JP2527147B2 - Relay overload protection relay method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an overload protection relay system for a transformer, and more specifically, it allows an overload operation of a main transformer of a power system to a limit of a transformer capacity. The present invention relates to a protection relay method using a digital relay that is enabled.

(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 / P _N = I / I _N ). Note that P and I represent actual load values, and P _N and I _N represent rated load values, respectively. In the past, the magnitude of the secondary current of the transformer was grasped stepwise 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 method, there is a possibility that the overload near the operating value limit of the overcurrent relay or the like may not be interrupted, and the overload that has a time margin for the transformer may occur. On the other hand, there is a case where the power must be shut off in a short time, and there is a disadvantage that the overload operation in the allowable range cannot be sufficiently performed 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 by the convergence calculation method using a predetermined temperature judgment formula, and the load is not cut off within this time (overload allowable time). By approximating the characteristics of the protection relay to the transformer overload characteristics to enable operation in an overload condition, the transformer capacity is maximized to enable a more stable supply of electric power and It is to provide an overload protection relay method.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides an overcurrent protection circuit that detects an overload condition of a transformer to be protected and performs a protection calculation for a load cutoff or the like by a digital protection relay. In the load protection relay method, based on the time constant of the oil temperature change of the transformer, the rated oil temperature ratio, the preload rate before overload and the overload rate, etc., the transformer interline temperature as a function of time. The temperature judgment formula that compares the added value of the differential value with the set value of the winding maximum temperature is calculated, and when comparing the added value and the set value with this temperature judgment formula, the actual time after overload is calculated. Using the corresponding time equivalent value to obtain the convergent body of the time equivalent value in the temperature determination formula, and then converting this convergent value into real time to overload until the winding maximum temperature reaches the settling value. Calculate the permissible time and calculate this overload permissible time The operation command such as load shedding is obtained before the passage 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. Detects that the transformer is overloaded from the input current of the protection relay and predicts the time from that point until the transformer maximum winding temperature reaches the limit value using the convergent calculation method. To do. Then, after the arrival time is calculated, a time obtained by subtracting a certain margin time is obtained, overload operation is allowed within this time, and after the lapse of that time, an alarm is issued or the load is cut off to transform the transformer. Protect.

As a result, the protective operation can be performed on the safer side while employing the overload operation of the transformer as much as possible.

(Example) An example of the present invention will be described below with reference to the drawings. First, the overload characteristic of the transformer to be protected is determined by the maximum temperature characteristic of the transformer winding and the life loss characteristic due to the winding temperature.Therefore, the protection relay characteristic to be obtained is based on both of these characteristics. It is necessary to approximate the overload characteristics.

First, Technical Report of the Institute of Electrical Engineers of Japan (Part I) No. 99 (issued in June 1969), Part I (Part I) No. 143 (issued in November 1986)
As is clear from "Oil-immersed transformer operation guideline" and the like, the winding highest point temperature Θ _H in a steady state when a constant load P is applied to the transformer is Θ _H = Θ _a + Θ _O + Θ _g ( ℃) ・ ・ ・ (1) Where Θ _O = Θ _ON {(K ² · R + 1) / (R + 1)} ^m (deg.)
…… (2) Θ _g = Θ _gN · K ²ⁿ (deg.) …… (3), Θ _a is the equivalent ambient temperature of the transformer (refrigerant temperature such as air), and Θ _O is the transformer oil. The maximum oil temperature rise, Θ _g is the difference between the winding maximum temperature and the maximum oil temperature, Θ _ON is the value at the rated load of Θ _O , and Θ _gN is the value at the rated load of Θ _g. Shown respectively. Furthermore, K is the overload rate (= P / P _N = I / I _N :
P and I are actual load values and P _N and I _N are rated load values. 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 in the transient state Θ _O and the winding maximum point temperature and the maximum oil temperature The difference Θ _g 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 when t = 0, τ
_O represents the time constant of change in oil temperature, and τ _g represents 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
It is assumed that the life is reached when = 1.

V ₁ = h ₁ / Y ₁ = (1 / Y ₀ ) ・ {h ₁ / (Y ₁ / Y ₀ )} (6) where Y ₀ is the maximum temperature when operating continuously at 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

Further, the maximum temperature Θ changes with time and Θ = f
The life loss V represented by (t) 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, the temperature characteristics of the highest winding point of the transformer and the life loss characteristics of the transformer due to the winding temperature are obtained from the above equations (1) to (3) and (8). , It is only necessary to approximate the transformer overload characteristics that are comprehensively determined by these, but it is difficult to calculate the life loss online, so here, only the winding maximum temperature characteristics are used. It is assumed that the time to reach the temperature limit point is predicted and calculated, and the life loss characteristic is considered 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 O (t) + Θ g (t) becomes a ... (9). Here, by substituting the equations (4) and (5), So if you organize this, Substituting the equations (2) and (3) into 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 obtained 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 _O (t) + Θ _g (t), of which Θ _O (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 _O (t) to Θ _ON is obtained. In addition,
Ku 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 _O (t).

