GB2417376A

GB2417376A - Voltage control by reactive current control in high-voltage power transformers

Info

Publication number: GB2417376A
Application number: GB0418719A
Authority: GB
Inventors: Jonathan Nicholas Hiscock; David John Goodfellow
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-08-21
Filing date: 2004-08-21
Publication date: 2006-02-22
Anticipated expiration: 2024-08-21
Also published as: GB2417376B; GB0418719D0

Abstract

A voltage control device [1] that uses an enhanced reactive current control method whereby circulating current is calculated from measurements of transformer current [6] and voltage [7] and a bias applied to the voltage measurement such that the circulating current is minimised. The scheme discriminates between circulating current that flows between transformers that are in parallel at a site calculated using the summed load current on the common bus-bar, and that which flows between transformers that are paralleled across a network calculated from the deviation of measured power factor from an assumed network value. The scheme provides for voltage control in the event of network changes such as embedded generation or remotely paralleled transformers where existing reactive current control schemes are not accurate, and also monitors the bus-bar arrangement of the transformer site to allow for the calculation of summed load current.

Description

A VOLTAGE CONTROL SCHEME FOR HIGH VOLTAGE POWER

TRANSFORMERS

This invention relates to voltage control of power transformers that is based on the reactive current control method.

Where transformers are connected in parallel to a common bus-bar, reactive circulating current can flow between them unless a number of conditions are met: The transformers have the same number of tap positions, tapping interval and nominal ratio The transformers are always at the same tap position The transformers have the same voltage applied to the primary winding connections The transformers have the same impedance and power rating These conditions put constraints on power system design but can be minimised using the reactive current control principle. Reactive control operates by the detection of the reactive circulating current component of the load and the application of a bias to the voltage measurement of the control system such that the tap position is altered to minimise this current. Use of this scheme allows transformers to be freely operated in parallel.

In existing reactive control schemes the magnitude of circulating current can be calculated by either of two methods, here referred to as method 1 and method 2.

Method I determines the magnitude of circulating current from a comparison of the measured transformer power factor with an assumed network power factor. This calculation will have an error if the real system power factor deviates from the assumed value. With the emergence of embedded generation in electrical power systems and seasonal changes in load characteristic, such network power factor deviations are becoming increasingly apparent.

Method 2 uses the summed load and the individual loads of grouped transformers at a site to calculate the circulating current. There are two limitations when using this method.

The first is that the calculation is incorrect if the configuration of the transformer site changes. The second is that this method cannot be used when networks are operated in parallel as the summed load at the site can also contain reactive current flowing to or from remote transformers.

This invention uses a combination of the methods described above together with an intelligent determination of the summed current for the calculation of circulating current but without any of the associated limitations. The scheme provides a voltage control system for transformers that are paralleled at a site or across a network that is acceptably accurate in the event of site configuration changes and network power factor deviations.

In accordance with the present invention, a method of voltage control for high voltage power transformers provides for the discrimination between power factor deviations caused by intra-group transformer tap changes and those caused by network activity that is remote to the transformer group, with the appropriate corrective action for the minimization of the reactive circulating current.

From hereon in the following definitions should be noted: VCR - voltage control relay Transformer site - transformers situated at a substation Transformer group - transformers connected to a common bus-bar at a site A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure I shows a VCR connected to a transformer Figure 2 shows four transformers at a site each with an attached VCR, with three of the transformers connected to a common bus-bar Figure 3 is a flow chart illustrating the processing stages involved in the calculation of voltage bias Figure 4 is a phasor diagram that shows how circulating current is calculated using method 1 Figure 5 is a flow chart illustrating how the configuration of the transformer site is determined Figure 6 is a phasor diagram that shows how circulating current is calculated using method 2 As shown in figure 1, the VCR [1] is connected to a transformer [2] that supplies a bus- bar [3] to which a load [4] is connected. The VCR controls tap-changers that are on the high-voltage side [5] of the transformer, and takes continuous readings of the transformer output current [6] and voltage [7] as measured by the respective current transformer [8] and voltage transformer [9].

Figure 2 shows an example of four transformers at a site, each of which have an attached VCR Three transformers T. [1], T2 [2] and T4 [4] are connected in parallel to a bus-bar A [5] that supplies load A [6] (equal to the vectorial sum of transformer load currents I, [7], I2 [8] and Is [101), and the other transformer T3 [3] is connected to bus-bar B [11] that supplies load B [12] (equal to transformer load current Is [9]).

The VCR uses the voltage and current measurements to calculate a voltage bias that will minimise reactive circulating current. The process flow for the calculation of the voltage bias is shown in figure 3. Following the measurement readings (as indicated by MEASURE) the VCR calculates the network coupled voltage Vncv. The VCR then establishes the configuration of the transformer site (as indicated by CONF1[G), which allows the calculation of the total load current on the bus-bar to which the transformer is connected, and the corrective coupling voltage Vccv. The bias applied to the VCR is the sum of VnCV and VcCv. This process is repeated continuously such that reactive circulating current is minimised at all times regardless of network activity or configuration changes.

The calculations are now considered in detail.

