JPH0783541B2 - Relay overload protection relay system - Google Patents

Description

TECHNICAL FIELD The present invention relates to a transformer overload protection relay system, and more particularly, to a transformer capacity of a main transformer of a power system when the overload varies. The present invention relates to a relay overload protection relay method that enables overload operation up to the limit.

(Prior Art) The overload characteristic of this type of transformer is determined by the winding maximum temperature curve of the transformer and the life loss curve due to the winding temperature, and is generally represented by FIG. In the figure, the vertical axis K on the graph indicates the overload rate (= P / _PN = I / _IN ). Note that P and I are actual load values, and P _N and I _N are rated load values, respectively. In the past, the magnitude of the secondary current of the transformer was grasped step by step by overload protection relays such as three overcurrent relays with different set values, and by combining it with a timer, the operating state could be in the dangerous range of overload characteristics. The protective relay method was adopted to cut off the load before reaching.

(Problems to be solved by the invention) However, according to this, there is a possibility that the overload near the operating value limit of the overcurrent relay or the like cannot be interrupted, and the fluctuation of the transformer load characteristic due to the load fluctuation is considered. Since it is necessary to set the characteristics of the protection relay on the safer side than the overload characteristics, it may be necessary to cut off the overload in a short time even for an overload that has a time margin for the transformer. There is a disadvantage that the overload operation in the allowable range cannot be performed sufficiently and the transformer capacity cannot be utilized to the maximum.

The present invention has been proposed in order to solve the above problems, and an object thereof is to detect that a transformer is overloaded from a change in input current to a digital overload protection relay, From that time, the time required to reach the maximum limit temperature of the transformer winding is predicted and calculated by a predetermined temperature judgment formula, and within this time (allowable overload time), operation is performed in the overload state without causing load shedding etc. The characteristics of the protection relay are approximated to those of the transformer overload so that the fluctuation of the overload can be achieved by correcting the temperature settling value of the temperature judgment formula when the transformer overload changes. An object of the present invention is to provide a transformer overload protection relay method that enables the stable supply of electric power by always maximizing the transformer capacity without impairing the life of the transformer.

(Means for solving problems) Time constant of oil temperature change of transformer to be protected, rated oil temperature,
Based on the preload rate and the overload rate before becoming overloaded,
Obtain a temperature judgment formula that compares the added value of the transformer winding temperature as a function of time and its differential value with the set value of the winding maximum temperature, and use this temperature judgment formula to determine the maximum winding temperature The allowable overload time until the value is reached is calculated.

If the overload fluctuates, find the winding maximum temperature at that point and subtract this value from the initial setting value to correct the setpoint value. Predictively calculate the time to reach the set value of.

(Operation) The present invention focuses on the fact that overload operation that maximizes the capability of the transformer is possible if the characteristics of the protection relay that approximates the overload characteristic of the transformer are obtained. The time from when the transformer overload is detected based on the input current of the protection relay to when the winding maximum temperature of the transformer reaches the limit value is calculated by the convergence calculation method. Predict calculation. In this predictive calculation, the temperature settling value corrected according to the load change is used, and after calculating the final arrival time, the time obtained by subtracting a certain margin time is calculated and the overload operation within this time is calculated. Allow and protect the transformer by issuing an alarm or breaking the load after that time.

As a result, even if a load change occurs, it is possible to realize a protection relay characteristic that is close to the overload characteristic of the transformer.

(Example) An example of the present invention will be described below with reference to the drawings. First,
The overload characteristics of the transformer to be protected are determined by the maximum temperature characteristics of the transformer windings and the life loss characteristics due to the winding temperature.Therefore, the protection relay characteristics to be obtained must also exceed these characteristics. It is necessary to approximate the load characteristics.

