FIELD OF THE INVENTION

[001] The present invention relates generally to the field of power management systems and more particularly to a method for load shedding during bus transfer and an intelligent electronic device thereof.

BACKGROUND OF THE INVENTION

[002] In industrial electrical power systems, unexpected power interruptions on continuous production lines are undesirable. In order to address this situation, conventionally power systems have employed two sources to supply power to a load. Under normal conditions, the load is connected to a main source. On failure of the main source, the load is transferred to a standby source. Attempts are typically made to reconnect the load to the standby source as quickly as possible, in order to provide uninterrupted supply to the load. For this purpose, fast bus transfer methods are used in power systems.

[003] There are various methods used for fast and efficient bus transfer, for example, Fast transfer, In-phase transfer, Residual transfer and Slow transfer. In some cases, due to unfavorable conditions a Fast or In-phase bus transfer may not be possible to perform. In such cases, the transfer can be performed either after the load voltage has fallen below a stable limit (Residual transfer) or after an adjustable time delay (Slow transfer). In such conditions, where the voltage of the load has fallen below a certain level, high inrush current is experienced resulting in unacceptable voltage and current transients on the standby source. [004] In order to avoid such transient phenomena, load shedding is implemented during a bus transfer. The purpose of load shedding is to improve the chance of successful transfer by eliminating unnecessary equipment and to minimize the stress placed upon equipment during the transfer. Shedding non-essential load reduces the magnitude and/or duration of the transient voltage decay. Present day bus transfer devices perform load shedding based on various load-shed modes during transfer. The various load shed modes are: Instantaneous, Voltage Base (load shedding done when voltage reaches a pre-defined threshold during the transfer) & Time base (load shedding done when pre-defined time has elapsed since bus transfer trigger command).

[005] However, bus transfer usually requires coordinated and integrated load shedding to be carried out. In the current methods of bus transfer with load shedding schemes, when the command is issued to close contacts for connecting the standby source to the load, the voltage at the load will have collapsed much below an acceptable level. Under this condition, once the contacts are closed completely, the standby source will experience a heavy inrush and tripping may take place for protection purposes. There are cases where the load shedding command is issued late and the voltage / frequency profile of load collapses significantly. After close command issued for connecting the standby source to the load, the frequency and voltage profiles continue to decay until the closing of the contacts is complete, resulting in high inrush followed by tripping for protection.

[006] Hence, there is a need for an improved method for load shedding during bus transfer by overcoming the drawbacks of the current methods mentioned above. SUMMARY

[007] The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification.

[008] In an embodiment, the present invention provides a method for load shedding in a multiphase electrical system controlled by at least one Intelligent Electronic Device during a bus transfer from a first feeder to a second feeder for powering a load bus connecting a plurality of loads with one of the first feeder and the second feeder, the at least one Intelligent Electronic Device: measuring a first voltage magnitude in a first voltage cycle from the load bus with a voltage measurement device connected at the load bus; measuring a second voltage magnitude in a second voltage cycle from the load bus with the voltage measurement device connected at the load bus; determining rate of decay of voltage based on measured first voltage magnitude and second voltage magnitude; predicting a decay in voltage of the load bus based on determined rate of decay of voltage; and operating at least one switching device for load shedding of at least one load from the plurality of loads connected in the load bus based on the predicted decay in voltage of the load bus during the bus transfer from the first feeder to the second feeder.

[009] In an embodiment, determining rate of decay of voltage comprises computing a first order gradient of voltage based on first voltage magnitude and second voltage magnitude. [0010] In an embodiment, the method described herein above further comprises determining rate of decay by computing a second order gradient of voltage based on the first order gradient of voltage.

[0011] In an embodiment, the bus transfer from the first feeder to the second feeder comprises computing a synchronization time based on an instantaneous angle of separation between a voltage of at least one of the first feeder or the second feeder and the voltage of the load bus using instantaneous voltage magnitude of the voltage of at least one of the first feeder or the second feeder and instantaneous voltage magnitude of the voltage of the load bus.

[0012] In an embodiment, the bus transfer from the first feeder to the second feeder comprises actuating the bus transfer from the first feeder to the second feeder for powering a load bus based on the synchronization time.

[0013] In an embodiment, the present invention also provides an Intelligent Electronic Device for controlling load shedding in a multiphase electrical system during a bus transfer from a first feeder to a second feeder for powering a load bus connecting a plurality of loads with one of the first feeder and the second feeder, the Intelligent Electronic Device configured to: measure a first voltage magnitude in a first voltage cycle from the load bus with a voltage measurement device connected at the load bus; measure a second voltage magnitude in a second voltage cycle from the load bus with the voltage measurement device connected at the load bus; determine rate of decay of voltage based on measured first voltage magnitude and second voltage magnitude; predict a decay in voltage of the load bus based on determined rate of decay of voltage; and operate at least one switching device for load shedding of at least one load from the plurality of loads connected in the load bus based on the predicted decay in voltage of the load bus during the bus transfer from the first feeder to the second feeder.

