Stability Enhancement of Power System by Controlling ... - IEEE Xplore

2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia

Stability Enhancement of Power System by Controlling HVDC Power Flow through the Same AC Transmission Line K. P. Basu, Senior Member, IEEE Faculty of Engineering Multimedia University Cyberjaya, Malaysia [email protected]

along with ac current. The added dc power flow does not create any transient stability problem. In EHV ac line the capacitive VAR of the line is very useful to compensate at least a part of the lagging VAR of the load. In HVDC line the capacitive VAR of the line cannot be tapped to compensate a part of the lagging VAR of the converters. The novel idea of simultaneous ac-dc power transmission through a single circuit ac transmission line is first proposed in reference [8]. For dc it is mono-polar power transmission with ground return. The idea is extended to bi-polar dc power transmission through a double circuit ac transmission line [9]. The presence of ac component of voltage produces capacitive VAR, which may be used for compensating a part of lagging VAR. The PSCAD/EMTDC simulation of single circuit ac-dc transmission is carried out in reference [10] and for the double circuit line in reference [11-12]. Power upgradation with steady state stability improvement is also presented in [9]. Improvement of transient stability limit in an interconnected power system with fast acting converter control of HVDC system is well documented [1, 13-15] and is also validated for large disturbances [1, 15]. Most severe transient stability criterion is tested by creating 3-phase fault near a heavily loaded generator in ac transmission [13, 16] and near the inverter in case of HVDC transmission [17]. Utilization of built-in short term overload capacity of converters in HVDC may improve the transient stability limit in an ac line operating in parallel with the dc line [18]. Large signal HVDC stabilizing torques may be developed to damp out oscillations in parallel ac line. In this paper a single-machine infinite bus system connected through a single circuit ac transmission line, being converted to simultaneous ac-dc transmission has been examined. The dc power flow being augmented with the stoppage of ac power flow temporarily just after a fault may improve the transient stability limit and may optimally damp the power swing. The basic idea is presented in reference [23]. This paper presents a detailed analysis of the proposed idea and the case of limited augmentation facility of dc power regulator is also considered.

Abstract— HVDC power transmission employing power electronic device provides a wide range of control in power transmission. Remote generators are sometimes used to supply power to infinite bus through a single circuit long ac transmission line. The system may loose stability after the clearance of a fault if the pre-fault power transfer or the fault clearance time is high. Simultaneous ac-dc power transfer through the transmission line may be used for power flow enhancement. It needs conversion of the line for simultaneous ac-dc power flow to keep the system stable with high values of pre fault power and fault clearance time. During the transient period after the clearance of the fault, ac power supply is kept off and the dc power flow is increased to produce a retarding torque to bring back the generator to its normal speed. HVDC current regulator having very fast control facility may be employed for this purpose. AC circuit breakers are then switched on and the ac power flow is resumed at its pre fault value without any oscillation. The power swing is low and the system is optimally damped at the end of first swing. If the augmentation facility of HVDC power controller is limited, proper governor control of remote generators may be used to reduce the total power flow and to maintain the dc power flow only. After the system becomes stable the ac power supply is switched-on and the power oscillation is minimized with HVDC power controller. Keywords — Transient stability; ac-dc power flow; dc power controller; fault clearing time; zigzag transformer

I.

INTRODUCTION

HVDC transmission lines in parallel with EHV ac lines may be used to improve transient stability as well as to damp out oscillations in power system [1-4]. Long EHV ac lines cannot be loaded to its thermal limit to keep sufficient margin against transient instability. Economic consideration and reliable operation necessitate EHV transmission lines to be loaded close to their thermal limits by using Flexible AC Transmission System (FACTS) components [5-7]. EHV ac line may be loaded to a very high value if the conductors are allowed to carry superimposed dc current

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TR2

TR4 Tr line

CB2

CB5

TR1 G

CB1 TR3

XD

XD

Inverter

Inf-bus

CB3 Rectifier Gen-bus

CB6 CB4

Load

Fig.1 Single machine connected to infinite bus through ac-dc transmission line

II.

BASIC SIMULTANEOUS AC-DC POWER FLOW

superimposed sinusoid ally varying ac voltage having rms value Va and the peak value being; Vmax = √2Vph = Vd + √2Va ------ (3) Electric field produced by any conductor voltage possesses a dc component superimposed with sinusoidally varying ac component. But the instantaneous electric field polarity changes its sign twice in a cycle if (Vd /Va) < √2. Therefore, higher creepage distance requirement for insulator discs used for HVDC lines are not required. Each conductor is to be insulated for Vmax but the lineto-line voltage of each line has no dc component and VLLmax = √6Va. Therefore, conductor to conductor separation distance of each line is determined only by rated ac voltage of the line. Allowing the asymmetrical voltage wave of each conductor with respect to ground to be zero at least once in each cycle; Vd = Vph/√2 and Va = Vph/2 ---------- (4)