Beginning, Technical Report of the Institute of Electrical Engineers of Japan (Part I) No. 99 (Showa 46
Monthly issue), Part I (No. 143) (issued in November 1986), "oil-filled transformer motion guidelines," as is clear, steady state when a constant load P is applied to the transformer The winding highest temperature Θ _{H at} is represented by Θ _H = Θ _a + Θ ₀ + Θ _g (° C.) (1). Where Θ ₀ = Θ _ON {(K ² · R + 1) / (R + 1)} ^m (deg.)
_{...... (2) Θ g = Θg} N · K 2n (deg.) ...... a (3), the best the Θa transformers equivalent ambient temperature (refrigerant temperature such as air), theta ₀ is transformer oil The oil temperature rise, Θg, shows the difference between the maximum winding temperature and the maximum oil temperature, Θ _ON shows the value of Θ ₀ under rated load, and Θ _gN shows the value of Θ _g under rated load. . Furthermore, K is the overload rate (= P / _PN = I / _IN : P, I is the actual load value, P _N and _IN are the rated load values, but the voltage is constant), and R is the The ratio of load loss and no-load loss at rated load, m and n are constants (≈0.8) determined by the transformer cooling method.

Next, when the load changes from P ₁ (= K ₁ P _N ) to P ₂ (= K ₂ P _N ), the maximum oil temperature rise Θ ₀ in the transient state and the difference between the winding maximum temperature and the maximum oil temperature Θg is as follows.

Here, Θ _Ou is the maximum oil temperature rise at any time t after the load changes from P ₁ to P ₂ , and Θ _Oi is the maximum oil temperature rise at t = 0 (when the load changes). , Θ _gu is the difference between the winding maximum temperature and the maximum oil temperature at any time t, Θ _gi is the difference between the winding maximum temperature and the maximum oil temperature at t = 0, and τ ₀
Indicates the time constant of change in oil temperature, and τ _g indicates the time constant of change in temperature difference between the winding and oil.

The life loss V ₁ when operating for ₁ hour at the coil maximum temperature Θ ₁ (° C) is as follows. This life loss is V ₁ = 1
When it becomes, it means that the life has come.

V ₁ = h ₁ / Y ₁ = (1 / Y ₀ ) ・ {h ₁ / (Y ₁ / Y ₀ )} …… (6) where Y ₀ is the interlocked operation at the maximum temperature of 95 ° C. , Ie, the normal life, and Y ₁ indicates the life at the time of continuous operation at the maximum temperature Θ ₁ , and the relational expression between the two is Becomes

In addition, the maximum temperature Θ changes with time and Θ = f (t)
The life loss V expressed by is as follows.

V = (1 / Y ₀ ) ∫ {1 / (Y / Y ₀ )} dt = (1 / Y ₀ ) ∫ {eb ^(θ-95) } dt (8) where Y is the highest point Shows the life when continuously operated at temperature Θ.

Then, according to the equations (1) to (3) and (8),
The winding maximum temperature characteristics of the transformer and the life loss characteristics of the transformer due to the winding temperature are required, and the characteristics of the protection relay should be approximated to the transformer overload characteristics determined comprehensively by these. However, since it is difficult to calculate the life loss online, it is assumed here that only the winding maximum temperature characteristics are used to predict and calculate the time to reach the winding maximum temperature limit point. For this, it was decided to consider this by correcting the settling value described later.

That is, the calculation principle by the protection relay will be described in detail below. First, when the subsequent steady-state winding maximum temperature Θ _H when the load changes is expressed as a function of time t, from the above equation (1), _H (t) = Θ _a + Θ ₀ (t) + Θ _g (t) (9) Substituting equations (4) and (5) above, So if you organize this, Becomes

Substituting equations (2) and (3) above this equation, Note that K _i represents a load factor (pre-load factor = I / I _N ) as a ratio of the current I _i before the load change and the rated current I _N. Here, assuming that m = n = 0.8 from the operation guideline of the oil-filled transformer, Therefore, this equation (13) becomes the ideal winding maximum temperature characteristic of the transformer. Also, the life loss of the transformer is Can be sought by.

