JP2001157365A - Power flow control device and power flow adjustment method - Google Patents

Abstract

(57) [PROBLEMS] To simplify a power flow control device and a power flow adjustment method using a rotary phase shift transformer. A power flow control device (20) is connected to a power transmission line (22) between a first power region (24) and a second power region (26). The power flow control device (20) has at least the first
Rotary phase-shift transformer (102 ₁ ) and the second rotary phase-shift transformer (1
02 ₂ ) The phase shift transformer circuit (44) is included. First rotary phase shift transformer (102 ₁ ) and second rotary phase shift transformer
The rotor of (102 ₂ ) is wound in the opposite direction. Also, the first
And a rotor of the _first rotary phase-shift transformer (102 ₁ ) to generate an effective phase difference or voltage difference between the power region (24) and the second power region (26). 2 rotary phase shift transformers (1
It has a driving device (306) for integrally rotating the rotor of ( ₂ )).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to power transmission between power systems or between power regions, and more particularly to a power flow control device and a power flow adjustment method for controlling power transmission.

[0002]

BACKGROUND OF THE INVENTION It is often necessary or necessary to transmit power between two power regions, for example, between two power companies or between power systems. No. 5,841,267 to Larsen, incorporated by reference, describes two rotary phase shift transformers.
ng transformer)
controller) is disclosed. The first terminals (either the stator or the rotor windings) of the two rotary phase-shifting transformers are connected to a single regulating transformer.
r) is connected in parallel. The second terminals of the two rotary phase shift transformers are connected in series to a series transformer. The controller supplies control signals to both rotary phase-shift transformers in order to set the phase angle between the rotor and the stator of each transformer to a predetermined angle. This gives 2
Effective phase differences and voltage ratios occur between two power regions.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to simplify a power flow controller and a power flow adjustment method using a rotary phase shift transformer.

[0004]

The power flow controller is connected to a transmission line between a first power area and a second power area. The power flow control device includes a rotary phase shifter circuit having at least a first rotary phase shift transformer and a second rotary phase shift transformer. The rotors of the first rotary phase shift transformer and the second rotary phase shift transformer are provided on a common drive shaft. To generate an effective phase difference between the first power region and the second power region, the drive unit includes a common drive shaft coupled to the rotor of the first rotary phase-shift transformer and the second rotary type. Rotate with rotor of phase shift transformer. The first rotary phase shift transformer and the second rotary phase shift transformer have opposite winding phases, ie, the electrical rotor angle of the first rotary phase shift transformer and the second rotary phase shift transformer. The rotary phase shift transformer is mounted with the electrical rotor angles always equal in magnitude and opposite in phase. The series transformer is connected to tie the net output voltage of the first rotary phase shift transformer and the second rotary phase shift transformer to the power line. The series transformer preferably has two sets of low-voltage windings and can be connected in a hexagonal shape in three phases. The regulating transformer supplies power to the first rotary phase shift transformer and the second rotary phase shift transformer. The adjusting transformer is a parallel transformer (shunt transfor
mer) is desirable. The controller supplies a control signal to the common drive shaft to generate an effective phase difference between the first power region and the second power region.

[0005]

DETAILED DESCRIPTION OF THE INVENTION FIG.
Transmission between grid 24 and a second power area or grid 26
Electric power flow control provided in electric circuit, that is, transmission line 22
The device 20 is shown. The transmission line 22 is divided into the region 24 and the region
26 to be one of the multiple transmission lines
it can. FIG. 1 also shows other (parallel) transmission lines.
You. The voltage of the system 24 is V₁And the voltage of the system 26 is V_Two
It is. As described below, the operator
The voltage V in series with the power transmission line 22 using the _STo
The injection regulates the power flow flowing into the grid 26.
Can be adjusted. The power flow control device 20
By injecting the pressure, the magnitude of the voltage on the transmission line 22 is increased.
The active power flow without significantly changing the
Shifts the phase of the voltage on the transmission line 22 to affect the
Can be Conversely, the power flow control device
The device 20 is configured to inject a series voltage to
Change the magnitude of the voltage without significantly shifting the phase
Can be designed (adjust the voltage ratio)
Wear. Using this latter method, mainly the reactive power flow
Affect. The transmission line 22 is a three-phase transmission line. Electric
The voltage transformer 36 is connected to the voltage V_TwoIs detected. Current
The transformer 38 controls the current I flowing into the system 26._TwoMeasure
You. Current I₁Flows from the grid 24 to the tide control device 20.
You.

