Zhihui Yuan

A FACTS Device: Distributed Power-Flow Controller (DPFC)

This paper presents a new component within the flexible ac-transmission system (FACTS) family, called distributed power-flow controller (DPFC). The DPFC is derived from the unified power-flow controller (UPFC). The DPFC can be considered as a UPFC with an eliminated common dc link. The active power exchange between the shunt and series converters, which is through the common dc link in the UPFC, is now through the transmission lines at the third-harmonic frequency. The DPFC employs the distributed FACTS (D-FACTS) concept, which is to use multiple small-size single-phase converters instead of the one large-size three-phase series converter in the UPFC. The large number of series converters provides redundancy, thereby increasing the system reliability. As the D-FACTS converters are single-phase and floating with respect to the ground, there is no high-voltage isolation required between the phases. Accordingly, the cost of the DPFC system is lower than the UPFC. The DPFC has the same control capability as the UPFC, which comprises the adjustment of the line impedance, the transmission angle, and the bus voltage. The principle and analysis of the DPFC are presented in this paper and the corresponding experimental results that are carried out on a scaled prototype are also shown.

Control scheme to improve DPFC performance during series converter failures

The Distributed Power Flow Controller (DPFC) is a new device within the FACTS family. It is emerged from the UPFC and has relatively low cost and a high reliability. The DPFC consists of two types of converters that are in shunt and series connected to grids. The common dc link between the shunt and the series converters is eliminated. The active power exchange between the shunt and series converters that is through the common dc link in the UPFC, is now though the transmission line at the 3rd harmonic frequency. The redundancy of the series converters provides the high reliability of the system. In this paper, the DPFC behavior during the failure of a single series converter unit is considered. A control scheme to improve the DPFC performance during the failure is proposed. The principle of the control is based on the facts that, the failure of single series converter will lead to unsymmetrical current at the fundamental frequency. By controlling the negative and zero sequence current to zero, the failure of the series converter is compensated. In this paper, the principle of the DPFC are firstly introduced, and followed by the behavior of the DPFC during the failure of a single series converter. The design of the control scheme and corresponding simulation are presented.

DPFC control during shunt converter failure

Distributed Power Flow Controller (DPFC) is a new device within the family of FACTS. The DPFC has the same control capability as the UPFC, however at much lower cost and with a higher reliability. The reliability of the DPFC is given by the redundancy of multiple series converters. The shunt converter is the bottleneck for remaining reliability, because there is only one shunt converter in a DPFC system. During the shunt converter failure, the DPFC continues to work as controlled impedance, and only control the active power flow through the line. This paper presents a control of the DPFC, which keeps the DPFC system stable during the shunt converter failure. Adapted control schemes are employed to every series converters, which can automatically switch the series converter between the full-control mode and limited-control mode.With the adapted control, the reliability of the whole DFPC system is further improved. The adapted control scheme is verified both by simulation and experiment.

A New FACTS component — Distributed Power Flow Controller (DPFC)

This paper presents a new concept for power flow control by distributed UPFC. The system, called distributed power flow controller (DPFC), consists of several low-power series converters and one shunt large-power converter without common dc link. Also new is that the power exchange between the shunt and series parts is through the existing transmission line at a harmonic frequency. This solution enables the DPFC to fully control all power system parameters, and it reduces the cost and increases the reliability of device at the same time.

Utilizing Distributed Power Flow Controller (DPFC) for power oscillation damping

Because of the power industry moving toward marketoriented, the power tends to be transmitted over longer distances. However, the capability of long, inter-regional power transmission is usually limited, and one of the limitations is caused by low-frequency power oscillations. One of the critical oscillation, known as Inter-area oscillation, is observed when a group of generates in one region swings against group in another region [1]. The traditional solution is to use power system stabilizers (PSSs) on generator excitation control systems [2]. However, PSSs are usually designed for local oscillation damping, and in large multi-area power systems it might be difficult to tune all the PSSs parameters. FACTS devices can be employed for inter-area power oscillation damping (POD), and they are proved to be effective [3], [4], [5].

