CN111579986B - Arc extinction method for single-phase earth fault of generator stator winding - Google Patents
Arc extinction method for single-phase earth fault of generator stator winding Download PDFInfo
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Abstract
The invention discloses an arc extinction method for a single-phase earth fault of a generator stator winding, which comprises the following steps: if the occurrence of the stator winding ground fault is detected, acquiring a reference electromotive force and detecting, namely applying a clamping voltage to the reference electromotive force to adjust the neutral point voltage to the clamping voltage, judging whether the voltage value at the stator fault coil is zero, and if the voltage value is zero, selecting the voltage at the neutral point of the clamped generator as the optimal clamping voltage; if not, selecting another reference electromotive force for detection; when the reference electromotive force is selected, detection and analysis are carried out backward turn by turn from the first turn coil on the same branch according to the same phase; if the voltage at the fault position is not adjusted to be 0 after the coils on one branch are searched and detected, analyzing the coils on the other branch in the same phase until the voltage at the fault coil position is adjusted to be 0. The voltage at the fault coil can be adjusted to be 0 through the invention, and the arc extinction problem of the single-phase earth fault is solved.
Description
Technical Field
The invention belongs to the technical field of generators, and particularly relates to an arc extinction method for a single-phase earth fault of a generator stator winding.
Background
Stator winding single-phase earth faults are the most common fault type of large generators and are often caused by insulation breakage between the stator winding and the core. Large-scale generating set at home and abroad generally adopts a neutral point non-effective grounding mode, the single-phase grounding fault current is limited within an allowable range, a protection unit only sends signals and does not trip, and the generator can continuously run for a period of time with faults, so that the energy balance of a power system is guaranteed. However, the zero sequence impedance of the neutral point non-effective grounding system is large, the fault signal is weak, and the arc extinction of the single-phase grounding fault of the stator winding is very difficult. During the fault period, the repeated charge and discharge process of the earth capacitor causes intermittent reignition of fault electric arc, fault current damages stator insulation, burns stator iron cores and possibly develops into more serious short-circuit fault, and the accident range is enlarged.
At present, two grounding modes of grounding through high resistance and grounding through arc suppression coils (resonance grounding) are commonly adopted by large generators in various countries in the world. The arc extinction idea of the method is to compensate or reduce the grounding fault current by increasing the zero sequence impedance of the system, thereby reducing the physical damage of the fault arc to the machine body. However, the method can only compensate the power frequency reactive residual current of the fault point, and cannot compensate the active current and the harmonic current; and the actual generator operation condition is complicated and changeable, the stator winding earth fault condition is different, and the earth current dynamic change during the fault period, so even if need realize the compensation control of power frequency reactive residual current completely also very difficult.
Different insights exist in countries around the world in defining the allowable value of the ground fault current for stator windings of large generators at different times. In the first half of the twentieth century, some european countries represented by germany and soviet union studied the burning loss of the stator core of the generator under different grounding current conditions, and the allowable value of the fault current was set to 20A to 5A in sequence, but the possibility of slight burning loss of the stator core was not excluded. In order to protect the stator core, stator winding ground fault experiments are carried out in the early stage of the 20 th century 70 th by the Czech dynamic research institute, and the ground current is recommended to be limited to 1.0-1.5A. The 5A safety current standard proposed by the Soviet Union has been used in the early days of China, and the allowable current value is discussed and revised by twenty-five key requirements for preventing major accidents in power production issued by the national grid company in 2000, and the specific requirements are shown in Table 1. It can be seen from the above development process that as the capacity of the single generator is continuously increased, the core position of the power supply safety in energy systems of various countries is strengthened, and the requirements on the safety index and the safe operation domain of the generator are increasingly stringent.
TABLE 1 Generator stator winding single-phase earth fault safe earth current allowed value
For hydrogen-cooled generator, 2.5A can be taken
In summary, the two grounding methods of high-resistance grounding and arc suppression coil grounding (resonance grounding) commonly adopted by the neutral point of the large-scale generator can compensate or reduce a part of grounding fault current, and meanwhile, the neutral point grounding branch circuit provides a discharge channel for transient energy excited by faults in the system, so that transient overvoltage is suppressed. However, the existing measures can not guarantee that the fault current is limited below the 1A standard required by regulations under the condition of high-resistance to low-resistance ground faults, and the existing measures can not treat intermittent arc ground faults.
