Electric Power Systems Research Power quality improvement with an ...

Electric Power Systems Research 79 (2009) 1553–1560

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Power quality improvement with an extended custom power park M. Emin Meral ∗ , Ahmet Teke, K. Cagatay Bayindir, Mehmet Tumay Cukurova University, Department of Electrical and Electronics Engineering, Balcali, 01330, Adana, Turkey

a r t i c l e

i n f o

Article history: Received 10 September 2008 Received in revised form 3 June 2009 Accepted 4 June 2009 Available online 4 July 2009 Keywords: Custom power Custom power park Power quality control centre STS APF DVR

a b s t r a c t This paper describes the operation principles of an extended custom power park (CPP). The proposed park is more effective when it is compared to the conventional power parks regarding the yield of improving both current and voltage quality of linear and nonlinear loads using dynamic voltage restorer (DVR), active power filter (APF), static transfer switch (STS) and diesel generator (DG). Moreover, a supervisory power quality control centre is presented to coordinate these custom power (CP) devices by providing pre-specified quality of power. A fast sag/swell detection unit is also presented to improve the system response. The ability of the extended CPP for power quality improvements is further analyzed using PSCAD/EMTDC through a set of simulation tests. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The control of most of the industrial loads is mainly based on semiconductor devices and microprocessors, which cause such loads to be more sensitive against power system disturbances such as voltage sag, voltage swell, current harmonics, interruption and phase shift. Thus, the prevention of negative effects of the PQ disturbances has gained more interest for the last twenty years [1,2]. CP is a power electronic based solution against PQ disturbances or electromagnetic disturbances. CP devices, namely DVR, APF and STS, are applied in the distribution system of an electric utility with the purpose of protecting an entire plant, feeder, a block of customers or loads [3]. CP devices include an acceptable combination of the following features; no (or rare) power interruptions, magnitude and the duration of voltage reductions within specified limits, magnitude and the duration of over voltages within specified limits and low harmonic currents [4]. The STS is used to transfer the load from the preferred source to an alterative healthy source. The DVR is capable of generating or absorbing independently the controllable real and reactive power at its ac output voltage in series with the distribution feeder in synchronism with the voltages of the distribution system. The APF is one of the CP devices and it is generally shunt connected to the system via a reactance. It can mitigate the harmonic currents generated by nonlinear loads by controlling the compensation current

∗ Corresponding author. Tel.: +90 322 3386868. E-mail addresses: [email protected] (M.E. Meral), [email protected] (A. Teke), [email protected] (K.C. Bayindir), [email protected] (M. Tumay). 0378-7796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2009.06.001

[3,5]. The concept of CPP has been introduced in order to meet customer’s needs. CPP concept means the integration of multiple CP devices within the Industrial/Commercial Park that offers the customers a high quality power at the distribution system voltage level [6]. In the literature, there are various studies about a high quality power park concept apart from CPP (unlike CPP). One of the most important studies is the power quality park (PQP) [7]. The classification of customers is the distinguishing feature of PQP and CPP. PQP does not classify their customers while CPP classifies the customers, so that each customer can be offered different tariff rates for required power quality needs. In this paper, an extended CPP is proposed and various PSCAD/EMTDC simulation studies are performed to validate the performance of the park. The designed park and case studies differ from the conventional power park studies in [8–10] from the following ways: • Power Quality Control Centre (PQCC) provides a coordination of extended CPP including CP devices and loads, thus resulting in a reliable distribution system and a required qualified power. • The extra functionality is added by integrating APF to the park and thus an extended CPP is performed. • A fast fault detection method is presented both for STS and DVR. • The coordination and interaction between the CP devices are presented comprehensively. The paper is organized as follows: after this introductory section, general operations of the CP devices in the CPP are described in Section 2. The innovative contributions of the study, the proposed CPP and power quality control centre are presented in Section 3. The

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M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560

Fig. 1. The single line diagram of the extended CPP.

