Power system operation & control
Al-Balqa Applied University
1 Dr Audih Department of Electrical Energy Engineering Part 3 Voltage Control in Power Systems
2 Dr Audih 1-Introduction:-
To maintain voltage level of the buses in power system the reactive power (absorb or generate) is connected automatically and adjusted voltage to be constant . -Reactive power compensating equipment is discussed , then voltage and reactive power continuous control devices are described, with a distinction between rotating electrical machines and static power electronic converters (i.e., static VAR compensator (SVC), static compensator (STATCOM) and unified power flow controller (UPFC)). the on-load tap changing transformer (OLTC), a voltage discrete control device, explaining in detail its operation and applications. a) The main equipment in a power system for reactive power is the synchronous generator, which is able to deliver or absorb a reactive power. b) The automatic voltage regulator (AVR) controls the generator’s excitation in order to maintain stator voltage at set-point value. 3 Dr Audih c) Control of MV/LV bus level equipment can be categorized as: . Shunt capacitors, shunt reactors, synchronous compensators and static compensators as well as series capacitors are passive compensation devices. . Equipment providing compensation of line inductive reactance . . Equipment providing variable ratio on transformer windings (tap-changer). . Shunt reactors and capacitors :
They can be permanently connected or they can be switchable. These devices are designed as part of the basic grid, in which control resources that support basic grid voltages by recovering voltage variations.
4 Dr Audih From here, the discussion mainly concerns “switchable” and therefore controllable reactive power resources. Stepping control of these devices is usually of a manual, local or remote type. Synchronous and static compensators are continuous, closed-loop units.
The above mentioned devices can be used alone or in any combination. Some are only suitable for constant or slow- varying compensation, whereas others allow for fast variation of reactive power .
Reactive power generated by the ac power source is stored in a capacitor or a reactor during a quarter of a cycle(charge) and in the next quarter of the cycle it is sent back to the power source (discharge) as a voltage. 5 Dr Audih
Why we need Reactive Power control ? For best describe the Reactive Power aspect consider a horse and a boat analogy . Visualize a boat on a canal, pulled by a horse on the side of the canal.
The horse’s objective (real power) is to move the boat straightly. Reactive power (Vars) is required to maintain the voltage to deliver active power (watts) through transmission lines. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines. 6 Dr Audih Reactive power compensation
Review of Complex Power:
S V Icos(VIVI ) j sin( ) , P jQ, V I *, P = Real Power (W, kW, MW), Q = Reactive Power (VAr, kVAr, MVAr), = magnitude of power into electric and magnetic fields, S = Complex power (VA, kVA, MVA), Power Factor (pf) = cos , If current leads voltage then pf is leading, If current lags voltage then pf is lagging. 7 Dr Audih Power Triangle |S| Q
P P 22 1 Q pf SPQ tan P PQ22 PP S P jQ S cos( ) pf 8 Dr Audih Relationships between real, reactive, and complex power: PS cos ,
QSSsin 1 pf2 ,
Example: A load draws 100 kW with leading of 0.85 Pf What are (power factor angle), QS and ? cos1 0.85 31.8 , P 100kW S 117.6 kVA, cos 0.85 QS.sin 117.6sin(31.8 ) 62.0 kVAr. S=P-jQ (for capacitive load) then Q=-62kVA Not : That means(minus) the load supplies ( 62 kVAr) capacitive load and consume 100kW 9 Dr Audih Example (RL)
Power flowing from source to load at bus
100 30o IA 20 6.9o 43 j SVI* 100 30 20 6.9 2000 36.9 VA, 36.9 pf = 0.8 lagging, ** SVIRIIRR ( ) 4 20 6.9 20 6.9 , 2 PIRQRR1600W ( 0), ** SLL V I ( jXI ) I 3 j 20 6.9 20 6.9 , 2 QIXL 1200VAr , (PL 0). 10 Dr Audih Resistors only consume real power: 2 PIRResistor Resistor , Inductors only "consume" reactive power: 2 QIXInductor Inductor L , Capacitors only "generate" reactive power: 2 1 QIXX . Capacitor Capacitor C C C 2 V Capacitor QCapacitor . (Note-some defineX C negative.) X C
11 Dr Audih Example
I
Vs VR
40000 0 V A cross the resistor I 400 0 Amps R 100 0 Note: Since we have series impedance this make change in voltage and no effected current
VsR V I. Z 40000 0 (5 j 40) 400 0 42000 j 16000 44.9 20.8 kV * SVIs s s 44.9k 20.8 400 0 17.98 20.8 MVA 16.8 j 6.4 MVA
12 Dr Audih Example Counts With inductive reactive load added)
Now add additional reactive power load and re-solve, assuming that load voltage is maintained at 40 kV.
