Shunt Compensator as Controlled Reactive Power Sources

SHUNT COMPENSATOR AS CONTROLLED REACTIVE POWER SOURCES Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

1. INTRODUCTION Problems with maintaining adequate levels of voltage in the power system nodes have been occurring practically since the first power systems were started. Increasing requirements regarding both the supply reliability and quality of supplied power force using more modern (faster, more reliable, with a broader range of applications) devices. This trend also concerns the devices used to control voltage or compensate reactive power. In order to cover the additional demand for reactive power and maintain the ability to control voltage within the target range, various sources of reactive power, e.g. shunt compensators, are used. In recent decades there has been significant progress in terms of equipment designed to improve the stability of voltage in power systems. This is mainly due to the development of power supply systems in the world, which requires seeking better ways of adjusting and controlling power flows and voltage levels. An increasing importance in this field has been gained by FACTS (Flexible Alternating Current Transmission Systems). The main feature of these systems that distinguishes them from other solutions is the high speed of operation at high control dynamics [1, 9]. This article contains a concise description of selected control features of shunt power systems such as SVC (Static Var Compensator) – static compensators of reactive power, STATCOM-type systems (Static Compensator) – static reactive power generators and systems that combine both these solutions, which are referred to as SVC based on STATCOM. So far, such systems have not been used in the National Power System (NPS). Given the need to improve the voltage safety of NPS, as well as the increasing requirements for energy quality, more interest in these systems should be expected.

2. THE ROLE OF SHUNT COMPENSATORS IN POWER SYSTEMS The world’s first FACTS compensator system for voltage of over 100 kV was launched in 1977 in the United States. It was a SVC for controlling voltage on 138 kV busbars in the node that caused huge problems with maintaining voltage in the right value range [3]. The first STATCOM was developed in Japan in 1991 [21]. Thanks to the applied technical solution, STATCOM is considered as one of the best power devices used in power systems for controlling voltage and reactive power levels. These devices are often referred to as the “younger brother” of SVCs, because they play the same roles in the system. Power compensator systems are designed primarily for carrying out the process of voltage and/or reactive power control at the connection point. These systems may also operate according to other criteria (fig. 1).

Abstract Based on the analysis of technical solutions used around the world, we present basic design features of a shunt power system. ,The article discusses the advantages

and disadvantages in terms of using these systems as controlled sources of reactive power in power systems.

13

14

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

The criteria for control used in FACTS shun compensators Voltage control Maintaining the reference voltage in connection node. This is the fundamental criterion for operation of these types of compensators. Power factor control Criterion used mainly in industrial plants in order to ensure that power factor is maintained within a certain range of values Reactive power control Criterion possible for implementation in order to maintain the value of reactive power at a certain level, not used in practice Damping of power oscillations Criterion whose task is to eliminate the oscillations of power e.g. after short circuits. Fig. 1. Control criteria used in FACTS shunt compensators

Thanks to their speed and continuous control, the role of shunt power compensators in the system is not limited to providing an additional source/receipt of reactive power and the associated voltage control process. They may also improve the quality of voltage in a power supply system. This refers to limiting rapid changes and voltage dips caused by connection processes or irregular receipts (e.g. steel mills, traction substations, chemical plants).

3. USED COMPENSATOR SOLUTIONS 3.1. Introduction The basic division of shunt compensators is shown in fig. 2. Shunt compensators

Electromechanical

Static

Power Fig. 2. The basic division of shunt compensators

Conventional

Shunt Compensator as Controlled Reactive Power Sources

Electromechanical compensators are primarily regulated synchronous machines that receive or supply reactive power. Currently, they are rarely found in the power system. Static compensators can be divided into conventional and power compensators. The main disadvantage of conventional compensators apart from the discrete control method is the fact that they use mechanical power switches. These switches have fairly long switching times and a limited number of connections (due to wear of mechanical components and contacts), which prevents the control process from being performed in fast-changing states. This group of compensators includes capacitors that are switched using electromechanical switches (MSC – Mechanically Switched Capacitor) and reactors that are also switched using similar switches (MSR – Mechanically Switched Reactor). Currently, compensators of this type are most widely used in the NPS. Shunt power compensators are the most modern group of compensators. They enable continuous control, both in steady and fast-changing states.

