Evaluation of Distribution System Losses Due to Unbalanced Load in Transformers a Case Study of Guinness 15MVA, 33/11KV, Injection Substation and its Associated 11/0.415kv Transformers in Benin City, Nigeria

Evaluation of Distribution System Losses Due to Unbalanced Load in Transformers a Case Study of Guinness 15MVA, 33/11KV, Injection Substation and its Associated 11/0.415kv Transformers in Benin City, Nigeria

Egwaile J. O; Onohaebi S.O; Ike S. A;

Department of Electrical/Electronic Engineering, University of Benin, Benin City, Nigeria.

Abstract

Distribution network losses can vary significantly depending on the load unbalance. Here, an analysis of distribution system losses is presented that considers load unbalance and the effect on the copper losses of a power distribution transformer. The study was carried by analyzing the load readings taken from all the public 11/0.415kv transformers fed from the Guinness injection substation. Comparison was made between the transformer copper losses calculated from the existing unbalanced load condition and the losses that would have resulted if the loads on the transformer were equally distributed among the phases

The result shows that high levels of load unbalance produced greater losses in the transformers, and the total transformer copper losses on both feeders considered can be reduced by about 6% if steps are taken to balance the loads on the phases of the transformer. This will ultimately lead to economic savings, and increase in the systems availability and reliability.

Keywords: Distribution networks, transformer, losses, load unbalance, three-phase load.substation, phase currents

1.0 Introduction

Energy efficiency, in limited energy resources scenario, is considered as a source of energy in a distribution system. [1]. This is particularly important in a country like Nigeria whose distribution System is faced with low voltage and high loss, these two problems of high voltage drop and losses in the distribution network varies with the pattern of loading on the distribution network. [2]. Since system losses represent a considerable cost for utilities and energy consumers, its evaluation and reduction have been recognized as of interest by researchers. There are many distribution network devices responsible for energy loss, these includes losses along distribution feeders, losses in transformer windings and losses associated with unbalanced loads connected to transformers.

Unbalanced load is a common occurrence in three- phase distribution systems. However, it can be harmful to the operation of the network components, its reliability and safety. Thus, a distribution system unbalance phenomenon has been the focus of research in recent decades [3]-[5]. Therefore, considering the importance of loss analysis, the objectives of this work is to evaluate losses due to unbalanced loading in a transformer. Transformers are the link between the generators of the power system and the transmission lines and between lines of different voltage levels.[13]. Transformers power losses can be divided into two main components: no-load losses (hysteresis and eddy current losses) and load losses (ohmic heat losses and conductor eddy current losses). There are, however, other two types of losses namely extra losses created by harmonic and unbalanced currents flowing in the transformer winding, respectively.

2.1 Transformer Losses

Three-phase power transformers are invariably used in transmission, sub transmission and distribution substations for essentially voltage transformation. The power transformer is a complex static electromagnetic machine with windings and a non-linear iron core .[6]. We will first present the transformer equivalent circuit in actual physical units then relate the losses in a transformer to these units.

Figure 1.0: Basic transformer equivalent circuit

An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers, energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those

rated for electricity distribution usually perform better than smaller ones.[7]

Transformer losses are produced by the electrical current flowing through the coils and the magnetic field

alternating in the core. The losses associated with the coils are called load losses, while the losses produced in the core are called no-load losses.

The no-load losses are basically the power required to keep the core energized. These are commonly referred to as core losses, and they exist whenever the unit is energized. No-load losses depend primarily upon the voltage and frequency, so under operational conditions they vary only slightly with system variations.

Load losses, as the terminology might suggest, result from load currents flowing through the transformer. The two components of the load losses are the I2R losses and the stray losses. I2R losses are based on the measured dc (direct current) resistance, the value of which is due to the winding conductors and the current at a given load. The stray losses is a term given to the accumulation of the additional losses experienced by the transformer, which includes winding eddy losses and losses due to the effects of leakage flux entering the internal metallic structures.

Auxiliary losses refer to the power required to run auxiliary cooling equipment, such as fans and pumps;

they are not normally included in the total losses as defined below.