The optimum bias voltage to apply to the control system such that circulating current is minimised is practically defined by the transformer nameplate impedance and full load current as follows: Vb," = z%XIC'rC/Io (1) Ig = MVAbluu/Vnom (2) where Vb,,', is the optimum voltage bias to minimise the circulating current ICIIC is the circulating current In is the transformer full load current Z% is the transformer nameplate impedance expressed as a percentage, equivalent to the full load winding voltage drop MVA is the transformer power rating VnOm is the transformer nominal voltage The network coupled voltage is applied to reduce the circulating current that may result from network activity remote to the transformer group, and is calculated based on the deviation of the transformer power factor from an assumed value. Figure 4 is a phasor diagram that shows how the circulating current Ic'= [1] is calculated following the reading of transformer current I [2] and voltage [3]. The following terms are used to derive the circulating current: p. trans = Cosine (qt) (3) p.net = Cosine (a) .. . (4) Ip = I x Cosine (q!)

. (5) Iq = I x Sine(q,) (6) Iqnet = Ip x Tan (a) (7) Icrc = Iq - Iqnet (8) where p.f.trans is the transformer load power factor p.f.net is the assumed network power factor Ip is the real element of the transformer current [4] Iq is the reactive element ofthe transformer current [5] Iqnet is the reactive element of the transformer current at assumed power factor [6] is the assumed phase angle [7] between transformer voltage and current Gt is the measured phase angle [8] between transformer voltage and current This method of calculating circulating current is the same as that described previously in mewed 1. There is an error in this calculation where the network power factor deviates from the assumed value. Such a network power factor deviation could be attributed to either a change in the load characteristic or to operations of remotely connected transformers, or both. The cause cannot be determined easily...DTD: If an observed power factor deviation is due to the load itself then the calculation of circulating current will be incorrect. If a bias were applied to the VCR voltage measurement to minimise the implied circulating current the result would be an error in system voltage. If however the power factor deviation is due to a remote transformer then the calculation is correct and the application of the bias will encourage the VCR to initiate a corrective operation that will minimise the circulating current and maintain the correct system voltage.

An attenuated bias of approximately 10% of the calculated optimum bias is applied to provide control in the event of both types of power factor deviation described above.

Where the power factor deviation is due to the load, the attenuated bias results in a correspondingly reduced and acceptable error in system voltage. Where the deviation is due to a remote transformer, the attenuated bias still affects the VCR to maintain the paralleled transformers on the optimum tap positions. Using equations (1) and (2) for optimum bias the network coupled voltage is written: Vnov O. 1 X Z% X loins / In. . . (9) where Ions is the circulating current calculated using equations (3) to (8) In is the transformer full load current Z% is the transformer nameplate impedance expressed as a percentage, equivalent to the full load winding voltage drop The configuration of the transformer site is required for the calculation of the corrective coupling voltage Vcv since it relies on an accurate determination of the summed group load current at the common bus-bar, which is not possible without knowledge of which transformers are connected in parallel. In order to establish the configuration and to calculate the summed group load current, communication is used between each VCR at the site.

Each VCR is equipped with inter-relay communications for communication with all others at the site. Any signal that is sent by a VCR is registered by all VCR's at the site.

The information that can be transmitted is as follows: UNIT_ID I Ip I Iq (signal I) UNIT_ID I tap change start (signal 2) UNIT_ID I tap change complete (Signal 3) UNTT_lD I change in Iq registered (signal 4) where UNIT_ID is an identifier for the VCR that sends the signal; Iq is the reactive element of the transformer current read by the VCR identified by

UNIT_ID

Ip is the real element of the transformer current read by the VCR identified by UNIT_ID Signal I is sent by each VCR following every measurement reading and every VCR at the site receives this information. At a site comprising n transformers, each VCR receives the following signals: UNIT_I I Ip I Iq UNIT_2 1 Ip 1 Id UNIT_3 1 Ip UNIT_n I lp I id In order to calculate the summed load current at the bus-bar to which it is connected, each VCR requires information regarding the group configuration. As an example consider the four transformers shown in figure 2. Transformer T. [1] is in parallel with transformer T2 [2] and T4 [4], T3 [3] operates alone. Each VCR holds a record of which transformers to include in the calculation of summed load current in the form of a lookup table. The configuration for the example considered is as follows: Transformer number Paralleled transformer Current at common bus-bar 1 2,4 1'+I2+I4 2 1,4 I2+1'+14 3 _ _ - __ I3 4 1,2 14 + I, + I2

Table 1

Figure 5 shows the CONFIG process flow for the determination of group configuration. It can be seen that the configuration is only established following a tap change operation. If any transformer at the site initiates a tap change operation the VCR attached to the operating transformer sends a signal of type signal 2. Following receipt of the signal each VCR stores a reading of the reactive element of the transformer load current Iq Once the tap change is complete the VCR sends a signal of type signal 3 and each VCR stores another reading of the reactive element of the transformer load current and compares it with the first reading.