100 j 100 Z 70.7 Load 100 j 100 40000 I 564 45 Amps , pf 0.7 lagging 70.7
Vjs 40000 (564 45 (5 40)) 59.7 13.6 kV * SVIs s. s 59.7 13.6 0.564 45 33.7 58.6 MVA 17.6j 28.8 MVA * V R SRRVj . 16 16MVA Z L Need higher source voltage to maintain load voltage magnitude when reactive power load is added to circuit. Current is higher. (next slide) 13 Dr Audih Example continue same result as before with power simulator
Previous circuit redrawn.
17.6 MW 16.0 MW 28.8 MVR -16.0 MVR
59.7 kV 40.0 kV Keep voltage 17.6 MW const. 16.0 MW 28.8 MVR 16.0 MVR Arrows are Generators are Transmission lines are shown as used to shown as circles a single line show loads
Key idea of reactive compensation is to supply reactive power locally. In the previous example this can be done by adding a 16 MVAr capacitor at the load.
14 Dr Audih Example continue with add capacitor compesation
16.8 MW 16.0 MW 6.4 MVR 0.0 MVR
44.94 kV 40.0 kV
16.8 MW 16.0 MW 6.4 MVR 16.0 MVR 16.0 MVR The result of compensation are:- .Compensated circuit is identical to first example with just real power load. .Supply voltage magnitude and line current is lower with compensation. .Reactive compensation decreased the line flow from 564 Amps to 400 Amps. This has advantages: –Lines losses, which are equal to I2 R, decrease, –Lower current allows use of smaller wires, or alternatively, supply more load over the same wires, –Voltage drop on the line is less. 15 Dr Audih Power Factor Correction Example Assume we have 100 kVA load with pf=0.8 lagging, and would like to correct the pf to 0.95 lagging Sj80 60 kVA cos1 0.8 36.9 1 PF of 0.95 requires desired cos 0.95 18.2
Snew80 j (60 Q cap ) 60 -Q cap tan18.2 60 Q 26.3 kVAr 80 cap
Qc Qcap 33.7 kVAr and C= 2 .V c
16 Dr Audih Series Compensation A simplified model of a transmission system with series compensation in order to improving voltage drop is shown in Figure. Series compensation is frequently found on long transmission lines used to improve voltage regulation. Due to the long transmission lines, voltage begins gradual decrease .Series compensation devices placed strategically on the line increase the voltage profile of the line to levels near 1.0 p.u.
17 Dr Audih Benefits of Series Compensation
Advantages of series compensation include: • Increases power transfer of existing lines • Voltage regulation • Reactive power balance • Raises transient stability • Suitable for many applications • increased transmission capabilities . • By improving voltage level it is possible to transmit over long distances without large voltage drops. Disadvantages of series compensation include: • High initial cost • Heavy dependence on protection • Past reinsertion problems following a disturbance • Sub-synchronous resonance (changed the electric circuits natural frequency to coincide with the mechanical system’s natural frequency, when these two frequencies matched up, oscillations at the resonance frequency of the system began).
Dr Audih 18 Series Compensation 1- Increases Power Transfer Capability
Since transmission lines are mostly inductive, adding series capacitance decreases its total reactance
Reducing XL increases PR
Compensation Level K is defined as the percent of XLoffset by the series capacitor
Example: For XL = 1 ohm, 30% compensation produces XL - XC = .7 ohm
19 Dr Audih 2- Improves Reactive Power Balance and Self-regulation
Transmission Line Reactive Reactive Power Balance For A 300 Mile 500kV Line Power Losses :
2 Qlosses=I Xline
Series Capacitor Reactive Power Output:
2 Qoutput=I Xcapacitor
As a transfer across the line increases, Qoutput partially offset Qlosses
20 Dr Audih 3- Improves Voltage Stability
Increasing compensation levels K provides greater
Qoutput capability
Maximum power transfer capability of the line is increased
Generator reactive power is made available for voltage control
Effect of Increasing Compensation Levels 21 Dr Audih Series Compensation example: In the circuit is become in below figure. For correction to unity power factor must be choose the capacitance impedance is
equal to the inductance impedance so XC =j8 [Ω].
