Voltage Control and Reactive Power Support in the Spanish by dyr60218


									 Voltage Control and Reactive Power Support in the
          Spanish Transmission Network
                                                     ı       a o
                                             Pablo Fr´as, Tom´ s G´ mez, David Soler
                                                                        o         o
                                                Instituto de Investigaci´ n Tecnol´ gica
                                                   Universidad Pontificia Comillas
                                                             Madrid, Spain

   Abstract— This paper presents a review of the reactive power        which has 33.197 km (400 kV and 220 kV), and 51.872 MVA
support and voltage control management in the Spanish trans-           of transformers capacity.
mission network. This is regulated by two market mechanisms:              The technical operation of the Spanish power system is
(i) the network constraint management, and (ii) the voltage
control ancillary service. Constraint management is conducted          regulated by different ”Operating Procedures” [4]. The ones
by the SO, who asks generators to modify their outputs to solve        related with voltage control are the following:
power system voltage problems. On the other hand, the ancillary           • P.O.-1.1 Operation and security principles.
service consists of: (i) an annual auction where reactive regulation
                                                                          • P.O.-1.3 Admissible voltages in the transmission network.
capacity VAR resources are agreed, and (ii) the scheduling and
real-time operation of these VAR resources. Nowadays, only the            • P.O.-3.2 System constraint management.
first mechanism is fully implemented, while the ancillary service          • P.O.-7.4 Voltage control ancillary service.
is partially established. Some improvements are proposed for              • P.O.-8.3 Voltage control principles.
future developments.
                                                                       This paper is divided into 4 sections. In Section 2 the two
                                                                       market mechanisms for reactive power and voltage control are
                       I. I NTRODUCTION
                                                                       presented. Section 3 describes the SO tools and the generator
   Nowadays, reactive power management and voltage control             controls for voltage and VAR management. Finally, in section
are a key issue for power system security and economy. A               4, an analysis of the current situation is carried out and
great research effort has been carried out to define new criteria       recommendations for future developments are proposed.
and voltage control schemes for the Spanish Power System [1].
   The Spanish electricity market [2], started on January 1st               II. VOLTAGE CONTROL AND VAR MANAGEMENT
1998, and it is technically and economically managed by                  The Spanish voltage control and reactive power manage-
two separate entities, the Market Operator (OMEL) and the              ment is based on two market mechanisms.
Transmission System Operator (REE). OMEL runs the day-                   • The constraint management is a mechanism used to solve
ahead energy market, where hourly generation and demand                    congestion and low-voltage problems in the transmission
bids are matched. On the other hand, REE is responsible for                system after the day-ahead market is cleared. To solve
the secure operation of the power system and manages the                   these constraints REE ask generators to modify their
ancillary services markets.                                                outputs according with their submitted bids.
                                                                         • The voltage control ancillary service is provided by a set
                            TABLE I
                                                                           of VAR resources, whose regulation capacity is assigned
                                                                           through an annual auction. The real-time operation of
                                                                           these resources is managed by REE.
              Technology      Capacity (%)   Energy (%)
                Hydro             24.3          11.8
                                                                       The following subsections describe these two mechanisms.
               Nuclear            11.5          25.3
                                                                       A. System Constraint Management
                 Coal             16.9          30.3
               Fuel/gas           10.1          3.1                       The operating procedure P.O.-3.2 [4] regulates the system
            Combined cycle        12.1          11.5                   constraint mechanism managed by REE. Network constraints
            Special regime         25            18                    may appear when generation dispatch coming from the day-
                                                                       ahead energy market is implemented. To solve congestion and
                                                                       voltage problems generation redispatch is performed by REE.
   The Spanish Power System had, in December 2004, a total             In the Spanish power grid low-voltage problems are the most
generation capacity of 68.425 MW, to cover an annual energy            frequent and relevant constraints. They usually are located in
demand of 235 TWh. The generation mix is shown in Table                Eastern and Southern Spain.
I, where special regime includes cogeneration and renewable               In case of network constraints, generation units submit
sources [3]. REE is also the owner of the transmission grid,           bids to solve them. Bids contain pairs of quantity and price.
These bids are independent on the ones submitted to the day-               – Generators can bid quantity, but not price, of ex-
ahead energy market. REE calculates the generation redispatch                 ceeding reactive power regulation capacity, both for
that minimizes system cost and variation of initial market                    generation and absorption. Different bids can be
positions, fulfilling security criteria [4], [5]. This redispatch              submitted for each month and main transformer
can also include the start-up of some needed units. Generation                tap ratios. Each bid includes two tables, one for
redispatch increments and started-up units are paid at their bid              reactive power generation capacity, and one for re-
price. On the other hand, generation redispatch decrements are                active power absorption capacity. Table II shows
paid their bid price, only if they are not responsible for the                an example. To verify the submitted values detailed
alleviated constraint.                                                        technical data from the generation plant is required