3.2. SVC compensators The largest group of shunt power compensators consists of the compensators belonging to the SVC group. They are characterised by a modular structure, so it is possible to have many types of these devices; the features of each type depend on the components used. Depending on individual needs, the use of configurations for discrete and continuous control is possible. Analysis of technical solutions in SVC encountered around the world allows us to divide them into several types: • TSC (Thyristor Switched Capacitor) is a capacitor switched using thyristor. Systems of this type consist of one or more cooperating three-phase sections of TSC, where each section includes capacitors, thyristor switches that are switched on or off depending on the total reactive power supplied by the entire device. • TSR or TCR. TSR and TCR are systems with induction components only. They consist of a TSR (Thyristor Switched Reactor) or TCR (Thyristor Controlled Reactor) section; TSR includes thyristor switched reactors, whereas TCR are reactors with thyristor controlled induction. A TSR-type compensator is composed of several three-phase TSR sections whose thyristor switches are switched on or off (discrete control) depending on reactive power that is to be received by the entire device from the system. A TCR-type compensator has a similar structure, but the basic difference between these devices lies in the fact that TCR has no ability to provide smooth control of inductance. • TCR-FC. These devices consist of two types of components. The first one is a TCR module that receives reactive power, and the second is FC (Fixed Capacitor), which include also higher harmonic filters. They are an essential element when it comes to the work of TCR. FC is a source of reactive power. • TCR-TSC-FC. These compensators consist of three groups of components. The first group consists of thyristor controlled reactors. The second group consists of TSC, which is the primary source of reactive power. The third group is higher harmonic filter (treated as fixed capacities – FC), which are an additional source of reactive power. Their presence in this system is necessary due to the need to eliminate the interferences caused by TCR. However, additional filters that do not come from the same compensator and can eliminate other interferences can be used. The discussed solution is identified as a typical structure of SVC. Fig. 3 presents the structure of this type of system with voltage controller, consisting of one branch of TCR, and one branch of TSC and higher harmonic filters.

15

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

16

HV � U�T� I�k�

TR� RU�

MV TCR�

�������

UT�z�

TSC�

on off

��

��

� � �

���

�

Fig. 3. Structure of SVC compensator, type TCR-TSC-FC: USS – susceptance control system, RU – voltage controller, TR – transformer WN/SN, α – thyristor ignition angle TCR, UTz – reference voltage, UT – controlled voltage, Ik – compensator current

• TSR-TSC. Compensators of this type consist of two groups of elements. The first group consists of thyristor switched inductors, while the second group consists of TSC. The system may provide only discrete control. Presented division of SVC is based on the divisions used by manufacturers of these systems [2, 4, 12, 13, 14, 17 and 18].

3.3. STATCOM – type compensators The second group of systems includes STATCOM-type compensators. These systems are characterized by a compact structure. Having the same available value of reactive power as SVC, STATCOM occupies much less space. In addition, they have better dynamics. Despite their properties, these systems have not supplanted SVC. One of the reasons is the installation cost – despite continuous technological development, STATCOM is still more expensive compared with SVC. In terms of design, there are two basic types of STATCOM. The first consists of VSI systems Voltage Source Inverter), in which the inverter is used as voltage converter. In this system, the capacitor is the inverter load. The more popular of the two possible methods for inverter control is the method of pulse phase modulation. In this case, it is required to maintain constant voltage in the capacitor which is the inverter load on the DC side. Fig. 4 presents the structure of such a system with voltage controller. HV � UT�� I�T�

TR�

Uk�� Ik��

MV �

��

�

-

+

UD� C�

�

UWT�

Uk��,��

RU�

UT� z�

U�DC�, ID� C�

�

Fig. 4. Structure of STATCOM compensator constructed on the basis of voltage converter VSI: TR – transformer WN/SN, RU – voltage controller, UWT – thyristor control system, α – inverter control signal, UTz – reference voltage UT – controlled voltage, IT – compensator current, Uk – inverter voltage, Ik – inverter current, UDC – circuit voltage DC, IDC – circuit current DC

Shunt Compensator as Controlled Reactive Power Sources

VSI-type STATCOMs have been used in power systems as devices designed to cooperate with wind farms, irregular receipts (e.g. steel mills, traction substations), and also to control the voltage in the power system nodes [10, 11, 15, 16, 19 and 20]. The second type of STATCOM is a device based on the current CSI (Current Source Inverter). This type of system has been used in power systems yet.

3. 4. Hybrid compensators The most recent group of shunt power compensators is a hybrid combination of SVC and STATCOM. Because of their construction, it is often called a STATCOM-based SVC. This is due to the fact that the structure of this system is based on the structure of SVC; however, thyristor controlled reactors (TCR) have been replaced with STATCOM (fig. 5). WN�

�� ��

UT Ik

TR�

SN Filters

�

TSC�

STATCOM�

�

� ���� � �� �� ���� �� �

on off

�

-

��

UDC

+

�

��

UT z

�

� � � �

U DC I DC

�

Fig. 5. Structure of STATCOM-based SVC: TR – HV/MV transformer, Controller – system controller, α – STATCOM inverter control signal, UTz – reference voltage, UT – controlled voltage, Ik – compensator current, UDC – circuit voltage DC, IDC – circuit current DC

Replacing TCR in SVC with STATCOM with the same rated power makes the range of reactive power generated in the entire system larger; at the same time the ability to consume this power has not changed. Furthermore, STATCOM can perform faster control than TCR and bring less interference to the supply system. However, despite many advantages, similar to STATCOM, the main drawback of these systems is the high price, so for now, their number in power systems remains low.

