JPH0783540B2 - Relay overload protection relay method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a transformer overload protection relay method, 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 system 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 overload allowable time until the value is reached is calculated, an operation command such as load shedding is obtained before the overload allowable time elapses, and the settling value is corrected in accordance with the change in the overload.

(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 prediction calculation, the corrected temperature settling value calculated each time the load fluctuates is used, and after calculating the final arrival time, the time obtained by subtracting a certain margin time is obtained, and within this time Allows overload operation and protects the transformer by issuing an alarm or breaking the load after that time.

As a result, even when an overload fluctuation occurs, the protection relay characteristic that is close to the overload characteristic of the transformer can be realized as a result.

(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.

First, 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 = Θ _gN · K ²ⁿ (deg.) …… (3) where Θa is the equivalent ambient temperature of the transformer (refrigerant temperature such as air) and Θ ₀ is the maximum of 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 Θg _N shows the value of Θg under rated load. . Furthermore, K is the overload rate (= P / P _N = I / I _N : P, I
Is the actual load value and P _N , I _N is the rated load value. However, the voltage is constant. ), R is the ratio of load loss to no load loss at the rated load, and 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 winding maximum point temperature and the maximum oil temperature The difference 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 overall overload characteristics of the transformer. 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 of 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, the above (1)
From the formula, Θ _H (t) = Θ _a + Θ ₀ (t) + Θ _g (t) (9) Substituting equations (4) and (5) above, So if you organize this, Substituting equations (2) and (3) above this equation, Note that K _i represents a load factor (pre-load factor = I _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 In this equation (15), R >> 1, so if erased, 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 =
Θ _ON + Θ _gN , and Θ _H Θ _a + Θ ₀ + Θ _g to Θ
_{Since H} − Θ _a = Θ ₀ + Θ _g , considering the winding temperature (t) normalized by (Θ _{HN −} Θ _a ) from these relationships, (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

The equation (21), since the load factor is indicative of the normalized winding temperature (t) every moment in the case of changes from K _i to Ku, by (21), winding the highest point 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 , in other words, By obtaining the overload allowable 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 characteristics of d (t) / dt can be variously changed, and the protection relay characteristics can be realized in which the life loss characteristics are sufficiently taken into consideration in addition to the winding maximum temperature characteristics of the transformer.

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 equation is assumed to be in the range from 0 to, for example, 15 hours, the time equivalent values t ^* corresponding to these upper and lower limits are set, and comparison with the settling value M is performed 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 passing the rated load (10
(0%), theoretically, it is possible to drive to infinity time, 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 equation (24) is calculated 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.

For this reason, in the present invention, a new settling value M ′ or M ″ is obtained by taking the winding temperature rise up to the point of change of overload into consideration and subtracting it from the initial settling value M, that is, the settling value. By correcting M, and using the corrected set values M ′ to M ″, the determination by the above equation (24) is performed to obtain the overload allowable time from the point of overload change, and the overload allowable time is exceeded. It was decided to determine the final allowable overload time by adding the time until the load changed.

First, in FIG. 4, the load P changes to P ₁ at time t ₀ (overload rate K ₁ ), and after the protection relay starts calculation, the load further increases at time t ₁ after a certain time TP ₁ has elapsed. Load factor and normalized winding temperature characteristics when the load changes such that the load increases to P ₂ (overload factor K ₂ ) and the load returns to P ₁ again at time t ₂ after a certain time TP ₂ has passed. It is the figure which predicted the state of change of.

In this case, if the overload P ₁ continues without change, (t) will reach the initial set value M after the overload allowable time T _M has elapsed. However, the load is increased to P ₂ at time t _1, the time t ₁ later can not be used setting value M when the load is P _1, it is necessary to adopt a new setting value M '.

The new settling value M ′ is the value (operation detection value) of the winding temperature (t) normalized by the equation (21) at time t ₁ and the time t ₀ (t = 0) when the overload occurs. ) Is subtracted from the settling value M to obtain the following setting.

That is, if the overload persists at P ₂ ,
(T) after time t _1, the time T _M 'to the time after a lapse of setting value M'
Will be reached.

However, at time t ₂ , the load returns to P ₁ again, so M ″ must be adopted as a new settling value.