Total losses in a transformer can be summed up as shown in equation 1.1

PTL = PNL + PL 1.1

Where

PTL = Total Losses PNL = No load Losses PL = Load Losses

2.1.1 No Load Losses

Early transformer developers realized that cores constructed from solid iron resulted in prohibitive eddy current and hysteresis loss (as high as 99% of the no-load losses), and their design mitigated this effect with cores consisting of stack layers of laminations, a principle that has remained in use.[8][9]. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost.

1. Hysteresis losses: Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. According to Steinmetz's formula [11][12], the heat energy due to hysteresis is given by

   , and, 1.2

   hysteresis loss is thus given by

   1.3

   where, f is the frequency, is the hysteresis coefficient and max is the maximum flux density, the empirical exponent of which varies from about

   1.4 to 1 .8 but is often given as 1.6 for iron.[11]

2. Eddy current losses: Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single short- circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse Square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores[10][12]. Referring to equivalent circuit of figure 1.0, Core loss and reactance is represented by the following shunt leg impedances of the model:

• Core or iron losses: RC

• Magnetizing reactance: XM.

  RC and XM are sometimes collectively termed the

  magnetizing branch of the model.

  2.1.2. Transformer load losses [7]

  These losses are commonly called copper losses or short circuitlosses. Load losses vary according to the

  transformer loading; they are composed of Ohmic heat losses, sometimes referred to as copper losses, since this resistive component of load losses dominates. These losses occur in transformer windings and are caused by the resistance of the conductors. The magnitude of these losses increases with the square of the load current and are proportional to the resistance of the windings. They can be reduced by increasing the cross section of conductor or by reducing the winding length. Using copper as the conductor maintains the balance between weight, size, cost and resistance; adding an additional amount to increase conductor diameter, consistent with other design constraints, reduces losses.

  Referring to equivalent circuit of figure 1.0 above, Winding joule losses and leakage reactance are

  represented by the following series loop impedances of the model:

  • Primary winding: RP, XP

  • Secondary winding: RS, XS.

Mathematically copper loss in a transformer is given by:

Copper losses = I2R 1.4

Where I is the load current and R is resistance of the transformer winding.

There is, however, another type copper loss created as a result unbalanced currents flowing in the phases of the transformer.

For a three phase transformer, let the secondary load currents flowing in each of the phases be

IR, IY, and IB respectively. Thus total load current =

and its associated 11/0.415kV transformers in Benin City, Nigeria

1. Network Overview

   Guinness Injection substation is located along the Benin Agbor road, immediately after Guinness Nigeria Limited Brewery premises. The substation is fed by the Ikpoba Dam 33kv feeder which radiates from the Benin 132/33kv transmission station along Benin/Sapele road, Benin City. The one line diagram showing the source of power for the Guinness injection substation is shown in figure 1.1

   132KV BUS

   60MVA, 132/33KV

   33KV BUS

   IT = IR + IY + IB 1.5

   Copper losses in each phase = R (Red Phase), R (Yellow Phase), R, (Blue Phase).

   Where R is the winding resistance of the transformer per phase. Therefore

   Total copper loss =

   R + R + R = + + ) 1.6

   If the load on the transformer is balanced,

   BDPA FEEDER

   IKPOBA DAM FEEDER

   33KV BUS

   GUINNESS 15MVA, 33/11KV INJECTION SUBSTATION

   11KV BUS

   ASABA ROAD

   then IR

   = IY

   = IB = I

   FEEDER

   Therefore equation 1.6 becomes:

   Total copper loss = R( I2 + I2 + I2)=3 I2R 1.7

   Equation 1.7 gives the total copper losses in a transformer under balanced load condition, while equation 1.6 gives the total copper losses for unbalanced load.

   Subtracting equation 1.7 from 1.6 yields:

   R( + + ) – 3 I2R = Ploss unbalanced load

   R(( + + ) – 3 I2) 1.8

   Equation 1.8 shows that the total losses in a transformer would be higher as a result of unequal current flowing through the different phases of the transformer under unbalanced load condition.