case studies and discussions showing power quality improvements are presented in Section 4. The main contributions and significant results of the study are summarized in Section 5. 2. The extended custom power park concept The extended CPP offers a high quality power (grades of A, AA and AAA) to customers and meets the needs of sensitive loads with an Industrial/Commercial business park. Fig. 1 shows the single line diagram of the proposed CPP including STS, DVR, APF, DG, the circuit breakers and loads. STS protects sensitive loads against voltage sags, swells and interruptions. STS ensures a continuous high quality power supply to sensitive loads by transferring, within a time scale of 4–8 ms, the load from a faulted bus to a healthy one [11]. STS with a make-before-break transfer strategy [12] is used to satisfy the uninterrupted transfer of the power to the critical loads in this study. The detection and transfer logic must function properly for all the possible operating conditions. In this study, the control method used for voltage compensation in [13] is developed for voltage sag/swell detection. By using this approach the detection time can be further improved with the respect to conventional methods using a low pass filter [14,15]. APF mitigates current harmonic disturbances and compensate the reactive power of nonlinear loads. The shunt connected voltage source inverter topology is used in the power circuit. The compensation signal is calculated using the concept of Instantaneous Reactive Power Theory (IRPT) [16], which is based on both load voltage and load current samples.

DVR is connected in series to the distribution circuit by means of a set of single-phase injection transformers and has capable of generating or absorbing the real and reactive power at its ac terminals. To maximize the dynamic performance, a direct feed-forward-type control [2] is applied to the control unit of DVR. With this control, a fast response time (approximately 1 ms) can be achieved to compensate the voltage disturbances. The voltage reference is obtained from the pre-fault line voltage and the compensation signal is calculated using the PQR theory [17]. The coordination of CP devices in the CPP is clearly described in the following sections. A detailed circuit diagram of the CPP system and the circuit parameters are given in Appendix A. 2.1. The profiles of CPP loads and grades of powers The loads in the park are divided into three categories. Loads L-A1, L-AA and L-AAA are balanced and harmonic-free, while Load L-A2 is a harmonic polluting load. L-AA and L-AAA are the sensitive loads and they require almost an uninterrupted electrical power. LAAA is the most critical load and cannot tolerate any disturbances. CPP has two incoming feeders designed for an improved grounding and insulation. Thus, all loads benefit from a high quality power supply. L-A1 (and also L-A2), L-AA and L-AAA receive the powers QP-A, QP-AA and QP-AAA, respectively, as shown in Fig. 2. The grades of the powers are explained below. 2.1.1. Qualified Power-A (QP-A) QP-A is a harmonic free and sag/swell free power. This is the least qualified power at the park. This grade power requires the use


Fig. 2. The grades of the powers at the CPP.

of STS and APF. STS reduces the duration of the voltage sag/swell or the interruption to 4–10 ms by rapidly transferring the loads to a healthy feeder. APF reduces the harmonic distortion at the CPP load bus created by nonlinear loads. 2.1.2. Qualified Power-AA (QP-AA) QP-AA is harmonic free and sag/swell free caused by the distribution faults and long interruption free. The grade of QP-AA is over from the grade of QP-A and it receives the benefit of a DG which can come up to about 5–10 s in the case of two feeder loss caused by the transmission line faults. 2.1.3. Qualified Power-AAA (QP-AAA) QP-AAA is a harmonic free, sag/swell free and long interruption free power. Grade QP-AAA is over grade QP-AA and it receives the benefit of DVR. Consequently, the loads of the CPP receive the superior quality power compared to the regular power of ordinary loads. In addition, a more sensitive load gets more power quality in the CPP as shown in Fig. 2. 3. Power quality control centre When different types of devices are used to solve multiple disturbances simultaneously, a coordination of these devices is needed. For the flexibility of the system, some control functions may be centralized [7]. On–Off states of the proposed CPP equipments are shown in Table 1 and these devices are controlled by the Power Quality Control Centre. The distribution system voltage is assumed faultless if the voltage is within ±10% of the nominal value. CP