The equivalent impedance for circuit is given by the equation:
The current in circuit is:
The apparent power S (series) is equal:
22 Dr Audih The drop voltage on resistance load Rload is
The series capacitance is:
Let's use a rounded capacitor value of 400 µF.
Note:-Series compensation devices are self regulating,
23 Dr Audih Shunt Compensation
The device that is connected in parallel with the transmission line is called the shunt compensator in order to improve voltage regulation. A shunt compensator is always connected in the middle of the transmission line. It can be provided by either a current source, voltage source or a capacitor. A simplified model of a transmission system with shunt compensation is shown in Figure.
24 Dr Audih Capacitors are connected either directly to a bus bar or to the tertiary winding (a third transformer winding) of a main transformer and are disposed along the route to minimize losses and voltage drops. The main advantages of shunt capacitors are low cost and flexibility of installation and operation.
Fig 2.1. Current-voltage characteristic of a capacitor
Disadvantage of shunt capacitor’s principal is: . Reactive power output reduction at low voltages is proportional to the voltage squared. . Switching reduces capacitor lifetime.
25 Dr Audih The output characteristic ( I–V ) is linear, defined by rated values of voltage and current, as shown
Therefore,
In the case of transmission systems, shunt capacitors are used to compensate for inductive ( ωLI2) losses and to ensure satisfactory voltage levels during heavy load conditions. Compensation schemes include both fixed and switchable capacitor banks. Capacitor banks are switched either manually or automatically by voltage relays. switching of capacitor banks provides a conventional means of controlling system voltages to recover large voltage deviation, typically due to the load difference from night to day or after a large contingency Shunt capacitors are sensitive to over-voltages and over-currents, which are limited by appropriate protections
26 Dr Audih EXAMPLE Calculate the complex power for the circuit of Figure , and correct the power factor to unity by connecting a parallel shunt reactance (capacitor) to the load .
Solution
VAr
Note: Use rms values for all phasor quantities in the problem.
27 Dr Audih To eliminate the reactive power due to the inductance, we will need to add an equal and opposite reactive power component−QL, To compute the reactance needed for the power factor correction, we observe that we need to contribute a negative reactive power equal to −118.5 VAR. This requires a negative reactance and therefore a capacitor with QC =−118.5 VAR. The reactance of such a capacitor is given by
If we consider 60 Hz then
Comments: You can see that it is possible to eliminate the reactive part of the impedance, thus significantly increasing the percentage of real power transferred from the source to the load.
28 Dr Audih 2.3 Shunt Reactors . Shunt reactors are used to compensate line capacitance effects by limiting voltage rise (when a circuit is open or when a load is light or lightning happened. . They are often used for EHV overhead lines longer than 150–200 km, where capacitive line-charging current flowing through high-value inductive reactance causes a voltage rise . .The output characteristic ( V–I) is linear in the operating range , and deviates from linearity due to saturation of iron-core or shrouded iron reactors as shown in Fig. 2.2 during linear performance,
Fig. 2.2 Voltage-current characteristic of a shunt reactor Reactive Power Compensation Devices
29 Dr Audih . Shunt reactors can be connected directly to electric line or through a transformer installed in the terminal station (Fig. 2.3).
Fig. 2.3 Connection configurations of shunt reactors: switchable and permanent reactors . Shunt reactors are permanently connected to the long electrical lines to limit temporary (lasting less than 1 s) or switching over-voltages up to 1.5 p.u. . Additional shunt reactors can be also used on electrical lines to limit over- voltages due to lightening. . During heavy load conditions, shunt reactors must be disconnected; for this reason they are equipped with switching devices.
