+ – positive inaccuracy of load estimation).

Substitution of tertiary power regulation in case of power production facility outage:

$$TR\,V\_{\text{vypbl}} = SR\,V\_{\text{RGCE}}\,\text{.}\tag{8}$$

Adjustment of the electricity market influence (this component can append value *TRV30MIN+* based on the historical data or expected changes depending on the electricity market).

The final value of *TRV30MIN+* is then calculated:

$$\text{TRV30MIN} + \sqrt{\text{(TRV}\_{\text{vypbl}}\text{)}^2 + \left(\text{TRV}\_{\text{raz}}\right)^2 + \left(\text{TRV}\_{\text{no}}\right)^2} \tag{9}$$

Negative tertiary power regulation *TRV30MIN-* is calculated according to [1] and consists of several components:

Inaccuracy of load estimation and influence of temperature

$$TR\,V\_{\text{no}} = \text{NP}\_{\phi} \cdot \text{MAX} \,/\, 100\,\tag{10}$$

(*NP* – inaccuracy of load estimation, *MAX* – maximum load). Stochastic load change:

$$TRV\_{\text{nx-}} = \text{NP}\_{\Phi^-} \cdot \text{MAX} \,/\, 100 \tag{11}$$

(*NP* – negative inaccuracy of load estimation). The final value of *TRV30MIN-* is then calculated:

$$TRV\\$0MIN\text{-}=\sqrt{\left(TRV\_{\text{nx}\text{-}}\right)^{2}+\left(TRV\_{\text{no}}\right)^{2}}\tag{12}$$

In view of some unpredictable fluctuations of RES power that can occur practically in the whole range of installed capacity, it is necessary to have sufficient regulation power of tertiary power regulation available at any time. Currently there are no statistical data for SPS which could be used to set starting values of increased tertiary regulation reserve caused by RES operation. That is why it would be suitable to use meteorological data as one of the supporting inputs for RES electricity production prediction.

Value of 30 minutes tertiary regulation reserve for RES considering a given degree of accuracy of RES production prediction is as follows:

$$TR\,V\_{\rm RES}^{30\,\mathrm{min}\,\pm} = k\_{\mathrm{NP}} \cdot P\_{\mathrm{inst\,RES}}\,\prime\,\tag{13}$$

where 30min *TRVRES* — increased 30 minutes tertiary regulation reserve caused by RES operation. When calculating final values of *TRV30MIN* it is necessary to distinguish winter and summer and also *TRV30MIN+* or *TRV30MIN-* services.

$$TRV\_{\text{Fin}}^{\text{30\,min}+} = TRV\_{\text{RES}}^{\text{30\,min}\pm} + TRV\text{30}MIN + \tag{14}$$

$$TRV\_{\text{Fin}}^{\text{30 min}-} = TRV\_{\text{RES}}^{\text{30 min}} + TRV \text{30}M \text{IN} - \tag{15}$$

Final values of tertiary regulation reserve *TRVFin* for different installed RES capacities and loads are in Table 4.


In view of some unpredictable fluctuations of RES power that can occur practically in the whole range of installed capacity, it is necessary to have sufficient regulation power of tertiary power regulation available at any time. Currently there are no statistical data for SPS which could be used to set starting values of increased tertiary regulation reserve caused by RES operation. That is why it would be suitable to use meteorological data as one

Value of 30 minutes tertiary regulation reserve for RES considering a given degree of

30min *TRV k P* RES NP inst RES

When calculating final values of *TRV30MIN* it is necessary to distinguish winter and

Final values of tertiary regulation reserve *TRVFin* for different installed RES capacities and

*P*inst RES (MW) 300 400 500 600 800 1000 1200 *TRV*no (MW) 85 85 85 85 85 85 85 *TRV*nz+ (MW) 82 82 82 82 82 82 82 *TRV*vypbl (MW) 62 62 62 62 62 62 62 *TRV30MIN*+ (MW) 133 133 133 133 133 133 133

(MW) 84 112 140 168 224 280 336

(MW) 217 245 273 301 357 413 469

= 30min *TRV*RES

= 30min *TRV*RES

— increased 30 minutes tertiary regulation reserve caused by RES operation.

(*NP* – inaccuracy of load estimation, *MAX* – maximum load).

of the supporting inputs for RES electricity production prediction.

accuracy of RES production prediction is as follows:

summer and also *TRV30MIN+* or *TRV30MIN-* services.

30min *TRV*Fin

30min *TRV*Fin

(*NP* – negative inaccuracy of load estimation). The final value of *TRV30MIN-* is then calculated:

Stochastic load change:

where 30min *TRVRES*

loads are in Table 4.

30min *TRV*RES

30min *TRV*Fin

*L*max (given) (MW) **1700**  *L*max (calculated) (MW) **2261**  *TRV NP MAX* no /100 (10)

*TRV NP MAX* nz /100 (11)

, (13)

+ *TRV MIN* 30 (14)

+ *TRV MIN* 30 (15)

2 2 nz- no *TRV MIN TRV TRV* 30 - ( ) ( ) (12)




Table 4. Final values of positive 30 minutes tertiary regulation reserve for different installed RES capacities and maximum loads *L*max