That is, by substituting Θ _gN based on equation (3) into equation (5), Therefore, if τ _{g is set} to almost 0 compared to τ _O , it can be approximated as Θ _g (t) / Θ _gN ≈ Ku ² …… (18) However, the maximum winding temperature at the rated load Given the _{Θ HN, Θ HN = Θ a} + Θ ON + Θ gN, ie, Θ _HN -Θ _a =
Since Θ _NO + Θ _gN and Θ _H = Θ _a + Θ _O + Θ _g to Θ _H − Θ _a = Θ _O + Θ _g , the relation normalized by (Θ _{HN −} Θ _a ) is used. Considering the line temperature (t), (t) = (Θ H -Θ a) / (Θ HN -Θ a) = (Θ O + Θ g) / {(Θ a + Θ ON + Θ gN) -Θ a} = (Θ _O + Θ _g ) / (Θ _ON + Θ _gN ) ... (19) This equation (19) is (t) = (Θ _ON / (Θ _ON + Θ _gN )} · (Θ _O / 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 _H
It is possible to find the time t until (t) reaches the set value M.

That is, if overload occurs at time t ₀ in FIG. 1, (t) = M on the curve (t).
A (set value) and a previous time T _M1 is the time that can tolerate the overload operation, because it is apparent that to reach the overload limit point of the transformer at time (t ₀ + T _M1), 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),
A value obtained by calculating a value d (t) / dt obtained by time-differentiating (t) and adding it to (t), that is, (t) + d (t)
By using / dt for the determination of the set value M, it was found that the protection operation can be performed on the safer side while allowing the overload operation of the transformer as much as possible.

Differentiating the above equation (21), Therefore, Becomes

Therefore, at time t ₀ when the overload P ₁ occurs, K _i and Ku are detected from the input current to the protection relay, and these K _i , Ku and τ _O , k are substituted into the equation (23) and (t ) + D
If (t) / dt is set equal to the set value M, the time t until the winding maximum temperature reaches the set value M as shown in FIG.
(= 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, , Ratio of input current I to protection relay and rated current I _N (= I / I _N )
Is a set value (for example, 8%
The calculation shall be started when it reaches 0%).

Therefore, τ _O , M, k and the margin time T _{N in the} equation (24) and the settling range of the calculation starting point I / I _N are as shown in Table 1 below.

In Table 1, by properly correcting the settling values of τ _O and k, the curve on the right side of equation (24), that is, (t)
The characteristics of + d (t) / dt can be changed variously,
It is possible to realize protection relay characteristics that fully consider the life loss characteristics in addition to the temperature characteristics of the winding maximum point of the transformer.

Now, as described 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, in the present invention, as described below. It is assumed that T _M is predicted by the convergence calculation method.

This convergence calculation method means that the time t in the equation (24) is 0.
, For example up to 15 hours, the time equivalent values t ^* corresponding to these upper and lower limits are set, and in this range, the settling value M is compared by the formula (24), and (24)
By converting the time equivalent value t ^* when the right side of the equation becomes M or more into the real time t, the time t which is the overload limit point
(= T _M ). Here, the purpose of setting the upper limit of the time t to 15 hours is, for example, from the overload characteristics of the transformer,
It is assumed that the maximum time that can be continued in an overload state is about 15 hours, and after 15 hours, theoretically, at the rated load (100%), the operation can be continued until infinity. Since it is possible, it means that 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.

By doing this, the time can be increased by a maximum of 10 times.
The T _M can be calculated.

(Effects of the Invention) As described in detail above, according to the present invention, a temperature determination formula that approximates the winding temperature curve of a transformer is obtained, and according to this determination formula, an arbitrary overload is transformed at the time of its occurrence. The time to reach the overload limit point of the transformer is quickly and accurately predicted by the convergence calculation method, and the characteristics of the protection relay based on this prediction calculation are obtained to protect the transformer. It is possible to carry out overload operation for a long time by making the best use of.

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. FIG. 4 is a graph for explaining the characteristics of a conventional overload protection relay.

Claims (1)

(57) [Claims] 1. An overload protection relay method for detecting an overload condition of a transformer to be protected and performing a protection calculation for load cutoff, etc. by a digital protection relay, when the oil temperature of the transformer changes. Based on constants, rated oil temperature ratio, preload rate before overload and overload rate, etc., set the winding temperature of the transformer as a function of time and the derivative of the winding temperature to set the winding maximum temperature. Obtain a temperature judgment formula to be compared with the value, and when comparing the added value and the settling value by this temperature judgment formula, the time equivalent value in the temperature judgment formula is used by using the time equivalent value corresponding to the actual time after overload. Find the convergence value of
After that, the convergence value is converted into a real time to calculate an overload allowable time until the winding maximum temperature reaches the settling value, and an operation command such as load shedding is given before the overload allowable time elapses. A relay method for overload protection of a transformer, characterized by being obtained.