5 Abstract This article discusses the issue of reactive power and higher harmonic compensation by means of clas- sic systems based on capacitor batteries and modern power electronics dynamic systems of the STATCOM and EFA type. The author, through the process of evaluating advantages and disadvantages, is strongly in favour of the modern dynamic solutions and underlines their benefits and usability in the attempts aimed at improving the quality and efficiency of electric energy usage. Modern Reactive Power and Higher Harmonic Compensation Through the Utilisation of STATCOM and EFA Dynamic Compensators MODERN REACTIVE POWER AND HIGHER HARMONIC COMPENSATION THROUGH THE UTILISATION OF STATCOM AND EFA DYNAMIC COMPENSATORS Bogdan Bałkowski / C&T Elmech Modern power systems are often compelled to deal with low energy quality and low energy efficiency pro- blems, in which case higher harmonic and reactive power compensation constitutes one of the most important issues. Even though, technically speaking, this problem is not new, its scale has been growing, which makes it more significant, and the results of low energy quality and low energy efficiency have become more severe. The above is- sues do not only refer to our domestic market. This issue is perceived as a global phenomenon and numerous coun- tries, including the EU Member States, have treated it as very important. However, in Poland, the energy efficiency of the economy is no less than two times lower than the European average. In this perspective, the Act promoting and supporting savings in the final energy utilisation has become indisputably vital 1 . This Act, to come into force at the beginning of 2011, is the fulfilment of regulations included in the 2006/32/EC European Parliament and Council directive and its main goal is to achieve 9% energy savings by 2016. This means that the problems related to energy quality are accompanied by issues related to energy utilisation efficiency. In such a dynamic economic environment (taking into account the energy prices increase), C&T Elmech suggests taking a comprehensive look at the issue of higher harmonic and reactive power compensation and appropriate new solutions. In this context it is worth comparing technical capabilities of conventional filter and compensating equip- ment (originating from the first half of the 20th century) and new solutions (originating from the turn of the 20th and 21st centuries), taking into account the advances in the area of power electronics which have been repe- atedly tested in the industry. This paper will discuss only low- and medium-voltage solutions, i.e. ones targeted at industrial recipients. The classic solutions in this range include, in particular, capacitor batteries for reactive power compensation and passive filters for selective higher harmonic compensation. Capacitor batteries may be divided into groups taking into account the method of their connection to power systems: • by means of mechanical connectors, i.e. contactors; • by means of static connectors, i.e. thyristors. The first capacitor battery with mechanical connectors was used in 1914 and the first static solutions did not appear before 1971. In both the above cases, the connectors are used for ON/OFF capacitor batteries con- nection (called compensation level). Taking into account the above, such compensation has a stepped characte- ristic and its precision depends on the value of the compensation level. This means that most usability features of such solutions depend on the type of used connector and compensation level value. The systems featuring mechanical connectors may be used in power systems with slowly variable loads and the activation of individual levels is usually accompanied by significant current surges and momentary unbalances. Such phenomena are also most often registered in switchgear protection systems. Due to the above, and taking into account the possibility of achieving higher dynamics of the capacitor batteries switching, it is much more beneficial to utilise systems featuring thyristor connectors and modern processor controllers. These systems causing no disadvantages using mechanical connectors are also most often used for follow-up compensation of reactive power. However, they are also limited by several factors resulting, among others, from relatively low semiconductor breakdown voltage. 1 See http://bip.mg.gov.pl/node/11629 for the bill and the justification.
Transcript
5
Abstract
This article discusses the issue of reactive power and higher harmonic compensation by means of clas-sic systems based on capacitor batteries and modern power electronics dynamic systems of the STATCOM and EFA type. The author, through the process of evaluating
advantages and disadvantages, is strongly in favour of the modern dynamic solutions and underlines their benefits and usability in the attempts aimed at improving the quality and efficiency of electric energy usage.