Disclosure of Invention
The invention aims to provide a generator stator winding single-phase earth fault arc extinction method, which restrains the voltage of a stator winding fault coil to zero, destroys fault arc restrike conditions, solves the problem of intermittent arc light earth fault, reduces the potential safety hazard of equipment, avoids frequent shutdown and unplanned shutdown maintenance, realizes operation with fault, and effectively prevents the occurrence of generator stator fault burnout and unit off-grid accidents; meanwhile, the invention ensures that the voltage at the fault coil is zero, no fault current exists, the potential safety hazard of equipment is reduced, frequent shutdown and unplanned shutdown maintenance are avoided, the fault operation is realized, the occurrence of fault burning of a generator stator and the occurrence of grid disconnection accidents are effectively prevented, and the fault current is far lower than the current value 1A allowed by the existing regulations.
A generator stator winding single-phase earth fault arc extinction method comprises the following steps:
step 1: monitoring zero sequence voltage of a neutral point of a generator stator in real time, judging whether a stator winding ground fault occurs or not based on the zero sequence voltage of the neutral point, if so, acquiring a fault phase and executing the step 2;
step 2: acquiring a reference electromotive force and applying a clamping voltage based on the reference electromotive force so that the neutral point voltage is adjusted to the clamping voltage;
wherein, the electromotive force from the tail end of the first turn coil on any stator branch on the fault phase to the neutral point is taken as the reference electromotive force in the first detection,
connecting a controllable voltage source to a neutral point for applying a clamping voltage, wherein the relation between the regulated neutral point voltage and the current reference electromotive force is as follows:
in the formula (I), the compound is shown in the specification,
is the adjusted neutral point voltage and is,
is the electromotive force from the end of the mth turn coil on the d-th branch to the neutral point on the failed phase;
after the neutral point of the generator is clamped to be the calculated voltage value, the terminal voltage of the coil corresponding to the reference electromotive force is clamped to be zero;
and step 3: detecting a voltage value at a stator fault coil after delaying the preset first power frequency duration, judging whether the voltage value at the stator fault coil is zero, if so, selecting the clamping voltage of the current neutral point of the generator as the optimal clamping voltage, and executing the step 5; if not, executing step 4;
and 4, step 4: judging whether the number of the coil turns corresponding to the reference electromotive force is less than or equal to the total number of turns of the coil windings on the same stator branch, if so, selecting the electromotive force from the tail end of the next coil turn to a neutral point as the next reference electromotive force in sequence, and returning to the step 2; otherwise, judging whether the coils on the remaining branches are not detected, if so, selecting the electromotive force from the tail end of the first turn coil on one branch to the neutral point as the reference electromotive force from any remaining branch, and returning to the step 2; otherwise, detecting again;
and 5: delaying the preset second power frequency duration, detecting the zero sequence voltage of the neutral point of the generator stator, and judging whether the stator winding earth fault occurs again, if so, the stator winding earth fault is a permanent earth fault, keeping the optimal clamped voltage of the neutral point of the generator, otherwise, stopping clamping the central voltage.
According to the principle of a generator stator, each branch of a stator winding is formed by connecting single-turn coils in series, and single-phase earth faults of the generator stator winding occur on the coils of the stator branches. Wherein the voltage at the fault coil is considered as the fault voltage. However, the failure point is unknown, and therefore, it is necessary to search for a coil failure point. Based on that each branch of the stator is formed by connecting single-turn coils in series, the electromotive force from the tail end of the ith turn of coil on one branch to the neutral point is equal to the accumulated electromotive force of the previous i turns of coils on the same branch, the invention adopts a mode of searching from the first turn of coil to the back in sequence aiming at each branch, if the current coil is a fault point, the voltage at the fault coil is directly zero after the neutral point voltage is clamped, and the problem of the current single-phase grounding fault is solved; if the current coil is not a fault point, the clamped neutral point voltage is changed into the clamped voltage, and the voltage at the fault coil is changed due to the fact that the neutral point voltage is changed, wherein the voltage at the fault coil is reduced and is smaller than the initial voltage at the fault coil after the neutral point voltage is clamped during each search because the same branch is searched backwards from the first turn of coil in sequence, and equipment safety is facilitated.