New Three-Phase Inductive FCL With Common Core and Trifilar Windings

Fault current limiters (FCLs) are expected to play an important role in the protection of future power grids. Inductive FCLs are particularly interesting due to their inherent reaction to a fault, but are not commercialized because of a too large amount of magnetic material and an induced overvoltage across dc windings. This paper introduces a new three-phase FCL with a common core and trifilar windings. With the new FCL topology, the phase windings are placed on a single core, resulting in significant reduction of the amount of required magnetic material. Furthermore, the phase windings are wound simultaneously, so that the flux coupling between the phases is increased considerably. It cancels out the magnetic field and reduces the FCL normal impedance significantly. The dc windings are used only to compensate for possible asymmetry in the system currents. Consequently, the number of dc turns and the induced overvoltage are considerably decreased. Testing of a scaled-down FCL lab prototype made it possible to verify the FE simulation results and demonstrated the principle of operation of the new topology. Experimental and simulation results matched very well.

A new concept of exchanging active power without common DC link for Interline Power Flow Controller (S-IPFC)

The separated interline power flow controller (S-IPFC) presented is a new concept for a FACTS device. The S-IPFC is an adapted version of the IPFC, which eliminates the common dc link of the IPFC and enable the separate installation of the converters. Without location constrain, more power lines can be equipped with the S-IPFC, which gives more control capability of the power flow control. Instead of the common dc link, the exchange active power between the converters is through the same ac transmission line at 3rd harmonic frequency. Every converter has its own dc capacitor to provide the dc voltage. This paper presents the basis theory of the S-IPFC, steady-state analysis, primary control loop and the corresponding simulation results.

UPFC with Eliminated Common DC Link Connection Between Shunt and Series Part

The UPFC is the most powerful power flow controller recently, and because of the exchange of active power between the shunt and series parts, they have to be located at the same place. This paper presents a new concept to transmit power without the common DC link of the UPFC, which gives the possibility of the separated UPFC. The exchange of active power is through the existing transmission line but at a different frequency, which is independent from the fundamental frequency component. The results of steady-state analysis of the separated UPFC are also presented.

DPFC design procedure - a case study using the KEPCO UPFC as an example

The Distributed Power Flow Controller (DPFC) is a new device within the FACTS family. It is emerged from the UPFC and has relatively low cost and a high reliability. The DPFC consists of two types of converters that are in shunt and series connected to grids. The common dc link between the shunt and the series converters is eliminated. The active power exchange between the shunt and series converters that is through the common dc link in the UPFC, is now though the transmission line at the 3rd harmonic frequency. The DPFC has the same control capability and the UPFC that is to adjust the line impedance, the transmission angle and the bus voltage simultaneously. Additionally, the DPFC has a lower cost due to the small rating of the series converters and a higher reliability due to the redundancy. In this paper, the design procedure of the DPFC is presented. A series of steps and corresponding equatio to determine the major parameters of the DPFCns are given and explained. A case study is presented which is using the KEPCO UPFC as an example. This paper ends with the comparisons between the DPFC and UPFC solutions considering the weight, cost and reliability.

A method to synchronize single-phase floating with grid without high voltage measurement or high bandwidth communication

Most of FACTS devices and grid connected converters need the information about the frequency and phase of the grid for synchronization and control. In some cases, the value to be measured that is used as synchronization signal is at remote bus or at a different voltage potential, which would require expensive high voltage measurements or high bandwidth communication. A new synchronization method for single-phase floating convertert is presented, which neither requires high voltage measurements nor high bandwidth communication. The method is applied to a new FACTS device — Distributed Power Flow Controller (DPFC) which is derived from the UPFC. Within the DPFC, multiple single-phase converters are distributed along the transmission line instead of one big 3-phase converter. The principle of this method is to use the line current as the rotation reference frame which enables the series converter to read the phase and frequency information locally. In this case, only the signals in dc quantity are communicated, and the system stability during communication failure is greatly improved.