Any change in Iq indicates that the transformer to which the VCR is attached is connected in parallel with the transformer which did the tap change, and is caused by circulating current flowing between the transformers. Following a change in Iq above a minimum value, the VCR sends a signal of type signal 4. This signal indicates which VCR's were affected by the tap change, and from this each VCR can determine to which group it belongs. This information is stored by each VCR in a lookup table that is updated following every tap change operation.

If during the CONFIG process flow another signal type 2 is received, i.e. if another transformer initiates a tap change operation, the process is aborted and the existing configuration is used. The configuration is only updated providing there is sufficient time between successive tap change operations for the first tap change operation to complete.

As an example of this process, consider again the four transformers shown in figure 2.

Assume that the transformers are identical, are all on the same tap position, and there is no circulating current flowing. If transformer 1 initiates a tap change operation as a result of low voltage (i.e. the transformer taps up), it sends the following signal which each of the VCR's receive: UNlT_I I tap change start Each VCR stores a reading of the reactive element of measured transformer current listart. When transformer I has completed its tap change each VCR receives the following signal: UNIT_1 Itap change complete Each VCR stores another reading of the reactive element of measured transformer current Iqfinish, and calculates the change in reactive current between the two readings, AIq as follows: AIq = Iqfinish - Iqstart If a VCR calculates a change above a threshold value, it transmits a signal of type signal 4. In this example the VCR's connected to transformers T. [1], T2 [2] and T4 [4] will calculate a change in reactive current since there will be circulating current flowing from T. to T2 and T4. The following signals are subsequently received by all VCR's: VCR_I l change in Iq registered VCR_2l change in Iq registered VCR_41 change in Iq registered From this the site configuration is established as that shown in table 1. If the bus-bar arrangement is changed, the new configuration will be established following the next tap change operation.

With reference to the transformer configuration, the summed load current at a bus-bar can be calculated. This is the basis of method 2 for calculating circulating current and is used to calculate the corrective coupling voltage V,v.

Figure 6 shows a phasor diagram of n transformers connected to a common bus-bar, each of which has an attached VCR. The phasor diagram shows transformer number n, Tn.

exporting circulating current lt,r, [1], and the constituent real and reactive components of the summed load current [2], Ipsum [3] and Iqum [4] respectively. The individual transformer load current ITn [5] has real element IpTn [6] and reactive element IqTn [7].

The VCR reads measurements of transformer voltage and current, and by virtue of the inter-VCR communications also receives readings of all other transformer load currents at the site as described previously. The following equations are used to derive the circulating current: IpTn = ITn X Cosine (<P) (10) IqTn ITn X Sine (q') (11) IPSUm = IPTI + 1PT2 + IPT3 + -IPTn. (12) Sum = IqTI + IqT2 + IqT3 + IqTn (13) pTn / Ipsum = IqTnsum / Iqeum (14) ctcc = IqTn IqTnsum (15) where n is the number of transformers connected to the common bus-bar q' is the measured phase angle [8] between transformer load current ITn [5] and voltage [9] IqTnsum [10] is the extrapolated reactive element of the transformer To load current that would flow in the absence of circulating current Using equations (1) and (2) for optimum bias the corrective coupling voltage is written: Vccv = Z%xIcirc/Ifl (16) where ICirc is the circulating current calculated using equations (10) to (15) In is the transformer full load current Z% is the transformer nameplate impedance expressed as a percentage, equivalent to the full load winding voltage drop The full voltage bias is applied to minimise the calculated circulating current. This contrasts with the earlier application of Vncv where an attenuated bias is applied. The corrective coupling voltage is applied to the voltage measurement to minimise the circulating current flowing between transformers in the same group.

The total bias applied to the voltage measurement of the VCR is calculated continuously and is written: Vbias = Vnev + VCCV

Claims

1. A method of voltage control for high voltage power transformers that provides for the discrimination between power factor deviations caused by intra-group transformer tap changes and those due to network activity that is remote to the transformer group, with the appropriate corrective action for the minimisation of the reactive circulating current

2. A method as claimed in Claim 1 wherein transformers connected to a common bus-bar are detected

3. A method as claimed in Claiml and Claim 2 wherein the transformer configuration at a site is determined and used to calculate the summed load on each bus-bar

4. A method as claimed in Claim 1 as dependent on Claim 3 wherein the calculation of the voltage bias used to minimise the circulating current caused by intra-group transformer tap changes is based on the summed group load

5. A method as claimed in Claim 1 wherein the calculation of the voltage bias used to minimise the circulating current caused by network activity that is remote to the transformer group is based on the deviation of measured transformer power factor from the assumed network power factor

6. A method as claimed in Claim 5 wherein the resulting error in voltage measurement is considerably smaller than that resulting from existing methods of voltage control that are based on the deviation of measured transformer power factor from the assumed network power factor

7. Apparatus for controlling high voltage power transformers comprising means for monitoring transformer output voltage and load current; communication means for the determination of the output voltage and load current of all other transformers at the site and the bus-bar configuration; means for the calculation of voltage bias to apply to the transformer voltage measurement to provide the appropriate corrective action for power factor deviations caused by network activity remote to the transformer group and those caused by intra-group tap changes