Preferably, when the next detection branch is selected from the remaining branches, an adjacent next branch of the branch where the current reference electromotive force is located is taken as the next detection branch.
Preferably, the relationship between the voltage at the stator fault coil, the neutral point voltage and the electromotive force at the fault coil is as follows:
in the formula of UfIs the value of the voltage at the faulty coil of the stator,
the value of the electromotive force at the stator fault coil to the neutral point,
is the neutral point voltage.
As can be seen from the above equation, in order to decrease the voltage at the stator fault coil to 0, the sum of the electromotive force value to the neutral point at the stator fault coil and the neutral point voltage is made to be 0.
Preferably, whether the stator winding ground fault occurs is judged based on the zero sequence voltage of the neutral point to judge whether the zero sequence voltage is larger than a zero sequence voltage starting threshold value Ut(ii) a If the voltage is larger than the preset value, the stator winding is in ground fault, otherwise, the stator winding is not in ground fault.
The start threshold may be set according to actual conditions, for example, 15% of the phase voltage.
Advantageous effects
1. The invention restrains the voltage of the fault point of the stator winding coil to zero, destroys the arc restriking condition of the stator fault, solves the problem of intermittent arc grounding fault, reduces the potential safety hazard of the equipment, avoids frequent shutdown and unplanned shutdown maintenance, and realizes fault operation to effectively prevent the occurrence of the fault burning of the generator stator and the unit off-line accident.
2. The invention reduces the voltage at the fault coil to zero, thereby having no fault current, realizing the full compensation of the earth fault current, not only compensating the power frequency reactive residual current at the fault point, but also compensating the active current and the harmonic current, and leading the fault current to be far lower than the current value allowed by the existing regulations by less than 1A.
3. Under the condition that the prior art cannot accurately locate the fault point, the searching method provided by the invention can quickly search and determine the clamped value of the fault neutral point to the ground voltage and achieve the effect of accurately inhibiting the fault current of the stator. In the whole process of searching the optimal neutral point clamping voltage value, the voltage value at the fault coil is always reduced, namely is smaller than the initial voltage at the fault coil, and the arc burning at the fault coil cannot be accelerated to prevent the existing insulation level at the fault coil from being further damaged.
Drawings
FIG. 1 is a diagram of a generator internal stator winding single phase ground fault configuration;
FIG. 2 is a graph of generator stator neutral point voltage phasors before and after regulation;
FIG. 3 is a single coil equivalent structure diagram of a generator stator branch winding;
FIG. 4 is a generator stator fault phase winding equivalent structure diagram;
FIG. 5 is a phasor relationship of coil electromotive force to neutral point voltage;
fig. 6 is a flow chart of a generator stator winding single-phase ground fault arc extinction method.
Detailed Description
The present invention will be further described with reference to the following examples.
The invention provides a generator stator winding single-phase earth fault arc extinction method which is mainly based on reasoning on the relation of the terminal voltage of each branch coil of a generator stator, the voltage of a neutral point and the electromotive force from the coil terminal to the neutral point. The theoretical relationship of the coil end voltage, the neutral point voltage, and the electromotive force of the coil end to the neutral point will be described first.