Now, according to the ideal temperature characteristics of the winding maximum point shown in equation (13), the winding maximum point temperature Θ _H (t) due to the overload factor K detected from the input current of the protection relay becomes the settling value M. It is possible to obtain the time t (time from the occurrence of overload) until they become equal.

However, since the above equation (13) is complicated and has a heavy load when performing arithmetic using a microcomputer, and it is expected that the calculation time will be prolonged, the equation (13) is further simplified in the present invention. An arithmetic expression shall be used.

That is, according to the above equation (9), Θ _H (t) = Θ _a + Θ ₀ (t) + Θ _g (t), of which Θ ₀ (t) and Θ _g (t) are divided into Ask like. First, by substituting Θ _ON based on equation (2) into equation (4), Becomes R >> 1, so if you erase it, Here, when approximating 1.6 to the power of 2, And the ratio of Θ ₀ (t) to Θ _ON is obtained. In addition,
K _u represents the load factor (= I _u / I _N ) as the ratio of the current I _u after the load change and the rated current I _N.

Next, Θ _g (t) is calculated in the same manner as Θ ₀ (t). That is, by substituting Θ _gN based on equation (3) into equation (5), When the 1.6th power is approximated by the second power, Therefore, when τ _{g is set} to almost 0 as compared with τ ₀ , it can be approximated as Θ _g (t) / Θ _{gN ≈K u} ² (18).

Then, considering the winding maximum temperature Θ _HN at the rated load, Θ _HN = Θ _a + Θ _ON + Θ _gN , that is, Θ _{HN −} Θ _a =
Since Θ _ON + Θ _gN , and Θ _H = Θ _a + Θ ₀ + Θ _g to Θ _H − Θ _a = Θ ₀ + Θ _g , the relation normalized by (Θ _{HN −} Θ _a ) is obtained. Considering the line temperature (t), (t) = (Θ _H -Θ _a ) / (Θ _NH -Θ _a ) = (Θ ₀ + Θ _g ) / ((Θ _a + Θ _ON + Θ _gN ) -Θ _a } = (Θ ₀ + Θ _g ) / (Θ _ON + Θ _gN ) ... (19) This equation (19) is expressed by (t) = {Θ _ON / (Θ _ON + Θ _gN )} ・ (Θ ₀ / Θ _ON ) + {Θ _gN / (Θ _ON + Θ _gN )} ・ (Θ _g / Θ _gN ). Then, substituting the equations (16) and (18) into Becomes Here, if Θ _ON / (Θ _ON + Θ _gN ) = k (rated oil temperature ratio), Becomes

This equation (21) shows the normalized winding temperature (t) every moment when the load factor changes from K _i to K _u. Temperature Θ
It is possible to find the time t until _H (t) reaches the set value M.

That is, if overload occurs at time t ₀ in FIG. 1, (t) = M on the curve (t).
(Setting value) and the time T _M1 until it Mari time acceptable overload operation, since at time (t ₀ + T _M1) it is apparent that to reach the overload limit point of the transformer, the time (t ₀
+ T _M1 ) Before + T _M1 ), it is sufficient to obtain the protection relay characteristic that issues a load cutoff command or alarm.

However, the inventors of the present invention not only (t),
The value d (t) / dt obtained by differentiating (t) with respect to time is calculated and added to (t), that is, (t) + d (t) / d
By using the judgment that t is the set value M, it was found that a protection relay characteristic more similar to the overload characteristic of the transformer can be obtained.

Differentiating the above equation (21), And (t) + d (t) / dt is as follows.