                                                                              by REE.
B. Voltage Control Ancillary Service                                       – Qualified consumers and distribution companies can
                                                                              offer their exceeding VAR capacity, structured in bids
   The operating procedure P.O.-7.4 [4] regulates voltage con-
                                                                              for each month and demand period (peak, flat, and
trol ancillary service. Nowadays, this procedure is not fully
                                                                              valley hours).
implemented. Regarding regulation capacity that should be
provided annually by VAR sources there are two different                 REE assigns in an annual auction the additional VAR
requirements. A first mandatory regulation capacity and VAR               regulation capacity to each service provider, trying to
obligations have been set for all market participants. There is          minimize additional payments.
not any remuneration for this part of the service. Additionally,      3) Service monitoring: REE supervises the performance of
market participants may offer an additional regulation capacity    the voltage control service using measures received at the
or VAR provision, submitting quantities at a regulated price in                                        e
                                                                   CECOEL (Centro de Control El´ ctrico). Active and reactive
the annual auction. This remuneration is not yet implemented.      power flows and voltages in the transmission buses are reg-
On the other hand, VAR sources are scheduled and controlled        istered every 5 minutes for generators and 10 minutes for
by REE in a daily basis.                                           consumers. In case of measurement errors, the values coming
   1) Service suppliers: Only market participants connected        from the state-estimator software are used instead.
to the transmission network can provide this service.                 REE will daily compute, after the day-ahead market, the
                                                                   hour-reference-voltages for every controlled transmission bus.
  •   Generators. The service is provided by generation units      Currently REE only provides voltage reference ranges for
      connected to the same transmission bus, with installed       some system areas and specific buses.
      capacity equal or exceeding 30 MW.                              The service is correctly fulfilled when at least 75% of the
  •   Transmission companies. This includes REE and other          measured voltage values in each hour comply with:
      companies who own part of the 220 kV grid.
                                                                      • Generators: (i) Voltage is within the permissible range
  •   Qualified non-regulated consumers, with contracted de-
                                                                         Vref ±2.5kV, (ii) if not, the unit has reached its assigned
      mand above 15 MW.
                                                                         maximum VAR regulation capacity.
  •   Distribution companies.
                                                                      • Qualified consumers and distributors: the power factor
  2) Service procurement: The mandatory regulation capacity              values meet the mandatory requirements.
requirements and additional procurement are differentiated.           • Transmission companies: follow REE instructions regard-
  •   Mandatory VAR generation and absorption regulation                 ing VAR sources dispatch in less than 10 minutes.
      capacity ranges are defined as follows:                          4) Service payments and penalties: Additional regulation
        – Generators: the mandatory VAR output expressed           VAR capacity will be computed through four hourly terms.
          as a percentage of the maximum MW output is a               • Available additional generation regulation capacity, mea-
          function of the transmission voltage (Solid lines in           sured in Mvar. The same for absorption capacity.
          Figures 1 and 2). Both magnitudes, VAR outputs              • Reactive energy produced using the additional VAR gen-
          and voltages, are specified for the transmission High           eration regulation capacity, measured in Mvar-h. The
          Voltage busbar of the generation substation. If the            same for absorbed reactive energy.
          generation unit cannot reach the mandatory VAR re-          To compute the remuneration, each one of the previous
          quirements, it must carry out the needed investments.    terms would be multiplied by a regulated price (Eur/Mvar-
        – Transmission companies: all the installed VAR re-        year, Eur/Mvar-h).
          sources should be available to provide the service.         In case that service providers do not comply with the agreed
        – Qualified consumers and distribution companies:           provision requirements they will be penalized. Penalties are
          power factor ranges are defined in three time frames,     established through a penalty cost times the reactive energy
          peak hours (cos φ ≥ 0.95 inductive), flat hours           generated or absorbed under the default situation. Penalty costs
          (cos φ ≤ 1 inductive) and valley hours (0.95l cos φl1    are different in case of violation of mandatory requirements,
          both inductive).                                         or in case of violation of the additional VAR provision.
  •   Additional VAR generation and absorption regulation             5) Generator voltage control example: A generation plant
      capacity regulation ranges are defined as follows:            producing 100 MW should keep voltages in the HV side
                           TABLE II                                                                                                                             40
  R EACTIVE POWER GENERATION AND ABSORPTION CAPACITY TABLES                                                                                                                                   Additional       Additional
                                                                                                                                                                                              production       regulation
                                                                                                                                                                                                 penalty       capacity
                                                                                                                                                                20                                             payment
  P-V        380                        384      388      392     396        400       404      408        412     416     420                                                 ASSIGNED