30 Dr Audih Static Reactive Reserves Extending the range of dynamic reactive resource response
Capacitors bank 31 Dr Audih Shunt reactors 2.4 Multiple Compensation Device Operating Point Fig. 2.4 Representing the linear characteristics of capacitor and reactor on the same V–I plane, where it is assumed the current enters at the positive sign,
Fig. 2.4 V–I characteristics of shunt reactor and shunt capacitor in parallel 32 Dr Audih The figure demonstrates two facts: a) The reactor absorbs current while the capacitor delivers current. According to the link between voltage and current discussed in 2..1 and 2.3
b) When the reactance values of the two passive components vC and vL are equal in absolute value, their algebraic sum is zero; then the operating point is fixed by the external voltage, with no impact of the shunts on the grid voltage ( I = 0); that is, the voltage axis also represents the resultant characteristic of the two shunts. In this case the full recirculation of reactive power between the two compensating devices is active, in the amount
33 Dr Audih This obvious result confirms that the two types of permanently connected compensating equipment does not make sense.
In fact, any switching of reactor or capacitor impacts the equivalent values of VE (equivalent voltage due to L and C) and X, thus changing the shape of the system load characteristic, as Fig. 2.6 shows.
34 Dr Audih Fig. 2.6 V–I shunt characteristics of capacitive and inductive dominant effects and trajectories following commutation from operating point A to B, taking into account consequent change in equivalent system load characteristic
The different shunt resultant characteristics can be seen in Fig. 2.6: .Case A, where the capacitive effect is dominant, and .Case B, where the inductive effect prevails ( frequently).
Starting from A and switching off a capacitor shunt, a new resultant shunt
characteristic B is determined, with current I changing from delivery IC to absorption IL . This produces not only a change in V but also in the equivalent VE and X values being the grid voltages less sustained by a change in the reactive power from injection into the grid to absorption from the grid. The result would be
cause lower VE value and/or a different slope of the system load characteristic, thereby determining a different operating point at lower voltage V (from V1 to V2 in the figure). 35 Dr Audih Static VAR Compensators (Capacitors and shunt reactors)
Dr Audih 36 3 Voltage and Reactive Power Dynamics Control Devices:
a) Synchronous Condensers . A synchronous condenser is dynamic reactive device and very similar to synchronous generator with the exception that it is not capable of producing any active power. It produces only reactive power.
. Synchronous condensers do not need a prime mover (steam or water turbine) ,so they operated like a motor. . The power system supplies the active power to turn the rotor and excitation to stator. . An excitation system is used to control the amount of MVAR produced by the synchronous condenser.
37 Dr Audih Synchronous Condensers
b) Synchronous Generators . Synchronous generators are primary voltage control devices and they are primary sources of reactive power reserve. Through excitation control they allow continuous fast control of their stator voltages and of reactive power delivered to or absorbed by the grid. . The synchronous generator is not simply a megawatt generator but also a VAR generator. . It allows the functional separation between the active and reactive power controls and the delivery or absorption of VARs up to limits without impact on the active power produced. . When Synchronous Generators does not able to deliver MW of power, then its a pure VAR generator. . A closed-loop control scheme with an automatic voltage regulator (AVR), shown in Fig. 2.7.
38 Dr Audih 39 Dr Audih Automatic Voltage Regulator(AVR)
The automatic voltage regulator (AVR) senses the voltage level at the generator terminals via a potential transformer (PT). If the measured voltage is lower than the set point, the AVR will cause the excitation system to increase the DC excitation current. This DC current is applied to the generator's rotor field winding. If the voltage measured is higher than the set point, the excitation system will lower the DC excitation current applied to the field winding.
Dr Audih 40 Excitation control systems
Excitation control systems (ECS) of synchronous generators can be classified as either “rotating” or “static”. The first category comprises rotating machines such as DC power amplifiers that feed the synchronous generator field. Rotating types include:
• ECS with exciting dynamo and electromechanical voltage regulator; • ECS with exciting dynamo and electronic\microprocessor-based voltage regulator; • ECS with alternator and rotating diodes, with electromechanical voltage regulator.
The second category considers as a DC power amplifier that feeds the synchronous generator field, a power electronic converter, typically thyristor-based. Static types include: • ECS with static exciter and electronic/microprocessor-based voltage regulator.
41 Dr Audih Exciter with automatic voltage regulator (AVR) 42 Dr Audih Transmission Lines voltage variation
Surge Impedance Level (SIL) Ferranti Rise Line Switching
Dr Audih 43 Transformer devices for voltage control
44 Dr Audih 1- Setup and step-down Transformers
Is a method to Increase or decrease voltage level Figure below illustrates the use of transformers in electric power transmission lines. The practice of transforming the voltage before and after transmission of electric power over long distances is very common.