The circuit diagram shown in Fig.1 shows the basic scheme for simultaneous ac-dc power flow through a single circuit ac transmission line. The dc power is obtained through the rectifier bridge and injected to the neutral point of the zigzag connected secondary of sending end transformer TR2, and is reconverted to ac again by the inverter bridge at the receiving end. The inverter bridge is again connected to the neutral of zigzag winding of the receiving end transformer. The transmission line caries both 3-phase ac and dc power. It is to be noted that a part of the total ac power at the sending end is converted into dc by the transformer TR3 connected to Rectifier Bridge. The same dc power is reconverted to ac at the receiving end by the receiving end transformer connected to the inverter bridge. Each conductor of each line carries one third of the total dc current along with ac current Ia. The return path of the dc current is through the ground. Zigzag connected winding is used at both ends to avoid saturation of transformer due to dc current flow and is used as an interfacing device of dc and ac supply. A high value of reactor XD is used to reduce harmonics in dc current. In the absence of zero sequence and third harmonics or its multiple harmonic voltages, under normal operating conditions, the ac current flow through the transmission line is restricted between the zigzag connected windings and the three conductors of the transmission line. Even the presence of these components of voltages may only be able to produce negligible current through the ground due to high value of XD. The total rms current per phase becomes: I = [Ia2 + (Id/3)2]1/2 ----- (1) Allowing the line to load to its thermal limit: Id = 3[Ith2 – Ia2]1/2 -------- (2) The instantaneous value of each conductor voltage with respect to ground becomes the dc voltage Vd with a

Assuming equal voltages at the two ends of the line the total power transfer through the line before conversion is; P/total = 3Vph2sin θ1/X1 = P/maxsinδ/0 -- (5) To keep sufficient stability margin, θ1 is generally kept low for long transmission lines and seldom exceeds 30°. With the increasing length of line, the loadability [15] of the line is decreased. The total power transfer through the converted line is; Ptotal=Pac + Pdc=3Va2sin θ2/X2 + VdId=Pmaxsinδ0 + VdId -(6) The power angle θ2 between the ac voltages at the two ends of the converted line may be increased to a high value because of the fast control facility of Pdc. The per unit value of the converted line reactance X2 is 4 times to that of the unconverted line as the ac voltage reduces to half of the original value. For a constant value of total power, Pdc may be modulated by fast control of the current controller of dc power converters. Its effect is similar to that of the FACTS controller and δ0 may be as

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high as 80° [19]. Protection: Preliminary qualitative analysis suggests that commonly used techniques in HVDC/AC system may be adopted for the purpose of the design of protective scheme, filter and instrumentation network to be used with the converted line for simultaneous ac-dc power flow. In case of a fault in the transmission system gate signals to all the SCRs are blocked and that to the bypass SCRs are released to protect rectifier and inverter bridges. Forcing the rectifier into inversion may also be used for protection of Converter Bridge. CBs are then tripped at both ends to isolate the complete ac-dc system. A surge diverter connected between the zigzag neutral and ground protects the converter bridge against any over voltage. Saturation of transformer core if any, due to asymmetric fault current, reduces line side current but increases primary current of transformers TR2 and TR4. In fact, any LG or LLG fault in the line produces unequal dc current through the zigzag windings of TR2 and TR4 and saturates the transformer cores. So these faults are seen as 3-phase fault by CB2 and CB5, which are designed to clear transformer terminal and winding faults, and isolate these faults easily. It is to be noted that the dc power flow may be maintained or resumed through the line even without any ac power flow when CB2 and CB5 are open but CB3 and CB6 are closed. Proper values of ac and dc filters as used in HVDC system may be connected to the busbar and zigzag neutral to filter out higher harmonics from dc and ac supplies. Dc current and voltages may be measured at zigzag winding neutral terminals by adopting common methods used in HVDC system. Ac component of transmission line voltage is measured with conventional cvts used in EHV ac lines. Superimposed dc voltage in the transmission line does not affect the working of cvts. Linear couplers with high air-gap core may be employed for measurement of ac component of line current. Dc component of line current is not able to saturate high airgap cores. Electronic signal processing circuits may be used to generate composite line voltage and current waveforms from the signals obtained for dc and ac components of voltage and current. Those signals are used for protection and control purposes. III.

PROBLEM FORMULATION

A conventional power system is considered with a single circuit long ac transmission line supplying power to infinite bus (Fig.1 without dc power flow). Any 3phase fault at the load terminal near the Gen-bus is cleared by CB4. But during the period of fault duration, power transfer through the line drops to zero as Gen-bus voltage collapses. The equal area criteria for stability study may be adopted to assess the transient stability limit of the system [20]. Fig.2 shows that the system becomes unstable for high value of pre-fault power Pm (= P/total), which is equal to the mechanical power input. If the fault clearing time t1 is not very fast then the corresponding

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angle δ1 becomes larger than the critical clearing angle δc and the system becomes unstable. Even if the system is stable, a large power swing occurs without the application of stabilizers. Similar phenomenon is observed for any temporary 3-phase or 1-phase fault in the transmission line which is cleared by opening CB2 and CB5 and subsequently reclosed after the time td. The system becomes unstable due to high value of Pm and td. The line is then converted for ac-dc simultaneous power flow (Fig.1). By applying rapid control to the current controller of the converter system, the dc power flow may be modulated to keep the system stable after the fault clearance. IV. CONTROL STRATEGY Under normal operating condition the power transfer through the line is Ptotal = Pm = Pac + Pdc. (refer Fig.3). When the fault occurs at the load terminal near the Genbus, the bus voltage reduces to a very low value. Both Pac and Pdc become zero. The fault is cleared after t1 second by tripping CB4. For independent control of dc power flow CB2 and CB5 are also commanded to trip at a time t ≤ t1, so Pac remains to be zero even when the fault is cleared. But dc current is allowed to flow through the line. At the instant of tripping of CB4 rapid control of the converter system increases the dc power flow to Pdc/ = Pm + ΔPdc. 1.4

For stability A1