As shown in FIG. 1, a power flow control device 20
Has a series transformer 40, a regulating transformer 42, a rotary phase shift transformer circuit 44, and a controller 46. Each device will be described in detail below. Series transformer 40 incorporates an output voltage u _S net series connected stator windings of the rotary phase shifting transformer circuit 44 in the transmission network. Series transformer 40
Has, for example, two sets of low-voltage windings in order to prevent a zero-phase current from flowing through the rotary phase-shift transformer circuit 44, and, for three phases thereof, those connected in a hexagonal shape. May be used. Adjusting transformer 42 is γ degrees (30 degrees, 90 degrees, etc.)
And a parallel Y-Δ transformer connected to produce a phase difference of The regulating transformer 42 is fed from the center tap of the series transformer 40 and minimizes the change in the magnitude of the voltage when the phase difference changes. The rotary phase shift transformer circuit 44 includes a first
Has a rotary phase shifting transformer 102 ₁ and the second rotary phase shifting transformer 102 _2. As will be described in detail below, first terminals (a winding of one of a stator and a rotor) of the first rotary phase-shift transformer 102 ₁ and the second rotary phase-shift transformer 102 ₂ . Is connected in parallel to the low-voltage side winding of the adjustment transformer 42. The first rotary phase shift transformer 102 ₁ and the second
The second terminal of the rotary phase shift transformer 102 ₂ (either the stator or the rotor winding) is connected in series with the low voltage winding of the series transformer 40, which is preferably connected in a hexagonal configuration. Is done.

The present invention is characterized in that, as shown in FIG. 2, the rotary phase shift transformers 102 ₁ and 102 ₂ are mounted on a common drive shaft 300 and have a common drive 306. is there. Rotary phase shift transformers 102 ₁ and 102 ₂
Are mounted on the common drive shaft 300 in a back-to-back state. Thus, the rotary phase shift transformer circuit 4
4 is a rotary phase shift transformer 102 ₁ and 102 ₂ and a common drive 306, also known as a position controller 306.
have. As shown in FIG. 2, the rotary phase shift transformer 1
02 ₁ and 102 ₂ have a rotor subassembly 110 and a stator 112.

As described above, the first rotary type phase shift transformer 10
2 ₁ and the second rotary phase shifting transformer 102 ₂ is a wound rotor machine can be moved 180 degrees rotor electrical angle (wound-rotor machine). With multi-pole windings, this change can be caused by a rather small mechanical movement. The phase angle of the stator voltage with respect to the rotor voltage can be adjusted by simple mechanical position control of the rotor. The rotor angles of the two rotary phase-shifting transformers determine the magnitude and phase of the series voltage V _S. To achieve this, if the amount of phase shift in the adjustment transformer 42 is 90 degrees and the rotors of the first and second rotary phase shift transformers are mounted as described above, the series voltage V _S , without changing the magnitude of the voltage so, to generate a phase difference between the main voltage V ₁ and the voltage V _2. When the phase shift amount in the adjustment transformer 42 is 0 degree, the voltage V
₁ and without much changing the phase difference between the voltage V _2, so that the main voltage ratio is changed.

FIGS. 3 (a) and 3 (b) show the electrical connections of the rotary phase shift transformers 102 ₁ and 102 ₂ as viewed from the common drive 306 at the position shown in FIG. 2, respectively. FIG. 3 (a) and 3 (b) schematically show the windings of each phase to explain the phase sequence and current, and show the voltage applied to each winding. For each of the rotary phase shift transformers 102 ₁ and 102 ₂ , a three-phase line RA, RB, RC is rotated by a collector ring 114 or a flexible conductor 116 (not shown). The child 110 is connected. On the other hand, the three-phase line S
A, SB, SC are connected to the stator 112, where A, B, C indicate the corresponding one of the three-phase lines. “R” indicates a rotor, and “S” indicates a stator. In each of the rotary phase shift transformers 102 ₁ and 102 ₂ , the current I _RA
Flows through winding RA, and current I _SA flows through winding SA. Voltage V _RA is the voltage of winding RA, and voltage V _SA is the voltage of winding SA.