Therefore, at time t ₀ when the overload P ₁ occurs, K _i and K _u are detected from the input current to the protection relay, and these K _i , k _u and τ ₀ , k are substituted into the equation (23) and (T) + d
If (t) / dt is set equal to the settling value M, the time t (= which is the time until the winding maximum temperature reaches the settling value M on the assumption that the overload P ₁ continues as shown in FIG. 2). T _M ) can be calculated. That is, the temperature judgment formula The minimum t that satisfies

Here, the time (t ₀ + T _M ) is the time when the overload P ₁ approaches the dangerous range inside the overload characteristic of the transformer when the overload P ₁ continues to be constant from the time t _0. By determining the load permissible time t (= T _M ), it is possible to realize the protection relay characteristic that is close to the overload characteristic of the transformer.

In FIGS. 1 and 2, time T _N is a margin time for outputting a load shedding command before time (t ₀ + T _M ) in anticipation of safety.

Further, in FIGS. 1 and 2, it is assumed that overload occurs at time t ₀ (P ₁ / P _N > 100%), and the protection relay calculation is started at this point, but in reality, , The ratio of the input current I to the protection relay and the rated current I _N (= I / I _N ) is
Set value set appropriately in the range of 0% to 100% (eg 80
%) Is reached, the calculation is started.

Therefore, τ ₀ , M, k in the _equation (24), the margin time T _N , and the settling range of the calculation starting point I / I _N are as shown in Table 1 below.

In Table 1, by appropriately correcting the settling values of τ ₀ and k, the curve on the right side of equation (24), that is, (t) +
The characteristic of d (t) / 2t can be variously changed, and the protection relay characteristic in which the life loss characteristic is sufficiently taken into consideration in addition to the winding maximum temperature characteristic of the transformer can be realized.

Now, as mentioned above, it is possible to obtain the overload allowable time T _M by substituting various numerical values into the formula (24), but since the calculation formula becomes complicated, T _M is predicted by the convergence calculation method as described below. It is suitable to calculate. This convergence method is (24)
The time t in the formula is assumed to be in the range of 0 to, for example, 15 hours, and the time equivalent value t ^* corresponding to these upper and lower limits is set ^.
And compare with the settling value M in this range,
Time equivalent value t ^* when the right side of equation (24) becomes M or more
Is converted into real time t and time t (=
T _M ).

Here, the purpose of setting the upper limit of the time t to, for example, 15 hours is that it is assumed from the overload characteristics of the transformer that the maximum time during which the overload state can be continued is about 15 hours, After that, theoretically, it is possible to operate until infinity at the rated load (100%), so T _M converges to infinity.

As a concrete example of the convergent calculation method, the upper limit of the time t corresponds to "1280" as the time equivalent value t ^* and the lower limit corresponds to "2" as shown in FIG. At the same time, the method of calculating equation (24) by adopting the time equivalent value t ^* from the median of these upper and lower limits (≈640) in the order of 320,160,80 ... The time equivalent value t ^* when the right side becomes equal to or comes closest to the settling value M is converted to the real time t to obtain T _M. As a result, the time T _M can be calculated by a maximum of 10 calculations.

Next, when the load fluctuates, the overload characteristic of the transformer also fluctuates. Therefore, in the present invention, the characteristic of the protection relay is also changed according to the load fluctuation. That is, for example, if the overload P ₁ that has continued to be constant from time t ₀ is
If changes to P ₂ at t _1, again at the time t _1, it is necessary to predict calculating overload permissible time by temperature determination expressions described above occurs, it determined using a temperature set-point M so far in this Is performed, the settling value M is restarted from the point of time t ₁ when the overload changes
The time required to reach is calculated. As a result, there is an inconvenience that the dangerous region of the transformer overload characteristic is reached before the newly calculated overload allowable time elapses.

FIG. 4 and FIG. 5 show the load fluctuation and the change in the normalized winding temperature due to the load fluctuation. First, in FIG. 4, the load P changes to P ₁ at time t ₀ (overload rate
K ₁ ), after the protection relay starts calculation, at time t ₁ when a certain time TP ₁ has elapsed, the load further increases to P ₂ (overload rate K ₂ ) and at a time t ₂ when a certain time TP ₂ has elapsed. in the case where the load has occurred load variation such as returns to P _1, a load P is changed to P ₁ Fig. 5 at time t ₀ (overload factor
K ₁ ), the load decreases to P ₃ (overload rate K ₃ ) at time t ₁ when a certain time TP ₁ has passed after the protection relay started calculation, and time t ₂ when a certain time TP ₂ has passed In each case, the load fluctuations such that the load returns to P ₁ are shown.