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devices are operated when the system voltage exceeds these limits as given in Table 1. DVR is designed to compensate maximum 50% sag as in similar studies [4,11,15,17]. The voltage sags higher than 50% are considered as an interruption, as given in Table 1. The voltage waveforms of the both feeders and the harmonic current-source load are monitored by the PQCC and power quality events are captured and managed for a periodic assessment of the service being provided. The DG shown in Fig. 1 normally stays off and is not connected to the CPP load bus. When both of the feeders are lost (more than 51% sag or interruption), the generator is started-up immediately and connected to the CPP load bus. It should take 5–10 s (condition 6 in Table 1) for the generator to come on line and pick up the loads of both L-AA and L-AAA [4]. L-AA and L-AAA experience power loss only for 5–10 s during this event. However, L-A1 and L-A2 do not receive power until one of the feeders is back in service (condition 7th in Table 1). When the 6th or 8th condition occurs, DVR protects L-AAA against voltage disturbances. This is the distinguishing feature of L-AAA from L-AA. During this condition, L-A1, L-A2 and L-AA are subject to these disturbances. During 4th and 5th conditions, CPP voltage remains at desired values by transferring the entire loads to an alternate feeder. However, for the conditions 1st, 2nd and 3rd, there is no need to transfer the loads because the CPP load bus voltage remains within desired values (90%–110% of nominal voltage) [18]. The APF can filter the current harmonics produced by the harmonic polluting load. It is “On” during the load L-A2 is “On” state. As explained above, for achieving all the conditions appropriately, the coordination of STS (STS a and STS p), DVR, DG, APF and the circuit breakers are needed. The flowchart of the proposed coordination scheme according to above conditions is shown in Fig. 3. A common fault detection method is used for the coordination of all the devices. The most important part of the PQCC is the sag/swell (fault) detection unit. In the proposed fault detection method shown in Fig. 4, the line-to-line supply voltages named as Vab , Vbc , Vca are firstly transformed into Stationary Reference Frame (STRF) using Clarke transformation [19] and then transformed into Synchronous Rotating Frame (SRF) using Park transformation [20]. dq voltages in the SRF and their relationship with ˛ˇ voltages in STRF are shown in Fig. 5. (3) is obtained in positive SRF by using Clarke and Park transformations

R() =

C=

1 0

cos() − sin()

sin() cos()

−1/2 √ 3/2

−1/2 √ − 3/2

(1)

(2)

Table 1 On–Off states of CPP devices and loads. On–Off states of park equipments Conditions

STS p

STS a

DVR

GEN

L-A1 and L-A2

L-AA

L-AAA

APF

Distribution line faults 1. Less than 10% sag/swell at preferred and alternate feeder (nominal operation) 2. Less than 10% sag/swell at preferred feeder between 10% and 50% sag/swell at alternate feeder 3. Less than 10% sag/swell at preferred feeder more than 50% sag/swell at alternate feeder 4. Between 10% and 50% sag/swell preferred feeder less than 10% sag/swell at alternate feeder 5. More than 50% sag/swell at preferred feeder less than 10% sag/swell at alternate feeder

On On On Off Off

Off Off Off On On

Off Off Off Off Off

Off Off Off Off Off

On On On On On

On On On On On

On On On On On

On On On On On

Transmission line faults 6. More than 50% sag or interruption at preferred and alternate feeder during start-up delay 7. More than 50% sag or interruption at preferred and alternate feeder after start-up delay 8. Between 10% and 50% sag at pref. feeder preferred and alternate feeder

On On On

Off Off Off

Off Off On

Off On Off

Off Off On

Off On On

Off On On

Off Off On

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Fig. 3. Flowchart for the coordination scheme of CPP.

Fig. 4. The block diagram of proposed fault detection method.


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Fig. 7. The filtered Vp and the Vqp signals for fault detection.

Fig. 5. Voltages in STRF and SRF.

Vd(p)

Vq(p)

⎛

Vab

Conventionally, (5) is used for fault detection [12]

⎞

VP =

2 ⎜ ⎟ = ∗ R(−) ∗ C ∗ ⎝ Vbc ⎠ 3 Vca

(3)

where = wt. R() rotates at the phase angle wt. The subscript (p) represents that this is the value in the positive sequence SRF. The subscripts d and q represent d-axis and q-axis values in SRF, respectively. For a positive sequence SRF, the positive sequence component rotates in the counter clockwise and the negative sequence component rotates in the clockwise direction in the STRF, so, the positive sequence becomes a DC component and the negative sequence has a 100 Hz (for 50 Hz network frequency) component as expressed in (4)

Vd(p) Vq(p)

=

Vdp Vqp

+ R(−2wt)

Vdn

(4)

The subscripts p and n show that related parameter is the value of original positive or negative sequence components, respectively. For balanced faults, there is no need to extract the original positive and negative sequence SRF. Using only the positive sequence q component is sufficient for fault detection since q component has a DC value for balanced faults. An unbalanced voltage sag/swell causes negative sequence components to appear in the feeder voltage. Fig. 6 shows q component for a 40% three phase balanced fault, and for a 40% single phase unbalanced fault.