45 Dr Audih 2- Tap Changing transformer
In this method, a number of tapping's are provided on the secondary (as effect ) of the transformer (tap changer is connected to primary). The tap selection may be made on automatic or manual tap changer mechanism. The number of tapping have been provided a variation voltage on the secondary. When the position of the tap is varied, the number of secondary turns is varied the voltage varied. The voltage control of the range + 15 to -15 % can be achieved by tap changing transformers. Tap-changing are either on-load or off load tap changers. . 46 Dr Audih
HV
LV Max and Min. relay
when the movable arm makes contact with lower positions such as 1, the secondary voltage is minimum this during the period of light inductive load. When the movable arm contact with higher position such as 8 ,the
secondary voltage is maximum this during the period of high inductive load, 47 Dr Audih Advantage of tap changing transformer
During high system load conditions, network voltages
are kept at highest practical level to
• minimize reactive power requirements.
• increase effectiveness of shunt capacitors to
compensated reactive power .
During light load conditions, it is usually required
to lower network voltages
• avoid under excited operation of generators
48 Dr Audih 3-Autotransformer
49 Dr Audih • Autotransformer has a single winding with two end terminals, one or more
terminals at intermediate tap points,
•The primary voltage is applied across terminals,
•The secondary voltage taken from two terminals,
•always having one terminal is common with the
primary and secondary
•The current flows directly from the input to the output. autotransformer .Autotransformers are frequently used in power transmission and distribution
50 Dr Audih in step down transformer the source is usually connected across
the full winding while the load is connected by a tap across the
desired voltage .
In a step up the source is connected to a tap across desired
voltage ,while load is attached across the full winding .
51 Dr Audih 4- Induction regulator
52 Dr Audih Induction Regulators
There are two types of induction regulators single phase and 3 phase.
The construction it is similar to a induction motor except that the rotor is
not allowed to rotate continuously but can be adjusted in any position
either manually or by a small motor.
The adjustable output voltage by varying the inductive coupling between
a rotor and a stator winding
induction Regulators are used for voltage control of distribution primary
feeders.
53 Dr Audih single induction Regulators
Single phase induction regulator. The primary winding terminals of the stator is connected across the supply line. The secondary winding is for rotor is connected in series with the line whose voltage is to be controlled.
54 Dr Audih Three phase induction regulator The primary windings either in star or delta are wound of the stator and are connected across the supply. The secondary windings are wound of the rotor and the six terminals are connected in series with the line whose voltage is to be controlled. Three phase induction regulators are used to regulate the voltage of feeders and connection with high voltage
55 Dr Audih Figure 6 : Three phase induction regulator Y_ Connected 56 Dr Audih A static synchronous compensator
(STATCOM)
57 Dr Audih STATCOM
A static synchronous compensator (STATCOM) . Is a regulating device used on alternating current electricity transmission networks. . It’s based on a power electronics voltage-source converter . can act as a source (supply) or sink (absorb) of reactive AC power to an electricity network. . If connected to a source of power it can also provide active AC power. . It is a member of the FACTS (flexible alternating current transmission system ) family of devices.
58 Dr Audih Usually contains a synchronous voltage source (see figure) that is driven from a dc storage capacitor and the synchronous voltage source is connected to the ac system bus through an interface transformer. The transformer steps the ac system voltage down such that the voltage rating of the SVC (static VAR compensator ) switches are within specified limit.
Basically, the STATCOM system is comprised of : 1- Power converters, 2- Set of coupling reactors or a step-up transformer, and 3- Controller
59 Dr Audih Principle of voltage control and operation of STATCOM
1- Increase of the grid voltage by injecting reactive power to the grid over-excited behavior of the FACTS capacitive (inductive grid)
2- Reduction of the grid voltage by absorbing reactive power from the grid under-excited behavior of the FACTS inductive (capacitive grid)
60 Dr Audih 61 Dr Audih 62 Dr Audih Some Important Conclusions
. The frequency is a common parameter throughout the system (next section)
. The voltage is controlled locally (actual section)
. The control mechanism for P (rotor angle) and Q (voltage amplitude) operate more or less separately
63