In the illustrated embodiment, the rotary phase shift transformer 1
The rotor and stator of each of 02 ₁ and 102 ₂ are wound with a phase of 60 degrees. It should be understood that the present invention is not limited to systems wound with 60 degrees phase.
Rather, the principles of the present invention are applicable to rotary transformer assemblies having two or more poles.

The rotary phase shift transformers 102 ₁ and 102 ₂ include:
They are mounted on the same common drive shaft 300, but in opposite winding phase order (see FIGS. 3 (b) and 3 (a)). Thus, the angle of the electrical rotor is always the same magnitude and opposite. FIG. 1 shows a rotary phase shift transformer 102.
The electrical angle of the rotor ₂ is δ, and the rotary phase shift transformer 10
Electrical angle of 2 ₁ of the rotor is - [delta. The rotor assembly 110 of the rotary phase shift transformers 102 ₁ and 102 ₂ may rotate clockwise CW or counterclockwise C around the common drive shaft 300.
1 CW from the position where the rotor winding and stator winding are aligned
Rotate up to 80 degrees. Rotation of the rotor assembly 110 is driven by a common drive 306.

[0012] The rotor drive device 306 is shown in FIG. 2 as a cylindrical portion attached to rotary phase shifting transformer 102 _{and second} rotor assembly 110. The common drive 306 of FIG. 2 represented in a cylindrical shape is the rotor subassembly 110 of both rotary phase shift transformers 102 ₁ and 102 _2.
Various alternatives and different configurations of the drive mechanism for simultaneously rotating are shown. A specific example of the common driving device 306 is named “Connecting Device of Electrical Systems Having Different Electrical Characteristics” and is described in US Pat. 08/55
No. 0,941. For example, in some embodiments, rotor drive 306 includes some form of coupling device (eg, gear) and actuator for connecting to rotor assembly 110. In some embodiments, drive 306 comprises a worm gear drive. In another embodiment, the rotor drive 306
Are a step motor that operates via a radial gear (for example, a spur gear), a direct drive device, a hydraulic actuator that rotates a gear on the rotor assembly 110, a pneumatic actuator that rotates a gear of the rotor assembly 110, and the like. Actuator. In another embodiment, the function of the position control device is achieved by providing two sets of windings on the rotor and the stator. The number of poles (for example, 2 poles) of the first set of windings of the two sets of windings is the second
The number of poles of the set of windings is different (eg, 4 poles or more). As mentioned above, it should be understood that any suitable drive mechanism can be used as the rotor drive 306.

Returning to FIG. 1, especially considering the connection of the rotary phase shift transformer circuit 44 to the transformers 40 and 42, since the transmission line 22 is three-phase, the windings of each transformer Is connected to the three-phase wire. In addition, these three-phase wires are connected to the rotor assembly 11 as in this example.
Either the zero or the stator 112 is connected to the appropriate and corresponding one of the A, B, C phase windings.

As shown in FIG. 1, the control device 46 includes a high precision control device 200 and a position control device 202.
The high-accuracy control device 200 may provide any combination of several controlled objects. In the illustrated embodiment, high-precision control device 200 is connected to the voltage transformer 36 (receiving the value of the voltage V ₂₎ and the current transformer 38 (receiving the value of the current I _2). Further, the high-accuracy control device 200 receives an operator set point input signal, in particular, an active power command signal P ₀ indicating a set value of active power passing through the line 22. The command (input) signal P ₀ is set by adjusting a knob or input data at an operator control panel or an operator workstation. The command (input) signal P ₀ is set by the control device 20.
It may be performed near zero or far.

The purpose of controller 46 is to regulate the active power flow through line 22. Using the active power command signal P ₀ and the detected V ₂ or I ₂ , the high-accuracy control device 200 generates a signal δ _ref . This signal δ _ref is provided to the position controller 202 via line 216. The signal δ _ref is the desired electrical angle δ of the rotor of the rotary phase shift transformer.
And is generated by the power flow control device 20. Other high-precision control devices having different purposes may be incorporated. For example, high-precision control device, may adjust the predetermined phase difference between the voltages V ₁ and V _2. Further, as shown in the following application example, the high-accuracy control device may adjust the balance of the power flow flowing through the multiple lines between the area # 1 and the area # 2.