   In the section that follows we will evaluate the total losses due to unbalanced transformer loading in the Guinness 15MVA, 33/11KV, Injection Substation

   Figure 1.0: One line Diagram Showing Power Source for the Injection Substation.

   As shown in figure 1.1, from the Guinness injection substation radiates two (2) feeders. Asaba Road feeder and BDPA feeder. The Asaba Road feeder has a total of fifty seven (57) 11/0.415KV distribution transformers of various ratings connected to it at various points along its length, while the BDPA feeder has a total thirty-seven (37) 11/0.415KV distribution transformers connected to it at various points along its length.

2. Collection of Data

The injection substation under review was visited and the following data was collected:

1. Single lines diagram of the Guinness 15MVA, 33/11/0.415KV injection substation and its associated feeders.

2. Document containing list of all 11/0.415kv transformers connected to the substation and their ratings.

Since the load reading for each of the 11/0.415 transformer in the network was not available, we embarked on taking load readings at each transformer (excluding privately owned transformers because these are not severely affected by unbalanced loads) at different times of the day, this we did between October 2011 September 2012) with the help of Mastech Digital Power Clamp; Model MS2203 capable of

measuring power (real and reactive), voltage, power factor and phase current. The average load reading for the period under review and other data collected for the two feeders is presented in tables 1.0 and 1.1 .

Table 1.0: Substation Parameters; (Asaba Road feeder)

			Average Load Current (Amps)
S/N	NAME OF SUBSTATION	Transformer Rating (KVA)	Red Phase	Yellow Phase	Blue Phase	TOTAL Load Current
1	ADUWAKA	300	124	121	40	285
2	OWIE	500	222	260	157	639
3	ODIONVBA	500	171	125	279	575
4	OGBESON PALACE	500	262	188	212	662
5	IYOBOSA	500	250	228	143	621
6	ADAZE	300	88	58	101	247
7	EBIKADE	500	150	317	197	664
8	OHOVBE PALACE	500	132	67	35	234
9	IGABOR	500	68	30	12	110
10	UNITY	200	25	21	32	78
11	UGOKPOLOR	500	220	280	207	707
12	LIBERTY	500	192	3	124	319
13	IGBINIDU	500	36	11	106	153
14	AZAGBA	300	129	65	196	390
15	SONOWE	300	62	56	70	188
16	PHILOVE JUNCTION	300	105	22	37	164
17	DAO	100	30	70	24	124
18	PIPELINE	500	294	155	133	582
19	UWAIMA II	500	181	116	8	305
20	UWAIMA II	500	80	110	100	290
21	UYIGUE	00	346	359	352	1057
22	AMUFI	500	179	159	8	346
23	UGBOZIGUE	500	220	230	243	693
24	JEHOVAH	300	228	76	261	565
25	AGBOWO 1	500	230	191	83	504
26	AGBOWO 11	500	191	118	92	401
27	IGUOMON 1	500	126	170	176	472
28	IGUOMON 11	500	130	160	180	470

29	IKHUENIRO 1	500	218	186	101	505
30	IKHUENIRO 11	500	216	197	87	500
31	NEPASCO 1	500	145	342	315	802
32	NEPASCO 11	300	112	123	116	351
33	ST MICHEAL	300	87	69	78	234
34	AFENGE	500	219	221	219	659
35	EHIKHIANMWEN	500	346	359	355	1060
36	BULLSEYE	500	115	135	177	427
37	GODIAC	500	219	253	165	637

Table 1.1: Substation Parameters (BDPA feeder)

			Average Load Current(Amps)
S/N	NAME OF SUBSTATION	Transformer Rating (KVA)	Red Phase	Yellow Phase	Blue Phase	TOTAL Load Current
1	ARUNDE	300	270	283	335	888
2	EVBADOLOYI	300	337	193	200	730
3	IHASE	500	85	126	178	389
4	ASOWATA	315	262	167	54	483
5	OSASUMWEN	200	126	133	109	368
6	ALAGHODARO	300	122	136	95	353
7	IFASUYI	300	209	258	108	575
8	OTABOR	500	146	185	58	389
9	OWANAZE	300	207	250	120	577
10	SUNNY FOAM	500	300	330	142	772
11	POGAH	500	300	268	330	898
12	POGAH RELIEF	300	268	200	190	658
13	UGBOKODU	300	206	240	119	565
14	ARMY SIGNAL	500	300	340	280	920
15	OBASUYI	500	306	269	340	915
16	OBASUYI 2	500	90	130	180	400
17	BDPA 1	500	340	260	350	950
18	BDPA 2	500	237	203	135	575
19	BDPA 3	500	240	124	122	486
20	OBANOSA	300	136	160	104	400