                                                                                                                                        Reactive power (Mvar)
  100     38.0                          37.2     36.4     35.6    34.8       34.0      33.2    32.4        31.6    30.8    30.0                                               REGULATION
  90      39.9                          39.1     38.2     37.4    36.5       35.7      34.9    34.0        33.2    32.3    31.5                                                CAPACITY
  80      41.0                          40.2     39.3     38.4    37.6       36.7      35.9    35.0        34.1    33.3    32.4
  70      42.2                          41.3     40.4     39.5    38.6       37.7      36.9    36.0        35.1    34.2    33.3
  60      43.3                          42.4     41.5     40.6    39.7       38.8      37.8    36.9        36.0    35.1    34.2
                                                                                                                                                                                               Additional                          MANDATORY
  100     12.0                          14.0     16.0     18.0    20.0       22.0      24.0    26.0        28.0    30.0    32.0
                                                                                                                                                                                               regulation                          REGULATION
  90      12.2                          14.3     16.3     18.4    20.4       22.4      24.5    26.5        28.6    30.6    32.6                                                                 capacity                            CAPACITY
  80      12.5                          14.6     16.6     18.7    20.8       22.9      25.0    27.0        29.1    31.2    33.3                                                                 payment
  70      12.7                          14.8     17.0     19.1    21.2       23.3      25.4    27.6        29.7    31.8    33.9
  60      13.0                          15.1     17.3     19.4    21.6       23.8      25.9    28.1        30.2    32.4    34.6

First row: voltages in kV; First column: MW output; Cells: reactive power                                                                                       -40
                                                                                                                                                                      370         380          390           400            410     420                      430
regulation capacity (Mvar) for generation (rows 2-6) and absorption (rows
7-11); All values are specified for the HV side busbar of the power plant.                                                                                                                             Bus voltage (kV)

                                                                                                                                                                            Fig. 2.     Voltage service wrong fulfillment

                                 20                                                           capacity
                                                ASSIGNED          reactive energy
         Reactive power (Mvar)

                                               REGULATION              production
                                                CAPACITY                 payment                                                                                REFERENCE VOLTAGES                            REAL-TIME MEASURES