Modern Reactive Power and Higher Harmonic Compensation Through the Utilisation of STATCOM and EFA Dynamic Compensators
MODERN REACTIVE POWER AND HIGHER HARMONIC COMPENSATION THROUGH THE UTILISATION OF STATCOM AND EFA DYNAMIC COMPENSATORS
Bogdan Bałkowski / C&T Elmech
Modern power systems are often compelled to deal with low energy quality and low energy efficiency pro-blems, in which case higher harmonic and reactive power compensation constitutes one of the most important issues. Even though, technically speaking, this problem is not new, its scale has been growing, which makes it more significant, and the results of low energy quality and low energy efficiency have become more severe. The above is-sues do not only refer to our domestic market. This issue is perceived as a global phenomenon and numerous coun-tries, including the EU Member States, have treated it as very important. However, in Poland, the energy efficiency of the economy is no less than two times lower than the European average. In this perspective, the Act promoting and supporting savings in the final energy utilisation has become indisputably vital1. This Act, to come into force at the beginning of 2011, is the fulfilment of regulations included in the 2006/32/EC European Parliament and Council directive and its main goal is to achieve 9% energy savings by 2016. This means that the problems related to energy quality are accompanied by issues related to energy utilisation efficiency. In such a dynamic economic environment (taking into account the energy prices increase), C&T Elmech suggests taking a comprehensive look at the issue of higher harmonic and reactive power compensation and appropriate new solutions.
In this context it is worth comparing technical capabilities of conventional filter and compensating equip-ment (originating from the first half of the 20th century) and new solutions (originating from the turn of the 20th and 21st centuries), taking into account the advances in the area of power electronics which have been repe-atedly tested in the industry. This paper will discuss only low- and medium-voltage solutions, i.e. ones targeted at industrial recipients. The classic solutions in this range include, in particular, capacitor batteries for reactive power compensation and passive filters for selective higher harmonic compensation.
Capacitor batteries may be divided into groups taking into account the method of their connection to power systems:
• by means of mechanical connectors, i.e. contactors;• by means of static connectors, i.e. thyristors.The first capacitor battery with mechanical connectors was used in 1914 and the first static solutions did
not appear before 1971. In both the above cases, the connectors are used for ON/OFF capacitor batteries con-nection (called compensation level). Taking into account the above, such compensation has a stepped characte-ristic and its precision depends on the value of the compensation level. This means that most usability features of such solutions depend on the type of used connector and compensation level value.
The systems featuring mechanical connectors may be used in power systems with slowly variable loads and the activation of individual levels is usually accompanied by significant current surges and momentary unbalances. Such phenomena are also most often registered in switchgear protection systems. Due to the above, and taking into account the possibility of achieving higher dynamics of the capacitor batteries switching, it is much more beneficial to utilise systems featuring thyristor connectors and modern processor controllers. These systems causing no disadvantages using mechanical connectors are also most often used for follow-up compensation of reactive power. However, they are also limited by several factors resulting, among others, from relatively low semiconductor breakdown voltage.
1 See http://bip.mg.gov.pl/node/11629 for the bill and the justification.
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Taking the above into account, they are connected directly to the power network with a voltage usually not exceeding 690 V or to power networks with higher voltages by means of a matching transformer. Solutions connected directly to power systems with a voltage equal to 17 kV are also available, however, their prices con-stitute significant economic barriers. Capacitor batteries are sensitive to harmonic voltage present in a power network so they are secured with chokes protecting them against undesirable frequencies. This feature may also be used for aligning with a selected harmonic frequency in order to compensate it.
Passive LC filters are uncontrolled systems activated by an operator. Their design is similar to that of a capacitor battery with mechanical connectors. Moreover, they are usually single-stage filters permanently inclu-ded into the power system. Additionally, the selection of their L and C elements results from the required power and the frequency of compensated harmonics. LC filters also have the reactive power compensation capability because in the case of the basic harmonic (50 Hz), the network “sees” the filter as capacity. When simultaneous filtration of several harmonics is required, several LC passive filters are installed and each of them is tuned to a different required resonance frequency (filtration).
The above description shows that capacitor batteries and passive filters are based on the same L and C elements. This results in the fact that they share numerous features, both advantageous and disadvantageous.