An application example of the present invention is shown in FIG.
In this case, it is used between two 500 kV buses. The purpose of the control device is to operate in a steady state or when one line is stopped.
To balance the power flow between two two-line transmission lines, ie, four transmission lines. In FIG. 4, the rotary type power flow controller (RPFC) has a rated power of 2000 MVA, and has a 10% series voltage (50 k
V). If the rotary phase shift transformer (RPST) is designed to inject a 10% series voltage, both the rotary phase shift transformer and the stator voltage are selected to be 25 kV. The RPFC element ratings are shown in Table 1. The element-based schematic leakage reactance is also shown in Table 1. The RPST (rotary phase shift transformer) has a considerably large excitation susceptance (about 1).
0%). Parallel capacitors can be provided on both sides of the RPST to compensate for the excitation susceptance. Table 1. RPFC element rating example

[Table 1]

[Effects]: The RPFC can well balance the power flow in the circuit within the rated capacity of the equipment in the steady state and when one line fails. -RPFC response is much faster than conventional phase shift transformers with mechanical switches. RPFCs are not as complex as power flow controllers based on power electronics technology and, like traditional phase shift transformers, respond to and recover from grid faults. • During a failure, an overvoltage of the fundamental frequency is generated internally as in all series devices. By using conventional methods based on high energy varistor technology, overvoltage duty can be significantly reduced. -The bypass operation and the insertion operation of the RPFC hardly disturb the system.

To illustrate the steady state behavior of a single-shaft RPFC, along with an example of the results for a simple system configuration,
The steady state vector equation is shown below. The relational expression in the steady state for the uniaxial RPFC is as shown in [Expression 1] when the rating of each device is expressed by pu using the symbol in FIG.

(Equation 1)

In [Equation 1], the ratio T_SIs the rated line power
Is the ratio of the maximum injected series voltage to the voltage, in this example
Is 0.1. To simplify the analysis, the excitation reactor
The closet and winding resistance are omitted. The resistance is small
Excitation reactance (important for RPST)
Is compensated by the parallel capacitor, so the winding
Omitting the resistance and excitation reactance is almost no problem.
No. Parallel transformer (X _T1), A series transformer (X_T2), Times
Inverting phase shift transformer (X_R) Is shown in Table 1.
It is the p.u. value of each listed device base. Injection series
The voltage Vs determines the amount of phase shift of the device,
Voltage V₁And current I_TwoUsing [Equation 2] to calculate
Can be.

(Equation 2)

To illustrate the steady state behavior of the RPFC, a simple circuit of FIG. 5 using the following parameter values is used.

(Equation 3) The RPFC parameter values correspond to the ratings of the devices used as examples shown in Table 1. It is assumed that the equivalent bus voltages V _1X and V _{2X of the system} and the phase angle between them are constant. The equivalent system reactances X ₁ and X ₂ are set to the RPFC passing power flow rating of 0.1 p.u. The parameters of this RPFC correspond to the rating examples of the elements shown in Table 1. It is assumed that the equivalent bus voltages V _1X and V _{2X of the system} and their angles are fixed. Reactances X ₁ and X ₂ are set to 0.1 p.u. of RPFC rated passing power.

FIGS. 6 to 11 show the effect of varying the electrical rotor angle (δ) of the RPST from its maximum range of 0 ° to 180 ° on some important variables. It is. A rotor angle of 0 degrees corresponds to the arrangement of the rotor and stator windings. At this angle, a maximum positive phase shift from the bus 1 to the bus 2 occurs. At a rotor angle of 180 degrees, a negative maximum phase shift occurs. In each of FIGS. 6 to 11, a thick line indicates a case where the phases of the voltages of two equivalent buses are equal (θ _2x = θ _1x ). Another two
Lines (dotted line and broken line) are obtained when the phases of the voltages between the equivalent buses are shifted by −10 degrees and +10 degrees, respectively.