4. VOLTAGE CONTROL USING SHUNT POWER SYSTEMS The quality and range of voltage control in a shunt compensator connecting node is dependent both on the control algorithm and available value of reactive power. In steady states, the control properties describe the external characteristics well. External characteristics of voltage and power in the discussed systems are presented in fig. 6. a)

b) U

U Umax

A B

cap.

ind.

cap.

Q

Umin ind.

Q

Fig. 6. Static voltage and power characteristics: a) SVC – with voltage controller, b) STATCOM – with voltage controller

17

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

18

Both systems are equipped with a voltage controller. In the control range, this allows obtaining the voltage characteristics of low gradient, corresponding to the assumed statics (usually 1-10 percent). In the linear range, both compensators behave similarly. The main difference is present in the case of too high or too low voltage. In SVC, the available power is changed in the voltage square as follows: • for capacitive part the changes in voltage correspond to parabola: Q = Bmax × U2

(1)

• for inductive part: Q = Bmin × U2

(2)

Where: Bmax – corresponds to capacitive susceptance occurring when all components of a capacitor bank are switched on and the reactors are switched off Bmin – corresponds to inductive susceptance occurring when all components of a capacitor bank are switched off and the reactors are switched on The controlled value in STATCOM is current. After reaching the limit values (points A and B), the current is kept at a constant level I = const, until voltage limiters become active (Umax , Umin ). Therefore, reactive power described by the relation: Q=I×U

(3)

will vary in direct proportion to voltage value. The properties described above are reflected in practice. Especially during operation outside the control range, mostly at voltages that differ significantly from rated conditions. Behaviours of various types of compensators are well illustrated in the curves. The following figures present example curves determined for a 400 kV node (fig. 7 and 8). General load / without compensator General load / MSC or TSC General load / SVC General load / STATCOM

,

Fig. 7. Effect of various installed types of compensators on the shape of the curves

Shunt Compensator as Controlled Reactive Power Sources

,

Switching capacitor banks of MSC or TSC

,

,

,

Switching capacitor banks of SVC

,

, ,

,

,

,

,

,

General load / without compensator General load / MSC or TSC General load / SVC General load / STATCOM

Fig. 8. Magnified fragment of fig. 7, covering the control range in compensators

Two curves were compared: for the nodes without compensator and with compensators, type: MSC, SVC and STATCOM. The curve for MSC also corresponds to the behaviour of TSC-type SVC. The curve described as SVC concerns the TSR-TSC-FC-type SVC. The FC-TCR-type SVC behaves in a very similar way (apart from interferences associated with connecting further sections of TSC). The curves were obtained for compensator systems with rated power of 200 MVA each. In terms of control, the results of operation of SVC and STATCOM are comparable. Voltage surges visible in waveforms for MSC are related to connection of further capacitor banks. It should be noted that MSC was designed to prevent a drop in voltage below 3 percent of the reference value, while SVC and STATCOM worked with a reference droop at 3 percent. The above assumption provided similar control ranges for all systems. When voltage was decreased to a value at which the process of voltage control in all systems was completed (MSC – switching on all the capacitor banks, SVC – switching off the reactor of TCR and switching on all TSC components, STATCOM – maximum current in the generation of reactive power), the impact on particular systems on the power system were changed. At voltage levels above 85 percent of the reference voltage (but below the control zone), SVC, STATCOM and MSC behaved very similarly. Nonetheless, small differences in favour of SVC and MSC in relation to STATCOM can be observed. However, STATCOM proves to be better for lower voltages. This is due to the fact that in capacitors (MSC and SVC behave similarly outside the control range), their generated reactive power depends on the voltage square, while in STATCOM its ability to generate reactive power depends linearly on the voltage (see relations 1, 2 and 3).

5. SUMMARY Power compensators belong to the group of compensators that enable fast automatic voltage control in the system. An important feature of these devices, especially in emergency situations, is their ability to maintain control also in fast-changing states. Such compensators should be considered as a very good solution for increasing voltage security in the system [6, 7 , 8]. Although these solutions are more expensive than conventional compensator systems, their properties make them worth considering for use in NPS. Control properties of power compensators can also be successfully used in distribution networks, e.g. to improve the quality of voltage in networks with high occurrence of wind farms.