Having analysed the characteristics shown in fig. 2 and properties of passive compensating and filtering systems based on LC elements, the following conclusions may be drawn:
• The compensating system power depends on the square value of the power network voltage fluctu-ations2. Thus, for example, a change of voltage by 10% causes a 21% change in the reactive power in a non-regulated compensating battery. This may also cause load overcompensation and, as a result,
Fig. 2. Voltage-current characteristic of capacitor batteries and passive filters
2 The value of power generated by the capacitor battery is expressed by the following equation Q = U2 wC.
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further voltage increase in the point where the compensation system is connected to the power ne-twork. When a non-regulated compensation battery or compensator which is not dynamic enough is used, this mechanism may cause instability of the power network including uncontrolled resonance.
• The increase in the power network voltage causes an increase in the compensation capacitor battery current, which in extreme cases, especially in the presence of the network voltage distortions, may result in their overload.
• The passive filter characteristic presented in fig. 1 shows that for frequencies higher than the resonance (damped) frequency an LC passive filter retains the damping properties, however, in the case of lower frequencies this filter is “seen” by the network as capacity, which may result in increased network in-stability in this frequency range.
• Higher harmonic passive filter systems are usually designed for selected harmonic frequencies charac-teristic for a given reception. When analysing fig. 3, one can see that the harmonic spectrum may be variable while maintaining an almost constant THD value. This causes additional problems for correct selection of compensation system passive elements. This is also one of the reasons for understating the estimates for the required power and the number of compensated harmonics and one of the re-asons why the filtration is ineffective and passive systems are overheated. For similar reasons, using simple passive filters, when it is possible that significant inter-harmonic components may be present, is not recommended either.
• Permanent inclusion of passive filters with load variability causes overcompensation (not appreciated in the power engineering industry), which, as mentioned above, creates conditions facilitating reso-nance.
Moreover, due to technical and safety reasons, in classic compensation systems it is rarely possible to achieve a power factor higher than tgø = 0.3. This is approximately 10% of the energy we pay for; however, it is not practically used because it is reactive energy. Nowadays, applicable regulations do not impose such a requirement. It is, however, worth analysing the new energy efficiency regulations which provide favourable conditions for taking actions aimed at a further increase in the tgø coefficient value.
Fig. 3. The figure shows an example variability of the WH voltage spectrum dependent on the control angle of a 6-pulse thyristor converter
Modern Reactive Power and Higher Harmonic Compensation Through the Utilisation of STATCOM and EFA Dynamic Compensators
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Does criticism expressed towards the classic compensation systems mean their elimination? Of course not. In the author’s opinion, classic compensating-filtering systems and active systems should
be merged in hybrid solutions utilising the most favourable features of both systems, i.e. by eliminating their disadvantages and focusing on advantages. Hybrid solutions, ensuring much better energy quality and its more efficient usage, are also characterised by an optimum relation between the price of a solution and results achie-ved.
Fig. 4 shows a concept of combining, in one compensation system, the advantages resulting from low pri-ces of LC and SVC compensators, compensating their disadvantages and providing them with positive features of STATCOM (STAtic COMpensator) dynamic compensators and EFA (power active filter), particularly the Xinus Q and Xinus D systems respectively which are manufactured by C&T Elmech.
The discussed concept uses an LC compensator permanently connected to the power network, which, in no load conditions, is compensated by STATCOM facilitating inductive and capacitive reactive power generation and load symmetrisation. The follow-up SVC capacitor battery is a quantified source of capacitive reactive power which ensures rough regulation of reactive power; however, the STATCOM system ensures smooth regulation and high dynamics of the compensations system within the range of one SVC compensator regulation stage.
Fig. 4. Compensation system concept using classic compensation systems with dynamic systems (type Xinus Q and Xinus D)
Cooperation of the three above compensation system elements allows for configuration of a fully regula-ted reactive power source with optimum costs, high dynamics, high speed of operation, smooth operation and resistance to transient conditions present in the power network. It also ensures precise filtration of the current reception harmonics.