                                                                                                                                                                                                                                      TRANSMISSION NETWORK
                                                                                                           MANDATORY                                                                    VOLTAGE, ACTIVE&REACTIVE POWER
                                                                          Additional                       REGULATION
                                                                          regulation                        CAPACITY
                                 -20                                                                                                                                            PLANT        AVR
                                                                           payment                                                                                                                 V-P-Q

                                                                                                                                                                GENERATION                         V-P-Q
                                 -40                                                                                                                                                         AVR
                                       370         380           390           400            410            420          430
                                                                         Bus v oltage (kV)                                                                                      PLANT               V-P-Q

                                             Fig. 1.     Voltage service correct fulfillment

substation busbar in the range of 405±2.5kV. Table II shows
the assigned additional VAR regulation capacity. Figures 1                                                                                                                       Fig. 3.     Voltage control diagram
and 2 represent both mandatory requirements and assigned
additional VAR capacity.
   Two different operation situations are analyzed in the fol-                                                                    Voltage Regulator (AVR) control, installed in each generator,
lowing.                                                                                                                           (ii) Secondary or power plant control, reactive power man-
   • Under the first situation, the generation unit is able to
                                                                                                                                  agement in each generation power plant, and (iii) tertiary or
     set the voltage at 406 kV, injecting 20 Mvar. The unit                                                                       SO control, where REE optimizes the system operation every
     is providing correctly the service. It would obtain a re-                                                                    hour, providing the reference voltages for each controlled bus,
     muneration for additional reactive energy production, and                                                                    and control actions for transmission VAR sources, according to
     also for additional generation and absorption regulation                                                                     the management market mechanisms described in the previous
     capacity, see Figure 1.                                                                                                      section, see Figure 3.
   • Under the second situation the generation unit cannot
                                                                                                                                  A. System Operator Tools
     maintain the voltage within the reference bandwidth. In
     this case the unit is not fulfilling with the service, as the                                                                   REE manages the operation and planning of VAR sources
     reactive power generated is less than the assigned value.                                                                    in the Spanish transmission grid. VAR real-time operation
     Independent on the regulation capacity remuneration, the                                                                     consists of:
     unit would receive a double penalty: (i) a penalty for not                                                                     • Connecting/disconnecting static compensating devices,
     reaching the mandatory requirement (18 Mvar), and (ii) a                                                                          such as reactors and some capacitor banks. Most of the
     penalty for not reaching the additional assigned regulation                                                                       shunt reactors are connected to the 400k V network,
     capacity (35 Mvar), see Figure 2.                                                                                                 and the rest to the tertiary winding of 400/220 kV
                                                                                                                                       transformers, see Table III.
        III. S YSTEM O PERATOR T OOLS AND G ENERATOR                                                                                • Switching lines, mainly once a day, during nights.
                      VOLTAGE C ONTROLS                                                                                             • Adjusting 400/220 kV transmission transformer taps.
   Voltage control in the Spanish transmission network is per-                                                                         Most of them have on-load tap changers with 17 inter-
formed in three time control loops: (i) Primary or Automatic                                                                           mediate positions.
                          TABLE III
                                                                    instance, a firm owning two nearby similar generating units
                                                                    would prefer, to earn a higher remuneration, to use only one
                         AUGUST 2005
                                                                    of them to provide reactive power, instead of using both,.
                                                                    However, in this case, the optimal solution for system security