In these figures, the original When continued remains overload factor K _1, after the same time T _M has elapsed and also FIG. 2 cases (t) is the initial settling without excessive load varies It can be seen that the value M is reached. However, since (t) changes with load changes, when calculating the allowable overload time, calculate (t) at the time when the load changes and subtract this value from the initial set value. It is necessary to perform the calculation again with the set value as a new set value.

Therefore, in the case of FIG. 4, time (TP ₁ + TP ₂ + T _M ″)
Is the overload permissible time corresponding to T _M in FIGS. 1 and 2, and the load increased halfway compared to the case in FIG. 2,
It is expected that the overload limit will be reached earlier than T _M. Further, in the case of FIG. 5, the load as compared to Figure 2 is reduced in the middle, T _M slower time than (TP
₁ + TP ₂ + T _M ″) is predicted to be reached.

Next, the method of calculating the winding temperature and the method of correcting the set value due to load fluctuations (changes in the input current to the protection relay) will be described in detail.

First, as a basic temperature calculation, a current value at an overload point is set as a lower limit value, a current is set as an upper limit value, and a difference between the current value and a time after the overload point is exceeded (for example, in FIG. 4 and FIG. 5). Calculate the current winding temperature from the time equivalent value corresponding to TP ₁ ). In the calculation, the above equations (21) and (23) are used. Then, the allowable overload time is obtained by using the value obtained by subtracting the winding temperature thus obtained from the initial set value M as a new set value.

For example, as shown in FIG. 6, when the overload is a change that increases stepwise and then maintains a constant value, the winding temperature (t) that increases every moment is subtracted from the set value M. A new settling value is obtained, and in the example shown in the figure, the temperature changes b ₁ and b ₂ are equal to a ₁ and a ₂ , respectively.

When the overload increases once and returns to the overload point as shown in FIG. 7, the winding temperature (t) rises as the load increases, and the settling value decreases accordingly. In order to return the increased winding temperature (t) to the temperature before exceeding the overload point, the winding temperature P at the time when the overload is eliminated is stored, and the temperature corresponding to the temperature lower than the temperature P is stored. The current setting value is obtained by subtracting the temperature after subtracting the setting value from the setting value M. That is, at the time when the temperature P becomes equal to the temperature lower than the temperature P, the settling value at that time becomes the initial settling value M. Here, the time until the temperature P becomes equal to the temperature lower than this (t _{b in} FIG. 7)
Is equal to the overload point exceeds the time t _a.

Further, the correction of the settling value by the rise and fall of the winding temperature corresponding to the change of the load current is not performed at the rise and fall of the load current, but when the current load current continues as shown in FIG. The winding final temperature P ′ that is expected to be reached is calculated, and this winding final temperature P ′ is compared with the current winding temperature (t). If the current winding temperature (t) is lower, the settling value is set. Is corrected in the decreasing direction (the winding temperature is increasing), and conversely, if the winding temperature (t) is higher, the settling value is corrected in the increasing direction (winding temperature is decreasing).

Next, FIG. 9 is a flowchart showing a procedure for correcting the settling value, and this procedure is constantly executed at a constant cycle. First, it is judged whether there is a change in the load current according to the input current to the protection relay (step S1). As a result, if there is a change in the current (when there is a load change), the rise and fall of the winding temperature are determined. Set the upper and lower limits of the current to be calculated (S2 of the same). If there is no change in current, the process proceeds to step S4 described below.

After setting the upper limit value and the lower limit value, the current winding temperature is calculated by the predetermined calculation formula as described above, and this is stored (at step S3). After that, the time-equivalent value from the time when the current changed last is detected regardless of the current change at the current time (S4).