[0014] In an embodiment, in the IED mentioned herein above wherein the voltage measuring device is a potential transformer.

[0015] In an embodiment, in the IED mentioned herein above wherein the switching device is a circuit breaker.

BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 illustrates a block diagram representation of an intelligent electronic device (IED) connected to a first feeder, a second feeder and a load bus;

[0017] Figure 2 illustrates a phasor diagram of load voltage after the main source is disconnected;

[0018] Figure 3 illustrates a phasor diagram of load voltage indicating advanced load shedding ;

[0019] Figure 4 illustrates the method for advanced load shedding during bus transfer;

[0020] Figure 5 illustrates load voltage profile after main source is disconnected without advance load shedding;

[0021] Figure 6 illustrates load voltage profile after main source is disconnected with advance load shedding; and [0022] Figure 7 illustrates a sampled load voltage signal along with a signal showing zero crossing time instances for estimating magnitude of load voltage.

DETAILED DESCRIPTION

[0023] The present invention is related to a method for load shedding during bus transfer and the steps of the method are being performed by a computing/processing device such as an intelligent electronic device (IED). IEDs are commonly used for control or protection functions in electrical networks.

[0024] The invention provides a method for load shedding in a multiphase electrical system during a bus transfer from a first feeder to a second feeder for powering a load bus. For example, the first feeder can be a main source of supply or main feeder and the second feeder can be a standby feeder. Usually, the energy required for auxiliary drives in a power station is supplied from a relevant generator group via a transformer provided for this purpose (main feeder). During starting up, running down, or in the event of a fault, the generator may fail to supply this energy. For this purpose a standby supply (standby feeder) is required. The process of switching from one feeder to the other is called bus transfer. The bus transfer is carried out by a bus transfer device like an intelligent electronic device (IED). Since bus transfer leads to high inrush current load shedding is adopted.

[0025] However, load shedding by using current methods gives rise to cases where the load shedding command is issued late and the voltage / frequency profile of load collapses significantly leading to inrush condition. The present invention provides a method for predicting a decaying voltage profile and cause an advanced load shedding of unnecessary loads. [0026] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments, which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized. The following detailed description is, therefore, not to be taken in a limiting sense.

[0027] Figure 1 illustrates a block diagram representation of an intelligent electronic device (IED) 100 connected to a first feeder 110, a second feeder 120 and a load bus 130. As shown in Figure 1, the voltage drop at the first feeder 110 can be measured by a voltage measuring device like a potential transformer 140, and the voltage drop at the second feeder 120 can be measured by another potential transformer 150. The first feeder 110 and the second feeder 120 are connected to the load bus 130 via a first switching device 160 and a second switching device 170 respectively. As it may be known to the person skilled in the art, the first switching device 160 and the second switching device 170 can be a circuit breaker. The IED 100 controls the bus transfer from the first feeder 110 to a second feeder 120 for powering the load bus 130. The bus transfer connects a plurality of loads 180 with one of the first feeder 110 and the second feeder 120. The load bus 130 is connected to a plurality of loads like motor loads 180 via a plurality of switching devices 190. As shown in Figure 1, a voltage measuring device 195 is connected at the load bus 130 for providing voltage magnitude information from the load bus 130.

[0028] Figure 2 illustrates a phasor diagram of load voltage after the main source is disconnected. Figure 2 shows a decaying voltage profile 200 of the load bus once the switching device 160 is opened and the first feeder 110 is disconnected from the load bus 130. The decaying phasor 200 indicates a phasor representation of voltage decay at the first feeder 110. As shown in Figure 2, the phasor 200 rotates and decays to a level of magnitude below a certain acceptable voltage level called the Residual Voltage 210. As shown in figure, the second feeder phasor representation is indicated by 220 with magnitude indicated at 230.

[0029] Figure 3 illustrates a phasor diagram of load voltage indicating advanced load shedding. As shown in Figure 3, the phasor voltage for the second feeder is indicated as 300 and the phasor voltage for the load bus is indicated as 310. The phasor 320 indicates the phasor for advanced load shedding. The phasor 330 indicates the phasor when the load bus voltage becomes less than or equal to the residual voltage. The phasor 340 indicates the instance when the command to close is issued to the second feeder.

[0030] Figure 4 illustrates a method 400 for advanced load shedding during bus transfer in a multiphase electrical system controlled by the Intelligent Electronic Device 100 during a bus transfer from a first feeder 110 to a second feeder 120. The bus transfer is carried out for powering the load bus 130 connecting a plurality of loads 140 with one of the first feeder 110 and the second feeder 120.