A controllable flexible voltage source is connected to a neutral point of the stator,
the generator stator protection equivalent topology for a flexible controllable voltage source connected to the neutral point is shown in fig. 1, wherein
For three-phase electromotive force, Z, of the stator winding of the generatorPIs the equivalent ground impedance value of the neutral point of the generator, Ca、Cb、CcRespectively three-phase stator winding earth capacitance, Ct is the capacitance value of external equipment at machine end to earth, and the single-phase stator winding is setThe earth fault occurs in phase A, the transition resistance is Rf。
A. b, C the terminal voltages of the cameras are:
in the formula
Voltage to earth, zero sequence voltage for each camera end of stator
Can be expressed as
The relation between the stator neutral point grounding mode zero sequence voltage and the neutral point voltage is as follows:
when the neutral point is connected to the controllable flexible voltage source,
the zero sequence controllable voltage source is added to the neutral point, so that the voltage of the neutral point can be forced to be a required voltage value. As shown in fig. 1, the relationship between the neutral point voltage and the applied controllable voltage source can be expressed as follows:
because the stator winding of the generator is formed by connecting a plurality of turns of coils in series, the earth electromotive force of each turn of coil is equivalent to the earth voltage value of a terminal capacitor, and when a certain position of the stator has an earth fault, the induced electromotive force from the tail end of the fault coil to a neutral point is
The voltage to ground at the fault coil can be expressed as:
as can be seen from fig. 1, the sum of the neutral point voltage and the electromotive force from the end of the fault coil to the neutral point is equal to the voltage to ground at the fault coil. When the permanent earth fault electric arc is stably burnt, the voltage at the fault coil is kept in a relatively stable state, and when the faults are processed, the earth fault is essentially eliminated, so that the voltage at the fault coil meets the requirement
Due to the fact that
Only need to adjust the induced electromotive force forced component of the generator stator
Then, transform equation (6) to obtain:
the neutral point position after movement is represented by N', namely the voltage source forces the neutral point voltage
And (4) offsetting, and satisfying the formula (7). From the above, it can be seen from the equations (6) and (7) that in order to make the fault voltage zero, the neutral point voltage needs to be adjusted, i.e. clamped by the controllable flexible voltage source connected to the neutral point. Because the fault point is unknown, namely which coil of the phase has an unknown fault, the invention sequentially processes the coils on the branches based on the mechanism, namely sequentially verifies the coils by adopting the formula (6) and the formula (7) aiming at the coils, if the clamping voltage aiming at the current coil leads the voltage at the tail end of the current coil to be zero and the fault voltage is also zero at the moment, the current coil is a fault coil; if the voltage at the fault coil is not zero, processing the next coil turn; according to this logic, the individual coils are processed in turn.
For a clearer description of equations (6) and (7), the phasor relationship of the neutral voltage to the voltage at the fault winding of the stator ground is shown in figure 2, moving the neutral potential is about to occur
Flexible control is carried out, and neutral point electromotive force is changed into
When (as a vector of the diagram O-N '), shift N point to N', effectively shifting the potential
And clamping to 0, and successfully eliminating the stator single-phase earth fault current.
Aiming at the problem of each coil on the stator branch, the invention adopts a generator stator capacitance quasi-distributed model to describe, the equivalent structure of a single-turn coil is shown in figure 3, Ri、LiRespectively the resistance and the inductance of a single-turn coil of the stator winding,
the electromotive force generated for the single turn coil,
is the ith turnThe voltage of the coil is connected to the ground,
is the neutral point voltage. CiIs a single turn coil capacitance to ground. As each branch of the stator is formed by connecting single-turn coils in series, the equivalent model of the A-phase winding of the fault phase is shown in figure 4, wherein the ith-turn coil is connected with the voltage to the ground
I.e. i turns of coils are connected in series to the voltage to ground before the phase A fault branch,
the electromotive force of the tail end to a neutral point when the phase k turn coil of the A phase fails, m is the total turn number of the phase A branch stator winding, and CaiIs the capacitance value of the A phase equivalent single-turn coil to the ground,
is the total phase a current to ground.
For the salient pole and non-salient pole generator, if the k-th winding coil has ground fault, the end of the k-th winding coil generates electromotive force to the neutral point
Can be represented as:
as can be seen from the above equation (6), the following is satisfied:
is the voltage to ground at the end of the k-th turn of the coil (voltage at the fault coil). In order to make the fault coil zero, the neutral point voltage and the end-to-neutral point electromotive force of the k-th coil are needed to be enabled
The sum is zero.