Then, in the next step, it is judged whether or not the winding temperature rises or falls, that is, whether or not there is a temperature change (same as above).
S5), if there is no change, set the initial values for the upper limit value and the lower limit value in step S2 (at step S6), and if there is a change, proceed to step S7. I.e. step
After S5 or S6, for example, after the last current change,
It is determined whether 15 hours have passed (S7). here,
The value of 15 hours is set for convenience and is not particularly limited, and another value may be used as long as it is an appropriate time interval at which it can be considered that the change in the winding temperature has disappeared.

If 15 hours has already passed in step S7, it is determined that the temperature change will no longer occur due to the current, and the process proceeds to step S11 without performing the subsequent temperature calculation to create a settling value for the current temperature. And finish. If 15 hours has not passed, the final temperature (P 'in Fig. 8) predicted to finally reach the current value is compared with the current winding temperature, and the difference is calculated. The value of the corresponding temperature rise is calculated as a negative value (when the current winding temperature is lower than the final temperature P ') (at step S8).

If the current winding temperature is lower than the final temperature P ′, it is necessary to correct the settling value in the direction of increasing the temperature as described above, so the judgment result in the next step (S9) is If the result is affirmative, a new settling value for the current winding temperature is created in the direction of decreasing the settling value in step S11.

On the other hand, when the current winding temperature is higher than the final temperature P ', the settling value needs to be corrected in the direction of decreasing the temperature (method of increasing the settling value), so the result of the determination in step S9 is denied. , The calculated value in step S8 is canceled and a new value of the temperature decrease is calculated as a positive value (at step S10). Then, a new settling value for the current winding temperature is created in the direction of increasing the settling value.

Based on the settling value corrected in this way, the overload allowable time is obtained, and if the operation command such as load cutoff or alarm is obtained at the time when the allowance time T _N is reduced, the overload characteristic of the transformer according to the load fluctuation can be obtained. It is possible to obtain a protection relay characteristic close to

(Effect of the invention) As described above, according to the present invention, a temperature judgment formula that approximates the winding temperature curve of the transformer is obtained, and according to this judgment formula, for any overload, the transformer The time to reach the overload limit point is quickly and accurately predicted, the characteristics of the protection relay based on this prediction calculation are obtained, and the temperature settling value of the temperature judgment formula is corrected according to the load fluctuation. It is possible to always perform long-time overload operation by making maximum use of the transformer capacity, even in a system in which the frequency fluctuates frequently.

Therefore, there is an effect that the electric power can be more stably supplied to the load as compared with the related art.

[Brief description of drawings]

FIG. 1 is a graph showing a normalized winding temperature characteristic of a transformer, FIG. 2 is a graph for explaining a protective relay characteristic when an overload is constant, and FIG. 3 is an explanatory diagram of a convergence calculation method.
4 and 5 are graphs showing load fluctuations and normalized winding temperature characteristics, and FIGS. 6 to 8 are graphs showing changes in winding temperature and set values according to load fluctuations. Is a flow chart showing the procedure for correcting the settling value,
FIG. 10 is a graph for explaining the conventional overload protection relay characteristic.

Claims (1)

[Claims] 1. A time constant of a change in oil temperature of a transformer to be protected,
Based on the rated oil temperature ratio, the preload ratio before overload and the overload ratio, etc., the added value of the transformer winding temperature as a function of time and its derivative value is used as the set point of the winding maximum temperature. Calculate the temperature judgment formula to be compared, calculate the overload allowable time until the winding maximum temperature reaches the set value using this temperature judgment formula, and perform operations such as load cutoff before the overload allowable time elapses. In an overload protection relay method that obtains a command, obtain the winding maximum temperature that has changed due to the fluctuation of the overload, and subtract this winding maximum temperature from the set value to obtain a new set value. A relay overload protection relay method characterized by.