2.4.1 Data Analysis

From tables 1.0 and 1.1, the copper losses for each transformer in the network was calculated using equation 1.6; i.e.

Total copper loss (unbalanced load condition) = R + R + R = R + +).

In doing this the winding resistance per phase is assumed to be unity since this value is the same and

constant for all phases of the transformer irrespective of loading.

Next balanced load condition was considered, which implies that the total load current will be shared equally among the phases of the transformer.

Under this condition equation 1.7 holds. i.e.

Total copper loss (balance load condition) = R ( I2 + I2 + I2) = 3 I2R. Thus the total copper losses under balanced load condition were also calculated.

The results for the two feeders are presented in table 1.3 and 1.4 respectively.

Table 1.3: Copper Losses for BDPA feeder
			Unbalanced Load Condition				Balanced Load Condition
S/N	NAME OF SUBSTATION	Transfor mer Rating (KVA)	Total CU Loss (Red Phase)	Total CU Loss (blue Phase)	Total CU Loss (yellow Phase)	Total loss in the Three Phases	Total CU Loss (Per Phase)	Total loss in the three Phases
1	ARUNDE	300	72900	80089	112225	265214	87616	262848
2	EVBADOLOYI	300	113569	37249	40000	190818	59211.11	177633.3
3	IHASE	500	7225	15876	31684	54785	16813.44	50440.33
4	ASOWATA	315	68644	27889	2916	99449	25921	77763
5	OSASUMWEN	200	15876	17689	11881	45446	15047.11	45141.33
6	ALAGHODARO	300	14884	18496	9025	42405	13845.44	41536.33
7	IFASUYI	300	43681	66564	11664	121909	36736.11	110208.3
8	OTABOR	500	21316	34225	3364	58905	16813.44	50440.33
9	OWANAZE	300	42849	62500	14400	119749	36992.11	110976.3
10	SUNNY FOAM	500	90000	108900	20164	219064	66220.44	198661.3
11	POGAH	500	90000	71824	108900	270724	89600.44	268801.3
12	POGAH RELIEF	300	71824	40000	36100	147924	48107.11	144321.3
13	UGBOKODU	300	42436	57600	14161	114197	35469.44	106408.3
14	ARMY SIGNAL	500	90000	115600	78400	284000	94044.44	282133.3
15	OBASUYI	500	93636	72361	115600	281597	93025	279075
16	OBASUYI 2	500	8100	16900	32400	57400	17777.78	53333.33
17	BDPA 1	500	115600	67600	122500	305700	100277.8	300833.3
18	BDPA 2	500	56169	41209	18225	115603	36736.11	110208.3
19	BDPA 3	500	57600	15376	14884	87860	26244	78732
20	OBANOSA	300	18496	25600	10816	54912	17777.78	53333.33