19

20

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

As demonstrated by voltage emergencies in recent years, it is becoming necessary to use additional sources of reactive power in the NPS. It is likely that conventional compensators will be installed more frequently due to the costs. However, shunt power compensators may be an alternative worth considering.

BIBLIOGRAPHY 1. Acha E., Fuerte-Esquivel C.R., Ambriz-Perez H., Angeles-Comacho C., FACTS Modelling and Simulaton in Power Networks, John Wiley & Sons, LTD, 2004. 2. Faruque M.O., Dinahavi V., Santoso S., Adapa R., Review of Electromagnetic Transient Models for Non-VSC FACTS, IEEE Transactions on Power Delivery, vol. 20, no. 2, April 2005. 3. Hingorani N.G., Flexible ac transmission, IEEE SPECTRUM April 1993. 4. Kodsi S.K.M., Cañizares C.A., Kazerani M., Rective current control through SVC for load power factor correction, Electric Power System Research 76, 2006. 5. Announcement of PSE-Operator SA on the final report from the tests regarding voltage breakdown on 26 June 2006 and the measures taken to prevent emergencies in the future. 6. Kowalak R., Małkowski R., Zajczyk R., Zbroński A., Instalowanie kompensatorów w sieci przesyłowej KSE, Konferencja Naukowo-Techniczna „Problematyka mocy biernej w sieciach dystrybucyjnych i przesyłowych”, Wisła, 7–8 December 2010. 7. Kowalak R., Szczeciński P. , Zajczyk R., Wpływ układów SVC na rozwój awarii napięciowej. Energetyka, zeszyt tematyczny nr XVII, październik 2008 (jako materiały konferencyjne III Międzynarodowej Konferencji Naukowej „Blackout a krajowy system elektroenergetyczny” 2008, Rosnówko near Poznań, 8–10 October 2008). 8. Kowalak R., Zajczyk R., Wpływ kompensatorów energoelektronicznych zainstalowanych w określonych punktach KSE na awarię napięciową, Energetyka, zeszyt tematyczny nr XX, 2010 (jako materiały konferencyjne IV Międzynarodowej Konferencji Naukowej „Blackout a krajowy system elektroenergetyczny” 2010, Rosnówko near Poznań, 16-18 June 2010). 9. Machowski J., Elastyczne systemy przesyłowe – FACTS, Przegląd Elektrotechniczny 7/2002. 10. Information materials, ABB Advanced Power Electronics, ABB Switzerland Ltd., Advanced Power Electronics, 3BH-S237242 ZAB E01, acquired on: July 2010. 11. Information materials: ABB STATCOM For flexibility in power systems, ABB Power Systems AB, A02-0165E, acquired on: July 2010. 12. Information materials, AMSCTM SVC Static Var Compensator, American Superconductor Corporation, 2008. 13. Information Materials, Modelling of SVC in Power System Studies, ABB Power Systems AB, information NR 500-026E, April 1996. 14. Information materials, Power Transmission and Distribution, Discover the World of FACTS Technology, Technical Compendium, SIEMENS AG Power Transmission and Distribution High Voltage Division, No. E50001-U131-A99-X-7600. 15. Information materials, STATCOM solutions for Wind Farm, ABB Switzerland Ltd., Advanced Power Electronics, 3BHT490587R0001, 2008. 16. Information materials, STATCOM, ABB Switzerland Ltd., Advanced Power Electronics, 3BHT490522R0001, 2006. 17. Information materials, SVC Configuration Optimisation, Nokian Capacitors Ltd., EN-TH18-03/2007, 2007. 17. Information materials, SVC Configuration Optimisation, Nokian Capacitors Ltd., EN-TH18-03/2007, 2007 18. Information materials, SVC Static Var Compensator, ABB Power Technologies AB, A02–0100E, acquired on: July 2010. 19. Information materials, Using Dynamic Reactive Compensation to Mitigate Voltage Sags at a Micron Technology Semiconductor Manufacturing Facility, American Superconductor Corporation, MCRN_CS_0610, 2010. 20. Oskoui A., Mathew B., Hasler J.P., Oliveira M., Larsson T., Petersson A., John E., Holly STATCOM – FACTS to replace critical generation, operation experience, materials acquired from the company ABB: July 2010. 21. Strzelecki R., Benysek G., Układy STATCOM i ich rola w systemie elektroenergetycznym, Międzynarodowa Konferencja Naukowo-Techniczna „Nowoczesne urządzenia zasilające w energetyce”, Kozienice, March 2004.