This new settling value M ″ is set as follows by subtracting from the settling value M ′ the difference between the operation detection value at time t _{2 and} the operation detection value at time t ₁ when overload occurs. I will.

In this way, since the final set value after correction is set as M ″, the temperature judgment formula of the formula (24) is Becomes

In other words, if we find the minimum T _M ″ that satisfies this judgment formula,
This is the final allowable overload time from time t ₀ (TP ₁ + TP ₂ +
It is clear that the overload limit will be reached after the time T _M ″) has passed, so as with the case of constant load, a certain margin time is obtained before the overload allowable time (TP ₁ + TP ₂ + T _M ″) elapses. The operation command such as load shedding is obtained with T _N.

The allowable overload time (TP ₁ + TP ₂ + T _M ″) in this case corresponds to T _M in FIGS. 1 and 2, and the load increased midway as compared with the case in FIG. , T _M is predicted to reach the overload limit in a shorter time.

Next, in FIG. 5, the load P changes to P ₁ (overload rate K ₁ ) at time t ₀ , and after the protection relay starts calculation, the load P changes at time t ₁ after a certain time TP ₁ has elapsed. ₃ (Overload rate K
₃ ), and at time t _{2 when} TP ₂ has passed for a certain period of time, the change in load factor and normalized winding temperature characteristics when a load fluctuation occurs such that the load returns to P ₁ again is shown. It is an expected figure.

In this case, first, the function (t) representing the normalized winding temperature characteristic is the time when the overload state is maintained at P _1.
The initial set value M will be reached after the lapse of T _M.

However, the load decreased to P ₃ at time t, so
After t ₁ , the settling value M when the load is P ₁ cannot be used, and in this case also, it is necessary to adopt a new settling value M ′. This M'is the same as the equation (25) above. Becomes

Since FIG. 5 shows an example in which the load P ₃ is reduced to the rated load or less, the winding temperature characteristic (t) becomes a damping characteristic. Therefore, if the load state continues to remain at P ₃ , (t) is always smaller than the settling value M ′.
The overload limit is never reached.

However, in the time t _2, the the load is restored P ₁ again, so that the must employ M "as more new setting value.

This new settling value M ″ is calculated by adding the difference between the motion detection value at time t _{2 and} the motion detection value at time t ₁ to the settling value M ′, as shown in equation (29) below. This equation sets K _i in equation (21) to K ₁ in order to make (t) a damping equation in consideration of the decrease in oil temperature due to the elimination of the overload condition. The set value M ″ after correction is set as follows so that it does not become larger than M.

Since the final settling value M ″ after correction is set in this way, the temperature judgment formula of the formula (24) is Becomes If the minimum T _M ″ that satisfies this judgment formula is obtained,
This is the final allowable overload time from time t ₀ (TP ₁ + TP ₂ +
It is clear that the overload limit will be reached at the time when T _M ″) has passed, and thus the overload allowable time (TP ₁ + TP ₂
It is sufficient to obtain an operation command such as load shedding with a margin time T _N before the lapse of + T _M ″).

And the allowable overload time in this case (TP ₁ + TP ₂ + T _M ″)
Also corresponds to T _M in Fig. 1 and Fig. 2, and the load decreases halfway compared to the case in Fig. 2, so it is predicted that the overload limit point will be reached at a time later than T _M. is doing.

In the above example, the overload condition is P ₁ → P ₂ → P ₁ (K ₁ →
Since K ₂ → K ₁ ) or P ₁ → P ₃ → P ₁ (K ₁ → K ₃ → K ₁ ), it is possible to directly use K _i as the preload, and the equation (24) is used. Only the settling value in the temperature judgment formula of M → M ″
It is possible to calculate the time T _M ″ until the winding maximum temperature reaches the limit value by the above-described convergent calculation method or the like simply by changing 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 predicted quickly and accurately, 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. Even in a system in which the load changes frequently, it is possible to perform overload operation for a long time by making maximum use of the transformer capacity.

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 according to the present invention, FIG. 3 is an explanatory diagram of a convergence calculation method, and FIG. And FIG. 5 is a graph showing a normalized winding temperature characteristic with respect to load fluctuation, and FIG. 6 is a graph for explaining a 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 hot water 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 setpoint value 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. An overload protection relay method for a transformer, characterized in that a command is obtained and the settling value is corrected in accordance with a change in the overload.