As can be seen from the above equation (8), the electromotive force from the coil end to the neutral point on the same branch is related to the position of the coil on the same branch, and therefore, there is a certain rule for the coil search path on the same branch. The invention adopts the sequence of searching from the first circle of the neutral point to the last circle of the connecting machine section for the coils on the same branch, so that the voltage at the fault coil is smaller than the initial voltage at the fault coil in the searching process, and the reasons are as follows: as shown in fig. 5, the stator phase winding is formed by connecting coils in series, and the electromotive force from the end of each turn to the neutral point is superimposed by the electromotive force of the front coil, in the figure,
is formed by
Are superposed. Assuming that the end of the third coil is the fault coil, if the search is started from the first coil, the neutral point voltage is adjusted to the reverse phase voltage of the electromotive force from the end of the first coil to the neutral point
Then the clamped neutral point voltage and the electromotive force from the end of the third turn coil to the neutral point
The sum of the vectors is
It is also the voltage at the fault coil at this time, which is much less than the terminal voltage on the third turn before neutral clamping
I.e. less than the voltage at the fault coil after clamping; since the voltage at the clamped fault coil is not 0, further searching is carried out to regulate the neutral point voltageThe electromotive force from the end of the second coil to the neutral point
The neutral point voltage of the third coil, and the end-to-neutral point electromotive force of the third coil
Is a sum of vectors of
I.e. the present voltage at the fault coil, whereby the voltage at the fault coil is known from
Is lowered to
I.e. searching backwards from the first turn of the coil will cause the voltage at the faulty coil to decrease. If the search is started from the seventh coil, the neutral point is adjusted to be the reverse phase voltage of the electromotive force from the tail end of the seventh coil to the neutral point
When the electromotive force at the fault is clamped to
As can be seen from FIG. 5, a
The formed vector triangle shows that the electromotive force at the clamped fault position is
Greater than the clamping voltage not applied
(side length relationship θ according to angle correspondence in triangle [ ]i1＜θi2) So that a search from back to front cannot be guaranteedThe sequence can always reduce the voltage value at the fault, so the invention selects the sequence from the first coil turn to the back on the same branch for analysis.
As shown in fig. 6, the present invention provides a method for extinguishing a single-phase ground fault of a generator stator winding based on the above principle, which comprises the following steps:
step 1: zero sequence voltage U for monitoring neutral point of generator stator in real time0Judging whether a stator winding ground fault occurs or not based on the zero sequence voltage of the neutral point, if so, acquiring a fault phase and executing the step 2; wherein the faulty phase can be determined using prior art techniques.
Step 2: acquiring a reference electromotive force and applying a clamping voltage based on the reference electromotive force so that the neutral point voltage is adjusted to the clamping voltage;
wherein, the electromotive force from the tail end of the first turn coil on any stator branch on the fault phase to the neutral point is taken as the reference electromotive force in the first detection,
the controlled voltage source is connected to the neutral point to apply the clamping voltage, and the relation between the regulated neutral point voltage and the current reference electromotive force is as follows:
in the formula (I), the compound is shown in the specification,
is the adjusted neutral point voltage and is,
is the electromotive force from the end of the mth turn coil on the d-th branch to the neutral point on the failed phase;
after a voltage value is calculated by clamping a neutral point of the generator, clamping the tail end voltage of the coil corresponding to the reference electromotive force to be zero;
and step 3: detecting a voltage value at a stator fault coil after delaying the preset first power frequency duration, judging whether the voltage value at the stator fault coil is zero, if so, selecting the voltage of a neutral point of a clamped generator as an optimal clamped voltage, and executing the step 5; if not, executing step 4; wherein the voltage value at the stator fault coil can be detected based on the prior art.
And 4, step 4: judging whether the number of the coil turns corresponding to the reference electromotive force is less than or equal to the total number of turns n of the coil winding on the same stator branch, if so, selecting the electromotive force from the tail end of the next coil turn to a neutral point as the next reference electromotive force in sequence, and returning to the step 2; otherwise, judging whether the coils on the remaining branches are not detected, if so, selecting the electromotive force from the tail end of the first turn coil on one branch to the neutral point as the reference electromotive force from any remaining branch, and returning to the step 2; otherwise, re-detection is performed.
In this embodiment, it is preferable that, when the next detection branch is selected from the remaining branches, the next detection analysis is performed using the branch adjacent to the branch where the current reference electromotive force is located.
And 5: delaying the preset second power frequency duration, detecting the zero sequence voltage of the neutral point of the generator stator, and judging whether the stator winding earth fault occurs again, if so, the stator winding earth fault is a permanent earth fault, keeping the optimal clamped voltage of the neutral point of the generator, otherwise, stopping clamping the central voltage.
The preset first power frequency time length and the preset second power frequency time length are both empirical values, and the invention is not particularly limited to this.