Table 1.4: Copper Losses for Asaba Road feeder
			Unbalanced Load Condition				Balanced Load Condition
S/N	NAME OF SUBSTATION	Transformer Rating (KVA)	Total CU Loss (Red Phase)	Total CU Loss (blue Phase)	Total CU Loss (yellow Phase)	Total loss in the Three Phases	Total CU Loss (Per Phase)	Total loss in the three Phases
1	ADUWAKA	300	15376	14641	1600	31617	9025	27075
2	OWIE	500	49284	67600	24649	141533	45369	136107
3	ODIONVBA	500	29241	15625	77841	122707	36736.11	110208.3
4	OGBESON PALACE	500	68644	35344	44944	148932	48693.78	146081.3
5	IYOBOSA	500	62500	51984	20449	134933	42849	128547
6	ADAZE	300	7744	3364	10201	21309	6778.778	20336.33
7	EBIKADE	500	22500	100489	38809	161798	48988.44	146965.3
8	OHOVBE PALACE	500	17424	4489	1225	23138	6084	18252
9	IGABOR	500	4624	900	144	5668	1344.444	4033.333
10	UNITY	200	625	441	1024	2090	676	2028
11	UGOKPOLOR	500	48400	78400	42849	169649	55538.78	166616.3
12	LIBERTY	500	36864	9	15376	52249	11306.78	33920.33
13	IGBINIDU	500	1296	121	11236	12653	2601	7803
14	AZAGBA	300	16641	4225	38416	59282	16900	50700
15	SONOWE	300	3844	3136	4900	11880	3927.111	11781.33
16	PHILOVE JUNCTION	300	11025	484	1369	12878	2988.444	8965.333
17	DAO	100	900	4900	576	6376	1708.444	5125.333
18	PIPELINE	500	86436	24025	17689	128150	37636	112908
19	UWAIMA II	500	32761	13456	64	46281	10336.11	31008.33
20	UWAIMA II	500	6400	12100	10000	28500	9344.444	28033.33
21	UYIGUE	500	119716	128881	123904	372501	124138.8	372416.3
22	AMUFI	500	32041	25281	64	57386	13301.78	39905.33
23	UGBOZIGUE	500	48400	52900	59049	160349	53361	160083
24	JEHOVAH	300	51984	5776	68121	125881	35469.44	106408.3
25	AGBOWO 1	500	52900	36481	6889	96270	28224	84672
26	AGBOWO 11	500	36481	13924	8464	58869	17866.78	53600.33
27	IGUOMON 1	500	15876	28900	30976	75752	24753.78	74261.33
28	IGUOMON 11	500	16900	25600	32400	74900	24544.44	73633.33
29	IKHUENIRO 1	500	47524	34596	10201	92321	28336.11	85008.33
30	IKHUENIRO 11	500	46656	38809	7569	93034	27777.78	83333.33
31	NEPASCO 1	500	21025	116964	99225	237214	71467.11	214401.3
32	NEPASCO 11	300	12544	15129	13456	41129	13689	41067
33	ST MICHEAL	300	7569	4761	6084	18414	6084	18252
34	AFENGE	500	47961	48841	47961	144763	48253.44	144760.3

35	EHIKHIANMWEN	500	119716	128881	126025	374622	124844.4	374533.3
36	BULLSEYE	500	13225	18225	31329	62779	20258.78	60776.33
37	GODIAC	500	47961	64009	27225	139195	45085.44	135256.3

1. Discussion

   The results from the copper losses calculations for both balanced and unbalanced load conditions and the resultant tables (table 1.3 & 1.4 above) show that:

   1. The copper losses of transformer varies considerably with the degree of load unbalance. (b)The total transformer copper loss in the Asaba road feeder is 3547002 units and 3318863 units for balance and unbalanced load conditions respectively.

      1. The total transformer copper losses in the BDPA feeder is 2937661units and 2802828units for balance and unbalanced load conditions respectively

      2. The total transformer copper losses on both feeders can be reduced by about 6% if steps are taken to balance the loads on the phases of the transformer.

      3. Unbalanced loading will reduce the capacity of the transformers since the protective devices of the overloaded phase will operate even before other phases senses overload.

2. Conclusion and Recommendations.

In this paper loss evaluation in distribution systems considering both unbalanced load and balanced load Scenarios in the transformers is presented.

The study shows that high levels of load unbalance produced greater losses in the transformers. This means that network reconfiguration considering load balancing is highly recommend in order to diminish overall system losses. It is recommended that balanced repartition of single-phase loads between the phases of the three-phase network should be vigorously pursued by the authorities concerned. The authorities concerned should also ensure that all the phases are always available to discourage consumers from shifting their loads when a phase fails. To this end, it is recommended that automatic phase monitors that would promptly report an open phase be installed in all the transformers in the network. This will not only help to reduce losses but it will also enhance the systems availability and reliability.

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