          Voltage       400 kV              220 kV                  is that both generators should provide the same amount of
         Resource       Number   Capacity   Number   Capacity       reactive power, so the available reactive margin for regulation
       Transformers      101     52.409       1        63           be larger than in the first situation.
       Shunt reactors     29      4.050       37      2.514
                                                                       In order to achieve a correct service procurement, all voltage
      Capacitor banks      -        -         13      1.083
                                                                    and reactive power measurements, both real-time and state-
Capacity measured in MVA.                                           estimation, used by REE to monitor the service should be
                                                                    shared on line with generation units and distribution compa-
                                                                    nies. Otherwise, VAR providers would be blind to effectively
  •   Defining reference voltages at generation controlled           control voltages within the required ranges.
      buses.                                                           Currently, the two implemented market mechanisms to
   In a longer term with a horizon of one year, the coordination    manage voltage control and reactive support (the constraint
of the different VAR resources is achieved through the so-          management and the voltage control ancillary services) are not
called ”voltage plan”. This plan is obtained for different volt-    coordinated. A generation unit can be paid twice for providing
age control areas in which is divided the Spanish transmission      a unique VAR regulation capacity, in the long term, through
system. The plan provides the voltage reference values at           the voltage control ancillary service, and in the short term,
the generation buses, the transformer tap positions and shunt       each time the unit is needed in the constraint management
reactors scheduling. To develop the corresponding studies,          mechanism.
optimal VAR dispatch algorithms are used.                              Finally, some changes in the present normative should be
                                                                    made to coordinate operation procedures for voltage con-
B. Generator Control                                                trol in both transmission and distribution networks. Current
   The most important reactive power resources are generators.      regulation establishes technical and remuneration differences
The VAR regulation capacity of a generation power plant             which can result in discriminatory and inefficient practices.
depends on different technical aspects: (i) generator VAR           For instance, generators based on renewable sources and
capacity curve, (ii) generator terminal voltage ranges, and (iii)   connected to distribution networks receive a compensation for
auxiliary services voltage ranges.                                  controlling their power factors within specific ranges, without
   The available reactive power capacity greatly depends on the     responsibility for voltage control. That is not the case for
taps of the main and auxiliary services transformers. So, load      generators connected to transmission networks. Furthermore,
tap changers control can increase the capability of the power       distribution operators currently are not able to manage all the
plant to provide VAR regulation capacity at the transmission        reactive power resources in their networks, such as distributed
network.                                                            generators, to maintain voltage and VAR requirements at their
   Additionally, a secondary loop control is installed in each      connection points with the transmission network.
power plant. The objective of this control is to distribute the
                                                                                                   R EFERENCES
reactive power output proportionally among the different units
in the plant, see Figure 3.                                                                    a              ı
                                                                    [1] J.L. Sancha, J.L. Fern´ ndez, F. Mart´nez, and C. Salle, Spanish practices
                                                                        in reactive power management and voltage control. IEE Colloquium on
                                                                        International Practices in Reactive Power Control, April 1993.
      IV. L ESSONS LEARNED AND RECOMMENDATIONS                      [2] Electricity Market Activity Rules, English Version. Available: http://
   The experience earned during the past years with the                 www.omel.es
                                                                               e               n                e            n
                                                                    [3] Red El´ ctrica de Espa˜ a, El sistema el´ ctrico espa˜ ol, 2004.(in Spanish).
implementation of the rules described in this paper has been            Available: http://www.ree.es
very positive for the Spanish electricity market. The operation                                        o                 e
                                                                    [4] Procedimimientos de operaci´ n del sistema el´ ctrico (in Spanish). Avail-
procedures have provided transparency and fair treatment for            able: http://www.ree.es/index_ope.html
                                                                    [5] E. Lobato, L. Rouco, T.G.S. Rom´ n, F. Echavarren, M.I. Navarrete, R.
the different market participants, and have facilitated REE to          Casanova, and G. Lopez, Hourly solution of voltage constraints in the
ensure a secure operation under the new deregulated struc-              Spanish electricity market. Power Engineering Society General Meeting,
ture. However, these first steps should be improved to cope              2003, IEEE. Volume 2, 13-17 July 2003.
with some drawbacks that have been identified in this initial
implementation period.
   Separation between a mandatory non-remunerated regula-
tion capacity and a voluntary remunerated one is not a good
practice. Studies on reactive power valuation have shown that
reactive power regulation provision has associated extra energy
losses costs and equipment investment. Therefore, the whole
provision of the service should be remunerated. This would
avoid market discrimination and economic inefficiency. For

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