FIG. 6 shows the phase difference (phase shift amount) of the voltage across the RPFC (θ _s = θ ₂ −θ ₁ ). For this system configuration and RPFC rating, a phase difference of about ± 5 degrees is obtained from the neutral point (neutral). Valid RPFC
The series impedance (X _S), because about 15% of the total system impedance, when the phase difference of 10 degrees ± is approximately 1.5 degrees phase is shifted curve. FIG.
4 shows a passing effective power flow (Series P Flow) flowing from the bus 1 to the bus 2 via the RPFC. RPFC is
The tidal current can be changed by about ± 800 MW from the neutral point. FIG. 8 shows the passing current flowing through the RPFC (Series
Current) and the pu value based on the RPFC's 2000 MVA rating. If the current is already greater than the value due to the phase angle of the system, attempting to increase the current may exceed the RPFC rating. FIG. 9 shows the current (Shunt Curr) flowing through the parallel transformer of the RPFC.
ent) is based on the 200MVA rating base of the parallel transformer.
Indicated by pu value. If the current through the series transformer is already large due to the phase angle of the grid, further attempts to increase the current may exceed the rating of the parallel transformer.
However, this is only the case if the current rating has already been exceeded. FIG. 10 shows the reactive power (Net Q Consumption) consumed by the leakage reactance of the RPFC element. When operating within the RPFC passing current rating, the reactive power consumption is less than or equal to 60 MVAR. FIG.
1 indicates the torque required to hold the two RPSTs in that position (RPST Torque) as a pu value based on the total rating of the two RPSTs. In the example symmetrical system arrangement, the torques of the two RPSTs are equal.
Since these are provided on the same axis, the values given in the figure are added.

The application of a power flow controller using rotary phase shift transformer (RPST) technology makes it attractive for devices requiring power electronics (PE) devices such as thyristor switch type phase shift transformers and UPFCs. Provide a simple alternative. Rotary power flow controller (RPFC)
It uses well-known transformer and rotating machine technology and offers a lower cost and more reliable approach. Some of the positive effects include:・ Continuous phase shift adjustment. -The thermal time constant is larger than that of the power electronics element (PE), and the compatibility with other distribution facilities is good. -There are no problems such as harmonics associated with the power electronics element (PE) and interaction with the generator. -Stable operation can be continued even in severe system transients.

Although the present invention has been described in terms of the presently most practical and preferred embodiment, the present invention is not limited to the disclosed embodiment. On the contrary, the invention is intended to cover various modifications and equivalents, including the spirit and scope of the appended claims. It should be noted that the “voltage ratio” in this specification includes the concept of “difference in voltage magnitude”.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a power flow control device of the present invention.

FIG. 2 is an external view of the rotary phase shift transformer circuit shown in FIG.

FIG. 3 is a diagram showing windings and current directions of a rotary phase shift type transformer constituting the power flow control device shown in FIG. 1;

FIG. 4 is a schematic diagram of a system to which the power flow control device shown in FIG. 1 is applied.

FIG. 5 is a schematic diagram of a system using the power flow control device shown in FIG. 1;

6 shows the effect on the phase shift amount θ _S when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from the maximum range of 0 ° to 180 °. FIG.

FIG. 7 shows the effect on the passing effective power flow (Series P Flow) when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from the maximum range of 0 degrees to 180 degrees. FIG.

FIG. 8 is a diagram showing an influence on a passing current (Series Current) when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from a maximum range of 0 ° to 180 °. .

9 shows the effect on the current (shunt current) flowing in the parallel transformer when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from the maximum range of 0 degree to 180 degrees. FIG.

FIG. 10 is a diagram showing the influence on reactive power consumption (Net Q Consumption) when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from the maximum range of 0 ° to 180 °. It is.

FIG. 11 is a diagram showing an influence on a torque (RPST Torque) applied to the RPST when the electric rotation angle of the power flow control device shown in FIG. 1 is changed from 0 degree which is a maximum range to 180 degrees. It is.