2 2 Vd(p) + Vq(p)

(5)

But, the term Vp has 100 Hz ripples. For effective fault detection, the original positive sequence components that have only DC value should be separated. A low pass filter (50 Hz) is used [12] to separate the DC component and ripples in conventional method. Nevertheless, the “original positive sequence component” cannot be obtained. Furthermore, the filter also causes in a certain amount of delay in an error signal. In order to overcome this problem, a differential controller is used in the proposed fault detection method. Eq. (6) is obtained by differentiating (4).

˙ Vd(p) V˙ q(p)

Vqn

= −2wR

2

R(−2wt)

Vdn

(6)

Vqn

Since the value of positive sequence is constant, the derivation of it becomes zero. (6) is rotated by 90◦ and divided by −2w, as follows: 1 − R 2w

V˙

d(p)

2

V˙ q(p)

= R()R(−2wt)

Vdn Vqn

(7)

Since the sum of a vector and the value of that vector shifted by 180◦ is zero, the sum of (4) and (7) leave an only positive sequence component. Thus all the negative sequence components are removed and the result is a DC component

Vd(p) Vq(p)

−

1 R 2w

V˙

d(p)

2

V˙ q(p)

=

Vdp Vqp

(8)

Finally, for obtaining original q component; following equation is used: Vqp = Vq(p) +

Fig. 6. q and d components for a balanced fault and an unbalanced fault.

1 V˙ 2w d(p)

(9)

Fig. 7 shows the filtered (with 50 Hz filter) Vp signal which is calculated by (5), and Vqp signal which is calculated by (9) in case of a single phase unbalanced fault occurred at 160 ms. As shown from Fig. 7, there is a certain delay because of the filtering on Vp . However, the Vqp is an ideal signal to obtain error signal. The obtained original positive sequence Vqp signal is compared with a DC reference and passed through a noise filter with a high cut-off frequency (greater than 1 kHz). Thus, the response time of the sag/swell detection is decreased compared to the conventional method [12]. An absolute value block is used for the swell detection (because, the value of Vqp is greater than 1 in the case of swell) and the hysteresis relay is used to generate the transfer signal.

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Fig. 8. CPP load bus currents in case of APF is offline and online.

4. Power quality improvements The extended CPP is tested under the different types of disturbances such as current harmonics, voltage sags/swells/interruption occurred on a preferred feeder and voltage sags/swells occurred on both of feeders. The circuit scheme of proposed CPP and the simulation parameters are given in more details in Appendix A. The following case studies are presented to test the power quality improvements with the proposed extended CPP. • Simulation results for APF which improves the quality of bus currents with mitigating the current harmonics drawn by nonlinear load. • Simulation results for STS which improves the quality of bus voltages with transferring bus to a healthy feeder. • Simulation results for DVR which improves the quality of most critical load voltages with compensating voltage sag. 4.1. Harmonic mitigation with shunt APF

Fig. 10. Load bus currents during transition from preferred feeder to alternate feeder.

The value of current THD should be smaller than the limits stated in IEEE Standard 519-1992 [21]. A considerable reduction of THD is obtained at the CPP load bus currents and L-A2 line currents as follows: • THD of L-A2 currents (%): 25.50 in case of APF-offline; 4.02 in case of APF-online. • THD of CPP load bus currents (%): 9.30 in case of APF-offline; 1.52 in case of APF-online. THD values are kept below the current distortion limits stated in [21] using APF. 4.2. Load bus transferring with STS As stated in Table 1, conditions 4th, 5th or 6th should be satisfied to transfer the loads to the alternate feeder.

L-A2 draws harmonic currents that give rise to a distortion of the bus currents and the bus voltages due to line resistance. This may affect other loads that are connected to the same bus. In order to overcome this problem and in order to provide a good power quality, active power filter is connected to the PCC. Fig. 8 shows the effect of harmonic current components drawn by the L-A2 to the bus current. Fig. 8 also shows the bus current when the APF is on line.

Fig. 9. Voltage waveforms of preferred feeder and CPP load bus.