[0031] The IED 100 performs the steps of the method and as depicted in step 410, firstly the IED measures a first voltage magnitude in a first voltage cycle from the load bus 130 with a voltage measurement device connected at the load bus. As mentioned herein above the voltage measuring device 195 can be a potential transformer.

[0032] Secondly, as depicted in step 420 a second voltage magnitude in a second voltage cycle is measured from the load bus 130 with the voltage measurement device 195 connected at the load bus. For example, the voltage cycles mentioned herein above can be a half cycle or a full cycle of voltage signal or any higher factor thereof. The voltage cycles can be determined with zero crossing detection. [0033] As shown in step 430, a rate of decay of voltage based on measured first voltage magnitude and second voltage magnitude is determined. The rate of decay of voltage may be obtained in various methods, for example in this invention the rate of decay of voltage can be determined by computing a first order gradient of voltage based on first voltage magnitude and second voltage magnitude. And then, computing a second order gradient of voltage based on the first order gradient of voltage gives the final rate of decay. It may be known to the person skilled in the art that, further higher order gradient of voltage can be determined for obtaining robust results on the rate of decay of the voltage profile.

[0034] As shown in step 440, a decay in voltage of the load bus based on determined rate of decay of voltage is predicted. The rate of decay in voltage of the load bus provides an estimate or prediction of a deteriorating voltage profile. This prediction is used to carry out the advance load shedding.

[0035] Finally as shown in step 450, a switching device is operated for load shedding of a load from the plurality of loads connected in the load bus based on the predicted decay in voltage of the load bus during the bus transfer from the first feeder to the second feeder. Shedding of unnecessary loads in advance of the closing of the switching device 170 for connecting the second feeder 120 to the load bus 130 reduces stress on the second feeder when the switching device 170 is closed. Essentially, the load shedding is performed in co-ordination with the bus transfer and avoids huge inrush flows. The critical decision for shedding unnecessary loads in an advanced manner can be taken in an efficient manner by using the method of the embodiments disclosed of this invention. The prediction of the decay profile of the load bus voltage by obtaining voltage gradient is paramount in deciding to perform advanced load shedding and avoid inrush. [0036] Figure 5 illustrates load voltage profile after main source is disconnected without advance load shedding. As shown in Figure 5, the voltage versus time plot i.e plot of terminal RMS voltage when advance load shedding is not performed is depicted. The plot in Figure 5 shows a deteriorated voltage profile. Figure 6 illustrates load voltage profile after main source is disconnected with advance load shedding. As observed in Figure 6 for the plot of RMS voltage with advanced load shedding, an improved voltage profile is indicated.

[0037] Figure 7 illustrates a sampled load voltage signal along with a signal showing zero crossing time instances for estimating magnitude of load voltage. Figure 7A illustrates a sampled voltage signal at the load bus for deriving at the RMS value of the load voltage from the voltage signal samples between two zero-crossing detection. The equation below indicates the RMS value of voltage:

calculated Rms Value Vi (1)

Where, ai, a2, ... , £¾v = Voltage signal samples between two zero-crossing detection and;

N= Number of samples taken between two consecutive zero crossing detection.

[0038] Figure 7B, illustrates a signal showing zero crossing time instances for estimating magnitude of load voltage. Figure 7B shows how the rate(s) of change of load voltage can be estimated from the calculated RMS value Vi. As shown below, equations 2A, 2B and 2C represent the first order voltage gradient obtained from the calculated RMS value of equation 1.

dV V v (2A) dt i-i (ti-l - ti-l ) dV ν _!+1 -ν, .(2C)

dt i ₊ i

[0039] Further from equations 2A, 2B and 2C the second order voltage gradient may be obtained as shown below in equations 3A and 3 C:

dV dV

d V _ dt dt

dt ² (t. - t,^ )

dV dV

d V _ dt i ₊ i dt

dt ² (t _M - t, )

[0040] The bus transfer from the first feeder to the second feeder may be performed by any method known in the art. For example, for attaining high speed transfer of bus, a synchronization time is computed. This synchronization time is based on an instantaneous angle of separation between a voltage of the first feeder and the voltage of the load bus. Alternately, instantaneous angle of separation between a voltage of the second feeder and the voltage of the load bus may also be computed. For this purpose an instantaneous voltage magnitude of the voltage of the first feeder or the second feeder and instantaneous voltage magnitude of the voltage of the load bus can be used. The bus transfer from the first feeder to the second feeder is performed by actuating the bus transfer from the first feeder to the second feeder for powering a load bus based on the synchronization time. Performing the bus transfer at the synchronization time ensures avoiding any fault due to out of phase components as they may further lead to torques and cause damage to the motor loads.

[0041] This written description uses examples to describe the subject matter herein, including the best mode, and also to enable any person skilled in the art to make and use the subject matter. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.