[Explanation of symbols]

Reference Signs List 20 power flow control device 22 transmission line 24, 26 power region 36 voltage transformer 38 current transformer 40 series transformer 42 regulating transformer 44 rotary phase shift transformer circuit 46 controller 102 ₁ , 102 ₂ rotary phase shift transformer Device 110 rotor assembly 200 high-precision control device 202 position control device 300 common drive shaft 306 common drive device

Continuation of front page (72) Inventor Richard, Joseph Pivco United States 12148 New York, Rexford, Grooms Road 962 (72) Inventor Hideki Fujita 20-1 Kitakanyama, Otakacho, Midori-ku, Nagoya-shi, Aichi 1 Chubu Electric Power (72) Inventor William, Warne Price Jr. United States 12302 New York, Scotia, Onderdonk Road 11

Claims (16)

[Claims] 1. A power flow controller connected to a transmission line between a first power area and a second power area, comprising: a first rotary phase-shift transformer; A phase shift transformer, a common drive shaft on which the rotors of the first rotary phase shift transformer and the second rotary phase shift transformer are mounted, and between the first power region and the second power region. And a driving device for rotating a common driving shaft for adjusting at least one of the phase difference and the voltage ratio of the power flow. 2. The power flow control device according to claim 1, wherein the first rotary phase-shift transformer and the second rotary phase-shift transformer have windings in reverse phase order. 3. The power flow control according to claim 2, wherein the electrical angles of the rotors of the first rotary phase shift transformer and the second rotary phase shift transformer are always the same and opposite directions. apparatus. 4. The system of claim 1, further comprising a series transformer connected to take in the net output voltage of the first rotary phase shift transformer and the second rotary phase shift transformer into a power line. Power flow control device as described. 5. The power flow control device according to claim 4, wherein the series transformer is a series transformer having two sets of low-voltage windings, and a low-voltage side is connected in a hexagonal shape in three phases. 6. The power flow control device according to claim 1, further comprising an adjusting transformer for supplying power to the first rotary phase shift transformer and the second rotary phase shift transformer. 7. The power flow control device according to claim 6, wherein the regulating transformer is a parallel transformer having a Y-Δ connection. 8. A control device for supplying a control signal to the common drive shaft, the control device adjusting a phase difference or a voltage ratio between the first power region and the second power region. 2. The power flow control device according to 1. 9. The control device includes a position control device and a high-precision control device. The high-precision control device supplies an instruction signal of a predetermined electric rotor angle to the position control device. 9. The power flow control device according to claim 8, wherein a mechanical position of the rotor is adjusted using an instruction signal of a predetermined electric rotor angle from the high precision control device. 10. A power flow adjustment method for adjusting a power flow of a transmission line between a first power region of a voltage V _{1 and} a second power region of a voltage V ₂ , wherein A first rotary phase-shift transformer and a second rotary phase-shift transformer between the second power region and a common drive of the rotors of the first and second rotary phase-shift transformers; A first step of providing a rotary phase-shift transformer circuit mounted on the shaft; and providing a position control signal to the rotary phase-shift transformer circuit and applying a voltage V _S generated in the rotary phase-shift transformer circuit. A second step of generating at least one of a predetermined phase difference and a voltage ratio between the first power region and the second power region. 11. The power flow adjusting method according to claim 10, wherein in the first step, the first and second rotary phase-shift transformers are mounted such that the windings have opposite phases. 12. In the first stage, the electrical angles of the rotors of the first and second rotary phase-shift transformers are always equal in magnitude,
The first rotary phase-shift transformer and the second
The power flow adjustment method according to claim 10, wherein the rotary phase shift transformer is mounted. 13. The power flow adjustment method according to claim 10, wherein in the first step, a series transformer for connecting output voltages of the first and second rotary phase shift transformers to a transmission line is connected. 14. The power flow regulation method according to claim 10, wherein in the first step, a regulation transformer for supplying power to the first and second rotary phase shift transformers is connected. 15. The method according to claim 10, further comprising using a control device for supplying a control signal to the common drive shaft to generate a phase difference or a voltage ratio between the first power region and the second power region.
Power flow adjustment method described in 1. 16. The control device includes a position control device and a high-precision control device, and further uses the high-precision control device to supply an instruction signal of a predetermined electrical phase difference to the position control device. The power flow adjustment method according to claim 15, wherein the position control device adjusts a mechanical position of the rotor using an instruction signal of a predetermined electric phase difference.