Fig. 11. Waveforms of CPP load bus voltages, L-AA voltages and L-AAA voltages.


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Table 2 Parameters of the simulated CPP. Symbol in Fig. 12

Description

Value/profile

S p and S a Sag/swell generator Z pref, Z alt VT CT BRK Z a1 TR GEN Z aa VSI apf L apf Lr Nonlinear load R a2 TR inj C filter and L filter VSI dvr DC s Z aaa

Preferred and alternate AC sources Disturbance generator Preferred and alternate feeder impedances Voltages measurement Currents measurement As circuit breakers, normally open or normally close Load L-A1 impedance/per phase For start-up delay Diesel generator Load L-AA impedance/per phase Voltage source inverter of APF Smoothing inductor Choke inductor Harmonic current source load Resistor as DC load Injection transformer Filter capacitor and inductor Voltage source inverter of DVR DC source of DVR Load L-AAA impedance/per phase

L–L 380 V – Negligible – – – 145 – L–L 380 V 145 Six pulse bridge inverter 25 mH 8 mH Thyristor bridge rectifier 90 Single phase, 1:1, 1 kVA 18 ␮F and 5 mH 1-phase H-bridge inverter 150 V 48

The voltage waveform of the preferred feeder is shown in Fig. 9. According to EN 50160 standards [18], the admissible maximum voltage variation should be within 10% of the nominal value. When the voltage on the bus is greater than 90% of nominal, there is no need to perform the source transfer. The feeder transfer is occurred at 360 ms because the preferred feeder voltage drops to 65% of the nominal voltage. STS instantaneously transfers the loads from the preferred feeder to the alternate feeder in a few ms when voltage sag occurs on the preferred feeder. Fig. 10 shows the transition of

load bus currents from the preferred feeder to an alternate feeder. When a fault is detected, the load is transferred to the alternate feeders and the preferred feeder currents are interrupted. CPP load bus voltage is almost kept constant. 4.3. Voltage compensation with DVR A transmission line fault causes a 25% voltage reduction on both alternate and the preferred feeder voltages at 240 ms. The

Fig. 12. The circuit scheme of simulated CPP.

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DVR starts to operate according to the condition 8th, as stated in Table 1. The CPP load bus and L-AAA voltage waveforms for this condition are shown in Fig. 11. A voltage sag to 80% of the nominal value ends at 350 ms. During this fault, the (all) loads except L-AAA are subject to a voltage sag and L-AAA voltage is almost kept constant. According to IEEE Standard 519-1992 [21], the voltage THD of a low voltage system should be smaller than 5%. DVR keeps the sensitive load voltage magnitude between 0.9–1.0 per unit (pu), and THD lower than 3.7%. 5. Conclusions An extended CPP for the improvement of power quality is presented in this paper. A high quality power and an improved power service are achieved with the extended CPP to satisfy the needs of customers in a power park. The loads of the CPP receive a superior quality power compared to the regular power of ordinary loads. In addition, more sensitive load gets more power quality and improved power service. The main contributions of this study are the coordination of CP devices with a supervisory PQCC and providing the rapid control of the switching devices with a fast sag/swell detection method. An extra functionality to CPP is also provided by introducing a Shunt APF to the park. It ensures the elimination of current harmonics drawn by nonlinear loads at load bus. Also, the DVR keeps the voltage of more sensitive load constant, the STS transfers all off the loads from a preferred feeder to a alternate feeder, and the DG protects the critical loads against the faults occurred in the transmission line. Consequently, the simulation results point out that the extended CPP with the new added functionalities has the ability to improve both voltage and current quality. The extended CPP provides an overall solution to most common power quality disturbances encountered in power systems. Acknowledgements The authors would like to acknowledge Electrical, Electronics and Informatics Research Group of the TUBITAK (Project No: EEEAG106E188) for full financial support. Appendix A. A.1. The scheme and parameters of simulated CPP The ratings of devices and loads are as follows: low voltage STS, DVR: 1.5 kVA, shunt APF: 1 kVA, generator: 9.5 kVA, L-A1: 1 kVA, L-AA: 1 kVA, L-A2: 3 kVA and L-AAA: 3 kVA. The sample time of simulation is 25 ␮s. PSCAD/EMTDC program is used to test the validity of proposed extended CPP. Table 2 gives the parameters of the CPP shown in Fig. 12.

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