In conclusion, the invention restrains the voltage of the fault point of the stator winding to be zero, thereby destroying the reignition condition of the stator fault arc, and fundamentally eliminating the earth fault current to realize the arc extinction of the earth fault of the stator winding; because the earth fault current generated at the earth fault re-earth point comprises reactive, active and harmonic components and the like, the invention can realize the full compensation of the earth fault current of the stator winding, not only compensate the power frequency reactive residual current of the fault point, but also compensate the active current and the harmonic current, realize the full compensation of the earth fault current, eliminate the earth fault or inhibit the further development of the single-phase earth fault of the generator, thereby reducing the fault current to be below the current value 1A allowed by the existing regulations. In addition, the existing generator stator winding ground fault positioning technology mainly adopts a means of measuring the included angle between the coil tail end electromotive force amplitude at a fault coil and a reference phasor to position a fault point, and in order to avoid the fault coil processing deviation caused by positioning error, the fault coil processing fast search algorithm can quickly determine the current value injected by processing the fault and achieve the accurate suppression of processing the stator fault current. During the whole process of searching the optimal clamping voltage value, the voltage value at the fault coil is smaller than the initial voltage at the fault coil, so that the arc burning at the fault coil is not accelerated to prevent the existing insulation level at the fault coil from being further damaged.
It should be emphasized that the examples described herein are illustrative and not restrictive, and thus the invention is not to be limited to the examples described herein, but rather to other embodiments that may be devised by those skilled in the art based on the teachings herein, and that various modifications, alterations, and substitutions are possible without departing from the spirit and scope of the present invention.
Claims (5)
1. A generator stator winding single-phase earth fault arc extinction method is characterized in that: the method comprises the following steps:
step 1: monitoring zero sequence voltage of a neutral point of a generator stator in real time, judging whether a stator winding ground fault occurs or not based on the zero sequence voltage of the neutral point, if so, acquiring a fault phase and executing the step 2;
and 2, step: acquiring a reference electromotive force and applying a clamping voltage based on the reference electromotive force so that the neutral point voltage is adjusted to the clamping voltage;
wherein, the electromotive force from the tail end of the first turn coil on any stator branch on the fault phase to the neutral point is taken as the reference electromotive force in the first detection,
acquiring a current reference electromotive force, and accessing a controllable voltage source at a neutral point based on the current reference electromotive force for applying a clamping voltage to adjust the neutral point voltage, wherein the relationship between the adjusted neutral point voltage and the current reference electromotive force satisfies:
in the formula (I), the compound is shown in the specification,
is the adjusted neutral point voltage and is,
is the electromotive force from the end of the mth turn coil on the d-th branch to the neutral point on the failed phase;
and step 3: delaying the preset first power frequency duration, detecting the voltage value of the stator fault coil, judging whether the voltage value of the stator fault coil is zero, if so, selecting the voltage of the neutral point of the clamped generator as the optimal clamped voltage, and executing the step 5; if not, executing step 4;
and 4, step 4: judging whether the number of the coil turns corresponding to the reference electromotive force is less than or equal to the total number of turns of the coil windings on the same stator branch, if so, selecting the electromotive force from the tail end of the next coil turn to a neutral point as the next reference electromotive force in sequence, and returning to the step 2; otherwise, judging whether the coils on the remaining branches are not detected, if so, selecting the electromotive force from the tail end of the first turn coil on one branch to the neutral point as the reference electromotive force from any remaining branch, and returning to the step 2; otherwise, detecting again;
and 5: and delaying the preset second power frequency duration, detecting the zero sequence voltage of the neutral point of the generator stator and judging whether the stator winding ground fault exists or not, if so, the stator winding ground fault is a permanent ground fault, and keeping the optimal clamped voltage of the neutral point of the generator, otherwise, stopping clamping the neutral point voltage.
2. The method of claim 1, wherein: when the next detection branch is selected from the remaining branches, the next branch adjacent to the branch where the current reference electromotive force is located is taken as the next detection branch.
3. The method of claim 1, wherein: the relationship among the voltage at the fault coil of the stator, the voltage at the neutral point and the electromotive force at the fault coil is as follows:
in the formula of UfIs the value of the voltage at the faulty coil of the stator,
is the value of the electromotive force at the stator fault coil,
is the neutral point voltage.
4. The method of claim 1, wherein: judging whether a stator winding ground fault occurs or not based on the zero sequence voltage of the neutral point, namely judging whether the zero sequence voltage is larger than a zero sequence voltage starting threshold value U or nott(ii) a If the voltage is larger than the preset value, the stator winding is in ground fault, otherwise, the stator winding is not in ground fault.
5. The method of claim 4, wherein: the starting threshold value UtIs 15% of the phase voltage.
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