Fig. 5a shows an output characteristic of a typical STATCOM system. It is evident that the generated compensating current is totally resistant to voltage fluctuations and the generated reactive power is fully con-trolled. This observation will facilitate the analysis of the simplified characteristic of a hybrid system (fig. 5b). As is evident, the suggested configuration has features characteristic both for classic and active systems. A hybrid compensator still is capable of providing automatic correction of the reactive power generation and facilitates generation of this power with a maximum value two times higher than the STATCOM system alone.
RECE
PTIO
N
LC c
ompe
nsat
or
C ba
tter
y
C ba
tter
y
STAT
COM
STAT
COM
Q power step regulator
Q power follow-up regulator
Q power precise regulator
MASTER power regulator
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Fig. 5. a) Comparison of IU characteristics of single STATCOM and b) in configuration with classic SVC type systems and LC compensator
Another element of the compensation system is a Xinus D power active filter of a relatively low power value. It facilitates precise and dynamic filtration (of the compensation) of current reception higher harmonics, regardless of its spectral composition and parameters of the power supply network. The Xinus D active filter may also support the STATCOM system operation, i.e. additionally compensate the reactive power and symmetrise the load within its power reserve rage.
The main uses of the suggested hybrid solution are:• energy quality improvement including the guarantee of a high power factor for the power system;• effective voltage stabilisation including the flicker phenomenon reduction; • load symmetrisation and active compensation of higher harmonics;• improvement of distribution lines power efficiency; • improvement of the system transmission capacity including improvement of power system stability
(oscillation damping).
Tab. 1. Comparison of basic features of classic and hybrid systems
Feature Classic system Hybrid system
Time of reaction to reactive power fluctuations 20 ÷ 200 ms 0.25 ÷ 0.5 ms
Resistance to voltage fluctuations in the network none full
Reaction to interferences possibility of resonance full resistance
Capability of generating capacitive and inductive power capacitive capacitive and inductive
Harmonic filtration limited, selective full with changing spectrum
Load symmetrisation none full
Flicker reduction poor effective
This paper is not only a purely theoretical discussion regarding existing possibilities. Even though a sys-tem with such a configuration has not been operated in Poland yet, it is worth analysing the reactive power and higher harmonics compensation system being constructed by C&T Elmech on the main winding machine in the KWK Ziemowit mine. C&T Elmech is the only Polish company which has ensured compensation of the extremely turbulent reception in the drive system of a winding machine. The experience gained will be used for construc-ting a compensation system in another machine. This time, C&T Elmech engineers intend to ensure cooperation of a classic SVC type follow-up compensation system with advantageous features of a Xinus D active power filter. Utilising high dynamics of an active filter and reactive power compensation capability in order to additio-
Vt Vmax
Vt Vmax
BC
ICmax 0 ILmax ICmax ID-STATCOMmax 0 ID-STATCOMmax
Lmax at Vmax
a) b)
Modern Reactive Power and Higher Harmonic Compensation Through the Utilisation of STATCOM and EFA Dynamic Compensators
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nally compensate the system is an example of a partial realisation of the concept under discussion. Fig. 6 shows a schematic diagram of the implemented project3 and fig. 7 shows a visualization of the newly designed active filter (power = 2 MVAd).
3 Elektroinfo: nr 12/2007 – Xinus active filters; no 9/2010 (87) – Reactive power and current higher harmonic compensation in medium-voltage networks using hybrid systems based on new XInus active filter solutions.
Fig. 6. Drive system for the northern and southern winding machines with the compensation system. Total reception power Smax = 19 MVA, S.r = 11 MVA, follow-up capacitor battery of power equal to Qbat = 6 MVAr (1 MVAr degree), Xinus D active power filter of power equal to D = 2 MVAd
Fig. 7. Visualisation of an active filter (power = 2 MVAd) with a 6 kV/l,l kV matching transformer
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It will be the first project of this type implemented in Poland and we are looking forward to its results. Fig.8 shows half of an operational reactive power and higher harmonic compensation system manufactured by C&T Elmech used on the S 1.2 winding machine in the LW Bogdanka coal mine.
Fig. 8. View of an operational Ultra system of an active Xinus D-lMVAd filter and capacitor battery (power = 1 MVar)
Modern Reactive Power and Higher Harmonic Compensation Through the Utilisation of STATCOM and EFA Dynamic Compensators
